Single Photocatalytic Hydrosilylation of Alkenes for the Synthesis of Bis(silyl) and Silaboryl Alkanes

Bis-(silyl) and silaboryl alkanes are of interest as bioactive compounds and highly functionalized synthetic building blocks, but conventional hydrosilylation reactions lack generality and/or selectivity for synthesizing multielement-containing alkanes. Here, we present a versatile photoactivated hydrosilylation reaction of silicon- or boron-containing alkenes using silylborane as a silyl radical source to construct bis-(silyl) or silaboryl alkanes. This method employs a single phenothiazine-based photocatalyst and does not require conventional transition-metal or hydrogen-atom-transfer (HAT) catalysts or cocatalysts. Consequently, it has high generality, including alkenes bearing acid/base-sensitive groups, and provides high chemo-/regio-selectivity, overcoming the limitations of existing methods. This methodology also enables cyclizative functionalization of dienes and arylsilylation of boryl alkenes, and we show that the obtained multielement-containing alkanes are useful synthetic building blocks. Mechanistic studies uncovered a novel photoactivation mechanism, in which the phenothiazine catalyst directly facilitates Si-B bond cleavage in the triplet state, in contrast to conventional redox or HAT mechanisms, enabling radical hydrosilylation with a broad scope of alkenes.