2-Pyridylnitrene−1,3-Diazacyclohepta-1,2,4,6-tetraene Rearrangements in the Trifluoromethyl-2-pyridyl Azide Series^1a

Photolysis of Ar matrix isolated trifluoromethyl-substituted 2-pyridyl azides/tetrazolo[1,5-a]pyridines at 12−18 K causes rapid and mostly clean conversion to the corresponding 1,3-diazacyclohepta-1,2,4,6-tetraenes (4D, 5D, 5,6D, and 4,6D) absorbing near 2000 cm-1 in the IR. In the latter case, the intermediate 3,5-bis(trifluoromethyl)-2-pyridylnitrene (4,6N) was observed by both ESR and IR spectroscopy and converted to the diazacycloheptatetraene 4,6D in the course of 90 min of UV irradiation. The 2-pyridylnitrenes were generally observable by ESR spectroscopy (|D/hc |∼1.05−1.10; |E/hc|∼0.0 cm-1) following both photochemical and thermal (FVP) generation from the 2-azidopyridines. Irradiation of the Ar matrix isolated mixtures of nitrenes and diazacycloheptatetraenes also caused development of weak carbene transitions (|D/hc|∼0.40−0.45; |E/hc| ∼0.006 cm-1) in the ESR spectra.将氩基质隔离的三氟甲基取代2-吡啶基叠氮化物/四唑并[1,5-a]吡啶在12~18开尔文条件下进行光解，可快速且基本实现无副产物的完全转化，得到对应的1,3-二氮杂环庚-1,2,4,6-四烯（4D、5D、5,6D及4,6D），该类产物在红外（Infrared, IR）光谱中于约2000 cm⁻¹处出现特征吸收峰。 针对后者（即四唑并[1,5-a]吡啶体系），中间体3,5-双(三氟甲基)-2-吡啶基氮宾（4,6N）可通过电子自旋共振（Electron Spin Resonance, ESR）与红外光谱同时观测到，并在90分钟紫外辐照过程中转化为二氮杂环庚四烯4,6D。 通常而言，由2-叠氮基吡啶经光化学或热解（闪真空热解，Flash Vacuum Pyrolysis, FVP）生成的2-吡啶基氮宾，均可通过电子自旋共振光谱观测，其零场分裂参数满足|D/hc|≈1.05~1.10 cm⁻¹，|E/hc|≈0.0 cm⁻¹。 对氩基质隔离的氮宾与二氮杂环庚四烯混合物进行辐照，还会在电子自旋共振光谱中出现微弱的卡宾跃迁信号，对应参数满足|D/hc|≈0.40~0.45 cm⁻¹，|E/hc|≈0.006 cm⁻¹。