Ru and Os catalysts for alcohol C-H activation reactions
The complexes [MCl2(dppf)(NN)] (M: Ru, Os, NN: ethylenediamine, 2-(aminomethyl)pyridine), containing 1,1’-bis(diphenylphosphino)ferrocene (dppf), have been found to catalyse the acceptorless dehydrogenation of alcohols and 5-en-3β-hydroxy steroids. The Ru and Os ampy derivatives display also high catalytic activity for related C-H bond activation reactions. Alcohol racemisation and ketone α-alkylation with primary alcohols are efficiently catalysed by these complexes by a careful choice of the catalytic conditions. The comparison of the catalytic activity of the Ru and Os complexes shows that osmium is a valid complement to ruthenium in organic reactions involving alcohols vs. carbonyl compounds.
The search of highly efficient transition metal catalysts, displaying high selectivity and productivity, is a current issue of industrial relevance for the synthesis of valuable organic compounds. Recently, much attention has been devoted to the development of sustainable transformations based on environmental acceptability. The catalytic dehydrogenation of alcohols (DHY) is a fundamental reaction for both hydrogen production and oxidation of alcohols, without the use of toxic oxidizing agents and formation of undesirable by-products. The activation of C-H bond close to the hydroxyl group is a crucial transformation for broadening the alcohol reactivity, giving access to C-C and C-N bond-forming reactions, via hydrogen borrowing (1).
Among the different transition metal complexes which catalyse the alcohol DHY, much of the research work has been concentrated on ruthenium (1a). It is worth noting that the well-known catalysts trans-[RuCl2(PP)(NN)] (PP: diphosphine; NN: diamine) (2, 3), which are highly active in the (asymmetric) hydrogenation (HY) of ketones, are expected to promote the DHY and related C-H activation reacti ...