ELECTROCHEMICAL OXIDATION OF CYCLOALKENES AND CYCLOALKANES INTO a,w-DICARBOXYLIC ACIDS OR INTO KETOCARBOXYLIC ACIDS AND CYCLOALKANONE COMPOUNDS

Abstract

The invention relates to a method for producing unsubstituted or at least monosubstituted α,ω-dicarboxylic acids or ketocarboxylic acids and unsubstituted or at least monosubstituted cycloalkanones by electrochemically oxidizing unsubstituted or at least monosubstituted monounsaturated or polyunsaturated cycloalkenes and unsubstituted or at least monosubstituted saturated cycloaliphatic hydrocarbons in the presence of an inorganic or organic nitrate salt in an electrolysis cell in a reaction medium in the presence of oxygen.

Description

[0001] Electrochemical oxidation of cycloalkenes and cycloalkanes to a,w-dicarboxylic acids or ketocarboxylic acids and cycloalkanone compounds

[0002] The invention relates to a process for the preparation of unsubstituted or at least monosubstituted α,ω-dicarboxylic acids or ketocarboxylic acids and unsubstituted or at least monosubstituted cycloalkanones by electrochemical oxidation of unsubstituted or at least monosubstituted, mono- or polyunsaturated cycloalkenes and unsubstituted or at least monosubstituted, saturated cycloaliphatic hydrocarbons in the presence of an inorganic or organic nitrate salt in an electrolysis cell in a reaction medium in the presence of oxygen. α,ω-Dicarboxylic acids, ketocarboxylic acids, and cycloalkanone compounds are important starting materials for organic synthetic chemistry and monomer building blocks for polymer syntheses and are therefore highly relevant for industrial applications.Conventional access to these substrates is achieved mainly from cycloalkanes and cycloalkenes via transition metal-catalyzed reactions and using chemical oxidants.

[0003] A method for the electrochemical oxidation of cycloalkanes to the corresponding ketones has not yet been described. Only a few examples of the electrochemical, oxidative double bond cleavage of cyclooctene and cyclododecene to form dicarboxylic acids or their methyl esters are known (II. Baumer, Electrochimica Acta 2003, 48, 489-495; II.-St. Bäumer, HJ Schäfer, J. Appl. Electrochem. 2005, 35, 1283-1292; DD Davis, DL Sullivan, Process for the Preparation of Dodecanedionic Acid, 1991 and US 5026461 A). The synthesis of dicarboxylic acids by oxidative double bond cleavage starts from pure cycloalkenes.

[0004] Furthermore, in these state-of-the-art processes, the synthesis often leads to the corresponding carboxylic acid esters, so that a further hydrolysis step is necessary to generate the free carboxylic acids, which requires additional time and resources.

[0005] The use of expensive transition metals as electrocatalysts or electrode materials, as well as the application of chemical oxidants, leads to reagent waste due to the increased material input, some of which must be disposed of or regenerated at great expense. Furthermore, the processes require a high overall material input due to the use of complex electrolyte systems and additional oxidants, which has an overall negative impact on the cost balance and economic viability of the processes.

[0006] It is an object of the invention to provide a sustainable and resource-saving process which enables the production of a,w-dicarboxylic acids and cycloalkanones.

[0007] This problem was solved by the subject matter of the patent claims and the description.

[0008] The present invention relates to a process for the preparation of unsubstituted or at least mono-substituted a,w-dicarboxylic acids or ketocarboxylic acids and unsubstituted or at least mono-substituted cycloalkanones by electrochemical oxidation comprising the process steps:

[0009] (a-1) providing at least one unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene;

[0010] (a-2) providing at least one unsubstituted or at least monosubstituted saturated cycloaliphatic hydrocarbon;

[0011] (b) providing at least one inorganic or organic nitrate salt;

[0012] (c) electrochemical oxidation of the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene provided in step (a-1) and the unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon provided in step (a-2) in the presence of the inorganic or organic nitrate salt provided in step (b) in an electrolysis cell in a reaction medium in the presence of oxygen.

[0013] Surprisingly, it was found that, in the process according to the invention, cyclic alkenes can be electrochemically oxidized in the presence of cyclic alkanes of the same ring size to α,ω-dicarboxylic acids or ketocarboxylic acids. Thus, the process according to the invention makes it possible to convert industrially produced cycloalkenes, which often contain a certain proportion of cycloaliphatic hydrocarbons, to α,ω-dicarboxylic acids. Cyclic ketones are obtained as further products, which can also be used in industrial applications.

[0014] The method according to the invention thus enables the simplification of industrially relevant processes and further leads to a possible process optimization under sustainable aspects.

[0015] The present invention enables a resource-efficient, synthetically relevant oxo-functionalization of basic chemicals, largely avoiding the use of environmentally harmful transition metals and oxidants. The selective conversion to the desired products and the effective use of the conductive salt and mediator in a dual function thus significantly reduce the generation of costly reagent waste. The present invention allows electrochemical synthetic access to aliphatic α,ω-carboxylic acids, ketocarboxylic acids, and cycloalkanones using an effective, convergent electrolysis, in which both electrode reactions fulfill a synthetic purpose.

[0016] The process according to the invention is characterized in particular by high selectivity, low use of auxiliary chemicals, the use of electric current as an oxidizing agent and, associated with this, by a reduced generation of waste products.

[0017] It has further been surprisingly found that the process according to the invention can be carried out under ambient pressure and ambient temperature, which also has an advantageous effect on energy efficiency and thus also on environmental compatibility.

[0018] In the process according to the invention, unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkenes that are monocyclic or bicyclic can be used. Preference is given to using unsubstituted or at least mono-substituted, mono- or polyunsaturated monocyclic cycloalkenes, with unsubstituted or at least mono-substituted, monounsaturated monocyclic cycloalkenes being particularly preferred. The position of the unsaturated bonds can be endocyclic or exocyclic, with endocyclic, unsaturated bonds being preferred. The unsubstituted or at least mono-substituted, mono- or polyunsaturated monocyclic cycloalkenes used in the process according to the invention can preferably have 5 to 12 C atoms, more preferably 6 to 12 C atoms, most preferably 8 to 12 C atoms in the ring system.These cycloalkenes can be mono- or polyunsaturated, with monounsaturated cycloalkenes being preferred. These cycloalkenes can each be unsubstituted or mono- or polysubstituted. If mono- or polysubstituted, they are preferably substituted by 1, 2, 3, 4, or 5 substituents, independently of one another, each selected from the group consisting of methyl, phenyl, or benzyl. The phenyl or benzyl substituents can themselves each be unsubstituted or mono- or polysubstituted by 1, 2, or 3 substituents, independently of one another, each selected from the group consisting of F, Cl, Br, and NO2.

[0019] The unsubstituted or at least mono-substituted, mono- or polyunsaturated bicyclic cycloalkenes used in the process according to the invention can preferably have 7 to 18 C atoms, more preferably 7 to 12 C atoms, most preferably 7 to 10 C atoms in the ring system. These cycloalkenes can be mono- or polyunsaturated, with monounsaturated cycloalkenes being preferred. These cycloalkenes can each be unsubstituted or mono- or polysubstituted. If they are mono- or polysubstituted, they are preferably substituted by 1, 2, 3, 4 or 5 substituents, independently of one another, each selected from the group consisting of methyl, phenyl or benzyl.Benzyl substituents may themselves be unsubstituted or mono- or polysubstituted with 1, 2 or 3 substituents, independently of one another, each selected from the group consisting of F, Cl, Br, and NO2.

[0020] If the mono- or bicyclic cycloalkenes or their substituents used according to the invention have alkyl radicals with more than one carbon atom in the side chain, undesirable side reactions occur at these substituents when carrying out the process according to the invention.

[0021] Most preferably, the monocyclic cycloalkene can be selected from the group consisting of cyclohexene, cycloheptene, cyclooctene, cyclononene, cyclodecene, cycloundecene, cyclododecene, and 1-phenylcyclohex-1-ene. Particularly preferred bicyclic cycloalkenes can be selected from the group consisting of bicylo[2.2.1]hept-2-ene, a-pinene, and carene.

[0022] In the process according to the invention, unsubstituted or at least monosubstituted, saturated cycloaliphatic hydrocarbons that are monocyclic or bicyclic, preferably bicyclic, can be used. Monocyclic cycloaliphatic hydrocarbons are particularly preferably used in the process according to the invention.

[0023] The monocyclic or polycyclic, in particular monocyclic or bicyclic, saturated cycloaliphatic hydrocarbons used in the process according to the invention can preferably have 5 to 18 C atoms in the ring system. These cycloaliphatic hydrocarbons can each be unsubstituted or mono- or polysubstituted. If they are mono- or polysubstituted, they are preferably substituted by 1, 2, 3, 4 or 5 substituents, independently of one another, each selected from the group consisting of methyl, phenyl or benzyl. The phenyl or benzyl substituents can themselves each be unsubstituted or mono- or polysubstituted by 1, 2 or 3 substituents, independently of one another, each selected from the group consisting of F, Cl, Br, and NO2.If the cycloaliphatic hydrocarbons or their substituents used according to the invention have alkyl radicals with more than one carbon atom in the side chain, undesirable side reactions occur at these substituents when carrying out the process according to the invention.

[0024] Particularly preferred unsubstituted or at least monosubstituted, saturated cycloaliphatic hydrocarbons used in the process according to the invention are monocyclic saturated hydrocarbons having 6 to 12 carbon atoms in the ring, preferably having 8 to 12 carbon atoms in the ring, which are unsubstituted or mono- or polysubstituted by 1, 2, 3, 4 or 5 substituents, independently of one another, each selected from the group consisting of methyl, phenyl or benzyl. Very particularly preferred monocyclic saturated hydrocarbons having 8 to 12 carbon atoms in the ring, which are unsubstituted or mono-, di- or tri-substituted by a methyl group, are used in the process according to the invention.Most preferably, the saturated monocyclic hydrocarbon is unsubstituted and is selected from the group consisting of cyclohexane, cycloheptane, cyclooctane, cyclononane, cyclodecane, cycloundecane and cyclododecane, even more preferably selected from the group consisting of cyclooctane, cyclononane, cyclodecane, cycloundecane and cyclododecane, most preferably the hydrocarbon is cyclododecane.

[0025] Particularly preferably, the cycloalkene is selected from the group consisting of cyclohexene, cycloheptene, cyclooctene, cyclononene, cyclodecene, cycloundecene, cyclododecene, 1-phenylcyclohex-1-ene, bicylo[2.2.1]hept-2-ene, a-pinene and carene and the saturated cycloaliphatic hydrocarbon is selected from the group consisting of cyclohexane, cycloheptane, cyclooctane, cyclononane, cyclodecane, cycloundecane and cyclododecane.

[0026] The cycloalkene cyclododecene and the saturated cycloaliphatic hydrocarbon cyclododecane are particularly preferred.

[0027] The provision of at least one unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene according to step (a-1) and the provision of at least one unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon according to step (a-2) can preferably be carried out in combination, particularly preferably as a mixture, in the process according to the invention. For example, precursors from large-scale industrial processes containing these two components can be used directly in the process according to the invention.

[0028] The quantitative ratio of the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and the unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon in the process according to the invention can vary over a wide range.

[0029] The molar proportion of the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene is preferably 40 to 95 mol%, preferably 45 to 55 mol%, particularly preferably 47 to 53 mol%, in each case based on the total amount of unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon used.

[0030] Likewise preferably, the molar proportion of the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene is > 60 mol%, preferably > 65 mol%, particularly preferably > 70 mol%, in each case based on the total amount of unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon used.

[0031] Most preferably, cyclododecene is used as the cycloalkene in an amount of 90 to 95 mol% and cyclododecane is used as the saturated cycloaliphatic hydrocarbon in an amount of 5 to 10 mol%, based on the total amount of cyclodecene and cyclodecane.

[0032] According to step (b) of the process according to the invention, at least one inorganic or organic nitrate salt is provided. This nitrate salt acts both as a conductive salt and as a mediator in the electrochemical oxidation process according to the invention. Preferably, an inorganic or organic nitrate of the general formula [cation + ][NO3'] is used, where the [cation + ] is selected from the group consisting of Na + , K + , ammonium ions with the general structure [R 1 R 2 R 3 R 4 N + ] with R 1 , R 2 , R 3 , R 4 , independently of one another, each selected from the group consisting of Ci- to Ci6-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched, imidazolium cations having the general structure (I) with R 1 and R 2, independently of one another, each selected from the group consisting of Ci- to Cis-alkyl, straight-chain or branched, in particular Ci- to Cs-alkyl, straight-chain or branched, and R 3 selected from the group consisting of H and Ci- to Cis-alkyl, straight-chain or branched, in particular from the group consisting of H and Ci- to Cs-alkyl, straight-chain or branched,

[0033] Pyridinium cations with the general structure (II) with R 1 selected from the group consisting of Ci- to Cis-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched and R 2 , R 3 and R 4 , independently of one another, each selected from the group consisting of H and Ci- to Cis-alkyl, straight-chain or branched, in particular from the group consisting of H and Ci- to Cs-alkyl, straight-chain or branched, and

[0034] Phosphonium ions with the general structure [R 1a R 2a R 3aR 4a P + ] with R 1a , R 2a , R 3a , R 4a , independently of one another, each selected from the group consisting of Ci- to Ci6-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched.

[0035] If an organic nitrate based on the imidazolium cations is used in the process according to the invention, preference is given to cations of the general formula (I) in which R 1 and R 2 , independently of one another, are each selected from the group consisting of Ci- to Cis-alkyl, straight-chain or branched, in particular Ci- to Cs-alkyl, straight-chain or branched and R 3 represents hydrogen. Particularly preferred imidazolium cations of the general formula (I) are those in which R 1 for methyl and R 2 represent ethyl or R 1 for methyl and R 2 represents methyl and R 1 for methyl and R 2 stands for butyl, and R3 each stands for hydrogen.

[0036] If a nitrate based on the pyridinium cations is used in the process according to the invention, preference is given to those cations of the general formula (II) in which R 1 represents Ci- to Cis-alkyl, straight-chain or branched, in particular represents Ci- to Cs-alkyl, straight-chain or branched. Particularly preferred are pyridinium cations of the general formula (II) in which R 1 represents Ci- to Cis-alkyl, straight-chain or branched, in particular represents Ci- to Cs-alkyl, straight-chain or branched, and the radicals R 2 , R 3 and R 4 , independently of one another, are each selected from the group consisting of Ci- to Cs-alkyl, straight-chain or branched, with a single substitution in the 2-, 3- or 4-position, a double substitution in the 2,4-, 2,5- or 2,6-position or a triple substitution in the 2,4,6-position being preferred.

[0037] In principle, two or more of the above-mentioned nitrate salts can also be used in the process according to the invention. Preferably, one nitrate salt according to the invention is used, in particular an organic ammonium nitrate salt of the composition [R 1 R 2 R 3 R 4 N + ][NC>3'] or an organic phosphonium salt of the composition [R 1a R 2a R 3a R 4a P + ][NO3'], wherein an organic ammonium nitrate salt of the composition [R 1 R 2 R 3 R 4 N + ][NC>3'] is particularly preferred.

[0038] The organic ammonium nitrate salt is most preferably tetra-n-butylammonium nitrate or methyltri-n-octylammonium nitrate. The organic phosphonium nitrate salt is most preferably tetra-n-butylphosphonium nitrate or methyltri-n-octylphosphonium nitrate. The organic imidazolium nitrate salt is preferably 1-butyl-3-methylimidazolium nitrate.

[0039] Tetra-n-butylammonium nitrate or methyltri-n-octylammonium nitrate is most preferably used as the organic nitrate salt in the process according to the invention.

[0040] The order in which the components used in the process according to the invention are provided may vary, as may the order in which the individual components are brought into contact with one another or with the respective reaction medium.

[0041] In one embodiment of the process according to the invention, the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and the unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon are initially introduced and brought into contact with the reaction medium, preferably at least partially or completely dissolved in the reaction medium or mixed therewith, and then the inorganic or organic nitrate salt is added thereto.

[0042] In a further embodiment of the process according to the invention, the inorganic or organic nitrate salt is initially introduced and brought into contact with the reaction medium, preferably at least partially or completely dissolved in the reaction medium or mixed therewith, and then the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and the unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon, preferably in combination, are added thereto.

[0043] It is also possible to initially charge the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and the unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon and the inorganic or organic nitrate salt and then combine them with the reaction medium, preferably to dissolve them at least partially or completely in the reaction medium or to mix them therewith. Furthermore, it is also possible that, in the process according to the invention, the unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and the unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon and the inorganic or organic nitrate salt are added to the reaction medium simultaneously or in chronological order, preferably to dissolve them at least partially or completely in the reaction medium or to mix them therewith.

[0044] The reaction medium used in the process according to the invention is liquid under the conditions under which the process is carried out and is suitable for partially or completely dissolving the components used, ie, in particular the unsubstituted or at least monosubstituted, saturated cycloaliphatic hydrocarbon and the inorganic or organic nitrate salt. If at least one of these components is used in liquid form, the reaction medium is preferably readily miscible with this component(s).

[0045] Preferably, a polar aprotic reaction medium is used in the electrochemical oxidation process according to the invention. This can be used in anhydrous form, in dried form, or in combination with water.

[0046] If an inorganic nitrate salt, in particular potassium nitrate or sodium nitrate, is used in the process according to the invention, the reaction medium advantageously contains water, with an aprotic reaction medium in combination with water being preferred. The water content in the reaction medium can vary. Preferably, the water content is up to 20 vol.%, more preferably up to 15 vol.%, most preferably up to 10 vol.%, and even more preferably up to 5 vol.%, in each case based on the total amount of reaction medium.

[0047] Preferably, the polar aprotic reaction medium is selected from the group consisting of aliphatic nitriles, aliphatic ketones, cycloaliphatic ketones, dialkyl carbonates, cyclic carbonates, lactones, aliphatic nitroalkanes, and dimethyl sulfoxide, esters and ethers or a combination of at least two of these components.

[0048] Particularly preferably, the reaction medium is selected from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, acetone, dimethyl carbonate, methyl ethyl ketone, 3-pentanone, cyclohexanone, nitromethane, nitropropane, tert-butyl methyl ether, dimethyl sulfoxide, gamma-butyrolactone and epsilon-caprolactone or a combination of at least two of these components.

[0049] Most preferably, the reaction medium is selected from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, dimethyl carbonate and acetone or a combination of at least two of these components.

[0050] The reaction medium is most preferably acetonitrile, isobutyronitrile or adiponitrile in dried or anhydrous form.

[0051] Also particularly preferred is the reaction medium acetonitrile, isobutyronitrile or adiponitrile, optionally in combination with water.

[0052] If one or more of the above-mentioned components are used in the reaction medium in combination with water, the water content is preferably up to 20 vol.%, particularly preferably up to 15 vol.%, very particularly preferably up to 10 vol.%, even more preferably up to 5 vol.%, in each case based on the total amount of reaction medium.

[0053] To carry out the process according to the invention, it may be advantageous to add further solubilizing components to the reaction medium. Suitable advantageous components can be determined by simple preliminary tests on solubility behavior. Suitable solubilizing components include, for example, primary alcohols, secondary alcohols, monoketones, or dialkyl carbonates, or mixtures of at least two of these components, optionally in combination with water. Aliphatic C 1-6 alcohols can preferably be used in the process according to the invention, with particularly preferred solubilizing components being selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol, or mixtures of at least two of these components, optionally in combination with water.

[0054] The use of dimethyl carbonate as reaction medium, optionally in combination with at least one Ci-6 alcohol, in particular selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol, optionally in combination with water, can be particularly advantageous.

[0055] If one or more of these solubilizing components are used in combination with water, the water content is preferably up to 20 vol.%, particularly preferably up to 15 vol.%, very particularly preferably up to 10 vol.%, even more preferably up to 5 vol.%, in each case based on the total amount of solubilizing component and water.

[0056] The solubilizing components can preferably be added in amounts of < 50 vol.%, particularly preferably < 30 vol.% and very particularly preferably < 10 vol.%, in each case based on the total amount of reaction medium.

[0057] The inorganic or organic nitrate salt is preferably used in the process according to the invention in an amount of 0.1 to 2.0, preferably 0.2 to 1.0, particularly preferably 0.3 to 0.8 and very particularly preferably 0.4 to 0.8 equivalents, based on the amount of unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene used and in an amount of 0.8 to 10.0, preferably 2.5 to 10.0, particularly preferably 3.0 to 10.0 and very particularly preferably 5.0 to 10.0 equivalents, based on the amount of unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon used.

[0058] According to the invention, the electrochemical oxidation of the unsubstituted or at least monosubstituted, saturated cycloaliphatic hydrocarbon takes place in the presence of the inorganic or organic nitrate salt in an electrolysis cell in a reaction medium in the presence of oxygen.

[0059] For this purpose, a gas atmosphere containing oxygen is advantageously provided in spatial connection with the reaction medium.

[0060] The proportion of oxygen in the gas atmosphere can vary. The proportion of oxygen in the gas atmosphere is preferably 10 to 100 vol.%, particularly preferably 15 to 30 vol.%, particularly preferably 15 to 25 vol.%, and most preferably 18 to 22 vol.%.

[0061] In one embodiment, the proportion of oxygen in the gas atmosphere can be 10 to 100 vol.%, particularly preferably 15 to 100 vol.%, particularly preferably 20 to 100 vol.%.

[0062] The gas atmosphere air is particularly preferred.

[0063] Advantageously, a gas exchange is enforced between the gas atmosphere and the reaction medium, preferably by introducing a gas atmosphere into the reaction medium or by stirring the liquid phase in the presence of the gas atmosphere.

[0064] The gas exchange between the gas atmosphere and the reaction medium, in particular stirring, for example via the geometry of the stirrer or the stirring speed, can be used to control the electrochemical oxidation.

[0065] Preferably, the amount of oxygen dissolved in the reaction medium is at least 1 mmol / L reaction medium, particularly preferably at least 5 mmol / L reaction medium.

[0066] Also preferably, the amount of oxygen dissolved in the reaction medium is at least 10 mmol / L reaction medium.

[0067] The process according to the invention for preparing unsubstituted or at least mono-substituted a,w-dicarboxylic acids or ketocarboxylic acids and unsubstituted or at least mono-substituted cycloalkanones by electrochemical oxidation of unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkenes and unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbons in the presence of an inorganic or organic nitrate salt in a reaction medium in the presence of oxygen can be carried out in both a divided and an undivided electrolysis cell, with the process being carried out in an undivided electrolysis cell being preferred.

[0068] The undivided electrolysis cell preferably used according to the invention has at least two electrodes. Anodes and cathodes made of conventional materials can be used, for example, glassy carbon, boron-doped diamond (BDD), or graphite. The use of glassy carbon electrodes is preferred.

[0069] The undivided electrolysis cell preferably has at least one glassy carbon anode or at least one glassy carbon cathode. Preferably, both the anode and the cathode are glassy carbon electrodes.

[0070] The distance between the electrodes can vary within a certain range. The distance is preferably 0.1 mm to 2.0 cm, more preferably 0.1 mm to 1.0 cm, and most preferably 0.1 mm to 0.5 cm.

[0071] Furthermore, the process according to the invention can be carried out batchwise or continuously, preferably in an undivided flow-through electrolysis cell.

[0072] The process according to the invention is preferably carried out with a charge quantity of at least 190 C (2 F) to 970 C (10 F), preferably 290 C (3 F) to 870 C (9 F), particularly preferably 330 C (3.5 F) to 820 C (8.5 F), very particularly preferably 380 C (4 F) to 775 C (8 F), most preferably 380 C (4 F) to 580 C (6 F), in each case for 1 mmol of unsubstituted or at least mono-substituted, mono- or polyunsaturated cycloalkene and unsubstituted or at least mono-substituted, saturated cycloaliphatic hydrocarbon used.

[0073] The electrochemical oxidation preferably takes place in the inventive

[0074] Process at constant current. The current density at which the process according to the invention is carried out is preferably at least 5 mA / cm 2 or at least 10 mA / cm 2 or at least 15 mA / cm 2 or at least 20 mA / cm 2or 20 mA / cm 2 up to 50 mA / cm 2 , where the area refers to the geometric area of ​​the electrodes.

[0075] A significant advantage of the process according to the invention is that electric current is used as the oxidizing agent, which is a particularly environmentally friendly agent when it comes from renewable sources, in particular from biomass, solar thermal energy, geothermal energy, hydropower, wind power or photovoltaics.

[0076] The process according to the invention can be carried out over a wide temperature range, for example at a temperature in the range from 0 to 60 °C, preferably from 5 to 50 °C, particularly preferably from 10 to 40 °C, most preferably from 15 to 30 °C.

[0077] The process according to the invention can be carried out at elevated or reduced pressure. If the process according to the invention is carried out at elevated pressure, a pressure of up to 16 bar is preferred, particularly preferably up to 6 bar.

[0078] The process according to the invention can also preferably be carried out under atmospheric pressure.

[0079] The products prepared by the process according to the invention can be isolated or purified by conventional methods known to the person skilled in the art, in particular by extraction, crystallization, centrifugation, precipitation, distillation, evaporation or chromatography.

[0080] The process according to the invention is preferably carried out without the addition of catalysts, in particular without the addition of transition metal catalysts.

[0081] The process according to the invention is also preferably carried out in such a way that no further oxidizing agents are added other than oxygen or atmospheric oxygen. The following examples further illustrate the present invention but are not to be construed as limiting the scope of the invention.

[0082] General information and methods

[0083] Analytical-grade chemicals were purchased and used from common suppliers (such as TCI, Aldrich, and Acros). Oxygen was purchased in 2.5 grade from NIPPON GASES Deutschland GmbH, Düsseldorf, Germany, and used directly.

[0084] Glassy carbon (SIGRADUR® G, from HTW Hochtemperatur Werkstoffe GmbH, Thierhaupten, Germany) was used as the electrode material.

[0085] High-performance liquid chromatography was performed on a Shimadzu HPLC-MS equipped with a SIL 20A HT autosampler, a CTQ-20AC column oven, two LC-20AD pump modules for eluent gradient adjustment, an SPD-M20A diode array detector, a CBM-20A system controller, and a Eurospher II 100-5 C18 column (150 x 4 mm, Knauer, Berlin). Eluent: acetonitrile (ACN) / water / formic acid (1 vol.%) (from 10% ACN to 90% ACN in 10 min + 10 min at 100% ACN). Mass spectrometry was performed on an LCMS-2020 Shimadzu, Japan. NMR spectrometry was performed by 1 H-NMR and 13 C-NMR spectra were recorded at 25 °C using a Bruker Avance II 400 (400 MHz, 5 mm BBFO head with z-gradient and ATM, SampleXPress 60 sample changer, Analytische Messtechnik, Karlsruhe, Germany).

[0086] Gas chromatography was performed on a Shimadzu GC-2025 (Shimadzu, Japan) equipped with an HP 5MS column (Agilent Technologies, Santa Clara, California; length: 30 m, inner diameter: 0.25 mm, film thickness: 0.25 pm, carrier gas: hydrogen). GC-MS measurements were performed on a Shimadzu GC-2010 (Shimadzu, Japan) equipped with an HP-1 column (Agilent Technologies, Santa Clara, California; length: 30 m, inner diameter: 0.25 mm, film thickness: 0.25 pm, carrier gas: helium). To prepare the samples for GC analysis, column filtration was performed over silica gel 60 M (0.04–0.063 mm, Macherey-Nagel GmbH & Co. KG, Düren, Germany). The undivided Teflon cells used for electrolysis are described in the literature (a) C. Gütz, B. Klöckner, SR Waldvogel, Org. Process Res. Dev. 2016, 20, 26-32; b) A. Kirste, G. Schnakenburg, F. Stecker, A. Fischer, SR Waldvogel, Angew. Chem. Int. Ed. 2010, 49, 971-975; Angew. Chem.2010, 122, 983-987. (see Sl).) The complete range of these cells is commercially available from IKA Screening System (IKA-Werke GmbH & Co. KG, Staufen, Germany). The dimensions of the electrodes were 7 cm x 1 cm x 0.3 cm.

[0087] The gas introduction was controlled by two Model 5850S ​​mass flow controllers (MFCs) from Brooks Instrument BV, Veenendaal, Netherlands. One controller each was used for the oxygen and nitrogen lines. The controllers were controlled using the Smart DDE software and Matlab R2017b. The volume flow was additionally controlled by a DK800 variable area flowmeter from KRÖHNE Messtechnik GmbH, Duisburg. For all experiments, the total volume flow was constant at 20 mL / min, which, limited by the MFCs used, also represents the maximum achievable volume flow. The percentage volume flows of the two gases were adjusted using the MFCs and their software. The gas cylinders used were from the following suppliers: Oxygen 2.5 from NIPPON GASES Deutschland GmbH, Düsseldorf, and Nitrogen 4.8 from Westfalen AG, Münster, or Nitrogen 5.0 from NIPPON GASES Deutschland GmbH, Düsseldorf. The gas distributor and the gas inlet covers of the electrolysis cells are described in the literature (M. Dörr, D. Waldmann, SR Waldvogel, GIT Labor-Fachz. 2021, 7-8, 26-28) and were purchased from IKA (IKA-Werke GmbH & Co. KG, Staufen, Germany).

[0088] General Working Instruction AAV1

[0089] In an undivided 5 mL Teflon pot cell, the cycloalkane (0.1 to 0.5 mmol), the cycloalkene (0.5 to 0.9 mmol, combined alkane and alkene to a total of 1 mmol), and tetrabutylammonium nitrate (0.5 eq.) were placed and dissolved in acetonitrile or isobutyronitrile (5 mL). The cell was equipped with glassy carbon electrodes spaced 0.5 cm apart. The immersion area of ​​the electrodes was 1.8 cm. 2 After the cell was fixed in a stainless steel block, galvanostatic electrolysis was carried out at a current density of 10 mA / cm 2at 22 °C. The application of the charge amount referred to the theoretical charge amounts for the oxidation of both components, depending on their ratio (alkane / alkene = 0.1 / 0.9: 7.6 F; 0.25 / 0.75: 7.0 F; 0.5 / 0.5: 6.0 F). After electrolysis, 1,3,5-trimethoxybenzene (10 mg) was added to the reaction solution as an internal standard. Three drops were taken and filtered through 60 M silica gel (eluent: ethyl acetate). The filtrate was collected in a GC vial and analyzed by gas chromatography. The solvent was first removed from the remaining reaction solution by distillation. Subsequently, the conducting salt was extracted using 10 mL of ethyl acetate and 10 mL of aqueous HCl solution (0.1 M). The solvent of the organic phase was removed by distillation, and the residue was taken up with an aqueous NaOH solution (1 M, 10 mL). The aqueous phase was washed with 10 mL of diethyl ether.After phase separation, the aqueous phase was adjusted to pH 1 with an aqueous HCl solution (1 M) and extracted with 2 x 10 mL of ethyl acetate. After drying the organic phase over sodium sulfate and removing the solvent by distillation, the dicarboxylic acid product was dried under high vacuum.

[0090] After applying the loading quantity, 10 mg of 1,3,5-trimethoxybenzene is added to the reaction solution as an internal standard. Three drops of the reaction solution are taken for gas chromatographic analysis and quantification of the product. These drops are eluted using ethyl acetate over approximately 330 mg of 60 M silica gel. Approximately 1.5 mL of the filtrate is collected in a GC vial, which is analyzed for oxidation products using GC-FID and GC-MS. Quantification is performed by prior calibration of the gas chromatograph.

[0091] Example 1

[0092] According to AAV1, the following co-electrolyses were carried out (Scheme 1 , Table 1). ° 10 mA / cm 2 Scheme 1.

[0093] Table 1 : Reactions for co-electrolysis

[0094] Products

[0095] Substrate mixing ratios

[0096] C: Cyclooctanone [dl D: Octanedioic acid [el

[0097] A:B = 1:9 (10 mol% A)[ a i 4% 40% n = 3 A:B = 1:3 (25 mol% A) [b] 2% 44%

[0098] A:B = 1:1 (50 mol% A) [c] 2% 45%

[0099] C: Cyclododecanone [dl D: Dodecanedioic acid [el

[0100] A:B = 1:9 (10 mol% A)[ a i 27% 57% n = 7 A:B = 1:3 (25 mol% A) [bl 13% 63%

[0101] A:B = 1 : 1 (50 mol% A)[ c i 9% 74%

[0102] For n = 3, acetonitrile was used, and for n = 7, isobutyronitrile. Cumulative charge amounts with respect to A and B: a) 7.6 F; b) 7.0 F; c) 6.0 F. d) Determination by external GC calibration (internal standard: 1,3,5-trimethoxybenzene). e) Isolated yields. All yields refer to the amounts of the respective starting materials used.

Claims

Process for the preparation of unsubstituted or at least singly substituted α,γ-dicarboxylic acids or ketocarboxylic acids and unsubstituted or at least singly substituted cycloalkanones by electrochemical oxidation comprising the process steps: (a-1) Provision of at least one unsubstituted or at least one simply substituted, mono- or polyunsaturated cycloalkene; (a-2) Provision of at least one unsubstituted or at least singly substituted saturated cycloaliphatic hydrocarbon, wherein the substituents of the singly or polyunsaturated cycloalkenes and the substituents of the saturated cycloaliphatic hydrocarbons are each selected independently from the group consisting of methyl, phenyl or benzyl, wherein the phenyl or benzyl substituents themselves are each unsubstituted or singly or polysubstituted, with 1, 2 or 3 substituents, each selected independently from the group consisting of F, CI, Br, and NO2, wherein the ring sizes of the cycloalkenes according to (a-1) and of the cycloaliphatic hydrocarbons according to (a-2) are the same; (b) Provision of at least one inorganic or organic nitrate salt, wherein the nitrate salt of process step (b) is a nitrate of the general formula [cation + ][NO3'] is present, with [cation+ ] selected from the group consisting of Na + , K + , Ammonium ions with the general structure [R 1 R 2 R 3 R 4 N + ] with R 1 , R 2 , R 3 , R 4 , independently of each other, each selected from the group consisting of Ci- to Cie-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched, Imidazolium cations with the general structure (I) with R 1 and R 2 , independently of each other, each selected from the group consisting of Ci- to Cis-alkyl, straight-chain or branched, in particular Ci- to Cs-alkyl, straight-chain or branched, and R 3 selected from the group consisting of H and Ci to Cis alkyl, straight-chain or branched, in particular from the group consisting of H and Ci to Cs alkyl, straight-chain or branched, Pyridinium cations with the general structure (II) with R 1 selected from the group consisting of Ci- to Cis-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched, and R 2 , R 3 and R 4 , independently of each other, each selected from the group consisting of H and Ci- to Cis-alkyl, straight-chain or branched, in particular from the group consisting of H and Ci- to Cs-alkyl, straight-chain or branched, and phosphonium ions with the general structure [R 1a R 2a R 3a R 4a P + ] with R 1a , R 2a , R 3a , R 4a , independently of each other, each selected from the group consisting of Ci- to Ci6-alkyl, in particular Ci- to Cs-alkyl, straight-chain or branched. (c) electrochemical oxidation of the unsubstituted or at least singly substituted, mono- or polyunsaturated cycloalkene provided in step (a-1) and of the unsubstituted or at least singly substituted, saturated cycloaliphatic hydrocarbon provided in step (a-2) in the presence of the inorganic or organic nitrate salt provided in step (b) in an electrolysis cell in a reaction medium in the presence of oxygen.The method of claim 1, wherein the unsubstituted or at least singly substituted, singly or polyunsaturated cycloalkene is monocyclic or bicyclic, wherein preferably the unsubstituted or at least singly substituted, singly or polyunsaturated monocyclic cycloalkene has 5 to 12 C atoms, particularly preferably 6 to 12 C atoms, most preferably 8 to 12 C atoms in the ring system and is unsubstituted or singly or polyunsaturated, preferably with 1, 2, 3, 4 or 5 substituents, independently. each other is substituted by 1, 2 or 3 substituents selected from the group consisting of methyl, phenyl or benzyl, wherein the phenyl or benzyl substituents themselves are unsubstituted or singly or multiply substituted, with 1, 2 or 3 substituents, independently of each other, each selected from the group consisting of F, CI, Br, and NO2 and / or the unsubstituted or at least singly substituted, singly or multiply unsaturated bicyclic cycloalkene has 7 to 18 C atoms, particularly preferably 7 to 12 C atoms, most preferably 7 to 10 C atoms in the ring system and is unsubstituted or singly or multiply substituted, preferably with 1, 2, 3, 4 or 5 substituents, independently of each other, each selected from the group consisting of methyl, phenyl or benzyl, wherein the phenyl or benzyl substituents themselves are unsubstituted or singly or multiply substituted, with 1, 2 or 3 substituents, independently of each other,each selected from the group consisting of F, CI, Br, and NO2. A process according to any one of the preceding claims, wherein the unsubstituted or at least singly substituted saturated cycloaliphatic hydrocarbon is monocyclic or bicyclic, wherein the monocyclic or bicyclic saturated cycloaliphatic hydrocarbon preferably has 5 to 18 C atoms in the ring system and is unsubstituted or singly or multiply substituted with 1, 2, 3, 4 or 5 substituents, independently of one another, each selected from the group consisting of methyl, phenyl or benzyl, wherein phenyl or benzyl may each be unsubstituted or singly or multiply substituted, with 1, 2 or 3 substituents, independently of one another, each selected from the group consisting of F, CI, Br, and NO2. A process according to any one of the preceding claims, wherein the unsubstituted or at least singly substituted,saturated cycloaliphatic hydrocarbon is a monocyclic saturated hydrocarbon with 6 to 12 carbon atoms in the ring, preferably with 8 to 12 carbon atoms in the ring, wherein this cycloaliphatic hydrocarbon is unsubstituted or singly or multiply substituted with 1, 2, 3, 4 or 5 substituents, independently of one another, each selected from the group consisting of methyl, phenyl or benzyl, particularly preferably a monocyclic saturated hydrocarbon with 8 to 12 carbon atoms in the ring, wherein this cycloaliphatic hydrocarbon is unsubstituted or singly, doubly or triply substituted with methyl.., A method according to any one of the preceding claims, wherein the cycloalkene is selected from the group consisting of cyclohexene, cycloheptene, cyclooctene, cycloonones, cyclodecene, cycloundecene, cyclododecene, 1-phenylcyclohex-1-ene, bicylo[2.2.1]hept-2-ene, α-pinene and carene, in particular preferably cyclododecene, and wherein the saturated cycloaliphatic hydrocarbon is selected from the group consisting of cyclohexane, cycloheptane, cyclooctane, cycloononane, cyclodecane, cycloundecene and Cyclododecane, preferably selected from the group consisting of cyclohexane, cycloheptane, cyclooctane, cyclononane, cyclodecane, cycloundecane and Cyclododecane, particularly preferably cyclododecane. A process according to any one of the preceding claims, wherein the provision of at least one unsubstituted or at least singly substituted, monounsaturated or polyunsaturated cycloalkene according to step (a-1) and the provision of at least one unsubstituted or at least singly substituted, saturated cycloaliphatic hydrocarbon according to step (a-2) are carried out in combination, preferably as a mixture. A process according to any one of the preceding claims, wherein the molar fraction of the unsubstituted or at least singly substituted, monounsaturated or polyunsaturated cycloalkene is 40 to 95 mol%, preferably 45 to 55 mol%, particularly preferably 47 to 53 mol%.-%, in each case based on the total amount of unsubstituted or at least singly substituted, mono- or polyunsaturated cycloalkene and unsubstituted or at least singly substituted saturated cycloaliphatic hydrocarbon used. A process according to any of the preceding claims, wherein the molar fraction of the unsubstituted or at least singly substituted, mono- or polyunsaturated cycloalkene is > 60 mol%, preferably > 65 mol%, particularly preferably > 70 mol%, in each case based on the total amount of unsubstituted or at least singly substituted, mono- or polyunsaturated cycloalkene and unsubstituted or at least singly substituted saturated cycloaliphatic hydrocarbon used. A method according to one of the preceding claims, wherein in the imidazolium cations of general formula (I) the residues R1 and R2 are each selected independently from the group consisting of O1- to O18-alkyl, straight-chain or branched, in particular O1- to C8-alkyl, straight-chain or branched, and R3 represents hydrogen, preferably R1 represents methyl and R2 represents ethyl, or R1 represents methyl and R2 represents methyl, and R1 represents methyl and R2 represents butyl, and R3 represents hydrogen.A method according to any one of the preceding claims, wherein in the pyridinium cations of general formula (II) the residue R1 represents O1- to O18-alkyl, straight-chain or branched, in particular O1- to C8-alkyl, straight-chain or branched, and residues R2, R3 and R4 are each independently selected from the group consisting of O1- to C8-alkyl, straight-chain or branched, wherein a simple substitution at the 2-, 3- or 4-position, a double substitution at the 2,4-, 2,5- or 2,6-position or a triple substitution at the 2,4,6-position is preferred. A method according to any one of the preceding claims, wherein the organic. The nitrate salt selected is from the group consisting of tetra-n-butylammonium nitrate, methyl-tri-n-octylammonium nitrate, and tetra-n-butylphosphonium nitrate. Methyl tri-n-octyl phosphonium nitrate and 1-butyl 3-methylimidazolium nitrate. A method according to any one of the preceding claims, wherein the The reaction medium is a polar aprotic reaction medium, optionally in combination with water, wherein the polar aprotic reaction medium is selected from the group consisting of aliphatic nitriles, aliphatic ketones, cycloaliphatic ketones, dialkyl carbonates, cyclic carbonates, lactones, aliphatic nitroalkanes, dimethyl sulfoxide, esters, and ethers, or a combination of at least two of these components. The process according to any one of the preceding claims, wherein the reaction medium is a polar aprotic reaction medium, in combination with water, wherein the water content is up to 20 vol%, preferably up to 15 vol%, particularly preferably up to 10 vol%, and even more preferably up to 5 vol%, in each case based on the total amount of reaction medium.

14. A method according to any of the preceding claims, wherein the reaction medium is a polar aprotic reaction medium selected from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, acetone, dimethyl carbonate, methyl ethyl ketone, 3-pentanone, cyclohexanone, nitromethane, nitropropane, tert-butyl methyl ether, dimethyl sulfoxide, gamma-butyrolactone and epsilon-caprolactone, preferably selected from the group consisting of acetonitrile, isobutyronitrile, adiponitrile, dimethyl carbonate and acetone, or a combination of at least two of these components, each optionally in combination with water.

15. Method according to any of the preceding claims, wherein the reaction medium comprises one or more solvent-mediating components.

16. The method of claim 15, wherein the solubilizing components are primary alcohols, secondary alcohols, monoketones or dialkyl carbonates or mixtures of at least two of these components, optionally in combination with water.

17. Method according to one of claims 15 or 16, wherein aliphatic C1-6 alcohols are present as one or more solubilizing components, preferably one or more alcohols selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol and mixtures of at least two of these components, optionally in combination with water.

18. Method according to one of the preceding claims, wherein the reaction medium is dimethyl carbonate, optionally in combination with at least one Ci-6 alcohol, preferably selected from the group consisting of methanol, ethanol, isopropanol, 2-methyl-2-butanol.

19. The method of claim 18, wherein the reaction medium comprises water.

20. Method according to any one of claims to 19, wherein one or more solubilizing components are added in an amount of < 50 vol.%, particularly preferably < 30 vol.% and most preferably < 10 vol.%, in each case based on the total amount of reaction medium. A method according to any of the preceding claims, wherein the inorganic or organic nitrate salt is used in an amount of 0.1 to 2.0, preferably 0.2 to 1.0, particularly preferably 0.3 to 0.8 and most preferably 0.4 to 0.8 equivalents, based on the amount of unsubstituted or at least singly substituted, mono- or polyunsaturated cycloalkene used, and in an amount of 0.8 to 10.0, preferably 2.5 to 10.0, particularly preferably 3.0 to 10.0 and most preferably 5.0 to 10.0 equivalents, based on the amount of unsubstituted or at least singly substituted saturated cycloaliphatic hydrocarbon used. A method according to one of the preceding claims, wherein a gas atmosphere containing oxygen is provided in spatial connection with the reaction medium, wherein the proportion of oxygen in the gas atmosphere is preferably 10 to 100 vol.%, particularly preferably 15 to 30 vol.%.-%, most preferably 15 to 25 vol%, most preferably 18 to 22 vol%. A process according to any one of the preceding claims, wherein the gas atmosphere is air. A process according to any one of the preceding claims, wherein gas exchange is forced between the gas atmosphere and the reaction medium, preferably by introducing gas atmosphere into the reaction medium or by stirring the reaction medium in the presence of the gas atmosphere. A process according to claim 24, wherein the stirring is used to control the electrochemical oxidation. A process according to any one of the preceding claims, wherein the amount of oxygen dissolved in the reaction medium is at least 1 mmol / L, preferably 5 mmol / L, more preferably 10 mmol / L. A process according to any one of the preceding embodiments, wherein no other oxidizing agents are added besides oxygen or atmospheric oxygen.Method according to one of the preceding claims, wherein the electrolysis cell is an undivided cell.

29. Method according to any of the preceding claims, wherein the electrolysis cell comprises a glassy carbon anode, a graphite anode or a BDD anode, preferably a glassy carbon anode.

30. Method according to any of the preceding claims, wherein the electrolysis cell comprises a glassy carbon cathode, a graphite cathode or a BDD cathode, preferably a glassy carbon cathode.

31. Method according to one of the preceding claims, wherein the distance between the electrodes in the electrolysis cell is 0.1 mm to 2.0 cm, preferably 0.1 mm to 1.0 cm, particularly preferably 0.1 mm to 0.5 cm.

32. A method according to any of the preceding claims, wherein the charge amount is at least 190°C (2°F) to 970°C (10°F), preferably 290°C (3°F) to 870°C (9°F), particularly preferably 330°C (3.5°F) to 820°C (8.5°F), most preferably 380°C (4°F) to 775°C (8°F), most preferably 380°C (4°F) to 580°C (6°F), in each case for 1 mmol of unsubstituted or at least singly substituted, mono- or polyunsaturated cycloalkene and unsubstituted or at least singly substituted saturated cycloaliphatic hydrocarbon used.

33. Method according to one of the preceding claims, wherein the electrochemical oxidation is carried out at constant current.

34. Method according to any of the preceding claims, wherein the current density is at least 5 mA / cm² 2 , preferably 10 mA / cm 2 , preferably 15 mA / cm 2 , especially preferably 20 mA / cm 2, where the area specification refers to the geometric area of ​​the electrodes.

35. Method according to one of the preceding claims, characterized in that the current density is at least 20 mA / cm² 2 up to 50 mA / cm 2 is, where the area specification refers to the geometric area of ​​the electrodes.

36. Method according to any of the preceding claims, wherein the current used for electrochemical oxidation is derived from renewable sources, in particular from biomass, solar thermal energy, geothermal energy, hydropower, wind power or photovoltaics.

37. Method according to one of the preceding claims, wherein the electrochemical oxidation takes place at a temperature in the range of 0 to 60 °C, preferably from 5 to 50 °C, particularly preferably from 10 to 40 °C, most preferably from 15 to 30 °C.

38. A method according to any of the preceding claims, wherein it is carried out under atmospheric pressure.

39. Method according to any of the preceding claims, wherein it is carried out under suppression.

40. Method according to one of the preceding claims, wherein it is carried out under overpressure, preferably up to 16 bar, particularly preferably up to 6 bar.

41. A method according to any of the preceding claims, wherein it is carried out in batches.

42. A method according to any of the preceding claims, wherein it is carried out without the addition of catalysts, in particular without the addition of transition metal catalysts.

43. Method according to one of the preceding claims, wherein it is carried out continuously in an undivided flow-through electrolysis cell.

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