Studies relevant to palladium-catalyzed carbonylation processes. Mechanisms of formation of esters and amides from benzylpalladium and (Phenylacetyl)palladium complexes on reactions with alcohols and amines

Benzylpalladium and (phenylacetyl)palladium complexes having two PMe3 or PPh3 ligands or a dppe ligand (P2-type complexes) have been prepared as models to study the mechanisms of carbonylation reactions to convert benzyl halides into single and double carbonylation products. Removal of one of the tertiary phosphine ligands in the P2-type monophosphine complexes, trans-PdCl(COCH2Ph)(PR3)(2) (R = Ph, Me), by treatment with PdCl2(PhCN)(2) led to the P1-type chloride-bridged dimers [PdCl(COCH2Ph)(PR3)](2), which were split on interaction with secondary amines to give amine-coordinated complexes, PdCl(COCH2Ph)(PR3)(NHEt2). Examination of the reactions of the (phenylacetyl)palladium complexes with secondary amines and alcohols provided evidence for operation of different types of mechanisms for yielding amides and esters. The amide formation proceeds faster when more basic secondary amine is used. The process is proposed to proceed through coordination of the amine to palladium followed by its nucleophilic attack on the acyl group aided by a base. On the other hand, the ester formation from the (phenylacetyl)palladium complex proceeds more readily when more acidic alcohols were used. The formation of the ester is compatible with the reaction mechanism proceeding through an intermediate acyl-alkoxide complex. For the amide formation from the benzylpalladium complexes on reactions with secondary amines under CO pressure, operation of two routes has been revealed; one involves the attack of an amine on the GO-coordinated benzylpalladium species to give a benzyl(carbamoyl)palladium intermediate, and the other involves (phenylacetyl)(amine)palladium species, each giving the amide on reductive elimination or amine migration to the acyl ligand. Competition reactions using mixtures of secondary amines and alcohols toward the acylpalladium complexes furnished the supporting evidence for the reaction mechanisms to give esters involving the reductive elimination of phenylacetyl-alkoxide intermediates.