Bis(allyl)-ruthenium(IV) complexes as highly efficient catalysts for the redox isomerization of allylic alcohols into carbonyl compounds in organic and aqueous media:: Scope, limitations, and theoretical analysis of the mechanism

The catalytic activity of the bis(allyl)-ruthenium(IV) dimer [{Ru(eta(3):eta(3)-C10H16)(mu-Cl)Cl}(2)] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) (1), and that of its mononuclear derivatives [Ru(eta(3):eta(3)-C10H16)Cl-2(L)] (L = CO, PR3, CNR, NCR) (2) and [Ru(eta(3):eta(3)-C20H16)Cl(NCMe)(2)][SbF6] (3), in the redox isomerization of allylic alcohols into carbonyl compounds, both in tetrahydrofuran and in water, is reported. In particular, a variety of allylic alcohols have been quantitatively isomerized using [{Ru(eta(3):eta(3)-C10H16)(mu-Cl)Cl}(2)] (1) as catalyst, the reactions proceeding in all cases faster in water. Remarkably, complex 1 has been found to be the most efficient catalyst reported to date for this particular transformation, leading to TOF and TON values up to 62 500 h(-1) and 1 500 000, respectively. Moreover, catalyst 1 can be recycled and is capable of performing allylic alcohol isomerizations even in the presence of conjugated dienes, which are known to be strong poisons in isomerization catalysis. On the basis of both experimental data and theoretical calculations (DFT), a complete catalytic cycle for the isomerization of 2-propen-1-ol into propenal is described. The potential energy surfaces of the cycle have been explored at the B3LYP/6-311+G(d,p) B3LYP/6-31G(d,p) + LAN2DZ level. The proposed mechanism involves the coordination of the oxygen atom of the allylic alcohol to the metal. The DFT energy profile is consistent with the experimental observation that the reaction only proceeds under heating. Calculations predict the catalytic cycle to be strongly exergonic, in full agreement with the high yields experimentally observed.