Ultrafast infrared and UV-vis studies of the photochemistry of methoxycarbonylphenyl azides in solution.
Ultrafast infrared and UV-vis studies of the photochemistry of methoxycarbonylphenyl azides in solution.
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溶液中甲氧基羰基苯基叠氮化物光化学的超快红外和紫外可见研究。
DOI：
10.1021/jp3025705
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发表时间：
2012
期刊：
影响因子：
--
通讯作者：
Platz,MatthewS
中科院分区：
文献类型：
--
作者：
Xue,Jiadan;Luk,HoiLing;Eswaran,SV;Hadad,ChristopherM;Platz,MatthewS
The photochemistry of 4-methoxycarbonylphenyl azide (2a), 2-methoxycarbonylphenyl azide (3a), and 2-methoxy-6-methoxycarbonylphenyl azide (4a) were studied by ultrafast time-resolved infrared (IR) and UV–vis spectroscopies in solution. Singlet nitrenes and ketenimines were observed and characterized for all three azides. Isoxazole species3gand4gare generated after photolysis of3aand4a, respectively, in acetonitrile. Triplet nitrene4eformation correlated with the decay of singlet nitrene4b. The presence of water does not change the chemistry or kinetics of singlet nitrenes2band3b, but leads to protonation of4bto produce nitrenium ion4f. Singlet nitrenes2band3bhave lifetimes of 2 ns and 400 ps, respectively, in solution at ambient temperature. The singlet nitrene4bin acetonitrile has a lifetime of about 800 ps, and reacts with water with a rate constant of 1.9 × 108L·mol–1·s–1at room temperature. These results indicate that a methoxycarbonyl group at either the para or ortho positions has little influence on the ISC rate, but that the presence of a 2-methoxy group dramatically accelerates the ISC rate relative to the unsubstituted phenylnitrene. Anortho-methoxy group highly stabilizes the corresponding nitrenium ion and favors its formation in aqueous solvents. This substituent has little influence on the ring-expansion rate. These results are consistent with theoretical calculations for the various intermediates and their transition states. Cyclization from the nitrene to the azirine intermediate is favored to proceed toward the electron-deficient ester group; however, the higher energy barrier is the ring-opening process, that is, azirine to ketenimine formation, rendering the formation of the ester–ketenimine (4d′) to be less favorable than the isomeric MeO–ketenimine (4d).
