Solute–solvent interactions in ordered phases: Asymmetric induction in cholesteric liquid crystals

Abstract

It is well known that diastereotopic solute–solvent interactions between a prochiral center and a chiral solvent can lead to some extent of asymmetric induction. A cholesteric liquid crystal is indeed a chiral organized medium having a macrostructural helical shape. The question at the start of this work was to determine if the macrostructural handedness of the cholesteric liquid crystal used as a solvent of a chemical reaction can control the stereochemistry of the reaction. Recently several papers pointed out a large controversy on such question. On the one hand, several research groups reported moderate extents of asymmetric induction during high temperature reactions conducted in cholesteric mesophases such as the Claisen rearrangement of O-allylarylethers [1], enantiomeric decarboxylation [2] or enantiomeric equilibration of sulfoxides [3]. On the other hand, Kagan and co-workers [4] did not succeed in reproducing these literature results and reported no detectable asymmetric induction during several photochemical processes. On the basis of these results these authors concluded by doubting that a cholesteric mesophase could afford appreciable asymmetric induction and that the effect of mesomorphic anisotropy ordering on asymmetric induction remains to be clearly established. Our own results dealing with Hofman pyrolysis of quaternary slats in cholesteric medium and enantiomeric equilibraition of trans-cyclooctene offer evidence that the stereochemical outcome of the reaction conducted in liquid crystals is dependent on the nature of the mesophase and that the asymmetric induction is governed by the ‘ local’ asymmetry of the mesophase and solute–solvent interactions and not by the macrostructural handedness of the mesophase. The photochemical synthesis of chiral hexahelicence in a compensated nematic phase confirms strongly these conclusions. Some typical results: A Trans-cyclooctene (−) R Yield e.e. Pyrolysis of Trimethylcyclooctyl Ammonium 180 °C-cholesteric A 46% 3.6% 130 °C-cholesteric B 52% 7.1% 162 °C-isotropic phase C 57% 0% Enantiomeric Equilibration of Racemic tran-Cyclooctene 180 °C-cholesteric C 53% 1.9% 185 °C-cholesteric D 61% 1.4% 185 °C-cholesteric E 64% 0% 180 °C-isotropic phase F 62% 0% Hexahelicene (+) P Yield e.e. Photo-asymmetric Synthesis of Hexalhelicene 27 °C-cholesteric G 75% 0.66% [5] 42 °C-compensated nematic H 75% 0.7% 57 °C-isotropic phase G 75% 0.2% A : 3-( p-anisyl)3,5-cholestadiene. B : a mixture of 44.5% of A, 40.2% of C and 15.3% of p-azoxyanisole. C : 3-phenyl-3,5-cholestadiene. D : 3-( p-tolyl)3,5-cholestadiene. E : cholesteryl p-nitrobenzoate. F : compound A diluted by decaline. G : mixture of cholesteryl nonanoate and chloride in the ratio 3 2 . H : mixture of cholesteryl chloride and myristate in the ratio 1.75 1 .