On the oxazaborolidine-catalyzed borane reduction of 1-tetralone-Cr(CO) 3 complexes: The control of the reagent over a strong substrate

Abstract

The oxazaborolidine-catalyzed enantioselective borane reduction (CBS reduction) of 1-tetralone-Cr(CO) 3 derivatives was investigated. The kinetic resolution of the racemic substrates was possible but the enantiomeric purity of the recovered ketone was only 40–48% e.e. (at 50–56% conversion). On complete conversion, a ca. 52:48-mixture of the endo- and exo-tetralol complexes was obtained (80–99 % e.e.). It was shown that each substrate enantiomer gives selectively rise to a particular diastereomer (reagent control). The formation of the endo-isomer involves an unprecedented hydride transfer from the complexed face of the ligand and was exploited for the (diastereoselective) preparation of enantiomerically pure exo-6,7-dimethoxytetralol-Cr(CO) 3.