Technical Field:
The present disclosure relates to a process for the preparation of a nematic compound 4-(4-propyl-cyclohexyl)-phenol from l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-benzene. The process of the present disclosure can be easily used for the industrial preparation of nematic compounds.
Background:
Nematic compounds of low viscosity & high clearing point are required in order to achieve TN-devices having fast response time. Chemically these compounds have linking of phenyl & cyclohexyl ring with or without additional carbon atoms in between them. One of these important compounds is n-alkyl cyclohexylphenol (Ref Takatsu, H.; Takeuchi, K.; Sato, H.; Molecular Crystals and Liquid Crystals (1969-1991); vol. Ill; (1984); p. 311 -320)
Very few methods are available for their preparation and these generally involve formation of C-C bonds. The general methods for C-C bonds formation are Negishi coupling, Kumada, Suzuki coupling etc. These invariably require transition metal complexes like 1,1'-bis(diphenylphosphino)ferrocene)palladium(II) as disclosed in Bumagin, Nikolai A.; Luzikova, Elena v.; Journal of Organometallic Chemistry; vol. 532; nb. 1-2; (1997); p. 271 -274. These complexes are very expensive and difficult to recover and recycle. Hence such processes are not suitable for industrial production.
US 4,198,130 discloses a liquid crystal mixtures having a positive anisotropy of llie dielectric constants comprising a nematogenic material having a clearing point above 60°C. It discloses a process where Cis or trans -l-methyl-4-(l-hydroxy-4-n-butylcyclohexyl)-benzene in rectified alcohol are hydrogenated in presence of Palladium/carbon catalyst until 1 mol uptake of hydrogen. The catalyst is removed by filtration. The cis or trans -l-methyl-4-(l-hydroxy-4-n-butylcyclohexyl)-benzene are used as starting material which is formed by grignard reaction of p-methyl-phenylmagnesium.
Summary;
An aspect of the present disclosure provides a process for the preparation of a nematic compound 4-(4-propyl-cyclohexyl)-phenol, comprising:
(a) reducing l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-benzene with hydrogen gas in
2
I p. I; UT ■? 7 - «■> - -7 Pii 1' K. 1 C- ! A 1

the presence of Pd/C catalyst and an alcohol at a temperature in the range of 80-90°C for a time period in the range of 10-12 hours to obtain a reaction mixture; and (b) isolating pure 4-(4-propyl-cyclohexyl)-phenol fi-om the reaction mixture. Another aspect of the present invention provides a process for producing l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-benzene by grignard reaction and reaction of resulting grignard solution with a solution of 4-n propylcyclohexanone.
An aspect of the present disclosure provides a process for preparation of a nematic compound 4-(4-propyl-cyclohexyl)-phenol, which overcomes the drawbacks of the processes of prior art.
The process of the present disclosure is suitable for production on industrial scale and the yield of the product is much higher than the yield of prior art processes. Further, in the process of the present disclosure the catalyst can be recycled/ recovered and no additional process is required for the de-protection of the protecting group.
These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the subject matter, nor is it intended to be used to limit the scope of the subject matter.
Detailed Description;
The present disclosure provides a process for the preparation of a nematic compound 4-(4-propyl-cyclohexyl)-phenol, comprising:
(a) reducing l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-benzene with hydrogen gas in the presence of Pd/C catalyst and an alcohol at a temperature in the range of 80-90°C for a time period in the range of 10-12 hours to obtain a reaction mixture; and
(b) isolating pure 4-(4-propyl-cyclohexyl)-phenol fi-om the reaction mixture.
In an embodiment of the present disclosure the isolating step of the process comprises:
(a) filtering the reaction mixture to obtain a filtrate; and
(b) concentrating the filtrate to obtain a residue, recrystallizing the residue in pentane to obtain pure 4-(4-propyl-cyclohexyl)-phenol.
3

In another embodiment of the present disclosure the alcohol used in the process is selected from
methanol, ethanol, n-propanol, iso-propanol, n-butanol and iso-butanol or mixture thereof.
The purity of 4-(4-propyl-cyclohexyl)-phenol is greater than about 95%, preferably greater than about
98%, more preferably greater than about 99.0%.
The catalyst used in the process of the present disclosure, i.e. palladium catalyst can be recycled or
recovered.
The process of the present disclosure is a convenient process which can be used for commercial
production of the nematic compounds. In the process of the present disclosure no additional process for
the de-protection of protection group is required.
In another embodiment of the present disclosure l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-
benzene is prepared by a process comprising:
(a) reacting l-bromo-4-methoxymethoxy-benzene with magnesium turnings in the presence of an organic solvent and refluxing to obtain a Grignard solution;
(b) adding 4-propyl-cyclohexanone to the Grignard solution and refluxing for a time period in the range of 1-3 hours to obtain a reaction mixture;
(c) cooling the resulting mixture and adding HCl to form an aqueous layer;
(d) extracting the aqueous layer with ethyl acetate to obtain a combined organic layer; and
(e) drying the combined organic layer and concentrating to obtain 1-methoxymethoxy-4-(4-propyl-cyclohex-1-enyl)-benzene.
In yet another embodiment of the present disclosure the organic solvent used in the process is selected from THF, dioxane and diethyl ether or mixture thereof.
The purity of l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-benzene obtained by the process of the present disclosure has a purity of at least 90%, preferably at least 95%.
Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the subject matter should not be limited to the description of the preferred embodiment contained therein.
Examples
The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of present disclosure. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are
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intended to provide further explanation of the subject matter.
Example 1:
30.9g of l-bromo-4-methoxymethoxy -benzene in 90 ml of THF was added drop wise with stirring to 3.5 g of magnesium turnings in 90 ml of THF. The resulting Grignard Solution is reflux ed. And, a solution of 20 g of 4-propylcyclohexanone in 90 ml of THF is added drop wise. After addition it was refluxed another 2 hours.


(1) Mg,dryTHF 0
0 O
/ l-Bromo-4-methoxymethoxy-benzene 4-Propyl-cyclohexanone /
„,, l-Methoxymethoxy-4-(4-propyl - "7 Pt-1 ^ i ^

stirring to 0.164 g of magnesium turnings in 15 ml of THF. The resulting grignard solution is refluxed. In another flask, a solution of 1.26 g of 4-bromo-4-Phenol in 10 ml of THF with 0.079 g of PdChCdppf) was refluxed and above prepared Grignard solution was added. After addition it was refluxed another 2 hours.
The reaction mixture was cooled and 20 ml of 5% HCl was added. The Aq. Layer was Extracted with ethyl acetate, combine organic layer was dried over sodium sulphate, concentrated on rota vapour to get crude product


Br
Br
HO-
HO-
\ /

\ /

PdCl;(dppf),THF
Mi '

MgBr

4-Bromophenol
Mol. Wl.: 173

4-(4-Propyl-cyclohexyl)-phenol Mol. Wt: 218

Weight = 1.45 g
Purity by GC: 40.7% of Trans
33.8% of Cis isomer
The yield of the products is quite less as well as the purity of the products is also very less as compare to the purity of the product obtained in example 1.

We Claim;
1. A process for the preparation of a nematic compound 4-(4-propyl-cyclohexyl)-phenol,
comprising:
(a) reducing l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-benzene with hydrogen gas in the presence of Pd/C catalyst and an alcohol at a temperature in the range of 80-90°C for a time period in the range of 10-12 hours to obtain a reaction mixture and
(b) isolating pure 4-(4-propyl-cyclohexyl)-phenol from the reaction mixture.

2. The process as claimed in claim 1, wherein the alcohol is selected from methanol, ethanol, n-propanol, iso-propanol, n-butanol and iso-butanol or mixture thereof
3. The process as claimed in claim 1, wherein the isolating step comprises:

(a) filtering the reaction mixture to obtain a filtrate; and
(b) concentrating the filtrate to obtain a residue, recrystallizing the residue in pentane to obtain pure 4-(4-propyl-cyclohexyl)-phenol.
4. The process as claimed in claim 1, wherein l-methoxymethoxy-4-(4-propyl-cyclohex-l-enyl)-
benzene is prepared by a process comprising:
(a) reacting l-bromo-4-methoxymethoxy-ben2ene with magnesium turnings in the presence of an organic solvent and refluxing to obtain a Grignard solution;
(b) adding 4-propyl-cyclohexanone to the Grignard solution and refluxing for a time period in the range of 1-3 hours to obtain a reaction mixture;
(c) cooling the resulting mixture and adding HCl to form an aqueous layer;
(d) extracting the aqueous layer with ethyl acetate to obtain a combined organic layer; and
(e) drying the combined organic layer and concentrating to obtain l-methoxymethoxy-4-(4-propyl-cyclohex-1 -enyl)-benzene.
5. The process as claimed in claim 4, wherein the organic solvent is selected from THF, dioxane
and diethyl ether or mixture thereof.