Photolysis of (3-Methyl-2H-azirin-2-yl)-phenylmethanone: Direct Detection of a Triplet Vinylnitrene Intermediate

Abstract

The photoreactivity of (3-methyl-2H-azirin-2-yl)-phenylmethanone, 1, is wavelength-dependent (Singh et al. J. Am. Chem. Soc. 1972, 94, 1199-1206). Irradiation at short wavelengths yields 2P, whereas longer wavelengths produce 3P. Laser flash photolysis of 1 in acetonitrile using a 355 nm laser forms its triplet ketone (T(1K), broad absorption with λ(max) ~ 390-410 nm, τ ~ 90 ns), which cleaves and yields triplet vinylnitrene 3 (broad absorption with λ(max) ~ 380-400 nm, τ = 2 μs). Calculations (B3LYP/6-31+G(d)) reveal that T(1K) of 1 is located 67 kcal/mol above its ground state (S(0)) and has a long C-N bond (1.58 Å), and the calculated transition state to form 3 is only 1 kcal/mol higher in energy than T(1K) of 1. The calculations show that 3 has significant 1,3-carbon iminyl biradical character, which explains why 3 reacts efficiently with oxygen and decays by intersystem crossing to the singlet surface. Photolysis of 1 in argon matrixes at 14 K produced ketene imine 7, which presumably is formed from 3 intersystem crossing to 7. In comparison, photolysis of 1 in methanol with a 266 nm laser produces mainly ylide 2 (λ(max) ~ 380 nm, τ ~ 6 μs, acetonitrile), which decays to form 2P. Ylide 2 is formed via singlet reactivity of 1, and calculations show that the first singlet excited state of the azirine chromophore (S(1A)) is located 113 kcal/mol above its S(0) and that the singlet excited state of the ketone (S(1K)) is 85 kcal/mol. Furthermore, the transition state for cleaving the C-C bond in 1 to form 2 is located 49 kcal/mol above the S(0) of 1. Thus, we theorize that internal conversion of S(1A) to a vibrationally hot S(0) of 1 forms 2, whereas intersystem crossing from S(1K) to T(1K) results in 3.

Vinylnitrene Formation from 3,5-Diphenyl-isoxazole and 3-Benzoyl-2-phenylazirine

“…36 The transient spectrum of vinylnitrene 4 is somewhat similar to the transient spectrum of vinylnitrene 10 (Scheme 9); they both have broad absorptions with a λ max at 360 nm that trail out to 500 nm. 18 The absorption in vinylnitrenes 4 and 10 are due to several electronic transitions between the lone pair on the oxygen, the half-filled p-orbital on the N-atom and the π-orbitals into the half-filled p-orbital on the N-atom, and the π*-orbitals. Vinylnitrenes 4 and 10 also have similar lifetimes of a few microseconds because they both decay by efficient intersystem crossing to their singlet states.…”

“…Ylide 5 has a lifetime similar to those of other ylides that decay by intramolecular reaction, but it is much shorter lived than those of ylides that decay by bimolecular reactions. 18,36 The transient absorption at 500 nm forms and decays with the same rate constants as the absorption at 360 nm.…”

“…Furthermore, the absorption intensity is higher at 360 nm in oxygen-saturated solution compared to argonsaturated solution. Kinetic analysis at 460 nm shows that in 35 The laser flash photolysis of 2 in argon-saturated acetonitrile produced a transient spectrum showing a broad absorption with the λ max at 360 nm ( Figure 12). We assign this absorption to vinylnitrene 4 and ylide 5 based on the following.…”

Triplet Sensitized Photolysis of a Vinyl Azide: Direct Detection of a Triplet Vinyl Azide and Nitrene

“…In 1c the CC and CN bond distances are 1.41 and 1.30 Å, respectively, similar to what we have reported previously for vinylnitrenes using DFT calculations. 3 The calculated spin density of vinylnitrene 1c demonstrates that the unpaired electrons are located mainly on the N atom (−1.41) and the Cβ atom (−0.66) ( Figure 2). The calculated rotational barrier around the CH 2 −CC−N bond in vinylnitrene 1c is displayed in Figure 3.…”

“…Vinylnitrenes decay by intersystem crossing to yield azirines, although vinylnitrenes that have a γ-carbonyl group intersystem cross to form isoxazole derivatives in competition with formation of azirines. 3,4,6 The singlet−triplet energy gap in vinylnitrenes has been calculated to be ∼15 kcal/mol. 7 On the basis of this, the lifetimes of triplet vinylnitrenes are limited by their rate constant for intersystem crossing to their singlet products, which for the triplet vinylnitrene is on the order of 10 5 −10 6 s −1 .…”