Phosphine–Amido Complexes of Ruthenium and Mechanistic Implications for Ketone Transfer Hydrogenation Catalysis

The air-sensitive phosphine–anilido complexes [RuR(η6-p-cymene)(P,N-Ph2PAr–)] (R = H, Et; Ar– = o-C6H4NMe–) have been prepared. While the precursor [RuCl(η6-p-cymene)(P,N-Ph2PAr–)] is a moderately active ketone transfer hydrogenation catalyst under basic conditions, the hydrido derivative is much less active, ruling out the possibility of an inner-sphere mechanism during catalysis. The possibility of an alternate mechanism, related to the Meerwein–Ponndorf–Verley–Oppenauer (MPVO) pathway, is discussed. While attempts to isolate intermediate alkoxo derivatives demonstrate their propensity toward β-hydride elimination to afford the hydrido complex, the ethyl derivative is remarkably stable, even in refluxing benzene, providing an interesting contrast between the labilities of the Ru–O and Ru–C bonds.