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 photolytic conditions affords polyfunctionalized alkenes--beta-arylthio-substituted acrylamides or acrylonitriles--in fair yields through a novel three-component radical cascade reaction. The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical to the isonitrile. A fast reaction, e.g., scavenging by a nitro derivative or beta-fragmentation, is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The stereochemistry of the reaction is discussed, particularly with respect to the stereochemical outcome of related hydrogen abstraction reactions by the same vinyl radicals. The lower or even inverted preference for either geometrical isomer observed in our cases with respect to that encountered in hydrogen abstraction reactions is explained in terms of transition-state interactions and/or isomerization of the final imidoyl radical. The latter possibility is supported by semiempirical calculations, which show that the spin distribution in the imidoyl radical can allow rotation of the adjacent carbon-carbon double bond prior to beta-fragmentation.

Cascade radical reaction of 2-alkynyl-substituted aryl radicals with aryl isothiocyanates: a novel entry to benzothieno[2,3-b]quinolines through alpha-(Arylsulfanyl)imidoyl radicals.

The novel cascade radical reaction of 2-(phenylalkynyl)aryl radicals with 4-Y-phenyl isothiocyanates (Y = H, OMe, Me, Cl, CN) provides a useful one-pot protocol for the production of 8-Y-substituted (12) and/or 9-Y-substituted benzothieno[2,3-b]quinolines (11). The whole process entails primary formation of an alpha-(2-alkynylarylsulfanyl)imidoyl radical that undergoes smooth 5-exo-dig cyclization onto the alkynyl triple bond. The derived 1-phenylvinyl radical then exhibits six-membered cyclization onto the isothiocyanate ring, to give 11, and/or five-membered ipso-cyclization to an azaspiro intermediate, whose eventual rearrangement affords 12. The overall findings clearly showed that the relative proportion of the outcoming isomeric benzothienoquinolines 11 and 12 can be markedly affected by the nature of the original isothiocyanate substituent. Moreover, the findings also furnished the first chemical evidence that enhancing the electrophilic power of the employed radical can properly enhance the reactivity of aryl radicals toward isothiocyanates.

Short intramolecular S...O interactions in S-substituted 2-mercaptoacetophenones.

The non-bonded S...O intramolecular interactions in the title compounds 2-(phenylthio)acetophenone [IUPAC: 2-[2-(phenylsulfanyl)phenyl]ethanone], C(14)H(12)OS, and 2-(benzylthio)acetophenone [IUPAC: 2-[2-(benzylsulfanyl)phenyl]ethanone], C(15)H(14)OS, are unusually short, indicating the contribution of heterocyclic oxathiole-type resonance structures to the overall bonding.