Reactivity and philicity of phthalimide-N-oxyl and benzotriazol-N-oxyl radicals in addition reactions to vinyl compounds

In this paper, we report the comparative investigation of reactivity of phthalimide-N-oxyl (PINO) and benzotriazole-N-oxyl (BTNO) radicals toward double bonds of vinyl compounds. The rate constants for radical addition (k_add) to alkenes were determined experimentally. It has been shown that k_add for PINO is significantly higher than for BTNO, and the reactivity of olefines increases in the series: CH₂ = CHCN < CH₂ = CHCOOH < CH₂ = CHC(O)OCH₃ < CH₂ = CHC(O)NH₂ < CH₂ = C(CH₃)C(O)OCH₃ < CH₂ = CHC₆H₅ < CH₂ = C(CH₃)C₆H₅. The plots of the dependence of the addition rate constants on the ionization potential of vinyl compounds gave a good linear correlation, which indicates the electrophilicity of the radicals and the significant influence of polar effects in the reactions studied. Polar effects operate at the level of the transition state, which is characterized by the transfer of electron from the alkene to the radical and the generation of a negative charge on the oxygen atom. The contribution of the polar factor to the energy of the transition state of reactions is adequately described by the Fukui–Klopman–Salem equation. The calculated indices of global electrophilicity of radicals (ω^o) confirm the electrophilic nature of PINO and BTNO, however ω^o BTNO (ω^o = 4.79 eV) is higher than that of PINO (ω^o = 4.00 eV). At the same time, the calculated spin density on the N–O fragment and O atom of the PINO radical is significantly higher than that of BTNO, which may be the reason for the different reactivity and selectivity of the radicals.