Copper(I)-based ionic liquid-catalyzed carboxylation of terminal alkynes with CO2 at atmospheric pressure

Tetrahedron Letters xxx (2015) xxx–xxx

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Copper(I)-based ionic liquid-catalyzed carboxylation of terminal alkynes with CO2 at atmospheric pressure Jia-Ning Xie a, Bing Yu b, Zhi-Hua Zhou a, Hong-Chen Fu a, Ning Wang a, Liang-Nian He a,⇑ a b

State Key Laboratory and Institute of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China Key Laboratory of Cluster Science, Ministry of Education School of Chemistry, Beijing Institute of Technology, 100081 Beijing, China

a r t i c l e

i n f o

Article history: Received 26 September 2015 Revised 4 November 2015 Accepted 9 November 2015 Available online xxxx Keywords: Copper(I)-based ionic liquids Terminal alkynes CO2 Carboxylation Homogeneous catalyst

a b s t r a c t An ionic liquid containing copper(I) proved to be an effective homogeneous catalyst for the carboxylation of terminal alkynes with ambient CO2. This developed procedure needs no external ligands and terminal alkynes with various groups proceeded smoothly at atmospheric CO2 pressure and room temperature. Interestingly, the ILs containing copper(I) in both the anion and the cation showed much higher activity in comparison with the counterparts incorporating copper(I) solely in the form of halocuprate, that is, copper(I) in the anion. Especially, activated effect of the terminal alkyne by the ionic liquid was also validated by the NMR technique. Ó 2015 Elsevier Ltd. All rights reserved.

Introduction CO2 is the most abundant greenhouse gas due to a constantly growing global demand for energy.1–3 As a consequence, the utilization of CO2 to energy-related products and commodity chemicals becomes a fascinating tactic, and meets the requirement for providing an alternative carbon source to chemical industry, which primarily relies on mineral oil, natural gas, and coal.4–6 Much importantly, chemical conversion of CO2 to value-added chemicals can minimize the need for consuming toxic and sensitive carbon sources such as carbon monoxide, phosgene, and carbon tetrachloride in view of organic synthesis.7,8 However, to overcome the thermodynamic stability and kinetic inertness of CO2, efficient catalyst is inescapable to ensure the conversion smoothly.9–12 Ionic liquids (ILs) are receiving growing notice for their tunability of chemical and physical properties by regulating the practicability of anion–cation combinations, provides a wealth of opportunities to apply ionic liquids to various fields13–17 such as organic chemistry, material science, electrochemistry, photochemistry and so on. Metal-containing ionic liquids, as a series of novel functionalized materials, have a built-in coordinating unit like a nitrile group which can bind to the metal ion, thus have advantages over metal salts in terms of solubility, moisture sensitivity, and catalytic efficiency, presumably originating from the incorpo⇑ Corresponding author. Tel./fax: +86 22 23503878. E-mail address: [email protected] (L.-N. He).

ration of metals into the cation or anion.18–20 Therefore, metal-containing ILs are generally regarded as catalytic ionic liquids (CILs), have been successfully employed as an effective catalytic agent to stimulate several reactions such as Friedel–Crafts acylation,21 cycloaddition reaction,22 Kharasch reaction23 and so on. However, metal-containing ionic liquids catalyzed carboxylation reaction with CO2 under mild conditions has not been reported yet to date. Traditional protocols to carboxylic acids usually employ strong oxidants or toxic cyanides to perform smoothly. Hence, from the viewpoint of atom economy and synthetic efficiency, the synthesis of carboxylic acids and derivatives using CO2 as the carboxylative reagent is an attractive approach. In recent years, the development of catalytic C H carboxylation of terminal alkynes with CO2 has attracted much attention because of its fundamental scientific appeal and promising application in organic synthesis. To date, several effective carboxylation of terminal alkynes strategies have been developed, promoted by Ag(I)/Cu(I) catalysis.24 In particular, copper(I) salts have been widely employed as C–C triple-bond activators because of the economical applicability.25,26 Since the pioneering catalytic system (Cu or Ag) reported by Inoue in 1994,27 the prominent copper-catalyst systems were reported by Lu (N-heterocyclic carbene copper(I))(Cu/NHC),28 Gooßen (phenanthroline phosphine copper(I) nitrate),29 Zhang (Cu/ NHC),30 and Kondo (Cu/PEt3),31 respectively. In these Cu(I) systems, ligands such as NHC, diamine, and phosphine are used to promote this transformation. Inspired by the preceding contribution, a greener process in which ethylene carbonate acted as not

http://dx.doi.org/10.1016/j.tetlet.2015.11.028 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

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J.-N. Xie et al. / Tetrahedron Letters xxx (2015) xxx–xxx

only the solvent but also the ligand was developed.32 Very recently, we also introduced the heterogeneous catalyst (Cu(I)@C) to the carboxylation of terminal alkynes with identified mechanism.33 In the framework of our continuous efforts on C–C bond construction with CO2, we speculated that the utilization of alternative type of Cu(I) complex (copper(I)-based ILs) as catalyst would lead to an effective procedure for direct carboxylation of terminal alkynes with atmospheric CO2 pressure at room temperature (Scheme 1), thanks to the good solubility and long alkyl chain-containing iminazole ligand. In this aspect, several copper(I)-based ILs (Scheme 2)34,35 were found to be practicable to promote the carboxylation of terminal alkynes with CO2. Among them, [Cu (Im12)2][CuBr2] (Im12 = 1-dodecylimidazole) proved to be the most efficient catalyst. This process features no external ligands, facile preparation of the catalyst with good solubility, high catalytic activity, and stability. Results and discussion For exploratory studies, we used phenylacetylene (1a) as the model substrate, N,N-dimethylformamide (DMF) as the solvent in the presence of copper catalyst (10% based on Cu), n-butyl iodide (1.2 equiv), and Cs2CO3 (1.2 equiv) with atmospheric pressure of CO2. In the control experiment, only 4% yield of 2a was obtained, suggesting the catalyst was essential to the carboxylation of terminal alkynes at room temperature (Table 1, entry 1). Subsequently, several commercial copper salts (CuCl, CuBr, CuI, and copper(I)thiophene-2-carboxylate (CuTC) were investigated. However, most of the tested copper compounds exhibited poor to moderate activity in yield of 42–74% without any additive (entries 2–5). In particular, the privileged N-heterocyclic carbene (NHC) copper(I) (IPrCuCl) only afforded 9% yield (entry 6). As depicted in Scheme 2, four types of copper(I)-containing ILs were easily prepared from commercially available starting materials and proved to be identical with authentic material reported in the literature.34,35 The catalytic performance of various ILs was evaluated in DMF/CO2 under the otherwise identical conditions. Interestingly, the ILs incorporating copper solely in form of halocuprate, that is, copper(I) in the anion (IL-1 and IL-2) gave a lower activity (entry 7 and 8); whereas, the ILs containing copper(I) in both the anion and the cation (IL-3 and IL-4), showed excellent activity, affording 94% and 99% yield of 2a, respectively (entries 9 and 10), presumably suggesting that the catalyst activity is remarkably affected by centrality copper(I) in the cation, originating from the coordination with 1-dodecylimidazole. As a result, copper(I)-based IL 4 ([Cu(Im12)2][CuBr2]) was confirmed to be the ideal catalyst (entry 10). Furthermore, CuBr with alternative ligands such as 1-dodecylimidazole or triphenylphosphine could also mediate the carboxylation of 1a with 81% and 88% yields (entries 11 and 12). We then examined the influence of the catalyst loading and reaction time, respectively. Catalyst loading could not be decreased to 5% because of noticeable decline in the yield (entries 13 and 14). When the reaction time was decreased to 9 h and 6 h, only 53% and 46% yield of 2a were observed, respectively (entries 15 and 16). As a result, the catalytic carboxylation proceeded

H R1

O +

CO2 (balloon)

+

n

BuI

Cu-IL RT

R

1

O n Bu

Scheme 1. Copper(I)-based ionic liquids-promoted carboxylation of terminal alkynes.

N N

N

Cu4 Cl5

N

11

CuCl2

Ph IL-1

IL-2

[Cu(Im12) 2 ][CuCl2 ]

[Cu(Im12) 2 ][CuBr2 ]

IL-3

IL-4

Scheme 2. Copper(I)-containing ionic liquids used in this study.

Table 1 Optimization of the reaction conditions for the Cu-IL catalyzed carboxylation of terminal alkynes a

H + CO2

Ph 1a

+

n

BuI

(balloon)

Cat., Cs2 CO3 DMF, RT, t

O Ph 2a

On Bu

Entry

Cat.

Cat. (mol %)d

t (h)

Yieldb (%)

1 2 3 4 5 6 7 8 9 10 11c 12c 13 14 15 16

— CuCl CuBr CuI CuTC IPrCuCl IL-1 IL-2 IL-3 IL-4 CuBr + Im12 CuBr + PPh3 IL-3 IL-4 IL-4 IL-4

— 10 10 10 10 10 10 10 10 10 10 10 5 5 10 10

12 12 12 12 12 12 12 12 12 12 12 12 12 12 9 6

4 42 74 53 46 9 20 53 94 99 81 88 43 43 53 46

a Reactions were performed with phenylacetylene 1a (0.0511 g, 0.5 mmol), Cs2CO3 (0.1955 g, 0.6 mmol), catalyst, nBuI (0.1104 g, 0.6 mmol), DMF (2.5 mL), CO2 (99.999%, balloon) at room temperature. b Determined by gas chromatography (GC) with biphenyl as the internal standard. c Physical mixture of CuBr (0.0072 g, 0.05 mmol) and ligand (0.05 mmol). d Based on Cu content.

efficiently with [Cu(Im12)2][CuBr2] (10 mol%) as the catalyst, Cs2CO3 as the base, and DMF as the solvent at room temperature for 12 h. With the optimized conditions in hand, the generality of this process was also examined (Table 2). Phenylacetylenes substituted with electro-rich groups at meta- or para-positions (e.g. Et-, Me-, Pentyl-, and MeO-), could be converted to the corresponding alkynoates 2a–2e in 88–96% yields (Table 2, entries 1–5). In addition, the substrate with weak electron-withdrawing group (e.g. bromo-) could also give 85% yield of 2f (entry 6). Good results were also achieved for the aliphatic alkyne, for example, 1-octyne, cyclopropyl acetylene, under standard conditions (entries 8 and 13). In general, the presence of a strong electron-withdrawing group could decrease the yield dramatically. This poor efficiency may be due to the dropped nucleophilicity of C–Cu bond. Accordingly, when raising the reaction temperature to 40 °C, terminal aromatic alkynes with electron-withdrawing group (F-, Cl-) and 3-ethynylpyridine were successfully converted into the related carboxylic esters 2i–2l in good to excellent yields (78–94%) with CO2 under mild reaction conditions (entries 9–12).

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J.-N. Xie et al. / Tetrahedron Letters xxx (2015) xxx–xxx Table 2 Cu-IL catalyzed carboxylation of various terminal alkynesa

H + CO2

R

n

BuI

(balloon)

1

Entry

+

RC„CH Ph

1

Ph

RT,12 h

2

On Bu

Yield (%)b

Product H 1a

Ph

Et

2

O

Cat. IL-4

CO2nBu 2a

Et

H

96

CO2nBu

1b

3

2c n

90

CO2nBu

H

4

2c

C5H11

n

H

CO2n Bu

C5H11

2d

1d

5

MeO

6

Br

CO2nBu

MeO

H

2e

1e H

Br

CO2nBu

Me

CO2nBu

1f

H

Me

7

1g n

8

C6H13

H

C6H13

CO2nBu 2h

F

N

1j H

Cl

11

85 70

61(80)

2i

COO Bu CO2nBu

Cl

Cl

35(78) 2j

1k

2k

64(93)

Cl

12

2l

1l

H

28(94)

CO2nBu

H

13

88

n

H

N

86

86

CO2nBu

1i

10

2f

2g n

H 1h

F

9

91

2b

Me

Me

n CO2 Bu

1m

14c

Ph

H 1n

Ph

CO2Et

15d

Ph

H 1o

Ph

CO2Me

2m

2n 2o

97 89 82

a Reaction conditions: Substrate (0.5 mmol), IL-4 [Cu(Im12)2][CuBr2] (0.019 g, 0.025 mmol, 10% based on Cu content), Cs2CO3 (0.1955 g, 0.6 mmol), nBuI (0.1104 g, 0.6 mmol), DMF (2.5 mL), CO2(99.999%, balloon), 25 °C, 12 h. b Isolated yield. Yields in parentheses correspond to the reaction operated at 40 °C. c EtBr (0.0654 g, 0.6 mmol) as esterification reagent. d MeI (0.0852 g, 0.6 mmol) as esterification reagent.

a

b

c

Figure 1. 13C NMR spectra of 1a (a); a mixture of 1a (0.0051 g, 0.05 mmol) with 1dodecyl iminazole (b) (0.0118 g, 0.05 mmol); a mixture of 1a (0.0051 g, 0.05 mmol) with ([Cu(Im12)2][CuBr2]) (0.019 g, 0.025 mmol), (DMSO-d6, 0.6 mL, 298 K) (c).

Other organic halides including MeI, EtBr were also employed in this procedure (entries 14 and 15). By using MeI, EtBr as reagents, good results were observed.

As a result, the copper(I)-based ionic liquid plays an important role in the carboxylation of various alkynes. To deeply explore the reaction mechanism, 13C NMR technique was employed to detect possible interaction of substrate 1a with the catalyst, 1-dodecyl imidazole, respectively. As illustrated in Figure 1a and c, the signal assigned to the C–C triple bond shifted from d = 83.4 and 78.6 ppm to 81.7 and 70.3 ppm, probably being ascribed to the strong interaction between [Cu(Im12)2][CuBr2] and 1a, leading to the phenylacetylene activation. Moreover, there is no peak change in Figure 1b in comparison with Figure 1a, indicating the promoting effect is from copper, rather than the ligand. On the basis of the NMR and experimental results, the possible reaction mechanism, similar to that proposed by Inoue27 is shown in Scheme 3. The terminal alkyne is first activated by copper, followed by copper acetylide generated with the aid of cesium carbonate. Subsequently, the insertion of CO2 into the sp-hybridized Cu–C bond forms the copper propynoate intermediate. Then the product ester is esterified using iodoalkane and copper catalyst is regenerated. What’s more, we speculated that ligand (Im12) in this study could further promote catalytic efficiency, thereby render the reaction run at mild conditions smoothly.

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J.-N. Xie et al. / Tetrahedron Letters xxx (2015) xxx–xxx

O

R

n

n

O Bu

R

IL-Cu(I)

H

BuI

Higher Education (20130031110013), the MOE Innovation Team (IRT13022) of China, and the ‘111’ Project of Ministry of Education of China (project No. B06005) for financial support. Supplementary data

O OCu

R

H Cu(I)

R

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.11. 028. References and notes

CO2 R

Cu

Cs2CO3

Scheme 3. The proposed mechanism for the IL-Cu(I) promoted carboxylation of terminal alkynes with CO2.

Conclusions In summary, we have developed the carboxylation of terminal alkynes with ambient CO2 using an alternative type of ILs containing copper(I) ([Cu(Im12)2][CuBr2]) to afford alkyl 2-alkynoates at room temperature. A range of terminal alkynes could undergo the coupling reaction smoothly with atmospheric CO2 pressure at room temperature. Profited by its good solubility, stability, long alkyl chain-containing iminazole ligand, [Cu(Im12)2][CuBr2] and terminal alkynes have an discernible synergistic effect on promoting the reactions. We believe that this kind of highly-efficient copper-based ionic liquid catalytic system has great potential of applications.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19.

20.

Experimental section General procedure for the copper(I)-based IL-catalyzed carboxylation of terminal alkynes: In a 10 mL Schlenk flask, the terminal alkyne (0.5 mmol), [Cu(Im12)2][CuBr2] (0.019 g, 0.025 mmol), Cs2CO3 (0.1955 g, 0.6 mmol), nBuI (0.1104 g, 0.6 mmol) and DMF (2.5 mL) were added. The flask was sealed. Then gas exchanging process was operated by the freeze–pump–thaw method. The reaction mixture was stirred at room temperature (proceeded at 40 °C for substrate i–l) for 12 h under an atmosphere of CO2 (99.999%, balloon). Water was added to the resulting mixture, which was extracted with acetic ether until no product was detected. The combined organic layer was washed with saturated aqueous NaCl and then dried over anhydrous Na2SO4, then the solvent was removed under vacuum. The crude product was purified by column chromatography on silica gel (200–300 mesh, eluting with 6:1 to 3:1 petroleum ether/ethyl acetate) to afford the desired product. The products were further identified by NMR and MS.

21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

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Acknowledgements We are grateful to the National Natural Sciences Foundation of China, the Specialized Research Fund for the Doctoral Program of

Please cite this article in press as: Xie, J.-N.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.11.028