Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) The first efficient electrophilic carbonylation of ethane with carbon monoxide Alexander V. Orlinkov,* Irena S. Akhrem and Sergei V. Vitt A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 117813 Moscow, Russian Federation. Fax: +7 095 135 5085; e-mail: cmoc@ineos.ac.ru Ethane reacts with CO without solvent in the presence of polyhalomethane·2AlBr3 superelectrophilic systems to form EtCOOR after alcoholysis together with small amounts of corresponding esters of 1- and 2-bromopropanoic acids.Ethane is the second alkane to methane in both inertness and abundance in natural gas. Therefore, the development of direct methods for ethane functionalization is one of the most important problems in the chemistry of alkanes.Among the numerous examples of electrophilic carbonylation of saturated hydrocarbons (mainly by HX–AlCl3 at elevated temperature and pressure with protic superacids under ambient conditions), only a very limited number of selective reactions have been described (see refs. 1–6 and references therein). These are reactions of propane (in the presence of polyhalomethane- containing promotors)3 and carbonylation of C5–C6 7 cycloalkanes, all in HF–SbF5 media.The polyhalomethane-based superelectrophilic systems were found to be good initiators for selective carbonylation of propane,8 butane and pentane,9 as well as C5–C6 10 cycloalkanes in organic media. These reactions occur probably via the Koch–Haaf mechanism involving the generation of carbocations followed by CO trapping to give acylium cations.11 The electrophilic carbonylation of ethane has not been reported earlier.On the contrary, catalytic carbonylation of methane, ethane, propane (nonselective) and cyclohexane in CF3COOH in the presence of a Pd(OAc)2 + CuCl2 mixture and K2S2O8 as an oxidant was performed.12 These reactions were carried out at 80 °C for 20 h and required large amounts of K2S2O8 and CF3COOH.For example, 100 g of Pd(OAc)2, 150 g of Cu(OAc)2, 8 kg of K2S2O8 and 17 dm3 of trifluoroacetic acid are needed to obtain 1 kg of EtCOOH.12 We report the first example of the efficient electrophilic carbonylation of ethane with CO. At 50 °C, ethane reacts with CO in the presence of polyhalomethane–AlBr3 superelectrophilic systems to form EtCOOBu 1 after treatment of the reaction mixture with n-butanol (Scheme 1).† With CBr4·2AlBr3 superacid as a promoter, the yield of 1 is 86% on a superelectrophile basis after 2 h (Table 1).In addition, esters 2 and 3 of 1- and 2-bromopropanoic acids are formed as by-products. The overall yields of ethane carbonylation products 1–3 are close to quantitative values with respect to the superelectrophile.Interestingly, under similar conditions, 50–60% yields of 1 can be achieved using CCl4·3AlBr3 and CHCl3·3AlBr3, † General procedure. To form a homogeneous liquid system, a mixture of appropriate amounts of AlBr3 and a polyhalomethane was heated and stirred without solvent in a 50 ml stainless steel autoclave (Parr Instrument Co.) at 80 °C for 5 min.After cooling to 50 °C, ethane and CO were supplied to the autoclave, and the reaction mixture was heated and stirred at 50 °C. After completion of the reaction, n-butanol was added at room temperature. The resulting mixture was poured into water, extracted, washed, dried and analysed by GC and GC–MS. while AlBr3 in CH2Br2 is completely inactive.In the case of CHCl3·3AlBr3, the ethane carbonylation occurs with a very high selectivity, although the yield of 1 is lower. The yields of 1 in the presence of the CBr4·2AlBr3 system strongly depend on temperature and the CO/C2H6 ratio (m). Higher yields of 1 were obtained at 50 °C. On going from m = 1 to m = 2.2, the yields of 1 considerably increase, while they fall down at m = 3 (Table 1).The yields and selectivities of formation of 1 also decrease on going from the hardest CBr4·2AlBr3 system to the milder CCl4·3AlBr3 and CHCl3·3AlBr3 systems. The proposed scheme of the ethane carbonylation involves generation of the ethyl cation followed by CO trapping to form the EtCO+ cation and, finally, EtCOOR 1 (Scheme 2). An increase in both the superelectrophile strenght and the CO/C2H6 ratio (within a certain range) is favourable to the generation of the ethyl cation and its addition to a CO molecule. For elucidation of the mechanism of formation of bromine-containing products 2 and 3, a further study is required.Thus, the new superelectrophilic systems allowed us to perform the efficient one-pot functionalization of ethane, which is an inert alkane.This work was supported by the Russian Foundation for Basic Research (grant nos. 97-03-32986, 96-15-97341 and 99-03-30006) and the U.S. Civilian Research & Development Foundation (grant no. RC1-274). C2H6 + CO COOBu COOBu COOBu Br Br i, E, 50 °C, 2 h ii, BuOH 1 2 3 E = CBr4·2AlBr3, CCl4·3AlBr3, CHCl3·3AlBr3 Scheme 1 aThe reactions were carried out at 65 °C. Table 1 Carbonylation of ethane with CO initiated by polyhalomethanebased superelectrophiles (E) at 50 °C.Run E PCO PC2H6 CO:C2H6:E t/h Products (mol%) 1 2 3 1a CBr4·2AlBr3 20 20 1:1:0.05 3 35 6 6 2a CBr4·2AlBr3 45 20 2.2:1:0.05 3 66 10 4 3a CBr4·2AlBr3 48 16 3:1:0.06 3 58 8 3 4 CBr4·2AlBr3 15 10 1.5:1:0.2 1 20 7 1 5 CBr4·2AlBr3 45 20 2.2:1:0.05 1 37 4 1 6 CBr4·2AlBr3 45 20 2.2:1:0.05 2 86 6 traces 7 CBr4·2AlBr3 48 16 3:1:0.05 3 47 6 2 8 CBr4·3AlBr3 45 20 2.2:1:0.05 2 82 12 2 9 CBr4·AlBr3 45 20 2.2:1:0.1 2 17 22 2 10 CCl4·3AlBr3 45 20 2.2:1:0.04 2 61 8 2 11 CCl4·3AlBr3 42 18 2.3:1:0.06 2 46 18 7 12 CHCl3·3AlBr3 42 18 2.3:1:0.06 1 50 traces traces 13 CH2Br2·2AlBr3 45 20 2.2:1:0.05 2 traces 0 0 CBr4 CBr3 Al2Br7 2AlBr3 C2H6 + CBr3 Al2Br7 C2H5 Al2Br7 + CHBr3 CO EtCO 1 ROH Scheme 2Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) References 1 H.Bahrmann, in New Syntheses with Carbon Monoxide, ed. J. Falbe, Springer, Berlin, 1980, ch. 5. 2 G. A. Olah, O. Farooq and G. K. S. Prakash, in Activation and Functionalization of Alkanes, ed. C. L. Hill, Wiley–Interscience, New York, 1989, ch. 2. 3 J. Sommer and J.Bukala, Account Chem. Res., 1993, 26, 370. 4 H. Hogeveen, in Advances in Organic Chemistry, ed. V. Gold, Academic Press, London, 1973, vol. 10, p. 29. 5 Q. Xu and Y. Souma, Topics in Catalysis, 1998, 6, 17. 6 I. Akhrem, Topics in Catalysis, 1998, 6, 27. 7 R. Paatz and G. Weisgerber, Chem. Ber., 1967, 100, 984. 8 I. S. Akhrem, A. V. Orlinkov, L. V. Afanas’eva and M. E. Vol’pin, Izv. Akad. Nauk, Ser. Khim., 1996, 1214 (Russ. Chem. Bull., 1996, 45, 1154). 9 I. Akhrem, A. Orlinkov, L. Afanas’eva, P. Petrovskii and S. Vitt, Tetrahedron Lett., 1999, 40, 5897. 10 I. S. Akhrem, S. Z. Bernadyuk and M. E. Vol’pin, Mendeleev Commun., 1993, 188. 11 M. Koch and W.Haaf, Org. Synth., 1964, 40, 1. 12 Y. Fujiwa, K. Takaki and Y. Taniguchi, Synlett., 1996, 7, 591. Received: 17th May 1999; Com. 99/1489