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

Broadband irradiation (lambda > 320 nm) of hemicarceplex H circle dot 1 between -74 degrees C and -84 degrees C, produces encapsulated didehydroazepine (2), triplet phenylnitrene ((PN)-P-3), 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 (Delta G(267K)(double dagger) = 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 (lambda = 334 nm) of matrix-isolated 2, which gave (PN)-P-3 (Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 5879-5881). Encapsulated (PN)-P-3 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(7.7 +/- 0.4) s(-1) x exp {( 13300 +/- 500 cal/mol)/RT}. The isolation of substantial amounts of a hemicarcerand lacking one acetal spanner suggests that (PN)-P-3 decays preferentially by inserting into an inward-pointing acetal C-H bond of H.