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

Low-temperature photolysis of phenyldiazirine, incarcerated inside a hemica rcerand which is built from two cavitands connected by four butane-1,4-diox y linker groups, yields transient phenylcarbene;this carbene then undergoes ring photochemical expansion to cycloheptatetraene in low yield, Competiti vely, the transiently formed phenylcarbene reacts with the surrounding hemi carcerand. 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 ins ertion into an acetal C-H(D) bond or an alpha -C-H bond of a butane-l,4-dio xy linker group have been isolated and characterized. The measured isotope effect for insertion into an acetal C-W(D) bond at 15.5 K is consistent wit h a reaction of singlet phenylcarbene. Incarcerated cycloheptatetraene is s table for a limited time at 100 degreesC and almost infinitely stable at ro om temperature in the absence of oxygen. NOESY experiments provide the dist ance ratio r(21)/r(23) = 1.134 +/- 0.01 between protons H1-H2 and H2-H3 of cycloheptatetraene which is consistent with its twisted structure. Low-temp erature photolysis of phenyldiazirine, incarcerated inside a chiral hemicar cerand which is built from two cavitands connected with three, butane-1,4-d ioxy and one (S,S)-2,3-O-isopropylidene-2,3-dihydroxybutane-1,4-dioxy linke r group yields two diastereomeric cycloheptatetraene hemicarceplexes in a 2 :3 ratio (30% total yield). Variable temperature H-1 NMR studies provided a lower limit of DeltaG(double dagger) = 19.6 kcal mol(-1) for the enantiome rization barrier of cycloheptatetraene, Incarcerated cycloheptatetraene rea cts rapidly with oxygen to yield benzene and carbon dioxide via the 1,2-dio xaspiro[2,6]nona-4,6,5-triene intermediate. Different mechanisms for the fo rmation of this spirodioxirane intermediate are discussed based on the meas ured rate of the oxygen addition. The activation parameters for the decarbo xylation of the spirodioxirane have been measured in different bulk solvent s. The free energy of activation shows very little solvent dependency Howev er, a strong propensity for enthalpy-entropy compensation due to a solvent reorganization that accompanies the reaction coordinate is observed.