Catalytic behavior of cationic hydridoruthenium(II) complex, [RuH(NH ₃)(PMe₃)₄]⁺, in H ₂-hydrogenation and transfer hydrogenation of imines

Catalytic hydrogenation under H₂ and transfer hydrogenation of imines by secondary alcohols with hydridoruthenium complexes bearing PMe ₃ and PPh₃ ligands have been examined. A cationic hydridoruthenium complex, cis-[RuH-(NH₃)(PMe₃) ₄]PF₆ (2), derived from cis-[RuH₂(PMe ₃)₄] (1) and NH₄PF₆, showed higher catalytic activity for H₂-hydrogenation of N-benzylideneaniline than neutral complexes such as 1 and cis- [RuClH(PMe₃)₄] (6). The effectiveness of cationic hydridoruthenium species for the catalytic H ₂-hydrogenation of benzylideneaniline was also demonstrated by a marked increase in the yield of N-benzylaniline on treatment of 6 with AgPF ₆. The cationic complex 2 was applicable to catalytic transfer hydrogenation of imines with secondary alcohols even in the absence of a base. Isotope labeling experiments using deuterated alcohols revealed that the hydrogen atom bound to the α-carbon of the donor alcohol was transferred exclusively to the imine carbon and alcoholic OD was transferred to the imine nitrogen. A rapid exchange between the alcoholic proton and the hydrido ligand of 2 was also confirmed by NMR investigation using (CH₃) ₂CHOD. On the basis of the experimental results the mechanisms of the H₂-hydrogenation and transfer hydrogenation are discussed.