Regiospecific metallation in palladium-hydrazone complexes

The dimethylhydrazone of pinacolone, ButMeC=N1N2Me2, reacts with [PdCl2(NCR)2] (R = Me or Ph) to give the complexes [PdCl2(ButMeC=NNMe2)2] and [{PdCl2(ButMeC=NNMe2)}2], in which the ligands co-ordinate preferentially through the aminic nitrogen, N2. Both species, in solution, carbopalladate regiospecifically on the Me group to give [{Pd[CH2C(=NNMe2)But]Cl}2]. In contrast, ButMeC=N1N2MePh reacts with palladium to give only the 1: 1 adduct [{PdCl2(ButMeC=NNMePh)}2] in which the ligand co-ordinates preferentially through the iminic nitrogen, N1. This complex is fairly stable in benzene or dichloromethane solution where it slowly decomposes to give only a small amount of the methylpalladated complex [{Pd[CH2(=NNMePh)But]Cl}2]; however, in methanol solution and in the presence of a base, NaO2CMe, it carbopalladates regiospecifically on the But group to give [{Pd[CH2C(CH3)2C(=NNMePh)Me]Cl}2]. The reason for different regioselectivity and the relative rate of carbopalladation on the Me and But group (for N2- and N1-co-ordinated hydrazones respectively) are discussed.