A general photo-induced wide-scope regioselective hydroboration of alkenes without using a photocatalyst or an external initiator

Abstract

Despite the recent development of boryl radical reactions, current methods to generate them by relying on photocatalysts or radical initiators cause contamination of the reaction. Herein, a novel system for generating boryl radicals has been developed without using photocatalysts or external radical initiators, and only a catalytic amount of thiol has been used. The success of this approach depends on the dual role of thiol as a proton donor and hydrogen atom transfer (HAT) catalyst. The thiol first reacts with NHC-borane to give boryl sulphides via a dehydrocoupling reaction, and then they undergo in situ cleavage of their B–S bond through photochemical reactions. The radicals obtained were later involved in a fast thiol-catalyzed HAT reaction, affording the hydroboration products. This strategy enables the regioselective hydroboration for scalable synthesis of α- or β-borylated products from many kinds of α,β-unsaturated alkenes and styrenes in good to excellent yields, with broad functional group compatibility at room temperature. The reaction could also be extended to radical-type nucleophilic addition to imines. This unprecedented protocol is anticipated to find potential application in other boryl radical-involved reactions by using boryl sulphides through photochemical reactions.