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

(Z)-beta-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 beta-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 beta,beta-disubstituted alkenylboranes has been significantly promoted by fluorides, especially nBu(4)NF(TBAF) or CsF, to give trisubstituted alkenes, i.e., (Z)-beta-Me-substituted 3-i-3-xi and (E)-beta-Ph-substituted 2b-i and 2b-ii. In all cases, each alkene product was formed in a >= 98% stereoselectivity. 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.