STUDIES ON THE TECHNOLOGICAL IMPROVEMENTS OF THE ...Chiral organocatalysts traditionally operate via...

POLITECNICO DI MILANO

Scuola di Ingegneria Industriale e dell’Informazione

Tesi di Laurea Magistrale in Ingegneria Chimica

Dipartimento di Chimica, Materiali e Ingegneria Chimica

"Giulio Natta"

STUDIES ON THE TECHNOLOGICAL IMPROVEMENTS

OF THE ORGANOCATALYTIC REDUCTION OF ENALS

Relatore: Dott. Alessandro SACCHETTI

Correlatore: Dott.ssa Arianna ROSSETTI

Roberto PESA

Matr. n. 801006

Anno Accademico 2013-2014

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Gli organocatalizzatori enantioselettivi sono composti di fondamentale importanza per la

sintesi asimmetrica di molecole chirali, in alternativa ai catalizzatori tradizionali, poiché

evitano l'impiego di costosi, e talvolta tossici, metalli di transizione. Solo negli ultimi dieci

anni, questo settore è cresciuto a un ritmo straordinario; da una piccola collezione di

reazioni uniche dal punto di vista chimico si è passati allo studio di reattività atipiche e

reazioni applicabili su larga scala. Inoltre, nuovi modi di attivazione dei substrati sono stati

verificati sperimentalmente, utilizzando catalizzatori organici che permettono quindi di

raggiungere livelli di selettività superiori o complementari rispetto a quelli ottenuti con

reazioni metallo-catalizzate.

Tuttavia, questi catalizzatori sono spesso utilizzati ad alte concentrazioni molari. Così,

nonostante la loro tossicità risulti essere inferiore rispetto a quella di catalizzatori contenenti

metalli, la loro separazione, recupero e riutilizzo sono sicuramente di estremo interesse. Ad

esempio, anche se molte combinazioni ligando/catalizzatore presentano elevata attività e

buona selettività, alcune problematiche relative al loro uso a livello industriale rimangono

irrisolte. Pertanto, gli studi che hanno affrontato le tematiche legate al riutilizzo e recupero

di questi complessi hanno attirato una notevole attenzione. Nella maggior parte dei casi, le

strategie sviluppate per superare ostacoli di questo tipo consistono nell'uso di supporti

polimerici funzionalizzati, facili da isolare una volta portata a termine la reazione.

Gli organocatalizzatori chirali operano tradizionalmente attraverso meccanismi semplificati

che mimano il comportamento di enzimi naturali, da cui emergono le necessità di operare in

condizioni di reazione molto blande e di sviluppare matrici polimeriche chimicamente il più

possibile inerti. Inoltre, sembra che la matrice polimerica influenzi sia l’attività che la

selettività del catalizzatore e, a differenza di quanto tradizionalmente si è sostenuto, il

supporto polimerico tende addirittura a migliorare le prestazioni del catalizzatore, fornendo

un microambiente favorevole attorno al composto attivo.

Nella presente tesi viene sfruttato il concetto di catalisi con ione imminio nelle più

significative tipologie di trasformazioni in cui questo concetto è stato applicato con

successo, tra cui ad esempio: idrogenazioni, alchilazioni di Friedel-Crafts e reazioni

SOMMARIO

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enantioselettive a cascata. Importanti risultati sono stati ottenuti realizzando le suddette

reazioni in ambiente acquoso, aprendo nuove strade nel campo delle reazioni in sequenza

condotte in presenza di organocatalizzatori ed enzimi.

I paragrafi successivi sono dedicati alla produzione e conseguente impiego di catalizzatori

supportati su matrici polimeriche. Il nostro lavoro si è concentrato sullo sviluppo sia di

polimeri insolubili, come le resine con derivati di polistirene (PS), e polimeri solubili, in

particolare glicole polietilenico (PEG) e acido poliacrilico (PAA). Grande interesse è stato

poi dedicato a reazioni di idrogenazione asimmetrica catalizzate da idrogel funzionalizzati

prodotti a partire da PAA e PEG supportati con organocatalizzatori. Mostrando elevata

stabilità, resistenza alle sollecitazioni meccaniche e una bassissima concentrazione di

catalizzatore, si confermano come migliori candidati per reazioni di organocatalisi condotte

in fase eterogenea e forniscono un promettente punto di partenza per applicazioni future.

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Enantioselective organocatalysts are compounds of central importance for the asymmetric

synthesis of chiral molecules, as alternatives to transition-metal catalysts, because they

avoid the use of expensive and sometimes toxic transition metals. In the last ten years only,

this field has grown at an extraordinary rate from a small collection of chemical unique

reactions to a thriving area of general concepts, atypical reactivities, and widely applicable

reactions. Moreover, novel modes of substrate activation have been achieved using organic

catalysts that can now deliver unique, or complementary selectivities in comparison to many

established metal-catalyzed transformations.24

However, these catalysts often are used at high molar concentration. Thus, while their

toxicity can be less than that of metal-containing catalysts, their separation, recovery, and

reuse are surely of interest. For example, although many ligand/catalyst combinations

demonstrate high activity and good selectivity, some concerns regarding the use of these

reactions in industrial applications still remain an unresolved problems. Therefore, studies

that address the reuse and recovery of these compounds have attracted much attention. In

most cases, the strategies that have been developed to bypass these problems involve the use

of polymeric functionalized supports easy to isolate after the reaction is completed.

Chiral organocatalysts traditionally operate via simplified enzyme-mimetic mechanisms,

implicating mild reaction conditions and an improved leverage for the chemical inertness of

the polymer matrix. In addition, the polymer matrix seems to influence both catalyst activity

and selectivity and, unlike what traditionally has been the case, the polymer scaffold can

now even enhance catalyst performance by providing a favourable microenvironment

around the organocatalysts.15

The present dissertation will take advantage to the concept of iminium catalysis in the most

significant types of transformations to which this concept has been successfully applied,

including transfer hydrogenations, Friedel–Crafts alkylations, and enantioselective

organocatalytic cascade reactions. Important results are achieved when all these reactions

are conducted in aqueous media, opening new pathways in the field of the organocatalyst-

enzyme reactions in flow chemistry.

ABSTRACT

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The following sections will describe the production and subsequent employment of

supported catalysts onto polymeric matrices. Our work deals with both insoluble polymers,

as the polystyrene-derived resins (PS), and soluble polymers, particularly poly(ethylene

glycol) (PEG) and poly(acrylic acid) (PAA). Most of the efforts have been employed to

transfer hydrogenation reactions catalyzed by functionalized hydrogels produced using

PAA- and PEG-supported catalysts. With a really high stability, an elevate resistance to

mechanical stresses, and an ultra-low catalyst concentration, they confirm to be great

candidates in heterogeneous organocatalysis and furnish a promising starting point for

future applications.

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1. INTRODUCTION .................................................................................................. 10

1.1 The Importance of Asymmetric Synthesis ............................................................. 10

1.1.1 Enamine Catalysis ........................................................................................... 11

1.1.2 Iminium Catalysis ............................................................................................ 12

1.1.3 Other Related Organic Hydride Donors .......................................................... 15

1.1.4 Thermodynamic Diagnosis of the Organic Hydride Sources .......................... 17

1.2 Organocatalitic Asymmetric Hydrogenation in Aqueous Media ........................... 19

1.3 Polymer-supported Chiral Organocatalysts ............................................................ 22

1.3.1 The Polystyrene-derived Resins ...................................................................... 22

1.3.2 The Acrylic Resins .......................................................................................... 24

1.4 Soluble Polymer-supported Organocatalysts .......................................................... 26

1.5 Polymer-Supported MacMillan Imidazolidinones ................................................. 28

1.5.1 Click Strategy for the Immobilization of MacMillan Catalysts ...................... 30

1.6 Hydrogel-supported MacMillan Organocatalysts ................................................... 33

1.6.1 Hydrogel Formulation ..................................................................................... 34

1.6.2 Steps of Hydrogels Preparation ....................................................................... 36

2. RESULTS AND DISCUSSION .................................................................................. 37

2.1 Preparation of Catalysts .......................................................................................... 37

2.1.1 Synthesis of First Generation MacMillan catalysts [30] and [33] ................... 38

2.1.2 Synthesis of Second Generation MacMillan Catalyst [46a] ........................... 39

2.2 Preparation of the Hydride Donors ......................................................................... 41

2.2.1 Synthesis of Hantzsch ester [1b] ..................................................................... 41

2.2.2 Synthesis of 2-phenyl-2,3-dihydrobenzothiazole [44] .................................... 42

CONTENTS

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2.3 Preparation of the Unsaturated Substrates .............................................................. 43

2.3.1 Synthesis of β-methyl cinnamaldehyde [36] ................................................... 43

2.3.2 Synthesis of 4-chloro-β-methyl cinnamaldehyde [38] .................................... 44

2.3.3 Synthesis of 4-methoxy-β-methyl cinnamaldehyde [40] ................................ 45

2.4 Enantioselective Organocatalytic Transfer Hydrogenation of Enals ...................... 46

2.5 Enantioselective Organocatalytic Hydride Reduction of Enones ........................... 49

2.6 Polymer-supported Organocatalyst: Polystyrene Resin ......................................... 51

2.7 Polymer-supported Organocatalyst: Poly(ethylene glycol) .................................... 56

2.7.1 PEG Functionalization via Ether Synthesis ..................................................... 56

2.7.2 Preparation of Azide and Alkyne Functionalized Poly(ethylene glycol) ........ 57

2.7.3 Preparation of Azide Organocatalyst .............................................................. 58

2.7.4 Azide-Alkyne Cycloaddition for the Synthesis of Supported Catalyst ........... 59

2.7.5 Results Obtained using Soluble Catalyst [52] or [57] ..................................... 60

2.7.6 Catalyst Recovery and Recycling Experiments .............................................. 61

2.8 Polymer-supported Organocatalyst: Poly(acrylic acid) .......................................... 64

2.8.1 Dialysis Technique .......................................................................................... 64

2.8.2 PAA amide-Imidazolidinone Click ................................................................. 65

2.9 Hydrogel Synthesis ................................................................................................. 67

2.9.1 Hydrogel AC-PEG-2000 ................................................................................. 67

2.9.2 Hydrogel AC-PEG-5000 ................................................................................. 68

2.9.3 Results Obtained using Gel Tablets in Aqueous Medium .............................. 69

2.10 Organocatalyst-Enzyme Coupled Reactions ....................................................... 70

2.11 Enantioselective Organo-Cascade Catalysis ....................................................... 73

2.12 Enantioselective Friedel-Crafts Reactions .......................................................... 74

3. EXPERIMENTAL SECTION ................................................................................... 76

3.1 General Remarks .................................................................................................... 76

3.2 Procedures ............................................................................................................... 78

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3.3 General Procedures ............................................................................................... 121

3.3.1 Enantioselective Reduction of Enales ........................................................... 121

3.3.2 Enantioselective Reduction of 3-methylcyclohex-2-en-1-one ...................... 130

3.3.3 Cascade Reactions ......................................................................................... 132

3.3.4 Asymmetric Friedel-Crafts Reactions ........................................................... 135

4. References .................................................................................................................. 137

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1.1 The Importance of Asymmetric Synthesis

Asymmetric catalysis represents one of the most important research areas in modern

synthetic chemistry since it provides quick access to a wide number of enantiopure

compounds employing only catalytic amounts of chiral molecules. Among various

successful asymmetric reactions, asymmetric hydrogenation has been one of the most

extensively studied in academia and widely applied in industry, which can be exemplified

by the fact that two prominent researchers in this field, Knowles and Noyori, were awarded

the Nobel Prize in Chemistry in 2001. In traditional asymmetric hydrogenation processes,

hydrogen gas is typically used as the reducing agent with transition metal-based chiral

catalysts, whereas in the case of asymmetric transfer hydrogenation, isopropanol and formic

acid are the most frequently employed hydrogen sources. Although most of these transition

metal-catalyzed processes show high reactivity and selectivity, some of them still suffer

from considerable drawbacks including limited substrate scope, difficulty in catalyst

separation and recycling as well as the danger in handling hydrogen gas under high

pressure. Nature, on the other hand, performs the asymmetric transfer hydrogenations

(ATH) in an amazing way in biological systems, taking advantage of enzymes and organic

hydride reduction cofactors which are hydride donors, such as NADH (Figure 1.1_a).

Inspired by the manner that nature conducts reduction, chemists have developed a

biomimetic ATH approach employing Hantzsch ester (Figure 1.1_b) and some other

heterocyclic compounds in the presence of catalytic amount of small organic molecules or

metal complexes. Over the past few years there have been significant improvements in this

field. Double bonds such as C=C, C=N and C=O can be successfully reduced affording

versatile chiral building blocks in high yields with typically excellent e.e. under mild

conditions.1

1. INTRODUCTION

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Figure 1.1 The structures of NADH (1a) and Hantzsch ester (1b)

Perhaps most crucial to the success of organocatalysis in the past decade has been the

invention or identification of generic methods for catalyst activation, induction and

reactivity. A generic activation mode consists in the description of a reactive species that

can participate in many reaction types with consistently high enantioselectivity. Indeed,

most of the 130 organocatalytic reactions that have been reported since 1998 are founded

directly on only five or six activation modes.

1.1.1 Enamine Catalysis

In 1971, there were two independent reports (the one by Zoltan Hajos and David Parrish2,

and the other by Rudolf Weichert, Gerhard Sauer and Ulrich Eder3) of an enantioselective

intramolecular aldol reaction that was catalyzed by proline in the synthesis of the Wieland-

Miescher ketone.

Mechanistically, enamine catalysis might be better described as a bifunctional one because

the amine-containing catalyst (proline in Figure 1.2) typically interacts with a ketone

substrate to form an enamine intermediate but simultaneously engages with an electrophilic

reaction partner through either hydrogen bonding or electrostatic attraction. This mode of

activation has now been used in a wide range of enantioselective carbonyl α-

functionalization processes.

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NH

Substrate Catalyst Activation mode

2

RO

Z

X Y

CO2HN

OO

R

ZHYX

Examples of reaction

• Aldehyde-aldehyde crossaldol coupling• Mannich reaction• Michael reaction• a-Amination• a-Oxygenation• a-Halogenation• a-Suphenylation

R = any organic chain orring system

X = C, N, O, SY = generic organic atomZ = alkyl, H

Figure 1.2 Enamine catalysis with some reaction examples

1.1.2 Iminium Catalysis

Iminium catalysis was the first organocatalytic activation mode to be designed (rather than

discovered) and introduced as a general strategy for asymmetric organic synthesis. It is

based on the capacity of chiral amines to function as enantioselective catalysts in several

transformations that traditionally use Lewis acid catalysts. The concept was founded on the

mechanistic hypothesis that the reversible formation of iminium ions from α,β-unsaturated

aldehydes and chiral amines might emulate the equilibrium dynamics and π-orbital

electronics that are inherent to Lewis acid catalysis (that is, lowest-unoccupied molecular

orbital (LUMO)-lowering activation). With its tailor-made family of imidazolidinone

catalysts, iminium catalysis is now used in more than 50 highly enantioselective protocols,

many of which have been developed by David MacMillan and Karl Anker Jorgensen’s

groups4.

Figure 1.3 Iminium catalysis with some reaction examples

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Although the Hantzsch ester-mediated reduction has been established for a long time, it

wasn’t until 2004 that the first catalytic ATH reaction of activated C=C double bond was

reported, owing to the emerging concept of iminium activation. By using the chiral

secondary amine (2S,5S)-3 as catalyst, in conjunction with trichloroacetic acid (TCA), List

and co-workers5 demonstrated the ATH reaction of α,β-unsaturated aldehydes with

Hantzsch ester as the hydride donor (Figure 1.3). Electron-poor aromatic substituted

substrates typically gave good yields and e.e. Almost at the same time, MacMillan et al.

reported a very similar strategy to accomplish the ATH reaction of α,β-unsaturated

aldehydes.

These reactions demonstrated high chemoselectivity due to the fact that the carbonyl group

of the α,β-unsaturated aldehydes is protected as imine during the reduction process.

Another attractive advantage of these reactions is the enantioconvergence, a so desired as

rare feature of a catalytic asymmetric reaction. The geometry of the double bond in the

starting materials has little effect on the enantioselectivity of the final products. It was

proposed that the E- and Z-α,β-unsaturated iminium ion intermediates are in a fast

equilibrium via a conjugate dienamine species. The subsequent rate-limiting hydride

transfer proceeds much easier with the E-isomer [k(E) > k(Z)], which mainly affords a

single enantiomer (Figure 1.4). This feature permits the use of starting materials with low

geometric purity, which undoubtedly enhances the practical utility of this operationally

simple asymmetric reduction.

Figure 1.4 Proposed mechanism of the ATH reaction catalyzed by chiral secondary amine and origin of

the enantioconvergence

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An obvious question after the succes of the ATH reaction of α,β-unsaturated aldehydes with

Hantzsch esters was whether these processes could be applied to α,β-unsaturated ketones,

despite their sterical hindrance and electronic deactivated toward iminium formation.

MacMillan and co-workers developed a simple protocol for the ATH reaction of α,β-

unsaturated ketones by using the furyl-derived imidazolidone catalyst (2S,5S)-4 A series of

cyclic α,β-unsaturated ketones were discovered to be suitable substrates, affording the

hydrogenated products in high yields and e.e. (Figure 1.5).

Figure 1.5 Catalytic ATH reaction of α,β-unsaturated ketones by MacMillan

The Hantzsch ester is able to transfer the hydride preferentially from the less hindered

bottom face of the iminium moiety with an anti mechanism, as shown in Figure 1.6. The

calculated energetic barrier of this transition state was 1.1 kcal·mol-1 lower than its

diastereomeric counterpart, which resulted in the experimentally observed

enantioselectivity.

Figure 1.6 Schematic structure of the transition state for the ATH reactions of cyclic α,β-unsaturated ketone

One of the main differences between the ATH reaction of activated C=C double bonds with

Hantzsch esters and the traditional transition-metal-catalyzed hydrogenation process is the

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stepwise nucleophilic addition of hydride, generating a nucleophile in situ that can be

captured by external electrophilic reagents. This reaction mode provides a unique

opportunity for quick construction of complex molecules with multi-chiral centers or

polycyclic structures through cascade reactions.

Jiao et al.6 reported a catalytic cascade of ATH reactions and alkylations of α,β-unsaturated

aldehydes. The authors employed secondary amine catalysts (30 mol% of (S)-5+ TFA) and

Hantzsch ester 1b as the hydride donor. A series of different α,β-unsaturated aldehydes 6

could be reduced and the in situ formed nucleophilic intermediate then reacted with the

carbocation species generated from diarylalcohols 7 to give the final alkylated products 8

with good e.e. (Figure 1.7). Similar to the reports from List and MacMillan, the geometry

of the double bonds in the starting materials has limited influence on the absolute

configuration of the reduced products.

Figure 1.7 The catalytic asymmetric reductive alkylation reaction by Jiao

1.1.3 Other Related Organic Hydride Donors

Great efforts have also been devoted to modifications of Hantzsch esters and therefore to the

development of other organic analogs as hydride sources for ATH reactions. In the last

decades, several groups across the world have synthesized numerous chiral NADH model

compounds. The design of these chiral hydride donors are generally according to three

aspects:

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• introducing chiral functional groups or sterically hindered side chains, in order to

replace the original substituent of dihydronicotinamide;

• directly incorporating a substituent at the reaction center, which is the “para”

position of the dihydronicotinamide ring;

• synthetizing compounds in a precise configuration to obtain stereoselective

reactions.

Procuranti and Connon7 proposed that transfer hydrogenation reactions conducted by

employing a sub-stoichiometric amount of organocatalyst incorporating both a substrate

activating moiety and an organic hydride donor; this hydrogen source can be generated and

recycled in situ with an inexpensive co-reductant.

In 2009, Zhu and Akiyama8 reported that benzothiazolines were highly efficient hydride

donors for the chiral phosphoric acid-catalyzed ATH reaction of ketimines. Compared to the

previous reports, where Hantzsch ester has been employed as reducing agent, various

aromatic and aliphatic ketimines were converted to the hydrogenated products using

benzothiazoline 9a with uniformly high enantioselectivity (95–98% e.e.) (Figure 1.8).

Moreover, the catalyst loading can be reduced to 2 mol% and the benzothiozoline can be

generated in situ.

One of the disadvantages of the Hantzsch ester-mediated transfer hydrogenation reaction is

the subsequent separation of the pyridine byproducts generated during the reduction. The

reaction using benzothiazolines 9a also faced similar isolation problems. Akiyama et al.

found that this difficulty could be overcome by using a hydroxyl group-substituted

benzothiazoline 9b as the hydride source because the benzothiazole byproduct could

precipitate in the reaction mixture and be readily removed by filtration. The yields and e.e.

with this improved reductant are maintained; for instance, good to excellent yields were

typically obtained with benzothiazolines 9c as the most efficient reductant.

Figure 1.8 Different kind of benzothiazoline

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1.1.4 Thermodynamic Diagnosis of the Organic Hydride Sources

Although plenty of work has emerged on the Hantzsch estermediated transfer hydrogenation

reactions in the literature, relatively little attention has been given to the fundamental

aspects of thermodynamic properties of Hantzsch esters and related organic hydride donors,

especially to the thermodynamic driving forces that leads these compounds to release

hydride anions, as well as the electron transfer abilities. However, this type of knowledge

will undoubtedly promote the fast development in the field of ATH reactions with organic

hydride donors.

For a long time, Zhu’s group has been devoted to measuring and determining the

thermodynamic parameters of various organic hydride donors and their reaction

intermediates by using titration calorimetry and electrochemical methods (as cyclic

voltammetry, CV). As shown in Figure 1.9, they envisaged that an organic hydride donor

could be oxidized (XH → X+) via either a concerted pathway (direct hydride transfer) or

multistep mechanisms such as e-–H+–e-, e-–H• and H•–e-. The standard state enthalpy

change (for example, ∆HHD(XH) means the standard state enthalpy change for XH to

release a hydride to generate X+) or standard oxidation potential values (for example,

Eox(XH) means the standard oxidation potential values for XH to release an electron to

generate XH+•) of each step were determined experimentally. With these valuable data, a

library can be constructed in order to outline the thermodynamic characteristic graphs

(TCGs) of various organic hydride donors.

Figure 1.9 Possible reaction pathways for various organic hydride donors to be oxidized

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The hydride-donating abilities of various organic hydride sources have been measured using

their ∆HHD(XH) values as criteria. As shown in Figure 1.10, most of the N-protected

dihydropyridine derivatives (BNAH analogues) belong to strong hydride donors with the

∆HHD(XH) values lower than 64 kcal mol-1. The widely used Hantzsch ester and N-methyl-

benzothiazolines are mid-strength hydride donors. The ∆HHD(XH) values are indeed from

69 to 77 kcal mol-1. Zhu and his co-workers9 also proved that the kinetics (log k2) of the

hydride transfer reaction have a linear relationship with the thermodynamic hydride-

releasing driving force, within a series of hydride donors having similar fundamental

structures. It is quite conceivable that this rank of hydride-donating abilities of various

organic hydride sources can be an important clue for synthetic chemists to choose the proper

reducing agent in combination of certain catalytic system when developing new catalytic

ATH reactions.

Figure 1.10 Thermodynamic driving force of various organic hydride donors to release hydride

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1.2 Organocatalitic Asymmetric Hydrogenation in Aqueous Media

Organocatalytic reactions are carried out in a one-pot operation by stirring a carbonyl

compound, an amine and an electrophile in conventional organic solvents, such as DMSO,

DMF, or CHCl3, which are toxic, flammable and volatile. Removal of water is not required

for the formation of enamine intermediates that proceed to react directly with an

electrophile. This water-tolerance is a desirable characteristic of an organocatalyst.

However, in the presence of bulk water aldolase-type organocatalytic reactions generally

result in very poor yield and stereoselectivity.

In 2006, the Hayashi’s and Barbas’ groups independently reported two distinct strategies for

aqueous organocatalytic direct cross-aldol reactions of various ketone and aldehyde donors

with aldehyde acceptors. trans-L-Siloxyproline 10 (Figure 1.11) was a key catalyst for high

diastereo- and enantioselectivities in the aldol reaction of cyclohexanone 12 with p-

nitrobenzaldehyde 13 in the presence of water (18 equiv.). Without organic solvent there is

lower diastereo- and enantioselectivity. This high efficiency is probably due to the solubility

of the catalyst 10 in organic solvent. Crude aldol product are easily isolated by removal of

water using centrifugal separation; no extraction and washing are needed. The recovered

catalyst as well as water can be used again.

In 2007, the Gryko’s group described a highly efficient aqueous aldol reaction: treatment of

4-nitrobenzaldehyde 13 with as little as 1.2 equivalents of cyclohexanone 12 in the presence

of the protonated thioamide catalyst 11 (2.5 mol%) affords the aldol product 14 in 97%

yield with high diastereo- and enantioselectivity.

O O

NO2

O OH

Ar

catalyst

H2O+

NH O

OH

TBSO

10

NH S

HN

11

Ph

+ Cl2CHCO2H

141312

Figure 1.11 Aldol reaction of cyclohexanone with p-nitrobenzaldehyde using two different water-compatible aldolase-type organocatalysts

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In addition to aldol reactions, in recent years, the development of organocatalytic direct

Michael reactions has received considerable attention. A direct asymmetric Michael reaction

of a stoichiometric amount of cyclohexanone 12 with β-nitrostyrene 16 that can be

performed in brine (55 equiv.) without the addition of organic solvents was developed by

Nobuyuki and Barbas’ group (Figure 1.12). A proline-derived catalyst 15 (10 mol%)

efficiently catalyzed Michael reactions and afforded product 17 in 79% yield with excellent

stereoselectivity (92% de, 91% ee), even when only an equimolar amount of the donor

acceptor was used.

Figure 1.12 Dyrect asymmetric Michael reaction of cyclohexanone with β-nitrostyrene using a proline-

derived catalyst

The activation of α,β-unsaturated carbonyl compounds as dienolephines via iminium

formation has been applied to Diels-Alder reactions. MacMillan studied the enantioselective

Diels Alder reaction through [4 + 2]-cycloaddition of cyclopentadiene 19 to enone 20 in the

presence of the iminium catalyst 18 (20 mol%) in water. Excellent levels of enantiofacial

discrimination (90% ee) were achieved by use of the heteroaromatic-substituted catalyst 18

(Figure 1.13).

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Figure 1.13 Cycloaddition of cyclopentadiene to enone in water medium using the iminium catalyst 18

Another emerging class of reactions attempts to combine multiple elements of reaction

economy, these are cascade-type reactions in water. Significantly, in the cascade

hydrogenation reaction, water was found to catalyze the reaction in the absence of

organocatalyst wherein proline catalysis was required when organic solvent was used.

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1.3 Polymer-supported Chiral Organocatalysts

Organocatalysis is entered its seconde decade after its rediscovery in 2000 through the now

already legendary publications of List, Lerner, Barbas10 and MacMillan11. It can no longer

be considered a new and immature field, but has proven itself as an established method

(alongside the more traditional approaches of bioenzymatic catalysis and transition metal

catalysis) for conducting catalytic asymmetric synthesis. At the same time, it is also more

than 50 years since the seminal work by R. Bruce Merrifield on solid phase synthesis12, and

more than 35 years since he received the Nobel prize for it. As has happened on numerous

occasions before, the discovery of new catalysts or reagents quickly converged with

Merrifield’s methodology for polymeric immobilization to create a subfield, namely the

polymer-supported versions of the catalysts or reagents in question, and in the case of

organocatalysis, the field of polymer-supported organocatalysts.

Every synthetic strategy, however, has its drawback. As far as solid-supported reagents are

concerned, a big limitation is their cost, which can be truly exorbitant. Although such price

can be quite acceptable for a recyclable catalyst, compared to a disposable reagent, the cost-

issues are nevertheless the single most dictating constraint for the widespread utility of

polymer-supported catalysts, especially for simple one.

With the advent of organocatalysis, a new situation has established itself. Chiral

organocatalysts traditionally operate via simplified enzyme-mimetic mechanisms,

implicating mild reaction conditions and an improved leverage for the chemical inertness of

the polymeric matrix. In addition, the polymeric matrix seems to integrate itself as a more

natural part of the overall catalytic system, influencing both catalyst’s activity and

selectivity. However, unlike what traditionally has been the case, the polymeric scaffold can

even enhance catalyst’s performance by providing a favourable microenvironment around

the organocatalysts.

1.3.1 The Polystyrene-derived Resins

After intensive researches with several polymeric materials, Merrifield introduced the

chloromethylated and divinylbenzene (DVB) crosslinked polystyrene (PS), known simply

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as the “Merrifield resin”.13 It has favorable swelling properties in typical nonpolar organic

solvents such as THF, halogenated and aromatic hydrocarbons.

Afterward, a large number of improvements were introduced (Figure 1.14). The simplest

approach has been the introduction of a longer and more flexible crosslinker to enhance

swelling characteristics, and Toy and Janda introduced tetrahydrofuran-derived crosslinkers

to give the so-called JandaJel™, now a commercial product. The crosslinker is prepared by

alkylating short oligomers of ethylene glycols with 4-vinylbenzyl chloride. A cheaper, but

chemically less inert, alternative is to use inexpensive tetraethylene glycol diacrylate as

crosslinker for PS.

Figure 1.14 The Merrifield resin and derivatives with modified crosslinkers15

Another intuitive way to enhance the swelling characteristics of crosslinked PS is to

incorporate elements that are more compatible with polar solvents. To this purpose, PS

chains have been traditionally mixed up with the versatile polyethylene glycol (PEG)

backbone. Already in 1982, PEGderivatized and crosslinked PS was prepared through

esterification, but it was only in the mid 1980s that the first commercial PEG-PS resins were

introduced, known as PEG-PS and TentaGel™ (Figure 1.15), the last one now being a well-

known commercial product.

24

Figure 1.15 Example of the PEG-PS family of polymer supports

1.3.2 The Acrylic Resins

In the mid-1970s, Atherton, Clive and Sheppard introduced a polyacrylamide support for

peptide synthesis.14 The support was prepared by inverse radical emulsion copolymerization

of dimethylacrylamide, N,N_-bis(acryloyl)ethylenediamine and a functionalized mono-

acryloylhexamethylenediamine in water/1,2-dichloroethane (Figure 1.16).

As in the case of the PS resins, the incorporation of PEG chains into the polymeric matrix

was also a natural extension in the development of the polyacrylamide supports. In 1992,

Meldal introduced the “PEGA” resin, a flow stable PEG–dimethylacrylamide resin useful

for continuous flow solid-phase peptide synthesis. Mono- and difunctionalized

aminopropyl-PEG derivatives are readily available, and mono- or diacylation with acryloyl

chloride gave acrylamides that were combined with dimethylacrylamide through either

radical precipitation copolymerization or inverse radical suspension copolymerization in

water/CCl4/heptane.

Probably the most innovative of the acrylic PEG resins is the “CLEAR™” (Cross-Linked

Ethoxylate Acrylate Resin) a family of resins introduced in the mid 1990s and now

important commercial products. It can be prepared by radical copolymerization of

trimethylolpropane ethoxylate triacrylates with one or more of PEG-dimethacrylate, PEG

ethyl ether methacrylate, trimethylolpropane trimethacrylate, allylamine and 2-aminoethyl

methacrylate·HCl. The resin is an ester-linked network, chemically less robust than the

acrylamide-based supports. “CLEAR™” was, however, unique in the way that it went

against the conventional wisdom of the field at the time since it is a fully crosslinked matrix

(its main constituent being trifunctional acrylates), previously thought to be unfeasible in

peptide synthesis since only a weakly crosslinked matrix was supposed to give the well-

solvated gels necessary for reactions to take place.

25

In fact, the notion that fully cross-linked resins can achieve excellent swelling

characteristics was used (even by our research group) to create polymer supports for

immobilized organocatalysts able to operate in aqueous solution, with lower alcohols or

MeCN percentage.15

Figure 1.16 Typical examples of acrylic resins depicted in one of several possible representative forms

26

1.4 Soluble Polymer-supported Organocatalysts

The use of soluble polymers as alternative supports respect to the insoluble ones used to

support catalysts, is receiving increasing attention in catalysis. The sorts of soluble polymer

support used can vary widely and can involve both synthetic and bioderived polymer

supports. Common examples of such supports include polyisobutylene (PIB) 22,

polyethylene glycol (PEG) 23, polystyrene 24, and polyethylene 25 (Figure 1.17). Insoluble

cross-linked polystyrene derivatives are polymeric supports that are insoluble before,

during, and after a reaction. Separation after a reaction thus requires only a filtration or

decantation and provides a solution, in which the product is dissolved, and an insoluble

solid, the supported catalyst. However, the insolubility of such supports before and during a

reaction rises other problems since catalyst characterization and reactivity can be affected

by it. Soluble polymer-bound catalysts can exhibit the same reactivity as their low-

molecular-weight analogues, and usually require the solution of product and soluble

polymer-bound catalyst to be separated other ways. For example, separations of a soluble

polymer-bound species and a low-molecular-weight product can be effected on the basis of

molecular size, using permselective membranes. However, this technique is still not widely

used for recovery/reuse of soluble polymer-bound catalysts. More often a solution

containing a polymer-bound catalyst dissolved in it is perturbed until it becomes biphasic.

Most commonly this approach involves a process that precipitates the polymer from the

product solution. In some cases, this can be accomplished by cooling or heating. In other

cases, this is accomplished by adding the solution of the soluble polymer and product to an

excess of a poor solvent for the polymer. In successful cases, this leads to selective polymer

precipitation. In either case, filtration or centrifugation allow to separate a solid polymer

from the solution of product. The other alternative is to perturb a solution of the soluble

polymer-bound catalyst and product so as to turn the solution into two liquid phases. If the

product is soluble in one phase and the polymeric catalyst phase selectively soluble in the

other, a gravity separation can be used to separate the product and catalysts and to recover

the polymer-bound catalyst, which permits its recycle in a subsequent reaction. The soluble

polymers used to support organocatalysts can be classified as either polar or non polar

ones.16

27

Figure 1.17 Common soluble polymer supports: polyisobutylene (PIB) [22], polyethylene glycol (PEG) [23], polystyrene (PS) [24], and polyethylene (PE) [25].

28

1.5 Polymer-Supported MacMillan Imidazolidinones

MacMillan’s research group published the use of chiral imidazolidinones as organocatalysts

for asymmetric Diels–Alder reactions.17 In the years that have followed, this family of

organocatalysts has enjoyed an ever-widening reaction scope. In addition to Diels–Alder

reactions, imidazolidinone catalysis has been used for 1,3-dipolar cycloadditions, Friedel–

Craft alkylations, α-halogenations, epoxidations etc. Although recycling of immobilized

MacMillan imidazolidinones has usually been coupled to a rather nonsatisfactory drop in

efficiency, immobilized imidazolidinones have nevertheless remained an active area of

research.

The first reports on polymer-supported MacMillan imidazolidinones surfaced in 2002.

Cozzi and co-workers in Milan, reported the PEG-supported imidazolidinone 29 (Figure

1.18): as a starting point, PEG-mesylate 28 was prepared from PEG by mesylation, followed

by a substitution with 3-(4-hydroxyphenyl)-1-propanol and then another mesylation.

Imidazolidinone 27 was prepared by treating L-tyrosine methyl ester·HCl with n-

butylamine to give tyrosineamide 26, which upon ring-closure with acetone in MeOH

furnished 27. Alkylation of the phenolic functionality of imidazolidinone 27 with PEG-

mesylate 28 gave PEGsupported MacMillan catalyst 29. This linear PEG catalyst is

compatible with a range of solvents because of its favourable solubility profile and was

tested in asymmetric Diels–Alder cycloaddition of acrolein and cyclohexadiene in

MeCN/H2O. Compared to the monomeric catalyst, in this case meaning either 27 or 27-

OMe, PEG catalyst 29 actually provided a better diastereoselectivity and enantioselectivity

(92% endo e.e. vs. 84% endo e.e.). However, the chemical yields for supported catalyst 29

are generally lower than for the monomeric ones and the reaction time was a considerable

40 h at 10 mol% of catalyst loading. Interestingly, recycling had only a modest effect on

enantioselectivity, decreasing from 92% to 85% e.e. in the fourth cycle, although yields

were lowered because of catalyst degradation. The catalyst was recovered in a modest 70–

80% by evaporation of solvent and precipitation with Et2O.

29

Figure 1.18 Imidazolidinone supported on linear PEG18

At the same time as the report on PEG-immobilized imidazolidinones appeared, Pihko and

co-workers reported a JandaJel™-supported MacMillan imidazolidinone (Figure 1.19). The

researchers chose amino-functionalized JandaJel™ as their starting point because of its

good compatibility with many solvents.

Catalyst was tested in asymmetric Diels-Alder cycloadditions of α,β-unsaturated aldehydes

and dienes. The enantioselectivities were very good, equalling or even surpassing those

obtained by soluble catalysts, but chemical yields were generally no more than moderate.

Using the catalyst from a previous run, easily recovered by simple filtration, results were

very close to those obtained with fresh catalyst, but no more extensive recycling was

apparently investigated.

Figure 1.19 JandaJel™-supported MacMillan imidazolidinone

30

1.5.1 Click Strategy for the Immobilization of MacMillan Catalysts

In recent years the copper-catalyzed alkyne azide cycloaddition (CuAAC) reaction as a tool

for the covalent immobilization of catalytic ligands and organocatalysts onto polystyrene

resins (PS) have been successfully used, and have shown that the 1,2,3-triazole linkers exert

highly beneficial effects on the performance of supported organocatalysts in both enamine

and iminium mediated processes.

Figure 1.20 Immobilization of the first generation MacMillan catalyst through CuAAC reactions

The target catalyst can be easily prepared following the procedure outlined in Figure 1.20.

Commercially available Merrifield resin (1% DVB, f = 1.3 mmol/g) is converted to

azidomethylpolystyrene, and the organocatalyst is immobilized through a CuAAC reaction

to afford catalyst 31.

When supported catalyst is compared with the homogeneous, first generation MacMillan

catalyst, important practical advantages of the heterogeneized species become evident; thus,

whereas the homogeneous catalyst requires reaction temperature from -60 to -30 °C, 31 can

be used at rt or 0 °C, respectively, with much shorter reaction times. Most notably, the

enantioselectivities depicted by the homogeneous MacMillan catalyst and 31 are essentially

identical in spite of the difference in reaction temperature.

The main practical advantage offered by heterogenized catalysts in comparison with their

homogeneous counterparts is their easy separation from the reaction medium for recycling

and reuse; for example polystyrene resins can be readily separated from the reaction

environment by filtration. After each cycle, the catalyst is treated with equimolar amount of

31

0.5 M TFA solution in order to reactivate the catalyst. For catalyst 31, a decrease in yield

can be observed after the third run. Very interestingly, enantioselectivity remains essentially

unchanged. Leaching of the functional monomers during the recycling can be almost

completely excluded, since elemental analysis after six cycles indicates a functionalization

almost identical to the initial one (f = 0.9 mmol/g). The decrease of activity during recycling

can be attributed to partial structural collapse of its rather fragile gel structure with repeated

use, making part of the catalytic sites unavailable to reagents.19

Since 2005 when Jørgensen2021 and Hayashi22 independently reported diarylprolinol silyl

ethers as effective organocatalysts for a variety of transformations, efforts have been made

to immobilize these catalysts to prepare recyclable catalysts and to more simply separate

these catalysts from the products. In 2010, Mager and Zeitler described how a PEG-

supported Jørgensen–Hayashi catalyst 32 could be used for this purpose. In this case, the

catalyst was prepared through a coppercatalyzed Fokin–Huisgen reaction of a PEG-azide

and a propargyl diarylprolinol silyl ether. The catalyst 32 so formed was then tested in

asymmetric Michael addition of nitromethane to α,β-unsaturated aldehydes in methanol

(Figure 1.21). The yields and enantioselective excess of the products formed in these

reactions were comparable to those of products prepared with low-molecular-weight

catalysts.

In these cases, the polymer-bound catalyst 32 could be precipitated and recovered by adding

the reaction mixture into ether, separating a recoverable form of the catalyst from the

products.

To examine the recyclability of 32, nitromethane and cinnamaldehyde can be chosen as the

substrates for an asymmetric Michael reaction. The catalyst is reused lot of times in this

Michael reaction providing products with good enantioselectivity. However, the yields of

the product are seen to decrease as the catalyst was recycled. In this instance, the catalyst

deactivation is attributed to the formation of a catalyst-product adduct. The authors found

that stirring of the recovered catalyst 32 with a solution of cinnamaldehyde (the substrate to

be used for the following cycle) can restore the catalytic activity.

32

Figure 1.21 Asymmetric Michael addition catalyzed by PEG-supported Jørgensen-Hayashi catalyst

33

1.6 Hydrogel-supported MacMillan Organocatalysts

Hydrogels are networks of hydrophilic cross-linked polymers, natural or synthetic, that

contains a large amount of water and maintain a distinctive three dimensional structure,

characterized by different grade of entanglement of the chains, depending on their

formulation.

According to Flory-Rehner theory and equations, the most important parameters that define

the structures and properties of hydrogels are:

• the polymer volume fraction in the swollen state, indicated as νs. It is defined as the

ratio between the polymer volume Vp and the swollen gel volume Vg. It is also the

reciprocal of volumetric swelling ratio Qv:

�� = ��

= 1�

where Qv can be defined as:

� = 1 + ��

(� − 1)

In this equation ρp is the dry polymer’s density and ρs the solvent’s one; Qm

represents the ratio between the weights of swollen polymer (Wswollen) and dry

polymer (Wdry ):

� = ��

• the effective molecular weight of the polymer chain between two following cross-

linking points, designated as Mc. It is related to the degree of gel cross-linking X

and the molecular weight of repeating monomer unit M0:

�� = ��2�

• the distance between sequential points of crosslink, ξ, which represents an estimate

of space between macromolecular chains accessible for drug or cell diffusion. It can

be calculated as:

34

� = � ∙ ��!"

# ∙ $��

%"&

where C is a constant for a given polymer-solvent system.

• cross-linkage density, νe, that is the ratio between polymer density and Mc:

�� = ��

In general, hydrogels can be prepared from either synthetic or natural materials. Water-

soluble linear polymers of both origins can be cross-linked to form hydrogels in different

ways, for example linking polymer chains via chemical reaction, using ionizing radiation to

generate main-chain free radicals which are able to recombine as crosslink junctions, or

polymerizing monomer on the backbone of a preformed polymer, activated by the action of

chemical reagent or high energy radiation treatment. It’s also possible to form hydrogels

through physical interaction such as entanglements or electrostatics and crystallite

formation.

The polymerization reaction starts in a system composed by monomer, initiator and cross-

linker. Then, the hydrogel mass needs to be washed to remove impurities left from the

preparation process: non-reacted monomers, initiators, cross-linkers in excess and unwanted

products obtained via side reactions.

1.6.1 Hydrogel Formulation

After the preparation of required functionalized polymers, it is possible to proceed to the

hydrogels synthesis. In this paragraph, methods used to produce 3D polymeric networks are

discussed. Starting materials used for hydrogel formulation are:

Phosphate Buffer Saline (PBS). It is a water-based salt solution containing sodium

phosphate, sodium chloride, potassium phosphate and minor amounts of carbonates and

other sodium salts. The buffer nature helps to keep pH constant in hydrogel synthesis.

35

Carbomer 974P. It is a cross-linked polyacrylic acid

containing carboxyl groups (65%) that make it a

ionizable molecule. The molecular weight is 1 million

Da. The structure, shown in Figure 1.22, evidences

the presence of a carboxyl group in each monomer.

Poly(acrylic acid) (PAA). Polymer of acrylic acid,

whose repeating units have a carboxyl group, as

reported in Figure 1.23, that can lose its proton thus

acquiring a negative charge. This makes PAA a

polyelectrolyte, with the ability of adsorbing and

retaining water, providing a swelling of many times

its original volume, which is a useful characteristic to

a functional hydrogel. In this work, PAA 35% w/w in

aqueous solution is used.

Poly(ethylene glycol) (PEG). Hydrophilic non-

degradable polymer of ethylene oxide. Due to its

hydrophilic and uncharged structure, it forms highly

hydrated layers. Its repeating unit is represented in the

Figure 1.24. PEGs with an average molecular weight

of 2000 g/mol was used in this synthesis.

Agarose ultra-low gelling temperature. It is a purified linear galactan hydrocolloid

isolated from agar or agar-bearing marine algae. Structurally, it is a linear polymer

consisting of alternating D-galactose and 3,6-anhydro-L-galactose units, as illustrated in

Figure 1.25. Agarose is used as gelling agent in biological applications, such as separation

of nucleic acids via electrophoresis, formation of gel plates and production of gel matrix.

The gel point is the temperature at which an aqueous agarose solution forms a gel: agarose

solutions exhibit hysteresis in the liquid-to-gel transition, that is, their gel point in not the

same as their melting temperature. As far as our work is concerning, it’s used agarose with

gel point in the range of 8-17° C.

Figure 1.22 Carbomer 974P

Figure 1.23 Poly(acrylic acid)

Figure 1.24 Poly(ethylene glicol)

36

Figure 1.25 Agarose structure

Moreover, PAA and PEG are functionalized with MacMillan catalysts via click reaction, as

discussed in the following paragraphs, whereas agarose is employed pure, without

introduction of any peptide groups in its chain. Anyway, the procedure to obtain hydrogels

has the same steps, when using or not functionalized polymers.

1.6.2 Steps of Hydrogels Preparation

Carbomer 974P and functionalized PAA are blended together in a PBS solution at room

temperature, until complete dissolution. PEG, both functionalized and pure, is subsequently

added and the system kept in stirring for 45 minutes; then the mixing is left to settle. The

introduction of two PEG’s types is justified by the need to have some polymer which is

active in chemical reactions (the task of PEG with MacMillan catalyst) and some able to

crosslink with other chains to generate hydrogel network and achieve good hydrophilicity

and permeability (the role of pure PEG).

NaOH 1 N is added to adjust pH to 7.4 and then, after the addition of agarose powder which

is not soluble at room temperature, the system is submitted to electromagnetic stimulation

(500 W irradiated power), heating up to 70-80° C to induce condensation reactions, through

interconnections of hydroxyl groups. Carbomer and PAA carboxyl groups constitute

crosslinking sites able to react with hydroxyl groups from agarose, forming ester bonds and

altogether giving rise to the three dimensional matrix. After irradiation, when the

temperature achieves 50° C, the mixture is diluted with water at 50/50 volumetric ratio; then

it is placed in steel cylinders (150 µl each) and left to rest at room temperature, until

reaching complete gelation and thermal equilibrium.23

37

In the realm of enantioselective hydrogenation, the use of molecular hydrogen or a hydride

donor in conjunction with a chiral metal catalyst system has emerged as the preeminent

strategy for asymmetric catalysis within the chemical community. It is intriguing to

consider, however, that the vast majority of C-H stereogenic centers that currently exist

globally were not created via organometallic catalysis. Indeed, this honor belongs to a series

of biochemical processes that create hydrogen-bearing stereocenters in biological cascade

sequences controlled by enzymes and hydride reduction cofactors. In our laboratory

enzymes and cofactors are replaced by imidazolidinone catalysts and Hantzsch ester

dihydropyridine systems.

2.1 Preparation of Catalysts

Preliminary experimental findings and computational studies demonstrated the importance

of four objectives in the design of a broadly useful iminium-activation catalyst:

• Efficient and reversible iminium ion formation.

• High levels of control of the iminium geometry.

• High levels of selective discrimination of the olefin π face.

• The ease of catalyst preparation and implementation.

The first catalyst to fulfill all four criteria was imidazolidinone (S)-5 (Figure 2.1). The

effectiveness of this compound as an iminium-activation catalyst was confirmed by its use

in enantioselective Diels–Alder reactions, nitrone additions, and Friedel–Crafts alkylations.

However, a diminished reactivity was observed when heteroaromatics such as indoles and

furans were used as π nucleophiles in similar Friedel–Crafts conjugate additions. To

overcome such limitations, MacMillan’s group embarked upon studies to identify a more

reactive and versatile amine catalyst. This led ultimately to the discovery of the “second-

generation” imidazolidinone catalyst (2S,5S)-3. They hypothesized that imidazolidinone 3

would form the iminium ion more efficiently and, hence, increase the overall reaction rate,

2. RESULTS AND DISCUSSION

38

since the participating nitrogen lone pair is positioned away from structural impediments (as

described in paragraph 1.1.2). Indeed, since their introduction in 2001, imidazolidinone

catalysts of type 3 have been successfully applied (> 90% e.e.’s, > 75% yields) to a broad

range of chemical transformations, including cycloadditions, conjugate additions,

hydrogenations, epoxidations, and cascade reactions.24

Figure 2.1 Examples of first generation MacMillan catalyst (S)-5 and second generation catalyst (2S,5S)-3

2.1.1 Synthesis of First Generation MacMillan catalysts [30] and [33]

Catalysts which are imidazolidinone analogues are the key chiral organocatalysts recently

developed by MacMillan’s group, and they have been used in a variety of highly

enantioselective reactions because of their great potential for industrial application.

Starting from (S)-tyrosine methyl ester hydrochloride, the 3-butyl-5-(4-hydroxybenzyl)-2,2-

dimethylimidazolidin-4-one 33 and 3-butyl-2,2-dimethyl-5-(4-(prop-2-yn-1-yloxy)benzyl)

imidazolidin-4-one 30, are easily obtained in 70% yield.

Figure 2.2 Synthesis of first generation MacMillan catalysts [30] and [33]

39

For the chemical synthesis of this compound, an excess of thionyl chloride is added to a

solution of (D)-Tyrosine in dry methanol at 0 °C. The stirring continues at room

temperature for about 24 hours then, after the reaction is completed, the volatiles are

removed under vacuum to afford (D)-tyrosine methyl ester hydrochloride 67 as a white

solid. The second step consists in adding the obtained compound to n-butylamine, and the

resulting mixture is stirred at room temperature for 48 hours. The excess of amine is

removed under vacuum and the resulting N-butyl amide 67 is subsequently added to stirring

acetone and methanol. Finally, the resulting solution is heated to 90 °C for 24 hours, cooled

to room temperature, and then concentrated in vacuum, yielding 33 as a white solid. The

desired product can be purified by using silica gel column chromatography.

As an alternative, it is possible to modify the fresh-made catalyst 33 adding K2CO3 and a

small excess of 3-bromoprop-1-yne in DMF (20 ml). The resulting mixture is stirred at

room temperature for 48 hours and, after complete consumption of the starting material, a

saturated solution of ammonium chloride and ethyl acetate are added and stirred for about

two hours. The organic layer is separated and the aqueous layer was back extracted with

ethyl acetate. In the last step, pure product 30 is purified using a silica gel column

chromatography (hexane/EtOAc, 30:70).

2.1.2 Synthesis of Second Generation MacMillan Catalyst [46a]

Following the same procedure described above, catalyst 46 can be obtained with few simple

passages. All steps are reported below:

Figure 2.3 Synthesis of second generation MacMillan catalyst [46a]

40

As previously described, thionyl chloride is added to a solution of (L)-Phenylalanine in dry

methanol at 0 °C. The stirring solution stay at room temperature for about 24 hours then, the

volatiles are removed under vacuum to afford (L)-phenylalanine methyl ester hydrochloride

70 as a white solid. The so obtained compound is threated with a solution of methylamine in

ethanol, and the resulting mixture is stirred at room temperature for additional 24 hours. The

excess of amine is removed under vacuum, then ethyl acetate and aqueous NaHCO3 are

added. The organic layer is separated and extracted with aqueous NaHCO3 before drying it

over Na2SO4 and removing all solvents, in order to afford compound 71.

Next steps consists in adding to a solution of 71 and p-toluenesulfonic acid monohydrate

(TsOH) in methanol, pure benzaldehyde. The new reaction mixture is heated to 50 °C and

stirred for 24 hours. Concentration of the reaction mixture followed by silica gel

chromatography (hexane/EtOAc, 30:70) afford the title compound (2S, 5S)-46a in 13.5

percent yield and the more quickly eluting (2R, 5S)-46b isomer in 34 percent yield.

41

2.2 Preparation of the Hydride Donors

The development of methods applicable to the selective reduction of organic functional

groups has been one of considerable interest to synthetic chemists. However, despite much

work in this field, little attention has been given to the use of heterocycles having hydrogen-

donating ability as practical reducing agents for such selective reduction. NADH coenzyme-

model reduction with 1,4-dihydropyridine derivatives such as diethyl 1,4-dihydro-2,6-

dimethyl-3,5-pyridinedicarboxylate (so-called Hantzsch ester 1b) have attracted much

attention in recent years and have been extensively studied with a wide variety of substrates.

During our study, we have also synthetize the 2-phenyl-2,3-dihydro benzothiazole 44 that is

a useful and convenient reducing agent for the selective reduction of carbon-carbon double

bonds of α,β-unsaturated aldehydes.25

2.2.1 Synthesis of Hantzsch ester [1b]

Figure 2.4 Synthesis of Hantzsch ester [1b]

Paraformaldehyde is added to a stirring solution of ethyl acetate and ammonium acetate.

The mixture, under low agitation, is warmed to 70 °C in a water bath, then, after round for

about 10 minutes, the mixture becomes a thick pale yellow paste. Within the next minute, a

highly exothermic reactions occurs resulting in the formation of a yellow solid which is

allowed to cool to room temperature. Water is added and the yellow suspension is let to stir

for 10 minutes at room temperature then the solid is filtered, washed thoroughly with water

and suspended in ethanol. The suspension is refluxed for 5 minutes and one more time

allowed to slowly cool back to room temperature. The solid obtained is filtered and washed

thoroughly with ethanol yielding 1b as a bright yellow solid.

42

2.2.2 Synthesis of 2-phenyl-2,3-dihydrobenzothiazole [44]

Figure 2.5 Synthesis of 2-phenyl-2,3-dihydrobenzothiazole [44]

As far as the synthesis of 44 is concerned, benzaldehyde is added to a solution of o-

aminothiophenol in ethanol and the mixture is stirred at room temperature. until, After about

30 minutes, pale yellow needles, formed in the reaction environment, are separated from the

mixture, which was then stirred for another 30 minutes. The resulting needles were collected

by filtration and recrystallized with n-hexane to give the pure thiazole 44 (6 g, 56% yield) as

pale yellow needles. It is stable in air but gradually oxidizes to 2-phenylbenzothiazole.

43

2.3 Preparation of the Unsaturated Substrates

We prepared three different kinds of β-methyl cinnamaldehydes starting from the

corresponding substituted ethyl cinnamates, prepared in turn by Horner condensation of

para-substituted acetophenone with the appropriate phosphonate ester. The desired

aldehydes are obtained from the cinnamate by benzyl chloride/LiAlH4 reduction followed

by MnO2 oxidation.26

2.3.1 Synthesis of β-methyl cinnamaldehyde [36]

Figure 2.6 Synthesis of β-methyl cinnamaldehyde [36]

Triethylphosphonoacetato is added dropwise under nitrogen atmosphere over a period of 5

minutes to a stirred suspension of NaH (60% on mineral oil) in dry THF, and the resulting

mixture is stirred for 30 minutes at 0 °C. A solution of acetophenone in dry THF is slowly

added and the reaction mixture is heated to reflux overnight. After cooling to room

temperature, the reaction is quenched with a saturated aqueous solution of NH4Cl and

extracted with EtOAc, then, the organic layer is washed with brine and dried over Na2SO4.

The so obtained ethyl cinnamate 78 is added dropwise to a stirred suspension of LiAlH4 and

benzyl chloride (BnCl) in dry THF at room temperature. After 2 hours the reaction is

completed so it is quenched with water, filtered and, the filtrate, dried over Na2SO4. The

solvent is evaporated under vacuum affording product 79. Finally, to the resulting alcohol

44

79 in chloroform is added an excess of MnO2 that permits to oxidize the charged reagent

into the two isomer 36a (76%) and 36b (24%).

2.3.2 Synthesis of 4-chloro-β-methyl cinnamaldehyde [38]

Figure 2.7 Synthesis of 4-chloro-β-methyl cinnamaldehyde [38]



mixture is stirred for 30 minutes at 0 °C. A solution of 4-chloroacetophenone in dry THF is

slowly added and the reaction mixture is heated to reflux overnight. After cooling to room









45

2.3.3 Synthesis of 4-methoxy-β-methyl cinnamaldehyde [40]

Figure 2.8 Synthesis of 4-methoxy-β-methyl cinnamaldehyde [40]



mixture is stirred for 30 minutes at 0 °C. A solution of 4-chloroacetophenone in dry THF is

slowly added and the reaction mixture is heated to reflux overnight. After cooling to room









46

2.4 Enantioselective Organocatalytic Transfer Hydrogenation of Enals

Figure 2.9 Asymmetric reduction of cinnamaldehyde using catalysts [30] or [33] in homogeneous phase

Entry Catalyst Solvent Conversion %

1 33 CHCl3 65

2 33 DCM 98

3 33 H2O/THF 6:1 99

4 30 CHCl3 55

5 30 DCM 95

6 30 H2O/THF 6:1 99

Table 2.1 Results obtained for cinnamaldehyde hydrogenation reaction conducted at 20 °C in 24 hours

We initiated studies to evaluate iminium catalyst and Hantzsch ester sources that would

effectively participate in the enantioselective hydrogenation of α,β-unsaturated aldehydes

(Table 2.1). Initial experiments were performed with cinnamaldehyde in a variety of

reaction media.

Conversions obtained with both imidazolidinone-catalyzed hydride reduction (catalyst 30

and 33) are higher in water medium in comparison of that obtained using organic solvents.

Particularly chloroform is the worst solvent for the hydrogenations of α,β-unsaturated

aldehydes conducted using MacMillan organocatalyst in an homogeneous phase.

We next examined the efficiency of the same catalysts, particularly catalyst 33, operating

the reduction of β-substituted aldehydes as shown in Table 2.2.

47

Figure 2.10 Asymmetric reduction of β-substituted unsaturated aldehydes using organocatalyst [33] in

homogeneous phase

Substrate Entry Solvent Conversion % e.e. %

1 DCM 93 81

2 H2O/THF 9:1 88 80

3 DCM 100 76

4 H2O/THF 9:1 79 74

5 DCM 87 69

6 H2O/THF 9:1 59 71

7 Diethyl ether 98 /

8 H2O/THF 9:1 75 /

Table 2.2 Results obtained forβ -substituted aldehydes using catalyst [33]. Reactions conducted at 20 °C in 24 hours

In our experiments we also tried to use different hydrogen sources (Table 2.3). Interestingly,

with respect to the evaluation of dihydropyridine analogs, it could easily be observed that 2-

phenyl-2,3-dihydrobenzo[d]thiazole 44 is a good hydrogen donor but, the ethyl substituted

Hantzsch ester (HEH) 1b prove to be superior (entry 1 and 3).

48

Substrate Entry Hydrogen source Conversion % e.e. %

1

99 /

2

90 /

3

93 81

4

90 73

Table 2.3 Results obtained for asymmetric reduction of aldehydes using different hydride donors. Reaction conducted at 20 °C in 24 hours

1b

44

1b

44

49

2.5 Enantioselective Organocatalytic Hydride Reduction of Enones

Inspired by the success on the enantioselective organocatalytic transfer hydrogenation of

enals, we tried to employ this biomimetic activation strategy to accomplish the enantio- and

chemoselective reduction of cyclic enones. How shown in Table 2.4, using catalysts 30 and

45 (entry 1-2-3-4-9-10), the conversion percentage is hairsbreadth due to the fact that

ketones are sterically and electronically deactivated toward iminium formation in

comparison of aldehydic carbonyls. On this basis, we modified the catalyst design and

examined the phenil imidazolidinone 46a; as expected this catalyst was indeed successful,

allowing the organocatalytic hydrogenation of 3-methyl-2-cyclohexenone and 3-penten-2-

one with good reaction efficiency.

Figure 2.11 Asymmetric reduction of unsaturated ketones using different organocatalysts in homogeneous

phase

50

Entry Substrate Catalyst Solvent Conversion %

1 47 30 Diethyl ether 0

2 47 30 H2O/THF 6:1 0

3 47 45 THF 0

4 47 45 H2O/THF 6:1 0

5 47 46a Diethyl ether 90

6 47 46a H2O/THF 6:1 5

7 47 46b Diethyl ether 0

8 47 46b H2O/THF 6:1 0

9 49 30 Diethyl ether 0

10 49 30 H2O/THF 6:1 0

11 49 46b Diethyl ether 38

Table 2.4 Results obtained for asymmetric reduction of enones using different catalysts and reaction conditions at 20 °C in 48 hours

51

2.6 Polymer-supported Organocatalyst: Polystyrene Resin

The supporting of homogeneous chiral organocatalysts is of great interest since it allows to

obtain heterogeneous catalytic materials which can be easily manipulated. Particularly,

polymers, in insoluble bead form, offer a great advantage in recovery of the supported chiral

catalyst by filtration. In fact, the main reason that prompted chemists to immobilize chiral

organocatalysts lies in the synthetic advantages for larger scale production of cheap, easily

available and recoverable materials, able to be reused for several cycles. The organic

catalyst, or its precursor, can be introduced after the macromolecular synthesis

(polymerization), or before it. The first case is described as post-modification strategy

whereas the second approach is referred as a bottom-up strategy. We develop recyclable

polystyrene-supported organocatalysts employing the post-modification strategy, starting

from commercially available polystyrene chloride (1% cross-linked with DVB).27 Firstly to

a solution of EtOH and water, catalyst 33 and an excess of Cs2CO3 are added. The resulting

mixture is stirred at room temperature for a few minutes, then ethanol and water were

removed and the obtained salt is added to a syringe containing polystyrene chloride and a

small amount of KI and 4-dimethylaminopyridine in DMF. Catalyst is added in small excess

with the purpose to functionalize all the polystyrene active sites, thus, in order to eliminate

any residue catalysts, the polymer was washed with AcOEt, MeOH/H2O and finally DCM.

All steps of functionalization process are reported in Figure 2.12:

Figure 2.12 Functionalization of polystyrene resin with MacMillan organocatalyst [33]

In this work we use the insoluble functionalized resin 61 mainly for the asymmetric

reduction of β-substituted unsaturated aldehydes in presence of an hydride donors and in

different reaction condition. The general reaction is reported on Figure 2.13.

52

Figure 2.13 Asymmetric reduction of a general -substituted unsaturated aldehyde

How can be seen from Table 2.5, conversion percentage decreases with the increase in

number of cycles, this could be due to the deactivation of the catalyst linked to the polymer

matrix or, more probably, to catalyst’ leaking.

Substrate 34 Cycle* Conversion %

1 52

2 50

3 48

4 42

5 33

Table 2.5 Decrease in conversion percentage with the number of cycles for asymmetric reduction of

cinnamaldehyde using catalyst [61]. Five cycles of reaction conducted at 20 °C in 24 hours

As already mentioned, the use of immobilized catalysts offers the inherent advantages of

easy recovery and reuse. After each run, the catalyst can be recovered by filtration and

0

20

40

60

80

100

1 2 3 4 5

Cycle

Cinnamaldehyde

53

reused in the next cycle by simple addition of fresh reactants and solvent. As demonstrated

in Table 2.6, resin 61 was recycled five times, affording the aldol products having constant

stereoselectivity and only marginal fall in conversion, with the same reaction time.

As far as β-substituted aldehydes’ reductions are concerned, enantiomeric excesses can be

calculated, in addition to conversion percentages. Data are reported below.

Substrate 36 Cycle Conversion % e.e. %

1 51 83

2 48 86

3 45 83

4 45 81

5 41 82

Table 2.6 Decrease in conversion percentage (blue) and e.e.(red) with the number of cycles for asymmetric

reduction of 3-phenylbut-2-enal using catalyst [61]. Five cycles conducted at 20 °C in 48 hours

0

20

40

60

80

100

1 2 3 4 5

Cycle

3-phenylbut-2-enal

54


1 94 74

2 88 74

3 84 71

4 86 74

5 75 70


reduction of 3-(4-chlorophenyl)but-2-enal using catalyst [61]. Five cycles conducted at 20 °C in 48 hours


1 23 60

2 19 62

3 15 60

4 15 64

5 14 61

0

20

40

60

80

100

1 2 3 4 5

Cycle

3-(4-chlorophenyl)but-2-enal

55


reduction of 3-(4-chlorophenyl)but-2-enal using catalyst [61]. Five cycles conducted at 20 °C in 48 hours

Substrate 38 [3-(4-chlorophenyl)but-2-enal] is the more reactive one, whereas substrate 40

[3-(4-methoxyphenyl)but-2-enal] is the less reactive. It is probably due to the different

functional group in para- position respect to the aromatic ring. As expected, the presence of

electro-withdrawing substituents (–Cl), give good conversion whereas when an electro-

donating group is present (–OMe), the reaction results slowered.

0

20

40

60

80

100

1 2 3 4 5

Cycle

3-(4-methoxyphenyl)but-2-enal

56

2.7 Polymer-supported Organocatalyst: Poly(ethylene glycol)

After using insoluble polymer like cross-linked Polystyrene, we have been interested in the

development of poly(ethylene glycol)-supported version of organic catalysts. The choice of

poly(ethylene glycol) (PEG) as the support is suggested by a number of consideration,

including the polymer’s low cost, commercial availability, easy functionalization, and, most

importantly, its very favorable solubility profile. Indeed, PEGs of Mw > 2000 Da are soluble

in many organic solvents and in water, and insoluble in a few other solvents, which, thus,

allows catalyzed reactions to be run under homogeneous conditions (with consequently best

performance), and the isolation and recovery of the catalyst as if it were bound to an

insoluble polymer.

As reported on chapter 3 we immobilized enantiopure imidazolidin-4-one on a derivative of

PEG hydroxide of average Mw = 2000 Da, and PEG monomethyl ether of average Mw =

5000 Da, to afford an efficient catalyst for the enantioselective α,β-unsaturated aldehydes

with dienes.28

2.7.1 PEG Functionalization via Ether Synthesis

Figure 2.14 PEG Functionalization via Ether Synthesis

57

To a solution of PEG monomethylether (Mw = 5000) dichloromethane was added an excess

of tri-n-butylamine and the reaction mixture is stirred for 30 min. Next, 4-toluenesulfonyl

chloride (indicated as TsCl) is slowly added to the stirring solution. After that the reaction is

completed, solvent is removed in vacuo and the resultant oily residue added dropwise to

stirring diethyl ether in order to precipitate the tosylated polymer. The precipitate is then

filtered and the white solid collected (PEG tosylate) is added to a solution of catalyst 33 and

K2CO3 in dry DMF. After stirring at 25 °C for 48 hours, the reddish-brown solution is

filtered, concentrated under vacuum and slowly added to cold stirring ether to precipitate the

polymeric catalyst. Finally, the solid is further washed with ether followed by iso-propanol

and dried in vacuo to yield pure PEG-supported imidazolidinone 52.

2.7.2 Preparation of Azide and Alkyne Functionalized Poly(ethylene glycol)

For this synthesis process, commercially available poly(ethylene glycol) (PEG) was used.

The hydroxyl group was quantitatively converted into an azide or alkyne functionality. The

azide end group was introduced by mesylation of the hydroxyl functionality and subsequent

substitution with sodium azide (Figure 2.15).

Figure 2.15 Synthesis of azide functionalized poly(ethylene glycol)

PEG (Mw = 2000) was dissolved in anhydrous THF followed by the addition of

trimethylamine (1 equiv.) and a solution of methanesulfonyl chloride (1 equiv.) in THF. The

reaction was stirred for 48 hours at room temperature and then, after the reaction was

completed, the residue was dissolved in acid water and extracted with dichloromethane in

order to eliminate the non-reacted reagents.

The mesylated polymer 53 was thus dissolved in DMF, followed by sodium azide (8 equiv.)

addition, and was stirred for about two days at room temperature. Dichloromethane was

then added, and the reaction mixture was washed five times with water and brine. Polymer

58

was recovered by precipitation into ether followed by filtration which permits to afford pure

PEG-azide 54.

Otherwise, terminal alkynes were introduced adding NaH and propargyl bromide to the

poly(ethylene glycol) monomethyl ether (Figure 2.16).

Figure 2.16 Synthesis of alkyne functionalized poly(ethylene glycol)

To a solution of PEG monomethylether (Mw = 5000) in toluene, were added NaH (80%

w/w solution in toluene, 2 equiv.) and 3-bromoprop-1-yne (80% w/w solution in toluene, 2

equiv.) at 0 °C. After the addition, the resulting mixture was stirred at room temperature for

overnight and desired product is recovered through extraction with DCM, filtrated and

concentrated under vacuum, to obtain 55 as a solid.

The thus obtained polymer building blocks were coupled via 1,3-dipolar cycloaddition

reactions between the azide and alkyne end groups using CuI and

Tetramethylethylenediamine (TMEDA) hence forming block copolymers described below.

2.7.3 Preparation of Azide Organocatalyst

Figure 2.17 Preparation of azide organocatalyst [81]

59

In double-necked round-bottom flask equipped with a condenser, a mixture consisting of

catalyst 33, 1,3-dibromopropane, an excess of K2CO3 and a catalytic amount of

tetrabutylammonium bromide (TBAB) in ACN is refluxed for 10 hours. After solvent

evaporation, the resulting foam is dissolved in CHCl3 and washed with water. The organic

layer was dried over anhydrous Na2SO4 and solvent evaporated.

Next step consists in adding NaN3 to a solution of the previously obtained product 80 in

DMF. The reaction mixture is heated at 100 °C for about 10 hours, then water and diethyl

ether are added to the reaction. The organic phase is separated, washed with brine solution

and dried with Na2SO4 before removed volatiles under vacuum. The crude product was

purified by column chromatography on silica gel (n-hexane/EtOAc, 60:40), affording pure

product 81.

2.7.4 Azide-Alkyne Cycloaddition for the Synthesis of Supported Catalyst

This reaction is properly referred to as “click reaction”. It involves the interaction between

an azide and an alkyne to form triazole, in presence of Cupper (I) catalyst. General schemes

of this type of reaction are represented in Figure 2.18.

Figure 2.18 General schemes of the azide-alkyne cycloaddition for the synthesis of supported catalysts

The preparation of respectively alkyne- and azide-organocatalyst we have used is described

in paragraph 3.2 (Products 30 and 81). A slight excess of PEG was used (1.2 equivalents) in

order to drive the click reactions to completion. This excess of PEG was easily removed

after the reaction via a washing step with methanol. Figure 2.19 shown final products

obtained.

60

Figure 2.19 Final catalysts obtained using the above-mentioned "click strategy"

2.7.5 Results Obtained using Soluble Catalyst [52] or [57]

Figure 2.20 Asymmetric reduction of aldehydes using PEG-supported catalysts [52] or [57]

Substrate Entry Catalyst Solvent Conversion % e.e. %

1 52 DCM 54 /

2 52 H2O 29 /

3 57 H2O 83 /

4 52 DCM 60 78

5 57 DCM 86 85

6 52 DCM 99 77

7 52 H2O/THF 9:1 29 70

8 52 DCM 85 67

9 52 H2O/THF 9:1 74 64

Table 2.9 Results obtained for asymmetric reduction of different unsaturated aldehydes. Reactions

conducted at 20 °C in 24 hours

61

A comparison of the results obtained with the PEG-supported catalyst in the 1,3-dipolar

cycloaddition reactions with those obtained by MacMillan catalysts in homogeneous phase,

indicates that the major difference between the PEG-supported and the non-supported

catalyst resides in the stereochemical, rather than the chemical, efficiency. Indeed, while the

supported catalyst gave values of e.e. higher than those obtained using the non-supported

one, the difference in chemical yields is large: conversion decrease of an average 10-20%.

2.7.6 Catalyst Recovery and Recycling Experiments

Catalyst immobilization on a polymeric matrix allow simple recovery from the reaction

environment and recycling. In this study, the separation of the catalyst is easily achieved by

concentrating the reaction mixture under vacuum, dissolving the residue in a small amount

of dichloromethane (1 µl/mg of catalyst), and adding a large excess of diethyl ether to the

mixture. The precipitated PEG-supported catalyst is then isolated by filtration in 50-90 %

yield and the diethyl ether phase is worked-up to obtain the product. The recovered catalyst

is the dried for a short time under vacuum to remove traces of solvent and then recycled.

In Table 2.10 are reported the results obtained using supported catalyst 57 in subsequent

reaction cycles.

Substrate 34 Cycle Conversion %

1 93

2 53

3 3

62

Table 2.10 Decrease in conversion percentage with the number of cycles for asymmetric reduction of

cinnamaldehyde using catalyst [57]. Three cycles conducted at 20 °C in 24 hours

Substrate 36 Cycle* Conversion % e.e. %

1 84 83

2 40 81

3 28 77

Table 2.11 Decrease in conversion percentage (blue) and e.e.(red) with the number of cycles for reduction of 3-phenylbut-2-enal using catalyst [57]. Three cycles conducted at 20 °C in 48 hours

0

20

40

60

80

100

1 2 3

Cycle

Cinnamaldehyde

0

20

40

60

80

100

1 2 3

Cycle

3-phenylbut-2-enal

63

The drastic downfall of the conversion percentage after only 3 cycles of reaction is probably

due to the catalyst’s deactivation or leaking but, principally, the most important cause, is the

huge catalyst lost during the recovery process. In fact it consist of a first step, in which the

supported polymer is precipitate in diethyl ether, and a second step of filtration and

washing. The recovery percentage vary between 60 and 90%.

We carried out the recovery and recycle operations only using PEG-supported catalyst 57

(in which catalyst has been linked with click reaction) and not using the one prepared via

ether synthesis (supported catalyst 52) because, in the stirred reaction environment, the

polymer matrix was subjected to a quite complete leaking of the organocatalyst. At the

contrary, in PEG-supported 57 the reduction of the activity due to catalyst loss was lower.

64

2.8 Polymer-supported Organocatalyst: Poly(acrylic acid)

Poly-acrylic acid (PAA) is functionalized with triple bond, in accordance with click reaction

requirements. This polymer is characterized by carboxyl groups, which can be used to

obtain an amide.

PAA reacts to propargylamine, in aqueous system containing HOBt, previously dissolved in

a solution composed by distilled water and CH3CN, and 1-ethyl-3-(3-dimethylaminopropyl)

carbodiimide (EDC) that is able to favor the attack of ammine on carbonyl group. The

reaction is shown in Figure 2.21:

Figure 2.21 Functionalization of poly-acrylic acid with a triple bond

Carboxyl groups are also important in hydrogel scaffold formation (because functionalized

PAA shall be used in hydrogel formulation), thus, not all of them must be functionalized

with triple bond, in a way to be able to react with hydroxyl group of PEG or agarose. This

aim is achieved using less amount of propargylamine than PAA. Then, desired product is

purified via dialysis against distilled water, in acidic conditions (pH=5÷6): this method is

based on osmotic principle and allows to separate salts and each compound which is not

linked to polymer chains because of their different diffusion rate through a membrane with

selective permeability. Finally, PAA functionalized is frozen and then lyophilized yielding

pure product 58.

2.8.1 Dialysis Technique

To dialyze reaction mixture, 2000 ml of distilled water in acidic conditions (pH = 5÷6) are

used, with the addiction of 11.2 g of NaCl to create an acid environment to molecular

diffusion. This solution is at room temperature.

65

Reaction mixture is put inside dialysis membrane, which is previously washed with distilled

water, and the membrane is closed in order to minimize air’s volume inside the same. Then,

it is submerged in a becher with the solution of distilled water at acid pH and stirred for

about 24 hours. In this way, diffusion occurs: smaller molecules than membrane cut-off,

such as formed salts or unreacted chemicals, are able to move from mixture to external

aqueous solution; while functionalized

polymer chains remain trapped inside,

because of their steric hindrance. After this

time, the membrane containing desiderated

product is token out and the dialysis

solution is substituted with other 2000 ml

of distilled water with the addiction of 4

HCl drops only. The membrane is put

inside again and left under stirring other 24

hours. This procedure is repeated one time

again, in order to purified the product via dialysis completely. Figure 2.22 illustrate the

dialysis’ principle. Finally, reaction solution remaining inside the membrane is dropped in a

flask and cover with para-film. Then it is stored at -21°C and lyophilized, as indicated

above.

2.8.2 PAA amide-Imidazolidinone Click

Catalyst-spacer-N3 (Product 59) is dissolved in THF, while functionalized polymer is

solubilized in distilled water; then they are blended together. Reaction occurs at 60°C and in

catalytic conditions: it is used CuI and sodium ascorbate as antioxidant agent because of the

oxidation of CuI (Cu+ → Cu2+) in experimental atmospheric conditions. For this reason, CuI

is added in excess compared to literature’s indication, in order to ensure its catalytic action.

Cu+ actives triple bond to interact with azide group to form cycle structure of triazole.

Figure 2.23 illustrates the reaction.

Figure 2.22 Dialysis principle

66

Figure 2.23 Click reaction between PAA amide and Imidazolidinone

Click product is obtained in aqueous phase, and it is dialyzed against distilled water at

pH=5÷6, to purified it and remove excess of catalyst and formed salts. Finally, the

compound is frozen at -40°C and lyophilized.

67

2.9 Hydrogel Synthesis

Two different hydrogel scaffolds are prepared: the first one (AC-PEG-2000) using the

products obtained by polymer click reactions, which are functionalized PAA 60 and

functionalized PEG 56 (Mw = 2000); the second one, labeled as AC-PEG-5000, is

constituted by polymers without functionalization, in which PEG-supported catalyst 57 (Mw

= 5000) is entangled. The procedures are illustrated below.

2.9.1 Hydrogel AC-PEG-2000

Figure 2.12 and 2.13 report all used chemicals and their relative amounts.

PBS

[ml]

Carbomer 974P

[mg]

PAA-catalyst 60

[mg]

PEG

[mg]

PEG-catalyst 56

[mg]

Agarose

[mg]

9 40 10 600 80 25

Table 2.12 Mass composition of AC-PEG-2000. Pure PEG is supposed to have an average molecular

weight of 2000 g/mol

Reacting carboxyl groups are provided by both Carbomer 974P and functionalized PAA.

Carbomer 974P due to its poly-acrylic molecular structure, is able to satisfy the requirement

of cross-linking formation in scaffold; for this reason, PAA pure is not added to the reaction

system, because Carbomer 974P can performs the same task yet. In addition, functionalized

PEG presents some hydroxyl groups able to intervene in scaffold formation, together with

the OH groups from pure PEG and agarose. In fact, as discussed in experimental

procedures, polymer chains are not completely functionalized via click reaction with

catalyst introduction, but some carboxyl or hydroxyl groups are preserved as free, in order

to interview chemically in hydrogel synthesis and to avoid to be trapped in network only

due to their steric hindrance.

Experimental procedure starts with dissolution of 50 mg of Carbomer 974P in 9 ml of PBS,

then 10 mg of PAA-catalyst 60 are added and left under stirring for 30 minutes, until

complete dissolution. 600 mg of pure PEG (Mw = 2000) and 80 mg of functionalized PEG

68

56 are added to the mixture, which is kept stirred for 45 minutes and then left to settle for 30

minutes. NaOH 1 N is dripped inside to adjust pH at value of 7.

To 5 ml of this mixture, 25 mg of agarose powder are added and this system is irradiated

with microwave radiation at 500 W until a temperature of about 80 °C is reached.

At this point the gel is liquid and condensation reactions begin. The mixture is carried to

50°C and 5 ml of PBS are added, in order to obtain a solution at 50/50 volumetric ratio.

Amounts of the latter, equal to 150 µl are placed in steel cylinders giving cylindrical shape

to the hydrogels, and left at rest for about 45

minutes until gelification is completed.

Hydrogels (illustrated in Figure 2.24) are finally

removed from the cylinders and put in a

multiwell plate, where distilled water is added in

order to submerge hydrogels and remove residual

air ball that can form during the synthesis

process because of electromagnetic stimulation

and subsequently gelation.

2.9.2 Hydrogel AC-PEG-5000

PBS

[ml]

Carbomer 974P

[mg]

PEG

[mg]

PEG-catalyst 57

[mg]

Agarose

[mg]

9 50 580 100 25

Table 2.13 Mass composition of AC-PEG-5000. Pure PEG is supposed to have an average mmolecular

weight of 2000 g/mol

PEG-catalyst 57 is a 100% functionalized poly(ethylene glycol) monomethyl ether (Mw =

5000), therefore it is not able to bond to Carbomer and PEG active sites. In this reactions

pure PAA and PEG polymers create a network in which compound 57 is entrapped because

of its high molecular weight.

Figure 2.24 Example of hydrogel tablets

69

2.9.3 Results Obtained using Gel Tablets in Aqueous Medium

Table 2.14 shows the results obtained using one or more functionalized gel tablets, after a

reaction time of 96 hours. Conversion percentages are very low, but it has to be considered

that the catalyst amount in every single tablet is equal to about 0.42 mg, corresponding to a

molar concentration of 8.5·10-5 mmol/ml for AC-PEG-2000 (catalyst molar ratio = 0.17 %)

and 4·10-5 mmol/ml for AC-PEG-5000 (catalyst molar ratio = 0.08 %).

Substrate Entry Catalyst Solvent Conversion %

1 AC-PEG-2000

(1 tablet) H2O 4

2 AC-PEG-5000

(1 tablet) H2O 8

3 AC-PEG-2000

(3 tablet) H2O 12

4 AC-PEG-2000

(1 tablet) H2O 7

5 AC-PEG-5000

1(tablet) H2O 6

6 AC-PEG-2000

(1 tablet) H2O 4

Table 2.14 Results obtained for asymmetric reduction of different substrates, using one or more gel tablets.

Reactions are conducted at 20 °C in 96 hours

The hydrogenation of all substrates starts despite the very small quantity of active catalyst in

the reaction environment. The simple way is to improve this promising method is to

increase the catalyst amount into the hydrogel tablets but, it has to be considered that the

addition of functionalized polymer cause a reduction in the number of active group (–OH)

able to create cross-linking bond. This unclaimed condition, could affect hydrogel’s

resistance and properties, leading to its collapse. An alternative path is to decrease the

reagents concentration dealing with other, not entirely negligible, problems.

70

2.10 Organocatalyst-Enzyme Coupled Reactions

After the success of the hydrogenation reactions of α,β-unsaturated aldehydes conducted in

aqueous media, we tried to combine the catalytic activities of MacMillan-derived

organocatalyst, helped by Hantzsch ester (HEH) as hydride donor, and alcohol

dehydrogenase in presence of NADH, glucose dehydrogenase and glucose, so as to carry on

the following reaction:

Figure 2.25 Organocatalyst-enzyme coupled reduction of cinnamaldehyde [34] to 3-phenylpropanol [35b]

To a solution of cinnamaldehyde 34 (50mM), catalyst 33 (20 mol%) and Hantzsch ester in

THF (200 µl), is added a buffer solution (1800 µl at different pH) of horse-liver alcohol

dehydrogenase (hl ADH), oxidized nicotinamide adenine dinucleotide (NAD+), glucose

dehydrogenase (GDH) and glucose. Table 2.15 shows that results are not so satisfying as

expected:

Entry pH Product 35 % Product 35b %

1 8 3 0

2 5 90 0

3 4 99 0

Table 2.15 Results obtained for asymmetric reduction of cinnamaldehyde, conducted at 20 °C in 24 hours

We discovered that, the excess of Hantzsch ester in the reaction environment, promote the

catalytic hydrogenation of the double bond C=C, but inhibits the enzymatic activity of the

alcohol dehydrogenase. Another important parameter that contribute to slow down the

alcohol formation is the pH; the more it is acid, the less conversion can achieve high values.

At the contrary the first step, that corresponds to the formation of 35, results facilitate at

acidic pH. This could be verify performing only the second step of the above-mentioned

reaction at different pH conditions and in presence of Hantzsch ester (Figure 2.26).

71

Figure 2.26 Reduction of 3-phenylpropanal to 3-phenylpropanol catalyzed by ADH in presence of

Hantzsch ester

In this case, to a buffer solution (at different pH conditions) are added horse-liver alcohol

dehydrogenase (hl ADH), oxidized nicotinamide adenine dinucleotide (NAD+), glucose

dehydrogenase (GDH) and glucose. Then is added a solution of cinnamaldehyde 34 in THF.

The presence of HEH in some reaction environment (entry 3) has an important influence on

the results obtained, as shown in Table 2.16:

Entry pH HEH Product 35b %

1 5 no 86

2 5 yes 0

3 4 no 65

4 4 yes 0

Table 2.16 Results obtained for reduction of 3-phenylpropanal to 3-phenylpropanol catalyzed by GDH.

Reaction are conducted at 20 °C in 24 hours, some of them in presence of HEH

After these results, we tried to bypass the “Hantzsch ester problem” directly substituting it

with NAD+. It is the hydrogen donor used by the alcohol dehydrogenase during the carbonyl

reduction reaction, thus, in theory, it could work pretty well with MacMillan organocatalyst

too. The reaction we toke-care consists in the reduction of cinnamaldehyde 34 to the

saturated aldehyde 35 as depicted in Figure 2.27.

Figure 2.27 Hydrogenation of cinnamaldehyde catalyzed by catalyst 33 using NAD+ as hydride donor

72

A solution of Cinnamaldehyde 34 in THF (50 mM) is added to a solution (buffering at

different pH) of horse-liver alcohol dehydrogenase (hl ADH), oxidized nicotinamide

adenine dinucleotide (NAD+), glucose dehydrogenase (GDH) and glucose.

Entry pH Product 35 %

1 5 7

2 4 20

Table 2.17 Results obtained for the hydrogenation of cinnamaldehyde using NAD+ as hydride donor.

Reactions conducted at 20 °C in 24 hours

Table 2.17 demonstrate that NAD+ can be successfully used as hydride donors in

asymmetric reduction of α,β-unsaturated aldehydes catalyzed by MacMillan organocatalyst,

but, considering that conversions are very low for long reaction time too, can surely be said

that more studies and improvements should be done to obtain better results.

73

2.11 Enantioselective Organo-Cascade Catalysis

The identification of new chemical strategies that allow increasingly rapid access to

structural complexity remains a preeminent goal for the chemical sciences. It has long been

known that biological systems produce elaborate molecules in continuous processes,

wherein enzymatic transformations are combined in highly regulated catalytic cascades. We

questioned whether these perfect biosynthetic pathways might be translated to a laboratory

“cascade catalysis” sequence. The generic reaction is described in Figure 2.28:

Figure 2.28 Generic cascade reaction of cinnamaldehyde with a para-substituted nitrovinylbenzene

As revealed in Table 2.18, the PS-supported catalyst 61 and the PEG-supported catalyst 57

permit to achieve good results in 48 hours of reaction.

Reagent N Catalyst Solvent Conversion %

57 DCM 38

NO2

F

63

33 DCM 18

61 (1° cycle) DCM 76


57 DCM 37

Table 2.18 Enantioselective organo-cascade reactions of cinnamaldehyde with different reagents.

Reactions conducted at 20 °C in 48 hours

74

2.12 Enantioselective Friedel-Crafts Reactions

The organo-catalyzed addition of aromatic substrates to electron deficient σ- and π-systems,

commonly known as Friedel-Crafts alkylation, has long been established as a powerful

strategy for C-C bond formation. The LUMO-lowering activation of α,β-unsaturated

aldehydes via the reversible formation of iminium ions with chiral imidazolidinones is a

valuable platform for the development of enantioselective organocatalytic Friedel-Crafts

reactions.29

We applied this strategy to the enantioselective Friedel-Crafts alkylation of indol with α,β-

substituted cinnamaldehyde in order to generate β-indolyl carbonyl.

Figure 2.29 General Friedel-Crafts reaction of different aldehydes with indole

Substrate Catalyst Solvent Conversion %

57 THF 95



57 THF 6

Figure 2.30 Friedel-Crafts reactions of two unsaturated aldehydes with indole, using different supported

catalysts. Reactions conducted at 20 °C in 48 hours

How revealed by Table 2.30, PEG-supported catalyst 57 and PS-supported catalyst 61,

successfully catalyze the enantioselective Friedel-Crafts alkylation of indol with β-

75

substituted aldehydes. On the contrary α-substituted cinnamaldehyde do not results a good

choice for this kind of reactions.

76

3.1 General Remarks

All reagents and solvents were purchased from commercial sources and used without further

purification. The reactions were carried out under atmospheric air unless otherwise

indicated, such as moisture-sensitive ones, for which a static nitrogen atmosphere was used.

Reactions were monitored mostly by thin-layer chromatography (TLC), performed on

Merck Kieselgel 60 F254 plates. Visualization was accomplished by UV irradiation at 254

nm and subsequently by treatment with alkaline KMnO4 reactant (an oxidant mixture of

KMnO4 , K2CO3 and NaOH 5% in water) or with phosphomolibdic reagent.

After dipping it into the reactant, the plate was heated: with KMnO4 the oxidizable

substances assumed a yellow colour on a violet background, whereas with phosphomolibdic

reagent they assumed a blue colour on a yellow background.

Each compound has been purified by silica gel column chromatography (230-400 mesh). 1H and 13C NMR spectra were recorded on a Bruker ARS 400 spectrometer ( 1H NMR, 400

MHz; 13C NMR, 100 MHz). Spectra are registered at room temperature, unless otherwise

indicated, in CDCl3 , with tetramethylsilane (TMS, δ=0.0 ppm) used as internal standard.

Peptides and some amino acids derivatives needed to be investigated in D2O, DMSO-d6,

CD3CN or CD3OD as solvents, as it will be indicated for each case. Chemical shifts are

reported as δ values in parts per million (ppm) in comparison to internal standards; the

coupling constants J are reported in Hz. 1H NMR spectra are tabulated as follows: chemical shift, multiplicity (s = singlet, d =

doublet, t = triplet, q = quartet, m = multiplet, br = broad signals), coupling constant and

number of protons.

GC-MS analyses were performed using an Agilent HP-6890 gas-chromatograph equipped

with a 5973 mass detector, and an Agilent HP-5MS column (30 m x 0.25 mm, 0.25 µm film

thickness) with the following temperature program: 60°C (1min) - 6°C/min - 150°C (1 min)

- 12°C/min- 280°C (5min); carrier gas, He; constant flow 1mL/min; split ratio, 1/30; tR

given in minutes.

3. EXPERIMENTAL SECTION

77

The enantiomeric excess values were determined by chiral GC analysis performed on a

DANI HT 86.10 gas chromatograph equipped with a CHROMPACK CPCHIRASIL-DEX

CB column, according to the following conditions: 90°C (1 min) - 6°C/min - 180°C (5 min).

ESI-MS spectra were recorded on a Bruker ESQUIRE 3000 PLUS spectrometer, whereas

high-resolution mass spectra were recorded on a FT-ICR (Fourier Transform Ion Cyclotron

Resonance), equipped with an ESI detector.

Optical rotations were determined at 20°C on a Jasco-DIP-181 digital polarimeter (at 589

nm) and values are given in ° cm-3 g-1 dm-1.

78

3.2 Procedures

Synthesis of diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate [1b]

Ethyl acetoacetate (65 g, 500 mmol), paraformaldheyde (7,5 g, 250 mmol) and ammonium

acetate (29 g, 375 mmol) were added to a 500 mL beaker equipped with a magnetic stirrer.

The beaker was loosely covered with a plastic recipient and under slow agitation, the

mixture was warmed to 70 °C in a water bath. After round 10 minutes, the mixture becomes

a thick pale yellow paste resulting in the lost of agitation. Within the next minute, a highly

exothermic reactions occurs resulting in the formation of a yellow solid. One minute after

the appearance of the solid, the mixture was allowed to cool to room temperature, diluted

with water (400 mL) and the yellow suspension was stirred for 10 minutes at room

temperature. The solid was filtered, washed thoroughly with water and suspended in EtOH

(250 mL). The suspension was refluxed for 5 minutes and allowed to slowly cool back to

room temperature with agitation. The solid was filtered and washed thoroughly with EtOH

yielding 1b as a bright yellow solid (44 g, 71 % yield). 1H NMR (400 MHz, CDCl3) : δ=1.26-1.29 (t, J=7,3 Hz, 6H), 2,186 (s, 6H), 3,265 (s, 2H),

4,140-4,193 (q, J= 7,3 Hz, 4H).

HRMS (ESI): calculated for C13H19NO4: 253.1314; found: 253.1322.

Elemental Analysis: C, 61.64%; H, 7.56%; found: C, 61.69%; H, 7.60%.

79

Synthesis of methyl (D)-tyrosine hydrochloride [67]

(D)-Tyrosine 66 (10.0 g, 55.24 mmol) was dissolved in dry methanol (100 ml). The stirred

solution was cooled in an ice bath and treated with thionyl chloride (10 ml, 139 mmol).

After the addition was complete, the cooling bath was removed and stirring was continued

at room temperature overnight. The volatiles were removed under vacuum to afford (D)-

tyrosine methyl ester hydrochloride 67 as a white solid (11.73 g, 91.6% yield).30 1H NMR (400 MHz, d6-DMSO): δ 9.49 (br s 1H), 8.66 (br s, 3H), 7.0 (d, J = 8.4 Hz, 2H),

6.76-6.68 (m, 2H), 4.19-4.06 (m, 1H), 3.65 (s, 3H), 3.11 (dd, J = 14.1 Hz, 5.7 Hz, 1H), 3.02-

2.95 (dd, J = 14.1, 7.1, 1H). 13C NMR (100 MHz, d6-DMSO): δ 35.08, 52.51, 53.47, 115.43, 124.34, 130.36, 156.70,

169.44.

HRMS (ESI): calculated for C10H14ClNO3: 231.0662; found: 231.0665.

Elemental Analysis: C, 51.84; H, 6.09; found: C, 51.80; H, 6.11.

80

Synthesis of (R)-2-amino-N-butyl-3-(4-hydroxyphenyl)propanamide [26]

67 (11.73 g, 50.6 mmol) was added to n-butylamine (35 ml, 354.5 mmol) and the resulting

mixture was stirred at room temperature for 48 hours. The excess of butylamine was

removed under vacuum until the N-butyl amide product 26 was obtained as a white solid

(10 g, 85% yield).27 1H NMR (400 MHz, Chloroform-d) δ 6.93 (t, J = 5.8 Hz, 1H), 6.26 (d, J = 8.3 Hz, 2H),

5.99 (d, J = 8.3 Hz, 2H), 2.70 (dd, J = 8.3, 4.8 Hz, 1H), 2.40 (q, J = 6.7 Hz, 2H), 2.20 (dd, J

= 13.6, 4.8 Hz, 1H), 2.00 (t, J = 7.4 Hz, 2H), 1.85 (dd, J = 13.6, 8.3 Hz, 1H), 0.81 (p, J =

7.9, 7.4 Hz, 2H), , 0.17 (t, J = 7.2 Hz, 3H).

HRMS (ESI): calculated for C13H20N2O2: 236.1525; found: 236.1522.


81

Synthesis of 3-butyl-5-(4-hydroxybenzyl)-2,2-dimethylimidazolidin-4-one [33]

Dry MeOH (80 ml) and dry acetone (15 ml) were added to 10 g (36.7 mmol) of the product

26. The resulting solution was stirred at 90 °C for 24 hours, cooled to room temperature,

and then concentrated in vacuum. Compound 33 was obtained as a white solid (7.3 g, 72%

yield) after silica gel chromatography (hexane/EtOAc, 30:70).27 1H NMR (400 MHz, CDCl3): δ 7.09-7.01 (m 2H), 6.79-6.69 (m, 2H), 3.74 (t, J = 5.2 Hz,

1H), 3.31 (ddd, J = 13.9 Hz, 9.5 Hz, 6.1 Hz, 1H), 3.10-2.96 (m, 2H), 1.34-1.22 (m,

2H),1.279 (s, 3H), 1.179 (s, 3H), 0.91 (t, J = 7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ 13.96, 20.56, 26.52, 28.11, 31.58, 35.88, 40.63, 59.14,

76.55, 115.87, 127.81, 130.95, 155.63, 174.32.



82

Synthesis of 3-butyl-2,2-dimethyl-5-(4-(prop-2-yn-1-yloxy)benzyl)imidazolidin-4-one [30]

Compound 33 (1 g, 3.62 mmol) was added to K2CO3 (1.5 g, 10.8 mmol), 3-bromoprop-1-

yne (80% w/w solution in toluene, 1.2 equiv., 4.34 mmol, 0.52 g) and DMF (20 ml) at room

temperature. The resulting mixture was stirred for 48 hours.

After complete consumption of the starting material, 30 ml of saturated solution of

ammonium chloride was added in the reaction mixture and stirred for 2 hours. Then 30 ml

of ethyl acetate was added and stirred for 30 minutes. The organic layer was separated and

the queues layer was back extracted with ethyl acetate (2 x 20 ml). The combined organic

layers were treated with brine solution and stirred for 1 hour. The organic layer was

separated and dried over Na2SO4. The pure product 30 (2.5 g, 73% yield) was obtained after

silica gel column chromatography (hexane/EtOAc, 30:70).27 1H NMR (400 MHz, CDCl3): δ 7.19-7.14 (m 2H), 6.91 (d, J = 8.6 Hz, 2H), 4.66 (d, J = 2.4

Hz, 2H), 3.72 (t, J = 5.3 Hz, 1H), 3.30 (ddd, J = 13.8 Hz, 9.4 Hz, 6.1 Hz, 1H), 3.04 (dd, J =

5.3 Hz, 2.3 Hz, 2H), 2.90 (ddd, J = 13.8 Hz, 9.4 Hz, 5.8 Hz, 1H), 2.50 (t, J = 2.4 Hz, 1H),

1.58-1.37 (m, 2H), 1.34-1.22 (m, 2H), 1.26 (s, 3H), 1.16 (s, 3H), 0.92 (t, J = 7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ 14.03, 20.61, 26.76, 28.33, 31.68, 36.22, 40.54, 56.11,

59.11, 75.66, 76.30, 78.81, 115.24, 130.13, 130.95, 156.75, 174.08.



83

Synthesis of (S)-3-butyl-5-(4-methoxybenzyl)-2,2-dimethylimidazolidin-4-one supported on

polystyrene [61]

Firstly add to a solution of EtOH (20 ml) and water (10 ml), 33 (1 g, 3.62 mmol) and

Cs2CO3 (1.2 g, 6.22 mmol). The resulting mixture was stirred at room temperature for a few

minutes, then ethanol and water were removed under vacuum.

Then a solution of the obtained salt in 5 ml of DMF was added to a syringe containing

polystyrene chloride (1 g), KI (0.2 g), DMAP (0.5 g).

In order to eliminate any residue catalysts, the polymer was washed with AcOEt (3 x 20

ml), then with MeOH/H2O (3 x 20 ml), MeOH/DCM (3 x 20 ml) and in the end with pure

DCM (3 x 20 ml). The volatiles were removed under vacuum obtaining the product 61 (1.27

g, theoretic loading = 1.23 mmol/g of support).

84

Synthesys of PEG-mesylate [68]

To a solution of PEG (Mw = 2000) (2.00 g, 1 mmol) in CH2Cl2 (30 ml) is added an excess

of triethylamine (420 µl, 3 mmol) and the reaction mixture was allowed to stir for 30 min.

Next, mesyl chloride (MsCl) (250 µl, 3 mmol) was added dropwise to the stirring solution

and the reaction mixture was left to stir at 25 °C for 48 hours. The solvent was then

removed in vacuo and the resultant oily residue was added dropwise to stirring diethyl ether.

The precipitate was filtered and the solid washed with diethyl ether (30 ml) and iso-

propanol (30 ml). The PEG mesylate was isolated as a white solid 68 (1.4 g, 70% yield). 1H NMR (400 MHz, Chloroform-d) δ 4.38 – 4.32 (m, 4H), 3.63 (s, 180H), 3.06 (s, 6H).

85

Synthesis of (S)-3-butyl-5-(4-methoxybenzyl)-2,2-dimethylimidazolidin-4-one supported on

PEG [69]

PEG mesylate 68 (500 mg, 0.25 mmol) was added to a solution of 33 (138 mg, 0.5 mmol)

and K2CO3 in acetone (20 ml), in a 100 ml round bottomed flask fitted with a magnetic stir

bar. After stirring at 25 °C for 48 h, the solution was filtered and then concentrated under

vacuum. The oily product obtained was slowly added to stirring cold diethyl ether (50 ml) in

order to precipitate the functionalized polymer, insoluble in this solvent. The pure product

69 was recovered as a brown-colored solid by filtration, and washed with diethyl ether (3 x

20 ml). 1H NMR (400 MHz, Chloroform-d) δ 7.08 (d, J = 8.3 Hz, 1H), 6.75 (d, J = 8.4 Hz, 1H),

4.09 (t, J = 5.0 Hz, 1H), 3.63 (s, 323H), 3.36 – 3.19 (m, 1H), 3.09 – 2.97 (m, 1H), 2.97 –

2.82 (m, 1H), 2.15 (s, 1H), 1.49 (dt, J = 14.9, 7.7 Hz, 2H), 0.92 (t, J = 7.3 Hz, 4H).

86

Synthesis of methyl L-phenylalanine hydrochloride [70]

(L)-Phenylalanine (15 g, 90.8 mmol) was dissolved in dry methanol (200 ml). The stirred

solution was cooled in an ice bath and treated with thionyl chloride (15 ml, 210 mmol).

After the addition was completed, the cooling bath was removed and stirring was continued

at room temperature overnight. The volatiles were removed under vacuum to afford (L)-

phenylalanine methyl ester hydrochloride 70 as a white solid (16.6 g, 84.8% yield).27 1H NMR (400 MHz, Deuterium Oxide) δ 7.52 – 7.40 (m, 3H), 7.37 – 7.31 (m, 2H), 4.67 (d,

J = 1.2 Hz, 3H), 4.48 (dd, J = 7.3, 6.1 Hz, 1H), 3.39 (dd, J = 14.6, 6.0 Hz, 1H), 3.29 (dd, J =

14.6, 7.4 Hz, 1H).

HRMS (ESI): calculated for C10H14ClNO2: 215.0713; found: 215.0709.


87

Synthesis of (S)-2-amino-N-methyl-3-phenylpropanamide [71]

Methylamine (33% w/w solution in ethanol, 5 equiv., 232 mmol, 21.8 g) was added

dropwise to a solution of 70 (10.0 g, 46.5 mmol) in ethanol (30 ml), and the resulting

mixture was stirred at room temperature for 24 hours. Then ethyl acetate (30 ml) and

aqueous NaHCO3 (30 ml) were added. The organic layer was separated and extracted with

aqueous NaHCO3 (2 x 30 ml).

The organic phase was separated and dried over Na2SO4. The pure product 71 (5 g, 60%

yield) was obtained after silica gel column chromatography (hexane/EtOAc, 30:70).27 1H NMR (400 MHz, Chloroform-d) δ 7.47 – 7.00 (m, 5H), 4.63 (s, 1H), 3.59 (dd, J = 9.3,

4.1 Hz, 1H), 3.26 (dd, J = 13.8, 4.1 Hz, 1H), 2.90 – 2.67 (m, 3H), 2.69 (dd, J = 13.7, 9.2 Hz,

1H).

HRMS (ESI): calculated for C10H14N2O: 178.1106; found: 178.1112.


88

Synthesis of 4-(3-bromopropoxy)benzaldehyde [72]

In double-necked round-bottom flask (250 ml) equipped with a condenser, a mixture

consisting of p-hydroxybenzaldehyde (40 mmol, 5 g), 1,3-dibromopropane (120 mmol),

K2CO3 (40 mmol, 5.7 g), and a catalytic amount of TBAB (tetrabutylammonium bromide,

0.2 g) in ACN (100 ml) was refluxed for 10 hours. After cooling and solvent evaporation,

the resulting foam was dissolved in CHCl3 (200 ml) and washed with water (3 x 200 ml).

The organic layer was dried over anhydrous Na2SO4 and solvent evaporated. The crude

product was purified by column chromatography on silica gel (n-hexane/EtOAc, 40:60),

affording a solid 72 (7.4 g, 76% yield).31 1H NMR (400 MHz, DMSO-d6/TMS): δ 9.87 (s, 1H), 7.80-7.20 (m, 4H), 4.02 (t, J = 5 Hz,

2H), 3.50 (t, J = 5 Hz, 2H), 2.13 (m, 2H). 13C NMR (100 MHz, DMSO-d6/TMS): δ 29.9, 32.9, 67.0, 106.8, 114.4, 114.9, 122.0,

128.3, 132.0, 190.2.

HRMS (ESI): calculated for C10H11BrO2: 241.9942; found: 241.9939.


89

Synthesis of 4-(3-azidopropoxy)benzaldehyde [73]

O

OBr

NaN3

O

ON3

7372

A mixture of 72 (7.4 g, 30 mmol) and NaN3 (5 g, 78 mmol) in DMF (120 ml) was heated at

100 °C for about 10 hours. When the reaction was over water (100 ml) and diethyl ether

(100 ml) were added, then the organic phase was separated and washed with brine solution

(3 x 100 ml). After drying with Na2SO4, volatiles were removed under vacuum to obtain

pure 73 (5.1 g, 82.8% yield). 1H NMR (400 MHz, CDCl3): δ 9.85 (s, 1H), 7.81 (d, J = 7.75 Hz, 2H), 6.98 (d. J = 7.0 Hz,

2H), 4.17 (t, J = 4.0 Hz, 2H), 3.58 (t, J = 3.5 Hz, 2H), 2.03 (tt, J = 2.0 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 190.6, 163.7, 131.9, 130.0, 114.7, 64.9, 48.0, 28.5.



90

Synthesis of (2S,5S)-5-benzyl-3-methyl-2-phenylimidazolidin-4-one [46a]

A solution of 71 (1.5 g, 8.4 mmol), benzaldehyde (1 ml, 9.8 mmol), and p-toluenesulfonic

acid monohydrate (160 mg, 0.84 mmol) dissolved in MeOH (20 ml) was heated to 50 °C for

24 hours. Concentration of the reaction mixture followed by silica gel chromatography

(hexane/EtOAc, 30:70) afforded the title compound (2S, 5S)-46a in 13.5 percent yield (300

mg, 1.13 mmol) and the more quickly eluting (2R, 5S)-46b isomer in 34 percent yield (750

mg, 2.8 mmol).32 1H NMR (400 MHz, CDCl3): δ 7.25 (m, 8H, ArH), 6.82 (m, 2H, ArH), 5.10 (m, 1H,

NCHN), 3.84 (dd, J = 4.5, 4.5 Hz, 1H, CHCO), 3.22 (dd, J = 14.1, 5.7 Hz, 1H, one of

CH2Ph), 3.11 (dd, J = 14.1,4.5 Hz, 1H, one of CH2Ph), 2.52 (s, 3H, CH3), 1.87 (br s, 1H,

NH). 13C NMR (100 MHz, CDCl3): δ 174.6, 138.8, 137.1, 130.1, 129.7, 129.2, 129.1, 127.4,

127.1, 60.7, 37.2, 27.5.

HRMS (ESI): calculated for C17H18N2O: 266.1419; found: 266.1421


91

Synthesis of (2S,5S)-2-(4-(3-azidopropoxy)phenyl)-5-benzyl-3-methylimidazolidin-4-one

[74a]

A solution of 71 (1.0 g, 5.6 mmol), 73 (1.15 g, 5.6 mmol) and p-toluenesulfonic acid

monohydrate (160 mg, 0.84 mmol) dissolved in MeOH (20 ml) was heated to 50 °C for 24

hours. Concentration of the reaction mixture followed by silica gel chromatography

(hexane/EtOAc, 30:70) afforded the title compound (2S, 5S)-74a in 12.6 percent yield (260

mg, 0.71 mmol) and the more quickly eluting (2R, 5S)-74b isomer in 39 percent yield (800

mg, 2.19 mmol).28 1H NMR (400 MHz, Chloroform-d) δ 7.39 – 7.15 (m, 5H), 5.09 (d, J = 1.6 Hz, 1H), 3.85 (t,

J = 5.2 Hz, 1H), 3.83 – 3.75 (m, 2H), 3.25 (dd, J = 14.2, 5.7 Hz, 1H), 3.14 (qd, J = 7.0, 4.5

Hz, 2H), 3.01 (dd, J = 14.2, 6.8 Hz, 1H), 2.75 (d, J = 0.7 Hz, 3H), 2.53 (s, 3H), 2.10 (s, 3H),

2.06 – 1.98 (m, 3H).



92

Synthesis of (2S,5S)-5-benzyl-2-(4-methoxyphenyl)-3-methylimidazolidin-4-one [75a]

A solution of 71 (3.0 g, 16.8 mmol), p-methoxybenzaldehyde (2.3 g, 16.8 mmol) and p-

toluenesulfonic acid monohydrate (200 mg, 1 mmol) dissolved in MeOH (30 ml) was

heated to 50 °C for 24 hours. Concentration of the reaction mixture followed by silica gel

chromatography (hexane/EtOAc, 30:70) afforded the title compound (2S, 5S) in 4 percent

yield (200 mg, 0.7 mmol) and the more quickly eluting (2R, 5S) isomer in 48 percent yield

(2.4 g, 8.1 mmol).28 1H NMR (400 MHz, Chloroform-d) δ 7.34 – 7.23 (m, 5H), 7.14 (d, J = 8.7 Hz, 1H), 6.88 (d,

J = 8.7 Hz, 2H), 4.83 (d, J = 1.8 Hz, 1H), 4.05 (t, J = 5.8 Hz, 1H), 3.79 (s, 3H), 3.12 (dd, J =

13.7, 4.0 Hz, 1H), 2.95 (dd, J = 13.7, 7.4 Hz, 1H), 2.61 – 2.47 (m, 3H).

HRMS (ESI): calculated for C18H20N2O: 296.1525; found: 296.1529.


93

Synthesis of (2S,5S)-5-benzyl-2-(4-hydroxyphenyl)-3-methylimidazolidin-4-one [76a]

A solution of 71 (3.0 g, 16.8 mmol), p-hydroxybenzaldehyde (2.5 g, 20.5 mmol) and p-

toluenesulfonic acid monohydrate (200 mg, 1 mmol) dissolved in MeOH (30 ml) was

heated to 50 °C for 24 hours. Concentration of the reaction mixture followed by silica gel

chromatography (hexane/EtOAc, 50:50) afforded the title compound (2S, 5S)-76a in 27

percent yield (1.3 g, 4.6 mmol) and the more quickly eluting (2R, 5S)-76b isomer in 6.1

percent yield (290 g, 1.03 mmol).28 1H NMR (400 MHz, Chloroform-d) δ 7.36 – 7.17 (m, 5H), 7.16 – 7.02 (m, 2H), 6.86 – 6.69

(m, 2H), 4.80 (d, J = 1.7 Hz, 1H), 4.06 (s, 1H), 3.11 (dd, J = 13.7, 4.1 Hz, 1H), 2.95 (dd, J =

13.7, 7.3 Hz, 1H), 2.55 (s, 2H).



94

Synthesis of (2S,5S)-5-benzyl-2-(4-(3-bromopropoxy)phenyl)-3-methylimidazolidin-4-one

[77]


consisting of 76a (500 mg, 1.77 mmol), 1,3-dibromopropane (358 mg, 1.77 mmol), K2CO3

(500 mg, 5 mmol), and a catalytic amount of TBAB (0.1 g) in ACN (30 ml) was refluxed

for 10 hours. After cooling and solvent evaporation, the resulting foam was dissolved in

CHCl3 (100 ml) and washed with water (3 x 100 ml). The organic layer was dried over

anhydrous Na2SO4 and evaporated. The crude product was purified by column

chromatography on silica gel (n-hexane/EtOAc, 50:50), giving a solid 77 (490 mg, 69%

yield).31 1H NMR (400 MHz, Chloroform-d) δ 7.32 (d, J = 8.6 Hz, 2H), 7.06 (d, J = 8.6 Hz, 2H),

5.00 (s, 1H), 4.24 (s, 1H), 3.30 (dd, J = 13.7, 3.9 Hz, 1H), 3.14 (dd, J = 13.7, 7.4 Hz, 1H),

2.73 (s, 3H), 2.57 – 2.36 (m, 2H), 1.38 – 1.19 (m, 1H).

HRMS (ESI): calculated for C20H23BrN2O2: 402.0943; found: 402.0950.


95

Synthesis of ethyl (E/Z)-3-phenylbut-2-enoate [78]

Triethylphosphonoacetato (17 g, 75.7 mmol) was added dropwise under nitrogen

atmosphere over a period of 5 minutes to a stirred suspension of NaH (60% on mineral oil; 3

g, 75.7 mmol) in dry THF (50 ml), and the resulting mixture was stirred for another 30

minutes at 0 °C. A solution of acetophenone (8.5 g, 70 mmol) in dry THF (10 ml) was

slowly added to the resulting mixture, and the reaction mixture was heated to reflux

overnight. After cooling to room temperature, the reaction was quenched with a saturated

aqueous solution of NH4Cl and extracted with EtOAc (3 x 30 ml). The combined organic

layers were washed with brine and dried over Na2SO4. Concentration under vacuum afford

product 78 (12.7 g, 95 % yield).33 1H NMR (400 MHz, CDCl3): δ 7.50-7.14 (m, 6H), 6.12 (q, J = 1.1 Hz, 1H), 4.26-4.16 (m,

2H), 2.61-2.53 (m, 3H), 1.31 (ddt, J = 7.9, 7.1, 0.9 Hz, 4H).

HRMS (ESI): calculated for C12H14O2: 190.0994; found: 190.0996.


96

Synthesis of (E/Z)-3-phenylbut-2-en-1-ol [79]

To a stirred suspension of LiAlH4 (150 mg, 4 mmol) in dry THF (10 ml), a solution of BnCl

(benzyl chloride; 495 mg, 4 mmol) in dry THF (3 ml) was added dropwise through dropping

funnel at room temperature. After the suspension was stirred for 15 min, a solution of 78

(500 mg, 2.6 mmol) in dry THF (2 ml) was added drowise to the suspension. The reaction

mixture was stirred at room temperature 2 hours. Then the reaction was quenched with

water, filtered and the filtrate was dried with Na2SO4. The solvent was evaporated under

vacuum to obtain 79 (300 mg, 77% yield).33 1H NMR (400 MHz, CDCl3): δ 7.43-7.40 (m, 2H), 7.36-7.30 (m, 2H), 7.29-7.25 (m, 1H),

5.98 (dt, J = 6.4, 1.2 Hz, 1H), 4.37 (d, J = 6.8 Hz, 2H), 2.08 (s, 3H), 1.87 (br s, 1H). 13C NMR (100 MHz, CDCl3): δ 142.7, 137.7, 128.2, 127.2, 126.4, 125.7, 59.8, 16.0.

HRMS (ESI): calculated for C10H12O: 148.0888; found: 148.0885.

Elemental Analysis: C, 81.04; H, 8.16; found: C,81.04; H, 8.18.

97

Synthesis of azide-PEG [54]

Commercial polymer (HO-PEG-OH) (5 g, 2.5 mmol) was dissolved in anhydrous THF (35

ml) followed by trimethylamine (1.01 g, 10 mmol) addition. The mixture was then added to

a solution of methanesulfonyl chloride (1 g, 8.75 mmol) in THF (50 ml), and stirred for 48

hours at room temperature with CaCl2 valve. After the reaction, THF was evaporated under

reduced pressure. The residue was redissolved in water (100 ml) and extracted with

dichloromethane (5 x 200 ml). The organic layers were combined and dried over Na2SO4.

After filtration and concentration, the polymer was recovered by precipitation into ether and

dried in vacuo, yielding a white solid.

This mesylated polymer (1.25 g, 0.23 mmol) was dissolved in DMF (20 ml), followed by

sodium azide (521 mg, 8.01 mmol) addition, and was stirred for two days at room

temperature. Dichloromethane (200 ml) was then added, and the reaction mixture was

washed five times with water and brine. The organic layer was dried over Na2SO4, filtered,

concentrated, and then reprecipitated into ether affording pure 54 (1 g, 71% yield).34 1H NMR : 400 MHz, CDCl3, δ in ppm: 3.46 (t, CH2-CH2-N3), 3.63 (s, O-(CH2)2-O).

98

PAA functionalized via amide synthesis [58]

Polyacrylic acid (500 mg, 6.94 mmol) was dissolved in 30 ml of distilled water and

propargylamine (63.2 g, 0.694 mmol) was added to the solution.

HOBt (hydroxybenzotriazole; 106 mg) was dissolved in a solution composed by H2O (10

ml) and CH3CN (10 ml) at about 50 °C because of its insolubility at room temperature and

added dropwise to the mixture. EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimmide)

powder (133 mg) was added.

Carboxyl groups are very important in hydrogel scaffold formation: thus, not all of them

must be functionalized with triple bond, in a way to be able to react with hydroxyl group of

PEG or agarose. This aim is achieved using less amount of propargylamine than PAA.

Then, desired product was purified via dialysis in distilled water (2000 ml), with the

addiction of 4 drops of HCl 37% w/w and NaCl (11.2 g) powder to create an acid

environment to molecular diffusion. This solution was stirred at room temperature. Dialysis

technique is described on paragraph 2.8.1. Finally, functionalized PAA was frozen at -40 °C

and lyophilized.23

99

PEG functionalization via click strategy [56]

54 (2.7 g, 1.35 mmol) was dissolved in water (20 ml) and 30 (424 mg, 1.35 mmol) in

acetone (20 ml), then they were blended together. About 3 mg of catalyst CuI and 3 mg of

sodium ascorbate were added to the mixture and the latter heated to 50°C, with reflux

system. In addition five drop of TMEDA (tetramethylethilenediamine) were added.

The mixture was mechanically stirred for about 36 hours. After returning to room

temperature, it was treated via extraction with DCM in a separating funnel for two times.

Separated organic phase was dried using Na2SO4, filtrated and the solvent evaporated using

rotary evaporator and vacuum pump. Synthetized product 56 is solid (2.5 g, 81.5% yield). 1H NMR (400 MHz, DMSO-d6) δ 7.80 (s, 1H), 7.13 (d, J = 8.1 Hz, 2H), 6.90 (d, J = 8.1

Hz, 2H), 4.52 (t, J = 5.1 Hz, 2H), 3.86 (t, J = 5.1 Hz, 2H), 3.62 (s, 183H), 3.44 (dd, J = 5.9,

4.0 Hz, 1H), 3.36 (t, J = 5.1 Hz, 1H), 3.00 (d, J = 5.1 Hz, 2H), 2.93 (s, 4H), 2.86 (s, 4H),

1.32 – 1.22 (m, 9H), 0.90 (t, J = 7.4 Hz, 3H).

100

Synthesis of (E)-3-phenylbut-2-enal [36]

To a stirring solution of 79 (600 mg, 4 mmol) in chloroform (10 ml), was added an excess

of MnO2 (1.75 g, 20 mmol). The reaction mixture was stirred at room temperature overnight

and filtered. Volatiles were removed under vacuum to obtain the product 36 (350 mg, 60%

yield).33 1H NMR (400 MHz, CDCl3): δ 10.17 (d, J = 7.6 Hz, 1H), 7.57-7.50 (m, 2H), 7.44-7.38 (m,

3H), 6.38 (dq, J = 7.6, 1.2 Hz, 1H), 2.56 (d, J = 1.2 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ 191.0, 157.4, 140.6, 130.0, 128.7, 127.2, 126.2, 16.3.

HRMS (ESI): calculated for C10H10O: 146.0732; found: 146.0739


101

Synthesis of 5-(4-(3-bromopropoxy)benzyl)-3-butyl-2,2-dimethylimidazolidin-4-one [80]


consisting of 33 (1 g, 3.6 mmol), 1,3-dibromopropane (732 mg, 3.6 mmol), K2CO3 (1 g, 7.2

mmol), and a catalytic amount of TBAB (tetrabutylammonium bromide, 0.1 g) in ACN (40

ml) was refluxed for 10 hours. After cooling and solvent evaporation, the resulting foam

was dissolved in CHCl3 (50 ml) and washed with water (3 x 200 ml). The organic layer was

dried over anhydrous Na2SO4 and solvent evaporated. The crude product was purified by

column chromatography on silica gel (n-hexane/EtOAc, 60:40), affording the product 80

(820 mg, 57% yield). 1H NMR (400 MHz, Chloroform-d) δ 7.36 (dd, J = 8.5, 4.0 Hz, 2H), 6.87 (dd, J = 8.5, 4.6

Hz, 2H), 4.31 (s, 1H), 4.05 (t, J = 5.8 Hz, 1H), 3.43 (d, J = 15.3 Hz, 1H), 3.10 – 2.93 (m,

1H), 2.34 – 2.18 (m, 1H), 2.16 (d, J = 0.5 Hz, 1H), 1.68 (s, 3H), 1.50 (s, 8H), 0.97 – 0.84

(m, 3H).

HRMS (ESI): calculated for C19H29BrN2O2: 396.1412; found: 396.1417.


102

Synthesis of 5-(4-(3-azidopropoxy)benzyl)-3-butyl-2,2-dimethylimidazolidin-4-one [81]

A mixture of 80 (800 mg, 2.02 mmol) and NaN3 (270 mg, 4.1 mmol) in DMF (15 ml) was

heated at 100 °C for about 10 hours. When the reaction was over water (20 ml) and diethyl

ether (20 ml) were added, then the organic phase was separated and washed with brine

solution (3 x 20 ml). After drying with Na2SO4, volatiles were removed under vacuum to

obtain pure 81 (550 mg, 75.7% yield). 1H NMR (400 MHz, Chloroform-d) δ 7.36 (dd, J = 8.5, 4.0 Hz, 2H), 6.87 (dd, J = 8.5, 4.6

Hz, 2H), 4.31 (s, 1H), 4.05 (t, J = 5.8 Hz, 1H), 3.12 (d, J = 15.3 Hz, 1H), 3.10 – 2.93 (m,

1H), 2.34 – 2.18 (m, 1H), 2.16 (d, J = 0.5 Hz, 1H), 1.68 (s, 3H), 1.50 (s, 8H), 0.97 – 0.84

(m, 3H).



103

Synthesis of ethyl (E/Z)-3-(4-chlorophenyl)but-2-enoate [82]

Triethylphosphonoacetato (5 g, 22.3 mmol) was added dropwise under nitrogen atmosphere

to a stirred suspension of NaH (60% on mineral oil; 800 mg, 20 mmol) in dry THF (40 ml),

and the resulting mixture was stirred for another 30 minutes at 0 °C. A solution of 4-

chloroacetophenone (2 g, 13 mmol) in dry THF (10 ml) was slowly added to the resulting

mixture, and the reaction mixture was heated to reflux overnight. After cooling to room

temperature, the reaction was quenched with a saturated aqueous solution of NH4Cl and

extracted with EtOAc (3 x 20 ml). The combined organic layers were washed with brine and

dried over Na2SO4. Concentration under vacuum afford product 82 (2 g, 69 % yield).33 1H NMR (400 MHz, CDCl3): δ 7.43-7.36 (m, 2H), 7.35-7.30 (m, 2H), 6.10 (q, J = 1.2 Hz,

1H), 4.25-4.17 (m, 2H), 2.54 (t, J = 1.0 Hz, 3H), 1.31 (td, J = 7.1, 0.7 Hz, 3H).

HRMS (ESI): calculated for C12H13ClO2: 224.0604; found: 224.0606.


104

Synthesis of (E/Z)-3-(4-chlorophenyl)but-2-en-1-ol [83]

To a stirred suspension of LiAlH4 (450 mg, 12 mmol) in dry THF (30 ml), a solution of

BnCl (benzyl chloride; 1.5 g, 12 mmol) in dry THF (8 ml) was added dropwise through

dropping funnel at room temperature. After the suspension was stirred for 15 min, a solution

of 82 (1.5 g, 6.7 mmol) in dry THF (3 ml) was added drowise to the suspension. The

reaction mixture was stirred at room temperature 2 hours. Then the reaction was quenched

with water, filtered and the filtrate was dried with Na2SO4. The solvent was evaporated

under vacuum to obtain 83 (1 g, 81.6% yield).33 1H NMR (400 MHz, CDCl3): δ 7.38-7.18 (m, 2H), 7.17-7.07 (m, 2H), 3.67-3.41 (m, 2H),

2.87 (hex, J = 8.2 Hz, 1H), 1.90-1.77 (m, 2H), 1.24 (d, J = 7.0 Hz, 3H).

HRMS (ESI): calculated for C10H11ClO: 182.0498; found: 182.0501.


105

Synthesis of (E/Z)-3-(4-chlorophenyl)but-2-enal [38]

To a stirring solution of 83 (1 g, 5.5 mmol) in chloroform (20 ml), was added an excess of

MnO2 (2.5 g, 28.7 mmol). The reaction mixture was stirred at room temperature overnight

and filtered. The crude product was purified by column chromatography on silica gel (n-

hexane/EtOAc, 90:10), giving pure 38a and 38b (400 mg, 40% yield).33 1H NMR (400 MHz, CDCl3): δ 9.46 (d, J = 8.1 Hz, 1H), 7.40 (d, J = 1.9 Hz, 2H), 7.23 (d, J

= 8.3 Hz, 2H), 6.16-6.10 (m, 1H), 2.31-2.26 (m, 3H).



106

Synthesis of ethyl (E)-3-(4-methoxyphenyl)but-2-enoate [84]

Triethylphosphonoacetato (5 g, 22.3 mmol) was added dropwise under nitrogen atmosphere

to a stirred suspension of NaH (60% on mineral oil; 800 mg, 20 mmol) in dry THF (40 ml),

and the resulting mixture was stirred for another 30 minutes at 0 °C. A solution of 4-

methoxyacetophenone (2 g, 13 mmol) in dry THF (10 ml) was slowly added to the resulting

mixture, and the reaction mixture was heated to reflux overnight. After cooling to room

temperature, the reaction was quenched with a saturated aqueous solution of NH4Cl and

extracted with EtOAc (3 x 20 ml). The combined organic layers were washed with brine and

dried over Na2SO4. Concentration under vacuum afford product 84 (1.7 g, 59 % yield).33 1H NMR (400 MHz, CDCl3): δ 7.47-7.41 (m, 2H), 6.91-6.84 (m, 2H), 6.10 (q, J = 1.3 Hz,

1H), 4.20 (q, J = 7.1 Hz, 2H), 3.82 (s, 3H), 2.55 (d, J = 1.2 Hz, 3H), 1.31 (t, J = 7.1 Hz, 3H).



107

Synthesis of (E)-3-(4-chlorophenyl)but-2-en-1-ol [85]

To a stirred suspension of LiAlH4 (450 mg, 12 mmol) in dry THF (30 ml), a solution of

BnCl (benzyl chloride; 1.5 g, 12 mmol) in dry THF (8 ml) was added dropwise through

dropping funnel at room temperature. After the suspension was stirred for 15 min, a solution

of 84 (1.5 g, 6.8 mmol) in dry THF (3 ml) was added drowise to the suspension. The

reaction mixture was stirred at room temperature for two hours. Then the reaction was

quenched with water, filtered and the filtrate was dried with Na2SO4. The solvent was

evaporated under vacuum to obtain 85 (1.1 g, 90.7% yield).33 1H NMR (400 MHz, Chloroform-d) δ 7.37 – 7.34 (m, 2H), 6.91 – 6.83 (m, 2H), 5.92 (tq, J

= 6.7, 1.3 Hz, 1H), 4.59 (d, J = 1.7 Hz, 1H), 4.35 (d, J = 6.8 Hz, 2H), 3.81 (s, 3H), 2.07 (dt,

J = 1.5, 0.8 Hz, 3H).



108

Synthesis of (E)-3-(4-methoxyphenyl)but-2-enal [40]

To a stirring solution of 85 (700 mg, 3.9 mmol) in chloroform (15 ml), was added an excess

of MnO2 (1.7 g, 19.7 mmol). The reaction mixture was stirred at room temperature

overnight and filtered. The crude product was purified by column chromatography on silica

gel (n-hexane/EtOAc, 90:10), giving pure 40 (330 mg, 48% yield).33 1H NMR (400 MHz, Chloroform-d) δ 9.68 (s, 1H), 7.55 – 7.50 (m, 2H), 6.95 – 6.89 (m,

2H), 6.38 (dq, J = 7.8, 1.2 Hz, 1H), 3.84 (s, 3H), 2.54 (d, J = 1.2 Hz, 3H).



109

Synthesis of 3-phenylbutan-1-ol [86]

To a stirred suspension of LiAlH4 (260 mg, 6.85 mmol) in dry THF (20 ml) at 0 °C, a

solution of 78 (1 g, 5.25 mmol) in dry THF (10 ml) was added drowise. Once the added was

completed, the reaction mixture was stirred at room temperature for 24 hours. Then the

reaction was quenched with water, filtered and the filtrate was dried with Na2SO4. The

solvent was evaporated under vacuum to obtain 86 (700 mg, 88.7% yield). 1H NMR (400 MHz, CDCl3): δ 7.32-7.15 (m, 5H), 3.61-3.48 (m, 2H), 2.93-2.82 (m, 1H),

1.85 (q, J = 6.8 Hz, 2H), 1.27 (d, J = 7 Hz, 3H), 1.26 (br s, 1H). 13C NMR (100 MHz, CDCl3): δ 146.9, 128.4, 126.9, 126.1, 61.2, 41.0, 36.5, 22.3.



110

Synthesis of 3-phenylbutanal [37]

To a stirred solution of 86 (390 mg, 2.6 mmol) in dichloromethane (30 ml) at 0 °C, was

added PCC (Pyridinium chlorochromate; 835 mg, 3.9 mmol). The suspension was stirred at

room temperature for about two hours then diethyl ether (30 ml) was added and the reaction

mixture was filtered on celite. Volatiles were removed under vacuum affording compound

37 (300 mg, 77 % yield).35 1H NMR (400 MHz, CDCl3): δ 9.71 (t, J = 2.1 Hz, 1H), 7.32-7.19 (m, 5H), 3.36 (h, J = 7.1

Hz, 1H), 2.79-2.61 (m, 2H), 1.33 (d, J = 7.0 Hz, 3H).



111

Synthesis of 2-phenyl-2,3-dihydrobenzo[d]thiazole [87]

Benzaldehyde (5.3 g, 50 mmol) was added to a solution of o-aminothiophenol (6.25 g, 50

mmol) in ethanol (10 ml) and the mixture was stirred at room temperature. After 30

minutes, pale yellow needles were separated from the reaction mixture, which was then

stirred for another 30 minutes. The resulting needles were collected by filtration and

recrystallized with n-hexane to give the pure thiazoline 87 (6 g, 56% yield) as pale yellow

needles.25 1H NMR (400 MHz, CDCl3): δ 7.55-7.03 (m, 5H), 6.95-6.45 (m, 4H), 6.26 (s, 1H), 4.19 (br,

1H).

HRMS (ESI): calculated for C13H11NS: 213.2980; found: 213.2975.


112

PAA functionalization via click strategy [60]

59 (150 mg, 0.42 mmol) was dissolved in 10 ml of THF. Referring to 10% functionalization

of PAA with triple bond, 100 mg of 58 were used and dissolved in 20 ml of distilled water.

Solutions are blended together, 3 mg of catalyst CuI and the same amount of sodium

ascorbate were added. The mixture is heated to 60°C and mechanically stirred for about 24

hours.

Then it is cooled at room temperature and dialyzed against distilled water, in acid

conditions. Dialysis occurs in the same way illustrated before: in 2000 ml of distilled water,

11.2 g of NaCl and 4 drops of HCl 37% w/w are added and then reaction mixture, put inside

dialysis membrane, is submerged in this acid solution. Dialysis takes place for three days,

with daily replacement of dialysis solution, re-prepared using 2000 ml of distilled water and

four drops of HCl only. Resulting mixture is frozen at -21°C and then lyophilized.23

113

Synthesis of PEG-tosylate [51]

To a solution of PEG monomethylether (Mw = 5000) (5 g, 1 mmol) in CH2Cl2 (50 ml) was

added an excess of tri-n-butylamine (3.34 g, 18 mmol) and the reaction mixture was allowed

to stir for 30 min. Next, TsCl (4-toluenesulfonyl chloride) (1.15 g, 6 mmol) was slowly

added to the stirring solution and the reaction mixture was left to stir at 25 °C for 24 hours.

The solvent was then removed in vacuo and the resultant oily residue added dropwise to

stirring diethyl ether. The precipitate was filtered and the solid collected and dissolved in a

minimal amount of CH2Cl2 and triturated with diethyl ether at 0 °C and vacuum filtered.

The PEG tosylate 51 was isolated as a white solid (4.6 g, 90.5% yield). 1H NMR (400 MHz, Chloroform-d) δ 7.82 – 7.78 (m, 3H), 7.19 – 7.07 (m, 3H), 4.21 – 4.07

(m, 2H), 3.87 – 3.77 (m, 3H), 3.64 (s, 603H), 3.57 (s, 4H), 2.44 (s, 3H).

114

PEG-supported via ether synthesis [52]

PEG tosylate 51 (2 g, 0.39 mmol) was added to a solution of the catalyst 33 (275 mg, 1

mmol) and K2CO3 (277 mg, 2 mmol) in dry DMF (20 ml), in a 100 ml round bottomed flask

fitted with a magnetic stir bar. After stirring at 25 °C for 48 hours, the reddish-brown

solution was filtered and then concentrated under vacuum to about half its original volume.

This solution was slowly added to cold stirring ether (200 ml) to precipitate the polymeric

catalyst. The solid was further washed in ether (50 ml) followed by iso-propanol (50 ml)

and dried in vacuo to yield 52 (1.8 g, 87% yield). 1H NMR (400 MHz, Chloroform-d) δ 7.08 (d, J = 8.3 Hz, 1H), 6.75 (d, J = 8.4 Hz, 1H),

4.09 (t, J = 5.0 Hz, 1H), 3.63 (s, 323H), 3.36 – 3.19 (m, 1H), 3.09 – 2.97 (m, 1H), 2.97 –

2.82 (m, 1H), 2.15 (s, 1H), 1.49 (dt, J = 14.9, 7.7 Hz, 2H), 0.92 (t, J = 7.3 Hz, 4H).

115

Synthesis of 3-(4-chlorophenyl)butan-1-ol [88]

OEt OH

LiAlH4

82 88

O

Cl Cl

To a stirred suspension of LiAlH4 (170 mg, 4.46 mmol) in dry THF (20 ml) at 0 °C, a

solution of 82 (1 g, 4.46 mmol) in dry THF (5 ml) was added drowise. Once the added was

completed, the reaction mixture was stirred at room temperature for 24 hours. Then the

reaction was quenched with water, filtered and the filtrate was dried with Na2SO4. The

solvent was evaporated under vacuum to obtain 88 (550 mg, 66.8% yield).33 1H NMR (400 MHz, CDCl3): δ 7.12 (m, 4H), 3.49 (m, 2H), 2.80 (m, 1H), 1.76 (m, 2H),

1.19 (m, 3H).



116

Synthesis of 3-(4-chlorophenyl)butanal [39]

To a stirred solution of 88 (400 mg, 2.16 mmol) in dichloromethane (30 ml) at 0 °C, was

added PCC (Pyridinium chlorochromate; 700 mg, 3.24 mmol). The suspension was stirred

at room temperature for about two hours then diethyl ether (30 ml) was added and the

reaction mixture was filtered on celite. Volatiles were removed affording compound 39 (100

mg, 25 % yield).29

1H NMR (400 MHz, CDCl3): δ 9.70 (t, J = 3.7, 2.0 Hz, 1H), 7.28 (d, J = 8.2 Hz, 2H), 7.14

(d, J = 8.0 Hz, 2H), 3.37-3.30 (m, 1H), 2.71 (ddd, J = 16.8, 7.0, 2.0 Hz, 1H), 2.64 (ddd, J =

17.0, 7.8, 2.0 Hz, 1H), 1.30 (d, J = 7.0 Hz, 3H).



117

Synthesis of PEG-propyne [55]

To a solution of PEG monomethylether (Mw = 5000) (5 g, 1 mmol) in toluene (30 ml), were

added NaH (80% w/w solution in toluene, 2 equiv., 80 mg, 2 mmol) and 3-bromoprop-1-

yne (80% w/w solution in toluene, 2 equiv., 298 mg, 2 mmol) at 0 °C. After the addition, the

resulting mixture was stirred at room temperature for overnight. Once the reaction was

completed solvent was removed under vacuum, then water (20 ml) was added to the

reaction mixture that was subsequently treated via extraction with DCM (2 x 30 ml) in a

separating funnel. Separated organic phase was dried using Na2SO4, filtrated and the solvent

evaporated using rotary evaporator and vacuum pump. Precipitation of 55 was obtained

adding diethyl ether. After filtration synthetized 55 was isolated as a white solid (3.8 g,

78.5% yield).23 1H NMR (400 MHz, Chloroform-d) δ 4.20 (d, J = 2.4 Hz, 2H), 3.64 (s, 479H), 3.37 (s, 3H),

2.42 (t, J = 2.4 Hz, 1H).

118

PEG functionalization via click strategy [57]

55 (2.1 g, 0.42 mmol) was dissolved in water (15 ml) and 81 (350 mg, 1 mmol) in THF (15

ml), then they were blended together. About 5 mg of catalyst CuI and 5 mg of sodium

ascorbate were added to the mixture and the latter heated to 50°C, with reflux system. In

addition five drop of TMEDA (tetramethylethilenediamine) were added.

The mixture was mechanically stirred for about 36 hours. After returning to room

temperature, it was treated via extraction with DCM in a separating funnel for two times.

Separated organic phase was dried using Na2SO4, filtrated and the solvent evaporated using

rotary evaporator and vacuum pump. Precipitation of 57 was obtained adding diethyl ether.

After filtration synthetized 57 was isolated as a brown solid (2.5 g, 82.5% yield). 1H NMR (400 MHz, Chloroform-d) δ 7.57 (s, 1H), 7.13 (s, 4H), 6.86 – 6.81 (m, 4H), 4.55

(t, J = 6.9 Hz, 2H), 3.99 (dt, J = 24.1, 5.9 Hz, 4H), 3.81 (t, J = 5.1 Hz, 3H), 3.64 (s, 429H),

3.37 (s, 3H), 2.94 – 2.83 (m, 0H), 2.37 (d, J = 6.4 Hz, 1H), 2.02 (d, J = 6.4 Hz, 1H), 1.15 (d,

J = 5.8 Hz, 5H).

119

Synthesis of diphenylmethanol [89]

To a stirring solution of benzophenone (3 g, 16.5 mmol) in ethanol at 0 °C, was added

sodium borohydride (680 mg, 18 mmol). The mixture was stirred at room temperature for

about 24 hours, then it was treated via extraction with ethyl acetate in a separating funnel for

two times. Separated organic phase was dried using Na2SO4, filtrated and the solvent

evaporated using rotary evaporator and vacuum pump, affording pure compound 89 (2.6 g,

88% yield). 1H-NMR (400 MHz, Chloroform-d) δ 2.21 (brs, OH), 5.79 (s, 1H), 7.22-7.7.37 (m, 10H).



120

Synthesis of (E)-(2-nitrovinyl)benzene [90]

Benzaldehyde (40 g, 0.38 mol) was added to a solution of aminoacetate (3 mg, 0.04 mol)

and CH3NO2 (25 ml, 0.46 mol) in glacial acetic acid (70 ml), in a 250 ml round bottomed

flask fitted with a magnetic stir bar. After stirring at 100 °C for 24 hours, the solution was

slowly added to water at 0 °C (200 ml) to precipitate the product. The solid was further

filtrated and washed with water (100 ml) followed by hexane (100 ml) to yield pure 90 (37

g, 66% yield). 1H-NMR (400 MHz, Chloroform-d) δ 8.02-7.98 (1H, d, J 13.6), 7.60-7.43 (6H, m).

HRMS (ESI): calculated for C8H7NO2: 149.0477; found: 149.0480.


121

3.3 General Procedures

3.3.1 Enantioselective Reduction of Enales

Procedure 1

In a 5 ml flask, organocatalyst 33 (0.025 mmol) was added to a solution of aldehyde A (0.25

mmol), hydride donor H (0.3 mmol) and trifluoroacetic acid in a solvent Y (3 ml). The

reaction mixture was stirred at room temperature for 24 hours affording product B with

different conversion percentages.

Procedure 2

In a 4 ml syringe, insoluble PS-supported organocatalyst 61 (0.025 mmol), was added to a

solution of aldehyde A (0.13 mmol) and hydride donor H (0.25 mmol) in dichloromethane

(2 ml). The reaction mixture was stirred at room temperature for 24 or 48 hours affording

product B with different conversion percentages. After the reaction mixture was discharged,

the polymer-supported was washed with dichloromethane (2 x 5 ml), methanol (2 x 5 ml)

and dichloromethane (2 x 5 ml) again. Catalyst was so reused for next cycle.

Procedure 3

In a 5 ml flask, soluble PEG-supported organocatalyst 52 or 57 (0.025 mmol), was added to

a solution of aldehyde A (0.25 mmol), hydride donor H (0.3 mmol) and trifluoroacetic acid

in a solvent Y (3 ml). The reaction mixture was stirred at room temperature for 24 hours

affording product B with different conversion percentages. After the reaction was completed

diethyl ether (20 ml) was added to the mixture. The purpose is to precipitate the pure

polymer to use in the next cycle.

122

Procedure 4

In a 10 ml syringe, one or more insoluble tablets of hydrogel-supported organocatalyst AC-

PEG-2000 or AC-PEG-5000 (150 µl each) (formulation reported in paragraphs 2.6.1 and

2.6.2), were added to a solution of aldehyde A (0.1 mmol) and hydride donor H (0.25

mmol) in water (2 ml). The reaction mixture was stirred at room temperature for 96 hours

affording product B with different conversion percentages.

Reduction of cinnamaldehyde [A1] to 3-phenylpropanal [6]

Accoding to Procedure 1

a) Cinnamaldehyde 34 (33 mg, 0.25 mmol) was added to a solution of Hantzsch ester

(75 mg, 0.3 mmol), trifluoroacetic acid (3 mg, 0.026 mmol) in water (2.5 ml) and

THF (0.5 ml).

Reaction time: 48 hours; Yield: 99%.

b) Cinnamaldehyde 34 (33 mg, 0.25 mmol) was added to a solution of Hantzsch ester

(75 mg, 0.3 mmol), trifluoroacetic acid (3 mg, 0.026 mmol) in chloroform (3 ml).


c) Cinnamaldehyde 34 (17 mg, 0.13 mmol) was added to a solution of 87 (50 mg, 0.25

mmol), trifluoroacetic acid (1.5 mg, 0.014 mmol) in dichloromethane (2 ml).


123

According to Procedure 2


(63 mg, 0.25 mmol) in dichloromethane (2 ml).

Number of cycles: 5; Reaction time: 24 hours; Yield: 33-55%.

According to procedure 3

a) Cinnamaldehyde 34 (32 mg, 0.25 mmol) was added to a solution of PEG-supported

catalyst 52 (60 mg, 0.025 mmol), Hantzsch ester (75 mg, 0.3 mmol), trifluoroacetic

acid (3 mg, 0.026 mmol) in dichloromethane (3 ml).

Reaction time: 24 hours; Yield: 54.7%.

b) Cinnamaldehyde 34 (32 mg, 0.25 mmol) was added to a solution of PEG-supported


acid (3 mg, 0.026 mmol) in water (3 ml).


c) Cinnamaldehyde 34 (17 mg, 0.13 mmol) was added to a solution of PEG-supported


acid (1.5 mg, 0.014 mmol) in dichloromethane (2 ml).


d) Cinnamaldehyde 34 (17 mg, 0.13 mmol) was added to a solution of PEG-supported


acid (1.5 mg, 0.014 mmol) in water (2 ml).

Number of cycles: 1; Reaction time: 24 hours; Yield: 83.11%.

124

According to procedure 4:

a) One tablet of AC-PEG-2000 was added to a solution of cinnamaldehyde 34 (13 mg,

0.1 mmol) and Hantzsch ester (63 mg, 0.25 mmol) in water (2 ml).


b) One tablet of AC-PEG-5000 was added to a solution of cinnamaldehyde 34 (13 mg,

0.1 mmol) and Hantzsch ester (63 mg, 0.25 mmol) in water (2 ml).


c) Three tablet of AC-PEG-2000 was added to a solution of cinnamaldehyde 34 (13

mg, 0.1 mmol) and Hantzsch ester (63 mg, 0.25 mmol) in water (2 ml).


Reduction of 3-phenylbut-2-enal [36] to 3-phenylbutanal [37]


a) 3-phenylbut-2-enal 36 (36 mg, 0.25 mmol) was added to a solution of Hantzsch ester

(75 mg, 0.3 mmol), trifluoroacetic acid (3 mg, 0.026 mmol) in dichloromethane (3

ml).


125


a) 3-phenylbut-2-enal 36 (19 mg, 0.13 mmol) was added to a solution of Hantzsch ester

(63 mg, 0.25 mmol) in dichloromethane (2 ml).


b) 3-phenylbut-2-enal 36 (19 mg, 0.13 mmol) was added to a solution of 87 (50 mg,

0.25 mmol) in dichloromethane (2 ml).

Number of cycles: 3; Reaction time: 48 hours; Yield: 84.5-90%.

c) 3-phenylbut-2-enal 36 (19 mg, 0.13 mmol) was added to a solution of Hantzsch ester

(63 mg, 0.25 mmol) in water (2 ml) and THF (200 µl).

Number of cycles: 1; Reaction time: 48 hours; Yield: 49%.


a) 3-phenylbut-2-enal 36 (19 mg, 0.13 mmol) was added to a solution of PEG-

supported catalyst 52 (60 mg, 0.025 mmol), Hantzsch ester (75 mg, 0.3 mmol),

trifluoroacetic acid (3 mg, 0.026 mmol) in dichloromethane (3 ml).


b) 3-phenylbut-2-enal 36 (19 mg, 0.13 mmol) was added to a solution of PEG-

supported catalyst 57 (70 mg, 0.013 mmol), Hantzsch ester (65 mg, 0.25 mmol),

trifluoroacetic acid (1.5 mg, 0.014 mmol) in dichloromethane (2 ml).



a) One tablet of AC-PEG-2000 was added to a solution of 3-phenylbut-2-enal 36 (13



126

b) One tablet of AC-PEG-5000 was added to a solution of 3-phenylbut-2-enal 36 (13



Reduction of 3-(4-chlorophenyl)but-2-enal [38] to 3-(4-chlorophenyl)butanal [39]


a) 3-(4-chlorophenyl)but-2-enal 38 (23 mg, 0.13 mmol) was added to a solution of

Hantzsch ester (60 mg, 0.25 mmol), trifluoroacetic acid (1.5 mg, 0.026 mmol) in

dichloromethane (2 ml).




Hantzsch ester (60 mg, 0.25 mmol) in dichloromethane (2 ml).




PEG-supported catalyst 52 (30 mg, 0.013 mmol), Hantzsch ester (60 mg, 0.25



127

b) 3-(4-chlorophenyl)but-2-enal 38 (23 mg, 0.13 mmol) was added to a solution of


mmol), trifluoroacetic acid (1.5 mg, 0.014 mmol) in water (1.8 ml) and THF (0.2

ml).


Reduction of 3-(4-methoxyphenyl)but-2-enal [40] to 3-(4-methoxyphenyl)butanal [41]


a) 3-(4-methoxyphenyl)but-2-enal 40 (23 mg, 0.13 mmol) was added to a solution of






Hantzsch ester (60 mg, 0.25 mmol) in dichloromethane (2 ml).


128






b) 3-(4-methoxyphenyl)but-2-enal 40 (23 mg, 0.13 mmol) was added to a solution of


mmol), trifluoroacetic acid (1.5 mg, 0.014 mmol) in water (1.8 ml) and THF (0.2

ml).



a) One tablet of AC-PEG-2000 was added to a solution of 3-(4-methoxyphenyl)but-2-

enal 40 (13 mg, 0.1 mmol) and Hantzsch ester (63 mg, 0.25 mmol) in water (2 ml).


Reduction of 2-methyl-3-phenylacrylaldehyde [64] to 2-methyl-3-phenylpropanal [65]


a) 2-methyl-3-phenylacrylaldehyde 64 (19 mg, 0.13 mmol) was added to a solution of


chloroform (2 ml).


129

b) 2-methyl-3-phenylacrylaldehyde 64 (19 mg, 0.13 mmol) was added to a solution of


water (1.8 ml) and THF (0.2 ml).


Reduction of (E)-3,7-dimethylocta-2,6-dienal [42] to 3,7-dimethyloct-6-enal [43]


a) (E)-3,7-dimethylocta-2,6-dienal 42 (20 mg, 0.13 mmol) was added to a solution of


diethyl ether (2 ml).


b) (E)-3,7-dimethylocta-2,6-dienal 42 (20 mg, 0.13 mmol) was added to a solution of




130

3.3.2 Enantioselective Reduction of 3-methylcyclohex-2-en-1-one

48

Catalyst M

Hantzsch ester

Solvent Y

O O

47

Procedure 1

In a 5 ml flask, organocatalyst M (0.025 mmol) was added to a solution of 3-

methylcyclohex-2-en-1-one 47 (27 mg, 0.25 mmol), Hantzsch ester (70 mg, 0.3 mmol) and

trifluoroacetic acid (3 mg, 0.026 mmol) in a solvent Y (3 ml). The reaction mixture was

stirred at room temperature for 48 hours affording product 48 with different conversion

percentages.

Reduction of 3-methylcyclohex-2-en-1-one to using catalyst 30

a) Catalyst 30 (7.5 mg, 0.025 mmol) was added to a solution of 47 (27 mg, 0.25 mmol),

Hantzsch ester (70 mg, 0.3 mmol) and trifluoroacetic acid (3 mg, 0.026 mmol) in



b) Catalyst 30 (7.5 mg, 0.025 mmol) was added to a solution of 47 (27 mg, 0.25 mmol),




131

Reduction of 3-methylcyclohex-2-en-1-one to using catalyst 45

a) Catalyst 45 (2 mg, 0.025 mmol) was added to a of solution 47 (27 mg, 0.25 mmol),




b) Catalyst 45 (7.5 mg, 0.025 mmol) was added to a solution of 47 (27 mg, 0.25 mmol),




Reduction of 3-methylcyclohex-2-en-1-one to using catalyst 46a

a) Catalyst 46a (6.5 mg, 0.025 mmol) was added to a of solution 47 (27 mg, 0.25

mmol), Hantzsch ester (70 mg, 0.3 mmol) and trifluoroacetic acid (3 mg, 0.026

mmol) in diethyl ether (3 ml).


b) Catalyst 46a (6.5 mg, 0.025 mmol) was added to a solution of 47 (27 mg, 0.25

mmol), Hantzsch ester (70 mg, 0.3 mmol) and trifluoroacetic acid (3 mg, 0.026

mmol) in water (1.8 ml) and THF (0.2 ml).


132

3.3.3 Cascade Reactions

Procedure 1

In a 5 ml flask, organocatalyst 33 (0.025 mmol) was added to a solution of cinnamaldehyde

34 (0.25 mmol), Hantzsch ester (0.3 mmol), reagent B (2.5 mmol) and trifluoroacetic acid in

dichloromethane (3 ml). The reaction mixture was stirred at room temperature for 24 hours

affording product C with different conversion percentages.

Procedure 2


solution of cinnamaldehyde 34 (0.13 mmol) reagent B (1.3 mmol) and Hantzsch ester (0.25

mmol) in dichloromethane (2 ml). The reaction mixture was stirred at room temperature for

48 hours affording product C with different conversion percentages. After the reaction

mixture was discharged, the polymer-supported was washed with dichloromethane (2 x 5

ml), methanol (2 x 5 ml) and dichloromethane (2 x 5 ml) again. Catalyst was so reused for

next cycle.

Procedure 3

In a 5 ml flask, soluble PEG-supported organocatalyst 57 (0.025 mmol), was added to a

solution of cinnamaldehyde 34 (0.25 mmol), reagent B (2.5 mmol), Hantzsch ester (0.3

mmol) and trifluoroacetic acid in a solvent Y (3 ml). The reaction mixture was stirred at

room temperature for 24 hours affording product C with different conversion percentages.

After the reaction was completed diethyl ether (20 ml) was added to the mixture. The

purpose was to precipitate the pure polymer used in the next cycle.

133

Synthesis of 2-benzyl-3-(4-fluorophenyl)-4-nitrobutanal [91]



(75 mg, 0.3 mmol), trifluoroacetic acid (3 mg, 0.026 mmol) and 1-fluoro-4-(2-

nitrovinyl)benzene 63 (200 mg, 2.5 mmol) in dichloromethane (3 ml).




(63 mg, 0.25 mmol) and 1-fluoro-4-(2-nitrovinyl)benzene 63 (100 mg, 1.3 mmol) in


Number of cycles:2; Reaction time: 48 hours; 56-75Yield: %.




acid (1.5 mg, 0.014 mmol) and 1-fluoro-4-(2-nitrovinyl)benzene 63 (100 mg, 1.3

mmol) in dichloromethane (3 ml).


134

Synthesis of 2-benzyl-4-nitro-3-phenylbutanal [92]




acid (1.5 mg, 0.014 mmol) and 2-benzyl-4-nitro-3-phenylbutanal 62 (100 mg, 1.3

mmol) in dichloromethane (3 ml).


135

3.3.4 Asymmetric Friedel-Crafts Reactions

Procedure 1

In a 5 ml flask, soluble PEG-supported organocatalyst 57 (0.025 mmol), was added to a

solution of aldehyde A (0.25 mmol), indole B (2.5 mmol), Hantzsch ester (0.3 mmol) and

trifluoroacetic acid in a solvent Y (3 ml). The reaction mixture was stirred at room

temperature for 24 hours affording product C with different conversion percentages.

Procedure 2


solution of cinnamaldehyde A (0.13 mmol) reagent B (1.3 mmol) and Hantzsch ester (0.25

mmol) in dichloromethane (2 ml). The reaction mixture was stirred at room temperature for

48 hours affording product C with different conversion percentages. After the reaction

mixture was discharged, the polymer-supported was washed with dichloromethane (2 x 5

ml), methanol (2 x 5 ml) and dichloromethane (2 x 5 ml) again. Catalyst was so reused for

next cycle.

Synthesis of 3-(1H-indol-2-yl)-3-phenylpropanal [93]

136



catalyst 57 (70 mg, 0.013 mmol), Hantzsch ester (65 mg, 0.25 mmol), trifloroacetic

acid (3 mg, 0.026 mmol) and 1H-indole (100 mg, 0.9 mmol) in THF (3 ml).




(63 mg, 0.25 mmol) and 1H-indole (100 mg, 0.9 mmol) in dichloromethane (3 ml).

Number of cycles:2; Reaction time: 48 hours; Yield: 62-81%.

Synthesis of 3-(1H-indol-2-yl)-2-methyl-3-phenylpropanal [94]


a) 2-methyl-3-phenylacrylaldehyde 34 (17 mg, 0.13 mmol) was added to a solution of


mmol), trifloroacetic acid (3 mg, 0.026 mmol) and 1H-indole (100 mg, 0.9 mmol) in



137

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