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 prese nce of HCO2CH3, NEt(3) and CO has been investigated and compared to th at of PhI. Methyl cinnamate was the only product observed. Model react ion 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(PP h(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 compl exes (2a and 3) in conjunction with [PdCl(PPh(3))(2)(CO2CH3)] (4). Bot h 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 aroylpal ladium(II) complexes. The activation pathway of HCO2CH3 has been studi ed and a dramatic influence oi CH3OH concentration in the reaction med ium has been found. These results suggest that CH3OH arising from smoo th decarbonylation of HCO2CH3 under the reaction conditions is the mai n alkoxy-transfer agent.