Generation of acyloxyl spin adducts from N-tert-butyl-a-phenylnitrone (PBN) and 4,5-dihydro-5,5-dimethylpyrrole 1-oxide (DMPO) via nonconventional mechanisms

The reaction between N-tert-butyl-alpha-phenylnitrone (PBN) and carboxylic acids has been studied. Two mechanisms are discernible: the generation of PBN.+ by oxidation of PEN with a photochemically produced excited state [from either 2,4,6-tris(4-methoxyphenyl)pyrylium ion 2(+) or tetrachlorobenzoquinone 4], followed by reaction with RCOOH, or the addition of RCOOH to PBN to give a hydroxylamine derivative, followed by thermal oxidation by a weak oxidant, The latter sequence is the Forrester-Hepburn mechanism, In this mechanism, neither 2(+) nor 4 is effective as an oxidant, whereas bromine could be used, Thus only oxidants with redox potentials greater than or equal to 0.1 V (SCE) are reactive enough to oxidize the intermediate hydroxylamine, This behaviour is in agreement with the redox reactivity of hydroxylamines. For the cyclic nitrone, 4,5-dihydro-5,5-dimethylpyrrole 1-oxide (DMPO), acyloxyl spin adducts have been prepared by the photochemical route. The reaction between dibenzoyl peroxide and PBN to give PhCOO-PBN. is not catalysed by added PhCOOH, It could be shown that the rate of formation of PhCOO-PBN. is compatible with the rate of thermal decomposition of dibenzoyl peroxide, Thus dibenzoyl peroxide does not support the Forrester-Hepburn mechanism, in agreement with its redox potential of ca. -0.2 V.