Matrix Isolation And Quantum Chemical Calculation Of The Heteroaroyl Nitrenes

As versatile reagents,nitrenes(R-N)have been fascinating chemists for decades.It has been broadly used not only in diverse chemical transformations such as C-H amination and transition-mental catalyzed synthesis of nitrogen-containing compounds,but also in materials science and biologial systems.Moreover,their fundamental properties including the spectra,molecular structures,ground state multiplicity,and rearrangement reactions have also been the focus of numerous computational and experimental studies.Herein,we reported a characterization of the two simplest pyrroylazides(1H-pyrrole-2-and 3-carbonyl azide)and pyridine-carbonyl azides(pyridine-3-and 4-carbonyl azide).In the meantime,the stepwise photodecomposition of both azides via the intermediacy of heteroaroyl nitrenes has been studies by combing matrix-isolation and quantum chemical calculations.1.The two simplest pyrroylazides,namely 1H-pyrrole-2-and 3-carbonyl azide,have been fully characterized by vibrational spectroscopy(IR and Raman)and X-ray crystallography.The photo-induced stepwise decomposition of both azides has been studied by combining matrix-isolation IR spectroscopy(N2-matrix at 15 K)and quantum chemical calculations.Upon a 266 nm laser irradiation,both pyrroylazides split off molecular nitrogen and yield the corresponding pyrroylnitrenes in the closed-shell singlet state.Subsequent irradiation of 1H-pyrrole-2-carbonyl nitrene with UV-light(365 nm)results in Curtius rearrangement to 2-pyrrylisocyanate,as followed by laser-induced(266 nm)CO-elimination and the formation of ring-opening products imines in Z and E-conformations.Similar rearrangement of 1H-pyrrole-3-carbonyl nitrene to 3-pyrrylisocyanate occurs under the green-light(532 nm)irradiation,and the latter eliminates CO and furnishes the elusive 1H-3-pyrrole nitrene in the triplet state.The spectroscopic identification of 3-pyrroylnitrene and 3-pyrrylnitrene has been supported by 15N-labeling experiments and quantum chemical calculations.The spectroscopy of pyrroylnitrenes and 3-pyrrylnitrene has been studied and mechanism for the decomposition in stepwise of both azides has been disclosed.2.Recently,pyridine-3-carbonyl nitrene has been generated from the photodecomposition of pyridine-3-carbonyl azide and used as the key intermediate in probing nucleobase solvent accessibility inside cells by Spitale et al.Following the 266 nm laser photolysis of pyridine-3-and 4-carbonyl azides in solid N2-matrices at 15 K,pyridine-3-and 4-carbonyl nitrenes form.Consistent with spectroscopic analysis and quantum calculation at the CBS-QB3 level,both aroyl nitrenes adopt closed-shell singlet ground state stabilized by significant intramolecular Nnitrene…O interactions.Upon subsequent visible-light irradiations,pyridine-3-carbonyl nitrene(400 ±20 nm)and pyridine-4-carbonyl nitrene(532 nm)undergo rearrangement to pyridyl isocyanates.Further dissociation of pyridyl isocyanates under 193 nm laser irradiation results in CO-elimination and formation of ketenimines via the ring-opening of elusive pyridyl nitrenes,respectively.In addition to the IR spectroscopic identification of 4-pyridine nitrene in the triplet ground state,its reversible photointerconversion with ring-expansion to diazacycloheptatetraene has been observed directly.The spectroscopic identification of the nitrene intermediates was aided by calculations at the B3LYP/6-311++G(3df,3pd)level,and the mechanisms for their generation in stepwise decompositions of azides are discussed in the light of CBS-QB3 calculations.