Assessment of the Intermediacy of Arylpalladium Carboxylate Complexes in the Direct Arylation of Benzene: Evidence for C-H Bond Cleavage by Ligandless Species

摘要

Palladium-catalyzed direct arylations of benzene have been proposed to occur by the generation of a phosphine-ligated arylpalladium pivalate complex LPd(Ar)(OPiv) and reaction of this complex with benzene. We have isolated an example of the proposed intermediate and evaluated whether this complex reacts with benzene to form the biaryl products of the catalytic process. In contrast to the proposed mechanism, no biaryl product was formed from cleavage of the benzene C-H bond by LPd(Ar)(OPiv). However, reactions of LPd(Ar)OPiv with benzene and additives that displace or consume the phosphine ligand formed the arylated products in good yield, suggesting that a “ligandless” arylpalladium(II) carboxylate complexes undergoes the C-H cleavage step. Consistent with this conclusion, we found that reactions catalyzed by Pd(OAc)2 without ligand occur faster than, and with comparable selectivities to, reactions catalyzed by Pd(OAc)2 and phosphine ligand.