Phosphinoyl Radicals: Structure and Reactivity. A Laser Flash Photolysis and Time-Resolved ESR Investigation

The photochemistry of a series of bis(acyl)phosphine oxides and the rate constants of the reactions of their phosphorus radicals with n-butylacrylate, thiophenol, bromotrichloromethane, oxygen, and methyl viologen have been investigated by laser flash photolysis. The results were compared to a mono(acyl)phosphine oxide ((2,4,6-trimethylbenzoyl)diphenylphosphine oxide). The variation in reactivity of the different phosphorus radicals was correlated with the degree of radical localization and s-character on the phosphorus atom, as reflected by the 31P hyperfine coupling constant, which was measured by time-resolved ESR. High 31P hyperfine splitting is observed for high degree of spin localization in a σ-orbital. It was shown that typical radical reactions (addition to monomers and oxygen, and atom abstractions) correlate well with the 31P hyperfine coupling. An inverse correlation was observed for electron-transfer reactions (reduction of methyl viologen), were a higher p-character of a localized orbital enhances the reaction.