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

本研究的目的是阐明[3+2]-光环加成芳烃和烯烃形成三重双自由基的特征。得到的结果如下：1。发现二甲基1,4 -萘二羧酸酯(1)与烯烃的[3+2]-光环加成可能发生在1的单线态激发态，可能是通过中性离子中间体。另一方面，在自由基清除剂（如氧和富甲腈）存在下，自由基清除剂添加的[3+2]-环加合物的光反应发生，同时[3+2]-环加合物的形成丧失。对照试验表明，[3+2]-加合物不是自由基清除剂加合物的前体。提出了一种由两性离子中间体产生三重态双自由基中间体的机理。通过对各种芳烃和烯烃的光化学反应的研究，发现具有芳香取代基（苯乙烯）的芳香族酯和烯烃在较低极性的溶剂（苯和四氯化碳）中，在低温下的光化学反应中，三重双自由基的形成是有效的。相应的分子内光反应表明，在低极性溶剂下，分子内反应也能有效地形成三重态双自由基。测定了烯烃对1的荧光猝灭速率常数，发现大的猝灭速率常数是三态双自由基有效形成所必需的。另一方面，自由基清除剂的猝灭作用未观察到有效的荧光猝灭。分子轨道计算表明，由烯烃单元靠近萘环1的1,8位得到的三重态双自由基比相应的单线态中性离子中间体更稳定（11-18 kcal/mol）。

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.