Orientational Control of Chemical Reactivity in Liquid Crystalline Solvents

Abstract

<p>The effect of cholesteric and smectic liquid crystalline solvent order on the regiochemical control of a thermal bimolecular reaction has been investigated. The cycloaddition reaction of cholesta-5,7-dien-3β-ylacetate with rigid enophiles has been shown to yield several 1:1 cycloadducts. The relative product yields for this reaction have been determined in a number of isotropic solvents, two cholesteric liquid crystals and a highly ordered smectic B phase. In ordered media, the relative yield of one product is enhanced at the expense of the others. The magnitude of this effect is highly dependent on the rod-like shape of the enophile and on the ordered nature of the solvent. We rationalize these results in terms of how the average orientation of the reactants within the liquid crystal influences the energetics of reaction. The enhanced adduct occurs via a transition state orientation that, compared to the other products, is much more compatible with liquid crystalline order. In ordered media, this pathway becomes energetically favoured. The largest effects on reactivity are seen when the reactants are rod-like in shape, the reactive centres are conformationally immobile and the solvent is highly ordered. This is explained in terms of how reactant shape and solvent order influence the average orientation of the solutes within the liquid crystal. It appears that more rod-like reactants are better oriented, resulting in a larger influence on reaction energetics. The more ordered smectic mesogen exerts a larger influence on regiochemical control than the more fluid cholesteric phases, presumably due to a better average orientation of the reactant(s). Activation parameters and thermal microscopy data indicate that while the orientational control of reactivity is greater in the smectic phase, the reactants may experience a heterogeneous environment.</p>