Kinetic Study of the Hydrogen Abstraction Reaction of the Benzotriazole-N-oxyl Radical (BTNO) with H-Donor Substrates

Abstract

The aminoxyl radical (>N−O^•) BTNO (benzotriazole-N-oxyl) has been generated by the oxidation of 1-hydroxybenzotriazole (HBT; >N−OH) with a Ce^IV salt in MeCN. BTNO presents a broad absorption band with λ_max 474 nm and ε 1840 M^-1 cm^-1, and spontaneously decays with a first-order rate constant of 6.3 × 10^-3 s^-1 in MeCN at 25 °C. Characterization of BTNO radical by EPR, laser flash photolysis, and cyclic voltammetry is provided. The spontaneous decay of BTNO is strongly accelerated in the presence of H-donor substrates such as alkylarenes, benzyl and allyl alcohols, and alkanols, and rate constants of H-abstraction by BTNO from a number of substrates have been spectroscopically investigated at 25 °C. The kinetic isotope effect confirms the H-abstraction step as rate-determining. Activation parameters have been measured in the 15−40 °C range with selected substrates. A correlation between E_a and BDE(C−H) (C−H bond dissociation energy) for a small series of H-donors has been obtained according to the Evans−Polanyi equation, giving α = 0.44. From this plot, the experimentally unavailable BDE(C−H) of benzyl alcohol can be extrapolated, as ca. 79 kcal/mol. With respect to the H-abstraction step, peculiar differences in the ΔS^⧧ parameter emerge between an alkylarene, ArC(H)R₂, and a benzyl alcohol, ArC(H)(OH)R. The data acquired on the H-abstraction reactivity of BTNO are compared with those recently reported for the aminoxyl radical PINO (phthalimide-N-oxyl), generated from N-hydroxyphthalimide (HPI). The higher reactivity of radical PINO is explained on the basis of the higher energy of the NO−H bond of HPI, as compared with that of HBT (88 vs ca. 85 kcal/mol, respectively), which is formed on H-abstraction from the RH substrate.