Chemistry and properties of cycloheptatetraene in the inner phase of a hemicarcerand

Low-temperature photolysis of phenyldiazirine, incarcerated inside a hemicarcerand which is built from two cavitands connected by four butane-1,4-dioxy linker groups, yields transient phenylcarbene; this carbene then undergoes ring photochemical expansion to cycloheptatetraene in low yield. Competitively, the transiently formed phenylcarbene reacts with the surrounding hemicarcerand. The yield of the photochemical ring expansion was increased when the photolysis was carried out inside a partially deuterated hemicarcerand. Two insertion products resulting from an intramolecular phenylcarbene insertion into an acetal C-H(D) bond or an α-C-H bond of a butane-1,4-dioxy linker group have been isolated and characterized. The measured isotope effect for insertion into an acetal C-H(D) bond at 15.5 K is consistent with a reaction of singlet phenylcarbene. Incarcerated cycloheptatetraene is stable for a limited time at 100°C and almost infinitely stable at room temperature in the absence of oxygen. NOESY experiments provide the distance ratio r₂₁/r₂₃ = 1.134±0.01 between protons H1 - H2 and H2 - H3 of cycloheptatetraene which is consistent with its twisted structure. Low-temperature photolysis of phenyldiazirine, incarcerated inside a chiral hemicarcerand which is built from two cavitands connected with three butane-1,4-dioxy and one (S,S)-2, 3-O-isopropylidene-2,3-dihydroxybutane-1,4-dioxy linker group yields two diastereomeric cycloheptatetraene hemicarceplexes in a 2:3 ratio (30% total yield). Variable temperature ¹H NMR studies provided a lower limit of ΔG≠ = 19.6 kcal mol^-1 for the enantiomerization barrier of cycloheptatetraene. Incarcerated cycloheptatetraene reacts rapidly with oxygen to yield benzene and carbon dioxide via the 1,2-dioxaspiro[2,6]nona-4,6,8-triene intermediate. Different mechanisms for the formation of this spirodioxirane intermediate are discussed based on the measured rate of the oxygen addition. The activation parameters for the decarboxylation of the spirodioxirane have been measured in different bulk solvents. The free energy of activation shows very little solvent dependency. However, a strong propensity for enthalpy-entropy compensation due to a solvent reorganization that accompanies the reaction coordinate is observed.