Mechanism of the 9,10-dicyanoanthracene-sensitized photooxidation of N,N-dibenzylhydroxylamine in polar and non-polar solvents

Abstract

The 9,10-dicyanoanthracene (DCA)-sensitized photooxidation of N,N-dibenzylhydroxylamine (DBH) gives α-phenyl-N-benzylnitrone (PBN) and hydrogen peroxide in quantitative yields at low conversions of the starting DBH both in oxygen-saturated polar solvents, acetonitrile and methanol, and in an oxygen-purged non-polar solvent, carbon tetrachloride. Thermodynamic and kinetic analyses of this sensitized oxidation in polar solvents indicate the involvement of superoxide (O₂˙^–) as the major oxidizing species in the process. Solvent deuterium isotope effects on the quantum yield for the appearance of PBN (Φ_PBN) provide supporting evidence supporting evidence for the preferential O₂˙^– mechanism by which the sensitized reaction in acetonitrile and methanol proceeds. In addition, a higher sensitized-oxidation efficiency is obtained in methanol (limiting quantum yield, Φ_{PBN, lim}= 0.37) than in acetonitrile (Φ_{PBN, lim}= 0.21). This finding is explained in terms of the much slower rate of back electron transfer within the initially-formed solvent-separated radical ion pair in methanol as compared to that in acetonitrile. On the other hand, a comparison of the DBH concentration dependence of Φ_PBN for the DCA- and tetraphenylporphine-sensitized photooxidations in carbon tetrachloride establishes the exclusive operation of a singlet oxygen (¹O₂) mechanism in this non-polar solvent. The chemical deactivation of ¹O₂ assisted by the hydroxylic hydrogen in DBH, taking place via a charge-transfer complex, is proposed to account for the efficient formation of PBN and hydrogen peroxide in carbon tetrachloride.