Oxidation of heteroleptic diarylpalladium compounds with tert-butyl hydroperoxide. Substituent effects in aromatic oxidation reactions

Abstract

A series of heteroleptic diarylpalladium compounds, containing both a naphthyl (1-C10H6CH2NMe2-2 or 1-C10H5CH2NMe2-2-Me-3) and a phenyl (1-C6H4CH2NMe2-2 or 1-C6H3CH2NMe2-2-Me-x, x = 3, 5, 6) monoanionic C,N-bidentate ligand, was reacted with tert-butyl hydroperoxide (TBHP) to give selective oxygen insertion into one of the C-Pd bonds. It was found that the order of reactivity of the palladium-carbon bonds decreases in the order 1-naphthyl-Pd > phenyl-Pd. Introduction of methyl substituents enabled us to fine-tune the regioselectivity of the oxidation reactions (i.e. the ratio of phenyl- versus naphthyl oxygenation was affected) but not to reverse the order. The methyl groups have a deactivating influence on the aromatic substrates that contain them, indicating an oxidatively induced nucleophilic pathway for the actual oxygenation step. The yields of mono-oxygenated product RPdOR' varied from 82 to 97% and were independent of the presence of a VO(acac)2 or [RhCl(cod)]2 catalyst, although the reaction rates were strongly enhanced in the presence of these catalysts.