Insertion of CO2 into a palladium allyl bond and a Pd(II) catalysed carboxylation of allyl stannanes

摘要

The complex 2,6-bis[(di-t-butylphosphino)methyl]phenyl allyl palladium ((PCPPd)-Pd-tBu-allyl, 3) reacts with CO2 in a very fast insertion reaction to give the corresponding butenoate complex. The reaction is thought to occur via a cyclic six-membered transition state (7), where the gamma-carbon of the allyl group is linked up with the CO2-carbon. A group of related PCP complexes were investigated as catalysts for the carboxylation of tributyl(allyl) stannane. A catalytic cycle is proposed for this reaction where the rate determining step is the transmetallation between tin and palladium. The carboxylation reaction is faster using less sterically crowded catalysts whereas the electron richness of the palladium complexes seems less important for reactivity. Thus, there was no apparent difference in reactivity between 2,6-bis[(di-phenylphosphino)methyl]phenyl palladium triflouroacetate (13) and resorcinolbis(diphenyl)phosphinite palladium triflouroacetate (10). Both of these complexes give high turnovers for the carboxylation of tributyl(allyl) stannane (80% in 16 h using a ca. 5% catalyst loading and 4 atm CO2 pressure). On the other hand complex 3 was inactive in the catalytic carboxylation reaction.