Synthesis and Properties of 3,5-Difluorotolane Liquid Crystal Compounds
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Trans-oxanes are important liquid crystals. the commonly used techniques for the synthesis were to react 2-substituted propylene glycols with substituted formaldehydes. Such process produces toxic cis-oxanes, which are harmful to the environment. The cis to trans isomerization of wasted cis-oxanes was studied in the presence of p-toluenesulfonic acid as catalyst. the yield of cis to trans conversion was over 70%, which was much higher than 42-69% when traditional methods were employed. The total yield of the new method was increased to 90%. Further investigation of effects of catalysts, reaction times, temperatures on the cis-trans conversion was carried out. proposed mechanism of this process for the conversion was discussed. One feature of trans-oxane materials is easy to synthesize, which is a key requirement for the applications in TN/STN and TFT modes of liquid crystal display 1,2. The class of trans oxane is among the key materials for liquid crystal display. During the past decades, the research on oxane liquid crystals, especially trans-oxanes, has attracted increasing attention in the field 3,4. The commonly used synthesis was to react substituted formaldehydes (a) with 2-substituted propylene glycols (b) for mixed cis-trans oxanes (e) 5,6 , followed by separation and purification to give pure trans-oxane monomer (d) (Fig. 1). Liquid crystal display requires highly pure trans-oxanes, and its cis-isomers have to be removed from their mixtures. The routine purification was to extract trans-isomers by recrystallization. However, such strategy was unsatisfactory 7,8. Some amount of trans-isomers remained in the mother liquor with cis-isomers, therefore the yield of trans-oxanes was relatively low (about 50% as calculated based on alkyl benzaldehyde). After recrystallization a large amount of mother liquor (f) was left over containing mainly cis-trans mixtures. There is no literature on recycling the mixtures and purifying the trans isomers, so mixture f was usually discarded causing not only environmental pollution, but also high cost of the trans isomers. Although cis-trans isomerization was reported 9,10 , there has been no reports on converting cis-oxanes (c) into trans-oxanes so far. In order to improve the yield of trans-oxanes and reduce the environmental pollution, a new synthetic route (Fig. 2) was employed to give trans-oxanes (d). As shown in Fig. 2, the cis-trans mixtures of oxanes (e) were synthesized by docking p-alkyl benzaldehyde (a) directly with 2-substituted propylene glycol (b). Pure trans-oxanes (d) monomers were obtained by either high vacuum distillation, chromatography or recrystallization. The latter left a large amount of cis-trans mixtures (f) with cis configuration dominant. The cis-trans mixture with trans configuration dominant (g) was obtained in the presence of p-toluenesulfonic acid as catalyst. Then the mixture (g) was purified further as shown in Fig. 2 to give pure trans-configuration of oxanes (d). The process was repeated several times until cis isomers were completely co...
Trans-oxanes are important liquid crystals. the commonly used techniques for the synthesis were to react 2-substituted propylene glycols with substituted formaldehydes. Such process produces toxic cis-oxanes, which are harmful to the environment. The cis to trans isomerization of wasted cis-oxanes was studied in the presence of p-toluenesulfonic acid as catalyst. the yield of cis to trans conversion was over 70%, which was much higher than 42-69% when traditional methods were employed. The total yield of the new method was increased to 90%. Further investigation of effects of catalysts, reaction times, temperatures on the cis-trans conversion was carried out. proposed mechanism of this process for the conversion was discussed. One feature of trans-oxane materials is easy to synthesize, which is a key requirement for the applications in TN/STN and TFT modes of liquid crystal display 1,2. The class of trans oxane is among the key materials for liquid crystal display. During the past decades, the research on oxane liquid crystals, especially trans-oxanes, has attracted increasing attention in the field 3,4. The commonly used synthesis was to react substituted formaldehydes (a) with 2-substituted propylene glycols (b) for mixed cis-trans oxanes (e) 5,6 , followed by separation and purification to give pure trans-oxane monomer (d) (Fig. 1). Liquid crystal display requires highly pure trans-oxanes, and its cis-isomers have to be removed from their mixtures. The routine purification was to extract trans-isomers by recrystallization. However, such strategy was unsatisfactory 7,8. Some amount of trans-isomers remained in the mother liquor with cis-isomers, therefore the yield of trans-oxanes was relatively low (about 50% as calculated based on alkyl benzaldehyde). After recrystallization a large amount of mother liquor (f) was left over containing mainly cis-trans mixtures. There is no literature on recycling the mixtures and purifying the trans isomers, so mixture f was usually discarded causing not only environmental pollution, but also high cost of the trans isomers. Although cis-trans isomerization was reported 9,10 , there has been no reports on converting cis-oxanes (c) into trans-oxanes so far. In order to improve the yield of trans-oxanes and reduce the environmental pollution, a new synthetic route (Fig. 2) was employed to give trans-oxanes (d). As shown in Fig. 2, the cis-trans mixtures of oxanes (e) were synthesized by docking p-alkyl benzaldehyde (a) directly with 2-substituted propylene glycol (b). Pure trans-oxanes (d) monomers were obtained by either high vacuum distillation, chromatography or recrystallization. The latter left a large amount of cis-trans mixtures (f) with cis configuration dominant. The cis-trans mixture with trans configuration dominant (g) was obtained in the presence of p-toluenesulfonic acid as catalyst. Then the mixture (g) was purified further as shown in Fig. 2 to give pure trans-configuration of oxanes (d). The process was repeated several times until cis isomers were completely co...
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