Photochemische Erzeugung und Reaktionen des Benzonitril‐benzylids. 42. Mitteilung über Photoreaktionen

Photochemical Generation and Reactions of Benzonitrile‐benzylide The low temperature irradiation of 2,3‐diphenyl‐2H‐azirine (1) in DMBP‐glass at −196° has been reinvestigated. It was possible to convert 1 nearly quantitatively into the dipolar species benzonitrile‐benzylide (3, Φ3 = 0,78), which exhibits UV.‐absorptions at 344 (ϵ = 48000) and 244 nm (ϵ = 28500) (Fig. 1, Tab. 1). Irradiation of 3 with 345 nm light at −196° resulted in almost complete reconversion to the azirine 1 (Φ = 0,15; Fig. 2). When the solution of 3 in the DMBP‐glass was warmed up to about −160° a quantitative dimerization to 1,3,4, 6‐tetraphenyl‐2,5‐diaza‐1,3,5‐hexatriene (8) occurred. This proves that 8 is not only formed by the indirect route 3 + 1 → 7 {article}{empty}{document}ʰᵛ{document} 11 → 8 known before (Scheme 1), but also by dimerization of 3 either by direct head to head coupling or via the intermediate e (p. 2675), followed by a fast thermal hydrogen transfer reaction. The occurrence of the dipolar intermediate 11 in the photochemical conversion of the bicyclic compound 7 to 8 could also be demonstrated by low temperature experiments: On irradiation at −196° 7 gave the cherry red dipolar intermediate 11 (λmax = 520 nm), which at −120° isomerizes to 8. It should be noted, that neither 7 nor 11 are formed by dimerization reactions of 3. Experiments carried out at room temperature demonstrate, that both processes for the formation of 8 may compete: Irradiation of a solution of 1 (DMBP, c = 8 × 10−4 to 5 × 10−3M) with 350 nm light of high intensity (which does not excite the bicyclic compound 7) leads to a relative high photostationary concentration of the dipolar species 3. Under these conditions the formation of 8 is due to dimerization of 3 (Φ8 = 0,19). With low light intensity only a very low stationary concentration of 3 can be obtained. Therefore the reaction of 3 with 1, leading to the bicyclic intermediate 7, becomes now predominant (Φ−1 = 1,55, which corresponds with the expected value of 2 × 0,8). Irradiation of 1 at −130° with 350 nm light of high intensity gives 8 with a quantum yield of 0,44. This is in agreement with the theoretical value Φ8 = 0,4 for an exclusive formation of 8 by dimerization of 3. The lower quantum yield for the formation of 8 at room temperature makes probable that under these conditions 3 not only dimerizes to 8, but also to another, so far unidentified dimer, e.g. 2,3,5,6‐Tetraphenyl‐2,5‐dihydropyrazine. By flash photolysis of a solution of 1 (cyclohexane, c = 10−4M, 25°) the disappearance of 3 could directly be measured by UV.‐spectroscopy: At relative high concentrations (c ≥ 10−7M) 3 disappeared according to a second order reaction with the rate constant k = 5 × 107M−1S−1. At lower concentrations (c ≤ 10−7M) the rate of disappearance of 3 follows first order kinetics. The rate constant of this pseudo first order reaction (3 + 1 → 7) has been determined to be 1 → 104M−1S−1. Using Padwa's table of relative rates for the cycloaddition of the dipolar species 3 to various dipolarophiles, including the azirine 1, an absolute rate constant of k ≈ 8 × 108M−1S−1 for the addition of 3 to the most active dipolarophile fumaronitrile could be estimated. In cyclohexane at room temperature, the diffusion controlled rate constant equals 6,6 × 109M−1S−1. In Table 1 the UV.‐maxima of several nitrile‐ylides, among them a purely aliphatic one, are given.