Arylethyne Bromoboration–Negishi Coupling Route to E‐ or Z‐Aryl‐Substituted Trisubstituted Alkenes of ≥98% Isomeric Purity. New Horizon in the Highly Selective Synthesis of Trisubstituted Alkenes

The hitherto unprecedented palladium‐catalyzed cross‐coupling of (Z)‐β‐bromo‐β‐arylethenylboranes can be made to proceed satisfactorily through (1) the use of highly catalytically active bis(tri‐tert‐butylphosphine)palladium or dichloro[N,N‐bis‐(2,6‐diisopropylphenyl)imidazol‐2‐yl](m‐chloropyridine)palladium and (2) conversion of the dibromoboryl group to the (pinacol)boryl group. Thus, a wide variety of carbon groups can be used to substitute bromine in ≥98% stereo‐ and regioselectivity, while suppressing the otherwise dominant β‐debromoboration. Together with the alkylethyne‐based protocols, the alkyne bromoboration–Negishi coupling tandem process has emerged as the most widely applicable and highly selective route to trisubstituted alkenes including those that are otherwise difficult to access.