Facile Access to 2-Substituted-1,2,3,4-Tetrahydroquinoxalines via Borane-Catalyzed One-Pot Tandem Cyclization/Hydrosilylation

Borane-catalyzed one-pot tandem cyclization/hydrosilylation of 1,2-diaminobenzenes and glyoxals for step-economic synthesis of 2-substituted-1,2,3,4-tetrahydroquinoxalines has been developed. The use of B(C6F5)(3) catalyst and polymethylhydrosiloxane provides convenient access to various 2-aryl- and 2-alkyl-1,2,3,4-tetrahydroquinoxaline products in good to excellent yields with wide substrate scope and broad tolerance of functional groups. The asymmetric variant of this reaction can be achieved by using the enantioenriched borane catalyst generated in situ from HB(C6F5)(2 )and a binaphthyl-based chiral diene and PhSiH3, affording a variety of chiral 2-substituted-1,2,3,4-tetrahydroquinoxalines with enantioselectivities ranging from 90% to >99% ee. The judicious selection of borane catalyst and hydrosilane has been demonstrated to be crucial for catalysis. This cascade reaction proceeds with simple operation, and can be easily scaled up. Preliminary mechanistic studies have been performed to elucidate the catalytic pathway.