Photochemical and thermal reactions of intermediates in the phenylnitrene rearrangement inside a hemicarcerand

Broadband irradiation (λ > 320 nm) of hemicarceplex H⊙1 between -74°C and -84°C, produces encapsulated didehydroazepine (2), triplet phenylnitrene (³PN), 2-azabicyclo[3.2.0]hepta-1,3,6-triene (6), and 4-azaspiro[2.4]hepta-1,4,6-triene (7). The highly strained anti-Bredt imine 6 is formed from 2 via a photochemical four-electron electrocyclization. Under the irradiation conditions, 6 rearranges further to azaspirene 7. In addition, 6 thermally rearranges to 7 via a 1,5-sigmatropic shift (ΔG ^‡_267K = 20.0 ± 0.5 kcal/mol), yielding a final equilibrium composed of [7]/[6] = 5 at room temperature. The observation of a photochemical rearrangement of 2 to 6 contrasts earlier results of narrow band irradiations (λ = 334 nm) of matrix-isolated 2, which gave ³PN (Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 5879-5881). Encapsulated ³PN is remarkably stable due to the prevention of its dimerization by the surrounding hemicarcerand. Above 255 K, it slowly decays with a rate constant k = 10^77±04 s^-1 x exp {(13300 ± 500 cal/mol)/RT}. The isolation of substantial amounts of a hemicarcerand lacking one acetal spanner suggests that ³PN decays preferentially by inserting into an inward-pointing acetal C-H bond of H.