Ruthenium-catalyzed redox isomerization/transfer hydrogenation in organic and aqueous media: A one-pot tandem process for the reduction of allylic alcohols

The hexamethylbenzene-ruthenium(II) dimer [{RuCl(μ-Cl)(η6-C6Me6)}2] 1 and the mononuclear bis(allyl)-ruthenium(IV) complex [RuCl2(η3:η2:η3-C12H18)] 2, associated with base and a hydrogen donor, were found to be active catalysts for the selective reduction of the CC bond of allylic alcohols both in organic and aqueous media. The process, which proceeds in a one-pot manner, involves a sequence of two independent reactions: (i) the initial redox-isomerization of the allylic alcohol, and (ii) subsequent transfer hydrogenation of the resulting carbonyl compound. The highly efficient transformation reported herein represents, not only an illustrative example of auto-tandem catalysis, but also an appealing alternative to the classical transition-metal catalyzed CC hydrogenations of allylic alcohols. The process has been successfully applied to aromatic as well as aliphatic substrates affording the corresponding saturated alcohols in 45–100% yields after 1.5–24 h. The best performances were reached using (i) 1–5 mol% of 1 or 2, 2–10 mol% of Cs2CO3, and propan-2-ol or (ii) 1–5 mol% of 1 or 2, 10–15 equivalents of NaO2CH, and water. The catalytic efficiency is strongly related to the structure of the allylic alcohol employed. Thus, in propan-2-ol, the reaction rate essentially depends on the steric requirement around the CC bond, therefore decreasing with the increasing number of substituents. On other hand, in water the transformation is favoured for primary allylic alcohols vs. secondary ones.