54

Chapter III

APPLICATIONS OF POLYMER-SUPPORTED

QUATERNARY AMMONIUM CHLORIDE AS PHASE

TRANSFER CATALYST IN ORGANIC REACTIONS

PART A. PREPARATION AND CHARACTERISATION OF POLYMER SUPPORTED QUATERNARY AMMONIUM SALTS

3.1 Introduction

Conventional polymer supports are based on either polar or nonpolar

chemical structures and pose some limitations in their applications.1-6 Last

decade witnessed the design and development of several tailor-made

macromolecular matrices with amphiphilic characteristics arising from both the

polar and nonpolar nature of the polymeric backbone. Using these polymers

reactions can be carried out in solvents having a wide range of polarity.7-10 The

amphiphilic copolymers provide a new impetus in the study and applications of

polymer supports and functional polymers due to their ease of production,

solvent substrate compatibility and simple reaction work-up. The chronic

limitations such as rigidity and incompatibility of the conventional Merrifield

resin have been overcome to some extent by using polar flexible

oligooxyethylene diacrylate crosslinking agents.11-20 These novel supports are

attractive due to their simple and easy preparation and derivatization. In this

55

chapter the preparation and characterization of polystyrene bound quaternary

ammonium chloride phase transfer catalysts are described. Modification of the

polymer supports by various means such as introducing alkyl chains and PEG

graft to the crosslinked support and investigations on the dependence of the

efficiency of the polymer bound phase transfer catalysts on the structural

characteristics of the polymer support, nature of the trialkylamine and the

reaction conditions are also discussed.

3.2 Prepara22tion of divinylbenzene (DVB) and 1,4-butanediol dimethacrylate (BDDMA) crosslinked polystyrene supports (1a1-1a8), (2a1-2a8) and (3a1-3a8)

Suspension polymerization is the most convenient method to obtain

polymers in the beaded form.21 Polymer beads having a size range of 200-

400 mesh are suitable for polymer supported reactions. Beads larger than 400

mesh have less total surface area and beads smaller than 200 mesh may clog

in the sintered disk and cause filtration problems.

DVB crosslinked polystyrenes (1a1-1a8) are prepared by radical initiated

suspension polymerization of the monomers styrene and divinylbenzene under

nitrogen atmosphere (Scheme 3.1). Suspension polymerization of water

insoluble monomers such as styrene involves the formation of a droplet

suspension of the monomers in water and direct conversion of the individual

monomer droplets into the corresponding polymer bead. Suspension was

stabilized using aqueous 1% PVA under constant stirring (600 rpm). Radical

initiator used was benzoyl peroxide and the temperature was maintained at

56

850C. In addition to the monomer and initiator, an inert liquid toluene was

used as diluent to the monomer phase. The diluent influences the pore

structure and swelling behaviour of the beaded resin.22-23 Toluene was found

to be an ideal diluent for the suspension polymerization of vinyl monomers.

The time of polymerisation depends on the initiator concentration,

temperature and composition of the monomer mixture. When the

polymerization is over, tough, insoluble and almost spherical crosslinked

beads of the polymer precipitates out. Since DVB is a rigid and nonpolar

crosslinking agent the polymer produced by the copolymerisation of styrene

and DVB is hard, rigid and hydrophobic.

Scheme 3.1 Suspension polymerisation of styrene and divinylbenzene

Using appropriate mole percentage of the monomer (styrene) and

crosslinking agent (DVB), polymers with 2, 4, 6, 8, 10, 12 and 20 mole

percent crosslink densities were prepared. The polymers were washed

thoroughly and subjected to Soxhlet extraction with acetone and methanol to

remove the linear polymers and finally dried at 700 C in an air oven. The

SuspensionPolymerisation

Bz2O2 / 85 C0+

1a1 = 1%

1a2 =2%

1a3 =4%

1a4 =6%

1a5 =8%

1a6 =10%

1a7 =12%

1a8 =20%

57

beads were weighed and the IR spectrum was recorded using KBr pellets. The

yields of the polymers obtained are given in Table 3.1.

Table 3.1 Details of the DVB crosslinked polystyrene resins prepared

Resin Cross link density

(mol %)

Vol. of monomers (mL) % yield Styrene DVB

1a1 1 11.30 0.238 68

1a2 2 11.20 0.476 70

1a3 4 10.98 0.973 73

1a4 6 10.75 1.429 77

1a5 8 10.52 1.905 79

1a6 10 10.29 2.381 82

1a7 12 10.06 2.858 86

1a8 20 9.15 4.763 92

3.2.1 BDDMA crosslinked polystyrenes (2a1-2a8)

When a functional moiety is attached to a polymer backbone the

microenvironment created within the polymer matrix can create specific

effects and change the reactivity of the bound group. Styrene copolymers with

a flexible cross linking agent BDDMA were also prepared. In styrene-

BDDMA copolymer the polar flexible BDDMA crosslinks can provide a local

environment different from that of the styrene-DVB copolymer.

BDDMA-PS copolymers (2a1-2a8) were prepared using styrene and

BDDMA in definite mole percentages by the same procedure as employed in

the preparation of DVB-PS resins (Scheme 3.2).

58

Scheme 3.2 Suspension polymerisation of styrene and butanediol dimethacrylate

Table 3.2 Details of the BDDMA crosslinked polystyrenes prepared

Resin Taken Crosslink density

(mol %)

Vol. of the monomers (mL )

% Yield of Polymers

Styrene BDDMA %

2a1 1 11.30 0.226 82

2a2 2 11.20 0.451 84

2a3 4 11.09 0.903 86

2a4 6 10.75 1.354 90

2a5 8 10.52 1.806 92

2a6 10 10.29 2.258 94

2a7 12 10.06 2.712 96

2a 8 20 9.15 4.516 98

HDODA-PS copolymers (3a1-3a8) were prepared using styrene and

HDODA in definite mole percentages by the same procedures as employed in

the preparation of DVB-PS and BDDMA-PS resins (Scheme 3.3) and results

are summarized Table 3.3.

+

SuspensionPolymerisation

Bz2O2 / 800C

O

O

O

O

OO

OO

OO

OO

2a1 – 1%

2a2 - 2%

2a3 – 4%

2a4 – 6%

2a5 – 8%

2a6 – 10%

2a7 – 12%

2a8 – 20%

59

Scheme 3.3 Suspension polymerization of styrene and hexanediol diacrylate

Table 3.3 Details of the HDODA crosslinked polystyrenes prepared

Resin crosslink density

(mol%)

Vol. of the monomers (mL) % Yield of

Polymers Styrene HDODA

3a1 1 11.30 0.224 87

3a2 2 11.20 0.448 89

3a3 4 10.98 0.896 91

3a4 6 10.75 1.344 92

3a5 8 10.52 1.792 93

3a6 10 10.29 2.240 95

3a7 12 10.06 2.668 96

3a8 20 9.15 4.480 98

3.3. Characterization of the BDDMA crosslinked polystyrene

The BDDMA crosslinked resins were characterized by IR (Fig. 3.1b),

solid state 13CP-MAS NMR (Fig. 3.2b) and scanning electron micrograph

(Fig. 3.3). The solid state NMR (Fig. 3.2) showed an intense peak at 130.48

ppm (BDDMA-PS) of the aromatic carbons and a small peak at 148.20 ppm

3a1 – 1%

3a2 - 2%

3a3 – 4%

3a4 – 6%

3a5 – 8%

3a6 – 10%

3a7 – 12%

3a8 – 20%

60

arising from the substituted ring carbon of styrene. The methylene carbons of

the polymeric backbone appear at 42.5 ppm and the carbonyl carbon around

200ppm. The IR spectrum of BDDMA resins showed a sharp band at 1720

cm-1 and 1125cm-1 corresponding to the ester group of the crosslinking agent.

Fig. 3.1 b IR of BDDMA-PS resin

Fig. 3.2 b Solid state 13C CP-MAS NMR spectrum of BDDMA-PS resin

CH CH2( )n

2 1

3

61

The scanning electron micrograph is an outstanding technique for

studying the morphological features and mechanism of formation of beaded

polymers.24-25 The SEM studies shows that 2% BDDMA crosslinked polystyrene

resin (Fig. 3.3) has spherical bead-like structure with smooth surface.

Fig. 3.3 SEM of 2% BDDMA-PS

3.4 Swelling studies

Swelling gives a measure of the solvation of the resin by a given

solvent. In crosslinked polymer supports the extent of swelling is determined

by factors like nature of the polymeric backbone, nature of the solvent and the

degree of crosslinking.26 Birr et al. carried out an extensive study on the

swelling characteristics of polystyrene resins and concluded that all the highly

crosslinked polystyrenes in good solvents exist in quasi-dissolved gel state.27

The swelling characteristics of the polymers were studied by

determining the swelling capacity and is defined as the ratio of the weight of

the solvent swollen polymer to the weight of the dry resin.

62

The swelling studies of the DVB-PS and BDDMA-PS resins with

various crosslink densities were carried out and the results were summarized

in Tables 3.4. and 3.5.

Table 3.4 Swelling properties of BDDMA-PS supports

Swelling capacity (mL/g of resin)

Crosslink density 1% 2% 4% 6% 8% 12% 20%

Dichloromethane 9.51 8.76 7.96 6.94 5.89 4.61 3.01

Dimethylformamide 7.25 6.82 6.02 5.21 4.46 3.82 2.54

Tetrahydrofuran 9.42 8.14 7.72 6.43 5.51 4.08 2.90

Toluene 8.71 7.12 6.95 6.32 5.21 3.92 2.81

Methanol 5.29 4.30 4.01 3.93 3.23 3.02 2.01

Water 4.45 3.11 2.98 2.62 2.41 2.02 1.72

Table 3.5 Swelling Properties of 2% DVB-PS, BDDMA-PS and HDODA-PS

Solvent Swelling capacity mL/g of resin

2% DVB-PS 2% BDDMA-PS 2% HDODA-PS

Dichloromethane 4.91 8.76 8.79

Dimethylformamide 3.52 6.82 6.85

Tetrahydrofuran 4.82 8.14 8.19

Toluene 4.51 7.12 7.08

Methanol 2.00 4.30 4.37

Water 1.71 3.11 3.16

weight of swollen resin weight of dry resin Swelling capacity =

Swelling capacity = Swelling capacity =

63

BDDMA crosslinked resins showed excellent swelling behaviour in

the commonly used organic solvents when compared to the rigid hydrophobic

DVB crosslinked resins. The amphiphilic nature of the BDDMA-PS resins

provides a hydrophilic/hydrophobic milieu in the polymeric backbone,

leading to a greater extent of swelling both in polar and nonpolar solvents.

The above mentioned studies reveal the significant influence of the nature of

the crosslinking on the physicochemical characteristics of the crosslinked

polystyrene systems.

3.5 Functionalisation of the polymer

The resins were functionalised by incorporating chloromethyl groups

by Friedel-Crafts reaction using chloromethyl methyl ether (CMME) in

presence of the Lewis acid catalyst anhydrous ZnCl2 (Scheme 3.4).28-30

Solvetov et al. demonstrated a study on the chloromethylation of DVB-PS

resin using ZnCl2 and reported that in spite of the heterogenity due to

insoluble polymer, the reaction takes place as in a homogenous phase swollen

polymer.31 In CMME polystyrene exhibits good solvation properties. CMME

acts both as the reagent and the solvent. IR and 13C CPMAS NMR studies

showed that the chloromethylation takes place at the para position of

styrene.32 Freshly prepared CMME and perfectly dry ZnCl2 in THF were used

in the chloromethylation of beaded polystyrene supports.

64

ClCl OCH3

ZnCl2/THF ,50 C0

1a1-1a8, 2a1-2a8 &3a1-3a8 1b1-1b 8 , 2b1 -2b8 &3b1 -3b8

Scheme 3.4 Chloromethylation of polystyrene resins

The details of the chloromethylation are summarized in Table 3.6.

Table 3.6 Details of chloromethylated resins prepared

Resin Crosslink density mole %

Reaction Time (h)

Chlorine capacity mmol Cl/g

DVB-PS

1b1 1 8 2.27

1b2 2 10 2.10

1b3 4 12 1.99

1b4 6 14 1.87

1b5 8 16 1.80

1b6 10 18 1.71

1b7 12 20 1.57

1b8 20 25 1.16

BDDMA-PS

2b2 2 6 2.40

2b3 4 7 2.39

2b4 6 8 2.31

2b5 8 9 2.29

2b6 10 10 2.21

2b7 12 12 2.11

2b8 20 14 1.85

HDODA-PS 3b2 2 6 2.46

65

The swelling capacities of the chloromethyl resins 1b2, 2b2 and 3b2

were studied. Analogous to the swelling capacities of free resin, chloromethyl

resins derived from the BDDMA-PS and HDODA-PS polymers showed

greater extent of swelling in the commonly used solvents (Table 3.7).

Table 3.7 Comparative swelling capacities of chloromethylated 2% crosslinked DVB-PS, BDDMA-PS and HDODA-PS

Solvent Swelling Capacity (ml/g)

DVB-PS (1b2) BDDMA-PS

(2b2) HDODA- PS (3b2)

Dichloromethane 4.84 7.42 7.49

Dimethylformamide 3.48 6.95 6.81

Tetrahydrofuran 4.75 8.23 8.28

Toluene 4.32 7.28 7.07

Methanol 1.97 3.53 3.32

Water 1.64 3.21 3.18

The chloromethylated resins were characterized by IR, solid state 13C

CPMAS, NMR and SEM. The IR spectrum showed (Fig. 3.4a) a band at 680

cm-1 for C-C1 stretching and H-C-Cl bending vibration at 1258cm-1. The 13C

NMR spectrum showed a peak at 49.3 ppm for methylene carbon of the

chloromethyl group and a small peak at 135.7ppm for the chloromethyl group

substituted carbon of the styrene (Fig. 3.5a).

66

Fig. 3.4a IR spectrum of chloromethylated BDDMA-PS

Fig. 3.5a Solid state 13C CPMAS NMR spectrum of chloromethylated BDDMA-PS resin.

67

Scanning electron micrographs of the chloromethylated BDDMA-PS

give clear indication of morphological changes (Fig. 3.6a). Functionalisation

brings about substantial changes to the surface features of the polymer. The

surface of the chloromethylated BDDMA-PS resin appears rough and porous.

The functionalised resin is more porous which allows better

permeation of solvents into the resin core. This is evidenced by the fact that

the functionalised resin swells to a higher degree in good solvent than the

unfunctionalised resin. The increase in the porosity is relevant in the

application of functionalised resins in solid phase organic synthesis.

Fig. 3.6b SEM of chloromethylated 2% BDDMA-PS resin (2000 magnification)

3.6 Preparation and characterization of polymer supported quaternary ammonium salt (PSQA) (1c1-1c5and2c1-2c5)

Quaternary ammonium salts are used as phase transfer catalysts in

aqueous organic biphasic systems. The attachment of phase transfer catalyst

residues to insoluble polymer supports simplifies their use in various

reactions. The immobilized quaternary ammonium salt is an efficient catalyst

68

having high lipophilic character to proceed the reaction in the organic phase.

Polymer bound quaternary ammonium chloride was prepared by stirring a

mixture of chloromethyl polystyrene resin in DMF with a trialkylamine at 800

C (Scheme 3.5).

Scheme 3.5 Quaternisation of chloromethylated DVB-PS, BDDMA-PS and HDODA-PS with different triakylamines

The beaded catalyst obtained was washed with DMF, DMF: water

(1:1, v/v), methanol and acetone. The characterization of the various PSQAs

was achieved by chemical methods such as chloride ion capacity

determination36, IR spectroscopy and SEM. The extent of quaternisation

using various trialkylamines was followed by the chloride ion capacity

determination and the observed results indicated the quantitative formation of

quaternary ammonium chloride from the chloromethylated resins. IR spectra

of the catalyst (Fig. 3.6b) confirmed the complete conversion by the absence

of C-Cl stretching and H-C-Cl bending bands. The appearance of a new band

at 1378cm-1 corresponding to C-N stretching confirmed the formation of the

quaternary ammonium salts.

69

Fig. 3.6b IR Spectrum of BDDMA-PS-TBA Catalyst

The SEM of the polymeric PTC revealed that the conversion of the

chloromethylated resin to the corresponding PSQA results in a change of

morphology of the polymer (Fig.3.8). From the Fig.3.8 it was observed that

the surface of the polymer becomes more rough and irregular.

Fig. 3.8 SEM of polymer supported benzyltributylammonium chloride

70

The kinetics of quaternisation was investigated by estimating the chloride

ion contents at definite intervals and a comparison has made between the

BDDMA and DVB crosslinked resins. The results are shown in Table 3.8.

It was found that the reaction period was reduced almost to half for

flexible BDDMA and HDODA crosslinked resins when compared to the rigid

DVB-PS polymer. This can be explained as due to the increased rate of diffusion

of soluble reagents through the swollen polymer matrix and polar character of

BDDMA resins. The reaction was completed after 12 h in the case of BDDMA-

PS and for DVB-PS complete reaction occurred only after 24 h.

Table 3.8 Kinetics of quaternisation of DVB (2.1 mmol Cl/g) & BDDMA (2.4 mmol Cl/g) crosslinked catalysts

Catalyst Time (h) Chloride ion capacity mmol Cl/g

% Quaternisation

DVB-PS-TBA(1c3)

4

6

10

14

18

20

22

0.71

0.83

1.05

1.24

1.43

1.55

1.62

43.82

51.23

64.81

76.54

88.27

94.59

100

BDDMA-PS-TBA

(2c3)

1

2

3

4

5

0.93

1.12

1.34

1.55

1.76

52.84

63.62

76.14

88.06

100

71

3.7 Effect of trialkylamine on rate of quaternisation

Chloromethylated BDDMA-PS was quaternised with different trialkyl

amines like trimethylamine, triethylamine, tributylamine, trioctylamine and

diisopropylethylamine (Scheme 3.5, Table 3.9) and it was found that the time

for quaternisation decreased with increase in the number of methylene groups

in the alkyl chain of the amine.

Table 3.9 Details of quaternisation of various trialkylamines

Resin Trialkylamine Time for complete

quaternisation (h)

Chloromethylated

2% BDDMA-PS

(2.4 mmol Cl/g)

Trimethylamine

Triethylamine

Tributylamine

Trioctylamine

Diisopropylethylamine

18

12

5

#

~

# chloride ion capacity first increases, reaches a maximum after 3 hrs and then decreases

~ The observed % of quaternisation was very low (5%) even after 72 h

The quaternisation was completed within 5 h in the case of

tributylamine whereas with triethylamine 12 h and with trimethylamine 18 h

were taken for 100% conversion. In the case of diisopropylethylamine very

low chloride capacity was observed which may be due to the steric hindrance

imparted by the bulky alkyl groups. But in the case of trioctylamine the

chloride ion capacity shows a maximum value of 0.6 mmol/g after three hours

and then decreases. This can also be explained by the steric factor. The

72

decrease in chloride ion capacity may be due to the detachment of the

trioctylamino group. This problem can be overcome to some extent by taking

resins of low chlorine capacity. But then the amount of catalyst required for

reactions will be large. The ease of formation of quaternary ammonium salts

was increased from trimethylamine to tributylamine. As the chain length

increases the lipophilic character of the amine increases which makes it more

compatible with the polymer support.

3.8 Effect of nature and extent of crosslinking on the rate of quaternisation

A comparative study on the influence of the nature of crosslinking on

the quaternisation reaction was carried out by immobilizing tributylamine to

the chloromethylated polymer supports DVB-PS and BDDMA-PS. The

observed results are summarized in Table 3.10.

Table 3.10: The details of quaternisation using tributylamine to the chloromethylated supports

Chloromethylated resin Time for 100% conversion

(h)

2% DVB-PS CH2Cl (1b2,2.1 mmol Cl/g)

2% BDDMA-PS CH2Cl (2b2, 2.4 mmol Cl /g)

2% HDODA-PS CH2Cl (2b2, 2.41 mmol Cl/g)

22

5

5

The results show that the use of hydrophilic flexible crosslinking in the

polystyrene has a significant influence on the rate of quaternisation and the

73

observed rate was three times greater than that of the conventional Merrifield

resin.

Similarly the effect of degree of crosslinking was investigated by

taking BDDMA–PS supports (2b1- 2b8) and the results are summarized in

Table 3.11 Effect of crosslink density on the quaternisation of chloromethylated BDDMA–PS supports (2b1- 2b8)

Resin Crosslink ratio (mol %) Time for 100 % quaternisation (h)

2b1 1 4.0

2b2 2 5.0

2b3 4 6.5

2b4 6 7.5

2b5 8 9.0

2b6 10 10.5

2b7 12 13.0

2b8 20 18.0

From the results of the above investigation it is clear that as the

crosslink density increased the time of quaternisation also increased. This

may be due to the increased rigidity or decreased chain mobility of the

polymer backbone.

74

3.9 Effect of nitro substituent on the polymer support

The reactivity of a polymeric reagent or catalyst is significantly

affected by its neighbouring groups. The effect of nitro group at the ortho

position of the benzene ring of the polymer matrix on the rate of

quaternisation reaction was investigated.

3-Nitro-4-chloromethyl polystyrene was prepared (Scheme 3.6) by

treating the chloromethyl polystyrene resin, 2 % BDDMA - PS CH2Cl (2b2,

2.4 mmol Cl/g) with fuming nitric acid at 0oC.

P CH2ClFuming HNO3

0 C0P CH2Cl

NO2

TBADMF

P CH2N+

(Bu )Cl-

NO2

3

Scheme 3.6 Preparation of catalyst with nitro substituent on the polymer support

The nitro substituted chloromethylpolystrene 2b2-NO2 was quaternised

with different trialkylamines like trimethylamine, triethylamine, tributylamine

and trioctylamine. It was found that the nitro group enhances the

quaternization reaction. A comparison of the quaternisation was made with

chloromethylated resins , without nitro group at the ortho position. Results are

given in Table 3.12.

75

Table 3.12 Effect of nitro substituent on quaternisation reaction

Resin Trialkylamine Time for 100 % quaternisation (h)

BDDMA-PS (2b2)

BDDMA – PS – NO2 – CH2Cl (2b2-NO2)

2%

TBA

TEA

TMA

TOA

5

12

18

#

4

10

16

#

# Complete quaternisation is not taking place.

It was observed that the presence of nitro group enhances the rate of

quaternisation. The nitro group can facilitate the nucleophilic attack by the

trialkylamine at the chloromethyl carbon.

3.10 Stability of the PSPTC

The stability of the catalyst was studied by thermogravimetric analysis

of the polymer supported benzyltributylammonium salt (2c3) derived from

BDDMA-PS resin (Fig. 3.9).

Fig. 3.9 Thermogram of polymer supported benzyltributylammonium chloride

76

Initially there is a small weight loss which may be due to the removal

of absorbed water. The thermogram of the PTC derived from

chloromethylated BDDMA-PS resin and TBA show that the polymer-bound

quaternary ammonium salt catalyst is stable up to 312.14oC. After that there

is gradual decomposition of the catalyst.

3.11 Swelling characteristics of PSPTC

The immobilised functional groups on the polymer supports can

significantly affect the swelling properties. During the chemical reactions

their swelling properties may change considerably as one functionality is

transformed into another. Thus the choice of the reaction solvent may not be

the same one as that commonly used in analogous reactions using low

molecular mass reactants. The swelling studies of the TBA incorporated 2%

DVB&BDDMA-PS PTC`s (1c3 & 2c3) were carried out. The swelling studies

were demonstrated using solvents having varying polarities such as DMF,

DCM, THF, and toluene. The details of the swelling studies are summarized

in Table 3.13.

77

Table 3.13 Swelling capacities of 2% DVB-PS-TBA (1c3) and 2%BDDMA-PS- TBA (2c3)

Solvent

Swelling capacity (mL/g of resin)

2% DVB-PS-TBA(1c3)

2 % BDDMA-PS- TBA(2c3)

Dichloromethane 4.93 8.79

Dimethylformamide 3.79 6.98

Tetrahydrofuran 4.95 8.37

Toluene 4.57 7.51

Methanol 2.16 3.69

Water 1.78 3.21

From the above results it was found that tributylamine incorporated

BDDMA-PS PTC has greater extent of swelling compared to DVB catalyst.

From the swelling studies it was found that DCM was the best solvent.

3.12 Application of polymer supported quaternary ammonium salt (PSQA)

The multifaceted applications of polymer supported quaternary

ammonium salts in organic synthesis have decisively contributed to the

establishment of organic catalysts as useful preparative tools. PSQA gained

utility and acceptance due to enhanced reaction rates, high yields of products

and convenience. Polymer supported quaternary ammonium catalysts

approach the catalytic efficiency of soluble catalysts under suitable optimized

reaction conditions. The use of PSQA in industry is significant and there is

78

considerable research towards the development of new fields of application.

The efficiency of PSQA places it in a unique position. When the catalyst is

immobilized on a solid support many of the problems associated with

separation and recovery of catalysts, formation of emulsions can be

overcome.37-38 The performance of the supported catalysts is strongly

dependent on physical and chemical nature of the polymer support.39

The following reactions were carried out using the polymer supported

quaternary ammonium salt as phase transfer catalyst.

a) Halogen Exchange reaction

b) O-Alkylation

c) O-Benzylation

d) N- Alkylation

e) Oxidation

f) Debromination

g) Insertion reaction

The Polymer supported quaternary ammonium salts require a

preliminary conditioning before the reaction. The conditioning process

involves swelling the resin in DCM solvent and equilibrating with the reagent

in the aqueous phase. The catalyst was first conditioned for five hours in the

presence of the aqueous solution of the alkali salt and the organic solvent.

Substrate was added to the conditioned catalyst.

79

3.12(a) Halogen exchange reactions

Phase transfer catalytic activity of the polymer bound catalyst in

halogen exchange reaction was investigated by taking the conversion of

1-bromooctane to 1-iodooctane as the model reaction. The catalyst was

conditioned first by swelling the catalyst (3 molar excess of the substrate) in

DCM for 15 minutes and potassium iodide solution was added to the swelled

resin and the mixture was stirred for 5 hours. To the conditioned catalyst,

1-bromooctane was added with constant stirring. The extent of reaction was

followed by TLC analysis. Results are summarized in Table 3.14.

Table 3.14 Conversion of 1-bromooctane to 1-iodooctane using various polymer supported quaternary ammonium salts

PSQA Catalyst (2%) Chloride ion capacity

mmol/g Time for 100% Conversion(h)

DVB –PS – TMA 1c1 1.61 53

DVB-PS – TEA 1c2 1.53 48

DVB-PS – TBA 1c3 1.62 40

DVB-PS – TOA 1c4 0.50 33

BDDMA-PS – TMA 2c1 1.57 25

BDDMA-PS – TEA 2c2 1.69 20

BDDMA-PS – TBA 2c3 1.76 15

BDDMA-PS – TOA 2c4 0.60 12

HDODA-PS-TMA 3c1 1.54 23

HDODA-PS-TEA 3c2 1.68 21

HDODA-PS-TBA 3c3 1.75 17

HDODA-PS-TOA 3c4 0.61 11

80

From the above results it was observed that the rate of the reaction

depends both on the nature of the trialkylamine and polymer support. The

catalytic activity was increased when the catalyst immobilized resin was

changed from DVB-PS to BDDMA-PS and HDODA-PS. With DVB-PS –

TBA (1c3) the time for complete reaction was 40 h while with catalyst

BDDMA-PS- TBA (2c3) the time taken was 15 h and with HDODA-PS-

TBA (3c3) the time required was 17 h.

The conversions of benzyl chloride to benzyl iodide, 1-bromoheptane

to 1-iodoheptane and 1-bromooctane to 1-chlorooctane were also effected in

aqueous–organic medium under phase transfer catalytic conditions using

PSQACs. The reactions were performed with 2% DVB-PS-TBA (1c3) and

2% BDDMA-PS-TBA (2c3). The details of the reactions are summarized in

Table 3.15.

Table 3.15 Halogen exchange reactions using 2% crosslinked benzyl tributylammonium chloride catalysts

Reaction Catalysts Reaction time (h)

Boiling Point of the Product (oC)

1-bromooctane to 1-iodooctane

1c3

2c3

40.0

15.0 219

1-bromoheptane to 1-iodoheptane

1c3

2c3

41.0

15.5 203

Benzyl chloride to benzyl iodide

1c3

2c3

49.0

20.0 117

1-bromooctane to 1-chlorooctane

1c3

2c3

55.0

41.0 183

81

From the above studies it was found that the conversion of 1-

bromooctane to 1-iodooctane and 1-bromoheptane to 1-iodoheptane

proceeded with more or less same rate. But with benzyl chloride the reaction

period was slightly more. The reaction period for conversion of 1-

bromooctane to 1-chlorooctane was also relatively high. Conversion of 1-

bromooctane to 1- chlorooctane was also carried out using aqueous sodium

chloride in presence of DVB and BDDMA catalysts.

3.12.1 Effect of the polymer parameters on the efficiency of the PSPTC

In polymer supported reactions the physicochemical characteristics of the

polymer support do affect the course and rate of the reactions. In order to

investigate the effect of the nature and structural characteristics of the polymer

support, nature of the trialkylamines used and reaction conditions, conversion of

1-bromooctane to 1-iodooctane was taken as the model reaction.

3.12.1(a) Effect of trialkylamine

The nature of the alkyl group in the trialkylamine moiety of the

PSQAS was found to influence significantly the rate of halogen exchange

reaction. The conversion of 1- bromooctane to 1-iodooctane was carried out

using PSQAS with different trialkylammonium groups and details are given

in Table 3.16.

82

Table 3.16 Conversion of 1-bromooctane to 1-iodooctane using various polymer supported quaternary ammonium salts

PSQA catalyst 2% crosslinked

Crosslinking agent Trialkylamine

Reaction Time (h)

1c1 DVB TMA 53

1c2 DVB TEA 48

1c3 DVB TBA 40

1c4 DVB TOA 33

2c1 BDDMA TMA 25

2c2 BDDMA TEA 20

2c3 BDDMA TBA 15

2c4 BDDMA TOA 12

3c1 HDODA TMA 24

3c2 HDODA TEA 21

3c3 HDODA TBA 17

3c4 HDODA TOA 12

From the above results it was observed that the rate of the halogen

exchange reaction depends on the nature of the trialkylamine moiety of the

PSQA. Here the time for complete conversion is reduced when the amine is

changed from triethylamine to trioctylamine and the lowering of the reaction

period is more significant in the case of BDDMA and HDODA crosslinked

resins. In the case of the reaction with polymer bound benzyltrioctylammonium

chloride catalyst, the time for complete conversion was considerably reduced

83

(12 h), but since the chloride ion capacity was very low a large amount of

catalyst was required for the reaction. Therefore polymer bound

benzyltributylammonium chloride was taken as the optimum catalyst.

3.12.1 (b) Effect of nature of crosslinking agent

Investigations on the halogen exchange reactions reveal that the nature

of crosslinking agent has a significant influence on the rate of the reaction. The

catalytic activity was increased when the polymer support was changed from

DVB-PS to BDDMA-PS. With catalyst 1c3 (DVB-PS-TBA) the time taken for

complete conversion of 1-bromooctane to 1-iodooctane was 40 h while with

catalyst 2c3 ((BDDMA-PS-TBA) and 3c3 the time taken was 15 h and 17 h

respectively. The long flexible BDDMA and HDODA crosslinks increase the

accessibility of the catalytic sites thereby reducing the reaction period to a

considerable extent when compared to the rigid DVB crosslinked catalysts

(Table 3.16). Similar was the case with reactions of 1-bromoheptane to 1-

1iodoheptane and benzyl chloride to benzyl iodide. The same observation was

found with reactions using catalysts with other trialkylamines (Table 3.16).

3.12.1(c) Effect of extent of crosslinking of the polymer support

Polystyrene supported catalysts were prepared from polymers with

different crosslink densities 1, 2, 4, 6, 8 and 10 mole percent (I to VI) under

identical conditions and employed for the conversion reaction of 1-

bromooctane to 1-iodooctane. The results obtained showed that increase in

the crosslink density reduces the activity of the catalyst. Increase in the degree

84

of crosslinking results in the reduction of the mobility of the polymer chains

which in turn reduces the accessibility of the catalytic sites on them to the

soluble reagents and substrates resulting in decreased reaction rates. Even

though increased reaction rates were observed, some problems were met with

the catalyst prepared from 1% crosslinked polymer that it does not possess

enough mechanical strength for stirring the reaction mixture for a long time.

The polymer undergoes deterioration and the separation of catalyst by

filtering the reaction mixture after the reaction was found to be time

consuming. The details of the studies are summarized in Table 3.17.

Table 3.17 Effect of crosslink density on the reactivity of PSPTC for the conversion of 1-bromooctance to 1-iodooctane

PSPTC Crosslink density

(%) Chloride ion

capacity (mmol/g)

Time for complete

conversion (h)

I 1 2.6 10

II 2 2.4 15

III 4 2.0 19

IV 6 1.8 24

V 8 1.6 30

VI 10 1.4 40

The results reveal that with 2% BDDMA crosslinked catalyst the time

taken for complete conversion of 1-bromooctane to 1-iodoctane is 15 h

whereas with 10 % crosslinked catalyst it is 40 h. From the studies it can be

85

concluded that 2 % BDDMA crosslinked PSQAC is the most effective one

among the catalysts prepared and possesses superior properties over other

analogues.

3.12.1(d) Effect of nitro group at the ortho position of the polystyrene support on the halogen exchange reaction

The effect of nitro group at the ortho position in the styrene moiety of

the polymer matrix on the efficiency of the catalyst was investigated by

carrying out the conversion of 1 – bromooctane to 1-iodooctane under similar

conditions. There is a small increase in the reaction rate when compared with

the unsubstituted catalyst and the results are summarized in Table 3.18.

Table 3.18 Effect of nitro group on the conversion of 1-bromooctane to 1-iodooctane

Resin Trialkylamine Time for 100 % Conversion

(h)

2%

BDDMA – PS – NO2 – CH2Cl

TBA

TEA

TMA

TOA

14

17

22

10

The increased reactivity of the catalyst may be due to the electron

withdrawing nitro group which can increase the ionic character of the

quaternary ammonium salt and can interact more effectively with the

substrate in the aqueous medium.

86

3.12.2 Effect of reaction conditions on the conversion of 1-bromooctane to 1-iodooctane

3.12.2.a Effect of solvent

Solvent plays a significant role in reactions using polymer supported

reagents and catalysts. When the solvent is compatible with the polymer

matrix the reaction will take place at considerable rates. The effect of solvent

on the activity of the PSPTC was investigated by following the conversion of

1-bromooctane to 1-iodooctane in different solvents with the aid of the

catalyst 2c3. The reaction was carried out at room temperature under identical

conditions in the solvents DCM, THF, toluene, DMF, dioxane, nitrobenzene

and hexane. The details are given in Table 3.19.

Table 3.19 Effect of solvent on the conversion of 1-bromooctane to 1-iodooctane

Solvent Time for 100% conversion (h)

DCM 15

THF 20

Toluene 26

DMF 30

Dioxane 35

Nitrobenzene 38

Hexane 40

87

Among the solvents maximum rate was observed when DCM was used

as solvent. The polymer backbone undergoes effective solvation in DCM.

Further the isolation of the product from DCM is relatively easy. Hence DCM

was used as the reaction medium in all studies.

3.12.2.b Effect of temperature

Due to the low boiling point of DCM, toluene was selected as the

solvent for temperature studies. The conversion of 1-bromooctane to 1-

iodooctane was carried out using the catalyst (2% BDDMA-PS-TBA, 2c3) in

toluene at temperatures 30, 40,50,60,70 and 800C. The time for complete

conversion was noted in each case. The observed results are summarized in

Table 3.20.

Table 3.20 Effect of temperature on the conversion of 1-bromooctane to 1-iodooctane

Temperature (0C) Time for 100% conversion (h)

30 26.0

40 25.5

50 25.0

60 24.0

70 21.0

80 17.0

88

As expected, with increase in temperature there is a decrease in the

time of reaction. When the temperature was increased from 30 to 800 C the

reaction period decreased from 26 to 17 hours. The rate of diffusion of the

soluble substrates and the reagents through the polymeric matrix increases at

elevated temperatures.

3.12.2.c Effect of the concentration of the catalyst

One of the important advantages of the polymer supported reactions

was the use of excess reagents without separation problem. Here also the

effect of the concentration of the catalyst on the phase transfer catalytic

reactions was investigated. The conversion of 1-bromooctane to 1-iodooctane

with the aid of catalyst 2c3 using different molar access of the catalyst was

studied. Initially the reaction was conducted with equimolar amount of the

catalyst and the time for complete conversion was noted. The reaction was

repeated with 2, 3, 4 and 5 molar excess of the polymer bound catalyst.

Table 3.21 Effect of concentration of catalyst on PSPTC

Catalyst 2c3 (molar excess) Time for 100% conversion (h)

Equimolar 30

2 20

3 15

4 15

5 15

89

From the above results it is seen that as the substrate-catalyst mole

ratio increases, the time for complete conversion decreases up to 1:3 ratio and

further increase in the concentration of the catalyst has no effect on the

reaction period.

3.12 (b) O-alkylation of phenols

The application of PSQAS in O-alkylation reactions was investigated

using DVB and BDDMA crosslinked catalysts. The catalyst was swelled in

DCM and conditioned with alkaline phenol at room temperature. To the

conditioned catalyst, the substrate was added and stirred. O-Alkylation

reaction of phenol, o-cresol, p-cresol and p-nitrophenol are carried out with 1-

bromooctane using DVB and BDDMA crosslinked catalysts. The reaction

was monitored by TLC and the results are summarized in Table 3.22.

Table 3.22 O-alkylation reaction of phenol using DVB and BDDMA crosslinked catalysts

Substrates Time for 100% conversion (hr)

DVB-PS –TBA(1c3) BDDMA-PS-TBA(2c3)

Phenol 40 28

o-Cresol 45 35

p-Cresol 45 35

p-Nitrophenol 32 20

From the above results it is seen that the time for O-alkylation is

decreased by the presence of electron withdrawing nitro group in the benzene

90

ring. The electron donating methyl group increases the reaction time. In the

case of phenol the reaction period was 28 h (BDDMA-PS-TBA). But with p-

nitrophenol the reaction time was reduced (20 h). Longer reaction time was

observed for o-and p- cresols (35 h).

3.12. (b).1 Effect of temperature on O-alkylation

O-Alkylation of phenol was carried out in solvent toluene at room

temperature, 400, 500, 600, 700, 800& 900C and results are summarized in

Table 3.23.

Table 3.23 Effect of temperature on O-alkylation of phenol with 1-bromooctane using BDDMA catalyst, 2c3

Temperature (0C) Time for complete reaction (h)

Room temperature 28

40 26

50 24

60 22

70 20

80 18

90 15

3.12. (c) O-Benzylation

O-Benzylation of phenol was also conducted and the results are

summarized in Table 3.24.

91

Table 3.24 O-Benzylation reaction of phenol using DVB and BDDMA catalysts

Substrates Time for 100% conversion (h)

Products DVB-PS -TBA BDDMA-PS-TBA

Phenol 45 33 Benzyl phenyl ether

o-Cresol 50 40 o-Tolyl benzyl ether

p-Cresol 50 40 p-Tolyl benzyl ether

p-Nitrophenol 37 25 p-Nitrophenyl benzyl ether

In this reaction also, the rate of reaction increased in the case of the

electron withdrawing nitro group and decreased with electron donating

methyl group.

3.12 (d) N-Alkylation

N-Alkylation of pyrrole and phthalimide with 1-bromooctane was done

using DVB and BDDMA catalysts. The substrate was stirred with NaOH, 1-

bromooctane and the catalyst at room temperature. TLC was used for

monitoring the reaction. Reaction was complete after 10 hours for pyrrole and

15 hours for phthalimide. The results are summarized in Table 3.25.

Table 3.25 N-alkylation of pyrrole and phthalimide using DVB and BDDMA crosslinked catalysts

Substrate Time for complete conversion (h)

DVB-PS-TBA BDDMA-PS-TBA

Pyrrole 15 10

Phthalimide 20 15

92

3.12. (e) Oxidation of alcohols

Alcohols were oxidized to the corresponding carbonyl compounds with

KIO4 in presence of DVB and BDDMA crosslinked PSQA catalysts at room

temperature. The catalyst was swelled in DCM and conditioned with an

aqueous solution of KIO4. Then the substrate was added to the conditioned

catalyst and the reaction mixture was shaken till the reaction was complete.

The reaction was monitored by TLC analysis. Oxidation of different alcohols

were carried out and the results are summarized in Table 3.26.

Table 3.26 Oxidation of alcohols using 2% DVB and BDDMA-PS-TBA catalysts (1c3 & 2c3)

Alcohols Product Reaction time (h)

DVB BDDMA

Benzoin Benzil 30 18

Benzhydrol Benzophenone 30 18

Benzyl alcohol Benzaldehyde 40 27

2- Nitrobenzyl alcohol 2-Nitrobenzaldehyde 28 16

4-Nitrobenzyl alcohol 4-Nitrobenzaldehyde 28 16

4-Cyanobenzyl alcohol 4-Cyanobenzaldehyde 29 16

Oxidation of 4-nitrobenzyl alcohol with KIO4 was also followed

spectrophotometrically at λmax 264 nm using both DVB-PS and BDDMA-PS

catalysts (1c3 & 2c3). The results are summarized in Table 3.27 and Fig.

3.10.

93

0

5

10

15

20

25

30T

ime

in h

2c3 1c3Catalysts 1c3-DVB-PS-TBA

2c3-BDDMA-PS-TBA

Time for completequaternisation

Time for 100%Conversion

Fig 3.10 Oxidation of 4-nitrobenzyl alcohol to 4-nitrobenzaldehyde

Table 3.27 Details of oxidation of 4-nitrobenzyl alcohol to

4-nitrobenzaldehyde.

3.12.(f)) Debromination reactions

Debromination of vicinal dibromides like dibromostyrene,

dibromostilbene, dibromocinnamic acid and dibromobenzilidene acetophenone

were done using DVB and BDDMA catalysts and the results are summarized in

Table 3.28.

Catalyst Time for complete quaternisation (h)

Time for 100% conversion (h)

BDDMA-PS-TBA (2c3)

DVB-PS-TBA (1c3)

5

22

16

28

2c3 1c3

94

Table 3.28 Dehalogenation using DVB and BDDMA crosslinked catalysts

Substrate Time for 100% conversion (h)

2% DVB-PS-TBA 2% BDDMA-PS-TBA

Dibromostyrene 20 11

Dibromostilbene 22 12

Dibromocinnamic acid 35 20

Dibromobenzilidene acetophenone

25 18

3.12.(g) Insertion reaction.

Insertion of dichlorocarbene to alkenes provides an attractive route to

the production of dichlorocyclopropanes. Styrene on reaction with chloroform

and alkali in presence of PSQAS yields 1,1-dichloro-2-phenylcyclopropane.

Similarly 1,1-dichloro-2, 3–diphenylcyclopropane was prepared from stilbene

by the same procedure and the results are summarized in Table 3.29.

Table 3.29 Reaction of styrene and stilbene with chloroform using different catalysts

Substrate Time taken for the reaction (h)

2% DVB-PS-TBA 2% BDDMA-PS-TBA

Styrene 38 20

Stilbene 40 22

95

PART B SPACER MODIFIED POLYMER SUPPORTED QUATERNARY AMMONIUM SALTS

Polymer supported quaternary ammonium salts with methylene spacers

and with poly(ethyleneglycol) grafted polystyrene resins were prepared and

the effect of spacer group on the activity of the polymer bound catalyst was

investigated. The catalytic activity of the immobilized quaternary ammonium

salts are comparable to the activity of soluble onium salts.3-13 The necessity

of prolonged reaction period and relatively large amount of catalyst were

observed to be the general limitation of these catalysts. This limitations can

be overcome to a large extent by separating the reactive function from the

polymer backbone with the help of a spacer arm.14 Introduction of methylene

and PEG spacers between polystyrene backbone and the quaternary

ammonium moiety can reduce the steric constraints imparted by the

macromolecular matrix on the active sites and can enhance the efficiency of

the polymer supported quaternary ammonium PTC (PSQA) significantly.15

Preparation of DVB and BDDMA crosslinked PS with methylene spacers and

PEG grafted polystyrene supported quaternary ammonium salts and the effect

of spacer grouping in polymer supported quaternary ammonium salts are

described in this section.

96

3.13.1 Preparation of polymer supported quaternary ammonium salts with methylene spacers

3.13.1a Preparation of catalyst with two methylene spacers 2c3a

Chloroethylated polystyrene resin was prepared by Friedel-Crafts reaction

of 2% crosslinked polystyrene resins with 1,2-dichloroethane using anhydrous

AlCl3 as the catalyst. The chloroethylated resin was quaternised with TBA to get

the quaternary ammonium chloride resin. The reaction sequence is depicted in

Scheme 3.7.

P + ClCH2CH2ClCH2Cl2

Anhy AlCl3P CH2CH2Cl

P CH2CH2N+(Bu)3Cl

-

TBADMF

Scheme 3.7 Preparation of catalyst with two methylene spacers

3.13. 1.b Preparation of catalyst with four methylene spacers 2c3b

Polymer bound catalyst with four methylene spacers was obtained by a

series of polymer analogous reactions starting from BDDMA crosslinked

resin. A ketopropanoic acid function was introduced into the polystyrene

matrix by Friedel-Crafts reaction with succinic anhydride. The keto function

in the resulting polymer was converted to methylene group by Clemmenson

reduction using zinc amalgam and HCl. The resulting polymer bound acid

resin was reduced by diborane in THF. The primary alcohol resin obtained

97

was treated with SOCl2 to yield the chlorobutylpolystyrene resin. The

chlorobutyl resin was quaternised with different trialkylamines. The reaction

sequence is depicted in Scheme 3.8.

P +CH2 CO

OCH2 CO CH2Cl2

Anhy AlCl3P COCH2CH2COOH

BH3/THF

P CH2CH2CH2CH2OH

SOCl2DCM

P CH2CH2CH2CH2Cl

TBADMF

P CH2CH2CH2CH2 N+(Bu)3Cl

-

Scheme 3.8 Preparation of catalyst with four methylene spacers

3.14.1 Preparation of PEG grafted polystyrene resins (1c3-PEG &2c3-PEG)

To prepare PS-PEG graft copolymers, monosodium derivative of

PEG600 was added to chloromethylated polystyrenes (1b2, 2.1 mmolCl/g, 2b2,

2.4 mmolCl/g) swelled in THF and refluxed for 42 h. The residual chlorine

capacity was determined by pyridine fusion method and found negligible

(.05mmol). The resin was characterized by IR spectral bands at 1150 cm-1 (C-

O stretching) and at 3180 cm-1 (O-H stretching).

98

The hydroxyl capacity of the PS-PEG resins were determined by

acetylation method and was found to be 1.21 mmol/g and 1.29 mmol/g.

+ N a O P E G O H

100 C 0

T H F

P C H 2 C l

P C H 2 O P E G O H S O C l 2

C H 2 O P E G C H 2 C l P T B A

D M F

P C H 2 O P E G C H 2 N ( B u ) 3 C l - +

Scheme 3.9 Preparation of PS-PEG graft quaternary ammonium chloride catalysts 1c3-PEG &2c3-PEG

3.14.1.b Conversion of PS-PEG-CH2OH to PS-PEG-CH2Cl

The DVB crosslinked PS-PEG-grafted polymer was swelled in DCM

and refluxed with SOCl2 for 3 h. The reaction mixture was filtered and

washed with different solvents until it is free from chloride ion. Chlorine

capacity of the resin obtained was determined by Volhard’s method and was

found to be 1.15 mmol Cl/g. Similarly BDDMA-PS-PEG-CH2Cl was

prepared and the chlorine capacity was found to be 1.19 mmol Cl/g.

99

3.14.1.c Quaternisation of the PS-PEG-CH2Cl , Preparation of spacer modified quaternary ammonium salts (1c3-PEG, 2c3-PEG)

DVB and BDDMA crosslinked spacer modified chlororesins were

swelled in DMF and treated with trialkylamines (TMA, TEA, TBA and TOA)

to obtain the corresponding quaternary ammonium chloride resin. The results

are shown in Table 3.30.

Table 3.30 Details of preparation of spacer modified catalysts with different trialkylamines

Resin Trialkyl amine

Time for 100% quaternisation (h)

Chloride capacity mmolCl/g

DVB- PS-P EG-CH2Cl ( 1.15 mmolCl/g)

1b2-PEG

TBA 22 0.94

TEA 24 0.89

TMA 30 0.86

TOA 20* 0.51

BDDMA-PS-PEG-CH2Cl,

(1.19 mmolCl/g)

2b2-PEG

TBA 3 1.03

TEA 5 0.95

TMA 7 0.94

TOA 2* 0.59

* quaternization was not complete.

In the case of PEG-grafted polystyrene resins also the structure of the

trialkylamine influences the rate of formation of the quaternary ammonium

salts. The reaction rate increases with increase in the number of methylene

group in the alkyl moiety. But steric effect also plays an important role in the

100

reaction. In the case of trioctylamine the chloride ion capacity first increases

rapidly, reaches a maximum and then decreases.

3.15 Halogen exchange reactions using spacer modified catalysts

The conversions of 1-bromooctane to 1-iodooctane, 1-bromoheptane to

1-iodoheptane, benzyl chloride to benzyl iodide and 1-bromooctane to

1-chlorooctane were effected in aqueous - organic medium under phase

transfer catalytic conditions using PS-PEG catalysts and the results are

summarized in Fig. 3.10 and Table 3.31.

2c3PEG 2b2(NO2) 2 c3 1 c3PEG1 c3

05

10

1520

2530

3540

45

Catalysts

Tim

e in

Ho

urs

The results clearly indicate that the reaction time was reduced when

PEG grafted polystyrene was used as the polymer support.

Fig. 3.10 Reaction time for complete conversion of 1-bromooctane to 1-iodooctane using various catalysts

101

Table 3.31 Halogen exchange reactions using 2% crosslinked spacer modified catalysts

Reaction Catalysts Reaction time (h) Boiling point of

the Product (oC)

1-bromooctane to 1-iodooctane

2c3a

2c3b

1c3-PEG

2c3-PEG

12.0

10.0

20.0

5.0

219

1-bromoheptane to 1-iodoheptane

2c3a

2c3b

1c3 -PEG

2c3 –PEG

13.0

11.0

21.0

6.0

203

Benzyl chloride to benzyl iodide

2c3a

2c3b

1c3-PEG

2c3-PEG

18.0

17.0

39.0

10.0

117

1-bromooctane to 1-chlorooctane

2c3 a

2c3 b

1c3-PEG

2c3-PEG

40.0

37.0

45.0

31.0

183

The results clearly indicate the reduction in the reaction period with the

catalysts derived from the PEG-grafted polymers. It was observed that the

reaction time was reduced when polyethylene graft polystyrene was used as

polymer support.

3.16 Oxidation of 4-nitrobenzyl alcohol

Optimization studies were carried out by the oxidation of 4-

nitrobenzylalcohol to 4- nitrobenzaldehyde. Oxidation of 4-nitrobenzyl alcohol

with KIO4 was carried out in the presence of different polystyrene-supported

quaternary ammonium chloride catalysts. The oxidation reaction was

102

followed spectrophotometrically at λmax 264nm. The results are summarized

in Fig. 3.11 and Table 3.32.

Table 3.32 Details of oxidation of 4-nitrobenzyl alcohol to 4-nitrobenzaldehyde

The results show that the efficiency of the PSQA catalyst depends on the

macromolecular characteristics of the polymer matrix to which the catalyst

function is bound. Catalyst (2C3-PEG) shows maximum efficiency and using this

catalyst the time for 100% oxidation in 11 h. Here the catalyst residue is attached

to BDDMA –PS resin. The polymer is more flexible and polar due to BDDMA

crosslinks and the active sites are made far apart from the polymer backbone by

the use of PEG spacer. DVB- PS polymer bound catalyst (1c3) has least

efficiency due to the rigid nature of the polymer backbone. But when the catalyst

site was separated by PEG spacer (1C3-PEG ) the time of oxidation was reduced

from 28 h to 18 h. The electron withdrawing nitro substituent on the polymer

matrix (2C3-N02) also exhibits significant rate enhancement by rendering the

catalyst site more ionic.

Catalyst Time for complete quaternisation (h)

Time for 100% conversion (h)

BDDMA-PS-PEG 2c3-PEG

DVB-PS-PEG 1c3-PEG

BDDMA-PS 2c3

DVB-PS 1c3

BDDMA-PS-NO2 2c3-NO2

3

12

5

22

4

11

18

16

28

13

103

3.16.1 Effect of solvent

Effect of solvent in the reactions using catalyst derived from PEG-grafted

polymers was investigated by following the oxidation of 4-nitrobenzylalcohol to

4-nitrobenzaldehyde. The oxidation reactions were carried out in solvents DCM,

THF, toluene, DMF and NMP at room temperature. DCM was found to be the

best choice of solvent for the reaction. From the results it is clear that for DVB-

PS-PEG catalyst the reaction rate is in the order DCM > THF>DMF>CHCl3

>Toluene>NMP for DVB-PS-PEG catalyst. But the order of reaction for

BDDMA-PS-PEG catalyst is DCM >THF >CHCl3 >DMF>Toluene > NMP. The

results are summarized in Table 3.33.

Table 3.33 Effect of solvent on the oxidation of 4-nitrobenzyl alcohol using DVB-PS-PEG and BDDMA-PS-PEG catalysts

Solvent Reaction time (h)

BDDMA-PS-PEG catalyst DVB-PS-PEG catalyst

DCM 11 18

THF 12 19

Chloroform 15 24

DMF 16 21

Toluene 20 25

`NMP 22 29

From the above results it is clear that DCM is the best solvent for DVB

and BDDMA crosslinked PS-PEG catalysts.

Oxidation of different alcohols were carried out using DVB-PS-PEG

and BDDMA-PS-PEG catalysts and results are summarized in Table 3.34.

104

Table 3.34 Oxidation of alcohols using DVB-PS-PEG (1c3-PEG) and BDDMA-PS-PEG catalysts (2c3-PEG)

Alcohols Product Reaction time (h)

1c3-PEG 2c3-PEG

Benzoin Benzil 19 12

Benzhydrol Benzophenone 19 13

Benzyl alcohol Benzaldehyde 25 20

2-Nitrobenzyl alcohol 2-Nitrobenzaldehyde 19 12

4-Nitrobenzyl alcohol 4-Nitrobenzaldehyde 18 12

4-Cyanobenzyl alcohol 4-Cyanobenzaldehyde 18 11

The results show that electron withdrawing groups in the substrate

decreased the reaction period.

3.16.2 Effect of temperature

The effect of temperature on the oxidation of 4-nitrobenzyl alcohol by the

BDDMA-PS-PEG catalyst was studied. The reactions were carried out in DCM

at temperatures 300, 350, 400, 450, 500 and at refluxing temperature (550 C).The

results are given in Table. 3.35.

Table 3.35. Effect of temperature on the oxidation of 4-nitrobenzyl alcohol

Temperature (0C) Reaction time (h)

30 11.0

35 10.5

40 10.5

45 9.5

50 9.0

Refluxing temperature (55) 9.0

From the above results it is clear that the effect of temperature on the

oxidation reactions is not drastic but a gradual increase in the reaction rate

was observed.

105

3.17 Recycling of the catalyst

The spent catalyst after the reaction was collected and washed with

DCM, toluene, DMF, DMF-water (1:1), water and finally with methanol.

After washing the resin was dried in a hot air oven at 500 C for 24 h .For

regenerating the catalyst, the dried resin was swelled in THF and treated with

concentrated HCl for five hours, washed with THF, DMF, DMF-water(1:1)

and methanol and dried in vacuo. The chloride capacity of the regenerated

catalyst was determined after each cycle and compared with the capacity of

the fresh catalyst (Table 3.36).The IR spectrum of the regenerated catalyst

showed the C-N stretching band at 1378 cm-1. The efficiency of the

regenerated catalyst was checked in the conversion of 1-bromooctane to 1-

iodooctane. There was only small change in the chloride ion content and

reactivity after five repeated cycles.

Table 3.36 Chloride ion content of 2c3 after recycling and time taken for the complete conversion of 1-bromooctane to 1-iodooctane

Number of cycles Chloride ion content (mmol Cl/g)

Reaction time (h)

1 2.42 15.0

2 2.39 15.0

3 2.34 15.0

4 2.25 16.0

5 2.11 18.0

106

The foregoing investigations indicate that polymer supported

quaternary ammonium salts are good catalysts in halogen exchange,

oxidation, O-alkylation, N-alkylation, debromination and insertion reactions

under suitable reaction conditions. As crosslinked polymers are insoluble in

all solvents and majority of the reactive sites are within the beads, it is clear

that for a reaction to take place the low molecular weight reactants must

diffuse into the polymer beads. Due to a number of factors related to the

diffusion of the low molecular reactant into and out of the polymer beads,

polymer supported reactions are often slower than that of their low molecular

weight counterparts and longer reaction periods are usually needed. This

limitation is overcome in this study to some extent using methylene and PEG

spacers to make the catalytic site distant from the macromolecular matrix. The

results obtained indicate that the rates of polymer supported reactions are

profoundly influenced by the spacer arms between the reactive group and the

polymer backbone. The macromolecular properties and morphological

characteristics of the polymer backbone are the decisive factors in

determining the rate of a polymer supported reaction.

107

References

1. Merrifield R. B., J.Am.Chem. Soc., 1963, 85, 2149.

2. Gisin B. F., Merrifield R. B., J.Am.Chem.Soc., 1972, 94, 6165.

3. Allum K. G., Hancock R. D., Howell I.V., McKenzie S., Pitkethly R. C.,

Robinson B., J. Organometallic Chem., 1975, 87, 203.

4. Unger K., Ang. Chem. Int. Edn., 1972, 11, 267.

5. Epton R., Holding S. R., Mc. Laren J. V., Polymer, 1976, 17, 843.

6. Coupek J., Krivakova M., Pakorny S., J.Polymer. Sci. Polymer

Symposia., 1973, 42, 185.

7. Renil M., Nagaraj R., Pillai V. N. R., Tetrahedron, 1994, 50, 6681.

8. Varkey J. T., Pillai V. N. R., J. Appl. Polymer Sci., 1999, 71, 19, 33.

9. Ajikumar P. K., Devaky K.S., Proc. Indian. Aca. Sci. (Chem. Sci.),

2000, 112, 465.

10. Renil M., Medal M., Tetrahedron Lett., 1996, 37, 6185.

11. Renil M., Medal M., Tetrahedron Lett., 1995, 36, 4647.

12. Varkey J. T., Pillai V. N. R., J. Peptide. Res., 1998, 51, 49.

13. Kumar K., Pillai V. N. R., Tetrahedron., 1999, 55, 10437.

14. Arunan C., Nagaraj R., Pillai V. N. R.Tetrahedron., 2000,56,3005.

15. Kumar K. S., Pillai V. N. R., Das M.R., J. Peptide. Res. 2000,56,88

16. Koruthu D. P., Madhusudhanan P.M., Pillai V. N. R.,Proc.Indian. Aca.

Sci. (Chem. Sci.), 1997, 109, 39.

108

17. Chandy M. C., Pillai V. N. R., J. Polymer Sci. Part A; Polym. Chem.,

1995, 33, 849.

18. Chandy M. C., Pillai V. N. R., Polymer, 1995, 36, 1083.

19. Arunan C., Pillai V. N. R., Tetrahedron, 2000, 56, 3005.

20. Varkey J. T., Aswathi S. K., Pillai V. N. R., Peptide and Protein Lett.

1999, 6, 209.

21. Arshady R., J. Chromatography, 1991, 586, 181.

22. Galina H., Colaz N.B., Wiezorek P.P., Bri. Polym. J., 1985, 17, 215.

23. Hiefz W., Adv. Polym..J., 1997, 23, 1.

24. Negre M., Bartholin M., Guyout A., Angew. Makromol. Chem., 1984,

126, 39.

25. Guyout A., Bartholin M., Prog. Polym. Sci. 1998, 8, 277.

26. Arshady R., J. Chromatography, 1991, 586, 199.

27. Birr C., Aspects of the Merrifield Peptide Synthesis, Springer-Verlag,

Hiedelberg, 1978,p-27.

28. Pepper K.W., Paisely H.M., Young M.A., J. Chem. Soc.,1953, 4097.

29. Feinberg R.S., Merrifield R.B., Tetrahedron, 1974, 30, 2209.

30. Sparrow J. T., Tetrahedron Lett., 1975, 4673.

31. Soveltov A. K., Demenkova T. N., Khomutov L. I., Vyasokomol

Soedin Ser. A., 1969, 11, 606; Chem. Abstr., 1969, 70, 1158.

109

32. Pinnell R. P., Khune G. D., Khatri W. A., Manatt. S., Tetrahedron

Lett., 1984, 25, 3511.

33. Stewart J. M., Young J.D., Solid Phase Peptide Synthesis, Pierce

Chemical Co., Rockford, Illinois, 1984.