Magnesium-Catalyzed Regioselective Reduction of Unprotected Indoles and Quinoxalines with Ammonia Borane

This study present a transfer hydrogenation method employing ammonia borane (H 3N·BH 3) as the hydrogen source and inexpensive magnesium as the catalyst for the selective reduction of unprotected indoles, quinoxalines, benzofurans, benzothiophenes, quinolines, and their derivatives, resulting the corresponding alicyclic heterocyclic compounds with desirable yields. This catalytic system is applicable to gram-scale syntheses and demonstrates compatibility with various functional groups, including fluorine, chlorine, bromine, trifluoromethyl, and hydroxyl. Deuterium labeling experiments show that the BH 3 counterpart of NH 3·BH 3 served as the hydride source, while the NH 3 counterpart of ammonia borane acted as a proton source. It offers a novel approach for the preparation of partially saturated heterocyclic derivatives.