Coordination chemistry and mechanisms of metal-catalyzed CC-coupling reactions . Part 7. Heck vinylation of aryl halides with n-butyl acrylate: relevance of PC bond cleavage to catalyst deactivation

Abstract

The Heck reaction between aryl halides ArX (X = Br, Cl) and n-butyl acrylate was studied in the presence of catalyst systems containing Pd(OAc) 2 and a tertiary phosphine ligand (PR 3, R 2P(CH 2)PR 2). It is shown that all tested arylphosphines except P( o-Tol) 3 and P(Mes) 3 undergo an extensive PC bond cleavage at temperatures higher than 120°C. As a consequence, vinylic side products are formed with concomitant catalyst deactivation. In the case of triarylphosphines side products originate from reaction intermediates of the type Pd II(PAr 3) 2(Ar)X. These species undergo an aryl-aryl exchange between the palladium center and coordinated phosphine ligands. Subsequent intermolecular phosphine scrambling leads to several isomerized arylpalladium species, all of which couple with n-butyl acrylate to give the corresponding cinnamic ester side products.