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

本研究的目的是阐明芳烃和烯烃通过[3+2]-光环加成生成三重态双自由基的特征。得到的结果如下：1.1，4-萘二甲酸二甲酯(1)与烯烃的[3+2]-光环加成反应是从1的单重态激发态开始的，可能是通过一种双离子中间体进行的。另一方面，在氧和富马腈等自由基清除剂存在下的光反应中，加入了自由基清除剂[3+2]-环加合物，但同时损失了[3+2]-环加合物的生成。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。提出了一种形成加合物的机理，涉及从两性离子中间体产生的三重态双自由基中间体。通过对各种芳烃和烯烃的光反应的研究发现，在低极性的溶剂(如苯和四氯化碳)中，具有芳香族取代基(如苯乙烯)的三线态双自由基的形成在低温下是有效的。相应的分子内光反应表明，在低温下，在低极性溶剂中的分子内反应中也能有效地形成三重态双自由基。测定了1的烯烃对荧光的猝灭速率常数，发现较大的速率常数是有效形成三重态双自由基所必需的。另一方面，在自由基清除剂的猝灭过程中，没有观察到有效的荧光猝灭。分子轨道计算表明，通过烯烃单元接近1的1，8-位得到的三重态双自由基比相应的单重态离子中间体更稳定(11-18kcal/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.