New reactions of triplet oxygen which avoid the spin barrier

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Abstract
The cation triphenylmethylium and the cation radical tris-(p-bromophenyl)ammoniumyl are effective catalysts for oxygenation of ergosteryl acetate (1; R = Ac) to the peroxide (2; R = Ac); the triphenylmethylium-catalysed reaction is a photo-oxygenation, whereas the cation radical reaction is thermal. Several other ‘forbidden’ additions of triplet oxygen to cisoid dienes have been demonstrated. A number of cation radicals have been examined for catalytic oxygenation ability. Ergosteryl acetate can also be converted into its peroxide by triplet oxygen in the presence of various Lewis acids. Two mechanisms for overcoming the spin barrier for triplet oxygen addition have been defined by experimental work. A mechanistic interpretation of the facts is proposed.