Modular Synthesis of (Z)-Allylsilanes via Copper-Catalyzed Cascade Silylation and Horner-Wadsworth-Emmons Reaction.

We report a copper-catalyzed cascade silylation/Horner-Wadsworth-Emmons (HWE) reaction for the stereoselective synthesis of (Z)-allylsilanes (up to 92% yield, Z/E > 20:1) through efficient three-component reactions of an unsaturated phosphonate, an aldehyde and a silylborane (Si-B) reagent. The reaction accommodates a variety of aldehydes and Si-B reagents to afford allylsilanes via copper-catalyzed silyl addition to unsaturated phosphonates, followed by aldehyde addition of the resulting copper enolate. The operational simplicity, improved step economy, and distinct stereochemical outcome of products highlight the synthetic utility of this cascade process and expand the scope of catalytic HWE-type olefination reactions. DFT calculations provided an enhanced understanding of the origin of diastereoselectivity of the reaction.