FORMATION OF TRIPLET BIRADICALS BY [3+2]-PHOTOCYCLOADDITION OF ARENES WITH ALKENES

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The purpose of this investigation is to clarify the features of the formation of triplet biradicals by [3+2]-photocycloaddition of arenes and alkenes.The results obtained are as follows :1. The [3+2]-photocycloaddition of dimethyl 1, 4-naphthalenedicarboxylate (1) and alkenes was found to occur from the singlet excited state of 1 possibly through a zwitter-ionic intermediate. On the other hand, the photoreactions in the presence of a radical scavenger, such as oxygen and fumaronitrile, radical scavengeradded [3+2]-cycloadducts were obtained in concomitant loss of the formation of the [3+2]-cycloadducts. Control examinations indicated that the [3+2]-adducts were not the precursors of the adducts of the radical scavenger. A mechanism on the formation of the adducts, involving a triplet biradical intermediate occurred from the zwitter-ionic intermediate, was proposed. From the investigations of the photoreactions of various arenes and alkenes, the formation of the triplet biradicals was found to be efficient in the photoreactions of aromatic esters and alkenes, having aromatic substituents, such as styrene, in less polar solvents, such as benzene and carbon tetrachloride, at low temperature.2. The corresponding intramolecular photoreactions revealed that the triplet biradicals were also effectively formed in the intramolecular reactions in less polar solvents at low temperature.3. The quenching rate constants of the fluorescence of 1 by the alkenes were measured and a large rate constant was found to be essential for the effective formation of the triplet biradicals. On the other hand, no effective fluorescence quenching was observed in the quenching by the radical scavengers.4. The molecular orbital calculation revealed that the triplet biradicals, obtained by approaching of the alkene units to the 1, 8-position of the naphthalene ring of 1, were more stable (11-18 kcal/mol) than the corresponding singlet zwitter-ionic intermediates.