Single-Isomer Trisubstituted Olefins from a Novel Reaction of (<i>E</i>)-β-Chloro-α-iodo-α,β-unsaturated Esters and Amides
2008-08-01T00:00:00Z (GMT) by
(<i>E</i>)-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 <i>E</i>-isomer. The process proceeds through (<i>E</i>)-β-chloro-α-aryl-α,β-unsaturated esters that are transformed efficiently into the corresponding <i>E</i>-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<sup>II</sup> is reduced to Pd<sup>0</sup> through the formation of biaryl-coupled products.
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