Diastereoselective Radical Hydrogen Transfer Reactions using N-Heterocyclic Carbene Boranes

Reported herein are the first diastereoselective and Lewis acid mediated radical reactions of N-heterocyclic carbene (NHC) boranes. We applied these reactions to the synthesis of four propionate diastereoisomers combining an aldol reaction, followed by a stereoselective radical-based reduction in which the NHC borane serves as the hydrogen donor, thus obviating the use of tin-based reagents. The 2,3-syn isomer is obtained by combining an NHC-borane and a Lewis acid (MgBr2.OEt2), while using a reverse polarity strategy provides the 2,3-anti isomer.