Structure of trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) in the isotropic and nematic phases: a computer simulation study

We have studied the single particle structural properties of the nematogen trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) by molecular dynamics simulation using realistic atom-atom potentials. On going from the isotropic phase at 390 K to the nematic phase at 350 K, the molecules become significantly longer and thinner, as indicated by the equivalent molecular moment-of-inertia spheroid and the distribution of trans and gauche bonds. This change is only partly accounted for by the lowering of the temperature, there being a significant quenching effect due to the change in the molecular environment. This quenching effect is also apparent in the distribution of molecular shapes seen in molecular width-breadth contour maps. In the nematic phase, at 350K, the distributions of alkyl tail bond orientations with respect to the director show a pronounced odd-even effect, with peaks in the distributions occurring alternately parallel to, and at an angle to, the director.     

Studies in stereochemistry—I Stereospecific routes to trans- anti- and cis- syn-Δ-dodecahydrophenanthrenes

Reduction of trans-1-oxo-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XI) by lithium tri-t-butoxyaluminohydride gave trans-1β-hydroxy-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XII) which on lithium—liquid ammonia reduction gave trans-anti-1β-hydroxy-7-oxo-Δ⁸⁽¹⁴⁾-dodecahydrophenanthrene (XIII). Reduction of cis-1-oxo-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XV) by sodium borohydride gave cis-1-hydroxy-7-methoxy-1,2,3,4,9,10,11,12-octahydrophenanthrene (XVI) which on lithium—liquid ammonia reduction gave cis-syn-1-hydroxy-7-oxo-Δ⁸⁽¹⁴⁾-dodecahydrophenanthrene (XVII).     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates

The effect on the liquid crystal transition temperatures of introducing various groups (for example incorporating C=C, O, CO₂ and CO) into the terminal alkyl chain of a weakly polar model compound 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pentane has been investigated systematically. Only the compound containing both an ester function and a trans-carbon-carbon double bond exhibited a wide-range nematic mesophase at elevated temperatures. Therefore, a wide variety of trans-4-substituted-cyclohexyl (E)-alk-2-enoates incorporating a carbon double bond with a trans-configuration (E) in the terminal alkyl chain has been synthesized. Nearly all the two-ring esters prepared exhibit a nematic phase over a wide temperature range (≤ 100°C) at elevated temperatures (≤ 200°C). The tendency to form smectic mesophases is often low. Comparisons with the corresponding derivatives incorporating either just a carboxy group (COO) or just a carbon-carbon double bond (C=C) in the same positions indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. The new compounds are easily prepared from known starting materials.     

Synthesis and mesomorphic properties of three- and four-ring liquid crystals containing cyclohexyl, phenyl and pyridyl units

Two series of 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl trans-4-n-alkylcyclohexylcarboxylates and 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl 4-(trans-4-n-alkylcyclohexyl)benzoates have been prepared. Their mesmorphic properties were observed and measured by polarizing optical microscopy and differential scanning calorimetry. The relationship between structures and properties is discussed, showing that the ferroelectric phase can be observed when chiral terminal chains exist.     

The synthesis and liquid crystal properties of certain 1-(4'-n-alkoxy-2,2',3,3',5,5',6,6'-octafluorobiphenyl-4-yl)- 2-(trans-4-n-pentylcyclohexyl)-ethanes and -ethenes

The synthesis and liquid crystal properties of eight members of the homologous series of 1-(4'-n-alkoxy-2,2',3,3',5,5',6,6'-octafluorobiphenyl-4-yl)-2-(trans-4-n-pentylcyclohexyl)-ethanes and eight members of the corresponding -ethanes, which have the E-configuration, are reported. These compounds are nematic, the ethenes having a considerably mesophase higher thermal stability than the analogous ethanes.     

Bis(2-bromoethyl) selenium dibromide as the selenium-introducing reagent: One-pot preparation of 2,5-bis(alkoxymethyl)tetrahydroselenophenes by the cyclization of 1,5-hexadiene

The reaction of bis(2-bromoethyl)selenium dibromide (1a) with 1,5-hexadiene (2) in methanol or ethanol affords 2,5-bis(alkoxymethyl)tetrahydroselenophene-1,1-dibromides (R = CH₃ (3b), R = C₂H₅ (3c)) via 2,5-bis(bromomethyl)tetrahydroselenophene-1,1-dibromide (3a). The reaction of 1a with 2 in 1-propanol, 2-methyl-1-propanol or 1-butanol in the presence of sodium carbonate gave 2,5-bis(alkoxymethyl)tetrahydroselenophene (R = C₃H₇ (4a), R = (CH₃)₂CHCH₂ (4b) and R = C₄H₉ (4c)) via 3a. The ratios of the trans and cis isomers of 3a–3c are 3:2. In addition, the structure of trans-2,5-bis(methoxymethyl)tetrahydroselenophene-1,1-dibromide (trans-3b) was determined by X-ray crystallography.     

Synthesis of new chiral amino ether derivatives: synthetic application of meso aziridinium ions prepared from β-amino alcohols

Methods of synthesis of new chiral amino ether derivatives through the opening of aziridinium ions, prepared in situ using trans-(±)-2-(1-N,N-dialkylamino)cyclohexyl mesylate with (R)-(+)-1,1′-bi-2-naphthol, are described. The (R,R,R)-diastereomer was obtained as the major product and isolated as an enantiopure salt, and characterized by single crystal X-ray analysis. The C₂-chiral (R,R,R,R,R)-diamino ether was obtained as the major product by opening of the aziridinium ion, prepared using trans-(±)-2-(1-pyrrolidino)cyclohexyl mesylate and (R)-(+)-1,1′-bi-2-naphthol in the presence of aq NaOH. This was also characterized by single crystal X-ray analysis.     

Four unit linking groups III. Liquid crystals of negative dielectric anisotropy

A series of 1-n-alkoxy-2,3-difluoro-4-[3-(trans-4-pentylcyclohexyl)propyloxy]-benzenes has been synthesized. Several homologues exhibit an enantiotropic nematic phase of moderately strong negative dielectric anisotropy (Δε ≈ 5) at or just above room temperature. An analogous series of three-ring diethers also including some (E)-3-allyloxy derivatives possesses enantiotropic, wide range nematic phases also of moderately strong negative dielectric anisotropy, but at elevated temperatures. The synthesis and liquid crystal transition temperatures of these 2,3-difluoroquinone derivatives are described and comparisons are made with the transition temperatures of the corresponding materials incorporating standard central linkages (-, C₂H₄, CH₂O and COO).     

2-(4-Alkylphenyl)-5-(alkenyloxy)pyrimidines: Synthesis, liquid crystal transition temperatures and some physical properties

We have recently reported the introduction of a carbon-carbon double bond into a wide variety of 5-n-alkyl-2-(4-n-alkoxyphenyl)pyrimidines to produce the corresponding alkenyloxy derivatives. The position and nature (E/Z) of the double bond were varied systematically and the effect on the liquid crystal transition temperatures studied. The position and nature (E/Z) of the double bond changed the conformation of the alkenyloxy chain substantially. This resulted in higher smectic C and nematic transition temperatures for compounds with a trans-double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) were observed for materials with a cis-double bond (Z) at an odd number of carbon atoms from the molecular core. We have now performed the same operation on the related 2-(4-n-alkylphenyl)-5-n-alkoxypyrimidines to produce the corresponding alkenyloxy derivatives. An interesting feature of the new results is the high melting points of the trans-substituted materials and the low melting points of the terminally substituted compounds. The smectic C transition temperatures of both series are high. No nematic phases could be observed. However, in admixture with other smectic C components, the new compounds lead to surprisingly fast switching times, high smectic C transition temperatures and low melting points/crystallization temperatures in experimental mixtures designed for electro-optic display devices based on ferroelectric effects.     

The remote effect of vicinal large groups on the anomeric equilibrium: 2-methoxy-cis- and -trans-5,6-dimethyltetrahydropyrans

The anomeric equilibrium of 2-methoxy-trans-5,6-dimethyltetrahydropyran was found to have an enhanced fraction of axial methoxyl compared to 2-methoxy-4- or 6-methyltetrahydropyran. The 2-methoxy-cis-5,6-dimethyltetrahydropyran did not. The enhanced anomeric effect is attributed to a steric repulsion between the trans vicinal Me groups. A similar effect is postulated for methyl glucoside and glucose pentaacetate.     

Four unit linking groups II. Some novel smectic C materials

About 40 diverse phenyl benzoate esters incorporating a trans-1-4-disubstituted cyclohexane ring joined to the central aromatic core by new four unit linking groups (C₄H₈, C₄H₆, C₃H₆O and C₃H₄O) have been synthesized. The effect of various lateral substituents (F, Cl, CN and Br) and especially two fluorine atoms in the 2,3-positions of the 4-n-alkoxybenzoate part of the esters has been investigated. Three homologous series of 5-n-alkyl-2-(4-phenyl)pyrimidines incorporating the same four unit linking groups have also been prepared. Many members of the ester and pyrimidine series exhibit enantiotropic smectic C mesophases at moderately elevated temperatures. Several esters and pyrimidines have been found to improve the surface alignment and temperature range of chiral smectic C mixtures for surface stabilized ferroelectric liquid crystal displays. The effect of the new four unit linking groups on the liquid crystal transition temperatures, rotational viscosity and spontaneous polarization of their host structures in a standard chiral C mixture has been studied and compared to that of analogous materials containing no central linkage and standard central linkages (-, C₂H₄, CH₂O and COO). Several lactate ester derivatives incorporating the four unit linking groups have also been prepared and been found to exhibit a moderately high spontaneous polarization.     

The active catalytic species and its isomerisation in the catalytic carbonylation of methanol — a density functional study

The Monsanto acetic acid process is one of the most effective ways to produce acetic acid industrially. This process has been studied experimentally but theoretical investigations are so far sparse. In the current work the active catalytic species [Rh(CO)₂I₂]⁻ (1) and its isomerisation has been studied theoretically using the hybrid B3LYP exchange and correlation functional. Similar calculations has been performed for the iridium complex [Ir(CO)₂I₂]⁻ (2) that also is catalytically active in the methanol carbonylation. Experimental work has confirmed the existence of the cis forms of the active catalytic species, but they do not rule out the possibility of the trans isomers. Our gas phase results show that cis-1 has 4.95 kcal/mol lower free energy than trans-1, and cis-2 has 10.39 kcal/mol lower free energy than trans-2. In the case of rhodium, trans-1 can take part to the catalytic cycle but in case of iridium this is not very likely. We have also investigated the possible mechanisms of the cis to trans conversions. The ligand association mechanism gave free energy barrier of 13.7 kcal/mol for the rhodium complex and 19.8 kcal/mol for iridium. Thus the conversion for the rhodium complex is feasible whereas for iridium it is unlikely.     

Synthesis and mesomorphic properties of some derivatives of 2-methylcyclohex-2-ene-4-one-1-carboxylic and cis-2,6-dimethyl cyclohex-2-ene-4-one-1 -carboxylic acids

Mesomorphic derivatives prepared from ethyl esters of 2-methylcyclohex-2-ene-4-one-1-carboxylic and cis-2,6-dimethylcyclohex-2-ene-4-one-1-carboxylic acids have been synthesized. It has been shown that the compounds obtained are characterized by a lower nematic—isotropic transition temperature (10-20°C) and a considerably narrower nematic range in comparison with the analogous derivatives of 4-alkylcyclohex-3-ene-carboxylic and trans-4-alkylcyclohexane-carboxylic acid.     

Enzymic resolution of 2-substituted cyclohexanols through lipase-mediated esterification

Several lipases were used for the kinetic resolution of the racemic cis- and trans-isomers of 2-(4-methoxybenzyl)cyclohexanol, by lipase-mediated esterification of the substrates to the corresponding acetate isomers. Conversion of the products and the remaining deracemized substrates into diastereoisomeric esters of 3,3,3-trifluoromethyl-2-methoxy-2-phenylpropanoic acid, their analysis by chiral HPLC and assignment of their absolute configurations through their ¹H and ¹⁹F NMR spectra, were the basis of evaluation of the studied enzymic process. Lipase from Rhizomucor miehei (RML) was found to be the most efficient enzyme regarding enantiomeric excess (ee) and yield of the desired products, while resolution by lipase from Rhizopus arrhizus (RAL) resulted in satisfactory ee and lower yields.     

A comparative ab initio study of the geometry and force field of thionformic acid with formic and thiolformic acids

The energy and force field for the planar cis and trans conformers of thionformic acid have been calculated using the 4–31 G basis set, augmented by a complete set of d-functions on the sulfur atom, with full geometry optimization. Extensive comparisons are made between the changes in geometry and selected force constants in going from cis- (chain) to the trans- (ring) structures of thionformic, thiolformic and formic acid. These changes are discussed in terms of a hydrogen bonding type of interaction in the O---HS, S---HO and O---HO structural units respectively. Of the thioacid conformers, the trans-thiol is found to be the most stable; the trans-thion and cis-thiol both about 10 kJ mol⁻¹ less stable; and the cis-thion the least stable by about 38 kJ mol⁻¹.     

Stereocontrolled synthesis of (RR)- and (RS)-5-hydroxy-2-methylhexanoic acid lactones : Pheromone of the carpenter bee via palladium-catalyzed reactions

(E,Z)-2,4-Hexadiene was transformed to the lactone cis-1 ( 93% cis)(pheromone of the carpenter bee) in a stereospedfic reaction sequence via a Pd-calalyzed 1,4-acetoxychlorination. The same reaction sequence applied to (E,E)-2,4-hexadiene afforded the isomeric lactone trans-1 (91% trans).     

An unexpected [2+2]-cycloaddition reaction of 4-methyldithieno-[3,4-b:3′,2′-d]pyridinium iodide with dimethyl acetylenedicarboxylate

An unexpected [2+2]-cycloaddition occured in the reaction of 4-methyldithieno-[3,4-6:3′,2′-d]pyridinium iodide (3)with two equivalents of DMAD, giving 4-(trans-1,2-dicarbomethoxy-2- iodovinyl)-5-methyl-6,7-dicarbomethoxy-4,5-dihydrothieno [23-c]quinoline (4) in 54% yield. 4 is formed via 4-methyl-5-(trans-1,2-dicarbomethoxy-2-iodo-4,5-dihydrothieno [3,4-b:3′,2′-d]pyridine (16), followed by [2+2]-cycloaddition. The primary adduct rearranges via a thiepin to an episulfide which eliminates sulfur to give 4.     

Cis- and trans-3-hexene: infrared spectrum in liquid argon solution, ab initio calculations of equilibrium geometry, normal coordinate analysis, and vibrational assignments

The infrared spectra of cis-3-hexene and trans-3-hexene dissolved in liquid argon have been obtained at temperatures from 93 to 120 K. The absorptions were observed with a low-temperature cell and a Fourier transform infrared spectrophotometer. Ab initio molecular orbital calculations were performed to obtain the equilibrium geometry, vibrational frequencies, force fields, and infrared intensities. The calculations were done at the Hartree-Fock level using 6-31G basis set. The Cartesian force fields from ab initio calculations have been converted to the force field in symmetry coordinates. The scale factors of ab initio calculated force fields were determined. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. Vibrational normal modes calculated for the lowest energy rotamers of cis- and trans-3-hexene have been assigned to infrared absorption bands observed in liquid argon solution. The assignments were based on calculated frequencies and potential energy distributions. The equilibrium geometries of the two lowest energy rotamers (symmetry C₂ and C_s) of cis-3-hexene and of the three lowest energy rotamers (symmetry C_i, C₂, and C₁) of trans-3-hexene were calculated. Variable temperature studies of the infrared spectrum of cis- and trans-3-hexenes dissolved in liquid argon were done to obtain the ΔH of conversion between the rotamers C₂ and C_s of cis-3-hexene and between the rotamers C_i, C₂, and C₁ of trans-3-hexene.     

Fluorocyclohexanes-II Cis- and trans- 1H:3H- and 1H:4H-decafluorocyclohexanes

An extension of the methods employed in the isolation of (trans) 1H/2H-decafluorocyclohexane,¹ (I) from the polyfluorocyclohexane mixture obtained by the vapour phase fluorination of benzene with cobaltic fluoride at about 150°, has afforded the four remaining members of the series of decafluorocyclohexanes [the cis- and trans-1H:3H- and 1H:4H-isomers (1H:3H/-(IV), 1H/3H-(III), 1H:4H/-(VII), and 1H/4H-(VIII), respectively)] and also the cis-1H:2H-decafluorocyclohexane (II), obtained previously^1,2 by the lithium aluminium hydride reduction of 1:2-dichlorodecafluorocyclohexane. The structures of the 1H:3H- and 1H:4H-decafluorides have been established by dehydrofluorination studies. The six decafluorocyclohexanes have been related to two new nonafluorocyclohexanes³ (IX and X) by further fluorination of the latter. 2H-Heptafluoroadipic acid has been obtained from 3H-nonafluorocyclohex-1-ene (V), one of the dehydrofluorination products of the 1H:3H-decafluorides.     

Novel liquid crystalline compounds having an enyne unit

Novel series of liquid crystalline compounds having an enyne unit, 1-(4-alkylphenyl)-4-(trans-4-alkylcyclohexyl)-but-3-en-1-yne and 1-(4-alkoxyphenyl)-4-(trans-4-alkylcyclohexyl)-but-3-en-1-yne are presented. The synthetic methods, mesomorphic phases, and some physical properties of these series are reported.