Matrix Isolation And Quantum Chemical Calculation Of Triplet Carbonyl Nitrenes

The decompositions of Me?H?NC?O?N₃,PhOC?O?N₃,and 3-PyC?O?N₃ were studied by laser photolysis and flash vacuum pyrolysis combined with a self-constructed matrix isolation system.The corresponding key intermediate carbonylnitrenes were first time directly observed by IR and EPR spectroscopy.To account for the different reactions with the three azides,the underlying mechanisms for their decomposition were explored with the quantum chemical calculations.1.The unsymmetrically substituted N-methylcarbamoyl azide Me?H?NC?O?N₃has been synthesized.We present a comprehensive study on the spectroscopy,conformation,structure,and photo-and pyrolysis decomposition of Me?H?NC?O?N₃from both aspects of experiment and quantum chemical calculations.The molecular structure of Me?H?NC?O?N₃ was established by X-ray crystallography for the first time.In the solid state,both N₃ and CH₃ groups in the molecule adopts a syn conformation with the C=O bond.In contrast,two conformers of the azide,syn-syn and anti-syn,differing mainly in the conformation of the CH₃ group and C=O bond were observed in cryogenic matrices.Upon flash vacuum pyrolysis at 800 K,the azide mainly decomposes into Me?H?NNCO/N₂ and MeNCO/HN₃ through Curtius rearrangement and a H-shift reaction,respectively.In contrast,193 or 266 nm laser irradiation of the azide in cryogenic matrices leads to stepwise Curtius rearrangement via the intermediacy of carbamoylnitrene Me?H?NC?O?N,for which two conformers with the CH₃ group being in syn and anti conformations to the C=O bond have been unambiguously identified by IR spectroscopy.Triplet multiplicity of Me?H?NC?O?N and another two carbamoylnitrenes H₂NC?O?N and Me₂NC?O?N has been further established by matrix isolation EPR spectroscopy.Subsequent visible light irradiation of Me?H?NC?O?N results in the exclusive formation of a novel carbamoyl isocyancte Me?H?NNCO.The molecular structures of Me?H?NC?O?N₃ and Me?H?NC?O?N,multiplicities of the nitrene,and the underlying mechanism for the decomposition of the azide are reasonable explained with quantum chemical calculations by utilizing the B3 LYP,CBS-QB3,CCSD?T?,and CASPT2 methods.2.The decomposition of PhOC?O?N₃ was studied by combining matrix isolation spectroscopy and quantum chemical calculations.Upon laser photolysis?193 and 266nm?,the azide isolated in cryogenic noble gas matrices decomposes into N₂ and a novel oxycarbonylnitrene PhOC?O?N in the triplet ground state which was identified by matrix-isolation IR spectroscopy and EPR spectroscopy for the first time.Subsequent visible-light irradiation of PhOC?O?N mainly causes isomerization into PhONCO which could secondary decomposed into PhO and NCO radical in the photochemistry.The radicals will further recombined in matrices.It is a completing stepwise Curtius rearrangement of the azide PhOC?O?N₃.In contrast to the photochemistry,flash vacuum pyrolysis of PhOC?O?N₃ at 550 K yields N₂ and furnishes the intramolecular C–H amination product 3H-benzooxazol-2-one.Pyrolysis at higher temperature?700 K?leads to further dissociation into CO₂,HNCO,and ring-contraction products.To account for the very different photolytic and thermal decomposition products,the underlying mechanisms for the Curtius rearrangement of the PhOC?O?N₃ and the intramolecular C–H amination of the nitrene in both singlet and triplet states are discussed with the aid of quantum chemical calculations using the B3 LYP,CBS-QB3,and CASPT2 methods.3.The photochemistry of 3-PyC?O?N₃ was studied by combining laser photolysis and quantum chemical calculations.Upon 266 nm irradiation,the azide decomposes into N₂ and a novel carbonylnitrene 3-PyC?O?N.3-PyC?O?N was directly observed of the singlet state with IR spectroscopy in solid N₂ and Ar matrices?15 K?,which agree well with the quantum chemical calculation.Subsequent visible-light irradiation causes rearrangement of the nitrene into 3-PyNCO.The stabilization of the singlet 3-PyC?O?N and the underlying mechanism for the decomposition of 3-PyC?O?N₃ were discussed with the aid of quantum chemical calculations by utilizing the B3 LYP and CBS-QB3 methods.