Photooxygenation of aziridines and some potential azomethine ylides

Bhat, Vikram; George, M. V.

doi:10.1021/jo01333a002

Cited by 11 publications

(1 citation statement)

References 5 publications

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“…1,3-Dipolar intermediates such as carbonyl and azomethine ylides play key roles in thermal and photochemical 1,3-cycloaddition reactions that are of considerable synthetic importance. − In addition, these intermediates are implicated in a variety of other phenomena and reactions, some examples of which are as follows: thermochromism/photochromism, isomerization and photooxygenation of small-ring heterocycles, − photoinduced electrocyclization of vinyl sulfides and enamines, − reactions of carbenes with molecules containing multiply bonded heteroatoms, − and ozonolysis of alkenes. , Since 1,3-dipoles are of transitory nature under ambient reaction conditions, their involvement in a chemical reaction is established through chemical trapping with dipolarophiles (leading to cyclic adducts) and with alcohols and HX-type reagents (e.g., carboxylic acids). Time-resolved techniques, such as laser flash photolysis and pulse radiolysis allow real-time observation of these intermediates in fluid solutions…”

Section: Introductionmentioning

confidence: 99%

Photochemical Ring-Opening in 2,3-Diphenyl Aziridines. Transient-Spectral and Kinetic Behavior of Azomethine Ylides and Related Photointermediates

2013

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By employing laser pulses at various wavelengths for nanosecond flash photolysis, a comprehensive time-resolved study has been performed on transient azomethine ylides photogenerated from several 2,3-diphenyl aziridines in fluid solutions under three different conditions, namely, by direct 266 nm excitation, under reversible electron-transfer sensitization by 1,4-dicyanonaphthalene singlet excited state, and via energy transfer from acetone triplet. Under each of the three conditions of photoexcitation, azomethine ylides are readily formed as transient species, characterized by broad, structureless absorption spectra with maxima at 470-500 nm and mostly complex decay kinetics in μs-ms time domain. Under acetone triplet sensitization, a second, shorter-lived transient species with absorption maximum at ∼360 nm is observed to grow and decay in the same time range as that of the growth of ylides. This species has been identified as the ring-opened precursor ylide triplet. The azomethine ylides are practically nonquenchable by oxygen, except that under acetone triplet sensitization in air-saturated acetonitrile, their decay is significantly enhanced. The latter is explained in terms of quenching through dipolarophilic reaction with singlet oxygen. A value of 1.6 × 10(9) M(-1) s(-1) has been estimated for the rate constant for reaction between singlet oxygen and ylide from trans-2,3-diphenylaziridine. We also report rate constants, in the range 2 × 10(3) to 4 × 10(9) M(-1) s(-1), for the quenching of azomethine ylides by two dipolarophiles, namely, maleic anhydride and dimethyl acetylene dicarboxylate. The dipolarophilic reactivity of ylides carrying bulky substituents on the N atom is relatively subdued. Acetic acid proved to be a modest quencher of ylides with rate constants close to 10(6) M(-1) s(-1).

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