Single-Isomer Trisubstituted Olefins from a Novel Reaction of (E)-β-Chloro-α-iodo-α,β-unsaturated Esters and Amides

(E)-β-Chloro-α-iodo-α,β-unsaturated esters are converted to single isomer trisubstituted olefins bearing three different carbon substituents by cross-coupling under reflux. Mechanistic investigations suggest that this process transfers a hydrogen from the boronic acid to the α-position of the substrate and then introduces an aryl group to the β-position of the intermediate template while replacing chloride. The reaction is highly stereoselective, showing preference for the E-isomer. The process proceeds through (E)-β-chloro-α-aryl-α,β-unsaturated esters that are transformed efficiently into the corresponding E-products through stereoselective Suzuki-type reactions giving single isomers. The observed stereochemistry is apparently enabled by the intermediacy of a palladium allenoate. The reaction involves a catalytic cycle in which Pd^II is reduced to Pd⁰ through the formation of biaryl-coupled products.