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Abstract:
Cyclotriveratrylene (CTV), or hexamethoxy tribenzocyclononatriene (hexamethoxy-TBCN) and other peripherally hexa-(or tri-)substituted TBCN were so far only known to exist in their crown form. Attempts to detect the corresponding saddle isomers failed, even though when structurally chiral these compounds undergo racemization, a process that is believed to proceed via the saddle form. We show that the saddle form of CTV (and other peripherally hexa-substituted TBCN) can be obtained by rapidly quenching a hot solution or the high-temperature melt to below room temperature. The saddle form of CTV was quantitatively separated from the quenched material by column chromatography, and some of its thermodynamic and kinetic properties in solution and in the solid state were determined by proton and carbon-13 NMR. Detailed measurement were performed in chloroform solutions, for which it was found that at room temperature the equilibrium constant, K = [saddle]/[crown] = exp[−(ΔH − TΔS)/RT, is ∼0.1 (with ΔH = 9.96 ± 0.5 kJ mol-1, ΔS = 13.8 ± 1.6 J mol-1 K-1). The isomerization half-life at room temperature is about 1 day with the rate constant for the crown to saddle transformation, (k(crown→saddle) = A exp(−Ea/RT), characterized by the kinetic parameters, Ea = 97.4 ± 4.8 kJ mol-1 and log[A (s-1)] = 11.0 ± 0.8). These parameters are consistent with those for the racemization rate of the isotopically chiral CTV-d9 measured by Collet and Gabard (J. Org. Chem. 1980, 45, 5400−5401). Attempts to freeze-out the fast pseudorotation of the saddle isomer by cooling a Freon solution to almost 100 K, failed, setting a lower limit of 106 to 107s-1 for the pseudorotation rate at 120 K. Carbon-13 MAS spectra indicate that the saddle isomer of CTV in the solid state is crystalline with two (nonequivalent and distorted) molecules per asymmetric unit.
