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

Article abstract of DOI:10.1021/ja00267a028

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) Angstroem, b = 9.943(3) Angstroem, c = 10.223(2) Angstroem α = 117.79(2) deg, β = 94.78(3) deg, γ = 105.58(3) deg, Z = 1.The molecules closely approximate C_6h 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 C_6h ground 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-d₁₄ in the presence of Hg2 gives 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 ae 22 kcal mol^-1 is 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.