MECHANISTIC INVESTIGATIONS OF PALLADIUM-CATALYZED SINGLE AND DOUBLE CARBONYLATION OF ARYL AND VINYL HALIDES BY METHYL FORMATE

The palladium-catalysed methoxycarbonylation of PhCH=CHBr in the presence of HCO2CH3, 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))(2)Br] (1), [(PhCH=CHCO)Pd(PPh(3))(2)Br] (2a), and [(PhCO)Pd(PPh(3))(2)I] (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 HCO2CH3 revealed the presence of the suggested aroylpalladium complexes (2a and 3) in conjunction with [PdCl(PPh(3))(2)(CO2CH3)] (4). Both stoichiometric and catalytic experiments indicated that complex 4 is a possible intermediate in the formation of both simple esters and cu-keto esters, but that the latter arise mainly from classical aroylpalladium(II) complexes. The activation pathway of HCO2CH3 has been studied and a dramatic influence oi CH3OH concentration in the reaction medium has been found. These results suggest that CH3OH arising from smooth decarbonylation of HCO2CH3 under the reaction conditions is the main alkoxy-transfer agent.