Direct Detection of a Triplet Vinylnitrene, 1,4-Naphthoquinone-2-ylnitrene, in Solution and Cryogenic Matrices

Abstract

The photolysis of 2-azido-1,4-naphthoquinone (1) in argon matrices at 8 K results in the corresponding triplet vinylnitrene (3)2, which was detected directly by IR spectroscopy. Vinylnitrene (3)2 is stable in argon matrices but forms 2-cyanoindane-1,3-dione (3) upon further irradiation. Similarly, the irradiation of azide 1 in 2-methyltetrahydrofuran (MTHF) matrices at 5 K resulted in the ESR spectrum of vinylnitrene (3)2, which is stable up to at least 100 K. The zero-field splitting parameters for nitrene (3)2, D/hc = 0.7292 cm(-1) and E/hc = 0.0048 cm(-1), verify that it has significant 1,3-biradical character. Vinylnitrene (3)2 (λmax ∼ 460 nm, τ = 22 μs) is also observed directly in solution at ambient temperature with laser flash photolysis of 1. Density functional theory (DFT) calculations support the characterization of vinylnitrene (3)2 and the proposed mechanism for its formation. Because vinylnitrene (3)2 is relatively stable, it has potential use as a building-block for high-spin assemblies.

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.…”

Photolysis of 3,5‐diphenylisoxazole in argon matrices

“…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 …”

“…[14][15][16] 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. [17] The most common precursors for forming triplet vinylnitrenes are azirines with built-in triplet sensitizers. Photolysis of azirines through triplet sensitizers cleaves the C-N bond of the azirine moiety to form triplet vinylnitrenes (Scheme 2).…”

Photolysis of 5-Azido-3-Phenylisoxazole at Cryogenic Temperature: Formation and Direct Detection of a Nitrosoalkene

“…Furthermore, they can be rendered stable at cryogenic temperatures by incorporating the vinyl bond into a cyclic structure (Scheme 8). The CASPT2 calculated singlet-triplet energy gaps of cyclic vinylnitrenes, reported to be between 10 and 13 kcal/mol, were similar to or slightly smaller than those calculated for nitrene 2 [36][37][38][39]. Thus, azidoisoxazole 1 reacts differently from isoxazole derivatives because it does not undergo cleavage of its C-O bond, as its azido moiety is more reactive.…”