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  1. Free, publicly-accessible full text available May 8, 2025
  2. Abstract

    Although alkyl azides are known to typically form imines under direct irradiation, the product formation mechanism remains ambiguous as some alkyl azides also yield the corresponding triplet alkylnitrenes at cryogenic temperatures. The photoreactivity of 3‐azido‐3‐phenyl‐3H‐isobenzofuran‐1‐one (1) was investigated in solution and in cryogenic matrices. Irradiation (λ = 254 nm) of azide 1 in acetonitrile yielded a mixture of imines 2 and 3. Monitoring of the reaction progress using UV‐Vis absorption spectroscopy revealed an isosbestic point at 210 nm, indicating that the reaction proceeded cleanly. Similar results were observed for the photoreactivity of azide 1 in a frozen 2‐methyltetrahydrofuran (mTHF) matrix. Irradiation of azide 1 in an argon matrix at 6 K resulted in the disappearance of its IR bands with the concurrent appearance of IR bands corresponding to imines 2 and 3. Thus, it was theorized that azide 1 forms imines 2 and 3 via a concerted mechanism from its singlet excited state or through singlet alkylnitrene11N, which does not intersystem cross to its triplet configuration. This proposal was supported by CASPT2 calculations on a model system, which suggested that the energy gap between the singlet and triplet configurations of alkylnitrene 1N is 33 kcal/mol, thus making intersystem crossing inefficient.

     
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  3. Broadband irradiation of 3,5‐diphenylisoxazole1in an argon matrix results in formation of azirine3. Further irradiation of the matrix reduces the amount of azirine3with concurrent formation of ylide4. Thus, it is theorized that the conversion of isoxazole1to azirine3goes through a triplet vinylnitrene2that does not intersystem cross to ketenimine6. Hence, the reactivity of triplet vinylnitrene2is different from similar vinylnitrene intermediates with α‐methyl substituents that intersystem cross to form corresponding ketenimines. Density functional theory calculations support the notion that the conjugation of the α‐phenyl group to the vinylnitrene moiety in vinylnitrene2renders it more flexible than vinylnitrenes with α‐methyl substituents, and therefore, vinylnitrene2intersystem crosses to azirine3, rather than ketenimine6. Copyright © 2016 John Wiley & Sons, Ltd.

     
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