Radical addition to isonitriles: A route to polyfunctionalized alkenes through a novel three-component radical cascade reaction

The reaction of aromatic disulfides, alkynes, and isonitriles under photoly tic conditions affords polyfunctionalized alkenes-beta-arylthio-substituted acrylamides or acrylonitriles-in fair yields through a novel three-compone nt radical cascade reaction. The procedure entails addition of a sulfanyl r adical to the alkyne followed by attack of the resulting vinyl radical to t he isonitrile. A fast reaction, e.g., scavenging by a nitro derivative or b eta-fragmentation, is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible p rocess. The stereochemistry of the reaction is discussed, particularly with respect to the stereochemical outcome of related hydrogen abstraction reac tions by the same vinyl radicals. The lower or even inverted preference for either geometrical isomer observed in our cases with respect to that encou ntered in hydrogen abstraction reactions is explained in terms of transitio n-state interactions and/or isomerization of the final imidoylradical. The latter possibility is supported by semiempirical calculations, which show t hat the spin distribution in the imidoyl radical can allow rotation of the adjacent carbon-carbon double bond prior to beta-fragmentation.