Ozone-mediated nitration of bicumene and derivatives with nitrogen dioxide. Preferential mesolytic bond cleavage over nuclear nitration in evidence for the electron transfer nature of the kyodai-nitration of arenes

Abstract

The ozone-mediated reaction of bicumene and some derivatives 1 with nitrogen dioxide in dichloromethane at low temperatures resulted in the facile cleavage of the central C–C bond to give the corresponding benzyl nitrate and its descendants 4–6. Mesolytic bond cleavage occurred almost exclusively over nuclear substitution at temperatures as low as -20 °C, especially at low concentration (2 × 10^-3 mol dm^-3). This result may be rationalized in terms of initial electron transfer between the aromatic substrate and nitrogen trioxide generated in situ to form the aromatic radical cation, which then undergoes C–C bond scission at the benzylic position. In contrast, bibenzyl 2a is simply nitrated on the aromatic ring under similar conditions, giving the expected nitration products 7 and 8a–c along with a small amount of benzaldehyde 9. Results obtained from semi-empirical calculations and cyclic voltammetry are also in accord with the electron transfer nature of the reaction. The C–Si bond fission of benzyltrimethylsilane, C–C bond fragmentation of cyclic acetals of aromatic carbonyl compounds as well as side-chain reactions of toluene and derivatives, have all previously been observed under certain conditions of the kyodai-nitration and can be understood on a similar basis as described above. The possible involvment of electron transfer processes in aromatic nitration with acetyl nitrate has also been suggested.