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

Abstract

The reaction between N-tert-butyl-α-phenylnitrone (PBN) and carboxylic acids has been studied. Two mechanisms are discernible: the generation of PBN^˙+ by oxidation of PBN 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 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.