The phenylnitrene rearrangement in the inner phase of a hemicarcerand

The photochemistry of phenyl azide 1 and ¹³C-labeled phenyl azide ¹³C-1 incarcerated inside a hemicarcerand 4 was investigated. Low-temperature photolysis of hemicarceplex 4·1 and 4·¹³C-1 yields incarcerated 1-azacyclohepta-1,2,4,6-tetraene 4·2 and 4·¹³C-2 (18-50%), respectively, which were characterized by low-temperature FT-IR and ¹H NMR and ¹³C NMR spectroscopy. After correction for the hemicarcerand-induced upfield shift, the ¹³C chemical shifts of incarcerated ¹³C-2 compare very well (Δδ ≤ 3.5 ppm) with the GIAO chemical shift tensors calculated at the PBE1PBE/6-311G++(2d,2p) level. Incarcerated 2 has a lifetime of 32 min at 194.4 K and decomposes by ring contraction to singlet phenylnitrene ¹PN. ¹PN intersystem crosses to triplet phenylnitrene, which reacts with the surrounding host. From the time dependence of the characteristic ketenimine IR absorption of 2 at 1886 cm^-1, the rate constant, k_obs, for decay of 2 was measured between 198.3 and 174.3 K. Under the assumption that the intersystem-crossing rate constant k_ISC and the ring-expansion rate constant k1 in pentane (Gritsan, N. P.; Zhu, Z.; Hadad, C. M.; Platz, M. S. J. Am. Chem. Soc. 1999, 121, 1202) and in the inner phase are identical, the rate constant k′ = 10^11.0 ± 0.7 × exp(-12.3 ± 0.6 kcal/mol/RT) for the ring contraction of 2 was calculated. The activation energy compares very well with those calculated at the CASPT2N/6-311G(2d,p) (Karney, W. L.; Borden, W. T. J. Am. Chem. Soc. 1997, 119, 1378) and B3LYP/6-311+G(2d,p) (Tsao, M.-L.; Platz, M. S. J. Am. Chem. Soc. 2003, 125, 12014) level of theory.