Chiral C 2 ‐Boron‐Bis(oxazolines) in Asymmetric Catalysis – A Theoretical Study of the Catalyzed Enantioselective Reduction of Ketones Promoted by Catecholborane

C 2 ‐Symmetrical boron complexes, prepared by the reactions of 2,2′‐methylenebis(oxazolines) (BOXs) with catecholborane (CATBH), can be used as catalysts (5–10 mol‐%) in the enantioselective reduction of prochiral ketones ( ee 72–86 %), giving the desired alcohols in satisfactory yields. We have theoretically investigated the mechanism of the reduction of chloroacetophenone at the DFT level and the computational results have provided a complete mechanistic picture, which explains the stereochemical outcome of the reaction. The B–BOXate complex binds both the reducing agent CATBH and the carbonyl compound, activating the former as a hydride donor and enhancing the electrophilicity of the latter. Moreover, the structure of two boron–BOX (BOXate) complexes has been confirmed by means of X‐ray diffraction techniques.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)