Catalytic asymmetric hydrogenation (CAH) is one of the most frequently employed reactions in chemical discovery and process research and widely used in the preparation of chiral agrochemicals and pharmaceutical drugs. We recently reported that borane-promoted rhodium-catalyzed asymmetric hydrogenations of oxime-directed tri- and tetrasubstituted alkenes proceed in high yield with excellent enantioselectivity. In addition, the borane-promoted reactions were found to proceed much faster compared to reactions in the absence of borane. Herein, we report studies on borane-promoted asymmetric hydrogenations of enamide substrates using substoichiometric amounts of the borane activator with BINOL-derived phosphoramidite chiral ligand. As illustrated below, the reaction affords reduced product in a complete conversion within 2 h and with enantiomeric ratios of up to 95:5 er. In contrast, the reaction in the absence of borane under otherwise identical conditions requires 12 hours and affords a 54:46 er. The role of the catalyst activator and other mechanistic aspects will be discussed.

On the mechanism of borane promoted rhodium catalyzed asymmetric hydrogenation

ACS Fall 2021

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