Chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of bulky aryl ketones with ammonia borane

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An asymmetric transfer hydrogenation of bulky aryl ketones with ammonia borane was successfully realized with chiral phosphoric acid (CPA) as catalyst and water as additive. A variety of optically active secondary alcohols were obtained in good to high yields with up to 77% ee. The reaction likely proceeded through a Brønsted acid-promoted double-hydrogen transfer between ketones and ammonia borane via a six-membered concerted transition state.