Highly(≥98%) Selective Trisubstituted Alkene Synthesis of Wide Applicability via Fluoride-Promoted Pd-Catalyzed Cross-Coupling of Alkenylboranes.

(Z)-β-bromo-1-propenyl(pinacol)borane(4), recently made available in 85% yield as a ≥98% isomerically pure compound via bromoboration of 1-propyne, has been converted to β-alkyl-, aryl-, and alkenyl-substituted (Z)-2-methyl-1-alkenyl(pinacol)boranes(2a) in ca. 75% yield based on propyne via Pd-catalyzed Negishi alkenylation with suitable organozinc bromide. The previously sluggish and modest-yielding Suzuki alkenylation of β,β-disubstituted alkenylboranes has been significantly promoted by fluorides, especially nBu4NF(TBAF) or CsF to give trisubstituted alkenes, i.e., (Z)-β-Me-substituted 3-i-3-xi and (E)-β-Ph-substituted 2b-i and 2b-ii. In all cases, each alkene product was formed in a ≥98% seteoselectivity. The propyne-based protocol nicely complements the widely used Zr-catalyzed alkyne methylalumination-Pd-catalyzed alkenylation by providing a highly stereoselective(≥98%) route to (Z)-Me-substituted alkenes.