Structure of (Z)-(5R)-methyl-2-(4-phenylbenzylidene)cyclohexanone as chiral component of liquid-crystalline systems

Abstract

The spatial structure of (Z ) (5R) methyl 2 (4 phenylbenzylidene)cyclohexanone prepared by photochemical isomerization of the E isomer was studied by analyzing the magnitudes and temperature dependence of the proton spin spin coupling constants obtained by 1 H NMR spectroscopy and the results of molecular modeling using semiempirical quantum chemical AM1 and PM3 methods and the density functional theory (DFT). Comparison of the results obtained for the Z and E isomers shows that in both cases the conformational equilibrium for both isomers is characterized by significant preference of the chair conformer having an equatorial methyl group, namely, -∆H (chair a chair e) = 1.98-2.12 and 1.36-1.54 kcal mole -1 for the Z and E isomers, respectively. Distinctions in the non planar ity of the enone fragment and cyclohexanone ring in the Z and E isomers under study following from the results of mathematical modeling were confirmed by the experimental values of the geminal spin spin coupling constants of protons of the methylene groups in α,α´ positions with respect to the enone group. Quantum chemical calculations of the Z isomer revealed the existence of intramolecular hydrogen bond between the carbonyl oxygen and the nearest aromatic proton in ortho position of the benzene ring. Possible reasons for different helical twisting power of (Z ) (5R) methyl 2 (4 phenylbenzylidene)cyclohexanone and the E and Z arylidene derivatives of 1R,4R isomenthone in the mesophase are discussed based on the results of molecular structure studies for these compounds.Key words: (Z ) and (E) (5R) methyl 2 (4 phenylbenzylidene)cyclohexanone, confor mational state, 1 H NMR spectroscopy, quantum chemical calculations, intramolecular hydro gen bond. Chiral α,β unsaturated ketones with six membered ring preset s cis configuration of enone fragment and a Z or E configuration relative to the double bond are of considerable interest as components of liquid crystalline (LC) systems with induced helical supramolecular order ing. 1-4 Photoinduced effects in LC compositions, which accompany a reversible photochemical E-Z isomeriza tion typical of these compounds, 5-9 strongly depend on the nature of both the nematic solvent and, especially, the chiral additive (CA), although the reasons for this depen dence are unclear.Photoactive CA include, in particular, 2 arylidene de rivatives of (1R,4R) isomenthone ((3R,6R) 6 isopropyl 3 methylcyclohexanone) 1*, which were studied in detail earlier. 5,6,10,11 UV Irradiation of E isomers in both iso tropic organic solvents 5,10 and LC media 5,11 causes a highly efficient E-Z photoisomerization. A feature of this photochemical transformation consists in a higher quan tum yield of the direct reaction compared to that of the reverse reaction. This predetermines a much higher pro portion of the Z isomer in the photostationary state. 10 An important macroscopic manifestation of this photo transformation in doped LC compositions is an increase in the pitch of the supramolecular helix (P), i.e., its un twisting. 5,11 A n...

New steroid α,β-unsaturated ketones as chiral components of induced cholesteric liquid crystal systems

“…5,8 At the same time, the high helical twisting power of Z isomers of 3R methylcyclohexanone arylidene derivatives, 16 which exist in the "chair" conformation and containing the methyl substituent in the equatorial position, was found. 17 It can be expected that an information on the influ ence of the molecular polarizability of chiral additives would be provided by the comparison of the helical twist Scheme 1 DEA is dehydroepiandrosterone; R 1 = H (1), Ас (2); substituents R 1 -R 4 for all compounds studied are given in Table 1. ing power of the compounds with the double bond in cyclohexane ring B of the steroid skeleton and without it.…”