Comparison of the Photochemistry of 3-Methyl-2-phenyl-2H-azirine and 2-Methyl-3-phenyl-2H-azirine

Abstract

Photolysis of 3-methyl-2-phenyl-2H-azirine (1a) in argon-saturated acetonitrile does not yield any new products, whereas photolysis in oxygen-saturated acetonitrile yields benzaldehyde (2) by interception of vinylnitrene 5 with oxygen. Similarly, photolysis of 1a in the presence of bromoform allows the trapping of vinylnitrene 5, leading to the formation of 1-bromo-1-phenylpropan-2-one (4). Laser flash photolysis of 1a in argon-saturated acetonitrile (λ = 308 nm) results in a transient absorption with λ(max) at ~440 nm due to the formation of triplet vinylnitrene 5. Likewise, irradiation of 1a in cryogenic argon matrixes through a Pyrex filter results in the formation of ketene imine 11, presumably through vinylnitrene 5. In contrast, photolysis of 2-methyl-3-phenyl-2H-azirine (1b) in acetonitrile yields heterocycles 6 and 7. Laser flash photolysis of 1b in acetonitrile shows a transient absorption with a maximum at 320 nm due to the formation of ylide 8, which has a lifetime on the order of several milliseconds. Similarly, photolysis of 1b in cryogenic argon matrixes results in ylide 8. Density functional theory calculations were performed to support the proposed mechanism for the photoreactivity of 1a and 1b and to aid in the characterization of the intermediates formed upon irradiation.

Photolysis of 3,5‐diphenylisoxazole in argon matrices

“…Although ketenimine formation is considered characteristic of the reactivity of triplet vinylnitrenes in cryogenic matrices, we did not observe any new IR bands that could be assigned to ketenimine formation following irradiation of 1 in argon matrices. The calculated IR spectrum of ketenimine 6 (Figure D) has a characteristic band corresponding to the C=C=N stretch at 2049 cm −1 after scaling.…”

“…Because triplet alkylnitrenes and arylnitrenes have high spin properties and are long‐lived, they have a potential as building blocks for high spin assemblies. In contrast, triplet vinylnitrene intermediates are short‐lived in solution, in spite of their electron‐donating substituents, because they decay by intersystem crossing to form azirines and isoxazoles . Triplet vinylnitrenes react differently from alkylnitrenes and arylnitrenes because they have significant 1,3‐biradical character, which makes rotation around the vinyl bond feasible, and it has been theorized that intersystem crossing is facilitated by this rotation …”

“…Photolysis of azirine derivatives with built‐in triplet sensitizers in cryogenic matrices does not result in triplet vinylnitrenes, but rather formation of the corresponding ketenimine derivatives (Scheme ) . It has been theorized that ketenimines are formed at cryogenic temperatures through efficient intersystem crossing of vinylnitrenes to the ketenimine products or via vibrationally hot vinylnitrenes, which highlight that the factors controlling the formation of ketenimines from vinylnitrenes are not well understood …”

Photoinduced reactions of both 2-formyl-2H-azirine and isoxazole: A theoretical study based on electronic structure calculations and nonadiabatic dynamics simulations

“…26,61,[64][65][66] The triplet vinylnitrenes were observed in recent experiments. 24,66,[98][99][100] In contrast, the n → π * excitation of the imine chromophore, promoted at shorter wavelengths, should result in the C 2 -C 3 bond cleavage and produce the nitrile ylide intermediate through an internal conversion to a vibrationally hot ground state. As shown in Figure 2(b), the isomerization from the nitrile ylide to oxazole owns a lower barrier (ca.…”

“…[21][22][23] In fact, the preference of the C-C over C-N photocleavage depends on the nature of substituents and their positions in the azirine ring, besides the applied wavelength. 24,25 With respect to the C-N single bond photocleavage, it has been proposed to proceed by a mechanism involving triplet intermediates that were formed through an intersystem crossing from an excited singlet state. [21][22][23][24][25][26] Our previous calculation, however, showed that the C-N single a) Author to whom correspondence should be addressed.…”