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

Article abstract of DOI:10.1016/S0022-328X(96)06614-4

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 (2K,5S)-2-o-anisyl-3-oxa-1-aza phosphabicyclo[3.3.0]octane-borane complex 1b and (2R,5S)-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 2mol% 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.