Novel diphosphine-modified palladiumcatalysts for oxidative carbonylation of styrene to methyl cinnamate

Abstract

The oxidative carbonylation of styrene has been achieved in MeOH with known and new palladium(II) catalysts of general formulae [Pd(P–P)(MeCN)₂][PF₆]₂, [Pd(P–P)(bipy)][PF₆]₂, Pd(P–P)(OAc)₂, [Pd₂(cyclo-tetraphos)(MeCN)₄][PF₆]₄, [Pd₂(cyclo-tetraphos)(bipy)₂][PF₆]₄, Pd₂(cyclo-tetraphos)(OAc)₄ (P–P = 1,2-bis(diphenylphosphino)ethane), meso-2,3-bis(diphenylphosphino)butane or rac-2,3-bis(diphenylphosphino)butane; cyclo-tetraphos = cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane; bipy = 2,2′-bipyridine; OAc = acetate). The influence of various catalytic parameters on the overall conversion of styrene into carbonylated products and on the product selectivity has been studied by systematically varying the type of palladium initiator, the concentrations of organic oxidant (1,4-benzoquinone) and protic acid (p-toluenesulfonic acid), and the CO pressure. This investigation has allowed a partial optimization of the process so as to obtain the largely predominant formation of either methyl cinnamate (highest selectivity 99%) or dimethyl phenylsuccinate (highest selectivity 88%).