GB2248059A

GB2248059A - Fluorocyclohexane derivatives and liquid-crystalline medium

Info

Publication number: GB2248059A
Application number: GB9118862A
Authority: GB
Inventors: David Coates; Mark Goulding
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 1990-09-20
Filing date: 1991-09-04
Publication date: 1992-03-25
Also published as: GB9118862D0

Abstract

Fluorocyclohexane derivatives of the formula I, <IMAGE> [in which n is 1 to 10; the ring B is trans-1,4-cyclohexylene, 1,4-phenylene, 3-fluoro-1,4-phenylene or a single bond; @@ X is alkyl or alkoxy with in each case up to 10 C-atoms, @@ or is F, Cl, -CHF2, -CF3, -CN, @@ -OCF3 or -OCHF2; Y and Z are in each case independently of one another H or F; r and s @@ are in each case independently of one another 0 or 1; and W is -CO-O, -CH2-O- or @@ -CH2CH2-] are suitable as components of liquid-crystalline media.n

Description

Fluorocyclohexane derivatives and liquid-crystalline medium The invention relates to new fluorocyclohexane derivatives of the formula I,

in which n is 1 to 10, the ring B is trans-1,4-cyclohexyl ene, 1,4-phenylene, 3-fluoro-1,4-phenylene or a single bond, X is alkyl or alkoxy with in each case up to 10 C-atoms, F, C1, -CHF2, -CF3, -CN, -OCF3 or -OCHF2, Y and Z are in each case indepen dently of one another H or F, r and s are in each case independently of one another 0 or 1 and W is -CO-O-, -CH2-O- or -CH2CH2-.

The invention furthermore relates to the use of these compounds as components of liquid-crystalline media and to liquid-crystal and electrooptical display elements containing such liquid-crystalline media.

The compounds of the formula I can be used as components of liquid-crystalline media, in particular for displays based on the principle of the twisted cell, the guest-host effect, the effect of the deformation of aligned phases or the effect of dynamic scattering.

An object of the invention is to provide new stable liquidcrystalline or mesogenic compounds which are suitable as components of liquid-crystalline media and in particular have simultaneously a comparativeiy low viscosity and a relatively high dielectric anisotropy.

It has now been found that compounds of the formula I are highly suitable as components of liquid-crystalline media. In particular, they have comparatively low viscosities. By employing these compounds, it is possible to obtain stable liquid-crystalline media having a bread mesophase range and advantageous values for optical and dielectric anisotropy.

These media furthermore have very good low-temperature behavior Liquid crystals of the formula

F Alkyl < -Alkyl have already been disclosed in EP 0,154,849.

However, these compounds have very high melting points and are less suitable for many practical applications.

In view of the wide range of applications for compounds having a high tf , it was desirable to provide further compounds having properties tailor-made for their particular applications.

In addition, the compounds of formula I significantly broaden the range of liquid-crystalline substances, which are suitable for use in the preparation of industrially useful liquid-crystalline mixtures.

The compounds of the formula I have a broad range of applications. Depending on the choice of substituents, these compounds can serve as a base material forming a major portion of a liquid-crystalline media; however, it is also possible to add a compound of the formula I to a liquid-crystalline base material from another class of compounds, in order, for example, to influence the dielectric and/or optical anisotropy of such a dielectric and/or to optimize its threshold voltage and/or its viscosity.

In the pure state, the compounds of the formula I are colourless and form liquid-crystalline mesophases in a temperature range favoured for electrooptical use. They'have very good chemical, heat and light stability.

Accordingly, the invention relates to compounds of the formula I and to the use of these compounds as components of liquid-crystalline media. The invention further relates to liquid-crystalline media containing at least one compound of the formula I and to liquid-crystal display elements, in particular electrooptical display elements, containing such media.

For the sake of simplicity below, A is a radical of the formula

Cyc a 1,4-cyclohexylene radical, Che a 1,4cyclohexenyl radical, Dio a 1,3-dioxane-2,5-diyl radical, Dit a l,3-dithiane-2,5-diyl radical, Phe a 1,4-phenylene radical, Pyd a pyridine-2,5-diyl radical, Pyr a pyrimidine-2,5-diyl radical and Bi a bicyclo[2.2.2octylene radical, it being possible for a Cyc and/or Phe radical to be ansubstituted or mono- or disubstituted by F or CN. Z is preferably F.

In embodiments in which X is an alkyl or an alkoxy radical, X or C B can be a straight-chain or branched. Preferably n 2n+l X and CnH2nsl are straight-chain radicals having l, 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly are preferably methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore octyl, nonyl, decyl, octoxy, nonoxy or decoxy.

Compounds of the formula I in which X and/or CnE2n+l are branched chain radicals R can be occasionally of importance due to better solubility in the conventional liquidcrystalline base materials, but in particular as chiral doping substances, provided they are optically active.

Smectic compounds of this type are suitable as components for ferroelectric materials.

Compounds of the formula I having SA phases are suitable, for example, for thermally addressed displays.

Branched groups of this type, preferably, do not contain more than on chain branching. Thus, preferred branched radicals R include isopropyl, 2-butyl (=. l-methylpropyl), isobutyl (= 2-methyl propyl), 2-methylbutyl, isopentyl (= 3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, l-methylheptoxy.

Compounds of the formula I containing branched groups R suitable for polycondensations are suitable for preparing liquid-crystalline polycondensation products.

The present invention not only relates to the racemates of the compounds of formula I but alse to the optical antipodes and mixtures thereof.

Of the compounds of formula I and their subformulae, the preferred compounds are those in which at least one of the radicals contained therein has one of the preferred meanings mentioned.

The most preferred compounds are those stereoisomers in which the rings cyc are trans-1,4-disubstituted.

A few very particularly preferred smaller groups of compounds are those of the subformulae 11 to I5:

Compounds of formula I are prepared by known methods, which are described in the literature (e.g. in the standard works such as Houben-Weyl, Methoden der Organischen Chemie (Methods of Organic Chemistry), Georg-Thieme-Verlag, Stuttgart Vol. IX. p. 867 ff), under reaction conditions known to be suitable and using compounds of formula III

as starting materials. These fluorinated acids can be made from the known unfluorinated acids by direct fluorination at low temperatures, e.g. 10 % fluorine in nitrogen, CC12F2 as solvent at -80 OC.

It is also possible to use variations known per se and not mentioned here.in more detail.

Precursors for the compounds according to the invention can be prepared, for example, by metalating a compound of the formula II

in accordance with the reaction scheme below and then reacting the product with a suitable electrophile: Scheme 1

F F 1 1. n-BuLi F 3r-O z 2. B(ONe)3 Br oo; Z 2. B(OMe)3 Z 3. H202 The target products where X is OCHF2 or OCF3 can be obtained from the thus formed phenol by known methods, for example by reaction with chlorodifluoromethane or carbon tetrachloride/ HF.

Scheme 2

F F 1. n-BuLi F Br-O Br F 2. N-chlorosuccinimide or F N-fluorotrimethylpyri dinium triflate THF/-600 (Y = F or Cl) Further methods of synthesis are evident to one skilled in the art.For example, 1,3-difluorobenzene compounds appropriately substituted in the 5 position can be converted in accordance with the above scheme to the 2-Y-1,3-difluoro

F compounds and the radical CnH2nr ~(W)s~ ss < t-, wherein t is O or 1, and &commat; is trans-l,4-cycloexylene, l4-phenylene or 3-f luoro-l,4-phenylene, can then be introduced by reactions customary in liquid-crystalline chemistry (e.g. esterification, etherification or cou pling reactions, for example as described in E. Poetsch, Kontakte (Darmstadt) 1988 (2), p. 15).

Precursors for the compounds according to the invention of the formula I in which Y is F and X is CF3 can be prepared by metalation of the unsubstituted 3,5-difluorophenyl compounds where n-BuLi, followed by reaction with iodine and reaction of the resulting iodine compound with sodium trifluoro acetate according to the following scheme:

F F n-BuLi F CF3COONa Br then 12 BrI BrF3 F F Cu-cat.

The compounds according to the invention where Z is H and X is CF3 can be prepared by converting 3-fluoro-4-iodobromobenzene to the benzotrifluoride compound with CF3COONa as follows:

The compounds of formula I wherein Z is -CH2CH2 - can be made by reaction of an aldehyde of the formula

with a corresponding phosphonium salt, in a Wittig reaction, followed by subsequent hydrogenation of the resulting ethane derivative.

Precursors which are suitable for the synthesis of the phosphorium salts can be obtained, for example, by the following reaction scheme:

The Grignard compound obtained from the corresponding bromobenzene derivative is reacted with chlorotrialkyl orthozirconate in accordance with WO 87/05599 to give the tertiary cyclohexanol. Elimination of water, hydrogenation of the double bond and isomerization give the trans-cyclohexanecarboxylic acid ester by customary methods. From the latter, customary standard processes can provide an aldehyde, which is suitable for providing compounds according to the invention through the aforementioned Wittig synthesis and subsequent hydrogenation of the double bond.

Some of the bromobenzene derivatives used as starting materials are known, and some can be obtained without difficulties from compounds known from the literature by standard methods of organic chemistry. For example, the OCF3 or OCHF2 compounds can be obtained by known processes from the corresponding phenols and the CF or CN compounds can be obtained from the corresponding benzoic acids. Compounds of the formula

or, alternatively, correpoading monofluorinated compounds can be obtained, for example, from known precursors where X = H by lithiation at low temperatures and a subsequent reaction with-a suitable electrophile.

The compounds with t = 1 are obtained analogously by using bromobiphenyls as starting materials instead of the bromo benzenes. Phe bromobiphenyls can be made by methods known from the literature, e.g. by noble metal catalyzed cross coupling reactions (E. Poetsch, Kontakte (Darmstadt) 1988 (2), p. 15). One possibility is given by example in the following:

The starting materials are either known or can be prepared in analogy with known compounds.

Esters of the formula I can also be obtained by esterification of the corresponding carboxylic acids (or reactive derivatives thereof) with alcohols or phenols (or reactive derivatives thereof) or by the DCC method (DCC = dicyclohexylcarbodiimide).

The corresponding carboxylic acids, alcohols and phenols are known or can be prepared in analogy with known processes.

The synthesis of a few particularly preferred compounds is detailed below: Scheme 8

F 1. BuLi F CF3COONa CH3-O- q > CH3O F 2. 12 F

In a further process for the preparation of the compounds of the formula I, an aryl halide is reacted with an olefin in the presence of a tertiary amine and a palladium catalyst (cf. R.F. Heck, Acc. Chem. Res. 12 (1979) 146). Examples of suitable aryl halides are chlorides, bromides and iodides, in particular bromides and iodides. The tertiary amines which are necessary for the coupling reaction to succeed, such as, for example, triethylamine, are also suitable as solvents.

Examples of suitable palladium catalysts are palladium salts, in particular Pd(II) acetate, with organic phosphorus(III) compounds, such as, Yor example, triarylphosphanes. The reaction can be carried out in the presence or absence of an inert solvent at temperatures between about 0 and 1500, preferably between 200 and 1000; examples of suitable solvents are nitriles, such as acetonitrile, or hydrocarbons, such as benzene or toluene. The aryl halides and olefins which are used as starting materials are commercially available in large numbers or can be prepared by processes known from the literature, for example by halogenation of the corresponding parent compounds or by elimination reactions performed on the corresponding alcohols or halides.

In this manner, it is, for example, possible to prepare stilbene derivatives. The stilbenes can furthermore be prepared by reaction of a 4-substituted benzaldehyde with the corresponding phosphorous ylide, using a Wittig synthesis.

However, it is also possible to prepare tolans of the formula I by using monosubstituted acetylene instead of the olefin (Synthesis 627 (1980) or Tetrahedron Lett. 27, 1171 (1986)).

Furthermore, in order to couple aromatics, aryl halides can be reacted with aryltin compounds. These reactions are preferably carried out with the addition of a catalyst, such as, for example, a palladium(0) complex, in an inert solvent, such as a hydrocarbon, at an elevated temperature, for example in boiling xylene, under an inert gas.

Ethers of formula I can be obtained by etherification of the corresponding hydroxy compounds, preferably of the corresponding phenols, in which the hydroxy compound is preferably first converted to the corresponding metal derivative, for example, by treatment with NaH, NaNH2, NaOH, KOH, Na2O3 or K2C03 to the corresponding alkali metal alcoholate or alkali metal phenolate. This derivative can then be reacted with the corresponding alkyl halide, alkyl sulfonate or dialkyl sulfate, preferably in an inert solvent, such as, for example, acetone, 1,2-dimethoxyethane, DMF or dimethyl solfoxide or even with excess aqueous or aqueous-alcoholic NaOH or KOH at temperatures between about 200 and 100 .

The starting materials are either known or can be prepared in analogy to known compounds.

The liquid-crystalline media according to the invention preferably contain, in addition to one or more compounds according to the invention, 2 to 40, in particular 4 to 30 further components. These media Most preferably contain, in addition to one or more compounds according to the invention, 7 to 25 components.These further components are preferably selected from nematic or nematogenic (monotropic or isotropic) substances in particular substances from the classes of azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylates, phenyl or cyclohexyl cyclohexylbenzoates, phenyl or cyclohexyl cyclohexylcyclohexanecarboxylates, cyclohexylphenyl benzoates, cyclohexanecarboxylates or cyclohexylcyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, cyclohexylcyclohexenes, cyclohexylcyclohexylcyclohexenes, 1,4-bis(cyclohexyl)benzenes, 4,4'bis(cyclohexyl)biphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes, 1-phenyl-2-cyclohexyl- ethanes, 1-cyclohexyl-2- (4-phenylcyclohexyl) -ethanes, l-cyclohexyl-2-biphenylethanes, 1-phenyl-2-cyclohexylphenyl- ethanes, halogenated or unhalogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinamic acids: The 1,4-phenylene groups in these compounds also be fluorinated.

The most important compounds which are suitable as further components of media according to the invention can be characterized by the formulae 1, 2, 3, 4 and 5: R' -L-E-R" 1 R' -L-COO-E-R" 2 R' -L-OOC-E-R" 3 R'-L-CH2CH2-E-R" 4 R'-L-CeC-E-R" 5 In the formulae 1, 2, 3, 4 and 5, L and E, which can be identical or different, in each case independently of one another, are a divalent radical from the group formed by -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- and their mirror images, in which Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc is trans-1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr is pyrim idine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5diyl and G is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl.

Preferably, one of the radicals L and E is Cyc, Phe or Pyr. E is perferably Cyc, Phe or Phe-Cyc. The media according to the invention preferably contain one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5, in which L and E are selected from the group comprising Cyc, Phe and Pyr and simultaneously one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5, in which one of the radicals L and E is selected from the group comprising Cyc, Phe and Pyr and the other radical is selected from the group comprising -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-, and, if desired, one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5, in which the radicals L and E are selected from the group -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-.

R' and R" in the compounds of the subformulae la, 2a, 3a, 4a and 5a are, in each case independently of one another, alkyl, alkenyl, alkoxy, alkenyloxy or alkanoyloxy having up to 8 carbon atoms. In most of these compounds, R' and R" are different from one another, one of these radicals in most cases being alkyl or alkenyl. In the compounds of the subformulae ib, 2b, 3b, 4b and 5b, R" is -CN, -CF3, F, C1 or -NCS; here, R' has the meaning given for the compounds of the subformulae la to Sa and is preferably alkyl or alkenyl. However, other variations, in addition to those detailed in subformulae la-5a and lb-5b, of the intended substituents in the compounds of the formulae 1, 2, 3, 4 and 5 are also customary. Many of these substances and also mixtures thereof are commercially available. All these substances can be obtained by methods known from the literature or analogous methods.

The media according to the invention preferably contain, in addition to components from the group of the compounds la, 2a, 3a, 4a and 5a (group 1) also components from the group of the compounds lb, 2b, 3b, 4b and 5b (group 2), the relative amounts of which are preferably as follows: Group 1: 20 to 90 %, in particular 30 to 90 % Group 2: 10 to 80 %, in particular 10 to 50 % the sum of the relative amounts of the compounds according to the invention and the compounds from groups 1 and 2 adding up to 100 %.

The media according to the invention preferably contain 1 to 40 %, particularly preferably 5 to 30 %, of the compounds according to the invention. Media containing more than 40 %, in particular 45 to 90 %, of the compounds according to the invention are furthermore preferred. The media preferably contain three, four or five compounds according to the invention.

The media according to the invention are prepared in a manner customary per se. As a rule, the components are dissolved in one another, preferably at an elevated temperature. By means of suitable additives, the liquid-crystalline phases according to the invention can be modified in such a manner that they can be used in all previously known types of liquid-crystal display elements. These types of additives are known to one skilled in the art and have been described in detail in the literature (H. Kelker/R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980). For example, pleochroic dyes for preparing coloured guest-host systems or substances for changing the dielectric anisotropy, the viscosity and/or the orientation of the nematic phases can be added.

The examples which follow are intended to illustrate the invention, without limiting it. Above and below, the percentages given are by weight. All temperatures are given in degrees Centrigrade. M.p. denotes the melting point and c.p.

the clearing point. Furthermore, C denotes crystalline state, N = the nematic phase, S = the smectic phase and I the isotropic phase. The numbers between these symbols are the transition temperatures. An is optic anisotropy (589 nm, 20 OC), and the viscosity (mm2/sec) was determined at 20 CC. "Conventionl work-up" means: if necessary, water is added, the product is extracted with methylene chloride, diethyl ether or toluene, the organic phase is separated off, dried, evaporated, and the product is purified by distillation under reduced pressure or crystallization and/or chromatography.

The following abbreviations are used: DAST Diethylaminosulfur trifluoride DCC Dicyclohexylcarbodiimide DDQ Dichlorodicyanobenzoquinone DIBALH Diisobutylaluminium hydride KOT Potassium tert.-butoxide THF Tetrahydrofuran pTSOH p-Toluenesulfonic acid TMEDA Tetramethylethylenediamine DCM Dichloromethane ExamPle 1: Step 1 Magnesium (6.4 g, 0.263 moles) and THF (150 ml) were stirred under nitrogen. l-bromo-3-fluoro-4-chlorobenzene (50 g, 0.239 moles) was added dropwise to the reaction mixture which was left to stir and reflux for 2 hrs. The reaction mixture was cooled to 0 OC, and trimethyl borate (30 ml, 0.263 moles) in THF (50 ml) was added keeping the temperature below 20 OC.

The reaction mixture was left to stir in the ice bath for 30 min, then quenched with acid solution (45 ml cHCl, 80 ml H2O). The reaction mixture was extracted into ether, washed well with H2O, then brine and finally the ether was evaporated to afford the crude boronic acid. The boronic acid was dissolved in ether (265 ml) and 30 % hydrogen peroxide solution (30 ml, 0.263 moles) was added dropwise over 30 minutes.

This was then left to stir at room temperature for 16 hrs.

The reaction mixture was extracted with ether, washed well with H20 then with ferrous sulphate to remove excess H202. The organic layer was extracted with 1 M NaOH solution and the aqueous phase acidified with cH2SO4. The acidic solution was re-extracted into ether, dried and run down. The crude product was distilled to afford the phenol, bp. 640/2.5 mbar.

SteP 2 The phenol (1.84 g, 0.013 moles) and trans-4-(4-fluoro-4-npropylcyclohexyl)-cyclohexane carboxylic acid (3.6 g, 0.0143 moles) were dissolved in DCM (30 ml) with trifluoroacetic anhydride (2 ml, 0.0143 moles). This was left to stand for 16 hrs. The reaction mixture was added to H2O (30 ml), the organic layer was washed with NaHCO3 solution, then with H2O and dried and run down. After customary work-up 3-fluoro4-chlorophenyl trans-4- (4-fluoro-4-n-propylcyclohexyl) -cyclo- hexane carboxylate is obtained.

ExamPles 2 to 6: In place of the used carboxylic acid and the fluorochlorophenol other starting materials can be used for preparation of the following compounds in analogy to example 1 (all have t = 0):

n -( w) -)r- - s- X Y Z (2) 3 7 -COO- F F H (3) 3 &commat; -COO- F F F (4) 3 '-' -COO- F H H (5) 3 r -COO- F F H (6) 3 S-, -COO- F F F

Claims

CLAIMS 1. A fluorocyclohexane derivative of the formula I,

in which n is 1 to 10, the ring B is a trans-1,4-cyclohexylene, a 1,4-phenylene, or a 3-f luoro-1,4-phenylene radical or a single bond, X is an alkyl or alkoxy radical with in each case up to 10 carbon atoms, or an F, C1, -CHF2, -CF3, -CN, -OCF3 or OCHF2 radical, Y and Z are in each case independently of one another a hydrogen or a flourine radical, r and s are in each case independently of one another 0 or 1 and W is a -CO-O-, -CH2-O- or -CH2CH2- group.
2. A fluorocyclohexane derivative as claimed in claim 1 wherein X is a straight chain or branched alkyl or alkoxy radical.
3. A fluorocyclohexane derivative as claimed in either claim 1 or claim 2 wherein X or CnH2n+1 is a straight chain radical having 1-7 carbon atoms.
4. A fluorocyclohexane derivative as claimed in any of claims 1-3 wherein X or C H2n+1 is a branched chain radical with a single branch and, preferably, an isopropyl, 2-butyl (= l-methylpropyl), isobutyl (= 2methylpropyl), 2-methylbutyl, isopentyl (= 3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2 -methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, l-methylhexoxy, l-methylheptoxy.
5. A fluorocyclohexane derivative as claimed in claim 1 comprising a compound of subformula I, I2, I3, I4 or I5.
6. A fluorocyclohexane derivative as claimed in any of the preceding claims in the form of a racemate, a substantially pure optical antipode or a mixture thereof.
7. A fluorocyclohexane derivative substantially as hereinbefore described as being in accordance with the invention.
8. Use of a compound as claimed in any of claims 1-7 as a component of a liquid-crystalline media.
9. A liquid-crystalline medium containing at least two liquid-crystalline components, characterized in that it contains at least one compound as claimed in any of claims 1-7.
10. A liquid-crystalline medium as claimed in claim 9, further comprising at least one compound. of formula 1, 2, 3, 4 or 5.
11. A liquid-crystalline medium as claimed in claim 10 comprising at least one compound of formula la, 2a, 3a, 4a or Sa.
12. A liquid-crystalline medium as claimed in claim 10 or 11 comprising at least one further compound of formula Ib, 2b, 3b, 4b or 5b.
13. A liquid-crystalline medium as claimed in claim 12 comprising 20 to 90% and preferably 30 to 90% of a compound or compounds of formula la, 2a, 3a, 4a or Sa, and 10 to 80% and preferably 10 to 50% of a compound or compounds of formula lb, 2b, 3b, 4b or 5b.
14. A liquid-crystalline medium as claimed in any of claims 9-13, comprising 1 to 40% and preferably 5 to 30% of a compound as claimed in any of claims 1-7.
15. A liquid-crystalline medium as claimed in any of claims 9-13, comprising 45 to 90% of a compound as claimed in any of claims 1-7.
16. A liquid-crystalline medium substantially as hereinbefore described as being in accordance with the present invention.
17. A liquid-crystal display element, characterized in that it contains a liquid-crystalline medium according to any of claims 9-16.
18. An electrooptic display element, characterized in that it contains, as dielectric, a liquid-crystalline medium according to any of claims 9-16.