Process for the purification of thiophenes

The invention relates to a process for the purification of thiophenes which are liquid at room temperature, the thiophenes purified by this process and their use.

Thiophenes are used, for example, for the production of conductive polymers. Of particular interest here are poly (3,4-alkylenedioxythiophenes), as described, for example, in EP-A 339 340. These connections are characterized by special properties such as high conductivity, high transparency and excellent long-term stability. Therefore, they have found increasing use in industry as organically conductive polymers. For example, important areas of application are: the through-contacting of printed circuit boards, the antistatic equipment of photographic films and the use as an electrode or solid electrolyte in solid electrolytic capacitors are described.

An important prerequisite for the production of the organic conductive polymers is the high purity of the starting materials necessary for their production. Impurities contained in the starting materials can negatively influence the polymerization in that the polymerization does not take place, only very slowly or incompletely, or is accelerated to an uncontrolled extent. As a result, the processing time of these monomers can decrease drastically, so that they can no longer be used in the processing processes.

In addition, the properties of the resulting polymers can also be negatively influenced by, for example, the impurities negatively changing the inherent color of the resulting polymer and thereby deteriorating the transparency which is essential for the use of the polymers, for example as transparent conductive or antistatic coatings.

Impurities that are also capable of polymerization can be incorporated into the polymer and thereby significantly reduce its conductivity. Further disadvantageous effects of impurities can be that the order of the conductive layers can be reduced by impurities, which results in poorer conductivities, that impurities accumulate on the surface of the polymer after the polymerization and thus result in undesired contact resistances, so that the function of the conductive layers Layer is restricted, or that the long-term stability of the conductive polymers is negatively influenced, for example by the impurities initiating the reaction of the conductive polymer with oxygen and thus significantly impairing the properties of the polymer.

Therefore, the raw materials necessary for the production of the organically conductive polymers, which are usually produced from raw materials by chemical reactions, are cleaned before they are used.

A number of cleaning operations are known to the person skilled in the art which are basically suitable for cleaning the monomers for the polymerization to give organic conductive polymers. Such purification methods are, for example, distillation, sublimation, extraction, crystallization, chromatography and adsorption. These cleaning methods have long been known to the person skilled in the art and are described in common textbooks.

Thiophenes which are liquid at room temperature and which are suitable for producing electrically conductive polymers are of particular importance owing to their ease of processing in the liquid form. In the purification of these thiophenes, the person skilled in the art offers the purification methods which are applicable to liquid substances, preferably distillation, which is also carried out on an industrial scale, extraction and chromatography.

The distillative purification of thiophenes as monomers for use in the production of electrically conductive polymers is known, for example, from EP-A 1 142 888. EP-A 1 142 888 teaches that the number and amount of by-products can be reduced by optimized reaction conditions and that, for example, 3,4-ethylenedioxythiophene is available in a purity of up to 97.7%. However, EP-A 1 142 888 further teaches that an additional extraction is necessary for further purification in order to remove water-soluble by-products and to achieve a purity of more than 99%. As a secondary component, ie contamination, occurs predominantly 3,4-dimethoxythiophene in this synthesis of 3,4-ethylenedioxythiophene.

A distillative removal of compounds is only possible if the components to be separated differ significantly in their boiling point, ie more than 1 ° C. The less the boiling points differ, the greater the expenditure on equipment for the separation, so that such separations can no longer be carried out economically. Since substituted thiophenes, such as, for example, alkylenedioxythiophenes, are preferably distilled under reduced pressure, the difference in boiling points is further reduced, which further increases the separation effort.

The purification of 3,4-alkylenedioxythiophenes, in particular 3,4-ethylenedioxythiophene, which are contaminated with 3,4-dimethoxythiophene, is a particular difficulty. For example, the 3,4-dimethoxythiophene obtained in the synthesis of 3,4-ethylenedioxythiophene can only be separated with very great effort owing to the molecular mass which differs by only two units and the very similar structure, which means purification by distillation from a certain point Purity no longer makes it economical. However, 3,4-dimethoxythiophene as an impurity has the disadvantage that it is incorporated into the polymer during polymerization and can thus adversely affect properties of the polymer, for example the conductivity.

The chromatographic purification of thiophenes as monomers for use in the production of electrically conductive polymers is also known. WO-A 02/79295 describes the production of liquid and solid chiral alkylenedioxythiophenes and mentions in examples the purification by chromatography on silicon dioxide. The compounds prepared according to WO-A 02/79295 have a purity of up to 99.7% after purification. However, the chromatographic separation also has disadvantages . Large amounts of solvents are required to carry them out, since the compounds to be separated must be in very dilute form in order to achieve the desired separation effect. Furthermore, the chromatographic separation using simple apparatus cannot be operated continuously, so that only small amounts of the desired purified thiophene are obtained in each case. A continuous separation of large amounts would therefore be associated with an extremely high expenditure on equipment, so that such purification of thiophenes can no longer be carried out economically.

The classic recrystallization, in which thiophenes solid at room temperature are brought into solution at elevated temperature, usually under reflux of the solvent and then recrystallized by cooling, for the purification of thiophenes as monomers for use in the production of electrically conductive polymers is also known and in WO-A 02/79295 described, however, is limited to thiophenes which are solid at room temperature.

A special form of crystallization can also be used for the crystallization of liquid thiophenes. This special form of crystallization, melt crystallization, is described, for example, in N. Wynn, Chem. Engineering (1986), 93 (8), 26-27 and in J. Ulrich and H. C Bülau, Editor (s): Myerson, Allan S. "Handbook of Industrial Crystallization (2nd Edition)" (2002), 161-179. Melt crystallization is essentially based on cooling a liquid substance until a melt is formed, from which only the substance to be purified crystallizes out. After crystallization, the mother liquor, which ideally contains all impurities, is separated off. If necessary, the crystallized material is heated so slightly that impurities adhering to the product can be removed together with part of the then melting material. However, this method is limited to substances or mixtures of substances which contain large amounts of impurities which can be separated off in liquid form. Small amounts of impurities can only be removed economically using this method, since large amounts of the desired compound have to be removed in order to wash out the small amount of impurities. In addition, the melt crystallization is critical with regard to the temperature control and is therefore complex in terms of equipment.

There was therefore still a need for a process for the purification of thiophenes which are liquid at room temperature, in which extremely high purity, preferably of more than 99.9%, is achieved and which does not have the disadvantages described above.

The object of the present invention was therefore to find a less complex process for the purification of thiophenes, by means of which high-purity 3,4-alkylenedioxythiophenes, preferably with a purity of more than 99.9%, can be produced.

The present invention relates to a process for the purification of thiophenes of the general formula (I),<chemistry id="chem0001" num="0001"><img file="EP1518859B1_D0001.tif" /></chemistry>wherein<dl id="dl0001" compact="compact"><dt>R¹ and R²</dt><dd>independently of one another for hydrogen, for an optionally substituted C which is optionally interrupted by 1 to 5 oxygen and / or sulfur atoms₁-C₂₀-Alkyl group or C₁-C₂₀-Oxyalkyl group or together for an optionally substituted C₁-C₂₀-Dioxyalkylene- or C₁-C₂₀-Dioxyarylene group stand,</dd></dl>characterized in that a thiophene of the general formula (I) which is liquid at room temperature is selected and prepared from a solution in a solvent or a solvent mixture or by introduction into a cooled solvent or solvent mixture or into a cooled solution of the same thiophene at a temperature below the melting temperature of the Thiophene is precipitated as a solid and separated.

In the context of the invention, thiophenes which are liquid at room temperature are to be understood as those thiophenes which have their melting point below + 40 ° C., preferably below + 30 ° C.

In the context of the invention, room temperature can be a temperature of 10 to 40 ° C., preferably 15 to 30 ° C., particularly preferably 18 to 25 ° C.

With the process according to the invention, preference is given to thiophenes of the general formula (I) as compounds of the general formula (II)<chemistry id="chem0002" num="0002"><img file="EP1518859B1_D0002.tif" /></chemistry>wherein<dl id="dl0002"><dt>A</dt><dd>for an optionally substituted C₁-C₅Alkylene radical or a C₁-C₁₂Arylene residue, preferably for an optionally substituted C₂-C₃Alkylene radical,</dd><dt>R</dt><dd>for a linear or branched, optionally substituted C₁-C₁₈-Alkylrest, preferably linear or branched, optionally substituted C₁-C₁₄Alkyl radical, an optionally substituted C₅-C₁₂-Cycloalkylrest, an optionally substituted C₆-C₁₄Aryl radical, an optionally substituted C₇-C₁₈Aralkyl radical, an optionally substituted C₁-C₄-Hydroxyalkylrest, preferably optionally substituted C₁-C₂-Hydroxyalkyl radical, or a hydroxyl radical,</dd><dt>x</dt><dd>represents an integer from 0 to 8, preferably from 0 to 6, particularly preferably 0 or 1 and</dd></dl>in the event that several R radicals are bonded to A, these may be the same or different, cleaned up.

The general formula (II) is to be understood such that the substituent R can be bonded to the alkylene or arylene radical A x times.

Preferred compounds of the general formula (II) are those of the general formula (IIa)<chemistry id="chem0003" num="0003"><img file="EP1518859B1_D0003.tif" /></chemistry>wherein<dl id="dl0003" compact="compact"><dt>R</dt><dd>has the meaning given in the general formula (II) and y represents 0, 1, 2, 3 or 4.</dd></dl>C.₁-C₅Alkylene radicals A are methylene, ethylene, n-propylene, n-butylene or n-pentylene in the context of the invention. C.₁-C₁₂Arylene residues A can be, for example, phenylene, naphthylene, benzylidene or anthracenylidene in the context of the invention. C.₁-C₁₈In the context of the invention, alkyl stands for linear or branched C.₁-C₁₈Alkyl residues such as methyl, ethyl, n- or isopropyl, n-, iso-, sec- or tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl, 1, 1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n- Tridecyl, n-tetradecyl, n-hexadecyl or n-octadecyl. C.₁-C₂₀Alkyl groups also include, for example, n-nonadecyl and n-eicosyl. C.₅-C₁₂Cycloalkyl stands for C in the context of the invention₅-C₁₂-Cycloalkyl radicals such as cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl or cyclodecyl, C₅-C₁₄Aryl for C₅-C₁₄Aryl radicals such as phenyl or naphthyl, and C₇-C₁₈Aralkyl for C₇-C₁₈Aralkyl radicals such as benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, 3,5-xylyl or mesityl. C.₁-C₂₀Oxyalkyl stands for C in the context of the invention₁-C₂₀-Oxyalkyl radicals such as methoxy, ethoxy, n- or iso-propoxy, n-, iso-, sec or tert-butoxy, n-pentyloxy, 1-methylbutyloxy, 2-methylbutyloxy, 3-methylbutyloxy, 1-ethylpropyloxy, 1,1 -Dimethylpropyloxy, 1,2-dimethylpropyloxy, 2,2-dimethylpropyloxy, n-hexyloxy, n-heptyloxy, n-octyloxy, 2-ethylhexyloxy, n-nonyloxy, n-decyloxy, n-undecyloxy, n-dodecyloxy, n-tridecyloxy , n-tetradecyloxy, n-hexadecyloxy, n-octadecyloxy, n-nonadecyloxy or n-eicosyloxy. The above list serves to explain the invention by way of example and is not to be regarded as conclusive. Numerous organic groups are suitable as further substituents of the alkylene or arylene radicals A, for example alkyl, cycloalkyl, aryl, halogen, ether, thioether, disulfide, sulfoxide, sulfone, sulfonate, amino , Aldehyde, keto, carboxylic acid ester, carboxylic acid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilane groups and carboxylamide groups.

If the thiophene to be purified has one or more stereocenters, the thiophene can be a racemate, an enantiomerically pure or diastereomerically pure compound or an enantiomerically enriched or diastereomerically enriched compound. An entantiomerically enriched compound is a compound with an enantiomeric excess (ee) of more than 50%. A diastereomer-enriched compound is understood to mean a compound with a diastereomer excess (de) of more than 30%. However, it can also be any mixture of diastereomers.

The thiophene to be purified preferably has a purity of more than 70%, particularly preferably a purity of more than 90%, before cleaning using the process according to the invention.

The thiophenes of the general formulas (I), (II) or (IIa) to be purified can be prepared by processes known to the person skilled in the art. For example, such a manufacturing process is described in EP-A 1 142 888.

Solvents used are those in which the thiophene to be purified dissolves and which have a sufficiently low melting point, preferably below -40 ° C. Examples of its suitable solvents are isobutyl methyl ketone, chloroform, methylene chloride, toluene, methanol, propanol, ethanol, acetone, isopropanol, n-butanol, sec-butanol, dimethylformamide, methyl tert-butyl ether, tetrahydrofuran, diethyl ether, hexane or pentane.

Preferred solvents are polar solvents, and alcohols are particularly preferred. Methanol or ethanol are very particularly preferred.

The solvent can also be a mixture of two or more solvents.

Mixtures of one or more alcohol (s), optionally with one or more further solvents (s), are preferred. It is not absolutely necessary for this that each individual solvent dissolves the thiophene and has a correspondingly low melting point; only the mixture must have these properties. A mixture of two alcohols is particularly preferred, very particularly preferably a mixture of methanol and ethanol.

The solvent is mixed with the thiophene in a ratio of 0.01: 1 to 10: 1, preferably in a ratio of 0.3: 1 to 3: 1 and very particularly preferably in a ratio of 1: 1.

The new procedure is, for example carried out in such a way that the thiophenes to be purified and at least one solvent are combined in any order, the solvent or solvents before the mixture with the thiophene or the solution obtained during or after the mixture is cooled to a temperature at which a mixture of a solid and a Forms liquid, the mixture of a solid and a liquid is stirred if necessary, and then the solid is separated off.

The solvent or solvents are preferably cooled to a temperature which is at least 10 ° C., preferably at least 20 ° C. below the melting temperature of the thiophene to be purified in pure form before being combined with the thiophenes or the solution obtained during or after combining. It is particularly preferably cooled to 0 ° C. or lower, very particularly preferably to -15 ° C. or lower.

The new method can be carried out, for example, in such a way that the thiophene is dissolved in the solvent (s) and then this solution is cooled down at least to such an extent that the purified thiophene precipitates or crystallizes out.

Here, the thiophene can be dissolved in the solvent (s) at a temperature above the melting point of the thiophene. A temperature between 0 ° C and + 40 ° C is preferred. A temperature between + 15 ° C and + 25 ° C is particularly preferred.

The solution obtained from the solvent and the thiophene is then cooled. The solution is cooled until the thiophene separates or crystallizes from the solution in the form of a solid. The solution is preferably cooled to a temperature of at least 20 ° C. below the melting temperature of the pure thiophene. Cooling to -15 ° C. or to a temperature of less than -15 ° C. is particularly preferred.

The solution is preferably cooled down at a rate such that the thiophene crystallizes out within a period of a few minutes to several hours. Cooling down to the desired temperature over a period of approximately one hour is preferred.

The cooling down can be brought about by external cooling or by introducing an inert cooling medium. Cooling down is preferably achieved by external cooling.

During the cooling phase, the thiophene separates from the solution as a solid, for example in the form of crystals. The solid obtained can contain the thiophene as a pure substance or consist of a mixture of the solvent (s) and the thiophene.

Alternatively, the new process can be carried out in such a way that the liquid thiophene is metered into the already cooled solvent, solvent mixture or into a cooled thiophene solution.

Here, the solvent is cooled to a temperature of at least 20 ° C below the melting temperature of the pure thiophene. Cooling to -15 ° C or a temperature lower than - 15 ° C is particularly preferred.

The liquid thiophene is then metered into the cooled solvent, preferably over a period of a few minutes to several hours. The metering rate is to be chosen so that the thiophene does not precipitate or crystallize too quickly and impurities are included in the solid. A metering time of at least 1 hour is preferred. Depending on the amount of thiophene that has to be added, dosing times of less than one hour can be sufficient. The solid obtained can also contain the thiophene as a pure substance or consist of a mixture of the solvent (s) and the thiophene.

The suspension obtained is then preferably stirred for a period of from 1 minute to 5 hours. A stirring time of about three hours is particularly preferred.

The subsequent stirring is carried out at a temperature of at least 20 ° C. below the melting temperature of the pure thiophene. A temperature of -15 ° C or a temperature of less than -15 ° C is preferred.

The precipitated or crystallized product is then separated off by known methods. This separation is preferably carried out by filtration. The filtration can be carried out under normal pressure or under pressure.

The filtration is preferably carried out with the aid of a temperature-regulating filter unit and is carried out in such a way that the product to be filtered is present as a solid during the filtration. The filtration is carried out at a temperature between 0 ° C and -20 ° C. The filtration is preferably carried out at -15 ° C or a temperature lower than -15 ° C.

The solid obtained can then be washed with one or more solvent (s) in order to remove residues of impurities from the filter cake. Polar solvents are preferably used for this. Alcohols are particularly preferably used, if appropriate as a mixture with one another and / or with further solvents. The solid is particularly preferably washed with ethanol or methanol or a mixture of these.

In the event that the filter cake is washed to remove contaminant residues adhering to the filter cake, it is useful to cool the detergent, ie the solvent used for washing, in order to prevent the dissolving of large amounts of purified thiophene in the detergent. The detergent has a temperature of less than 0 ° C during washing. The detergent is preferably cooled down to -15 ° C. or lower for washing.

The solid then obtained is heated to a temperature above the melting point of the thiophene over a period of between 5 minutes and 5 hours. The solid is preferably allowed to melt over a period of 1 hour.

After melting, the molten solid may still contain residues of the solvent added before crystallization or residues of the detergent. These residues can be removed by methods known to those skilled in the art, for example by simple distillation. During the distillation, the solvent is distilled over. The distillation can be carried out at normal pressure or under reduced pressure. It is preferably carried out under reduced pressure at temperatures between 30 ° C. and 150 ° C., preferably between 50 ° C. and 100 ° C.

The thiophene obtained in the sump thus obtained preferably has a purity of at least 99.50%, preferably at least 99.9%, after the solvent has been completely distilled off. For example, thiophenes, which were synthesized using 3,4-dimethoxythiophene or whose synthesis produces 3,4-dimethoxythiophene as a by-product, contain less than 0.05% by weight of 3,4-dimethoxythiophene after purification with the process according to the invention. Such a low 3,4-dimethoxythiophene content cannot be achieved with conventional purification processes, such as simple distillation, or can only be achieved with a very high loss in yield for the desired thiophene.

Unless otherwise stated, all purity data are percentages by weight.

The thiophene remaining in the sump after distillation can also be distilled over to separate color-forming traces. As a rule, this gives a thiophene that is colorless to the eye. The thiophene is also preferably distilled under reduced pressure.

Depending on the amount of solvent used in relation to the amount of thiophene used and depending on the temperature during the precipitation and optionally during washing, for example up to 70%, preferably up to 90%, particularly preferably up to 95% and very particularly preferably almost 100% of the used Thiophene can be obtained in purified form. The possibly remaining part of the thiophene used remains in the mother liquor, ie for example dissolved in the filtrate separated in the filtration, or optionally dissolved in the detergent. Since it is desirable to recover almost 100% of the thiophene used in a purified form, the cleaning method can also be carried out in a preferred embodiment in such a way that the mother liquor from a previous precipitation or crystallization and / or the detergent as solvent or together with the solvent the process for the purification of further thiophene can be used.

The process according to the invention enables the purification of thiophenes in a simple procedure. The products are also obtained in good yield.

Due to their high purity, the thiophenes purified by the process according to the invention are outstandingly suitable for the production of conductive polymers or for the production of organic semiconductors, for example in the production of capacitors, printed circuit boards, antistatic layers, transparent conductive layers, displays, electrochromic glazing and integrated semiconductor circuits are suitable. This use is a further subject of the invention.

The following compounds may be mentioned as examples of compounds which can be purified using the process according to the invention:<ul id="ul0001" list-style="none" compact="compact"><li>3,4-ethylenedioxythiophene, 3,4-methylenedioxythiophene; R, S-3,4- (1'-hydroxymethyl) ethylenedioxythiophene, S-3,4- (1'-hydroxymethyl) ethylenedioxythiophene; R-3,4- (1'-hydroxymethyl) ethylenedioxythiophene, 3,4- (2'-hydroxy) propylenedioxythiophene; 3,4- (1'-methyl) ethylenedioxythiophene, 3,4- (3'-tert-butyl) benzodioxythiophene; 3,4- (1'-n-hexyl) ethylenedioxythiophene; 3,4- (1'-ethyl) ethylenedioxythiophene; 3,4- (1'-n-propyl) ethylenedioxythiophene; 3,4- (1'-butyl) ethylenedioxythiophene; Thieno [3,4-b] -1,4-oxathiine; 3,4-ethylenedioxythiophene-1-methyl-N-methyl carbamate; 3,4-ethylenedioxythiophene-1-methyl-N-ethyl carbamate; 3,4-ethylenedioxythiophene-1-methyl-N-hexyl carbamate; 3,4-ethylenedioxythiophene-1-methyl-N-phenylcarbamate; 3,4-ethylenedioxythiophene-1-methyl-N-tolylcarbamate; (3,4-ethylenedioxythiophene-1-methyl) methyl ether; (3,4-ethylenedioxythiophene-1-methyl) ethyl ether; (3,4-ethylenedioxythiophene-1-methyl) propyl ether; (3,4-ethylenedioxythiophene-1-methyl) hexyl ether; 3-hexylthiophene; 3 octylthiophene</li></ul>

<u style="single">Examples</u>

Example 1:

Purification of 3,4-ethylenedioxythiophene

1800 g of 3,4-ethylenedioxythiophene with a purity of 98.4% and a content of 3,4-dimethoxythiophene of 0.3% and a slightly yellowish color were stirred in a sulfonation beaker with 2400 ml of ethanol. The solution was cooled down to a temperature of -15 ° C. by external cooling and stirred at -15 ° C. for 3 h. The resulting solid was separated off using a suction filter and washed with ethanol pre-cooled to -15 ° C. The filter cake was heated to a temperature of + 20 ° C. In a distillation apparatus consisting of a receiver flask, a distillation bridge and a condensation flask, the solvent was first distilled off at a pressure of 16 hPa and a temperature of 50 ° C. and then at a temperature of 90 ° C. and a pressure of 16 hPa 3, Distilled 4-ethylenedioxythiophene. 1374 g of 3,4-ethylenedioxythiophene (76% of theory) were obtained in a purity of 100%. The colorless product no longer had 3,4-dimethoxythiophene.

Example 2:

Purification of 3,4-ethylenedioxythiophene

1800 g of 3,4-ethylenedioxythiophene with a purity of 70% and a content of 3,4-dimethoxythiophene of 0.3% and dark brown color were stirred in a sulfonation beaker with 1800 g of ethanol. The solution was cooled down to a temperature of -23 ° C. by external cooling and stirred at -23 ° C. for 3 h. The resulting solid was separated off using a suction filter and washed with ethanol pre-cooled to -15 ° C. The separated filter cake was heated to a temperature of + 20 ° C. In a distillation apparatus consisting of a receiver flask, a distillation bridge and a condensation flask, the solvent was first distilled off at a pressure of 12 h Pa and a temperature of 50 ° C and then at a temperature of 90 ° C and a pressure of 12 hPa 3 , 4-ethylenedioxythiophene distilled. 718 g of 3,4-ethylenedioxythiophene (55% of theory) were obtained in a purity of 99.2%. The product no longer had 3,4-dimethoxythiophene.