Search

Antipodal substitution of the [closo-1-CB11H12](-) anion with a 4-pentylquinuclidinium fragment and alkyl groups in positions C(1) and B(12) gave polar zwitterions 1[n] and 2[n]. The molecular structure of 1[5] was established using X-ray diffraction (XRD) methods: P1̅, a = 15.162(2) Å, b = 16.546(3) Å, c = 19.794(3) Å; α = 84.871(2)°, β = 84.057(2)°, γ = 84.058(3)°; Z = 8. All compounds exhibit high temperature in-plane ordered smectic phases that are stabilized by dipolar interactions. The ordered phases were investigated by powder XRD methods. Thermal and dielectric parameters for two derivatives, 1[0] and 1[6], were evaluated in nematic hosts, ClEster and BPhF. The dielectric data were analyzed with the Maier-Meier formalism augmented with density functional theory methods, and the results were compared to those for similar zwitterions previously reported.

Several 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes (1-4) were synthesized on > or = 0.1 mol scale. The heat-induced retro Diels-Alder (rDA) decomposition of these derivatives was studied by thermal analysis, and the kinetics of the rDA were measured for 4. First-order rate constants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures between 124 and 150 degrees C, were used to calculate Arrhenius activation parameters Ea (34.5 +/- 0.5 kcal/mol) and ln A (1.77 +/- 0.03 x 10(4)). The observed activation energy was significantly larger (by 9.5 kcal/mol) than that previously measured for the maleic anhydride adduct 1, and this was attributed to the difference in LUMO energies for the two dienophiles. Modeling of the activation parameters found for 4 with density functional theory (DFT) calculations for similar compounds 5 and 6 gave close quantitative correlations for deltaH double dagger, deltaG double dagger, deltaS double dagger. The rDA reactions studied were found to be entropy-driven.

A triphenylsilyl group is used as an auxiliary in the synthesis of heterodisubstituted p-carboranes via triphenylsilyl-p-carborane (1). The preparation of 1 is statistical, but with recovery of the starting p-carborane, the effective conversion to 1 is about 90%. Carborane 1 has been easily converted to its lithium and copper derivatives, which were reacted with a range of electrophiles including alkyl halides: an aryl iodide, an acetylene bromide, and a sulfenyl chloride. The derivatives of 1 are crystalline and UV active, which facilitates their isolation and purification. The Ph(3)Si group is efficiently removed with fluoride to give monosubstituted p-carboranes I, which upon further nucleophilic substitution yield p-carboranes II. The yields of heterodisubstituted products II are higher than for direct, "statistical" syntheses. For example, 12-pentyl-1, 12-dicarba-closo-dodecaborane-1-carboxylic acid was synthesized from p-carborane via 1 in 62% overall yield, a considerable improvement over the 44% yield obtained by direct methods.