Novel diphosphine-modified palladium catalysts for the oxidative carbonylation of styrene to methyl cinnamate

The oxidative carbonylation of styrene has been achieved in MeOH with known and new Pd(II) catalysts of general formulae [(P-P)Pd(MeCN)2](PF6)2, [(P-P)Pd(bipy)](PF6)2, (P-P)Pd(OAc)2, [(P2-P2)Pd2(MeCN)4](PF6)4, [(P2-P2)Pd2(bipy)2](PF6)4, (P2-P2)Pd2(OAc)4 (P-P = 1,2-bis(diphenylphosphino)ethane), meso-2,3-bis(diphenylphosphino)butane, rac-2,3-bis(diphenylphosphino)butane; P2-P2 = 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 to carbonylated products and on the product selectivity has been studied by systematically varying the type of the Pd initiator, the concentrations of organic oxidant (1,4-benzoquinone) and protic acid (p-toluenesulfonic acid), and the CO pressure. This investigation has allowed us to bring about a partial optimisation of the process so as to obtain the largely predominant formation of either methyl cinnamate (highest selectivity 99%) or dimethyl phenylsuccinate (highest selectivity 88%).