GB2118934A - Production of liquid crystal cyanophenylethanes - Google Patents

Abstract

A method of preparing a compound <IMAGE> is characterised in that a compound <IMAGE> is treated so as to introduce directly or indirectly a cyano group into the para- position of its unterminated benzene ring, wherein <IMAGE> where R1 is an alkyl group and R2 is an alkyl, alkoxy, alkylcarbonyloxy or alkoxycarbonyloxy group. The compounds of Formula 1, particularly where <IMAGE> are useful as liquid crystal materials.

Description

SPECIFICATION Methods of producing liquid crystal compounds exhibiting a positive dielectric anisotropy The present invention relates to liquid crystal compounds exhibiting a positive dielectric anisotropy and to methods of their production.

The use of liquid crystal materials to exhibit electrnoptic effects in display devices such as digital calculators or watches is now well-known. One of the parameters of a liquid crystal material which is important in relation to electro-optical operation is the dielectric anisotropy (AE) of the material. This is the difference, for a given frequency and temperature, between the average dielectric constant measured parallel (E,,) and perpendicular (E1) to the molecules.

The sign of the dielectric anisotropy of a given liquid crystal material is one of the major parameters which determine the kinds of electro-optic device in which that material may be used.

For example, materials having a positive dielectric anisotropy may, depending on their chirality or twist in their molecular arrangement, be used in known twisted nematic effect devices, Fréedericksz effect devices or cholesteric-to-nematic phase change effect devices. Such materials normally consist of a mixture of compounds at least one of which is a liquid crystal compound of high positive dielectric anisotropy.

Compounds of formula (I) as follows: where

Formula (I) is a trans-l ,4-disubstituted cyclohexane: where R is an alkyl group, as disclosed in published United Kingdom Patent Application No GB 2,023,1 36A, are known compounds of high positive dielectric anisotropy. These compounds exhibit liquid crystal phases (mesophases) at temperatures at or near to room temperature and have relatively low viscosities which make them highly attractive for use in liquid crystal materials for the effects mentioned above.

The method of preparing the compounds of Formula (I) disclosed in the above specified UK Patent Application (Example 5) is a form of the general method which has previously been used for the preparation of other classes of cyano-terminated ethyl bridged compounds, viz the other classes specified in that Patent Application and those specified in UK Patent Specification No. 1,551,043.

Basically, the general route is Route 1 as follows: Route 1

Step A1 A-CH2.COOH SfepA A - CH2. CO - B - Br Step Ci IStep Si A-CH2.CH2-B-CN A-CH2.CH2 -B-Br where A is a substituted cycloalkyl or aryl ring system, and B is an aryl ring system.

The specific route disclosed in the case of compounds of Formula (I) is Route 2 (which is an Example of Route 1) as follows: Route 2

CH2.COOH StepA R {i}tH2.CO{i}Er Step B2 R{+CH2-CH2{CN Step C2 R 2.tH2XBr Unfortunately, when compounds of Formula (I) are prepared by Route 2 the yields obtained are very low. For example, in the case where R=n-C5H11 and the procedure of Example 5 (based on Example 3) of UK Patent Application No. GB 2,023,1 36A is followed using dichloromethane as the solvent for the bromobenzene in the step corresponding to Step A2 above the yield obtained is generally small, e.g. less than 10%.The yield may be increased, e.g. to about 15%, by using nitrobenzene as the solvent for bromobenzene in place of dichloromethane and by carrying out the reaction in Step A2 above at a higher temperature (room temperature) for a longer period of time (e.g.

2 days). However, yields of about 15% are still considered too low.

According to the invention in a first aspect there is provided a method of preparing a compound of Formula 1 as follows:

Formula 1 which is characterised in that a compound of Formula 2 as follows:

Formula 2 is treated so as to introduce directly or indirectly a cyano group into the para-position of its unterminated benzene ring, wherein

where R1 is an alkyl group and R2 is an alkyl, alkoxy, alkylcarbonyloxy or alkoxycarbonyloxy group,

is a 1,4-disubstituted bicyclo(2,2,2)octane ring,

is a trans-1 4-disubstituted cyclohexane ring,

is a trans-2,5-disubstituted-1 ,3-dioxan ring and

is a 2,5-disubstituted pyrimidine ring.

For compounds where

the intermediates of Formula 2 i.e. compounds of Formula (II) as follows:

Formula II are the subject of copending UK Patent Application No. 8222513.

The compound of Formula (2) may be converted directly into the cyanide compound of Formula (I) e.g. using cyanogen bromide. However, as this involves the use of extremely toxic material indirect methods of forming the cyanide are preferred.

Preferably, the compound of Formula 2 is converted into the cyanide of Formula 1 by an indirect route in which an acid halide of Formula (III) as follows:

Formula (III) where X is a halogen, preferably chlorine, is first prepared from the compound of Formula (2). The acid halide may then be converted to the cyanide of Formula (1) either directly, e.g. using phosphonitrilic chloride, or preferably, indirectly by first producing an amide of Formula (IV) as follows:

Formula (IV) the amide then being dehydrated to give the cyanide.

Indirect conversion of the acid halide of Formula (III) into the compound of Formula 1 via the amide of Formula (IV) is preferred to direct conversion since: (i) the reagent phosphonitrilic acid used in the direct reaction has first to be prepared or specially purchased for use in that reaction; (ii) the conversion of the acid halide of Formula (III) to the amide of Formula (IV) is a very simple step in which the additional chemicals required to treat the compound of Formula (III) are readily available laboratory materials; (iii) the acid halide of Formula (III) is not normally purified in its method of production, so preparation of the intermediate amide of Formula (IV) allows that product to be purified prior to preparation of the cyanide of Formula 1.

Thus, the compounds of Formula (1) are preferably prepared by the novel method of Route 3 as follows: Route 3

RCH2.CH2 Step A3 R#-CH2.Ch2EO.Ct Step B3 RM(H2.CH7mN StepC3 R{ss}{H2.CH4}{QNH2 It has been found that by producing compounds of Formula 1, in particular those of Formula (I) by the method according to the first aspect of the invention preferably by Route 3, that much higher overall yields of the pure product of Formula (I), e.g. typically 40%, are possible than by using the prior art Route 2 specified above.

Furthermore, as the compounds of Formula (II) are themselves useful as constituents of liquid crystal materials (as described in the copending UK Patent Application No. 8124045) the cost of the starting materials may be reduced by commercial demand for those materials.

According to the present invention in a second aspect there are provided novel compounds useful as intermediates in the production of liquid crystal compounds which novel compounds are the compounds of Formula (Ill) above.

According to the present invention in a third aspect there are provided novel compounds useful as intermediates in the production of liquid crystal compounds which novel compounds are the compounds of Formula (IV) above.

An example of the preparation and properties of compounds having Formula (I) will now be given.

The symbols used in the example have the following meanings: N=nematic liquid crystal.

I=isotropic liquid.

N-l=nematic to isotropic liquid transition temperature (clearing point).

square brackets [ ]around a temperature value indicate a virtual transition.

mp=melting point.

bp=boiling point.

=viscosity.

The preparative route used in this example is as follows:

R CO2H Step 1 R CO.CL Step2 -CH20H &verbar;Step3 R4}CH2.Co2H StepS R- CH2CN Step4 R -CH28r RCH2Br Step Step 6 I2.CO.CL P Step 7~ R-aa 9 R HCH1CH2O R CH2.CH2- CO.NH2 S~tp 10 R- StL, R {+CH2 CH2{i}CO NH StAp 10 Stet 11 -CH2 CH2{i} \tep 11 CH2.CH2 CN R as defined above.

Steps 1 to 5 These steps are known and may be carried out by methods described in the literature.

Steps 6 to 8 These steps are as described in copending UK Patent Application No. 8124045. They may be carried out as follows.

Steps 6 and 7 The production of trans-4-alkylcyclohexylmethyl phenyl ketones from trans-4alkylcyclohexylacetic acids This production may be carried out essentially by the same method as used for the Friedel-Crafts acylation of benzene using the appropriate trans-4-alkylcyclohexylacetic acid, which is first converted to the acid chloride in Step 6, as described in the literature. See for example, Al Vogel, Textbook of Practical Organic Chemistry, 4th Edition, 770 (1978).

An example of such a product is trans-4-n-propylcyclohexylmethyl phenyl ketone, mp 460 C.

Step 8 The production of 1-(trans-4'-alkylcyclohexyl)-2-phenylethanes from trans-4alkylcyclohexylmethyl phenyl ketones This step may be carried out essentially by one of the methods described in the literature for the reduction of a ketone to a hydrocarbon-see for example the method described by D. Coates and G. W.

Gray, J. Chem. Soc., Perkin 11, 7, 867 (1976) which uses a 4-(4"-n-alkoxyphenylacetyl)-4'- bromobiphenyl.

Examples of such products are:

bp=113 C at 0.5 mm Hg N-l=[-550C] (by extrapolation) 17200C=8.5 cSt (isotropic liquid state) 77200C=2.2 cSt (nematic state-extrapolated) mp=30C

bp=12O0C at 0.1 mm Hg N-l=[-300Cj (by extrapolation) 112O0C=13.4 cSt (isotropic liquid state) x120 C=7.7 cSt (nematic state-extrapolated) mp=--1.SOC

bp=1 5O0C at 4 mm Hg N-l=[-250C] (by extrapolation) 1200C=18.6 cSt (isotropic liquid state) r1200C=9.9 cSt (nematic state-extrapolated) mp=70C Step 9 The preparation of 4-(p-trans-4'-alkylcyclohexylethyl)benzoyl chlorides Oxalyl chloride (0.027 mole) is added over 5 min to a stirred suspension of aluminium (III) chloride (0.0135 mole) in dry dichloromethane (27 cm3) at 50C.Asolution of the 1-(trans-4'alkylcyclohexyl)-2-phenylethane (0.0135 mole) in dry dichloromethane (40 cm3) is added dropwise to the stirred mixture over 1 hour at 5"C.

The mixture is reduced to half its original volume in vacuo to remove any unused oxalyl chloride/phosgene. Then fresh, dry dichloromethane (40 cm3) is added and the mixture is cooled to OOC. The solution is poured onto an efficiently stirred mixture of calcium chloride (1.9 g) and crushed ice (34 g) ensuring that the temperature is kept below 50C. The organic layer is quickly separated and dried over Na2SO4. The drying agent is removed by filtration and the solvent removed in vacuo. The residual oil is taken up in sodium-dried ether (50 cm3), cooled to OOC, and shaken with an aqueous solution of 5% potassium hydroxide (50 cm3), precooled to OOC, and water (50 cm3) precooled to OOC.

The organic products are separated, dried (Na2SO4), filtered, and the solvent removed in vacuo.

The product was obtained as a yellow oil and is used in the next step (amide formation) without further purification.

Step 10 Preparation of 4-l,B-trans-4'-alkylcyclohexylethyl)benzamides A solution of 4-(ss-trans-4'-alkylcyclohexylethyl)benzoyl chloride (0.1 mole) in ethylene glycol dimethyl ether (30 cm3) is added dropwise to a vigorously stirred solution of aqueous ammonia (d=0.88; 125 cm3) at OOC.

The solution is shaken with ether (4x70 cm3) and the combined ethereal extracts are washed with water (3x50 cm3), dried (Na2SO4), filtered and the solvent removed in vacuo. The pure amide is obtained upon crystallisation from ethanol as a white powder.

Examples of such products are:

n-C3H7 Q CH2.CH2 NH2 yield 720/oj mp mp 215QC.

n-C5Hll{0}CH2 CO.NH2 yield 70%/ mp 210.50C.

n-C7H15- CH2.CH2 CO.NH2 yield 61%/ mp 1980C.

Step 11 Preparation of I -(trnns-4'-alkylcyclohexyl)-2-(4"-cyanophenyl)ethanes Thionyl chloride (0.10 mole) is added dropwise to a stirred solution of 4-(P-trans-4'- alkylkcyclohexylethyl)benzamide (0.025 mole) in dimethylformamide (120 cm3) at 200 C. After 1 5 hours sufficient dilute hydrochloric acid is added to destroy the excess of thionyl chloride and the mixture is poured onto ice-water (80 g). The organic material is extracted into ether (3x60 cm3) and the combined ethereal extracts are washed with water (3 x40 cm3), dried (Na2SO4), filtered, and the solvent removed in vacuo. The pure product is obtained by column chromatographing the crude material (using silica gel as the stationary phase and chloroform as the eluent) followed by crystallisation from light petroleum (bp 4060 C).

Examples of such products are:

n-C3H7- yeW 66%1 C-N: 360C? N-I = 44.50C. n-C5H11HCH1CH2OCN yield 66%; C-N=28 C; N-I = 49 C. n-C7H15H CH2.CH2O (N yield 71 %; C-N=65 C; N-I 54.50.

Examples of the uses of these compounds as liquid crystal materials are described in published UK Patent Application GB 2,023,136A.

Claims (13)

Claims

1. A method of preparing a liquid crystal compound of Formula (I) as follows:

Formula 1 where

is as defined hereinbefore, is characterised in that a compound of Formula 2 as follows:

Formula 2 is treated so as to introduce directly or indirectly a cyano group into the para-position of its benzene ring.

2. A method as claimed in claim 1 and wherein the compound of Formula 2 is converted into the cyanide compound of Formula 1 indirectly by first producing an acid halide of Formula (III) as follows:

Formula (Ill) where X is a halogen by direct addition of the group CO . X to the compound of Formula 2.

3. A method as claimed in claim 2 and wherein X is chlorine.

4. A method as claimed in claim 2 or claim 3 and wherein the acid halide of Formula (III) is converted indirectly into the cyanide of Formula 1 by first converting the acid halide into an amide of Formula (IV) as follows:

Formula (IV) and then dehydrating the amide of Formula (IV).

5. A method as claimed in claim 2 or claim 3 and wherein the acid halide of Formula (III) is converted directly into the cyanide of Formula 1.

6. A method as claimed in claim 5 and wherein the conversion is carried out using phosphonitrilic acid.

7. A method as claimed in claim 1 and wherein the compound of Formula 2 is converted directly into the cyanide compound of Formula 1.

8. A method as claimed in claim 7 and wherein the conversion is carried out using cyanogen bromide.

9. A method as claimed in any one of the preceding claims and wherein

where R, is an n-alkyl group.

10. A method as claimed in claim 4 and wherein the method is substantially as described in Steps 9 to 11 of the specific example given hereinbefore.

11. A method as claimed in any one of the preceding claims and wherein R is n-C3H7.

12. A method as claimed in any one of claims 1 to 10 and wherein R is NCsH,1.

13. A method as claimed in any one of claims 1 to 10 and wherein R is n-C7H15.