Organic reactions in liquid crystalline solvents. 1. The thermal cis–trans isomerization of a bulky olefin in cholesteric liquid crystalline solvents

Abstract

The effect of cholesteric liquid crystalline solvents on the energetics of rotational thermal cis–trans isomerization of olefins has been examined. Rate constants have been obtained over a 70-degree temperature range for isomerization of trans-1,2-di-(4-cyanophenyl)-1,2-diphenylethylene in two isotropic solvents and three cholesteric liquid crystals and the Arrhenius parameters determined. The rates of isomerization are found to be consistently slower in the liquid crystalline phases compared to the isotropic solvents. The Arrhenius parameters for isomerization of the olefin in the isotropic solvents (Ea = 34.8 ± 0.3 kcal/mol; ΔS≠ = −1.5 ± 0.5 e.u.) compare favourably with reported values for its isomerization in benzene solution. In the cholesteric phases, Ea is consistently 1–1.5 kcal/mol higher and ΔS≠ slightly more positive than the corresponding values for the isotropic solvents. The results are tentatively rationalized in terms of disruption of liquid crystalline order as the olefin twists from its pseudo-planar, ground state geometry through the globular, twisted transition state. The magnitude of this effect is proposed to depend on both the difference in steric bulk of the ground and transition states and the "tightness" of the solvation shell seen by the isomerizing molecule. It is believed that in the present case the observed effects are somewhat truncated as a result of rather poor solvation of the bulky olefin in the liquid crystalline phases.