Radical annulations and cyclisations with isonitriles: the fate of the intermediate imidoyl and cyclohexadienyl radicals

Abstract

The reaction of 4-methoxyphenylisonitrile with phenylacetylene and AIBN produces a novel cyclopenta-fused quinoxaline through addition of 2-cyanoprop-2-yl radical to the alkyne; the resulting vinyl radical attacks isonitrile to afford an imidoyl radical, which gives rise to a tandem 5-exo, 6-endo cyclisation. The whole process entails a new example of a rare 4 + 1 radical annulation. The cyanopropyl radical can also attack isonitrile to yield small amounts of quinolines deriving from 4 + 2 and 3 + 2 annulation between the resulting imidoyl radicals and phenylacetylene. The oxidation step leading to the final aromatic products involves the starting isonitrile, which is converted to an α-unsubstituted imidoyl radical and affords 2-unsubstituted quinolines. This behaviour was also found in cyclisations of biphenyl-2-ylisonitrile under various radical conditions. Finally, the title reaction gives small amounts of an α,β-unsaturated nitrile, which can arise from a spirocyclohexadienyl radical through fragmentation and subsequent β-scission of the resulting iminyl. This could be the first, direct evidence of the intermediacy of iminyl radicals in the rearrangements of the spirocyclohexadienyls obtained by 3 + 2 annulation between imidoyl radicals and alkynes.