Asymmetric Addition of Allylsilanes to Aldehydes: A Cr/Photoredox Dual Catalytic Approach Complementing the Hosomi-Sakurai Reaction

The allylation of aldehydes is a fundamental transformation in synthetic organic chemistry. Among the multitude of available reagents, allylsilanes especially have been established as a preferred allyl source. As initially reported by Hosomi and Sakurai, these non-toxic and highly stable reagents add to carbonyls via an open transition state upon Lewis acid activation. Herein, we report a general strategy to access a variety of valuable homoallylic alcohols in opposite chemo-and diastereoselectivity to the established Hosomi-Sakurai conditions by switching to photocatalytic activation in combination with a closed transition state (chromium catalysis). Moreover, this dual catalytic approach displays a straightforward way to introduce excellent levels of enantioselectivity and its mild conditions allow for a broad substrate scope including chiral boron-substituted products as a highlight. To emphasize the synthetic utility, our method was applied as the key step in the synthesis of a bioactive compound and in the late-stage functionalization of steroid derivatives. Detailed mechanistic studies and DFT calculations hint toward an unprecedented photo-initiated chain being operative.