Stabilization of Triplet Diphenylcarbene by Ortho Aryl Groups

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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.