Synthesis of Tri- and Tetrasubstituted Olefins by Palladium Cross-Coupling Reaction

Abstract

Tri- and tetrasubstituted olefins were obtained in high yields and regioselectivities using stilbene as starting material. First, stilbene was converted into (E)-bromostilbene by a bromination-dehydrobromination sequence. Then, (E)-bromostilbene was coupled with arylboronic acids at room temperature and low loading of Pd catalyst precursor (0.5-0.05 mol%) to afford selectively (E)-1-aryl-1,2-phenylethylenes in high yields (87-98%). Bromination of triphenylethylene afforded directly the bromotriphenylethylene that also underwent coupling reactions with arylboronic acids under mild conditions to afford tetrarylethylene (88-90% yield). Under the same conditions attempted Suzuki cross-coupling reactions of (E)-bromostilbene or 1,1,2-triphenylethene with alkylboronic acids were unsuccessful. However, the alkyl group could be introduced under mild conditions and high yields by using a Pd-catalyzed Negishi coupling protocol.