New insights into N-tert-butyl-α-phenylnitrone (PBN) as a spin trap. Part 2.1 The reactivity of PBN and 5,5-dimethyl-4,5-dihydropyrrole N-oxide (DMPO) toward N-heteroaromatic bases

Abstract

The reactions of N-tert-butyl-α-phenylnitrone (PBN) and 5,5-dimethyl-4,5-dihydropyrroline N-oxide (DMPO) with a series of N-heteroaromatic bases have been studied in different solvents (benzene, dichloromethane, acetonitrile) in the presence of such oxidants as PbO₂, chloranil and tetrabutylammonium dodecatungstocobalt(III)ate. In all cases spin adducts were formed and their corresponding EPR signals were recorded. The formation of the spin adducts, which mostly involves nucleophilic addition of a base to the nitrone and subsequent oxidation (Forrester–Hepburn mechanism) is also discussed and compared to conventional spin trapping and to ‘inverted spin trapping'. In order to obtain further evidence for the mechanism involved some reactions were carried out using other oxidants, such as galvinoxyl and 2,2-diphenyl-1-picrylhydrazyl (DPPH).The reaction mechanisms are discussed on the basis of the redox potentials of the studied nucleophiles. Particular attention is given to the benzotriazolyl–DMPO˙ spin adduct, whose generation and decay rates were determined in the presence of some benzoquinones having different redox potentials.