Palladium hydrazonato complexes and their role in the Pd-catalyzed cross-coupling reactions of hydrazones as carbene precursors

Abstract

Hydrazones are common carbene precursors in many palladium-catalyzed cross coupling reactions of carbenes as a coupling partner, but their interaction with palladium has been overlooked. We have found that hydrazonato ligands readily coordinate to Pd aryl complexes leading to [PdAr(L–L){(TolSO₂)N–NCHR}] (Ar = Ph, C₆F₅; L–L = dppe, dppf; R = CHCHPh, Ph). Ligand substitution reactions on [PdAr(dppe)X] (X = Br, TolSO₂) show that the hydrazonato ligand coordinates preferentially so the hydrazonato complexes are likely resting states in catalytic carbene coupling reactions using hydrazones as reactants. The decomposition of the hydrazonato moiety to a diazoalkane is needed during the catalysis and the analysis of the evolution of the hydrazonato complexes shows that it is not promoted by coordination to the metal and it does not occur in the coordination sphere of palladium. The substitution of diazoalkane for the metal-bound hydrazonato is possible and the steps that follow to form a new C–C bond, including the carbene migratory insertion, are fast.