Conformational Dynamics of Polyarylbenzenes. Buttressing Effects

The free energies of activation for rotational stereoisomerization of a series of pentaarylbenzenes have been measured. The results reveal that substantial steric effects are transmitted around the circumference of the central aryl ring from sites remote from the rotating rings. The free energies of activation for compounds bearing various substituents on the central ring range from 15.5 o 18.7 kcal/mol and obey a linear relationship with respect to -ΔG° for the same substituents in the axial-equatorial cyclohexane equilibrium. This relationship permits the estimation of -ΔG° for additional substituents. The value for the 1-hydroxyethyl group was found to be 1.8 kcal/mol, whereas a lower limit of 5.6 kcal/mol was determined for the terf-butyl group.