FR2629474A1 - Process for electrosynthesis of benzyl ketones - Google Patents

Abstract

Process for electrosynthesis of an arylmethyl ketone by electrosynthesis of a mixture of an organic acid anhydride and of a heteroatomic arylmethyl compound chosen from the group consisting of quaternary arylmethylammonium, arylmethylsulphonium and quaternary arylmethylphosphonium salts, arylmethyl thiocyanates and esters of arylmethyl alcohols which are more easily electroreducible than the anhydride. The anode, which is consumed during the electrosynthesis, is made of a metal chosen from the group consisting of reducing metals and their alloys, preferably magnesium, zinc or aluminium. This process is very simple to use and makes it possible in particular to avoid the use of arylmethyl halides. Arylmethyl ketones are advantageous synthesis intermediates in the pharmaceutical field.

Description

Electrosynthesis method of aryl methyl ketones
The present invention relates to a method for the eletrosynthesis of arylmethyl ketones by electrolysis of a mixture of an arylmethyl heteroatom compound in which the arylmethyl group is directly attached to a heteroatom by a single covalent bond and an acid anhydride. organic process used in an electrolysis cell in an organic solvent medium.

dans laquelle Ar est un cycle ou hétérocycle aromatique et R1 et
R2 l'hydrogène ou des groupements hydrocarbonés.By arylmethyl group is meant any monovalent arylmethyl group of general formula

in which Ar is an aromatic ring or heterocycle and R1 and
R2 is hydrogen or hydrocarbon groups.

Arylmethyl ketones are particularly useful synthesis intermediates in the pharmaceutical field, in particular for the synthesis of amphetamines and analogous derivatives
FR 2 579 626, which the Applicant claims, inter alia describes the electrosynthesis of arylmethyl ketones by electrolysis of a mixture of an arylmethyl halide and an organic acid anhydride in an organic solvent medium containing a supporting electrolyte. The anode, consumed during the electrochemical reaction of which it is the seat, is preferably zinc, aluminum or magnesium.

The process is preferably carried out in a non-compartmentalized cell. Although this process is simple and applicable to the electrosynthesis of many ketones, it is in practice greatly limited by the toxicity and / or instability of the starting organic halides as well as by the difficulties of access to these compounds.

Most arylmethyl halides are lachrymatory, irritant and corrosive. The most reactive are particularly unstable: paramethoxyarylmethyl chloride, chloromethyl and chloroethyl thiophenes spontaneously polymerize at room temperature with a large gas release of hydrochloric acid.

Alpha arylchloroethanes often undergo dehydrochlorination reactions leading to undesirable styrenic derivatives.

In addition, access to arylmethyl halides is difficult. The most direct route of synthesis is the chloromethylation of aromatic or aromatic heterocyclic compounds (synthesis of chloromethylthiophene, chloromethylnaphthalene).

The formation of highly carcinogenic by-products considerably limits their application.

In general, the introduction of a halogen into an organic molecule requires the use of a dangerous and corrosive reagent such as hydrochloric acid, hydrobromic acid, thionyl chloride, phosphorus chlorides, chlorine, bromine.

The process according to the invention makes it possible, in comparison with the process described in FR 2 579 626, to retain all the advantages thereof, and in particular those mentioned in FR 2 579 626, without having the disadvantages thereof and in particular those mentioned above relating to the use arylmethyl halides. It also allows, in most cases, to overcome the addition of a supporting electrolyte, which greatly simplifies its implementation. According to the invention, the electrosynthesis method of an arylmethyl ketone by electrolysis of a mixture of an arylmethyl heteroatomic compound in which the arylmethyl group is directly connected to a heteroatom and of an organic acid anhydride, used in an organic solvent medium in an electrolysis cell provided with electrodes, is characterized in that the anode is a metal selected from the group consisting of reducing metals and their alloys and that the heteroatomic arylmethyl compound is selected from the group consisting of arylmethyl quaternary ammonium salts, arylmethyl sulfoniums, arylmethyl quaternary phosphoniums, arylmethyl thiocyanates, and arylmethyl alcohol esters more easily electroreducible than the The anion of the above-mentioned salts may be any. It is preferably monovalent, such as, for example, a halide, the methylsulfate anion or the tetrafluoroborate anion.

In the context of the invention, only the esters of arylmethyl alcohols that are more easily electroreducible, under the conditions of electrosynthesis, than the organic acid anhydride are suitable.

arylméthyle, comporte en général un groupement activateur -A, facilitant la rupture entre le groupement arylméthyle et le groupement acyloxy

lors de l'électrosynthèse. Comme exemple de tels groupements activateurs, on peut citer les groupements aromatiques, trifluorométhyle et carbonylés tels que les esters, les amides et les cétones.To do this, the arylmethyl alcohol ester, which can be represented by the general formula

arylmethyl, generally comprises an activator group -A, facilitating the breakage between the arylmethyl group and the acyloxy group

during electrosynthesis. Examples of such activating groups include aromatic, trifluoromethyl and carbonyl groups such as esters, amides and ketones.

Such an arylmethyl alcohol ester or such an acyloxy group may be termed "activated".

dans laquelle Ar représente un cycle ou hétérocyclique aromatique, substitué ou non substitué, de préférence un cycle phényle, thiophène, Nméthylpyrole, indole ou pyridine.According to a particular embodiment of the invention, the heteroatomic arylmethyl compound has the general formula

wherein Ar represents a substituted or unsubstituted aromatic or heterocyclic ring, preferably a phenyl, thiophene, N-methylpyrole, indole or pyridine ring.

 R1 and R2, identical or different, represent a hydrogen atom or a hydrocarbon group, preferably containing 1 to 12 carbon atoms, substituted or unsubstituted.

Preferably R1 represents a hydrogen atom and R2 represents a hydrogen atom or a methyl, ethyl or isopropyl group.

In a particularly preferred manner Ar represents a phenyl ring and R1 and R2 a hydrogen atom.

ayant la signification précitée).
YY is an organic group selected from the group consisting of monovalent groups of quaternary ammonium salts, quaternary phosphoniums and sulphoniums whose free valence is located respectively on nitrogen, phosphorus and sulfur, as well as by thiocyanate groups (-SCN) and activated acyloxy

having the above meaning).

dans laquelle Ra, Rb et Rc, identiques ou différents, sont des groupements hydrocarbonés aliphatiques, aromatiques ou hétérocycliques, substitués ou non substitués. Ils peuvent également former des cycles, entre eux ou avec R1, R2 ou Ar.The preferred monovalent groups of quaternary ammonium salts have a cation corresponding to the general formulas

in which Ra, Rb and Rc, which may be identical or different, are substituted or unsubstituted aliphatic, aromatic or heterocyclic hydrocarbon groups. They can also form cycles with each other or with R1, R2 or Ar.

dans laquelle Ra et Rb, identiques ou différents, ont la signification précitée.The preferred monovalent sulfonium salt moieties have a cation corresponding to the general formula

in which Ra and Rb, identical or different, have the meaning mentioned above.

dans laquelle Ra, Rb, Rc, identiques ou différents, ont la signification précitée. De façon préférée Ra, Rb, Rc sont identiques et représentent un groupement phényle ou une chaîne alkyle ayant 3 à 8 atomes de carbone.The preferred monovalent groups of quaternary phosphonium salts have a cation corresponding to the general formula

in which Ra, Rb, Rc, identical or different, have the meaning mentioned above. Preferably Ra, Rb, Rc are identical and represent a phenyl group or an alkyl chain having 3 to 8 carbon atoms.

préférés ont un groupesent activateur (-A) choisi dans le groupe constitué par les groupements aromatiques, phényle par exemple, subsbtitués ou non substitués, le groupement trifluorométhyle (-CF3) et les groupements carbonylés tels que les cétones

les amides

et les esters

Activated acyloxy groups

Preferred are an activating group (-A) selected from the group consisting of aromatic groups, for example phenyl, substituted or unsubstituted, trifluoromethyl (-CF3) and carbonyl groups such as ketones.

the amides

and esters

R being any organic radical, aliphatic or aromatic, substituted or unsubstituted, preferably an alkyl chain preferably having 1 to 12 carbon atoms, optionally substituted with an aromatic group.

dans laquelle
R3 représente - une chaîne aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée, - un groupement aryle, substitué ou non substitué, - un hétérocycle aromatique, substitué ou non substitué, comme par exemple le cycle furanne, thiophène ou pyridine et R4 représente - une chaîne aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée, - un groupement aryle, substitué ou non substitué, - un hétérocycle aromatique, substitué ou non substitué, comme par exemple le cycle furanne, thiophène ou pyridine, - un groupement -OR5 dans lequel R5 représente
* une chaine aliphatique ou cycloaliphatique, substituée ou non substituée, saturée ou non saturée,
* un groupement aryle, substitué ou non substitué,
* un hétérocycle aromatique, substitué ou non substitué, comme par exemple le cycle furanne, thiophène ou pyridine, ou bien encore R3 et R4 forment au moins un cycle, substitué ou non substitué, comme c'est le cas par exemple pour 1' anhydride phtalique ou pour l'anhydride succissique. According to another particular embodiment of the invention, the organic acid anhydride corresponds to the general formula

in which
R3 represents - a substituted or unsubstituted, saturated or unsaturated aliphatic or cycloaliphatic chain, - a substituted or unsubstituted aryl group, - a substituted or unsubstituted aromatic heterocycle, for example the furan, thiophene or pyridine ring, and R4 represents - a substituted or unsubstituted, saturated or unsaturated aliphatic or cycloaliphatic chain, - a substituted or unsubstituted aryl group, - a substituted or unsubstituted aromatic heterocycle, for example the furan, thiophene or pyridine ring, a group -OR5 in which R5 represents
an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated,
an aryl group, substituted or unsubstituted,
a substituted or unsubstituted aromatic heterocycle, such as, for example, the furan, thiophene or pyridine ring, or else R 3 and R 4 form at least one ring, substituted or unsubstituted, as is the case, for example, for anhydride; phthalic or for succic anhydride.

When R4 represents a group OR5, the corresponding anhydrides are then mixed anhydrides of carboxylic acids and carbonic acid.

In all other cases, they are carboxylic acid anhydrides.

Preferably, R3 and R4 represent a linear or branched alkyl chain, preferably containing 1 to 12 carbon atoms.

According to another preferred variant, R3 and R4 are identical.

In a particularly preferred manner, R3 and R4 are identical and represent a linear or branched alkyl chain, as is the case, for example, for acetic anhydride.

dans laquelle Ar, R1, R2 et Y ont la signification précitée et lorsque l'anhydride d'acide organique répond à la formule générale

dans laquelle R3 et R4 ont la signification précitée, on obtient une cétone arylméthylique de formule générale

Ar, R1, R2 et R3 ayant la signification précitée.When the heteroatomic arylmethyl compound meets the general formula

wherein Ar, R1, R2 and Y have the above meaning and when the organic acid anhydride has the general formula

in which R3 and R4 have the abovementioned meaning, an arylmethyl ketone of general formula is obtained

Ar, R1, R2 and R3 having the above meaning.

on obtient une cétone arylméthylique acide carboxylique de formule

In the very particular case of anhydrides in which R 3 and R 4 form a ring, anhydrides can be represented by the formula

an arylmethyl carboxylic acid ketone of formula

In general, Ar, R1, R2, R3, R4 and R5 may carry functional groups that are not electroreducible or more difficult to reducible than the CY bond of the heteroatomic arylmethyl compound, under the experimental conditions of electrosynthesis, and none of the functions carried by Ar, R1, R2, R3, R4 and R5 must be more electrophilic than the anhydride function itself.

Examples of functional groups that can be worn by
Ar, R 1, R 2, R 3, R 4 and R 5 include nitrile groups, tertiary amines, amides, esters, ethers and fluorine.

sont généralement facilement accessibles par les méthodes classiques de la chimie organique. Leur synthèse ne présente pas de difficulté particulière, même à l'échelle industrielle et peut souvent être mise en oeuvre sans passer par l'intermédiaire halogéné instable ou toxique. On peut par exemple réaliser la synthèse des sels d'ammoniums quaternaires arylméthyliques selon l'un des trois schémas réactionnels suivants

The aforementioned heteroatomic arylmethyl compounds of the general formula

are generally easily accessible by conventional methods of organic chemistry. Their synthesis does not present any particular difficulty, even on an industrial scale and can often be implemented without going through the unstable or toxic halogenated intermediate. For example, the synthesis of aryl methyl quaternary ammonium salts can be carried out according to one of the following three reaction schemes.

The quaternization step can optionally be carried out directly in the electrolysis solvent, which avoids the isolation of the quaternary ammonium salt.

généralement stable et peu toxique.The synthesis of the arylmethyl compounds in which Y represents a trifluoroacetate group may for example be carried out by esterification of the arylmethyl alcohol.

generally stable and not very toxic.

présente un anion monovalent comme par exemple un anion halogénure, tosylate ou méthylsulfate.Ar, R 1, R 2, Y, Ra, Rh, Rc have the above meaning and

has a monovalent anion such as an anion halide, tosylate or methylsulfate.

According to the invention, the anode is made of a metal chosen from the group consisting of reducing metals and their alloys.

By their alloys is meant any alloy containing at least one reducing metal.

Preferably, the anode is a metal selected from the group consisting of magnesium, aluminum, zinc and their alloys.

This anode may have any shape and in particular all the conventional forms of metal electrodes: twisted wire, flat bar, cylindrical bar, square section bar, plate, renewable bed, canvas, grid, ribbon, beads, shot, powder, etc. .

Preferably, a cylindrical bar of diameter adapted to the dimensions of the cell is used.

Before use, it is best to clean the surface of the anode chemically or mechanically.

The purity of the metal or alloy constituting the anode is not an important parameter and the industrial qualities are suitable.

During electrosynthesis, the anode is consumed by the electrochemical reaction of which it is the seat. Such a process is commonly referred to as "soluble anode" or "consumable anode".

The cathode is graphite or any metal such as stainless steel, nickel, platinum, gold, copper.

It is constituted, preferably, by a grid or a cylindrical plate disposed concentrically around the anode.

The electrodes are supplied with direct current by means of a stabilized supply.

The organic solvents used in the context of the present invention are all the low protic solvents usually used in organic electrochemistry. Mention may be made, for example, of hexamethylphosphoric triamide (HMPT), tetrahydrofuran (THF), N-methylpyrrolidone (NMP), tetramethylurea (TMU), dimethylformamide (DMF), acetonitrile and mixtures of these solvents, for example THF-HMPT mixtures.

The support electrolytes used to make the medium conductive or more conductive are those usually used in organic electrochemistry. There may be mentioned, for example, the salts whose anion is a halide, a carboxylate, an alcoholate, a perchlorate, a hexafluorophosphate, a fluoroborate and the cation a quaternary ammonium, a quaternary phosphonium, lithium, sodium, potassium, magnesium, zinc, aluminum.

Among these salts, there may be mentioned tetraethylammonium tetrafluoroborate (BF4NBu4), lithium perchlorate (LiClO4), tetrabutylammonium iodide (INBu4), tetraethylammonium chloride (ClNEt4), tetrabutylammonium perchlorate (ClO4NBu4), tetrabutylammonium bromide (BrNBu4).

The addition of a carrier electrolyte is not necessary when the heteroatomic arylmethyl compound is itself ionic, as is the case when this compound is an arylmethyl quaternary ammonium, arylmethylsulphonium or arylmethyl quaternary phosphonium salt. .

When a carrier electrolyte is added, its concentration in the organic solvent is preferably between 10 -3 and 10-2M.

The concentration in the organic solvent of the heteroatomic compound arylmethyl is preferably between 0.01M and 1M.

The ratio of the concentrations in the organic solvent between the organic acid anhydride and the heteroatomic arylmethyl compound may be arbitrary, but preferably an excess of anhydride and in particular a concentration ratio of between 1.5 and 10 is used.

In some cases, the anhydride can act as a cosolvent and is then in very large excess.

It has moreover been found that, unexpectedly, the efficiency of the reaction is improved by operating in the presence of an esterification catalyst, especially in the presence of aminated pyridines, for example N, N'-dimethylaminopyridine.

This also makes it possible to obtain the same efficiency by only using a much lower amount of current, which is of obvious economic interest.

The electrolysis is preferably conducted in a non-compartmentalized cell, under an inert atmosphere (nitrogen, argon for example), after degassing of the solution 1) at a temperature generally between 0C and 800C, preferably between 15 and 30 * C, for practical reasons of simplicity, 2) under a current density on the cathode generally between 10 and 80 mA / cm 2.

The operation is generally carried out at a constant intensity, but it is also possible to operate at constant voltage at controlled potential or with variable voltages and voltages, with stirring of the solution, for example by means of a magnetized bar.

After electrolysis, the desired products are extracted, isolated, purified, identified and analyzed in a conventional manner, for example as indicated in FR 2 579 626 mentioned above.

The invention is illustrated by the following nonlimiting examples.

To achieve these examples, using a conventional electrolytic cell, having only one compartment and equipped with pipes for the arrival and the exit of the inert gas, the possible samples of solution during electrolysis, as well as the electrical passages .

The anode is formed by a cylindrical bar 1 cm in diameter. It is introduced into the cell through a central tubing and is thus located approximately in axial position with respect to the cell.

The cathode is constituted by a cylindrical metal felt arranged concentrically around the anode. Its working surface is of the order of t dm2.

All these tests were carried out under an argon atmosphere.

Examples 1 3 21 - Synthesis of various arylmethyl ketones
Approximately 250 cm3 of a solution of 10 g of heteroatomic arylmethyl compound according to the invention are prepared in an electrolysis cell in 60 cm 3 of DMF and 60 cm 3 of acetic anhydride.

The anode is magnesium and the cathode is nickel.

For Examples 14 and 18, the conductive solution is made by adding 2 g of tetrabutylammonium iodide.

The temperature is about 30 ° C for Examples 1 to 10 and 14 to 19 and about 60 ° C for Examples 11 to 13.

The electrolysis is conducted under a current of constant intensity of 2 A until the amount of electricity used is 386 103 C (4 Faraday) per mole of arylmethyl compound.

After evaporation of the solvent and hydrolysis with 100 cm3 of 18% hydrochloric acid, the mixture is extracted with ethyl ether.

After evaporation of the ether, the residue obtained is dissolved in 50 cm 3 of ethanol.

Then add 10 cm3 of 37% hydrochloric acid and then heated at reflux for about 1 hour.

The arylmethyl ketone is then extracted with ethyl ether and then purified by chromatography on a silica column.

Its structure is determined by mass spectrometry (MS), infra-red (IR) and nuclear magnetic resonance (NMR).

Its purity is verified by gas chromatography (GC).

The nature of the starting heteroatomic arylmethyl compound, that of the corresponding arylmethyl ketone obtained as well as the yield of pure pure product expressed relative to the heteroatomic arylmethyl compound are shown in Table 1 below, for each example

EX <SEP> RENDER
N <SEP> COMPOUND <SEP> HETEROATOMIC <SEP> ARYLMETHYLIC <SEP> KETONE <SEP> ARYLMETHYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 1 <SEP>#<SEP>#<SEP> 69
<tb> 2 <SEP>#<SEP>#<SEP> 78
<tb> 3 <SEP>#<SEP>#<SEP> 78
<tb> 4 <SEP>#<SEP>#<SEP> 61
<Tb>

EX <SEP> RENDER
N <SEP> COMPOUND <SEP> HETEROATOMIC <SEP> ARYLMETHYLIC <SEP> KETONE <SEP> ARYLMETHYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 5 <SEP>#<SEP>#<SEP> 64
<tb> 6 <SEP>#<SEP>#<SEP> 50
<tb> 7 <SEP>#<SEP>#<SEP> 50
<tb> 8 <SEP>#<SEP>#<SEP> 41
<Tb>

EX <SEP> RENDER
N <SEP> COMPOUND <SEP> HETEROATOMIC <SEP> ARYLMETHYLIC <SEP> KETONE <SEP> ARYLMETHYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb><SEP> 9 <SEP>#<SEP>#<SEP> 16
<tb> 10 <SEP>#<SEP>#<SEP> 80
<tb> 11 <SEP>#<SEP>#<SEP> 66
<tb> 12 <SEP>#<SEP>#<SEP> 78
<Tb>

EX <SEP> RENDER
N <SEP> COMPOUND <SEP> HETEROATOMIC <SEP> ARYLMETHYLIC <SEP> KETONE <SEP> ARYLMETHYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 13 <SEP>#<SEP>#<SEP> 82
<tb> 14 <SEP>#<SEP>#<SEP> 62
<tb> 15 <SEP>#<SEP>#<SEP> 55
<tb> 16 <SEP>#<SEP>#<SEP> 87
<Tb>

EX <SEP> RENDER
N <SEP> COMPOUND <SEP> HETEROATOMIC <SEP> ARYLMETHYLIC <SEP> KETONE <SEP> ARYLMETHYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 17 <SEP>#<SEP>#<SEP> 66
<tb> 18 <SEP>#<SEP>#<SEP> 88
<tb> 19 <SEP>#<SEP>#<SEP> 82
<tb> 20 <SEP>#<SEP>#<SEP> 78
<Tb>

EX <SEP> RENDER
N <SEP> COMPOUND <SEP> HETEROATOMIC <SEP> ARYLMETHYLIC <SEP> KETONE <SEP> ARYLMETHYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 21 <SEP>#<SEP>#<SEP> 20
<Tb>
Example 22 - Synthesis of 3,4-dimethoxyphenylacetone
The procedure is the same as for Example 3 but a 1500 cm3 cell with a stainless steel cathode is used. The quantities of products used are quintupled.

The intensity of the electrolysis current is 5 A.

The yield of isolated pure dimethoxyphenylacetone is 80%.

Example 23 Synthesis of 2-thienylacetone
The procedure is the same as for Example 1 but the benzyl quaternary ammonium indure is prepared in situ in DMF by mixing an equimolar amount of methyl iodide and dimethyl, thienylmethylamine.

The yield of isolated pure 2-thienylacetone is 72 t.

Example 24 - Synthesis of 3,4-dimethoxyphenylbutanone
The procedure is the same as for Example 3, but the acetic anhydride is replaced by propionic anhydride.

pure isolée est de 72 t. The yield of 3, 4-dimethoxyphenylbutanone of formula

pure isolated is 72 t.

Example 25 Synthesis of (trifluoromethyl-3-phenyl) -5-acid
4-oxo-pentanoic
The procedure is the same as for Example 12 but the acetic anhydride is replaced by succinic anhydride.

pur isolé est de 80 %.The yield of trifluoromethyl-3-phenyl-5-oxo-4-pentanoic acid of formula

pure isolated is 80%.

Example 26 - Synthesis of phenylacetone
The procedure is the same as for Example 19 but the anode is aluminum.

The yield of isolated pure phenylacetone is 85%.

Example 27 - Synthesis of phenylacetone
The procedure is the same as for Example 19 but the amount of current used is 241 103 C (2.5 Faraday) instead of 386 103 C (4 Faraday).

The yield of isolated pure phenylacetone is 65%.

Example 28 - Synthesis of phenylacetone
The procedure is the same as for Example 27 but it is also carried out in the presence of 6 g of N, N-dimethylaninopyridine.

The yield of isolated pure phenylacetone is 84%.

Claims (10)

claims 1. Electrosynthesis method of an arylmethyl ketone by electrolysis of a mixture of an arylmethyl heteroatomic compound in which the arylmethyl group is directly connected to a heteroatom and an organic acid anhydride, used in a solvent medium organic compound in an electrolysis cell provided with electrodes characterized in that the anode is in a metal selected from the group consisting of reducing metals and their alloys - the heteroatomic arylmethyl compound is chosen from the group consisting of arylmethyl quaternary ammoniums, arylmethyl sulfoniums, arylmethyl quaternary phosphoniums, arylmethyl thiocyanates, and arylmethyl alcohol esters more easily electroreducible than anhydride. 2. Method according to claim 7 characterized in that the anode is a metal selected from the group consisting of magnesium, aluminum, zinc and their alloys and in that the electrolysis cell comprises only one only compartment. 3. Method according to any one of claims 1 and 2 characterized in that the heteroatomic arylmethyl compound meets the general formula

 in which - Ar represents a substituted or unsubstituted aromatic ring or heterocycle, preferably a phenyl, thiophene, N-methylpyrrole, indole or pyridine ring; R1 and R2, which may be identical or different, represent a hydrogen atom or a substituted hydrocarbon group; or non-substituted, preferably containing 1 to 12 carbon atoms - Y represents an organic group chosen from the group consisting of monovalent groups of quaternary ammonium salts, quaternary phosphoniums and sulfoniums whose free valence is located respectively on the nitrogen, phosphorus and sulfur, as well as activated acyloxy groups and thiocyanate. 4. Process according to claim 3, characterized in that R1 represents a hydrogen atom and that R2 represents a hydrogen atom or a methyl, ethyl or isopropyl group. 5. Method according to any one of claims 3 and 4 characterized in that Ar represents a phenyl ring and R1 and R2 a hydrogen atom. 6. Process according to claim 3, characterized in that the monovalent groups of quaternary ammonium salts have a cation corresponding to the general formula

 in which Ra, Rb and Rc, which may be identical or different, are substituted or unsubstituted aliphatic, aromatic or heterocyclic hydrocarbon groups which may optionally form rings with each other or with R 1, R 2 or Ar.   in which Ra and Rb, identical or different, have the meaning mentioned above.

the monovalent groups of sulphonium salts have a cation corresponding to the general formula Rat Rb and Rc, identical or different, have the meaning mentioned above.  in which

the monovalent groups of quaternary phosphonium salts have a cation corresponding to the general formula  wherein R is an organic, aliphatic or aromatic, substituted or unsubstituted radical.

 in which -A represents an activating group selected from the group consisting of substituted or unsubstituted aromatic groups, trifluoromethyl group and ester groups of general formula

the activated acyloxy groups correspond to the general formula 7. Process according to any one of the preceding claims, characterized in that the organic acid anhydride corresponds to the general formula

 in which R3 represents - a substituted or unsubstituted, saturated or unsaturated aliphatic or cycloaliphatic chain, - a substituted or unsubstituted aryl group, - a substituted or unsubstituted aromatic heterocycle, and R4 represents an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated, - a substituted or unsubstituted aryl group, - a substituted or unsubstituted aromatic heterocycle, - a group -OR5 in which R5 represents  an aliphatic or cycloaliphatic chain, substituted or unsubstituted, saturated or unsaturated,  an aryl group, substituted or unsubstituted,  an aromatic heterocycle, substituted or unsubstituted, or else R3 and R4 form at least one ring, substituted or unsubstituted. 8. Method according to any one of the preceding claims characterized in that a support electrolyte is added to make the medium conductive or more conductive. 9. Process according to any one of the preceding claims, characterized in that an excess of organic acid anhydride is used relative to the heteroatomic arylmethyl compound, preferably a molar excess of between 1.5 and 10. 10. Process according to any one of the preceding claims, characterized in that it is also carried out in the presence of an esterification catalyst, preferably an aminopyridine.