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

Abstract

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).

Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

“…The work presented in this report was performed in a continuation of our interest in time-resolved kinetic investigations of photochemical reactions initiated by electron transfer (ET). − In our work as well as that by several other groups, cyanoaromatic systems, such as 1,4-dicyanonaphthalene (DCN), 1,2,4,5-tetracyanobenzene (TCB), and 9,10-dicyanoanthracene, have been widely used as excited-state electron acceptors. − ,− In the work reported herein, using nanosecond laser flash photolysis, we studied the fragmentation behavior of radical cations produced as a result of electron transfer from several phenyl-substituted alkanes and alcohols to the lowest singlet excited state of two cyanoaromatics, namely, DCN and TCB. In particular, this undertaking was desirable to resolve an issue regarding the fate of the radical cation of 1,1,2,2,-tetraphenylethane (TPE).…”

“…Chemistry via photoinduced electron transfer (PET) can be very rich owing to the involvement of a variety of transient intermediates, namely, singlet and triplet excited states, radical ion pairs, exciplexes, solvated radical ions, and radicals. − In addition, secondary transient products can be formed and implicated as a result of bimolecular interaction (quenching) of these intermediates with various species, including oxygen. In the recent past, considerable interest has been shown ,− in using time-resolved techniques such as laser flash photolysis and pulse radiolysis to elucidate the mechanisms of chemical transformations arising from photoassisted electron transfer. Much of the focus ,,− in recent fast-kinetic studies of photoinitiated electron transfer has been on the dynamics of contact and solvent-separated radical ion pairs in picosecond and shorter time domains.…”

2-Azaallyl Anions, 2-Azaallyl Cations, 2-Azaallyl Radicals, and Azomethine Ylides

“…As outlined in Scheme , there are several different synthetic strategies for the generation of N-protonated azomethine ylides ( 289 ) and their N-alkylated cousins ( 290 ) including (1) ring-opening reactions, (2) deprotonation, (3) 1,2-prototropic rearrangement, (4) decarboxylation, (5) disilylation/destannylation, and (6) via carbenes and carbenoids. The most classical method involves the thermal (or photochemical) retro -pericyclic ring opening of aziridines ( 291 ). − The mechanism of this ring-opening process and the ensuing 1,3-dipolar cycloaddtions have been investigated thoroughly. − More recent innovations include the use of Lewis acids to mediate the ring-opening of aziridines to azomethine ylides at lower temperatures. − Dihydrooxazoles ( 292 , X = O), ,− dihydrothiazoles ( 292 , X = S), and related heterocycles ,, also can produce N -alkyl azomethine ylide dipoles via ring-opening processes.…”