Intramolecular CN bond formation in the reductive elimination of ortho-palladated arylhydrazones of acetophenone

Carbonylation in dichloromethane at room temperature of the dimeric ortho-palladated complex [Pd{C6H4C(CH3)NNHC6H5}(μ-Cl)]2 yields in a first step the monocarbonyl [Pd{C6H4C(CH3)NNHC6H5}Cl(CO)] which very slowly (days) undergoes in solution reductive elimination of palladium to give high yield of the isoindolinone derivative, 2-anilino-3-methylene-isoindolin-1-one. Addition of the stoichiometric amount of NaOMe to the monocarbonyl complex [Pd{C6H4C(CH3)NNHC6H5}Cl(CO)] in solution leads instantaneously to reductive elimination and affords the indazole derivative, 3-methyl-1-phenyl-indazole, in high yield. Carbonylation of the related complex [Pd{C6H4C(CH3)NNHC6H4-p-NO2}(μ-Cl)]2 yields in solution the monocarbonyl [Pd{C6H4C(CH3)NNHC6H4-p-NO2}Cl(CO)] which in minutes affords high yield of the isoindolinone derivative, 2-(4-nitroanilino)-3-methylene-isoindolin-1-one. Such different behaviour sheds light on the mechanistic grounds that govern the intramolecular CN bond formation and the cyclization of ortho-palladated acetophenonearylhydrazones, and shows that the acidity of the NH bond controls the rate of the reactions. The requirement of the formation of a palladium–nitrogen amido bond cis to a palladium–carbon bond is supported for these results. We suggest that a tautomeric equilibrium of the hydrazonato ligand accounts for the two alternate pathways.