1‐Silyl‐1‐boryl‐2‐alkenes: Reagents for Stereodivergent Allylation Leading to 4‐Oxy‐(E)‐1‐alkenylboronates and 4‐Oxy‐(Z)‐1‐alkenylsilanes

Silylboryl reagents for organic synthesis: 1‐silyl‐1‐boryl‐2‐alkenes (2) were prepared efficiently by gem‐silylborylation of α‐chloroallyllithium compounds from (dimethylphenylsilyl)(pinacolato)borane (1; see scheme, LDA=lithium diisopropylamide) and were demonstrated to allylate acetals and aldehydes in the presence of a Lewis acid to produce (E)‐4‐alkoxy‐alkenylboronates. Heating the reagents with aldehydes in the absence of Lewis acid afforded (Z)‐4‐hydroxy‐alkenylsilanes stereospecifically.