Ruthenium(II) Terdentate CNN Complexes: Superlative Catalyst for the Hydrogen-Transfer Reduction of Ketones by Reversible Insertion of a Carbonyl Group into the Ru-H Bond

we have shown that terdentate [RuX- (CNN)(PP)] (X=Cl, H) complexes are highly efficient catalysts in transfer hydrogenation involving 2-propanol to afford quantitative reduction of different ketones with very low loading and in a short time. Reduction of ketones apparently takes place by insertion of the substrate into the RuH bond, thus leading to a RuII alkoxide. The reversibility of this step has been observed in the case of the stoichiometric reaction of complex 2 with benzophenone. This fact provides strong evidence that a Ru alkoxide derivative is most probably an intermediate in the formation of the hydride 2, the putative active catalyst, from the relevant chloride 1. As the structure of the terdentate pyridine ligand is well suited for a modular synthetic approach, which allows for a fine tuning of the stereoelectronic properties of the CNN complexes, this new class of ruthenium derivatives holds the promise for a broad application in organometallic chemistry and in homogeneous asymmetric catalysis.