Photoredox-Catalyzed Radical Alkylation/Cyclization of Isocyanides with Organoboron Reagents for Diversified Synthesis of N-Heteroaromatics

Abstract

The utility of alkyl boronic acids as alkyl radical precursors in photoredox-catalyzed reactions has been limited due to their low reactivity. A photoredox-catalyzed radical alkylation/cyclization of functional arylisocyanides with alkyl boronic acids is reported herein. This strategy allows the formation of alkyl radicals from boronic acids containing primary, secondary, and tertiary alkyl groups, which are subsequently transformed into structurally diverse alkylated products, including quinoxalines, phenanthridines, benzothiazoles, benzoselenazoles and quinolines. This transformation features readily available radical precursors and a broad substrate scope. The reaction mechanism is investigated by fluorescence quenching, light on/off, and cyclic voltammograms (CV) experiments. The combination of alkyl boronic acids with DBU may lead to the formation of the alkyl radical via a single electron transfer (SET) process, while the arylisocyanides may undergo an energy transfer (EnT) process with photocatalyst under visible light to generate the carbon radical or act as a radical acceptor to react with the alkyl radical.