Reduction of Benzils Promoted by a New Class of Bifunctional Organocatalysts

Significance

Procuranti and Connon developed a new class of bifunctional organocatalysts (1) incorporating both a substrate-activating thiourea moiety and an organic hydride donor as NADH analogue, which is (re)-generated in situ using sodium dithionate as a reductant. Racemic catalyst 1a was successfully applied for the reduction of a range of symmetrical and unsymmetrical substituted benzils 2 of variable steric and electronic characteristics to the corresponding benzoins 3 in good to excellent yields. NMR studies confirmed that the transfer hydrogenation proceeds via intramolecular hydride donation from the in situ generated hydropyridine moiety to the thiourea-bound diketone. One example of enantio­selective reduction of benzyl 2 promoted by the enantiopure catalyst 1b is also given, although the enantiomeric ratio (62.5:37.5) is modest.