Inverted spin trapping. Part III. Further studies on the chemical and photochemical oxidation of spin traps in the presence of nucleophiles

Inverted spin trapping, denoting the reaction between the radical cation of a spin trap (ST˙+) and a nucleophile (Nu–), has been further explored with respect to the photochemical generation of ST˙+ and the use of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) and 3,3,5,5-tetramethyl-1-pyrroline 1-oxide (TMPO) as ST. It is shown that photoexcitation of N-benzylidene-tert-butylamine N-oxide (PBN) in the presence of a weak electron acceptor, tetrabutylammonium 12-tungstocobaltate(III), and a nucleophile gives the corresponding spin adducts, PBN–Nu˙, most likely via the intermediate radical cation, PBN˙+. This species could also be generated by sensitized photoxidation and made to react with Nu–.The cyclic N-oxides DMPO and TMPO are more difficult to oxidize than PBN(by 0.2–0.3 V), They both engage in inverted spin trapping, using both thermal and photochemical oxidation, but in a more restricted way than for PBN.