New chiral 3-aryl-7-arylmethylidene-3,3a,4,5,6,7-hexahydroindazoles: Synthesis, structure, and twisting power in nematic liquid crystals

Abstract

3R)-2,6-Bis(arylmethylidene)-3-methylcyclohexanones and 3-(4-X-aryl)-7-(4-X-arylmethylidene)-3,3a,4,5,6,7-hexahydro-2,6-and -2,4-dimethyl-2H-indazoles (X = Cl, HO, AlkO, AlkOCO, MeOC 6 H 4 COO) as potential chiral additives to liquid crystalline systems were synthesized starting from (3R)-3-methylcyclohexanone. Relations between the twisting power and molecular structure of the synthesized compounds are discussed.Modern compositions for imaging devices based on liquid crystalline materials usually contain several components, among which chiral additives are important. Such additives are capable of inducing formation in mesophase of helical supramolecular structures via transformation of nematic (N) and smectic-C (SmC) phases into cholesteric and ferroelectric SmC* [1-4]. Diversity of requirements to chiral components of liquid crystals makes search for new chiral additives an important problem. Biological activity of substituted hexahydroindazoles and their N-methyl and sulfonyl analogs was studied [5][6][7][8], and their chemical properties were also examined.The present study was aimed at synthesizing photostable chiral additives with a strong twisting power on the basis of (3R)-3-methylcyclohexanone (I). A general procedure for the synthesis of 2-substituted hexahydroindazoles is based on reaction of benzylidenecyclohexanones with hydrazine derivatives in the presence of hydrochloric [5,9] or acetic acid [6], as well as in the absence of a catalyst in alcoholic medium [7]. Syntheses under basic conditions are less common [10]; in these cases, alternative products are formed [11][12][13].We found that the reactivity of initial ketones II toward various hydrazine derivatives is considerably lower than the reactivity of 2,6-bis(arylmethylidene) cyclohexanones. Both ketones II and 2,6-bis(arylmethylidene)cyclohexanones readily reacted with hydrazine and methylhydrazine, whereas the former failed to react with phenylhydrazine and thiosemicarbazide. Hexahydroindazoles obtained by the reaction with hydrazine readily undergo oxidation with atmospheric oxygen. Even though N-acylation stabilizes these compounds, their twisting power in liquid crystals strongly weakens. We considered compounds III and IV as target components of liquid crystalline compositions. They were synthesized under conditions of base catalysis using triethylamine (in the synthesis of IIIb) [14] or excess methylhydrazine (IIIa-IIIe) to avoid tarring in acid medium. In the general case, the reaction with methylhydrazine gives two groups of regioisomeric products III and IV (Scheme 1), depending on the double bond (C 2 =C or C 6 =C) involved in the formation of pyrazole fragment; in addition, cis-trans isomerism with respect to the C 3 -C 3a bond is possible; therefore, eight regio-and stereoisomers may be formed.In fact, the 1 H NMR data showed the presence in the reaction mixtures of appreciable amounts of three or four compounds, which were identified by the signal from the methyl group on the cyclohexane ring. In the reaction with ketone IIb we su...