Regiospecific metallation in palladium–hydrazone complexes

Abstract

The dimethylhydrazone of pinacolone, Bu^tMeCN¹N²Me₂, reacts with [PdCl₂(NCR)₂](R = Me or Ph) to give the complexes [PdCl₂(Bu^tMeCNNMe₂)₂] and [{PdCl₂(Bu^tMeCNNMe₂)}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 [{[graphic omitted]Me₂)Bu^t]Cl}₂]. In contrast, Bu^tMeCN¹N²MePh reacts with palladium to give only the 1 : 1 adduct [{PdCl₂(Bu^tMeCNNMePh)}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 [{[graphic omitted]MePh)Bu^t]Cl}₂]; however, in methanol solution and in the presence of a base, NaO₂CMe, it carbopalladates regiospecifically on the Bu^t group to give [{[graphic omitted]NMePh)Me]Cl}₂]. The reason for different regioselectivity and the relative rate of carbopalladation on the Me and Bu^t group (for N²- and N¹-co-ordinated hydrazones respectively) are discussed.