Boron-Catalyzed Hydrogenative Reduction of Substituted Quinolines to Tetrahydroquinolines with Hydrosilanes

Published as part of the Cluster Silicon in Synthesis and Catalysis

Abstract

A metal-free procedure for the hydrogenative reduction of substituted N-heteroaromatics has been developed by using hydrosilanes as reducing agents. The optimized conditions were successfully applied to the reactions of quinolines, quinoxalines, and quinoline N-oxides. They were also effective for the reduction of quinolines bearing amino or hydroxy groups, where H₂ was evolved through dehydrogenative silylation of the amine or hydroxy moieties. Preliminary mechanistic studies revealed that the initial step in the catalytic cycle involves 1,4-addition of the hydrosilane to the quinoline to give a 1,4-dihydroquinoline; this is followed by (transfer) hydrogenation to deliver the tetrahydroquinoline as the final product.

Key Words

Lewis acids - boranes - quinolines - quinoxalines - hydrosilanes - hydrogenation

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