Enantiomeric excess, ferroelectrics

Asymmetric epoxidation is applied to the synthesis of the novel ferroelectric liquid crystals 99 that have the chiral trans-2, >-e, o y hexyl group as a core moiety (Scheme 31). The (25, 35 )-epoxy alcohol 98, conveniently obtained in 86% ee, is transformed into the desired material in two steps [99]. A formal synthesis of Brefeldin A (102), which shows a variety of biological activity represented by antitumor, antifungal, and antiviral activity, is accomplished via a highly enantioselective intramolecular hydroacylation of racemic pentanal 100 with 0.9 % of cationic Rh[(S)-binap] BF4. A 1 1 mixture of trans- and cis-cyclopentanones 101 is obtained with a high enantiomeric excess of 96% for each (Scheme 32). In the following step, the undesired cw-isomer is converted into the thermodynamically favored tran -isomer for further transformation [100]. 

Amino acids are readily available in high enantiomeric excess and have probably been the most used type of chiral precnrsor for the preparation of chiral liquid ciystals and chiral dopants (especially for ferroelectric applications). Two particularly interesting applications of amino acids e.g., 132) in the generation of chiral materials for ferroelectric liquid crystal mixtures (see Chapter 6) are shown in Schemes 31 and 32. 

By mixing a chiral liquid-crystal compound with its optical antipode, systems possessing arbitrary values of the enantiomeric excess can be designed. If a chiral compound shows smectic-C and smectic- I phases, the racemate, i.e., the 1 1 mixture of the two antipodes, also exhibits these phases but the ferroelectric properties of the smectic-C phase and the electroclinic effect in the smectic- phase are lost. This offers the unique possibility to study a given system with and without ferroelectridty or with a variable markedness of its ferroelectric properties. 

The ferroelectric liquid-crystal compounds which have been studied in chiral-racemic systems possess large values of the spontaneous polarization Ps, i.e., these compounds show a strong bilinear coupling between tilt angle and polarization. The behavior in chiral-racemic systems of these compounds can be well described assuming a simple proportionality of the bilinear P-9 coupling constant C and the enantiomeric excess Xee- This applies also for the electroclinic effect in the smectic- phase which has been studied in [74], [77]. Figure 8.12 shows the electroclinic tilt susceptibility /51 as a function of Xee at constant temperature difference to the transition to the smectic-C phase. The observed proportionality between xe ee is vvell in... 

In contrast to the ferroelectric switching time in the smectic-C phase, the dynamics of the electroclinic effect does not depend on the chirality. The soft mode relaxation frequency was observed to show the same linear dependence on (r — Tag) in several mixtures possessing different values of the enantiomeric excess [77]. 

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