Oxazaborolidines from boracycles through the intramolecular azide insertion

Abstract

Optically active 2-azido alcohols react with boracycles such as 9-BBN-H and borinane to produce air stable oxazaborolidines, namely 3-oxa-6-aza-2- boratricyclo[5.3.3.0]tridecanes and 10-oxa-7-aza-1-borabicyclo[5.3.0]decanes, respectively. Nopyl azide reacts with 9-BBN-H forming a novel N-substituted-9-aza-10-borabicyclo[3.3.2]decane through a B-C nitrenoid insertion. This intermediate undergoes intramolecular hydroboration at 200 °C to produce a pinene-derived azaborapentacycle. These compounds were examined in the asymmetric reduction of acetophenone under the conditions employed for CBS-type catalytic processes. The enantioselectivities obtained were very low due to the lack of formation of the oxazaborolidine-borane complex, which is essential in orchestrating this process.