Highly Enantioselective Bromocyclization of Allylic Amides with a P/P=O Double-Site Lewis Base Catalyst

The enantioselective bromocyclization of allylic amides catalyzed by phosphorus-containing Lewis bases was examined in detail. A series of control experiments and NMR studies showed that a partially oxidized bis-phosphine generated in situ serves as the actual enantioselective catalyst. The reaction mechanism involves distinct roles of two Lewis basic sites, P and P=O, with P⁺ Br serving as a fine-tuning element for substrate fixation in the chiral environment, and P⁺ OBr as the Br⁺ transfer agent to the olefin. Catalyst loading could be reduced to as little as 1 mol %, and the reaction affords enantioenriched oxazolines with up to >99.5 % ee.