Stabilization of Triplet Diphenylcarbene by Ortho Aryl Groups

合成了邻位含苯基的二苯基重氮甲烷(DDMS)，并对它们光解生成的二苯基卡宾进行了表征，不仅用产物分析，而且用ESR和UV/Vis光谱表征了它们在刚性有机基质中的低温和室温下的激光闪光光解.产物分析表明，相应的荧烯几乎是完全形成的。荧烯很可能是通过单重态卡宾攻击苯基取代基的邻位碳生成异荧烯，然后是1，5-H移动而产生的。在77K下，在2-甲基四氢呋喃(2-MTHF)基质中照射苯基化的DDMS，得到的ESR信号可归因于三重态卡宾。在相同的光照条件下，获得了三重态卡宾的UV/Vis光谱。然而，在室温下，DDMS在脱气苯中的激光闪光光解显示出与77K下在2-MTHF基质中的光解完全不同的瞬时吸收带，基本上在相应的荧烯的LFP中观察到相同的瞬时吸收带。苯基化的DDM在未脱气的苯中的LFP出现了可归因于三线态卡宾和相应的羰基氧化物的瞬时吸收带。测得氧猝灭三重态卡宾的速率常数为1.9×10~(-7)M^~(-1)，无氧时寿命为16μS。与含有4-联苯基、3，5-二(三氟甲基)苯基、2-氟苯基和4-吡啶基团的DDMS的研究结果基本一致，表明这些取代基对整个反应路径几乎没有影响。从这些研究中可以看出，三重卡宾也被邻苯环捕获，最终得到荧烯。

Diphenyldiazomethanes (DDMs) having phenyl groups at the ortho positions were prepared and the corresponding diphenylcarbenes photolytically generated from them were characterized not only by product analysis but also by ESR and UV/vis spectroscopy in a rigid organic matrix at low temperature and in laser flash photolysis in solution at room temperature. Product analysis indicated that the corresponding fluorenes are formed almost exclusively. Fluorene is most likely produced by the attack of singlet carbene on an ortho carbon of the phenyl substituent to generate isofluorene, followed by 1,5-H shift. Irradiation of the phenylated DDMs in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K gave ESR signals ascribable to triplet carbenes. UV/vis spectra of the triplet carbenes were obtained by irradiating the DDMs under identical conditions. However, laser flash photolysis of the DDMs in degassed benzene at room temperature showed transient absorption bands completely different from those observed in photolysis in the 2-MTHF matrix at 77 K. Essentially the same transient band was observed in LFP of the corresponding fluorene. The LFP of the phenylated DDM in non-degassed benzene gave transient absorption bands ascribable to triplet carbene and the corresponding carbonyl oxide. The quenching rate constant of the triplet carbene by oxygen and the lifetime of the triplet carbene in the absence of oxygen were 1.9 x 10^7 M^<-1>s^<-1> and 16 μs, respectively. Similar studies with DDMs having 4-biphenylyl, 3,5-bis (trifluoromethyl) phenyl, 2-fluorophenyl, and 4-pyridyl groups gave essentially identical results, indicating that those substituents exhibit little effect on overall reaction pathway. From those studies, it is suggested that the triplet carbenes are also trapped by the ortho-phenyl ring to give eventually fluorenes.