On the Regioselectivity of Imidoyl Radical Cyclisations

Abstract

The previously reported tandem cyclisation of N-aryl α-(2-cyanophenyl)sulfanyl imidoyl radicals affords one quinoxaline derivative arising from exclusive 1,6-cyclisation of the final iminyl radical onto the N-aryl ring. When the imidoyl radicals are generated by addition of photolytically generated (2-cyanophenyl)sulfanyl radicals to isocyanides, the reaction also gives small amounts of a by-product that is formed by an analogous route and whose X-ray crystallographic data are reported here. The formation of this product entails a rare ortho-selective photo-Fries rearrangement of the starting disulfide, followed by addition to the isocyanide and regioselective 1,5-cyclisation of the resulting imidoyl onto only one of the two available radical acceptors, i.e. the cyano group and the sulfide moiety. Semiempirical MNDO-d calculations were performed in order to throw some light on the factors affecting these competitive cyclisations.