The metal-ligand bifunctional catalysis: A theoretical study on the ruthenium(II)-catalyzed hydrogen transfer between alcohols and carbonyl compounds

The ternary system consisting of [RuCl2(eta(6)-benzene)](2), N-tosylethylenediamine or ethanolamine, and KOH (Ru:amine:KOH = 1:1:2 molar ratio) catalyzes reversible hydrogen transfer between alcohols and carbonyl compounds. The use of chiral amine auxiliaries effects asymmetric transformation. The theoretical calculations using methanol/formaldehyde transformation as the model indicates the operation of a novel metal-ligand bifunctional catalysis, which is contrary to currently accepted putative pathways. The results reveal that: (1) KOH is necessary for the generation of a formal 16-electron Ru complex, Ru(NHCH2CH2Y)(eta(6)-benzene) (Y = O or NH) (catalyst), from an 18-electron Ru chloride, RuCl(NH2CH2CH2Y)(eta(6)-benzene) (precatalyst), by a Dcb elimination of HCl, and not for increasing alkoxide concentration; (2) Ru alkoxides do not intervene in transfer hydrogenation; (3) the Ru alkoxide, even if formed, serves merely as a reservoir of the 16-electron catalyst; (4) the key 18-electron Ru hydride, RuH(NH2CH2CH2Y)(eta(6)-benzene) (reducing intermediate), is generated by dehydrogenation of methanol with coordinatively unsaturated Ru(NH2CH2CH2Y)(eta(6)-benzene); (5) this process and reverse hydrogen delivery from RuH(NH2CH2CH2Y)(eta(6)-benzene) to formaldehyde take place by a pericyclic mechanism via a six-membered transition structure, (6) neither carbonyl oxygen not alcoholic oxygen interacts with Ru throughout the hydrogen transfer; (7) the carbonyl oxygen atom interacts with NH on Ru and the hydroxy function with the amido nitrogen via hydrogen bonding; (8) the Ru center and nitrogen ligand simultaneously participate in both forward and reverse steps of the hydrogenation transfer. The ethanolamine- and ethylenediamine-based complexes behave similarly. In the asymmetric transformation catalyzed by chiral Ru complexes, the stereochemical bias originates primarily from the chirality of the heteroatom-based five-membered chelate rings in the transition structure. The calculated mechanism explains a range of experimental observations including the ligand acceleration effect, the structural characteristics of the isolated Ru(II) complexes, the role of the NH or NH2 end of auxiliaries, the effect of a strong base cocatalyst, the kinetic profile, the reactivities of hydrogen donors and accepters, the C=O vs C=C chemoselectivity, and the origin of enantioselection. This metal-ligand bifunctional catalysis is in sharp contrast to many other metal-centered catalyses.