New insights on N-tert-butyl-α-phenylnitrone (PBN) as a spin trap. Part 1. Reaction between PBN and N-chlorobenzotriazole

Abstract

N-Chlorobenzotriazole (BT–Cl) and N-tert-butyl-α-phenylnitrone (PBN) react thermally or photochemically at room temperature with the formation of two isomeric benzotriazolyl spin adducts and the chloro spin adduct, as monitored by EPR spectroscopy. The benzotriazolyl spin adducts are subsequently converted into benzotriazolyl phenyl ketone O-benzoyloxime while the latter is converted into the N-benzoyl-N-tert-butyl aminoxyl radical. Normal solvents do not substantially influence the spin adduct distribution, but the reaction becomes faster in the order of benzene < dichloromethane < acetonitrile. The reaction is autocatalytic in benzotriazole, provided a suitable H atom donor is present, e.g. the solvent or added toluene. In 1,1,1,3,3,3-hexafluoropropan-2-ol, a solvent which strongly suppresses nucleophilic processes, only the benzotriazolyl spin adduct is detectable. The macroscale reaction between BT–Cl and PBN confirms the fate of the radicals detected by EPR spectroscopy. The structures of (Z)- and (E)-benzotriazolyl phenyl ketone O-benzoyloxime were determined by X-ray analysis.