Static and Dynamic Stereochemistry of Hexaisopropylbenzene: A Gear-Meshed Hydrocarbon of Exceptional Rigidity

Abstract

The crystal and molecular structure of hexaisopropylbenzene (1) has been analyzed by X-ray methods at 99 K: space group P1, a = 6.400 (2) Å, b = 9.943 (3) Å, c = 10.223 (2) Å, α = 117.79 (2)°, β = 94.78 (3)°, γ= 105.58 (3)°, Z = 1. The molecules closely approximate C6h symmetry in the crystal. A crystallographic orientational disorder around the molecular sixfold axes was modeled with the help of simple structural and geometrical considerations. The molecular model thus derived accords well with the C6Hground state calculated by the empirical force field (EFF) method. The relative energies of the nine conformers of 1 have been estimated by EFF calculations, and a one-to-one mapping of the conformers of 1 and hexaethylbenzene is developed. An improved approach for the synthesis of 1 has been devised, based on a novel method for obtaining the precursor, diisopropylacetylene. Cotrimerization of diisopropylacetylene with an elevenfold molar excess of diisopropylacetylene-dl4in the presence of Hg[Co(CO)4]2gives a mixture of isotopomers in which 1 -d28 is the major isomer observable by 1H NMR spectroscopy. From the noncoalescence of the resonance-doubled signals of methyl and methine protons, a ΔG* value of ≥22 kcal mol-1is estimated for the process of internal rotation in 1. EFF calculations serve to rule out correlated rotation of the isopropyl groups. A reaction graph for stepwise rotation has been constructed and saddle points calculated for the individual transitions. These calculations indicate that the process of homomerization by rotation of all six isopropyl groups requires ca. 35 kcal mol-1. © 1986, American Chemical Society. All rights reserved.