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Title: Integration of CO2 Reduction with Subsequent Carbonylation:Towards Extending Chemical Utilization of CO2

Authors: Xian-Dong Lang and Liang-Nian He

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To be cited as: ChemSusChem 10.1002/cssc.201800902

Link to VoR: http://dx.doi.org/10.1002/cssc.201800902

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Integration of CO2 Reduction with Subsequent Carbonylation:

Towards Extending Chemical Utilization of CO2

Xian-Dong Lang,[a] and Liang-Nian He*[a, b]

Currently, it still remains a challenge to amplify the spectrum of

chemical fixation of CO2, though enormous progresses have been

witnessed in this field. In view of the widespread applications of CO

in a myriad of industrial carbonylation processes, we proposed an

alternative strategy to integrate CO2 reduction to CO and

carbonylations with CO ex situ generated, efficiently affording

pharmaceutically and agrochemically attractive molecules. As such,

CO2 in this study was efficiently reduced by triphenysilane with CsF

to CO in a sealed two-chamber reactor; Subsequently, the

palladium-catalyzed aminocarbonylation and carbonylative

Sonogashira coupling of aryl iodides, rhodium(I)-mediated Pauson-

Khand-type reaction proceeded smoothly to furnish the amides,

alkynones and bicyclic cyclopentenones, respectively. Furthermore,

the formed alkynones can further be successfully converted to a

series of heterocycles e.g. pyrazoles, 3a-hydroxyisoxazolo[3,2-

a]isoindol-8-(3aH)-one derivatives and pyrimidines in moderate

yields. The salient features of this protocol include operational

simplicity, high efficiency, relatively broad application scope, which

represents an alternative avenue for CO2 transformation.

With the growing recognition of low carbon economy and

sustainable development, much attention has been focused on

the chemical transformation of CO2, which bears attractive

characteristics such as low cost, nontoxicity, non-flammability,

easy accessibility and so on as a valuable C1 resource.[1-3]

Though recycling CO2 into chemicals, fuels and polymers has

been flourished, broadening the scope of conversion of CO2 still

remains a significant challenge due to its thermodynamic

stability and kinetic inertness.

Compared to CO2, CO has been universally recognized as

an industrially important gas in oxidative coupling to give diethyl

oxalate, Fischer-Tropsch process, Monsanto process, Reppe

reaction, and so on.[4-7]

Furthermore, tremendous improvements

have also been achieved in the palladium-catalyzed

carbonylations of aryl halides with CO to furnish a variety of

carbonyl compounds such as aldehydes, amides, esters, etc.[8-10]

However, the high toxicity and gas pressure, along with gaseous

nature of CO remains a formidable obstacle in the activity of

both academia and industrial sectors. To circumvent such

problems and to extend the scope of CO2 utilization, CO2 has

been exploited as a CO precursor for conventional

carbonylations. For example, the CO formed through a reverse

water-gas shift reaction (rWGSR) can be consumed in situ in a

[a] Dr. X.-D. Lang, Prof. L.-N. He

State Key Laboratory and Institute of Elemento-Organic Chemistry

College of Chemistry, Nankai University

Tianjin, 300071 (P. R. China)

E-mail: heln@nankai.edu.cn

[b] Prof. L.-N. He

Collaborative Innovation Center of Chemical Science and Engineering

Nankai University, Tianjin, 300071 (P. R. China)

E-mail: heln@nankai.edu.cn

Supporting information for this article is given via a link at the end of the

document.((Please delete this text if not appropriate))

range of carbonylative functionalization of alkenes, delivering

value-added molecules such as carboxylic acids,[11]

esters,[12]

amines[13-14]

and alcohols[15-19]

efficiently. Recently, Ding and co-

workers firstly reported the exclusive synthesis of aldehydes via

rhodium-catalyzed one-pot hydroformylation of olefins with CO2,

poly(methyl-hydrosiloxane) (PMHS), and H2, and the

mechanistic investigations disclose that the presence of CO2

inhibites further reduction of the thus formed aldehydes.[20]

Furthermore, Skrydstrup and co-workers have elegantly

designed a connected two-chamber system (CO ware), in which

CO2 is efficiently reduced to CO using disilane as the reductant

under transition metal-free conditions, while the thus liberated

CO gas go through the metal promoted carbonylations to afford

the pharmaceutically relevant molecules[21]

(Scheme 1). On the

other hand, some CO-releasing molecules (CORMs) including

Mo(CO)6,[22-23]

formates,[24-25]

silacarboxylic acids,[26-27]

acid

chlorides,[28]

N-formylsaccharin,[29-30]

and chloroform[31]

can

readily deliver CO upon its activation. Besides, some of them

can be recognized as the bridge molecules from CO2 to CO,

since they can be readily prepared from CO2.

Scheme 1. Integration of reduction of CO2 with carbonylation reaction.

As part of our research endeavors on hydrogenation and

reductive functionalization of CO2,[32]

along with the inspiration

by the readily decarbonylation of formates and the

thermodynamically favorable hydrosilylation of CO2 into silyl

formats[33]

, we speculated whether CO2 can be used directly with

hydrosilane to evolve CO with silyl formate as an intermediate,

for the carbonylative reactions. In this aspect, fluoride salts have

been demonstrated to be efficient catalyst for the hydrosilylation

of CO2 to silyl formate, due to the inherent high fluorophilicity of

the silicon atom.[34-35][43]

Therefore, a common fluoride source e.g.

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KF was selected to evaluate the effect of the hydrosilane on the

reaction in a sealed two chamber reactor which is summarized

in Table 1, and the thus generated CO gas was trapped by the

Pd-catalyzed aminocarbonylation from p-iodoanisole and

cyclohexylamine at 80 °C for 24 h and quantified by the yield of

corresponding product 2a. To our surprise, the type of

hydrosilane has a substantial influence on the reaction, with

Ph3SiH gave up to 86% yield of N-cyclohexyl-4-

methoxybenzamide 2a with excellent chemoselectivity, and

other hydrosilanes such as PMHS, (EtO)3SiH, PhSiH3, Ph2SiH2,

Et3SiH and (CH3)2PhSiH did not provide satisfactory conversion

(entries 1-7). We deduced that the silyl formates readily form

from the hydrosilanes and CO2, but release CO more difficult

than Ph3SiH. Notably, any side reaction such as double

carbonylation was not observed. We then switched our attention

to the dependence of 2a yield on the catalyst such as fluoride

and oxygen-containing bases owing to their dynamic interaction

with the silane to promote the reduction of CO2. The data in

Table 1, entry 8 illustrates that the reaction did not occur in the

absence of any catalyst and the starting material of

carbonylative reaction was recovered quantitatively, implying

that catalyst play a significant role in both the generation and the

decarbonylation of silyl formate HCOOSiPh3. Compared with KF,

CsF rendered a slight higher yield of 2a (entry 9 vs entry 5),

whereas TBAF (tetrabutylammonium fluoride) (1M solution in

THF) displayed a lower activity under otherwise identical

conditions (entry 10 vs entry 5). For the catalyst with a fixed

anion, the catalytic activities followed the order of NH4OAc >

NaOAc, suggesting the pivotal role the cation played in the

solubility and catalytic performance (entries 11-12). For

comparison, Na2CO3 was tested to be inactive (entry 13).

The influence of reaction parameters including solvent,

temperature and reaction time on the reaction was then

investigated. Solvents with high polarity might be favorable for

the reaction (entries 9, 14-15 vs 16-19), and DMF was proved to

be an appropriate solvent. The poor yield in entry 19 may be

derived from the fact that THF inhibit the reaction evidently with

the lone pair in THF preventing the interaction between fluoride

and hydrosilane (entry 19). Subsequently, decreasing the

temperature from 130 oC to 110

oC caused lower yield of 2a,

whereas unchanged yield was obtained when enhance the

temperature (entries 9, 20-21). Therefore, 130 oC was selected

as the optimal temperature for further optimization of the

reaction times. A noticeable decrease in 2a yield was witnessed

when shortening the reaction times from 24 h to 16 h (entries 9,

22-23). Next, an evaluation of the amount of Ph3SiH in the step

of CO2 splitting into CO confirmed that 2 mmol Ph3SiH seems to

be most suitable (entries 9, 24-26). Subsequently, Pd(dba)2 was

confirmed to be the optimum catalyst for the carbonylation

reaction in this study through the investigation of various

palladium catalysts including Pd(OAc)2, PdCl2, Pd(PPh3)2Cl2,

Pd(PPh3)4, Pd/C and so on (for details, see Table S1 in

Supporting Information).

Blank runs were also conducted to demonstrate the

carbonyl fragment in 2a originates from CO2. Firstly, the

aminocarbonylation reaction in chamber B did not occur in the

absence of CO2 (entry 27). Besides, it has been reported that

DMF can also be employed as CO source at high

temperature.[36-37]

To rule out the abovementioned possibility, the

reaction was performed just in the presence of DMF or in the

presence of both DMF and CsF, the starting material remain

intact after 24 h (entries 28-29), which further underscore the

significance of CO2 as a CO surrogate.

Table 1. Catalyst screening and reaction conditions optimization.[a]

Entry Hydrosilane

(mmol) Catalyst Solvent T (

oC) t (h)

Yield

2a

(%)[b]

1 PMHS (2) KF DMF 130 24 12

2 (EtO)3SiH (2) KF DMF 130 24 18

3 PhSiH3 (0.667) KF DMF 130 24 15

4 Ph2SiH2 (1) KF DMF 130 24 24

5 Ph3SiH (2) KF DMF 130 24 86

6 Et3SiH (2) KF DMF 130 24 4

7 (CH3)2PhSiH

(2) KF DMF 130 24 3

8 Ph3SiH (2) - DMF 130 24 0

9 Ph3SiH (2) CsF DMF 130 24 95

10 Ph3SiH (2) TBAF DMF 130 24 30

11 Ph3SiH (2) NH4OAc DMF 130 24 63

12 Ph3SiH (2) NaOAc DMF 130 24 32

13 Ph3SiH (2) Na2CO3 DMF 130 24 <1

14 Ph3SiH (2) CsF DMSO 130 24 34

15 Ph3SiH (2) CsF NMP 130 24 51

16 Ph3SiH (2) CsF CH3CN 130 24 11

17 Ph3SiH (2) CsF toluene 130 24 10

18 Ph3SiH (2) CsF dioxane 130 24 <1

19 Ph3SiH (2) CsF THF 130 24 <1

20 Ph3SiH (2) CsF DMF 120 24 74

21 Ph3SiH (2) CsF DMF 110 24 59

22 Ph3SiH (2) CsF DMF 130 20 87

23 Ph3SiH (2) CsF DMF 130 16 73

24 Ph3SiH (1.5) CsF DMF 130 24 65

25 Ph3SiH (1) CsF DMF 130 24 37

26 Ph3SiH (0.5) CsF DMF 130 24 22

27c Ph3SiH (2) CsF DMF 130 24 <1

28d - - DMF 130 24 <1

29e - CsF DMF 130 24 <1

[a] Reaction conditions: chamber A was loaded with hydrosilane (0.5208 mmol), KF (0.0116 g, 10 mol%, 0.2 mmol), DMF (1.0 mL), and CO2 (approx. 20 mL, 0.892 mmol), 100

oC, 24 h; chamber B was loaded with Pd(dba)2 (5

mol%, 0.0229 g), PPh3 (10 mol%, 0.0131 g), p-iodoanisole (0.117 g, 0.50

mmol), cyclohexylamine (0.150 g, 1.5 mmol), Et3N (0.101 g, 1.0 mmol, 0.14 mL), dioxane (2.0 mL), 80

oC, 24 h. [b] GC yields were given using biphenyl as

the internal standard. [c] In the absence of CO2. [d] In the atmosphere of argon.

[e] In the atmosphere of argon.

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Afterwards, the catalytic protocol using connected two-

chamber reactor was then found to be applicable to the

aminocarbonylation of various aryl iodides with cyclohexylamine

as a fixed nucleophile, which is summarized in Scheme 2.

Various electron-withdrawing or electron-donating groups at the

phenyl ring such as F, Cl, OCH3, CH3 and tBu were well

tolerated to give the desired amides 2a-2j in excellent yields.

Moreover, 29% isolated yield of 2a was also obtained when

using 4-bromoanisole as the starting material, which is in

accordance with the general order of reactivity of aryl halide in

carbonylative reactions (aryl iodide > aryl bromide > aryl

chloride). Negligible difference was observed between the

activity of 2-iodofuran and 2-iodothiophene, and the same

happened to the activity of iodobenzene and 1-iodonaphthalene

(yield: 2k ≈ 2l, 2f ≈ 2m). To our delight, the catalytic system can

be extended to the synthesis of moclobemide 2p, an inhibitor of

the MAO-A isozyme, which is a pharmacological and therapeutic

useful molecule. Furthermore, aliphatic amines, such as

butylamine and benzyl amine provided good to excellent yields

of the corresponding product (2n-2o), whereas; secondary

amine, i.e. piperidine afforded lower yield of 2q. We then

switched our attention to the exploitation of benzylamines as

starting materials. 4-F, 4-Cl, 4-OCH3 substituted iodobenzene

showed almost the same activity compared with iodobenzene

when reacted with aniline (2s-2u). And high yields of 2v and 2w

(81% and 83%) were also obtained when p-chloroaniline and p-

anisidine were used as nucleophile, respectively.

[a] Reactions were carried out with: chamber A was loaded with Ph3SiH

(0.5208g, 2 mmol), CsF (0.030 g, 10 mol%, 0.2 mmol), DMF (1.0 mL), and

CO2 (approx. 20 mL, 0.892 mmol), 130 oC, 24 h; chamber B was loaded with

Pd(dba)2 (5 mol%, 0.0229 g), PPh3 (10 mol%, 0.0131 g), aryl iodides (0.50

mmol), amines (1.5 mmol), Et3N (0.101 g, 1.0 mmol, 0.14 mL), dioxane (2.0

mL), 80 oC, 24 h. [b] Isolated yields.

Scheme 2. Scope of aminocarbonylations of various aryl iodides.

In addition, carbamate salts 1aa was proved to be excellent

CO2 and cyclohexylamine surrogate for the aminocarbonylation,

rending the reaction proceed smoothly under gas-free conditions

with negligible influence on 2a yield. The protocol is easily

handled, practical and high yielding as shown in Scheme 3.

Scheme 3. Synthesis of 2a under gas-free conditions.

Considering the abundant occurrence in a wide variety of

biologically active molecules and their pivotal role as key

synthetic intermediates,[40-42]

the methodology was then applied

to the synthesis of α,β-alkynyl ketones, which is depicted in

Scheme 4. The carbonylative coupling reactions of iodobenzene

with various para-substituted phenylacetylenes was firstly

investigated. And the results showed that all the reactions

proceeded smoothly, with electron-donating groups substituted

ones at para position showing slightly higher reactivity. When

comes to the aliphatic alkynes, e.g. cyclopropylacetylene and 1-

hexyne, higher catalyst loading was required to obtain a satisfied

yield. Then the reaction scope was expanded to various

substituted aryl iodides with phenylacetylene as a fixed coupling

reagent. The results discovered that the electron-withdrawing or

electron-donating groups at the phenyl ring generally have little

influence on the reactivity, affording the corresponding products

3g-3k in 81-90% yields. 1-iodonaphthalene and 2-iodothiophene

were also proved to be suitable substrates under the optimized

conditions.

[a] Unless otherwise specified, all the reactions were performed with: chamber

A was loaded with Ph3SiH (0.5208g, 2 mmol), CsF (0.030 g, 10 mol%, 0.2

mmol), DMF (1.0 mL), and CO2 (approx. 20 mL, 0.892 mmol), 130 oC, 24 h;

chamber B was loaded with PdCl2 (5 mol%, 0.0045 g), Xantphos (10 mol%,

0.029 g), aryl iodides (0.50 mmol), alkyne (0.65 mmol), Et3N (0.101 g, 1.0

mmol, 0.14 mL), dioxane (2.0 mL), 80 oC, 24 h. [b] Isolated yields. [c] PdCl2

(7.5 mol%, 0.0067 g), Xantphos (15 mol%, 0.0435 g).

Scheme 4. CO gas-free carbonylative Sonogashira coupling reaction to

prepare α,β-alkynyl ketones.

N-containing heterocycles represent a synthetically useful

structural motif found in numerous biologically active molecules.

To further extend the application of current methodology, we

found that CO2 can also be successfully integrated into N-

containing heterocycles with the thus prepared alkynones as

important building blocks. For example, treatment of 3i and 3m

with hydrazine hydrate and phenylhydrazine provided

substituted pyrazoles 4i and 4m in high isolated yields through

dehydration and subsequent 1,4-addition process, respectively

(Scheme 5a-b). Furthermore, [3+2] annulation between 3a and

NHPI (N-hydroxyphthalimide) provided pharmaceutically

attractive 3a-hydroxyisoxazolo[3,2-a]isoindol-8-(3aH)-one

derivatives 4a in up to 89% yield, which is definitely confirmed

by X-ray analysis (Figure 1). Finally, the tandem reaction was

also extended to the preparation of pyrimidines via the

cyclization of 3a/3e with acetamidine hydrochloride (Scheme 5d).

All the above-mentioned results indicated that this methodology

is practical to valuable molecules preparation.

Recently, CO gas free version of Pauson-Khand reaction

has become a promising field for the construction of

cyclopentenones and their derivatives.[38-39]

The combination of

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reduction of CO2 to CO and rhodium(I)-catalyzed Pauson-

Khand-type reaction was proved feasible, delivering the diester-

linked and oxygen-tethered products 4f and 4g in 36% and 41%

yields, respectively (Scheme 6).

Scheme 5. Transformations of alkynones into N-containing heterocycles.

Fig. 1 The ORTEP diagram of compound 4a.

Scheme 6. Application of the protocol to rhodium(I) catalyzed synthesis of cyclopentenones.

A plausible mechanism for the reduction of CO2 to CO and

its consumption in a palladium-catalyzed aminocarbonylation is

illustrated in Scheme 7. The nucleophilic interaction between

fluoride anion and triphenylsilane leads to the hypervalent silicon

intermediate A, which makes the insertion of CO2 into the

polarized Si-H bond more easily to form the corresponding silyl

formate C.[43]

C is thermosensitive to give CO gas and silanol.[44]

Then, a carbonylative reaction with CO ex situ generated occurs

to give the carbonyl compounds. Firstly, intermediate D

generates via the oxidative addition of Pd(0) and iodobenzene.

Subsequently, the evolved CO gas from C inserts into Pd-C

bond in D to form the intermediate E. Subsequently, nucleophilic

attack by amine or alkyne in the presence of base produces the

amide and alkynone, respectively.

In summary, we have demonstrated the feasibility of

combining the two process i.e. reduction of CO2 to CO using

hydrosilane as reductant and transition metal catalyzed

carbonylation in a connected two chamber reactor, providing

amides, alkynones and bicyclic cyclopentenones in nice yields.

Furthermore, the thus formed alkynones can also act as starting

materials for the synthesis of a series of heterocycles e.g.

pyrazoles, 3a-hydroxyisoxazolo[3,2-a]isoindol-8-(3aH)-one

derivatives and pyrimidines in moderate yields. This protocol is

practical, CO gas free, which amplifies the scope of CO2

transformation.

Scheme 7. Possible mechanism for the ex situ reduction of CO2 to CO and

subsequent carbonylative reaction.

Experimental Section

General procedure for the two chamber carbonylative

reactions

Take the synthesis of 2a under optimized conditions as an

example, in the CO producing chamber, Ph3SiH (0.5208 g, 2

mmol) and CsF (0.030 g, 10 mol%, 0.2 mmol) were added. Then

this chamber was connected with another chamber, namely CO

consuming chamber, which is charged with Pd(dba)2 (5 mol%,

0.0229 g), PPh3 (10 mol%, 0.0131 g), p-iodoanisole (0.117 g,

0.50 mmol). Subsequently, the whole setup was charged with

CO2 with Schlenk technique. Afterwards, DMF (1 mL) was

added to the CO producing chamber, and cyclohexylamine

(0.150 g, 1.5 mmol), Et3N (0.101 g, 1.0 mmol, 0.14 mL), dioxane

(2.0 mL) were added to another chamber. The two chambers

were conducted under 130 oC and 80

oC for 24 h, respectively.

After the reaction, 10 mg diphenyl was used as internal standard

for GC yield determination. To obtain pure products, 20 mL

water was added to the reaction mixture of CO consuming

chamber, extracted with ethyl acetate. Then the organic layers

were washed with saturated NaCl solution, dried with anhydrous

Na2SO4, column chromatography on silica gel (petroleum

ether/ethyl acetate = 3/1) was conducted. Similar procedure was

performed in the carbonylative Sonogashira coupling of aryl

iodides, rhodium(I)-catalyzed Pauson-Khand-type reaction. For

the characterization data of the products, see supporting

information.

Acknowledgements

This work was financially supported by the National Program on

Key Research Project (2016YFA0602900), National Natural

Science Foundation of China (21472103, 21421001, 21672119),

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the Natural Science Foundation of Tianjin Municipality

(16JCZDJC39900).

Keywords: amides • carbon dioxide fixation • homogeneous

catalysis • reduction • synthetic methods

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[44] The existence of CO in the resultant gas sample was confirmed by:

CO + PdCl2 + H2O = CO2 + Pd + 2HCl

In a 100 mL baker, PdCl2 (100 mg, 0.56 mmol) and anhydrous CuCl2 (150

mg, 1.11 mmol) were added. Subsequently, water (20 mL) and SiO2

(chromatographic grade, 35-75 µm) were added to form the solidified

yellow mixture. In order to detect the CO gas, the gas sample which is

harvested from the reaction tube was added via syringe to the

microcentrifuge tube containing approximately 100 mg of the yellow solid.

The yellow solid instantaneously turned black, demonstrating the

presence of CO gas in the sample.

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COMMUNICATION

COMMUNICATION

Ex situ reduction of CO2 to CO for carbonylations: CO2 can be efficiently

reduced by triphenysilane to CO in a sealed two-chamber reactor, which subsequently

runs diverse carbonylation reactions to afford amides, alkynones and bicyclic

cyclopentenones. Furthermore, the thus formed alkynones can successfully be used as

scaffold for the synthesis of heterocycles e.g. pyrazoles and pyrimidine derivatives.

X.-D. Lang, L.-N. He*

Page No. – Page No.

Integration of CO2 Reduction with

Subsequent Carbonylation: Towards

Extending Chemical Utilization of CO2

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