Imidoyl Radicals in Organic Synthesis

Imidoyl radicals (R1N=C.R2) are very attractive intermediates that can be readily produced by several methods, i.e. hydrogen atom abstraction from aldimines, homolytic fragmentation of certain imidoylic precursors, and addition of both carbon- and heteroatom-centered radicals to isonitriles and isothiocyanates. As far as their synthetic potential is concerned, they have been shown to perform smooth intra- and intermolecular additions to double and triple carbon-carbon bonds, as well as cyclizations onto aromatic rings, sulfur atoms, and cyano groups. They have been therefore efficiently employed in cyclizations, annulations, and cascade reactions leading to the construction of various nitrogen-containing heterocyclic compounds, e.g. indoles, phenanthridines, pyrrolidines, quinolines, quinoxalines, and fused polycyclic derivatives. They have been also used as key intermediates in the synthesis of carbonyl compounds, amides, and nitriles and as precursors of alkyl radicals in tin-free reactions. In this review we discuss thoroughly the generation methods, the structural features, and, above all, the reactivity of imidoyl radicals, with particular attention devoted to their synthetic applications. Keywords: isothiocyanates, di-tert-butyl peroxide, electron withdrawing groups, azo-bis-iso-butyronitrile, benzophenone