Light-Induced Iminyl Radicals: Generation and Synthetic Applications

The photochemistry of the carbon-nitrogen double bond was first explored in depth in the seventies (Padwa, 1977; Pratt, 1977). The low photochemical reactivity of this bond is due to the deactivation of the excited state of the imine by E–Z isomerization processes, which do not have any synthetic utility because of their very low energy barrier for thermal conversion (Padwa & Albrecht, 1974a, 1974b). However, the presence of an electronegative atom on the nitrogen opens up the field of reactivity. Studies on the energies of the N–O bonds in acyloximes show that this bond can easily undergo homolytic cleavage induced by ultraviolet light, a process that leads to iminyl radicals (Okada et al., 1969). The use of radicals has proven to be a useful tool in organic synthesis (Renaud & Sibi, 2001; Togo, 2004; Zard, 2003). In particular, the cyclization of nitrogen-centred radicals, such as aminyl or iminyl radicals (Chart 1), is a valuable procedure for the preparation of nitrogen heterocycles (Fallis & Brinza, 1997; Zard, 2008). It is therefore necessary to have effective methods for the production of these radicals.