FORMATION AND REACTIVITY OF SIGMA-RADICAL CATION INTERMEDIATES IN THE C-C COUPLING REACTION OF PHENYLDIAZOMETHANES BY ONE-ELECTRON OXIDATION

One-electron oxidation of phenyldiazomethanes afforded cis-stilbene predominantly. The reaction was independent of the oxidation methods, e.g., electrolysis, copper(II), triarylaminium salts, or photosensitized one-electron oxidations. The C-C coupling reaction was retarded by introducing alpha-substituents on phenyldiazomethane. The ESR spectra of diazoalkane radical cations could be obtained during the electrolysis at low temperature and the resulting spectra revealed their unique electronic structure as sigma-radicals for most cases. When a bulky tert-butyl group was substituted, the corresponding pi-radical cation was observed, but the C-C coupling reaction did not occur. The novel HOMO-LUMO switching by one-electron removal from the HOMO pi-orbital of diazomethane is explained by the interaction of phenyl group with the C-N-N sigma-radical moiety. The C-C coupling reaction proceeds via facile [4 + 2] cycloaddition between the diazomethane and sigma-radical cation, and the preferential formation of cis-olefins is based on the secondary orbital interaction between the two phenyl groups. The structure and the stability of radical cation intermediates are rationalized on the basis of ab initio calculations.