Use of new chiral tricoordinated phosphorus borane complexes in enantioselective borane reduction of ketones: Complexes structure and mechanistic studies

New tricoordinated phosphorus borane complexes were synthesized and their use as catalysts in enantioselective borane reduction of prochiral aromatic and aliphatic ketones was investigated. The structure of (2R,SS)-2-o-anisyl-3-oxa-1-aza phosphabicyclo[3.3.0]octane-borane complex Ib and (2R,SS)-2,3-diphenyl-1,3-diazaphosphabicyclo[3.3.0]octane-borane complex 6a was established by X-ray diffraction analysis. A relationship has been established between the structure of the oxazaphospholidine borane complexes and the enantioselectivity obtained in the reduction of acetophenone, both with 2 mol% and one equivalent of the catalyst. Among the different oxazaphospholidine borane complexes tested, only the complexes 1-3, including 3-oxa-1-azaphosphabicyclo[3.3.0]octane and 3-oxa-1-azaphosphabicyclo[4.3.0]nonane moieties, were efficient catalysts. A rational mechanism is proposed according to the experimental results, especially from a deuterium labelling study.