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Ultrafast Infrared and UV-vis Studies of the Photochemistry of Methoxycarbonylphenyl Azides in Solution

Content Provider	PubMed Central
Author	Xue, Jiadan Luk, Hoi Ling Eswaran, S. V. Hadad, Christopher M. Platz, Matthew S.
Abstract	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 species 3g and 4g are generated after photolysis of 3a and 4a, respectively, in acetonitrile. Triplet nitrene 4e formation correlated with the decay of singlet nitrene 4b. The presence of water does not change the chemistry or kinetics of singlet nitrenes 2b and 3b, but leads to protonation of 4b to produce nitrenium ion 4f. Singlet nitrenes 2b and 3b have lifetimes of 2 ns and 400 ps, respectively, in solution at ambient temperature. The singlet nitrene 4b in acetonitrile has a lifetime of about 800 ps, and reacts with water with a rate constant of 1.9 × 108 L·mol−1·s−1 at 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. An ortho 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 towards 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 to be less favorable than the isomeric MeO-ketenimine.
Related Links	http://dx.doi.org/10.1021/jp3025705
Ending Page	5336
Page Count	12
Starting Page	5325
File Format	PDF
ISSN	10895639
e-ISSN	15205215
Journal	The Journal of Physical Chemistry. a
Issue Number	22
Volume Number	116
Language	English
Publisher Date	2012-06-07
Access Restriction	Open
Subject Keyword	Physical and Theoretical Chemistry Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Medicine Physical and Theoretical Chemistry

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