Stabilization of Triplet Diphenylcarbene by Ortho Aryl Groups

制备了邻位有苯基的二苯基重氮甲烷（DDMs），并对其在刚性有机基质中低温和在室温溶液中激光光解下光解生成的二苯基羰基甲烷进行了产物分析、ESR和紫外/可见光谱的表征。产品分析表明，相应的芴几乎完全形成。芴最可能是由单线态碳攻击苯基取代基的邻位碳生成异芴，然后发生1,5- h移位。在77 K下，在2-甲基四氢呋喃（2-MTHF）基质中辐照苯基化DDMs，可获得属于三重态碳的ESR信号。在相同的条件下，用DDMs辐照得到了三联体碳烯的紫外可见光谱。然而，室温下激光在脱气苯中光解DDMs的瞬态吸收带与在77 K下2-MTHF基质中光解所观察到的完全不同。在相应芴的LFP中观察到基本相同的瞬态带。在未脱气的苯中，苯基化DDM的LFP具有瞬态吸收带，可归因于三联态羰基和相应的羰基氧化物。三态卡宾在缺氧条件下的猝灭速率常数为1.9 × 10^7 M^<-1>s^<-1>，寿命为16 μs。对含有4-联苯基、3,5-双（三氟甲基）苯基、2-氟苯基和4-吡啶基的DDMs进行类似研究，结果基本相同，表明这些取代基对整个反应途径影响不大。这些研究表明，三联体碳也被邻苯基环捕获，最终生成芴。

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.