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(−E a/RT), characterized by the kinetic parameters, E a = 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-d 9 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.

Nona-alkanoyloxy tribenzocyclononene (CTV-n, where n is the number of carbons in the side chains) were prepared for n = 2 to 14. The homologues of this series appear in two stable isomeric forms, rigid crown and flexible saddle. We report on their isomerization equilibria and dynamics in solution and on their mesomorphic properties in the neat state. The crown-saddle equilibrium and interconversion kinetics of the CTV-8 isomers were studied in dimethyl formamide solutions using high-resolution (1)H NMR in the temperature range from 50 to 130 degrees C. At lower temperatures, the isomerization is too slow to measure. In this range the equilibrium saddle fraction increases from approximately 0.40 to approximately 0.65, whereas the isomerization rate increases from approximately 10(-)(4) to approximately 1 s(-)(1). The saddle isomer undergoes fast pseudorotation at room temperature, but below about -50 degrees C, it becomes slow enough to affect the NMR line width. The rate parameters for this process were estimated from the carbon-13 spectra in methylene chloride solutions to be, k(p)(-100 degrees C) approximately 1.7 x 10(3) s(-)(1) and E(a) approximately 9.6 kJ/mol. The slow crown-saddle isomerization at room temperature (half-life of about one year) allows quantitative separation (by chromatography) of the two isomers and their separate investigation. When the alkanoyloxy side chains are sufficiently long both isomers are mesogenic (n >or= 4 for the saddle and n >or= 5 for the crown), exhibiting hexagonal columnar mesophases. The structure, dynamics, and mesomorphic properties of these mesophase were investigated by X-ray diffraction, optical polarizing microscopy, differential scanning calorimetry, and NMR. The lattice parameters of the crown and saddle mesophases of corresponding homologues are almost identical and increase monotonically with increasing length of the side chains. The clearing temperatures of the saddle isomers are consistently lower than those of the corresponding crowns. Within each series, the clearing temperatures are almost independent of the length of the side chains (156 to 170 degrees C for the crown and 115 to 148 degrees C for the saddle). The thermal and kinetic properties of the neat compounds lead to peculiar phase sequences, as observed in the polarizing microscope and in the DSC thermogram, involving repeated, back and forth, interconversion between the two isomers. Carbon-13 MAS NMR measurements of the crown and saddle mesophases of several homologues were carried out. The spectra of the crown mesophase exhibit dynamic features consistent with planar 3-fold molecular jumps about the column axes. A quantitative analysis for the CTV-8 crown homologue yielded the following Arrhenius parameters, A = 3.1 x 10(22)s(-)(1) and E(a) = 130.1kJ/mol. These unusually high values suggest that the barrier to the jump process is temperature dependent, decreasing with increasing temperature. The rate of this 3-fold jump process is slower for the lower homologues and faster for the higher ones. I...