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.