Liquid Crystal Control of Bimolecular Thermal Reactions. Highly Regioselective Pericycloaddition of Fumarates to 2,6-Dialkoxyanthracenes in Liquid−Crystalline Media

The ability of liquid crystalline solvent phases to control the
stereochemical course of bimolecular thermal
reactions of 2,6-dialkoxyanthracenes with a series of fumarates
conducted at 130−180 °C has been examined, primarily
with respect to the structural compatibility of the solutes with the
solvent mesogens. For the case of the model
thermal [4 + 2] cycloadditions of
2,6-bis(decyloxy)anthracene to bis
(trans-4-cyclohexylcyclohexyl) and cholesteryl
trans-4-cyclohexylcyclohexyl fumarates at 130−150 °C,
cholesteryl 2,4-dichlorobenzoate(CDCB) and bis(4-pentyloxyphenyl)
trans-1,4-cyclohexanedicarboxylate(BPCD) serve well as
cholesteric and smectic liquid crystalline
solvents and result in the preferential formation of
syn-isomers with an extremely high level of regioselection
(syn/anti ≥ 20/1). In contrast, the isotropic solvents with
closely related structures give isomer ratios of only
≥3/1.
Structural similarities between the solutes and the solvent
mesogens appear to play a key and influential role in
controlling the stereochemical course of the reaction. The
temperature dependence for the isomer distribution affords
an estimate of the differences of solvation enthalpy and entropy
between syn and anti transition states in the
anisotropic
media.