Regenerable Dihydrophenanthridine via Borane-Catalyzed Hydrogenation for the Asymmetric Transfer Hydrogenation of Benzoxazinones.

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The highly enantioselective transfer hydrogenation of benzoxazinones with chiral phosphoric acids under H2 was successfully achieved, where boranes promoted the hydrogenation of phenanthridine for the regeneration of dihydrophenanthridine as the hydrogen donor. A variety of dihydrobenzoxazinones were obtained in high yields with up to 99% ee. The current work provides a promising solution to unreactive substrates for frustrated Lewis pair-catalyzed asymmetric hydrogenation.