Mechanistic investigations of palladium-catalysed single and double carbonylation of aryl and vinyl halides by methyl formate

Abstract

The palladium-catalysed methoxycarbonylation of PhCHCHBr in the presence of HCO 2CH 3, NEt 3 and CO has been investigated and compared to that of PhI. Methyl cinnamate was the only product observed. Model reaction studies were conducted on the possible intermediates [(PhCHCH)Pd(PPh 3) 2Br] ( 1), [(PhCHCHCO)Pd(PPh 3) 2Br] ( 2a), and (PhCO)Pd(PPh 3) 2I] ( 3). The results suggest that the high alcoholysis rate of complex 2a is responsible for the lack of double carbonylation product from PhCHCHBr. NMR studies of the catalytic systems in the presence of HCO 2 CH 3 revealed the presence of the suggested aroylpalladium complexes ( 2a and 3) in conjunction with [PdCl(PPh 3) 2(CO 2CH 3)] ( 4). Both stoichiometric and catalytic experiments indicated that complex 4 is a possible intermediate in the formation of both simple esters and α-keto esters, but that the latter arise mainly from classical aroylpalladium (II) complexes. The activation pathway of HCO 2CH 3 has been studied and a dramatic influence of CH 3OH concentration in the reaction medium has been found. These results suggest that CH 3OH arising from smooth decarbonylation of HCO 2CH 3 under the reaction conditions is the main alkoxy-transfer agent.