DE4218978A1 - New di:hydro-furo:pyridine cpds. used in liquid crystal medium - for electro=optical display esp. matrix display, liquid crystal switch or LCD, giving stable chiral tilted smectic phase - Google Patents

Abstract

New 3-substd.-2,3-dihydro-furo(2,3-b)pyridine cpds. are of formula (I), where R1 = 1-15C alk(en)yl, opt. with one CN, halogen or CF3 substit., in which one or more -CH2- gps. may be replaced by -S-, -O-, -CO-, -CO-O-, -O-CO- or -O-CO-O- but S and/or O atoms are not linked to one another directly; R2 = H or 1-15C alkyl; A1-2 = (a) trans-1,4-cycohjexylene (Cycy), in which one or more non-adjacent -CH2- gps. may be replaced by -O- and/or -S-; (b) 1,4-pheylene (Phe), in which one or two CH gps. may be replaced by N; or (c) 1,3-cyclobutylene, 1,3-bicyclo(1.1.1)pentylene, 1,4-cyclohexenylene, 1,4-bicyclo(2.2.2)octylene, piperidin-1,4-diyl, naphth-2,6-diyl, decahydronaphth-2,6-diyl and 1,2,3,4-tetrahydronaphth-2,6-diyl; and (a) and (b) may be substd. by CN or halogen; Z1-2 = -CH2-CH2-, -CC-, -CH2O-, -OCH2-, -CO-O-, -O-CO-, -CH=N-, -CH2S, -SCH2-, a single bond or 3-6C alkylene, in which one -CH2- gp. may be replaced by -O-, -CO-O-, -O-CO-, -CH(halogen)- or -CH(CN)-; L = H, F or Cl; m = 0, 1 or 2. L = F; R2 = H. USE/ADVANTAGE - (I) are used in liquid crystal (LC) media, esp. dielectric media, for electrooptical displays, esp. matrix displays, and in LC switches and LCDs. (I) have negative dielectric anisotropy, large dielectric constant perpendicular to the long axis and a wide useful range. They give chemically stable chiral tilted smectic LC phases with favourable ferroelectric phase range and viscosity and can be supercooled to temps. below 0 deg.C. Phases with medium to high spontaneous polarisation can be produced and also LC SA phases for the electroclinic effect.

Description

The invention relates to 2,3-dihydro-furo (2,3-b) pyridines Formula I,

in which
R ^{1 is} an unsubstituted or a simply substituted by CN, halogen or CF ₃ alkyl or alkenyl radical having up to 15 carbon atoms, one or more CH ₂ groups in each of these radicals being independently of one another by —S—, —O -, -CO-, CO-O-, O-CO- or O-CO-O- can be replaced so that S and / or O atoms are not directly linked to each other,
R ^{2 is} H or alkyl having 1 to 15 carbon atoms,
A ¹ and A ² each independently

• a) trans-1,4-cyclohexylene radical, in which one or more non-adjacent CH ₂ groups can also be replaced by -O- and / or -S-,
• b) 1,4-phenylene radical, in which also one or two CH- Groups can be replaced by N,
• c) residue from the group 1,3-cyclobutylene, 1,3-bicyclo [1.1.1] pentylene, 1,4-cyclohexenylene, 1,4-bicyclo [2.2.2] octylene, piperidine-1,4-diyl , Naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
  where the residues (a) and (b) can be substituted by CN or halogen,
  Z ¹ and Z ² each independently of one another CH ₂ -CH ₂ -, -C≡C-, -CH ₂ O-, -OCH ₂ -, -CO-O-, -O-CO-, -CH = N-, -CH ₂ S-, -SCH ₂ -, a single bond or an alkylene group with 3 to 6 C-atoms, in which also a CH ₂ group by -O-, -CO-O-, -O-CO-, - CH-halogen or -CHCN- can be replaced,

LH, F or Cl,
m 0, 1 or 2,
4 mean.

Chiral tilted smectic liquid crystalline phases with ferroelectric properties can be produced by making base mixtures with one or more tilts smectic phases with a suitable chiral dopant (L.A. Beresnev et al., Mol. Cryst. Liq. Cryst. 89, 327 (1982); MR. Brand et al., J. Physique 44 (lett.), L 771 (1983). Such phases can be used as dielectrics for fast switching displays are used on the Clark and Lagerwall described the principle of SSFLC technology (N.A. Clark and S.T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980); USP 4,367,924) based on the ferroelectric Properties of the chirally tilted phase are based. In this Phase, the elongated molecules are arranged in layers net, with the molecules tilting to the layer norm len. As you move from layer to layer the tilt direction changes by a small angle with respect to an axis perpendicular to the layers, so that a helical structure is formed. In displays that are based on the principle of SSFLC technology smectic layers perpendicular to the plates of the cell arranged. The helical arrangement of the tilt directions of the Molecules are created by a very small distance between the plates (approx. 1-2 µm) suppressed. As a result, the longitudinal axes of the Forced molecules to be parallel to each other in one plane Arrange plates of the cell, making two excellent Tilt orientations arise. By creating a suitable one alternating electrical field can be a spontaneous polar sation liquid crystal phase between them be switched back and forth in both states. This Shifting is much faster than conventional ones twisted cells (TN-LCD's) based on nematic liquid crystals based.  

A major disadvantage for many applications of the currently available materials with chirally tilted smectic phases (such as Sc *, but also S _H *, S _I *, S _J *, S _G *, S _F *) is their low chemical level , thermal and photo stability. Another disadvantageous property of displays based on currently available chirally tilted smectic mixtures is that the spontaneous polarization has values that are too small, so that the switching time behavior of the displays is influenced unfavorably and / or the pitch and / or the tilt and / or the viscosity of the Phases do not meet the requirements of display technology. In addition, the temperature range of the ferroelectric phases is usually too small and is predominantly at too high temperatures.

It has now been found that the use of compounds of the formula I as components of chirally tilted smectic mixtures can substantially reduce the disadvantages mentioned. The compounds of the formula I are therefore particularly suitable as components of chirally tilted smectic liquid-crystalline phases. In particular, with their help, chemically particularly stable chirally tilted smectic liquid crystalline phases with favorable ferroelectric phase ranges, favorable widths for the viscosity, in particular with wide Sc * phase ranges, excellent supercoolability down to temperatures below 0 ° C. without crystallization occurring and medium to high values for the spontaneous polarization can be produced for such phases. P is the spontaneous polarization in nC / cm ² . However, the compounds of the formula I are also suitable for liquid-crystalline S _A phases for the electroclinic effect.

The compounds of formula I have a negative anisotropy of the dielectric constant with a large dielectric constant perpendicular to the longitudinal axis of the molecule (ε _⟂ ) and therefore have a wide range of applications. Depending on the selection of the substituents, these compounds can serve as base materials from which liquid-crystalline smectic phases are predominantly composed; However, it is also possible to add compounds of the formula I to liquid-crystalline base materials from other classes of compounds, for example to improve the dielectric and / or optical anisotropy and / or the viscosity and / or the spontaneous polarization and / or the phase range and / or the tilt angle and / or the To vary the pitch of such a dielectric.

The invention thus relates to the pyridines of the formula I, in particular in which at least one of the radicals A ¹ and A ^{2 is} 1,4-phenylene, 1,4-cyclohexylene, pyrimidine-2,5-diyl or pyridine-2,5 optionally substituted by fluorine -diyl means.

The compounds of formula I include the compounds of Formula I1-I3 with 2 rings,

wherein R¹, R², A² and Z² have the meaning given and
Compounds of the formulas I4 to I6 with 3 rings,

and compounds of the formulas I7 to I9 with 4 rings.

The invention also relates to optically active 2,3-dihy dro-furo (2,3-b) pyridines of the formula I *,

wherein

R ² alkyl with 1-15 C atoms

mean and R¹, A ¹ , A², Z ¹ , Z², m and L have the meaning given in claim 1.

The invention also relates to liquid crystalline Phases containing at least one compound of Formula I. The invention further relates to ferroelek trical liquid crystalline phases containing at least least one compound which is a group of formula

where L is F or Cl, has as structural element, and liquid crystal display elements, in particular ferroelek trical electro-optical display elements, such phases contain.

The phases according to the invention preferably contain at least at least two, in particular at least three, connections of the Formula I. Chiral according to the invention are particularly preferred tilted smectic liquid crystalline phases, their  achiral base mixture in addition to compounds of the formula I at least another component with negative or amount contains small positive dielectric anisotropy. These further component (s) of the achiral base mixture can 1 up to 50%, preferably 10 to 25%, of the base mixture chen. As further components with a small posi tive or negative dielectric anisotropy are suitable Compounds of formula IV, which the compounds of Sub-formulas IVa to IVi include:

R ⁴ and R⁵ are each preferably straight-chain alkyl, alkoxy, alkanoyloxy or alkoxycarbonyl, each having 3 to 12 carbon atoms. X is preferably O. In the compounds of the formulas IVa, IVb, IVd, IVe, IVf and IVg, a 1,4-phenylene group can also be substituted laterally by halogen or CN, particularly preferably by fluorine.

Particularly preferred are the compounds of the sub-formulas IVa, IVb, IVd and IVf, in which R ⁴ and R ⁵ each represent straight-chain alkyl or alkoxy each having 5 to 10 carbon atoms. Particularly preferred individual compounds are given in Table I below:

Table I

The compounds of the sub-formulas IVc, IVh and IVi are suitable as additives for lowering the Schinelzptsts Lowering and are normally added to the base mixtures with not more than 5%, preferably 1 to 3%. R⁴ and R ⁵ in the compounds of the sub-formulas IVc, IVh and IVi are preferably straight-chain alkyl having 2 to 7, preferably 3 to 5, carbon atoms. Another class of compounds suitable for lowering the melting point in the phases according to the invention is that of the formula

wherein R⁴ and R ^{5 have} the preferred meaning given for IVc, IVh and IVi.

As further components with negative dielectric aniso Compounds containing that are also suitable in tropics Structural element A, B or C.

Preferred compounds of this type correspond to the formulas Va, Vb and Vc:

R 'and R'' each preferably represent straight-chain alkyl or alkoxy groups each having 2 to 10 carbon atoms. Q ¹ and Q² each represent 1,4-phenylene, trans-1,4-cyclohexylene, 4,4'-biphenylyl, 4- (trans-4-cyclohexyl) phenyl, trans, trans-4,4'-bicyclohexyl or one of the groups Q ¹ and Q ^{2 is} also a single bond.

Q³ and Q ⁴ each represent 1,4-phenylene, 4,4'-biphenylyl or trans-1,4-cyclohexylene. One of the groups Q³ and Q ⁴ can also mean 1,4-phenylene, in which at least one CH group has been replaced by N. R '''is an optically active residue with a
asymmetric carbon atom of the structure

Particularly preferred compounds of formula Vc are those gene of the formula Vc ′,

wherein A is 1,4-phenylene or trans-1,4-cyclohexylene and n is 0 or 1 means.

Such ferroelectric liquid crystalline phases containing at least one achiral halopyridine of the formula I1, at least one achiral phenylpyrimidine of the formula Vc ', in which R''' is an alkyl or alkoxy radical having up to 18 carbon atoms, are particularly preferred as the base material with a broad S _C phase and at least one chiral halopyridine of the formula I as an optically active dopant. Also preferred are those ferroelectric liquid-crystalline phases which, in addition to the compounds of the formula I1, Vc 'and I mentioned, have at least one phenyl pyridine of the formula Vd and / or a 2-fluoro- (L = H) or 2,3-difluorophenylpyrimidine Formula Ve (L = F) and / or a phenylpyrimidine of the formula Vf

contain.

The compounds of formula I include those below preferred binuclear and trinuclear materials listed of the formulas Ia-Ij:

R 1 and R ² in the formulas above and below preferably have 1-15 C atoms, in particular 3-12 C atoms. Compounds of the formula I in which R ¹ and R ^{2 have} 1-7 C atoms, preferably 1-5 C atoms, are particularly suitable for liquid-crystalline phases for display elements based on the ECB effect. Compounds of formula I, however, in which R ¹ and R ^{2 have} 6-15 carbon atoms, preferably 6-12 carbon atoms, are suitable for liquid-crystalline phases with ferroelectric properties. One or two CH ₂ groups can also be replaced in R ¹ . Preferably only one CH ₂ group is replaced by -O-, -S-, CO-O- or O-CO-, in particular by -O-.

Also preferred are those compounds in which 2, 3 or non-adjacent CH ₂ groups have been replaced by -O-.

In the formulas above and below, R ₁ preferably denotes alkyl, alkoxy or another oxaalkyl group.

If R ^{1 is} an alkyl radical in which one ("alkoxy" or "oxaalkyl") or two ("alkoxyalkoxy" or "dioxaalkyl") non-adjacent CH ₂ groups can be replaced by O atoms, then they are straight or branched. They are preferably straight-chain, have 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly preferably mean ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pen toxy, hexoxy or heptoxy, furthermore Methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy, tridecoxy or tetradecoxy.

Oxalkyl preferably means straight-chain 2-oxypropyl (= Methoxymethyl), 2- (= ethoxymethyl) or 3-oxabutyl (= 2-Me thoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxa hexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl, 1,3-dioxabutyl (= Methoxymethoxy), 1,3-, 1,4-, 2,4-dioxapentyl, 1,3-, 1,4-, 1,5-, 2,4-, 2,5- or 3,5-dioxahexyl, 1,3-, 1,4-, 1,5-, 1,6-, 2,4-, 2,5-, 2,6-, 3,5-, 3,6- or 4,6-dioxaheptyl.

The compounds of the formula I which contain one or two dioxa- or have trioxaalkyl group are particularly suitable as complex-forming pedestals to reduce concentration free ions in liquid crystalline mixtures, in particular for ferroelectric displays. In particular, with With the help of these connections so-called "ghost images" (cf. e.g. B. J. Dÿon et al., SID conference, San Diego 1988, pp 2-249) can be suppressed.

If R ^{1 is} an alkyl radical in which a CH ₂ group has been replaced by -S-, this can be straight-chain and branched ver. This thiaalkyl radical is preferably straight-chain and means 2-thiapropyl, 2- or 3-thiabutyl, 2-, 3- or 4-thiapentyl, 2-, 3-, 4- or 5-thiahexyl, 2-, 3-, 4- , 5- or 6-thiaheptyl, 2-, 3-, 4-, 5-, 6- or 7-thiaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-thianonyl or 2 -, 3-, 4-, 5-, 6-, 7-, 8- or 9-thiadecyl.

Particularly preferred are alkyl radicals R ₁ in which the CH ₂ group adjacent to the group A ¹ , A ² and / or A ^{3 is} replaced by -S- and thus preferably means methylthio, ethylthio, propylthio, butylthio, pentylthio, hexylthio, heptylthio , Octylthio, nonylthio or decylthio.

If R ^{1 is} an alkenyl radical, this can be straight-chain or branched. It is preferably straight-chain and has 2 to 10 carbon atoms. Accordingly, it means especially vinyl, prop-1-, or prop-2-enyl, but-1-, 2- or but-3-enyl, pent-1-, 2-, 3- or pent-4-enyl, hex -1-, 2-, 3-, 4- or hex-5-enyl, hept-1-, 2-, 3-, 4-, 5- or hept-6-enyl, oct-1-, 2-, 3-, 4-, 5-, 6- or oct-7-enyl, non-1-, 2-, 3-, 4-, 5-, 6-, 7- or non-8-enyl, Dec-1 -, 2-, 3-, 4-, 5-, 6-, 7-, 8- or Dec-9-enyl.

If R _{1 is} an alkyl radical in which a CH ₂ group has been replaced by -O-CO or -CO-O-, this can be straight-chain or branched. It is preferably straight-chain and has 2 to 6 carbon atoms. Accordingly, it means in particular acetyloxy, propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetyl oxymethyl, propionyloxymethyl, butyryloxymethyl, pentaoyloxy methyl, 2-acetyloxyethyl, 2-propionyloxyethyl, 2-butyryl oxyethyl, 3-acetyloxypropyl, 3-acetyloxypropyl, 3-propyloxy propyl, 3-propyloxy propyl Methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonyl methyl, 2- (methoxycarbonyl) ethyl, 2- (ethoxycarbonyl) ethyl, 2- (propoxycarbonyl) ethyl, 3- (methoxycarbonyl) propyl, 3- ( Ethoxycarbonyl) propyl, 4- (methoxycarbonyl) butyl.

Compounds of formula I with branched wing groups R¹e and / or R² can occasionally because of better solubility speed in the usual liquid crystalline base materials from Be important, but especially as chiral dopants, if they are optically active. Smectic connections of these Art are suitable as components for ferroelectric materials lien.

Branched groups of this type usually contain no more than one chain branch. Preferred branched radicals R 1 and / or R ² are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl), 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, 1-methylheptoxy, 2-oxa-3-methylbutyl , 3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-do decyl, 6-methyloctoxy, 6-methyloctanoyloxy, 5-methylheptyl oxycarbonyl, 2-methylbutyryloxy, 3-methylvaleryloxy, 4-Me thylhexanoyloxy, 2-chloropropionyloxy, 2-chloro-3-methylbutyryloxy, 2-chloro-4-methylvaleryloxy, 2-chloro-3-methylvaleryl oxy, 2-methyl-3-oxapentyl, 2-methyl-3-oxahexyl.

The compounds of the formula I with L = F and R ¹ alkyl with up to 15 C atoms are particularly preferred, in which one or more CH ₂ groups can also be replaced by O atoms. R 1 is preferably alkoxy having up to 12 carbon atoms.

The chiral halopyridines of the formula I are suitable excellent as dopants for induction of ferroelectricity in a smectic base material. You stand out particularly characterized by a high level of spontaneous polarization.

The chiral compounds of formula I accordingly include the Compounds of the sub-formulas Ic1 to Ic4,

wherein R ¹ , R ² , L, Z ¹ and Z ^{2 have} the meaning given.

Particularly preferred are those of the sub-formulas Ic1 to Ic3, in which R¹ is alkyl or alkoxy having 1 to 12 C atoms, or R ^{2 is} H or alkyl having 1 to 12 C atoms, and A¹ or A² is 1,4-phenylene means in which 1 or 2 CH groups can also be replaced by N.

The compounds of formula I are known per se Methods as described in the literature (e.g. in the standard works like Houben-Weyl, methods of organic chemistry, Georg-Thieme-Verlag, Stuttgart) are described poses.

If desired, the starting materials can also be in situ are formed such that they can be removed from the reaction mixture not isolated, but immediately to the connections of the Formula I implements.

The compounds of the present invention can be one of the following Simply create reaction schemes.

Scheme 1

Scheme 2

Is used instead of difluoropyridine as a starting material dichloropyridine sets, the compounds of Formula I, where L = H.

Scheme 3

The starting compounds can be, for example, as follows produce:

Scheme 4 (Y = F or Cl)

Scheme 5

Scheme 6

Scheme 7

The compounds of formula I are also suitable as components nematic liquid crystalline phases for ECB displays. This nematic liquid crystalline according to the invention Phases consist of 2 to 25, preferably 3 to 15 components ten, including at least one compound of formula I. Die other ingredients are preferably selected from the nematic or nematogenic substances, especially the known substances from the classes of azoxybenzenes, Benzylidenanilines, biphenyls, terphenyls, phenyl or cyclo hexyl benzoates, cyclohexane carboxylic acid phenyl or cyclohexyl esters, phenylcyclohexanes, cyclohexylbiphenyls, cyclohexyl cyclohexanes, cyclohexylnaphthalenes, 1,4-bis-cyclohexylben zole, 4,4'-bis-cyclohexylbiphenyls, phenyl- or cyclohexyl pyrimidines, phenyl- or cyclohexylpyridazines and their N-oxides, phenyl or cyclohexyl dioxanes, phenyl or cyclo hexyl-1,3-dithiane, 1,2-diphenylethane, 1,2-dicyclohexyl ethanes, 1-phenyl-2-cyclohexylethanes, optionally halogen nated stilbenes, benzylphenyl ether, tolanes and substitutes ten cinnamic acids.

The main as components of such liquid kri Stalline phases can be considered characterize by the formula I′′

R′-L-G-E-R ′ ′ I ′ ′

where L and E each have a carbocyclic or heterocyclic ring system from that from 1,4-disubstituted benzene and cyclohexanrin gene, 4,4'-disubstituted biphenyl, phenylcyclohexane and Cyclohexylcyclohexane systems, 2,5-disubstituted pyrimidine and 1,3-dioxane rings, 2,6-disubstituted naphthalene, di- and tetrahydronaphthalene, quinazoline and tetrahydroquinazoline educated group,

or a single CC bond,
Y halogen, preferably chlorine, or -CN, and
R 'and R''alkyl, alkoxy, alkanoyloxy, alkoxycarbonyl or alkoxycarbonyloxy with up to 18, preferably up to 8 carbon atoms, or one of these radicals also mean CN, NC, NO ₂ , CF ₃ , F, Cl or Br.

For most of these compounds, R 'and R' 'are from one another which is different, one of these residues usually being an alkyl or alkoxy group. But other variants of the pre The substituents seen are common. Many such sub punches or mixtures thereof are commercially available. All of these substances are based on methods known from the literature available.

The phases according to the invention contain about 0.1 to 99, preferably 10 to 95%, one or more compounds of formula I. Furthermore preferred according to the invention liquid crystalline phases containing 0.1-40, preferably 0.5-30% of one or more compounds of the formula I. The phases according to the invention are produced in yourself in the usual way. As a rule, the components dissolved in one another, expediently at elevated temperature.  

The liquid-crystalline phases can be added by suitable additives be modified according to the invention so that they in all Types of liquid crystal that have become known so far display elements can be used.

Such additives are known to the person skilled in the art and in the lite described in detail. For example, Leit salts, preferably ethyldimethyldodecylammonium-4-hexyloxy benzoate, tetrabutylammonium tetraphenylboranate or complex salts of crown ethers (see, for example, I. Haller et al., Mol. Cryst.Liq.Cryst. Volume 24, pages 249-258 (1973)) for Verbes conductivity, pleochroic dyes Manufacture of colored guest host systems or substances for Change in dielectric anisotropy, viscosity and / or the orientation of the nematic phases added will.

Such substances are e.g. B. in DE-OS 22 09 127, 22 40 864, 23 21 632, 23 38 281, 24 50 088, 26 37 430, 28 53 728 and 29 02 177.

The mixtures described are particularly suitable as components for liquid crystal switching and display directions, in particular for the SSFLC described above Cell. In particular, this applies to the so-called "virgin liche (virgin) liquid crystal texture "in which the smekti layers are angled ("chevron structures") and for the "bookshelf" or "quasi-bookshelf" geometry, at which the smectic layers are vertical or almost vertical (see Y. Sato et al., Jap. J. Appl. Phys. Vol. 28, 483 (1989)).  

The nematic mixtures containing are also suitable Compounds of formula I due to their wide nematic Phase ranges and their high negative dielectric ani sotropy in particular as components for higher ver Drilled liquid crystal displays such as STN (e.g. GB 2,123,163) or OMI (e.g. M. Schadt, F. Leenhouts, Appl. Phys. Lett. 50, 236 (1987)), with very high steepness of the characteristic can be achieved (e.g. WO 89/08691).

In particular the compounds of the formula IT

are ideal as components for ECB displays (e.g. M. Schiekel, K. Fahrenschon, Appl. Phys. Lett. 19 391 (1971)) that they have very large values of optical anisotropy (Δn 0.22) and comparatively high negative values of have dielectric anisotropy (Δε -5.0).

The following examples are intended to illustrate the invention without limiting it. F. = melting point, K. = clearing point. Percentages above and below mean percentages by weight; all temperatures are given in degrees Celsius. "Normal refurbishment" means:
water is added, the mixture is extracted with methylene chloride, the mixture is separated off, the organic phase is dried, evaporated and the product is purified by crystallization and / or chromatography.

Furthermore mean:
K crystalline
N nematic
S smectic
I isotropic

The numbers between these symbols indicate the respective phase transition temperature in ° C. The following abbreviations are also used:
n-BuLi n-butyllithium
DAST Diethylaminosulfur trifluoride
DCC dicyclohexylcarbodiimide
DEAD diethylazodicarboxylate
DMF dimethylformamide
LDA lithiandiisopropylamide (made from equimolar amounts of n-BuLi and diisopropylamine) THF tetrahydrofuran
TP triphenylphosphine
TPP tetrakis (triphenylphosphine) palladium (O)

example 1

0.6 mol of diisopropylamine are dissolved in 60 ml of THF and 0.6 mol of n-BuLi are added at -10 ° C. The mixture is then cooled to -70 ° C. and 2,6-difluoropyridine is added dropwise. The mixture is then stirred for 15 min and 0.6 mol of B (OCH ₃ ) _{3 is} added at -60 ° C. The mixture is allowed to warm to -30 ° C. and hydrolyzed by adding half-conc. HCl. The organic phase is separated off and worked up as usual.

0.05 mol of p-trans-4-pentylcyclohexylphenyl bromide, 0.06 mol of boronic acid, 100 ml of toluene ', 40 ml of ethanol, 50 ml of 2 M Na ₂ CO ₃ solution and 1.25 g of Pd (0) catalyst are 2 hours heated to 60 ° C. The mixture is allowed to cool, filtered through silica gel and the filtrate is worked up as usual.

10 mmol of 3- [4-trans-4-pentylcyclohexyl) phenyl] -2,6-di fluoropyridine is dissolved in 70 ml THF under protective gas and mixed with 11 mmol n-BuLi at -100 ° C. Subsequently is stirred for 10 minutes and then at -100 ° C to the mixture of 20 mmol ethylene oxide, dissolved in 20 ml THF, added dropwise. After 5 minutes of stirring, 10 mmol Boron trifluoride diethyl ether added dropwise. It is left at 0 ° C heat, hydrolyze and add methylbuty lether. The organic phase is separated off and how worked up as usual.  

device

1 H-NMR (CHCl₃, MH)

10 mmol 2 - [(4-trans-4-pentylcyclohexyl) phenyl] - [2,6- difluoropyridyl] ethanol are dissolved in toluene, with Potassium tert-pentylate added and heated. Subsequently the reaction mixture is treated with methyl iodide. Man lets cool down and works up as usual.

The following compounds of the formula become analogous

produced.

Claims (10)

1. 2,3-dihydro-furo (2,3-b) pyridines of the formula I. in which
R _{1 is} an unsubstituted or an alkyl or alkenyl radical which is substituted by CN, halogen or CF ₃ and has up to 15 C atoms, in which radicals one or more CH ₂ groups are each independently of one another by -S-, - O-, -CO-, -CO-O-, -O-CO- or -O-CO-O- can be replaced so that S and / or O atoms are not directly linked,
R² H or alkyl with 1-15 C atoms,
A ¹ and A ² each independently

• a) trans-1,4-cyclohexylene radical, in which one or more non-adjacent CH ₂ groups can also be replaced by -O- and / or -S-,
• b) 1,4-phenylene radical, in which one or two CH groups can also be replaced by N,
• c) residue from the group 1,3-cyclobutylene, 1,3-bicyclo [1.1.1] pentylene, 1,4-cyclohexenylene, 1,4-bicyclo [2.2.2] octylene, piperidine-1,4-diyl , Naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
• where the residues (a) and (b) can be substituted by CN or halogen,

Z ¹ and Z ² each independently of one another -CH ₂ -CH ₂ -, -C≡C-, -CH ₂ O-, -OCH ₂ -, -CO-O-, O-CO-, -CH = N-, CH ₂ S-, SCH ₂ , a single bond or an alkylene group with 3 to 6 carbon atoms, in which also a CH ₂ group by O-, CO-O-, -O- CO-, CH-halogen or -CHCN - can be replaced,
LH, F or Cl
m 0, 1 or 2,
mean. 2. 2,3-dihydro-furo (2,3-b) pyridines according to claim 1, wherein
LF, and R ² H means. 3. Use of the compounds of formula I as components liquid crystalline media for electro-optical display elements. 4. Liquid-crystalline medium with at least two components ten, characterized in that at least one compo nente is a compound of formula I. 5. Ferroelectric liquid-crystalline medium with at least one chiral component and at least one achiral component, characterized in that at least one component contains a liquid-crystalline compound which is a group of the formula where LF or Cl denotes as a structural element. 6. Medium according to claim 5, characterized in that it at least one compound of formula I according to claim 1 contains. 7. Electro-optical display, characterized in that it as a dielectric after a liquid-crystalline medium Claim 4 contains. 8. Matrix liquid crystal display according to claim 6, characterized characterized in that it is a liquid as a dielectric contains crystalline medium according to claim 4. 9. liquid crystal switching and display device containing a ferroelectric liquid-crystalline medium, carrier plates, electrodes, at least one orientation layer and optionally additional auxiliary layers, characterized in that the medium contains at least one liquid-crystalline compound which is a group of the formula where LF or Cl denotes as a structural element.