EP0005406B1 - Electrochemical catalytic system and process for the electrochemical selective dimerisation of conjugated diolefins - Google Patents

Description

The present invention relates to a process for the selective dimerization of conjugated diolefins by electrochemical means. Using this process, the conjugated diolefins will exclusively give dimers with a cyclohexene structure. Thus, 1,3-butadiene will lead only to vinyl-4-cyclohexene and isoprene to a mixture of isomers with a cyclohexene structure: limonenes (methyl-isopropenylcy-clohexenes) and dimethylvinylcyclohexenes.

It is known to selectively dimerize the conjugated diolefins chemically, using catalysts or catalytic systems based for example on dinitrosylfer chloride. Thus, French patent n ° 1 502141 teaches the use of a catalyst composed of a dinitrosylfer halide, a reducing agent (organo-magnesian, organo-aluminum) and a third component (arsine, stibine, phosphine , ether, sulfide, nitrogen derivative). US Pat. Nos. 3,655,793 and 3,767,593 describe as catalyst the dinitrosylfer halide / reducer couple (organo-aluminum, borohydride, aluminum hydride). French Patent No. 2,225,401 describes the use of the dinitrosylfer halide / carbonyl metal catalytic couple. French Patent No. 2,217,299 describes the use of a catalyst comprising ferrous or ferric compounds (for example iron chloride), organo-aluminum compounds and nitrogen oxide NO.

It is also known, from French Patent No. 2,080,556, that one can trimerize butadiene to 1,5,9-cyclododecatriene or dimerize this butadiene or 1,5-cyclooctadiene and / or into linear dimer, by electrochemical, using a system comprising an oxidizable metal anode (for example aluminum), an iron or platinum cathode, an inert electrochemical solvent (for example propylene carbonate) and an electrolyte containing a compound chlorinated iron (in particular iron chloride), forming in situ transition metal complexes and operating at constant voltage and electrolysis intensity. No electrochemical conditions are taught to selectively dimerize butadiene to 4-vinylcyclohexene.

However, it may be advantageous to have an electrochemical dimerization process which avoids the manipulation of compounds, for the most part toxic, which requires little energy and which selectively leads to the dimer of cyclohexene structure sought.

The Applicant has found that these aims could be achieved provided that the dimerization of a conjugated diolefin is carried out in an electrochemical cell with an imposed potential between cathode and reference electrode, determined as will be seen in the following description, in the presence of dinitrosylfer chloride in very small quantities, or in the presence of iron chloride and nitrogen monoxide NO, also in very small quantities.

The reaction is carried out in the presence of an adequate solvent and optionally of a conductive salt and, if one operates on a concentrated solution of diolefin to be dimerized, the reaction mixture is separated into two phases, the upper phase not containing practically than the dimer sought.

The subject of the present invention is an electrochemical catalytic system comprising an oxidizable metal anode, a cathode of a compound chosen from platinum, iron and vitreous carbon, an inert electrochemical solvent and an electrolyte comprising a chlorinated iron compound, characterized in that the chlorinated iron compound is either a dinitrosylfer chloride or a mixture of iron chloride and nitrogen monoxide NO.

The present invention also relates to a process for the selective dimerization of conjugated diolefins into dimers having a cyclohexene structure, characterized in that one operates electrochemically by bringing into contact, in an electrochemical cell comprising an anode of oxidizable metal, a cathode and a reference electrode: the diolefin to be dimerized, a compound chosen from dinitrosylfer chloride and the mixture of iron chloride and nitrogen monoxide NO, a solvent inert with respect to the reaction medium, and optionally a conductive salt , then by bringing the cathode, relative to the reference electrode, to a potential corresponding to the last wave of reduction of said compound and by keeping this potential constant throughout the duration of the reaction.

Preferably the potential is kept constant by means of a potentiostat which automatically supplies the voltage and the current between anode and cathode.

A conductive salt (electrolyte) can be used, if desired, which makes it possible to work, initially, with a lower voltage between anode and cathode.

The electrochemical cell therefore has 3 electrodes: a cathode, an anode and a reference electrode. The cathode is made of a material inert with respect to the reaction medium; a platinum cathode is preferably used and better still the cathode is a grid of intertwined platinum wires. The cathode can also be made of glassy carbon or iron. The anode is made of oxidizable metal; aluminum was preferably chosen in the case of the use of dinitrosylfer chloride, and preferably iron in the case of the use of the iron chloride + NO mixture. According to a preferred embodiment of the invention, the anode is a hollow aluminum or iron cylinder and the cathode a cylindrical grid made of wires and an external plate concentric with the anode. We have noticed that it was useless to separate anode and cathode by compartmentalization.

The reference electrode can be chosen from the electrodes known as the Ag / AgCl electrodes. Ag / Ag ⁺ , Hg / Hg ₂ C1 ₂ (calomel). It will preferably be placed near the cathode.

The electrochemical solvent must be inert both with respect to the reactants present and under the operating conditions adopted. It is preferable that it has a sufficiently high dielectric constant. Preference is given to propylene carbonate.

If a conductive salt (electrolyte) is used, it must be soluble in the solvent used. It can be a lithium salt or a quaternary ammonium salt. Tetrabutylammonium perchlorate is ideal. Preferably, and in order to minimize the junction potential, it is possible to add to the reference electrode the same conducting salt as that which is dissolved in the reaction medium, and at the same concentration (except in the case of the electrode at calomel which will be used as is).

The Applicant has found that the equally high yields can be obtained, in the case where the reaction medium does not contain a conductive salt, by adding (except in the case of the calomel electrode) or not of the conductive salt in the lipid. junction of the reference electrode.

les radicaux R, R₁, R₂, R₃, R₄ et R₅ étant tous des hydrogènes ou bien partiellement des hydrogènes et partiellement des radicaux alkyle. On utilise par exemple le butadiène-1,3, l'isoprène, le pipérylène. Ces produits sont à l'état pur ou bien compris dans une fraction pétrochimique ; par exemple le butadiène-1,3 peut être contenu dans une fraction appelée « coupe C4 » provenant du craquage à la vapeur de naphta. Cette coupe C4 peut être soumise telle quelle à dimérisation suivant le procédé de l'invention. Bien évidemment seul le butadiène sera dimérisé, les autres constituants demeurant intacts et pouvant être séparés en fin de réaction, du vinylcyclohexène formé, par exemple par distillation.The general formula of the diolefin to be dimerized is:

the radicals R, R ₁ , R ₂ , R ₃ , R ₄ and R ₅ being all hydrogen or else partially hydrogen and partially alkyl radicals. For example, 1,3-butadiene, isoprene and piperylene are used. These products are pure or well included in a petrochemical fraction; for example butadiene-1,3 may be contained in a fraction called “C4 cut” originating from the steam cracking of naphtha. This C4 cut can be subjected as such to dimerization according to the method of the invention. Obviously only the butadiene will be dimerized, the other constituents remaining intact and being able to be separated at the end of the reaction from the vinylcyclohexene formed, for example by distillation.

ou par réaction de NO sur le mélange Fe + FeC1₃ dans le tétrahydrofuranne. Le produit obtenu peut, si on le désire, être purifié par une seconde sublimation.Dinitrosylfer chloride is prepared by any known means, in particular according to the reaction:

or by reaction of NO with the Fe + FeC1 ₃ mixture in tetrahydrofuran. The product obtained can, if desired, be purified by a second sublimation.

The iron chloride is either ferrous chloride FeCl ₂ or ferric chloride FeCl ₃ ; if necessary, these products are dried before use. Nitrogen monoxide NO is preferably used in the gaseous state, obtained from liquefied gas delivered commercially in bottles. The Applicant has found that it was preferable to operate with numerous ratios of moles of NO of between 2 and 12, the number of gram atoms of _Fe including yields being improved in this case.

The Applicant has found that any condition of voltage and electrolysis current cannot be used successfully. Thus, a comparative example will be described below where the rate of transformation of isoprene into its dimers with cyclohexene structure (selectivity: 100%) is only 3% after 5 hours. It has been found that, for the process according to the invention to lead to high yields, the electrochemical dimerization had to be carried out at controlled potential. The plotting of the voltammetric curve of the mixture solvent + dinitrosylfer chloride, or solvent + iron chloride + NO, with or without conducting salt, using a conventional apparatus, shows that there are two waves of reduction in the case of Fe (NO) ₂ CI and FeCl ₂ + NO and three in the case of FeCI ₃ + NO. And it was found that the method according to the invention had to be implemented while keeping the potential of the cathode constant with respect to the reference electrode at a value situated on the bearing of the last reduction wave, whatever or the reference electrode used, it being understood that the electrode used in the process is of the same nature as that which served to draw the reduction curve.

Thus, if Fe (NO) ₂ Cl is used and an Ag / AgCI reference electrode, the potential of the cathode, relative to the reference, must be - 100 to - 400 mV; with an Ag / Ag ⁺ reference electrode it must be - 1,500 to - 1,800 mV and with a calomel reference electrode it must be - 700 to - 1,000 mV, always with Fe (NO) ₂ CI.

In the case of the use of the Ag / AgCl reference electrode with ferric chloride FeCl ₃ and NO, the potential of the cathode with respect to said reference electrode will be between - 600 and - 1600 mV.

To keep the potential of the cathode constant with respect to the reference electrode, a potentiostat is used which, in a manner known per se, delivers a working voltage and current, between anode and cathode, functions of the chosen reference potential. and functions of the characteristics of the medium (dielectric constant of the solvent, resistivity of the reaction mixture).

Thus, at the start of the reaction, the potentiostat generates, after the reference potential has been set, a current, between anode and cathode, which has been found to be a function of the concentration of iron compound. In the case of dinitrosylfer chloride it is between 40 and 100 milliamps for concentrations of Fe (NO) ₂ CI between 5 and 40 millimoles per liter, under a voltage which has been found to be an inverse function of the concentration of conductive salt and close to 2 to 5 volts for tetrabutylammonium perchlorate concentrations close to 0.16 to 0.12 mol per liter of mixture reactive. These intensity and voltage values correspond to a distance between anode and cathode of approximately 1 cm.

It goes without saying that one can, if desired, reduce the concentration of conductive salt. The voltage between anode and cathode will have a higher value than those indicated above. In the absence of conductive salt, the starting voltage between anode and cathode is around 20 to 35 volts. Also, the Fe (NO) ₂ Cl concentration can be reduced, which will have the consequence of reducing the value of the initial intensity. It is preferred to use a concentration of Fe (NO) ₂ CI such that one works at the start with an intensity of 40 to 150 mA.

It is also possible, if desired, to modify the distance between anode and cathode.

In all cases, the values of the intensity and of the working voltage decrease as a function of time, reaching in a few hours the respective values of a few hundred microamperes and a few millivolts, in the case of tests in the presence of salt. conductor, and a few milliamps and a few volts in the absence of conductive salt, the reference potential always being at its value imposed at the start.

The process is preferably carried out at ambient temperature and under the pressure corresponding to the vapor pressure of the reaction medium at the temperature considered. This pressure does not generally exceed 1 bar.

It is imperative to operate in the absence of oxygen.

Finally, the reaction medium must be constantly agitated so as to accelerate the diffusion phenomena. The agitation can be produced mechanically, in a manner known per se. It is also possible to renew the liquid layer in contact with the working electrodes using a rotating anode and cathode.

The nonlimiting examples below are intended to illustrate the invention.

The process is carried out in an electrochemical glass cell able to work under a maximum pressure of 1 bar and which includes inlet and outlet pipes for the products, magnetic stirring, a hollow cylindrical anode with a diameter of approximately 2 cm and of height about 6 cm, a cathode concentric with the anode of the same height and 4 cm in diameter, a reference electrode placed as close as possible to the cathode.

In the case of butadiene, the raw material is cooled to -10 ° C. before introducing it into the electrochemical cell. The temperature of the reaction medium is then allowed to rise to ambient (about 20 ° C).

Examples 1 to 10 Use of Fe (NO) ₂ CI; aluminum anode; platinum cathode (grid).

Example 1

• 100 ml de carbonate de propylène,
• 6,8 g (0,02 mole) de perchlorate de tétrabutylammonium,
• 24,5 g (0,45 mole) de butadiène-1,3,
• 130 mg (0,86 millimole) de chlorure de dinitrosylfer, exprimé en Fe(NO)₂Cl.

In the electrochemical cell (Ag / AgCI reference electrode):

• 100 ml of propylene carbonate,
• 6.8 g (0.02 mole) of tetrabutylammonium perchlorate,
• 24.5 g (0.45 mole) of 1,3-butadiene,
• 130 mg (0.86 millimole) of dinitrosylfer chloride, expressed as Fe (NO) ₂ Cl.

Using the potentiostat, a potential of - 100 mV is applied to the cathode, relative to the reference electrode. The potentiostat automatically generates a voltage of 4 V and a current of 40 mA between anode and cathode. These values become, after 20 minutes: 2 V and 25 mA and after 3 hours a few tenths of a volt and 1 mA.

After these 3 hours, 85% of the starting butadiene have been selectively transformed into vinyl-4 cyclohexene (ehromatographic assay).

Example 2

• 100 ml de carbonate de propylène,
• 6,8 g (0,02 mole) de perchlorate de tétrabutylammonium,
• 54 g (1 mole) de butadiène-1,3,
• 102 mg (0,67 millimole) de Fe(NO)₂CI.

In the electrochemical cell (reference electrode Ag / Ag ⁺ ):

• 100 ml of propylene carbonate,
• 6.8 g (0.02 mole) of tetrabutylammonium perchlorate,
• 54 g (1 mole) of 1,3-butadiene,
• 102 mg (0.67 millimole) Fe (NO) ₂ CI.

Using the potentiostat, a potential of - 1,500 mV is applied to the cathode relative to the reference electrode. The potentiostat automatically maintains this constant potential throughout the duration of the test. At the start, the working voltage is 2 V and the intensity of 40 mA. The reaction is stopped after 5 hours. The reaction medium forms two phases. Chromatographic analysis of the upper phase shows that it contains 17.3% by weight of butadiene and 82.7% by weight of vinylcyclohexene.

The lower phase contains, in addition to propylene carbonate and the conducting salt: 5.5% by weight of butadiene and 5.7% by weight of vinylcyclohexene. The conversion rate of butadiene into vinylcyclohexene is 75%. The reactor is cooled to -10 ° C., the upper phase is drawn off, the butadiene is distilled and rectifies the yinylcyclohexene formed. 36 g of chromatographically pure vinyl-4-cyclohexene are thus collected.

Example 3

• 110 ml d'une solution 0,2 molaire de perchlorate de tétrabutylammonium dans le carbonate de propylène,
• 20 g d'une fraction provenant du craquage à la vapeur de naphta et appelée « coupe C4 », contenant 50 % en poids de butadiène-1,3 (les autres constituants sont du méthyl-2 propène, du butène-1, les butènes-2 cis et trans et du butane),
• 273 mg (1,8 millimole) de Fe(NO)₂Cl.

In the electrochemical cell (reference electrode to calomel):

• 110 ml of a 0.2 molar solution of tetrabutylammonium perchlorate in propylene carbonate,
• 20 g of a fraction originating from the vapor cracking of naphtha and called “C4 cut”, containing 50% by weight of 1,3-butadiene (the other constituents are 2-methyl propene, 1-butene, butenes -2 cis and trans and butane),
• 273 mg (1.8 millimole) Fe (NO) ₂ Cl.

Using a potentiostat, a potential of - 750 mV is applied to the cathode relative to the reference electrode. The potentiostat automatically generates, at time zero, a voltage between anode and cathode, of 2 V and a current of 80 mA.

After 5 h of reaction, the voltage has become negligible (the potential of the cathode relative to the reference electrode is always equal to - 750 mV) and the current is approximately 2 mA.

Chromatographic analysis of the reaction medium shows that it contains 5.5 g of vinylcyclohexene and no other product formed.

In this case, the selective dimerization of 1,3-butadiene was carried out with a conversion rate of 1,3-butadiene of 55%. The other constituents of the "C4 cup are unaltered.

Example 4

• 110 ml d'une solution 0,2 molaire de perchlorate de tétrabutylammonium dans le carbonate de propylène,
• 44,4 g (0,653 mole) d'isoprène,
• 485 mg (3,2 millimoles) de Fe(NO)₂CI.

In the electrochemical cell (Ag / AgCI reference electrode):

• 110 ml of a 0.2 molar solution of tetrabutylammonium perchlorate in propylene carbonate,
• 44.4 g (0.653 mole) of isoprene,
• 485 mg (3.2 millimoles) of Fe (NO) ₂ CI.

Using a potentiostat, a potential of - 100 mV is applied to the cathode relative to the reference electrode. The potentiostat generates between initial cathode and anode an initial current of approximately 100 mA under a voltage of approximately 5 V.

After 5 h, the reaction is stopped. The reaction medium occurs in two phases. The chromatographic analysis of these two phases shows that the isoprene is selectively transformed into its dimers with cyclohexenic structure (the product formed contains 88% of the two dimethylvinylcyclohexenes and 12% of the two limonenes) with a conversion rate of 50%.

The upper phase contains 15% isoprene and 85% dimers which can be purified by distillation and rectification.

Example 5

The conditions of Example 4 are repeated using 33.3 g (0.49 mole) of isoprene.

The working voltage is initially 3 V and the current 70 mA.

After 20 h, the conversion rate of isoprene into its dimers with cyclohexene structure is 67%.

Example 6 (comparative)

The conditions of Example 4 are repeated, but by applying to the cathode a potential of + 200 mV relative to the reference electrode Ag / AgCI. This value corresponds to the first wave of reduction that can be determined voltammetrically on the mixture of propylene carbonate + tetrabutylammonium perchlorate + Fe (NO) ₂ Cl (Ag / AgCI reference electrode).

After 5 h of reaction, the conversion rate of isoprene into its dimers is only 3%.

Example 7

• 100 ml de carbonate de propylène,
• 20,4 g (0,3 mole) d'isoprène,
• 197 mg (1,3 millimole) de Fe(NO)₂Cl.

The following are placed in the electrochemical cell (Ag / AgCI reference electrode, the junction liquid of which contains 0.2 mole / liter of tetrabutylammonium perchlorate):

• 100 ml of propylene carbonate,
• 20.4 g (0.3 mole) of isoprene,
• 197 mg (1.3 millimole) Fe (NO) ₂ Cl.

Using the potentiostat, a potential of -100 mV is imposed on the cathode relative to the reference electrode. The potentiostat generates an initial voltage and current of 35 V and 90 mA. We observe, after 1 h 30 min, the appearance of a second liquid phase. The reaction is stopped after 3 h and the rate of conversion of isoprene to its dimers with cyclohexene structure is 99 _% .

Example 8

The conditions of Example 7 are repeated without introducing tetrabutylammonium perchlorate into the reference electrode and using 144.7 mg (0.96 millimole) of Fe (NO) ₂ Cl.

Initially, the voltage and current between anode and cathode are 30 V and 68 mA respectively.

After 3 h of reaction, the transformation rate of isoprene is 97%.

Example 9

• 70 ml de carbonate de propylène,
• 40,8 g (0,6 mole) d'isoprène,
• 341 mg (2,25 millimoles) de Fe(NO)₂Cl.

In the electrochemical cell (Ag / AgCI reference electrode not containing tetrabutylammonium perchlorate):

• 70 ml of propylene carbonate,
• 40.8 g (0.6 mole) of isoprene,
• 341 mg (2.25 millimoles) of Fe (NO) ₂ Cl.

From the outset, the mixture was heterogeneous.

After a potential of -100 mV relative to the reference electrode has been imposed on the cathode, the potentiostat generates an initial current between anode and cathode of 135 mA at an initial voltage of 34 V.

The reaction is stopped after 3 p.m. at the end of this time, the transformation rate of isoprene is 99%.

Example 10

• 80 ml de carbonate de propylène,
• 59 g (1,09 mole) de butadiène-1,3,
• 100 mg (0,66 millimole) de Fe(NO)₂Cl.

In the electrochemical cell (Ag / AgCI reference electrode not containing tetrabutylammonium perchlorate):

• 80 ml of propylene carbonate,
• 59 g (1.09 mole) of 1,3-butadiene,
• 100 mg (0.66 millimole) of Fe (NO) ₂ Cl.

A potential of - 100 mV is applied to the cathode relative to the reference electrode, using the potentiostat which automatically generates an initial current between anode and cathode of 44 mA at a voltage of 20 V.

After 1 h 30 min, the appearance of a second liquid phase is observed. The reaction is stopped after 3 h and the two liquid phases are separated. The upper phase contains 92% by weight of vinyl-4-cyclohexene and 8% by weight of butadiene. The lower phase contains, in addition to propylene carbonate, 12% of vinyl-4-cyclohexene and 3% of butadiene.

The conversion rate of 1,3-butadiene to vinyl-4-cyciohexene is 90%.

The selectivity of this transformation is, as in all the other examples, 100%.

Examples 11 to 22

In all the examples given in the table below, the following were used: FeCl ₃ and nitrogen monoxide, and a reference electrode Ag / AgCI and the cathode was brought, with respect to said reference electrode, to a potential - 700 mV kept constant by means of a potentiostat which delivers the working current under the working voltage between anode and cathode, values appearing in the table below. The intensity at the end of the operation is also shown in this table.

In all of these examples, the solution of FeCl ₃ in propylene carbonate was prepared in advance. This solution is put in place in the electrochemical cell, the diene (or the petrochemical cut containing it) is added; optionally, the electrolyte; the medium is purged with nitrogen then the nitrogen monoxide is introduced and the potentiostat is adjusted. We operate at room temperature.

The reaction is stopped after a time appearing in the table, time after which the corresponding degree of conversion into dimer is obtained (the selectivity is always 100%, which means that higher rates of conversion could be obtained in increasing the reaction time).

(See table p. 7)

Claims (19)

1. Electrochemical catalytic system comprising an anode of oxidizable metal, a cathode of a compound chosen from platinum, iron and vitreous carbon, an inert electrochemical solvent and an electrolyte comprising a chlorinated iron compound, characterised in that the chlorinated iron compound is either a dinitrosyllron chloride, or a mixture of iron chloride and nitrogen monoxide NO.

2. Method for the selective dimerlsation of conjugated diolefins into dimers having a cyclohexenic structure, characterised In that one proceeds by the electrochemical method by combining, in an electrochemical cell comprising an anode of oxidizable metal, a cathode and a reference electrode : the diolefin to be dimerlsed, a compound chosen from dinitrosyliron chloride and the mixture of iron chloride and nitrogen monoxide NO, a solvent which is inert with respect to the reaction medium and possibly a conducting salt, then by bringing the cathode, with respect to the reference electrode, to a potential corresponding to the last reduction wave of said compound and by keeping this potential constant throughout the duration of the reaction.

3. Method according to claim 2, characterised in that a potentiostat which automatically provides the voltage and the current between the anode and cathode is used for keeping the potential of the cathode constant with respect to the reference electrode..

4. Method according to claim 2, comprising the use of the mixture of iron chloride and nitrogen monoxide NO, characterised in that the ratio of the number of moles of NO to the number of gramme- atoms of Fe, coming from the iron chloride, present in the reaction medium, is comprised between 2 and 12.

5. Method according to claim 2, characterised in that the iron chloride used is ferric chloride FeCl₃.

6. Method according to claim 2, characterised in that the iron chloride used is ferrous chloride FeCl₂.

7. Method according to one of claims 2 to 6, characterised in that the anode is of aluminium.

8. Method according to one of claims 2 to 6, characterised in that the anode is of iron.

9. Method according to one of claims 2 to 8, characterised in that the cathode is of platinum.

10. Method according to one of claims 2 to 8, characterised in that the cathode is of iron.

11. Method according to one of claims 2 to 8, characterised in that the cathode is of vitreous carbon.

12. Method according to one of claims 2 to 11, characterised in that the solvent which is inert with respect to the reaction medium is propylene carbonate.

13. Method according to claim 2, comprising the use of dinitrosyliron.chloride, characterised in that the reference electrode is an Ag/AgCI electrode and the potential applied to the cathode, with respect to said reference electrode, is comprised between - 100 and - 400 mV.

14. Method according to claim 2, comprising the use of dinitrosyliron chloride, characterised in that the reference electrode is an Ag/Ag⁺ electrode and the potential applied to the cathode, with respect to said reference electrode, is comprised between - 1 500 and - 1 800 mV.

15. Method according to claim 2, comprising the use of dinitrosyliron chloride, characterised in that the reference electrode is a calomel electrode and the potential applied to the cathode, with respect to said reference electrode, is comprised between - 700 and - 1 000 mV.

16. Method according to claim 2, comprising the use of the mixture of ferric chloride and nitrogen monoxide, characterised in that the reference electrode is an Ag/AgCI electrode and the potential applied to the cathode, with respect to said reference electrode, is comprised between - 600 and - 1 600 mV.

17. Method according to one of claims 2 to 16, characterised in that the reference electrode contains a conducting salt.

18. Method according to one of claims 2 to 17, characterised in that the conducting salt is tetrabutylammonium perchlorate.

19. Method according to one of claims 2 to 18, caracterised in that the quantities of solvent and dioiefins used are such that at the beginning of the reaction, said diolefin is not completely dissolved in said solvent..