2-Aryloxytetrafluoropropionic esters, process for their preparation, and their use in liquid-crystalline mixtures

In addition to nematic and cholesteric liquid crystals, optically active 
tilted smectic (ferroelectric) liquid crystal phases have also been used 
recently in commercial display devices. 
Clark and Lagerwall have been able to show that the use of ferroelectric 
liquid crystal systems in very thin cells results in optoelectrical 
switching or display elements which have response times faster by a factor 
of 1000 compared with conventional TN ("twisted nematic") cells (cf., for 
example, EP-A 0 032 362). On the basis of this and other favorable 
properties, for example the possibility of bistable switching and the 
virtually viewing angle-independent contrast, FLCs are fundamentally 
highly suitable for the abovementioned areas of application, for example 
via matrix addressing. 
For electro-optical or fully optical components, either compounds are 
required which form tilted or orthogonal smectic phases and are themselves 
optically active, or ferroelectric smectic phases can be induced by doping 
compounds which, although forming such smectic phases, are not themselves 
optically active, with optically active compounds. The desired phase 
should be stable over the broadest possible temperature range. 
In order to achieve good contrast in electro-optical components, a uniform 
planar alignment of the liquid crystals is necessary. Good alignment in 
the S.sub.A and S.sup.*.sub.C phase can be achieved, for example, if the 
phase sequence of the liquid-crystal mixture is, with decreasing 
temperature: 
EQU isotropic.fwdarw.N.sup.* .fwdarw.S.sub.A .fwdarw.S.sup.*.sub.C 
The prerequisite is that the pitch of the helix in the N.sup.* phase is 
very large (greater than 10 .mu.m) or even better is fully compensated 
(see, for example, T. Matsumoto et al., pp. 468-470, Proc. of the 6th Int. 
Display Research Conf., Japan Display, Sep. 30-Oct. 2, 1986, Tokyo, Japan; 
M. Murakami et al., ibid. pp. 344-347). This is achieved by adding one or 
more optically active dopes which induce a right-hand helix to the chiral 
liquid-crystal mixture which has, for example, a left-hand helix in the 
N.sup.* phase, in such amounts that the helix is compensated. 
A further prerequisite for the use of the SSFLCD effect (surface-stabilized 
ferroelectric liquid-crystal display) of Clark and Lagerwall for uniform 
planar alignment is that the pitch in the smectic C.sup.* phase is 
significantly greater than the thickness of the display element (Mol. 
Cryst. Liq. Cryst. 94 (1983), 213-134 and 114 (1984), 151-187). As in the 
case of the cholesteric pitch, this is achieved by using dopes having the 
opposite rotation of the helix. 
Ferroelectric liquid-crystal displays can also be operated by utilizing the 
DHF (distorted helix formation) effect or the PSFLCD effect 
(pitch-stabilized ferroelectric liquid-crystal display, also known as 
SBF=short pitch bistable ferroelectric effect). The DHF effect has been 
described by B. I. Ostrovski in Advances in Liquid Crystal Research and 
Applications, Oxford/Budapest, 1980, 469 ff.; the PSFLCD effect is 
described in DE-A 39 20 625 and EP-A 0 405 346. In contrast to the SSFLCD 
effect, utilization of these effects requires a liquid-crystalline 
material having a short S.sub.C pitch. 
The optical response time T[.mu.s] of ferroelectric liquid-crystal systems, 
which should be as short as possible, depends on the rotational viscosity 
of the system Y[mPas], the spontaneous polarization P.sub.S [nC/cm.sup.2 ] 
and the electric field strength E[V/m], in accordance with the equation 
##EQU1## 
Since the field strength E is determined by the electrode separation in the 
electro-optical component and by the applied voltage, the ferroelectric 
display medium must have low viscosity and a high spontaneous polarization 
to achieve a short response time. 
Finally, in addition to thermal, chemical and photochemical stability, a 
small optical anisotropy .DELTA.n, preferably .apprxeq.0.13, and a low 
positive or preferably negative dielectric anisotropy .DELTA..epsilon. are 
required (see S. T. Lagerwall et al., "Ferroelectric Liquid Crystals for 
Displays", SID Symposium, October Meeting 1985, San Diego, Calif., USA). 
The totality of these requirements can only be achieved by means of 
mixtures comprising a plurality of components. The base (or matrix) used 
preferably comprises compounds which if possible themselves already have 
the desired phase sequence I.fwdarw.N.fwdarw.S.sub.A .fwdarw.S.sub.C. 
Further components of the mixture are frequently added in order to reduce 
the melting point and to broaden the S.sub.C and usually also the N phase, 
to induce optical activity, for pitch compensation and to match the 
optical and dielectric anisotropy; however, the rotational viscosity, for 
example, should if possible not be increased. 
Since the development of ferroelectric liquid-crystal mixtures in 
particular can in no way be regarded as complete, the manufacturers of 
displays are interested in a very wide variety of components for mixtures. 
Another reason for this is that only the interaction of the 
liquid-crystalline mixtures with the individual components of the display 
device or of the cells (for example the alignment layer) allows 
conclusions to be drawn on the quality of the liquid-crystalline mixtures 
too. 
The object of the present invention was therefore to provide novel 
compounds which are suitable in liquid-crystalline mixtures for improving 
the property profile of these mixtures. 
2-Phenoxytetrafluoropropionic acid and the phenyl ester thereof are 
disclosed in DE-A-1 593 046. However, the preparation process described 
therein gives only unsatisfactory yields and is technically complex. There 
is no mention of the use of such substances in liquid-crystal mixtures. 
Surprisingly, it has now been found that aryl, heteroaryl and cyclohexyl 
esters of 2-aryloxytetrafluoropropionic acid are suitable as components of 
liquid-crystal mixtures. 
The invention therefore relates to the use of 2-aryloxytetrafluoropropionic 
esters of the formula (I) 
EQU R.sup.1 (--A.sup.1).sub.a (--M.sup.1).sub.b 
(--A.sup.2)--O--CF(CF.sub.3)--COO--(--A.sup.3)(--M.sup.2).sub.c 
(--A.sup.4).sub.d --R.sup.2 (I) 
in which the symbols and indices have the following meanings: 
R.sup.1 and R.sup.2 are identical or different and are hydrogen, --CN, --F, 
--Cl or a straight-chain or branched alkyl radical having 1 to 20 carbon 
atoms (with or without asymmetrical carbon atoms), where one or more 
--CH.sub.2 -- groups may also be replaced by --O--, --S--, --CO--, --CS--, 
--CH.dbd.CH--, --C.tbd.C--, cyclopropane-1,2-diyl, --Si(CH.sub.3).sub.2 
--, 1,4-phenylene, trans-1,4-cyclohexylene or trans-1,3-cyclopentylene, 
with the proviso that oxygen atoms and sulfur atoms (referred to as 
chalcogens below) must not be bonded directly to one another, and/or one 
or more H atoms of the alkyl radical may be substituted by --F, --Cl, 
--Br, --OR.sup.3, --SCN, --OCN or --N.sub.3, or are alternatively one of 
the following chiral groups: 
##STR1## 
R.sup.3, R.sup.4, R.sup.5, R.sup.6 and R.sup.7 are identical or different 
and are hydrogen or a straight-chain or branched alkyl radical having 1-16 
carbon atoms (with or without asymmetrical carbon atoms), where one or 
more --CH.sub.2 -- groups may also be replaced by --O-- or --CH.dbd.CH--, 
with the proviso that oxygen atoms must not be bonded directly to one 
another, and/or one or more H atoms of the alkyl radical may be 
substituted by --F or --Cl; R.sup.4 and R.sup.5 may also together be 
--(CH.sub.2).sub.4 -- or --(CH.sub.2).sub.5 -- if they are bonded to an 
oxirane, dioxolane, tetrahydrofuran, tetrahydropyran, butyrolactone or 
valerolactone system; 
Q is a single bond, --CO--O-- or --CH.sub.2 --O--; 
M.sup.1 and M.sup.2 are identical or different and are --O--, --S--, 
--CO--, --CO--O--, --O--CO--, --O--CO--O--, --CO--S--, --S--CO--, 
--CS--O--, --O--CS--, --S--CS--S--, --O--C S--O--, --S--CO--S--, --CS--, 
--CH.sub.2 --O--, --O--CH.sub.2 --, --CH.sub.2 --S--, --S--CH.sub.2 --, 
--CH.dbd.CH--, --C.tbd.C-- or a single bond; 
A.sup.1, A.sup.2, A.sup.3 and A.sup.4 are identical or different and are 
1,4-phenylene, in which one or more H atoms may be replaced by F, Cl 
and/or CN, pyrazine-2,5-diyl, in which one or two H atoms may be replaced 
by F, Cl and/or CN, pyridazine-3,6-diyl, in which one or two H atoms may 
be replaced by F, Cl and/or CN, pyridine-2,5-diyl, in which one or more H 
atoms may be replaced by F, Cl and/or CN, pyrimidine-2,5-diyl, in which 
one or two H atoms may be replaced by F, Cl and/or CN, 
trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by CN 
and/or CH.sub.3, 1,3,4-thiadiazole-2,5-diyl, 1,3-dioxane-2,5-diyl, 
1,3-dithiane-2,5-diyl, 1,3-thiazole-2,4-diyl, in which one H atom may be 
replaced by F, Cl and/or CN, 1,3-thiazole-2,5-diyl, in which one H atom 
may be replaced by F, Cl and/or CN, thiophene-2,4-diyl, in which one H 
atom may be replaced by F, Cl and/or CN, thiophene-2,5-diyl, in which one 
or two H atoms may be replaced by F, Cl and/or CN, piperazine-1,4-diyl, 
piperazine-2,5-diyl, naphthalene-2,6-diyl, in which one or more H atoms 
may be replaced by F, Cl and/or CN, bicyclo[2.2.2]octane-1,4-diyl, in 
which one or more H atoms may be replaced by F, Cl and/or CN, or 
1,3-dioxaborinane-2,5-diyl; 
a, b, c and d are zero or one; 
as components of liquid-crystal mixtures. 
If R.sup.1 and/or R.sup.2 are one of the optically active or racetalc 
groups mentioned, the indices a and d are preferably zero. 
Preference is furthermore given to the use of compounds of the formula (I) 
in which the symbols and indices have the following meanings: 
R.sup.1 and R.sup.2 are identical or different and are hydrogen, --CN, --F, 
--Cl or a straight-chain or branched alkyl radical having 1 to 18 carbon 
atoms (with or without asymmetrical carbon atoms), where one or more 
--CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CH.dbd.CH--, 
--C.tbd.C--, cyclopropane-1,2-diyl, --Si(CH.sub.3).sub.2 -- or 
trans-1,4-cyclohexylene, with the proviso that oxygen atoms must not be 
bonded directly to one another, and/or one or more H atoms of the alkyl 
radical may be substituted by --F, --Cl, --OR.sup.3, --OCN or --N.sub.3, 
or are one of the following chiral groups: 
##STR2## 
R.sup.3, R.sup.4, R.sup.5, R.sup.6 and R.sup.7 are identical or different 
and are hydrogen or a straight-chain or branched alkyl radical having 1-16 
carbon atoms (with or without asymmetrical carbon atoms), where one or 
more --CH.sub.2 -- groups may also be replaced by --O-- or --CH.dbd.CH--, 
with the proviso that oxygen atoms must not be bonded directly to one 
another, and/or one or more H atoms of the alkyl radical may be 
substituted by --F or --Cl; R.sup.4 and R.sup.5 may also together be 
--(CH.sub.2).sub.4 -- or --(CH.sub.2).sub.5 -- if they are bonded to an 
oxirane, dioxolane, tetrahydrofuran, tetrahydropyran, butyrolactone or 
valerolactone system; 
Q is a single bond, --CO--O-- or --CH.sub.2 --O--; 
M.sup.1 and M.sup.2 are identical or different and are --O--, --CO--, 
--CO--O--, --O--CO--, --O--CO--O--, --O--CS--O--, --CH.sub.2 --O--, 
--O--CH.sub.2 --, --CH.dbd.CH--, --C.tbd.--C-- or a single bond; 
A.sup.1, A.sup.2, A.sup.3 and A.sup.4 are identical or different and are 
1,4-phenylene, in which one or more H atoms may be replaced by F, Cl 
and/or CN, pyrazine-2,5-diyl, in which one or two H atoms may be replaced 
by F, Cl and/or CN, pyridazine-3,6-diyl, in which one or two H atoms may 
be replaced by F, Cl and/or CN, pyridine-2,5-diyl, in which one or more H 
atoms may be replaced by F, Cl and/or CN, pyrimidine-2,5-diyl, in which 
one or two H atoms may be replaced by F, Cl and/or CN, 
trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by CN 
and/or CH.sub.3, 1,3,4-thiadiazole-2,5-diyl, 1,3-dioxane-2,5-diyl, 
thiophene-2,4-diyl, in which one H atom may be replaced by F, Cl and/or 
CN, thiophene-2,5-diyl, in which one or two H atoms may be replaced by F, 
Cl and/or CN, naphthalene-2,6-diyl, in which one or more H atoms may be 
replaced by F, Cl and/or CN, or 1,3-dioxaborinane-2,5-diyl; 
a, b, c and d are zero or one. 
Very particular preference is given to the use of compounds of the formula 
(I) in which the symbols and indices have the following meanings: 
R.sup.1 and R.sup.2 are identical or different and are hydrogen, --CN, --F, 
--Cl, or a straight-chain or branched alkyl radical having 1 to 16 carbon 
atoms (with or without asymmetrical carbon atoms), where one, two or three 
--CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CH.dbd.CH--, 
cyclopropane-1,2-diyl, --Si(CH.sub.3).sub.2 -- or trans-1,4-cyclohexylene, 
with the proviso that oxygen atoms must not be bonded directly to one 
another, and/or one or more H atoms of the alkyl radical may be 
substituted by --F, --Cl or --OR.sup.3, or are alternatively one of the 
following chiral groups: 
##STR3## 
R.sup.3, R.sup.4, R.sup.5, R.sup.6 and R.sup.7 are identical or different 
and are hydrogen or a straight-chain or branched alkyl radical having 1-14 
carbon atoms (with or without asymmetrical carbon atoms), where one or 
more --CH.sub.2 -- groups may also be replaced by --O-- or --CH.dbd.CH--, 
with the proviso that oxygen atoms must not be bonded directly to one 
another, and/or one or more H atoms of the alkyl radical may be 
substituted by --F or--Cl; R.sup.4 and R.sup.5 may also together be 
--(CH.sub.2).sub.4 -- or --(CH.sub.2).sub.5 -- if they are bonded to an 
oxirane or dioxolane system; 
Q is a single bond, --CO--O-- or--CH.sub.2 --O--; 
M.sup.1 and M.sup.2 are identical or different and are --O--, --CO--, 
--CO--O--, --O--CO--, --O--CO--O--, --CH.sub.2 --O--, --O--CH.sub.2 --, 
--CH.dbd.CH-- or a single bond; 
A.sup.1, A.sup.2, A.sup.3 and A.sup.4 are identical or different and are 
1,4-phenylene, in which one, two or three H atoms may be replaced by F, Cl 
and/or CN, pyrazine-2,5-diyl, in which one or two H atoms may be replaced 
by F, Cl and/or CN, pyridazine-3,6-diyl, in which one or two H atoms may 
be replaced by F, Cl and/or CN, pyridine-2,5-diyl, in which one or two H 
atoms may be replaced by F, Cl and/or CN, pyrimidine-2,5-diyl, in which 
one or two H atoms may be replaced by F, Cl and/or CN, 
trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by CN 
and/or CH.sub.3, 1,3,4-thiadiazole-2,5-diyl, naphthalene-2,6-diyl, in 
which one or two H atoms may be replaced by F, Cl and/or CN, or 
1,3-dioxaborinane-2,5-diyl; 
a, b, c and d are zero or one. 
Very particular preference is given to compounds of the formula (I) in 
which the alcohol and ester components are identical. The compounds of the 
formula (I) are employed as optically active components or as the 
racemate. 
The invention furthermore relates to compounds of the formula (I), in which 
the abovementioned definitions apply, but phenyl 
phenoxytetrafluoropropionate is excluded. 
The novel compounds can be prepared by the process described in DE-A-1 593 
046. 
However, preference is given to a novel preparation process, which is 
likewise a subject-matter of the present invention. 
In this process, hexafluoropropene oxide is passed at atmospheric pressure 
into an arylate solution, and the aryl 2-aryloxytetrafluoropropionate 
formed is isolated, saponified if desired and, if desired, re-esterified 
in a further step. 
Preferred arylates are metal or ammonium salts, preferably alkali metal or 
alkaline earth metal salts, particularly preferably sodium salts, of 
compounds of the formula (II) 
EQU R.sup.1 (--A.sup.1).sub.a (--M.sup.1).sub.b (--A.sup.2).sub.c --OH (II) 
in which the symbols and indices have the same definitions and preferences 
as in the formula (I). 
Hexafluoropropene oxide is commercially available. 
in order to carry out the reaction, the arylate is introduced into a 
suitable solvent, and hexafluoropropene oxide is passed into the solution 
at room temperature. In general, an exothermic reaction immediately takes 
place, which can be controlled, for example, by throttling the 
hexafluoropropene oxide stream. When the reaction is complete, unreacted 
hexafluoropropene oxide begins to condense in the reflux condenser, thus 
indicating the end of the reaction. 
The reaction is generally carried out at temperatures of from 10.degree. to 
-30.degree. C., preferably from 0.degree. to -10.degree. C., particularly 
preferably -10.degree. C. 
The solvents used are generally dipolar aprotic solvents, preferably 
ethers, particularly preferably glycol ethers, in particular diglyme and 
tetraglyme. 
The work-up can be carried out by standard methods, for example by pouring 
into an ice/water mixture, followed by extraction with a suitable solvent, 
for example diethyl ether, methylene chloride or t-butyl methyl ether, 
followed by drying, for example by means of magnesium sulfate. Removal of 
the solvent gives the crude product, which can be purified, for example by 
column chromatography, for example on SiO.sub.2, using a suitable solvent 
(for example heptane, methylene chloride, methanol, ethyl acetate or 
mixtures thereof). 
In order to prepare compounds of the formula (I) using different ether and 
ester components, the resultant ester is cleaved and the acid liberated or 
a salt thereof is esterified, for example using a compound of the formula 
(III), 
EQU R.sup.2 --(A.sup.4 --).sub.g (M.sup.3 --).sub.f (--A.sup.3)--X (III) 
where the symbols and indices have the meanings and preferences given in 
the formula (I), 
X is OH or a reactive group which is capable of esterification. 
The ester cleavage is carried out, for example, by hydrolysis in an acidic, 
neutral or basic medium, preferably in a basic medium. The subsequent 
esterification by means of the compound (III) is carried out by general 
esterification reactions known to the person skilled in the art. 
Esterification methods are described, for example, in A. Hassner and V. 
Alexanian, Tet. Lett. 46 (1978) 4478, (a) R. B. Turner, J.Am. Chem. Soc., 
75 (1953) 3489; (b) E. Vowinkel, Chem. Ber., 100, (1967) 16; (c) H. Staab, 
Angew. Chem. Internat. Ed. 1 (1962) 351; (d) J. M. Tedder, Chem. Revs., 55 
(1955) 787; (e) H. Brechbuehler, H. Buchl, E. Hatz, J. Schreiber and A. 
Eschenmoser, Helv. Chim. Acta, 48 (1965) 1746; (f) S. Masamune, S. Kamata 
and W. Schilling, J. Am. Chem. Soc. 97 (1975) 3515; (g) E. J. Corey and D. 
J. Brunella, Tetrahedron Letters (1976) 3409; and (h) T. Mukalyama, M. 
Usui and K. Saigo, Chem. Lett. 1976, 49, or in the German Patent 
Application P 43 26 336.4 with the title "Verfahren zur Veresterung von 
Carbonsauresalzen" [Process for the esterification of carboxylic acid 
salts]. 
The yields in the novel process are significantly greater than those 
described hitherto, and the process can also be used for substituted 
phenols and can be carried out in a technically simple manner by the 
pressureless reaction procedure. 
The synthesis of the R.sup.1 (--A.sup.1).sub.a (--M.sup.1).sub.b 
(--A.sup.2) and (A.sup.1)(M.sup.2).sub.c (--A.sup.4).sub.d --R.sup.2 
radicals or suitable reactive derivatives thereof is carried out by 
methods known to the person skilled in the art. 
The preparation is carried out under reaction conditions which are known 
and suitable for said reactions. Use may also be made here of variants 
which are known per se, but are not described here in greater detail. 
For example, reference may be made to DE-A 23 44 732, 24 50 088, 24 29 093, 
25 02 94, 26 36 684, 27 01 591 and 27 52 975 for compounds containing 
1,4-cyclohexylene and 1,4-phenylene groups; DE-A 26 41 724 for compounds 
containing pyrimidine-2,5-diyl groups; DE-A 40 26 223 and EP-A 03 91 203 
for compounds containing pyridine-2,5-diyl groups; DE-A 32 31 462 for 
compounds containing pyridazine-3,6-diyl groups; EP-A 309 514 for 
compounds containing 1,3,4-thiadiazole-2,5-diyl groups; WO-A 92/16500 for 
naphthalene-2,6-diyl groups; DE-A 37 10 890 for 
bicyclo[2.2.2]octane-1,4-diyl groups; K. Seto et al., Journal of the 
Chemical Society, Chemical Communications 1988, 56, for 
dioxoborinane-2,5-diyl groups. 
The preparation of disubstituted pyridines, disubstituted pyrazines, 
disubstituted pyrimidines and disubstituted pyridazines is also given, for 
example, in the corresponding volumes in the series "The Chemistry of 
Heterocyclic Compounds" by A. Weissberger and E. C. Taylor (Editors). 
Dioxane derivatives are expediently prepared by reaction of a corresponding 
aldehyde (or a reactive derivative thereof) with a corresponding 1,3-diol 
(or a reactive derivative thereof), preferably in the presence of an inert 
solvent, such as benzene or toluene, and/or in the presence of a catalyst, 
for example a strong acid, such as sulfuric acid, benzenesulfonic acid or 
p-toluenesulfonic acid, at temperatures between about 20.degree. C. and 
about 150.degree. C., preferably between 80.degree. C. and 120.degree. C. 
Primarily suitable reactive derivatives of the starting materials are 
acetals. 
Some of said aldehydes and 1,3-diols and reactive derivatives thereof are 
known and some can be prepared without difficulty by standard methods of 
organic chemistry from compounds known from the literature. For example, 
the aldehydes are obtainable by oxidation of corresponding alcohols or by 
reduction of nitriles or corresponding carboxylic acids or derivatives 
thereof, and the diols are obtainable by reduction of corresponding 
diesters. 
Compounds in which an aromatic ring is substituted by at least one F atom 
can also be obtained from the corresponding diazonium salts by replacement 
of the diazonium group by a fluorine atom, for example by the methods of 
Balz and Schiemann. 
As far as the linking of ring systems to one another is concerned, 
reference may be made, for example, to: 
N. Miyaura, T. Yanagai and A. Suzuki in Synthetic Communications 11 (1981 
), 513-519 DE-C-39 30 663, M. J. Sharp, W. Cheng, V. Snieckus in 
Tetrahedron Letters 28 (1987) 5093; G. W. Gray in J. Chem. Soc. Perkin 
Trans II 1989, 2041 and Mol. Cryst. Liq. Cryst. 172 (1989) 165, 204 (1991) 
43 and 91; EP-A 0 449 015; WO-A 89/12039; WO-A 89/03821; EP-A 0 354 434 
for the direct linking of aromatics and heteroaromatics; DE-A 32 01 721 
for compounds containing --CH.sub.2 CH.sub.2 -- bridges, and Koji Seto et 
al. in Liquid Crystals 8 (1990) 861-870 for compounds containing 
--C.tbd.C-- bridges. 
Esters of the formula (I) can also be obtained by esterification of 
corresponding carboxylic acids (or reactive derivatives thereof) using 
alcohols or phenols (or reactive derivatives thereof) or by the DCCl 
method (DCCl=dicyclohexylcarbodiimide). 
The corresponding carboxylic acids and alcohols or phenols are known and 
can be prepared analogously to known processes. 
Particularly suitable reactive derivatives of the said carboxylic acids are 
the acid halides, especially the chlorides and bromides, furthermore the 
anhydrides, for example also mixed anhydrides, azides or esters, in 
particular alkyl esters having 1-4 carbon atoms in the alkyl group. 
Particularly suitable reactive derivatives of said alcohols and phenols are 
the corresponding metal alkoxides or phenoxides, preferably of an alkali 
metal, such as sodium or potassium. 
The esterification is advantageously carried out in the presence of an 
inert solvent. Particularly suitable solvents are ethers, such as diethyl 
ether, di-n-butyl ether, THF, dioxane or anisole, ketones, such as 
acetone, butanone or cyclohexanone, amides, such as DMF or 
hexamethylphosphoric triamide, hydrocarbons, such as benzene, toluene or 
xylene, halogenated hydrocarbons, such as tetrachloromethane, 
dichloromethane or tetrachloroethylene, and sulfoxides, such as dimethyl 
sulfoxide or sulfolane. 
Ethers of the formula (I) are obtainable by etherification of corresponding 
hydroxyl compounds, preferably corresponding phenols, where the hydroxyl 
compound is expediently first converted into a corresponding metal 
derivative, for example into the corresponding alkali metal alkoxide or 
alkali metal phenoxide by treatment with NaH, NaNH.sub.2, NaOH, KOH, 
Na.sub.2 CO.sub.3 or K.sub.2 CO.sub.3. This can then be reacted with the 
corresponding alkyl halide, sulfonate or dialkyl sulfate, expediently in 
an inert solvent, such as acetone, 1,2-dimethoxy-ethane, DMF or dimethyl 
sulfoxide, or alternatively with an excess of aqueous or aqueous/alcoholic 
NaOH or KOH at temperatures between about 20.degree. and 100.degree. C. 
Regarding the synthesis of specific radicals R.sup.1, reference may 
additionally be made, for example, to EP-A 0 355 008 for compounds 
containing silicon-containing side chains and to EP-A 0 292 954 and EP-A 0 
398 155 for compounds containing cyclopropyl groups in the side chain. 
The provision of compounds of the formula (I) very generally considerably 
broadens the range of liquid-crystalline substances which are suitable, 
from various application points of view, for the preparation of 
liquid-crystalline mixtures. 
In this connection, the compounds of the formula (I) have a broad range of 
applications. Depending on the choice of substituents, they can be used as 
base materials from which liquid-crystalline phases are predominantly 
composed; however, compounds of the formula (I) can also be added to 
liquid-crystalline base materials from other classes of compound, in 
order, for example, to modify the dielectric and/or optical anisotropy of 
a dielectric of this type and/or to optimize its threshold voltage and/or 
its viscosity. Optically active compounds of the formula (Ia) are 
preferably used as dopes in ferroelectric and/or antiferroelectric 
liquid-crystal mixtures. 
The invention also relates to the use of compounds of the formula (I) in 
liquid-crystal mixtures, preferably ferroelectric, antiferroelectric and 
nematic, in particular in ferroelectric and antiferroelectric mixtures. 
The invention furthermore relates to liquid-crystal mixtures, preferably 
ferroelectric, antiferroelectric and nematic mixtures, in particular 
ferroelectric and antiferroelectric mixtures, containing one or more 
compounds of the formula (I), preferably of the formula (Ia). 
The liquid-crystal mixtures according to the invention generally contain 
from 2 to 35, preferably from 2 to 25, particularly preferably from 2 to 
20 components. 
They generally contain from 0.01 to 80% by weight, preferably from 0.1 to 
60% by weight, particularly preferably from 0.1 to 30% by weight, of one 
or more, preferably 1 to 10, particularly preferably 1 to 5, very 
particularly preferably 1 to 3, of the compounds of the formula (I) 
according to the invention. 
Further components of liquid-crystal mixtures containing compounds of the 
formula (I) according to the invention are preferably selected from known 
compounds having smectic and/or nematic and/or cholesteric and/or 
antiferroelectric phases. These include, for example: 
derivatives of phenylpyrimidine, as described, for example, in WO 86/06401 
and U.S. Pat. No. 4,874,542, 
meta-substituted aromatic compounds having a six-membered ring, as 
described, for example, in German Patent Application P 42 22 565, 
silicon compounds, as described, for example, in EP-A 0 355 008, 
mesogenic compounds containing only one side chain as described in EP-A 0 
541 081, 
hydroquinone derivatives, as described, for example, in German Patent 
Application P 4 243 705, 
pyridylpyrimidines, as described, for example, in WO 92/12974, 
phenylbenzoates, as described, for example, in P. Keller, Ferroelectrics 58 
(1984), 3, and J. W. Goodby et al., Liquid Crystals and Ordered Fluids, 
Vol. 4, New York, 1984, and 
thiadiazoles as described, for example, in EP-B 309 514. 
Examples of suitable chiral, non-racemic dopes are: 
optically active phenylbenzoates, as described, for example, in P. Keller, 
Ferroelectrics 58 (1984), 3, and J. W. Goodby et al., Liquid Crystals and 
Ordered Fluids, Vol. 4, New York, 1984, 
optically active oxirane ethers, as described, for example, in EP-A 0 263 
437 and WO-A 93/13093, 
optically active oxirane esters, as described, for example, in EP-A 0 292 
954, 
optically active dioxolane ethers, as described, for example, in EP-A 0 351 
746, 
optically active dioxolane esters, as described, for example, in EP-A 0 361 
272, and 
optically active tetrahydrofuran-2-carboxylic esters, as described, for 
example, in EP-A 0 355 561. 
The mixtures can in turn be used in electro-optical or fully optical 
elements, for example display elements, switching elements, light 
modulators, elements for image processing and/or signal processing or 
generally in the area of nonlinear optics. 
Liquid-crystalline mixtures containing compounds of the formula (I) are 
particularly suitable for use in electro-optical switching and display 
devices (displays). These displays are usually constructed in such a way 
that a liquid-crystal layer is enclosed on both sides by layers which are 
usually, in this sequence starting from the LC layer, at least one 
alignment layer, electrodes and a limiting sheet (for example of glass). 
In addition, they contain spacers, adhesive frames, polarizers and, for 
color displays, thin color-filter layers. Other possible components are 
antireflection, passivation, compensation and barrier layers and electric 
non-linear elements, such as thin-film transistors (TFTs) and 
metal-insulator-metal (MIM) elements. The structure of liquid-crystal 
displays has already been described in detail in relevant monographs (see, 
for example, E. Kaneko, "Liquid Crystal TV Displays: Principles and 
Applications of Liquid Crystal Displays", KTK Scientific Publishers 1987). 
The mixtures are furthermore suitable for field treatment, i.e. for 
operation in the quasi-bookshelf geometry (QBG) (see, for example, H. 
Rieger et al., SID 91 Digest (Anaheim), 1991, p. 396). 
The mixtures according to the invention are likewise suitable for use in 
ferroelectric liquid-crystal displays which are based on utilization of 
the DHF effect or the PSFLCD effect (pitch stabilized ferroelectric 
liquid-crystal display, also known as SBF, short pitch bistable 
ferroelectric effect). 
In addition, the compounds of the formula (I) can also be used as 
antiferroelectric liquid crystals or as components of corresponding 
mixtures. 
Antiferroelectric liquid crystals are described, for example, in A. D. L. 
Chandani et al., Jap. J. of Appl. Phys. 27 (1988) L 729-L 732; A. D. 
Chandani et al., Jap. J. of Appl. Phys. 28 (1989) L 1265; H. Orihara et 
al., Jap. J. of Appl. Phys 29 (1990) L 333. 
Electro-optical devices based on antiferroelectric liquid crystals are 
described, for example, in U.S. Pat. No. 5,046,823. 
The novel compounds are furthermore suitable for achieving the so-called 
electroclinic effect in the chiral smectic A phase (S.sup.*.sub.A phase) 
which can be utilized for light modulation (S. Garoff and R. B. Meyer, 
Phys. Rev. Lett. 38, 848 (1977)). 
If an electric field is applied parallel to the layers of a chiral S.sub.A 
phase (S.sup.*.sub.A phase), the molecules in the phase, which is 
orthogonal per se, tilt. The tilt angle .theta. between the director n and 
the layer perpendicular z is proportional to the applied field E. The 
electroclinic coefficient (d.theta./dE) gives the strength of the linear 
coupling between the tilt coordinates and the field. 
The electroclinic effect proceeds from a state which is monostable in the 
field-free space and gives, when a field is applied, a linear 
electro-optical characteristic line for small angles and a slightly curved 
electro-optical characteristic line for large angles. By contrast, the 
characteristic line for ferroelectric switching is highly nonlinear. 
The electroclinic effect can be regarded as a change in the tilt angle 
which is continuously controlled by the field. 
For practical utilization of the electroclinic effect in electro-optical 
switching and display elements, liquid-crystalline media which form an 
S.sup.*.sub.A, S.sup.*.sub.B or S.sup.*.sub.E phase are all the more 
suitable the larger the electroclinic coefficient d.theta./dE and the 
higher the electroclinic limiting frequency f.sub.G. 
A device based on utilization of the electroclinic effect is given, for 
example, in Andersson et al., Appl. Phys. Lett. 1987, 51,640. 
The invention is described in greater detail by means of the examples, but 
this is not intended to represent a limitation.