FR2632978A1 - Process for electrosynthesis of benzyl carbinols - Google Patents

Abstract

Process for electrosynthesis of a benzyl carbinol by electrolysis in an organic solvent medium, of a mixture of a carbonyl compound chosen from the group consisting of ketones and aldehydes and of a heteroatomic benzyl derivative chosen from the group consisting of benzyl quaternary ammonium, benzylsulphonium and benzylquaternary phosphonium salts, benzyl thiocyanates and esters of benzyl alcohols which are more easily electroreducible than the carbonyl compound. The anode, 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 benzyl halides. Benzylcarbinols are advantageous synthesis intermediates in the pharmaceutical field and in perfumery.

Description

Electrosynthesis process of benzyl carbinols
The present invention relates to a method of electrosynthesis of benzylic carbinols by electrolysis of a mixture of a benzylic heteroatomic compound in which the benzyl group is directly connected to a heteroatom by a single covalent bond and a carbonyl compound selected from the group consisting of ketones and aldehydes, a process carried out in an electrolysis cell in an organic solvent medium.

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

wherein Ar is a substituted or unsubstituted aromatic ring or heterocycle, and R1 and R2, which are the same or different, are hydrogen or hydrocarbon groups.

Benzyl carbinols are particularly useful synthesis intermediates in the pharmaceutical and perfume industries.

FR 2,579,627 describes inter alia the electrosynthesis of benzylic carbinols by electrolysis of a mixture of a benzyl halide and a ketone or an aldehyde, in an organic solvent medium containing a supporting electrolyte. The anode, consumed during the reaction of which it is the seat, is in 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 alcohols, it is in practice strongly limited by the toxicity and / or instability of the starting organic halides as well as by the difficulties of access to these compounds.

Most benzyl halides are lachrymatory, irritating and corrosive. The most reactive are particularly unstable: paramethoxybenzyl chloride, chloromethyl and chloroethyl thiophenes polymerize spontaneously 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 benzyl 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 2 579 627, to retain all the advantages thereof, and in particular those mentioned in FR 2 579 627, without having the disadvantages thereof. especially those mentioned above related to the use of benzyl halides. It also allows, in most cases, to overcome the addition of a carrier electrolyte, which greatly simplifies its implementation.

According to the invention, the electrosynthesis method has a benzylic carbinol by electrolysis of a mixture of a benzylic heteroatomic compound in which the benzyl group is directly attached to a heteroatom and a carbonyl compound selected from the group consisting of the ketones and the aldehydes, used in a solvent-organic medium in an electrolysis cell provided with electrodes, are characterized in that the anode is in a metal chosen from the group consisting of reducing metals and their alloys, and that the benzylic heteroatomic compound is selected from the group consisting of benzyl quaternary ammonium salts, benzylic sulfoniums, benzyl quaternary phosphoniums, benzyl thiocyanates, and esters of benzyl alcohols more easily electroreducible than the carbonyl compound. The anion of the above-mentioned salts may be some. It is preferably monovalent as. for example a halide, the methylsulfate anion or the tetrafluoroborate anion.

In the context of the invention, only the esters of benzyl alcohols more easily electroreducible, under the conditions of electrosynthesis, than the carbonyl compound selected from the group consisting of ketones and aldehydes.

benzyle comporte en général un groupement activateur--A facilitant la rupture entre le groupement benzyle 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 esters1 les amides et les cétones.To do this, the benzyl alcohol ester, which can be represented by the general formula

benzyl generally comprises an activator-A group facilitating the rupture between the benzyl 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 a benzyl 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, N méthylpyrrole, indole ou pyridine.According to a particular embodiment of the invention, the benzylic heteroatomic compound corresponds to the general formula

in which . Ar represents a substituted or unsubstituted aromatic or heterocyclic ring, preferably a phenyl, thiophene, N methylpyrrole, indole or pyridine ring.

 . R1 and R21, which may be identical or different, represent a hydrogen atom or a hydrocarbon group, preferably containing 1 to 12 carbon atoms, which may be 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.

-A ayant la signification précitée).. Y 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

-A having the above meaning).

dans lesquelles 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, 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 of the general formula

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

dans laquelle Ra, Rb et 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 and 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 groupe ment 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, eg 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 by an aromatic group.

dans laquelle R3 et R4, identiques ou différents, représentent - un atome d'hydrogène - une chaîne aliphatique ou cycloaliphatique, saturée ou non saturée, non substituée ou substituée par des groupes aliphatiques ou aromatiques - 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 par la cyclohexanone.According to another particular embodiment of the invention, the carbonyl compound chosen from the group consisting of ketones and aldehydes corresponds to the general formula

in which R3 and R4, identical or different, represent - a hydrogen atom - an aliphatic or cycloaliphatic chain, saturated or unsaturated, unsubstituted or substituted by aliphatic or aromatic groups - an aryl group, substituted or unsubstituted - a substituted or unsubstituted aromatic heterocycle, such as, for example, the furan, thiophene or pyridine ring, or alternatively R3 and R4 form at least one ring, substituted or unsubstituted, as is the case for example with cyclohexanone.

Preferably, R 3 and R 4 represent hydrogen or a linear or branched alkyl chain, preferably containing 1 to 12 carbon atoms.

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

Particularly preferably, R 3 and R 4 are identical and represent hydrogen or a linear or branched alkyl chain containing 1 to 12 carbon atoms, as is the case, for example, for acetone and formaldehyde.

The process according to the invention can be represented by the following reaction scheme

Ar, R1, R2, R31 R4 and Y having the above meaning.

In a particularly preferred manner, the carbonyl derivative is a ketone, that is to say that R3 and R4- are different from hydrogen and then tertiary alcohols are obtained or the carbonyl derivative is formaldehyde, that is to say say that R3 and R4 represent hydrogen and we then obtain primary alcohols.

When the carbonyl derivative is an aldehyde other than formaldehyde, secondary alcohols are obtained.

In general, Ar, R11, R2, R3 and R4 may carry functional groups that are not electroreducible or more difficult to reducible than the CY bond of the benzylic heteroatom compound, under the experimental conditions of electrosynthesis, and none of the functions carried by Ar, Art R2, R3 and R4 must be more electrophilic than the carbonyl group of the ketone or aldehyde.

Examples of functional groups that can be worn by
Ar, R 1 and R 21, R 3 and R 1, 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 rsans passer par l'intermédiaire halogéné instable ou toxique. On peut par exemple réaliser la synthèse des sels d'ammoniums quaternaires benzyliques selon l'un des trois schémas réactionnels suivants

The aforementioned benzyl heteroatomic 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 passing through the unstable or toxic halogenated intermediate. For example, the synthesis of benzyl 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 benzyl heteroatomic compounds, in which Y represents a trifluoroacetate group, may for example be carried out by esterification of benzyl alcohol.

generally stable and not very toxic.

Ar, R1, R2, Y, Ra, Rb, Rc have the above meaning and # represents a monovalent anion such as a halogenated anion, tosylate or methylsulfate.

According to the invention the anode is a metal selected 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. .

In a preferred manner, 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 industrial conditions 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" expendable 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 fed with direct current through 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. For example, mention may be made 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 benzylic heteroatomic compound is itself ionic, as is the case when this compound is a benzyl quaternary ammonium, benzylsulphonium or benzyl quaternary phosphonium salt. .

When a carrier electrolyte is added, its concentration in the organic solvent is preferably between 5 103M and 5 102M.

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

The ratio of the molar concentrations in the organic solvent between the carbonyl compound and the benzylic heteroatom compound may be arbitrary, but preferably a molar excess of the carbonyl compound and in particular a molar concentration ratio of between 1.5 and 10 is used.

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

The electrolysis is preferably conducted in a cell having only one compartment, under an inert atmosphere (nitrogen, argon for example), after degassing the solution: - 1) at a temperature generally between 0 C and 1000C.

2) under a current density on the cathode generally between 10 and eo 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 intensity and potential, and 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 2 579 627 cited 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 tubings for the inlet and the outlet of the inert gas, the possible samples of solution during electrolysis, and 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 1 dm2.

All these tests were carried out under an argon atmosphere.

Examples 1 to 11 - Synthesis of various benzyl carbinols ter
tiaries
For Examples 1 and 3 to 6, a solution of 10 g of the benzylic heteroatom derivative in 80 ml of acetone and 50 ml of DMF at 1 μl is electrolyzed. The intensity of the current is 2A. The anode is aluminum and the cathode is nickel.

The electrolysis is conducted until the amount of electricity used is 290 ° C. (3 Faraday) per mole of benzyl derivative.

The solvent is then evaporated off, the residue is hydrolyzed with 100 ml of 18% hydrochloric acid and then extracted with 100 ml of ethyl ether.

After concentrating the organic phase, the product obtained is purified by chromatography on a silica column.

For example 6, the conductive solution is made by adding 2 g of tetrabutylammonium iodide.

For Example 2, the procedure is as for Example 1 but the anode is magnesium and the amount of benzyl heteroatom derivative of 7 g.

For Examples 7 and 8, the procedure is as for Example 1 but the amounts of acetone and DMF are different namely that 15 g of acetone and 95 g of DMF are used.

For Example 9, the procedure is as for Example 7 but the solution is made conductive by the addition of 2 g of tetrabutyl amm.onlum iodide.

Examples 11 and 11 are carried out as in Example 7 but acetone is replaced by 3-pentanone.

The nature of the benzyl heteroatomic derivative and of the starting carbonyl compound, that of the benzyl carbinol obtained as well as its yield of isolated pure product, relative to the benzylic heteroatomic derivative, are shown in Table 1 below, for each example. TABLE 1
SYNTHESIS OF VARIOUS TERTIARY BENZYLIC CARBINOLS

EX <SEP> RENDER
N <SEP> DERIVATIVE <SEP> HETEROATOMIC <SEP> BENZYLIC <SEP> DERIVATIVE <SEP> CARBONYL <SEP> CARBINOL <SEP> BENZYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb><SEP> 1 <SEP> 70
<tb><SEP> 2 <SEP>#<SEP> CH3COCH3 <SEP>#<SEP> 65
<tb><SEP> 3 <SEP>#<SEP> CH3COCH3 <SEP> 40
<tb><SEP> 4 <SEP>#<SEP> CH3COCH3 <SEP>#<SEP> 70
<tb><SEP> 5 <SEP>#<SEP> CH3COCH3 <SEP>#<SEP> 53
<tb> TABLE 1 CONTINUATION
SYNTHESIS OF VARIOUS TERTIARY BENZYLIC CARBINOLS

EX <SEP> RENDER
N <SEP> DERIVATIVE <SEP> HETEROATOMIC <SEP> BENZYLIC <SEP> DERIVATIVE <SEP> CARBONYL <SEP> CARBINOL <SEP> BENZYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb><SEP> 6 <SEP>#<SEP> CH3COCH3 <SEP>#<SEP> 50
<tb><SEP> 7 <SEP>#<SEP> CH3COCH3 <SEP>#<SEP> 51
<tb><SEP> 8 <SEP>#<SEP> CH3COCH3 <SEP>#<SEP> 62
<tb><SEP> 9 <SEP>#<SEP> CH3COCH3 <SEP>#<SEP> 42.5
<tb> TABLE 1 CONTINUATION
SYNTHESIS OF VARIOUS TERTIARY BENZYLIC CARBINOLS

EX <SEP> RENDER
N <SEP> DERIVATIVE <SEP> HETEROATOMIC <SEP> BENZYLIC <SEP> DERIVATIVE <SEP> CARBONYL <SEP> CARBINOL <SEP> BENZYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 10 <SEP>#<SEP> 54
<tb> 11 <SEP>#<SEP>#<SEP>#<SEP> 49.5
<Tb>
Examples 12 and 13 - Synthesis of secondary benzyl carbinols
For example 12, a solution of 10 g of the benzylic heteroatomic derivative and 50 g of benzaldehyde in 60 g of electrolysis is electrolyzed.
DMF at 240C.

The intensity of the current is 2A. The anode is aluminum and the cathode is nickel.

The electrolysis is conducted until the amount of electricity used is 290 ° C. (3 Faraday) per mole of benzyl derivative.

The DMF is then evaporated and then the residue is hydrolysed with 18% hydrochloric acid and then extracted with ethyl ether.

After concentration of the organic phase, the product obtained is purified by chromatography on a silica column.

For Example 13 the procedure is as for Example 12 but the benzaldehyde is replaced by butyraldehyde (15 g).

The nature of the benzyl heteroatomic derivative and of the starting carbonyl compound, that of the benzyl carbinol obtained as well as its yield of pure pure product, expressed relative to the benzylic heteroatomic derivative, are shown in Table 2 below, for each example.

TABLE 2
SYNTHESIS OF BENZYL SECONDARY CARBINOLS

EX <SEP> RENDER
N <SEP> DERIVATIVE <SEP> HETEROATOMIC <SEP> BENZYLIC <SEP> DERIVATIVE <SEP> CARBONYL <SEP> CARBINOL <SEP> BENZYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 12 <SEP>#<SEP>#<SEP>#<SEP> 58
<tb> 13 <SEP>#<SEP>#<SEP>#<SEP> 43
<Tb>
Examples 14 and 15 - Synthesis of primary benzyl carbinols
A solution of 10 g of the benzylic heteroatom derivative, 9 g of paraformaldehyde (HCHO) n and 3 g of tetrabutylammonium iodide in 110 ml of DMF was electrolyzed at 75 ° C.

The intensity of the current is 2A. The anode is aluminum and the cathode is nickel.

The electrolysis is carried out until the amount of electricity used is 290 ° C. (3 Faraday) per mole of benzyl derivative.

The DMF is then evaporated and then the residue is hydrolyzed with 18% hydrochloric acid and then extracted with ethyl ether.

After concentration of the organic phase, the product obtained is purified by chromatography on a silica column.

The nature of the benzyl heteroatomic derivative and of the starting carbonyl compound, that of the benzyl carbinol obtained as well as its yield of pure pure product, expressed relative to the benzylic heteroatomic derivative, are shown in the following table 3, for each example. TABLE 3
SYNTHESIS OF PRIMARY BENZYLIC CARBINOLS

EX <SEP> RENDER
N <SEP> DERIVATIVE <SEP> HETEROATOMIC <SEP> BENZYLIC <SEP> DERIVATIVE <SEP> CARBONYL <SEP> CARBINOL <SEP> BENZYLIC <SEP> OBTAINED <SEP> MENT <SEP> (%)
<tb> 14 <SEP>#<SEP> HCHO <SEP>#<SEP> 72
<tb> 15 <SEP>#<SEP> HCHO <SEP>#<SEP> 80
<Tb>

Claims (9)

Claims 1. A method of electrosynthesis of a benzylic carbinol by electrolysis of a mixture of a benzylic heteroatomic compound in which the benzyl group is directly attached to a heteroatom and a carbonyl compound selected from the group consisting of ketones and aldehydes, used in an organic solvent medium in an electrolysis cell provided with electrodes, characterized in that the anode is in a metal chosen from the group consisting of reducing metals and their alloys - the heteroatomic compound benzyl is selected from the group consisting of benzyl quaternary ammonium salts, benzylic sulfoniums, benzyl quaternary phosphoniums, benzyl thiocyanates and benzyl alcohol esters more easily electroreducible than the carbonyl compound. 2. Method according to claim 1 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 a single compartment. 3. Method according to any one of claims 1 and 2 characterized in that the benzylic heteroatom 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 hydrocarbon group, preferably comprising 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 sulphoniums whose free valence is located respectively on 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 formulas

 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 R11, R2 or Ar.  wherein A represents a group selected from the group consisting of substituted or unsubstituted aromatic groups, trifluoromethyl group and carbonyl groups such as ketones, amides and esters.

 in which R ,, Rb-and Rc r identical or different, have the meaning mentioned above - the acyloxy-activated groups correspond to the general formula

 in which Ra and Rh, identical or different, have the abovementioned meaning - the monovalent groups of the phosphonium-quaternary salts have a cation corresponding to the general formula

the monovalent groups of sulphonium salts have a cation corresponding to the general formula 7. Process according to any one of the preceding claims, characterized in that the carbonyl compound chosen from the group consisting of ketones and aldehydes corresponds to the general formula

 in which R3 and R41, which may be identical or different, represent a hydrogen atom, a saturated or unsaturated aliphatic or cycloaliphatic chain, not substituted or substituted by aliphatic or aromatic groups, an aryl group, substituted or unsubstituted, a substituted or unsubstituted aromatic heterocycle, or alternatively R3 and R4 form at least one substituted or unsubstituted ring. 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 a molar excess of the carbonyl compound relative to the benzylic heteroatomic compound is used, preferably an excess such as the ratio of the molar concentrations in the organic solvent between the carbonyl compound. and the benzylic heteroatomic compound is between 1.5 and 10.