1,3 bis(4 methylbenzyl)imidazol 2 ylidene silver(i) chloride catalyzed carboxylative coupling of terminal alkynes, butyl iodide and carbon dioxide

1,3 Bis(4 methylbenzyl)imidazol 2 ylidene silver(I) chloride catalyzed carboxylative coupling of terminal alkynes, butyl iodide and carbon dioxide Accepted Manuscript Original article 1,3 Bis(4 methyl[.]

Original article

1,3-Bis(4-methylbenzyl)imidazol-2-ylidene silver(I) chloride catalyzed

carbox-ylative coupling of terminal alkynes, butyl iodide and carbon dioxide

Zhi-Zhi Zhang, Rui-Jie Mi, Fang-Jie Guo, Jing Sun, Ming-Dong Zhou,

Xiang-Chen Fang

DOI: http://dx.doi.org/10.1016/j.jscs.2017.02.001

Please cite this article as: Z-Z Zhang, R-J Mi, F-J Guo, J Sun, M-D Zhou, X-C Fang, 1,3-Bis(4-methylbenzyl)imidazol-2-ylidene silver(I) chloride catalyzed carboxylative coupling of terminal alkynes, butyl

2017.02.001

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1,3-Bis(4-methylbenzyl)imidazol-2-ylidene silver(І) chloride catalyzed carboxylative coupling of terminal alkynes, butyl iodide and carbon dioxide

Xiang-Chen Fang1,3*

and Technology, Shanghai 200237, China Email: fxc@ecust.edu.cn

Road 1, Fushun 113001, China E-mail: mingdong.zhou@lnpu.edu.cn

113001, China

Abstract

The N-heterocyclic carbene silver(I) complex 1,3-bis(4-methylbenzyl)imidazol-2-ylidene silver chloride was applied as the effective catalyst for the three-component carboxylative coupling of terminal alkynes, butyl iodide and carbon dioxide The reaction proved to be highly efficient when using 2 mol% of 1,3-bis(4-methylbenzyl)imidazol-2-ylidene silver(І) chloride as the catalyst in the presence of

substituent-loading capability, in which various functionalized 2-alkynoates were obtained in good yields under very mild conditions

Keywords: Carboxylative coupling; 2-Alkynoates; Carbon dioxide; N-heterocyclic carbene silver (I) complex

1 Introduction

one of perspective research field in terms of sustainable chemistry In the last decade, great efforts have been made both in academia and chemical industry [1-4] Some

alkynes with CO2 and so forth [5-7] Among various catalytic transformations, the carboxylation of alkynes with CO2 to produce functionalized propiolic acids or 2-alkynoates has been received considerable attentions owing to the importance of

propiolic acids and 2-alkynoates in organic synthesis [8-11] Nolan [12,13], Grooβen [14-16], Zhang [17-19], Lu [20-22], He [23-26], and other groups [27,28] have made significant efforts in this rapidly emerging field Generally, such a transformation can

be smoothly proceeded by using copper(I) or silver(I) as the catalyst in the presence

of a strong base such as Cs2CO3 or K2CO3 under mild conditions Comparing to copper(I), silver(I) seems to be more advantageous as it is more stable and active Moreover, the catalyst loading can also be highly reduced when using silver(I) instead

of copper(I) On the other hand, it has been found that the involvement of N-heterocyclic carbene (NHC) ligands to the catalytic carboxylation system can widen

via the formation of an intermediate CO2 adduct [29-32] Nevertheless, the study concerning Ag(I) - or Cu(I)-NHC catalyzed carboxylation of terminal alkynes with

have been prepared in our laboratory and they have been successfully applied as the catalyst for the carboxylation of aryl / alkyl terminal alkynes with CO2 to afford various functionalized propiolic acids [33] Previous studies indicated that 1,3-bis(4-methylbenzyl)imidazol-2-ylidene silver(І) chloride is one of the most effective catalyst among various examined Ag-NHC complexes In continuation our study on

1,3-bis(4-methylbenzyl)imidazol-2-ylidene silver(І) chloride also displayed excellent catalytic activity towards this transformation Moreover, the reaction could be also proceeded under rather mild conditions (40ºC, 1atm) Therefore, we wish to report our

findings of this work herein

2 Experimental Section

2.1 General Remarks

All manipulations were performed using standard Schlenk techniques under a dry

IR spectra were recorded on a Spectrum GX FT-IR spectrometer HRMS (ESI) determinations were carried out on a Bruker Daltonics McriOTOF II mass spectrometer The spectra were collected from 55 to 600m/z at an acquisition rate of 1- 2 s per scan The product (10 mg) was dissolved in acetonitrile (200 mL) The

samples were injected at a flow rate of 1.1 mL·min-1 of 80 % acetonitrile + 0.1% HCOOH for the MS detection under the basic conditions The cesium carbonate was dried for 12 h in vacuo at 120 °C prior to use CO2 (99.999%) was dried by 4Å molecular sieves before use Solvents before used were dehydration under standard

used as received

2.2 General procedure for the carboxylative coupling reaction

A 50 mL oven dried Schlenk flask was charged with Ag-NHC (8.4 mg, 0.01 mmol),

mL dry DMF Then, the reaction mixture was stirred at 40oC for 48 h under an atmosphere of CO2 (99.999%, balloon) After the reaction, water was added to the mixture, then extracted with diethyl ether (3×10 mL) The combined organic layer

filtered, and finally concentrated in vacuo The pure products were obtained by column chromatography (ethyl acetate/petroleum ether=1:50)

Butyl 3-phenylpropiolate (2a) [25]

(m, 2H), 4.23 (t, J=6.5 Hz, 2H), 1.72-1.64 (m, 2H), 1.47-1.37 (m, 2H), 0.95 (t, J=7.5

Butyl 3-(p-tolyl)propiolate (2b) [25]

4.23 (t, J=6.5 Hz, 2H), 2.37 (s, 3H), 1.73-1.66 (m, 2H), 1.48-1.40 (m, 2H), 0.96 (t,

Butyl 3-(4-Propylphenyl)propiolate (2c)

4.23 (t, J=6.5 Hz, 2H), 2.60 (t, J=7.5 Hz, 2H), 1.72-1.60 (m, 4H), 1.46-1.40 (m, 2H),

Butyl 3-([1,1'-biphenyl]-4-yl)propiolate (2d)

7.47-7.41 (m, 2H), 7.40-7.34 (m, 1H), 4.25 (t, J=6.5 Hz, 2 H), 1.74-1.66 (m,

154.19, 139.72, 133.41, 128.90, 128.07, 127.14, 127.05, 118.32, 86.03, 81.28, 65.90,

(ESI, m/z) calcd for C19H19O2 [M+H]+: 279.1380, found: 279.1385

Butyl 3-(4-methoxyphenyl)propiolate (2e) [25]

4.23 (t, J=6.5 Hz, 2H), 3.83 (s, 3H), 1.72-1.66 (m, 2H),1.46-1.40 (m, 2H), 0.96 (t,

Butyl 3-(4-fluorophenyl)propiolate (2f) [25]

(KBr) 2959, 2216, 1709, 1601, 1597, 1509, 1471

Butyl 3-(3-fluorophenyl)propiolate (2g)

7.19-7.12 (m, 1H), 4.25 (t, J=6.5 Hz, 2H), 1.74-1.66 (m, 2H), 1.49-1.39 (m, 2H), 0.96 (t,

130.26 (d, J=9.1 Hz), 128.78 (d, J=3.6 Hz), 121.48, 119.57, (d, J=23.5 Hz), 118.02 (d,

Butyl 3-(4-chlorophenyl)propiolate (2h) [25]

4.24 (t, J=6.5 Hz, 2H), 1.73-1.65 (m, 2H), 1.48-1.38 (m, 2H), 0.96 (t, J=7.5 Hz, 3H);

Butyl 3-(4-cyanophenyl)propiolate (2i) [28]

1464

Butyl 3-(4-(trifluoromethyl)phenyl)propiolate (2j)

4.26 (t, J=6.5 Hz, 2H),1.73-1.67 (m, 2H), 1.48-1.41 (m, 2H), 0.97 (t, J=7.5 Hz, 3H);

(q, J=3.6 Hz), 123.49 (q, J=270.9 Hz), 123.48, 83.65, 82.28, 66.10, 30.36, 18.95,

C14H13F3O2 [M+H]+: 271.0940, found:271.0944

Butyl hept-2-ynoate (2k) [34]

1.70-1.61 (m, 2H), 1.60-1.55 (m, 2H), 1.48-1.37 (m, 4H), 0.97-0.90 (m, 6H); IR (cm-1) (KBr) 2962, 2236, 1715, 1470

Butyl non-2-ynoate (2l) [25]

3 Results and discussion

1,3-bis(4-methylbenzyl)imidazol-2-ylidene silver(І) chloride (Ag-NHC) was synthesized according to the published procedures [33] Scheme 1 represents the molecular structure of 1,3-bis(4-methylbenzyl)imidazol-2-ylidene silver(І) chloride (Ag-NHC) X-ray single crystal diffraction data indicates a dinuclear solid-state structure [33], however it exists as a monomer in the presence of polar solvents owing to the weak bridging Ag…Cl bond of the dimer [35, 36] The Ag-NHC complex was applied as the catalyst for the carboxylative

effective base (Table 1) The terminal alkyne 1-phenylethyne 1a was initially

examined as the substrate for reaction optimizations Blank experiments showed that the reaction could not proceed in the absence of the metal catalyst

or the base (Table 1, entries 1, 2) The reaction was firstly studied using different dry solvents under the condition of 2 mol% catalyst, 1.5 equiv of

accordance with most carboxylative coupling reactions reported in the literatures (entries 3-5) [23-26, 28] However, the reaction could not proceed in

reducing the catalyst loading from 2 to 1 mol%, the product yield was also significantly reduced (entry 8) Increasing the amount of catalyst to 3 mol%, the reaction only led to a similar yield as that of 2 mol% (entry 9) Moreover, the

10-12) Finally, the examined three-component carboxylative coupling reaction

led to a significant decrease of 2a yield, which may due to the low concentration of

Based on the above studies, the scope of this carboxylation reaction was then

Cs2CO3 and 2 mol% of Ag-NHC in DMF at 40oC for 48 h To our delight, the reaction proved to be applicable for terminal aryl alkynes bearing various functional groups on the phenyl ring Good to excellent 2-alkynoateyields were obtained for terminal aryl alkynes bearing electron-donating groups such as methyl, propyl phenyl

and methoxy groups (2b-2e) Halo (F or Cl) substituted terminal aryl alkynes also resulted in satisfactory yields of desired 2-alkynoates (2f-2h) Comparable good

substituted terminal aryl alkynes (2i, 2j) Finally, the reaction was also found to be applicable for linear alkyl terminal alkynes (2k, 2l) Therefore, this reaction shows

good substituent-loading capability Notably, all the reactions resulted in the

formation of 2 as the only isolated products, thus showing a good selectivity

Based on the literature precedents [24, 33, 37], a possible catalytic mechanism outlined in Scheme 2 is proposed The coordination of alkyne to Ag-NHC is assumed

to occur at first, thus leading to a more acidic alkyne The subsequent deprotonation

of alkyne by strong basic Cs2CO3 affords a CsCO3- ligated intermediate Such an

acetylide The following insertion of CO2 into Ag-C bond of the silver(I) acetylide affords a silver(I) propiolate intermediate Finally, silver(I) propiolate might interact with butyl iodide to afford the desired 2-alkynoate product, regenerating the Ag-NHC catalyst

4 Conclusions

The three-component carboxylative coupling of terminal alkynes, butyl iodide and carbon dioxide were successfully achieved using 1,3-bis(4-methylbenzyl)imidazol-2-ylidene silver(І) chloride as the catalyst and Cs2CO3 as the base in DMF under ambient temperature and atmospheric pressure of CO2 Good to excellent yields of various 2-alkynoates were afforded as the isolated products The possible reaction mechanism is also discussed

Acknowledgements

M.D.Z thanks the National Science Foundation of China (21101085), Natural Science Foundation of Liaoning Province (2015020196), Fushun Science & Technology Program (FSKJHT 201423), and Liaoning Excellent Talents Program in University (LJQ2012031) for the financial supports

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Table 1．Optimization of reaction conditions.a