EP0756588A1

EP0756588A1 - Method for carboxylating a phenol ether

Info

Publication number: EP0756588A1
Application number: EP96904138A
Authority: EP
Inventors: Pascal Metivier; Emmanuel Dos Santos
Original assignee: Rhone Poulenc Chimie SA
Current assignee: Rhodia Chimie SAS
Priority date: 1995-02-20
Filing date: 1996-02-14
Publication date: 1997-02-05
Also published as: FR2730731A1; FR2730731B1; AU4834696A; JPH09512285A; CN1146761A; WO1996026176A1

Abstract

A method for carboxylating a phenol ether, particularly guaiacol and guetol, wherein said phenol ether in salt form is reacted with carbon dioxide at a temperature below 150 °C in an aprotic, polar and basic organic solvent.

Description

CARBOXYLATION PROCESS OF A PHENOL FTHFR

The present invention relates to a process for the carboxylation of a phenol ether.

In a preferred variant, the invention relates to the carboxylation of guaiacol (or 2-methoxyphenol) and of guetol (or 2-ethoxyphenol).

The processes of carboxylation of phenols or ethers of phenols having an industrial vocation, use carbon dioxide.

The carboxylation is carried out according to the Kolbe-Schmitt or Marasse reactions.

In the first case, the phenol or phenol ether is reacted in the form of a salt, generally sodium or potassium, with carbon dioxide, under pressure. An illustration of this type of reaction is given by

EP-A 327 221 which describes the carboxylation of sodium guaiacolate at 250 ° C under CO2 pressure. Such a process is suitable when it is desired to obtain 2-hydroxy-3-methoxybenzoic acid, that is to say to carry out a carboxylation in the ortho position of the hydroxyl group. It has also been proposed to conduct the reaction according to the conditions of

Marasse. Thus, O. BAINE et al [J. Org. Chem. there, pp. 510 (1954)] described obtaining 2-hydroxy-3-methoxybenzoic acid, by carboxylation at 200 ° C, of guaiacol by CO2, in the presence of an excess of potassium carbonate.

However, the yield obtained is not satisfactory since it is only 47%. The objective of the present invention is to provide a para carboxylation process of a phenol ether which makes it possible to overcome the aforementioned drawbacks.

It has now been found and this is what constitutes the object of the present invention, a process for the carboxylation of a phenol ether which consists in reacting said phenol ether in salified form with carbon dioxide, said process being characterized in that the reaction is carried out at a temperature below 150 ° C., in an aprotic, polar and basic organic solvent.

The term “phenol ether” denotes an aromatic compound, the aromatic nucleus of which carries a hydroxyl group, a hydrogen atom in para position of the OH group and of which a hydrogen atom directly linked to the aromatic nucleus is replaced by an ether group.

In the description which follows of the present invention, the term "aromatic" means the classic notion of aromaticity as defined in the literature, in particular by Jerry MARCH, Advanced Organic Chemistry, 4 ^istrymθ edition, John Wiley an Sons, 1992, pp 40 and following.

More specifically, the subject of the present invention is a process for the carboxylation of a phenol ether of general formula (I):

in which :

- A symbolizes the remainder of a cycle forming all or part of an aromatic, monocyclic or polycyclic carbocyclic system, system comprising at least one OR 'group: said cyclic residue being able to carry u or more substituents,

- R represents a hydrogen atom or one or more identical or different substituents,

- R 'represents a hydrocarbon radical having from 1 to 24 carbon atoms, which may be a linear or branched saturated or unsaturated acyclic aliphatic radical; a saturated, unsaturated or aromatic, monocyclic or polycyclic cycloaliphatic radical; a saturated or unsaturated, linear or branched aliphatic radical carrying a cyclic substituent,

- n is a number less than or equal to 3.

In the present text, the term “ether group” denotes, in a simplified manner, groups of the R'-O- type in which R 'has the meaning given above. R 'therefore represents both an acyclic or cycloaliphatic, saturated, unsaturated or aromatic aliphatic radical as well as a saturated or unsaturated aliphatic radical carrying a cyclic substituent.

The phenol ether which is involved in the process of the invention corresponds to the formula (I) in which R "represents an acyclic, saturated or unsaturated, linear or branched aliphatic radical.

More preferably, R 'represents a linear or branched alkyl radical having from 1 to 12 carbon atoms, preferably from 1 to 6 carbon atoms, the hydrocarbon chain possibly being interrupted by u heteroatom (for example, oxygen), by a functional group (for example -CO-) and / or carrying a substituent (for example, a halogen).

The acyclic, saturated or unsaturated, linear or branched aliphatic radical may optionally carry a cyclic substituent. The term “ring” preferably means a saturated, unsaturated or aromatic carbocyclic ring, preferably cycloaliphatic or aromatic, in particular cycloaliphatic comprising 6 carbon atoms in the ring or benzene.

The acyclic aliphatic radical can be linked to the ring by a valential bond, a heteroatom or a functional group and examples are given above.

The ring can be optionally substituted and, by way of examples of cyclic substituents, it is possible, among others, to consider substituents such as R, the meaning of which is specified for formula (la).

R 'can also represent a saturated carbocyclic radical or comprising 1 or 2 unsaturations in the ring, generally having from 3 to 8 carbon atoms, preferably 6 carbon atoms in the ring; said cycle being able to be substituted with substituents such as R.

R 'can also represent an aromatic carbocyclic radical, preferably a monocyclic radical generally having at least 4 carbon atoms, preferably 6 carbon atoms in the ring; said cycle being able to be substituted with substituents such as R.

The process of the invention applies very particularly to the ethers of phenols of formula (I) in which R ′ represents a linear or branched alkyl radical having from 1 to 4 carbon atoms or a phenyl radical. As examples of radicals R 'preferred according to the invention, mention may be made of methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl or phenyl radicals.

In the general formula (I) of phenol ethers, the residue A may represent the remainder of an aromatic, monocyclic carbocyclic compound having at least 4 carbon atoms and preferably 6 carbon atoms or the remainder of a polycyclic carbocyclic compound which can be constituted by at least 2 aromatic carbocycles and forming between them ortho- or ortho- and pericondensed systems or by at least 2 carbocycles of which at least one of them is aromatic and forming between them ortho- or ortho- and pericondensed.

The remainder A can carry one or more substituents on the aromatic ring. Examples of substituents R are given below, but this list is not limiting. Any substituent can be present on the cycle as long as it does not interfere with the desired product.

The remainder A can, inter alia, carry several alkoxy groups, it is possible according to the process of the invention to carboxylate polyalkoxylated compounds.

The process of the invention applies more particularly to the phenol ethers of formula (la):

in which :

- n is a number less than or equal to 3, preferably equal to 0 or 1,

the radical R ′ represents an alkyl radical, linear or branched, having from 1 6 carbon atoms, preferably from 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-buty or phenyl,

- the radical (s) R represent one of the following atoms or groups:

. a hydrogen atom,

. an alkyl radical, linear or branched, having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms, such as methyl ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl,

. a linear or branched alkoxy radical having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms such as l methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, se butoxy, tert-butoxy, radicals. a halogen atom, preferably a fluorine atom, bromine chlorine, a trifluoromethyl radical.

- the radicals R 'and R and the 2 successive atoms of the benzéniq ring can form between them, a ring having from 5 to 7 atoms, possibly included another heteroatom. When n is greater than or equal to 1, the radicals R ′ and R and the 2 successive atoms of the benzene ring can be linked together by a radi alkylene, alkenylene or alkenylidene having from 2 to 4 carbon atoms in order to form a saturated heterocycle, unsaturated or aromatic having 5 to 7 carbon atoms. One or more carbon atoms can be replaced by another heteroatom, preferably oxygen. Thus, the radicals R 'and R can represent a methylene dioxy or ethylene dioxy radical.

The process of the invention is particularly applicable to ethers of phenols of formula (la) in which n is equal to 1, the radicals R and R ′ both representing, identical or different, alkoxy radicals. It is more preferably suitable for the phenol ethers of formula (la) in which n is equal to 0, the radical R ′ representing an alkoxy radical.

By way of illustration of compounds corresponding to formula (I), there may be mentioned more particularly: - monoethers such as guaiacol, 3-methoxyphenol, guetol, 3-ethoxyphenol, 2-isopropoxyphenol, 3-isopropoxyphenol, 2-methoxy-5-methylphenol, 2-methoxy-6-methylphenol, 2-methoxy-6-tert-butylphenol, 3-chloro-5-methoxyphenol, 2,3-dimethoxy-5-methylphenol, 2-ethoxy-5- (1-propenyl) phenol, 2-methoxy-1-naphthol, - diethers such as 2,3-dimethoxyphenol, 2,6-dimethoxyphenol,

3,5-dimethoxyphenol.

The compounds to which the process according to the invention applies more particularly advantageously are guaiacol and guetol.

The phenol ethers are involved in the process of the invention in salified form. They are preferably the salts of the metallic elements of group (la) of the periodic table or an ammonium salt.

For the definition of the elements, reference is made below to the Periodic Table of the Elements published in the Bulletin of the Chemical Society of France, n ° 1 (1966). From a practical and economical point of view, sodium or potassium salts are used.

In accordance with the process of the invention, a polar aprotic organic solvent is used which exhibits certain characteristics of polarity and basicity; the presence of said solvent making it possible to improve the regio-selectivity of the reaction.

Several requirements govern the choice of organic solvent. A first characteristic of the organic solvent is that it is aprotic and stable in the reaction medium.

By aprotic solvent is meant a solvent which, in Lewis theory has no protons to release.

Excluded from the present invention are solvents which are not stable in the reaction medium or which are liable to react under the reaction conditions. According to the invention, a polar organic solvent is used. An organic solvent is chosen, in accordance with the invention, which has a dielectric constant greater than or equal to 15. The upper bound has no critical character. It is preferred to use an organic solvent having a high dielectric constant, preferably between 25 and 7. In order to determine whether the organic solvent meets the dielectric constant conditions set out above, reference may be made, among others, to tables of the book: Techniques of Chemistry, H - Organic solvents - p. 536 following, ^3rd edition (1970). Another condition which governs the choice of solvent is that it must meet certain basicity characteristics. Indeed, said solvent must be basic. To determine whether a solvent satisfies this requirement, its basic is assessed by referring to the "donor number". A pola organic solvent is chosen having a donor number greater than 20, preferably greater than or equal to 25. The upper limit has no critical character. An organic solvent is preferably chosen having a donor number of between 25 75 and preferably between 25 and 50.

With regard to the requirements concerning the basicity of the organic solvent to be used, it will be recalled that the "donor number" or "donor numb abbreviated as DN, gives an indication of the nucleophilic character of the solvent and reveals its ability to give its doublet .

In Christian REINHARDT's book, [Solvents and Solvent Effects

Organic Chemistry - VCH p.19 (1988)], we find the definition of "donor numb which is defined as the negative (-ΔH) of the enthalpy (Kcal / mol) of the interaction between the solvent and the pentachloride d 'antimony, in a dilute dichloroethane solution.

As examples of polar, aprotic organic solvents which meet the aforementioned basicity characteristics, which can be used in the process of the invention, mention may be made more particularly of linear or cyclic carboxamids such as N, N-dimethylacetamide (DMAC). , N, diethylacetamide, dimethylformamide (DMF), diethylformamide or methyl-2-pyrrolidinone (NMP); dimethyl sulfoxide (DMSO) hexamethylphosphotriamide (HMPT); tetramethylurea. It is also possible to use a mixture of solvents. Among the aforementioned solvents, linear or cyclic carboxamides are preferably used.

The amount of organic solvent to be used is determined depending on the nature of the organic solvent chosen. It is determined so that the concentration of the substrate in the organic solvent is preferably between 1 and 50% by weight and more preferably between 10 and 40%.

In accordance with the process of the invention, the phenol ether in the salified form and the carbon dioxide are reacted in an organic solvent as defined.

A salified form of an extemporaneously prepared phenol ether can be used, but it can also be prepared in situ by reacting the phenol ether and the base. A base which can be mineral or organic therefore intervenes in the process of the invention.

A strong base is preferably chosen, that is to say a base having a pKb greater than 12: the pKb being defined as the cologarithm of the dissociation constant of the base measured, in aqueous medium, at 25 ° C. Particularly suitable for carrying out the process of the invention, inorganic bases such as the alkali metal salts, preferably an alkali metal hydroxide which may be sodium or potassium hydroxide.

It is also possible to use a quaternary ammonium hydroxide.

As examples of quaternary ammonium hydroxide, use is preferably made of tetralkylammonium or trialkylbenzylammonium hydroxides in which the identical or different alkyl radicals represent a linear or branched alkyl chain having from 1 to 12 carbon atoms, preferably from 1 to 6 carbon atoms.

Tetramethylammonium hydroxide, tetraethylammonium hydroxide or tetrabutylammonium hydroxide is preferably chosen.

It is also possible according to the invention to use trialkylbenzylammonium hydroxides and in particular trimethylbenzylammonium hydroxide.

For economic considerations, one chooses from all the bases, preferably sodium or potassium hydroxide.

The concentration of the basic starting solution is not critical. The alkali metal hydroxide solution used has a concentration generally between 10 and 50% by weight.

The amount of base introduced into the reaction medium takes account of the amount necessary to salify the hydroxyl function of the phenol ether. If the said compound has salifiable functions other than hydroxyl grou, the quantity of base necessary to salify all the salifiable functions is therefore introduced.

Generally, the amount of base expressed relative to the ether of phe varies between 90 and 120% of the stoichiometric amount.

The phenol ether is prepared in salified form by reacting it with base at a temperature advantageously between 25 ° C and 100 ° C.

Before introducing the carbon dioxide, the water formed at the neck of the salification reaction is removed by distillation under atmospheric pressure under a reduced pressure of between 1 mm of mercury and atmospheric pressure or by drying. When there is no more water in the medium, introduce carbon dioxide.

The quantity of carbon dioxide to be used, expressed in terms of molar ratio between carbon dioxide and the phenol ether, varies between 1,100 and more preferably between 1 and 2.

The process of the invention is advantageously carried out at a temperature of less than 150 ° C, and preferably less than 140 ° C and still more preferably less than 120 ° C. To obtain better reaction selectivity, the reaction temperature is chosen between 90 ° C and 110 ° C. It is generally carried out at atmospheric pressure, causing carbon dioxide to bubble in the reaction medium maintained under agitation.

It is also possible to carry out the reaction under pressure of carbon dioxy varying between atmospheric pressure and about 100 bar. prefer a pressure between 1 and 20.

A preferred practical embodiment of the invention consists in using the solvent, the phenol ether, the base, in removing the water by distillation and then introducing the carbon dioxide.

In a last step, the carboxylated phenol ether is recovered para from the reaction medium, in a manner known per se.

At the end of the reaction, the pH is brought to between 5.0 and 8.0, by adding an aqueous solution of a mineral acid such as, for example, hydrochloric, sulfuriq or nitric acid. Hydrochloric acid or sulfuric acid are preferred. acid concentration is not critical. It preferably corresponds to the concentration of the commercial form, for example, 37% by weight for hydrochloric acid, 92 or 96% for sulfuric acid.

The unreacted phenol ether settles. It is eliminated by separation of the organic and aqueous phases. The aqueous residual solution is acidified to a pH below 3, preferably between 1 and 2, by addition of an acidic aqueous solution as previously described so as to precipitate the substituted 4-hydroxybenzoic acid. The acid obtained is recovered from the reaction medium according to conventional solid / liquid separation techniques, preferably by filtration.

The process of the invention allows easy access to 4-hydroxybenzoic acids, the aromatic nucleus of which carries at least one ether group, which can be used, as intermediates for the manufacture of the corresponding 4-hydroxybenzaldehydes, by reduction of the carboxylic function to aldehyde function.

To carry out the reduction of the 4-hydroxybenzoic acid obtained according to the invention, to the corresponding aldehyde, it is possible in particular to use the process described in EP-A-0 539 274. The process described consists in carrying out the reduction with hydrogen, in the vapor phase and in the presence of a bimetallic catalyst of the ruthenium / tin type.

It is thus possible according to the invention to prepare vanillin and ethyl vanillin by reduction respectively of p-vanillic acid and of 4-hydroxy-3-ethoxybenzoic acid, by implementing the aforementioned process.

Another variant application of the invention consists in preparing the alkyl esters having from 1 to 8 carbon atoms and preferably from 1 to 4 carbon atoms, 4-hydroxybenzoic acids in which the aromatic nucleus carries at least one ether group. obtained according to the process of the invention, according to any method known to those skilled in the art.

There are several ways of preparing esters.

A first variant consists in reacting said acid with the appropriate alcohol.

It is also possible to carry out the ester if icati on in the presence of an organic solvent. The organic solvent is chosen such that it forms an azeotrope with water and that the boiling point of its azeotrope with water is lower than that of the alcohol involved. Examples of solvents that may be mentioned include toluene, cumene or pseudocumene.

Preferably, it is chosen to use a direct esterification process, in the absence of organic solvent, for alcohols having from 1 to 5 carbon atoms.

In the case of heavy alcohols having more than 5 carbon atoms, it is preferable to conduct the reaction, in the presence of an organic solvent. The different reactions are carried out in a conventional manner, presence of a conventional acid type catalyst. Mention may in particular be made of sulfuric acid, hydrochloric acid, p-toluene sulfonic acid, alkyl titanates, preferably isopropyl or n-butyl titanate, antimony oxy.

It is thus easy to obtain the esters of 4-hydroxybenzoic acids, the aromatic nucleus of which carries at least one ether group, in particular the preferably methyl esters of p-vanic acid and of 4-hydroxy-ethoxybenzoic acid.

The examples which follow illustrate the invention without however limiting it.

In the examples, the yields mentioned correspond to the following definition: number of moles of 4-hydroxy-3-methoxybenzoic acid formed Yield: RR =% number of moles of sodium guaiacolate introduced

EXAMPLES The operating protocol which is given in the different examples is given below.

1 - Synthesis of potassium guaiacolate

62.05 g (0.5 mol) of guaiacol and 50 ml of toluene are loaded into a 1000 ml tric equipped with central agitation, a vigorous column and a 100 ml dropping funnel.

97.8 g of a 28.6% by weight aqueous potassium hydroxide solution are poured in at 0:30 hours at room temperature.

There is a white precipitate in suspension in the medium. The mixture is heated under reflux: the medium is homogeneous in greenish color.

The water / toluene azeotrope is distilled. After distilling 91% of the committed and formed water, the medium is pasty: agitation is difficult. Cooled under a stream of nitrogen.

Filtered on sintered glass No. 4. The precipitate is washed with 50 ml x 2 of anhydrous toluè sold by Aldrich.

Dried, at constant weight, in an oven at 100 ° C under 1-2 mm mercury.

We grind in a mortar. 75.61 g of a fine brown precipitate are recovered. The yield is 100%.

We store in a desiccator containing phosphoric anhydride

P2θ ₅ .

2 - Synthesis of sodium guaiacolate

62.09 g (0.5 mol) of guaiacol and 500 ml of toluene are loaded into a 1000 ml three-necked flask equipped with a central agitation, a vigorous column and a 100 ml dropping funnel. 65.2 g of an aqueous sodium hydroxide solution at 30.8% by weight are poured in at 0.25 h at room temperature.

There is a white precipitate in suspension in the medium. It is heated under reflux. The water / toluene azeotrope is distilled: 13.5 / 86.5% at 84 ° C. After distilling the committed and formed water, the medium is pasty: agitation is difficult.

Cooled under a stream of nitrogen.

Filtered on sintered glass No. 4. The precipitate is washed with 50 ml x 2 of anhydrous toluene sold by Aldrich. Dried to constant weight, in an oven at 100 ° C under 1-2 mm of mercury.

We grind in a mortar.

68.18 g of precipitate are recovered in the form of fine white needles. The yield is 93.3%. It is stored in a desiccator containing phosphoric anhydride.

3 - Carboxylation of sodium guaiacolate under Cθ £ pressure

2.79 g (19.1 mmol) of anhydrous sodium gaiacolate are loaded into a Burton Corbelin 50 ml reactor in hastelloy B2, equipped with a paddle turbine, with 20 ml of anhydrous 1-methyl-2-pyrrolidinone marketed by Aldrich.

The reactor is purged by a stream of CO2. There is a slight exotherm.

The mixture is heated at 100 ° C for 7 h 00, while maintaining the CO2 pressure at 20 bar.

After the reactor has cooled to room temperature, 20 ml of water are added. A 5N hydrochloric acid solution is poured until an approximately 2.0 is obtained. There is precipitation.

Acetonitrile is added in order to make the reaction medium homogeneous.

Dose by high performance liquid chromatography: Lich Cart RP 18 column - 5 μm, 250/4 mm sold by Merck - eluent: 800 H2O / 2OO ml CH3CN / 3.5 ml H3PO4 - flow rate: 1 ml.mn- ¹ - UV 240 detection μm ambient temperature.

4 - Carboxylation of potassium guaiacolate under a stream of CO at atmospheric pressure

5.3011 (32.7 mmol) of dry anhydrous potassium gaiacolate is charged into a 100 ml three-necked flask equipped with a glass turbine, a CO2 inlet dip tube and a straight ascending cooler, with 30 ml of anhydrous 1-methyl-pyrrolidinone sold by Aldrich. It is heated to 100 ° C. under a stream of CO2 having a flow rate of approx.

3.0 lh ^"1 for 7 hours.

It is rapidly cooled to room temperature in an ice water bath. 20 ml of water are added.

A 5N hydrochloric acid solution is poured in until a p of approximately 2.0 is obtained. There is precipitation.

Acetonitrile is added in order to make the reaction medium homogeneous. Dose by high performance liquid chromatography: Lichr Cart RP 18 column - 5 μm, 250/4 mm sold by Merck - eluent: 800 H2O / 2OO ml CH3CN / 3.5 ml H3PO4 - flow rate: 1 ml.mn ^"1 - UV 240 detection μm ambient temperature.

Examples 1 to 6 Comparative test a

A series of tests is carried out by reproducing the operating protocol given above, using 1-methyl-2-pyrrolidinone, polar and basic solva and by varying the reaction temperature, the pressure and the nature of the starting guaiacolate.

The operating conditions and the results obtained are recorded in table (I). It is noted that a too high reaction temperature causes the selectivity of the reaction to drop.

Examples 7 and 8 Comparative tests b to d

In the two examples 7 and 8 which follow, the carboxylation reaction of guaiacol is carried out in polar and basic aprotic solvents, such as N, N-dimethylformamide (Example 7) and N, N-dimethylacetamide (Example 8). Examples of embodiments of the carboxylation of guaiacol are given, by way of comparison, in different solvents which are not suitable for the invention:

- protic solvent such as butanol (test b),

- weakly polar and slightly basic aprotic solvent such as toluene (test c), - weakly polar and basic aprotic solvent such as pyridine (test d).

The operating conditions and the results obtained are listed in Table (II).

Table (II)

Ref. Type of solvent Pressure Temperature Gaiacolate Duration RR para RR ortho Selectivity ex. (bar) (° C) (h) (%) (%) p / p + o

(%)

7 N, N-dimethylformamide 20 100 Na 7 h 00 12.0 1, 9 86

8 N, N-dimethylacetamide 20 100 Na 7 h 00 27.5 5.1 84 b 1-butanol 20 100 Na 7 h 00 7.5 0 0 ec toluene 20 100 Na 7 h 00 4.4 8.8 33 d pyridine 20 100 Na 7:00 a.m. 1, 2 9.6 1 1

It appears from the comparison of the results obtained that the selectivity of the para carboxylation reaction is very good under the conditions of the invention but very poor in the comparative tests.

Claims

1 - Process for the carboxylation of a phenol ether which consists in reacting said phenol ether in salified form with carbon dioxide, said process being characterized in that the reaction is carried out at a temperature below 150 ° C, in an aprotic, polar and basic organic solvent.

2 - Method according to claim 1 characterized in that the phenol ether corresponds to the general formula (I):

in which :

A symbolizes the remainder of a cycle forming all or part of an aromatic, monocyclic or polycyclic carbocyclic system, system comprising at least one OR 'group: said cyclic residue being able to carry one or more substituents,

- R represents a hydrogen atom or one or more substituents, identical or different, - R 'represents a hydrocarbon radical having from 1 to 24 carbon atoms, which can be an acyclic saturated or unsaturated, linear or branched aliphatic radical; a saturated, unsaturated or aromatic, monocyclic or polycyclic cycloaliphatic radical; a saturated or unsaturated, linear or branched aliphatic radical, carrying a cyclic substituent, - n is a number less than or equal to 3.

3 - Method according to one of claims 1 and 2 characterized in that the phenol ether corresponds to the general formula (I) in which R 'represents:

- an acyclic, saturated or unsaturated, linear or branched aliphatic radical, preferably a linear or branched alkyl radical having from 1 to 12 carbon atoms, preferably from 1 to 6 carbon atoms: the hydrocarbon chain can possibly be interrupted by a heteroatom, a functional group and / or carrier of a substituent,

- an acyclic, saturated or unsaturated, linear or branched aliphatic radical carrying an optionally substituted cyclic substituent: said acyclic radical can be linked to the cycle by a valential link, a heteroatom or a functional group,

- a saturated carbocyclic radical or comprising 1 or 2 unsaturations in the ring, generally having 3 to 8 carbon atoms, preferably 6 carbon atoms in the ring; said cycle can be substituted.

- an aromatic carbocyclic radical, preferably monocyclic, with generally at least 4 carbon atoms, preferably 6 carbon atoms in the ring; said cycle can be substituted.

4 - Process according to claim 1 characterized in that the phenol ether corresponds to the general formula (I) in which R 'represents a linear or branched alkyl radica having from 1 to 4 carbon atoms, preferably a methyl radical or a phenyl radical.

5 - Method according to one of claims 1 to 4 characterized in that the phenol ether corresponds to the general formula (I) in which the remainder represents the remainder of an aromatic, monocyclic carbocyclic compound having at least 4 atoms of carbon and preferably 6 carbon atoms or the remainder of a polycyclic carbocyclic compound: the remainder A being able to carry one or more substituents on the aromatic nucleus.

6 - Method according to one of claims 1 to 5 characterized in that the phenol ether corresponds to the formula (la):

in which :

- n is a number less than or equal to 3, preferably equal to 0 or 1,

the radical R ′ represents an alkyl radical, linear or branched, having from 1 6 carbon atoms, preferably from 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl or phenyl,

- the radical (s) R represent one of the following atoms or groups:

. a hydrogen atom, . an alkyl radical, linear or branched, having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl,. a linear or branched alkoxy radical having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms such as the methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, tert-butoxy radicals,

. a halogen atom, preferably a fluorine, chlorine or bromine atom, a trifluoromethyl radical. - The radicals R 'and R and the 2 successive atoms of the benzene ring can form between them, a ring having 5 to 7 atoms, optionally comprising another heteroatom.

7 - Process according to claim 6 characterized in that the phenol ether corresponds to the formula (la) in which n is greater than or equal to 1, the radicals R 'and R and the 2 successive atoms of the benzene ring can be linked together by an alkylene, alkenylene or alkenylidene radical having 2 to 4 carbon atoms to form a saturated, unsaturated or aromatic heterocycle having 5 to 7 carbon atoms in which one or more carbon atoms may be replaced by. a heteroatom, preferably oxygen: the radicals R 'and R preferably forming a methylene dioxy or ethylene dioxy radical.

8 - Method according to one of claims 1 and 7 characterized in that the phenol ether corresponds to formula (la) in which n is equal to 0, the radical

R 'representing an alkoxy radical.

9 - Process according to claim 1 and 2 characterized in that the phenol ether is guaiacol or guetol.

10 - Method according to one of claims 1 to 9 characterized in that the polar organic solvent has a dielectric constant greater than or equal to 15, preferably between 25 and 75.

11 - Method according to one of claims 1 to 10 characterized in that the polar organic solvent has a donor number greater than 20, preferably greater than or equal to 25, and more particularly between 25 and 75 and even more preferably between 25 and 50. 12 - Method according to one of claims 1 to 11 characterized in that the polar organic solvent is chosen from linear or cyclic carboxamides; dimethyl sulfoxide (DMSO); hexamethylphosphotriamide (HMPT) tetramethylurea.

13 - Process according to claim 12 characterized in that the polar organic solvan is chosen from N, N-dimethylacetamide (DMAC), N, N diethylacetamide, dimethylformamide (DMF), diethylformamide or 1 methyl-2- pyrrolidinone (NMP).

14 - Method according to one of claims 10 to 13 characterized in that the amount of solvent is determined so that the concentration of substra in the organic solvent is preferably between 1 and 50% by weight and more preferably between 10 and 40%.

15 - Method according to one of claims 1 to 14 characterized in that the phenol ether is in salified form, preferably in the form of metallic elements of group (la) of the periodic table, preferably of salt sodium or potassium or an ammonium salt.

16 - Process according to claim 15 characterized in that the phenol ether in salified form is prepared by reacting said compound with a base, preferably sodium or potassium hydroxide or a quaternary ammonium hydroxide then in eliminating the water formed during salification.

17 - Process according to claim 16 characterized in that the amount of base expressed relative to the phenol ether varies between 90 and 120% of the stoichiometric amount.

18 - Method according to one of claims 16 and 17 characterized in that the temperature of the salification reaction is between 25 ° C and 100 ° C.

19 - Method according to one of claims 1 to 18 characterized in that the amount of carbon dioxide to be used expressed by the molar ratio between the carbon dioxide and the phenol ether varies between 1 and 100, e more preferably between 1 and 2. 20 - Method according to one of claims 1 to 19 characterized in that the temperature at which the carboxylation takes place is a temperature below 120 ° C, preferably between 90 ° C and 110 ° C.

21 - Method according to one of claims 1 to 20 characterized in that the carbon dioxide pressure varies from atmospheric pressure and about 100 bar, preferably between 1 and 20 bar.

22 - Method according to one of claims 1 to 21 characterized in that one uses the solvent, the phenol ether, the base, that the water is removed by distillation and that one introduces carbon dioxide.

23 - Method according to one of claims 1 to 22 characterized in that the phenol ether of para-carboxylated phenol is recovered from the reaction medium.

24 - Use of 4-hydroxybenzoic acids in which the aromatic nucleus carries at least one ether group obtained according to the process described in one of claims 1 to 23, as intermediates for the manufacture of the corresponding 4-hydroxybenzaldehydes.

25 - Use of p-vanillic acid and 4-hydroxy-3-ethoxybenzoic acid obtained according to the process described in one of claims 1 to 23, as intermediates for the manufacture of vanillin and ethyl vanillin respectively .

26 - Use according to one of claims 24 and 25 characterized in that the reduction of the carboxylic function in aldehyde function is carried out by hydrogen, in the vapor phase and in the presence of a bimetallic catalyst of ruthenium / tin type.

27 - Use of 4-hydroxybenzoic acids in which the aromatic nucleus carries at least one ether group obtained according to the process described in one of claims 1 to 23, as intermediates for the manufacture of alkyl esters, preferably having from 1 to 4 atoms carbon of the above acids.

28 - Use of p-vanillic acid and 4-hydroxy-3-ethoxybenzoic acid obtained according to the process described in one of claims 1 to 23, as intermediates for the production of esters, preferably methyl esters of vanillic acid and 4-hydroxy-3-ethoxybenzoic acid.