WALDVOGEL SIEGFRIED (DE)
SCHUETZ JAN (CH)
BONRATH WERNER (CH)
GOY ROMAN (CH)
| JPH0565682A | 1993-03-19 | |||
| US4963234A | 1990-10-16 |
| Claims 1. A process for producing a compound of formula (I), comprising electrochemical oxidation of a compound of formula (II) to obtain the compound of formula (I): wherein: R is H or C1-C6 alkyl, optionally substituted with at least one substituent; X is H, OH, halogen or C1-C6 alkoxy, optionally substituted with at least one substituent; and R', at each occurrence, is independently C1-C6 alkyl, aryl, C1-C6 alkoxy, cyano or halogen, optionally substituted with at least one substituent, or two adjacent R’, together with the atoms to which they are attached, form a fused C3-C8 cycloalkyl ring, a fused 5- or 6-membered heterocycloalkyl ring, a fused aryl ring or a fused 5- or 6- membered heteroaryl ring, optionally substituted with at least one substituent; and the subscript n is 0, 1, 2, 3 or 4, wherein the anode used for the electrochemical oxidation is made of the material selected from the group consisting of dimensional stable anodes, boron-doped diamond, and carbon- based materials such as graphite and glassy carbon. 2. The process of claim 1, wherein the dimensional stable anode contains a coating of at least two metal oxides selected from the group consisting of RuO2, IrO2, PtO2, TiO2 and TaOx (x is 1 or 2), preferably selected from the group consisting of RuO2, IrO2 and TiO2, more preferably the dimensional stable anode contains a coating of RuO2 and IrO2. 3. The process of claim 1 or 2, wherein the electrochemical oxidation is carried out in a medium which is water or a non-aqueous solvent or mixture thereof. 4. The process of claim 3, wherein the non-aqueous solvent is selected from the group consisting of alkylene carbonates (such as propylene carbonate and butylene carbonate), polyethylene glycol, N-methyl-2-pyrrolidone, N-butylpyrrolidone (NBP), alcohols (such as methanol, ethanol and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)), acetone, acetic acid, sulpholane, dimethylsulphoxide (DMSO), tetrahydrofuran (THF), 2-methyl-tetrahydrofuran (MeTHF), dimethylformamide (DMF), hexa-methylphosphoramide, acetonitrile (MeCN), dichloromethane (DCM), dimethoxyethane (DME), pyridine and hexafluoro-2-propanol, and mixture thereof. 5. The process of claim 1 or 2, wherein the electrochemical oxidation is carried out in a medium selected from the group consisting of acetic acid, MeCN, DME or HFIP, or mixture thereof. 6. The process of any one of claims 1-5, wherein the supporting electrolyte is selected from the group consisting of HCl, H2SO4, Na2SO4, NaCl, NaHSO4, alkyl- or arylsulfonic acids (such as methanesulfonic acid and p-toluenesulfonic acid), phosphoric acid, phosphates, and tetraalkylammonium salts (such as tetrabutylammonium acetate (NBu4OAc), tetrabutylammonium tetrafluoroborate and tetrabutylammonium hexafluorophosphate).. 7. The process of any one of claims 1-5, wherein the supporting electrolyte is an acid such as HCl, H2SO4, phosphoric acid or mixture thereof. 8. The process of claim 6 or 7, wherein the concentration of the supporting electrolyte in the medium is up to 5 M, preferably from 0.01 M to 4 M, more preferably from 0.05 M to 3 M, the most preferably from 0.5 M to 1.5 M. 9. The process of any one of claims 1-8, wherein an oxygen source is added into the reaction. 10. The process of claim 9, wherein the oxygen source is selected from the group consisting of water, methanol, acetic acid, tert-butanol, isopropanol, ethanol, hydrogen peroxide, tert-butyl hydroperoxide and oxygen. 11. The process of claim 9, wherein the oxygen source is selected from the group consisting of water, methanol, acetic acid, tert-butanol, isopropanol and ethanol, preferably is water and/or methanol. 12. The process of claim 9, wherein the oxygen source is included in the medium in an amount of from 0.1 vol% to 20 vol%, preferably from 0.5 vol% to 15.0 vol%, more preferably from 1.0 vol% to 10 vol%. 13. The process of any one of claims 1-12, wherein no terminal oxidant is used. 14. The process of any one of claims 1-13, wherein the electrochemical oxidation is carried out in a continuous way. |
Technical Field
The present invention relates to a process for producing benzoquinones by electrochemical oxidation of phenols.
Background of the Invention
Benzoquinones represent a versatile class of compounds with manifold fields of applications. They are ubiquitous in nature and play crucial roles in biological redox processes, making them pivotal for life. Moreover, their unique redox properties often equip them with remarkable biochemical activity. This allowed them to gain attention as a privileged motif for pharmaceutical agents. Furthermore, they serve as multifunctional building blocks and intermediates in organic chemistry e.g., in the synthesis of food additives from the vitamin E and K.
The most intuitive route of synthesis of benzoquinones is the oxidation of 1 ,4-hydroquinones. This pathway experiences some severe drawbacks, which are linked to the limited commercial and synthetic availability of these substrates. Therefore, a method for the direct synthesis from abundant phenols is desirable.
To convert phenols into benzoquinones, current state-of-the-art synthesis resorts to transition metal catalysts, inorganic oxidizing agents, fluorinated solvents, and/or the application of high- pressure reactors. As a result, they suffer from a magnitude of deficiencies, such as limited scope of substrates, application of expensive or toxic catalysts or reagents, low yield and/or safety concerns.
Electric current is an excellent and sustainable oxidant for the phenol oxidation since it serves as a traceless oxidant. Electrochemistry is now widely recognized as alternative “green” synthesis protocols. However, methods for the direct electrochemical oxidation of readily available phenols to benzoquinones are underdeveloped.
Summary of the Invention
The present invention provides an electrochemical process for producing a benzoquinone compound of formula (I): Wherein: R', at each occurrence, is independently C1-C6 alkyl, aryl, C1-C6 alkoxy, cyano or halogen, optionally substituted with at least one substituent, or two adjacent R’, together with the atoms to which they are attached, form a fused C3-C8 cycloalkyl ring, a fused 5- or 6-membered heterocycloalkyl ring, a fused aryl ring, or a fused 5- or 6-membered heteroaryl ring, optionally substituted with at least one substituent; and the subscript n is 0, 1, 2, 3 or 4. The process of the present invention is versatile in converting hydroquinone and a broad range of phenols such as 4-halophenols, 4-methoxyphenols and 4-methoxyanisoles into the benzoquinone compound of formula (I) in an attractive yield and selectivity without the need to modify the protocol. In addition, the process of the present invention avoids cost-intensive catalysts, reagents and oxidants and does not provide reagent waste, so it is cost effective and environment friendly. Detailed description of the Invention In the present invention, the term “C1-C6 alkyl”, employed alone or in combination with other terms, refers to a saturated hydrocarbon group that may be straight-chain or branched, having 1 to 6 carbons. In some embodiments, the alkyl group contains from 1 to 5 carbon atoms or from 1 to 4 carbon atoms, or from 1 to 3 carbon atoms. Examples of the alkyl moieties include, but are not limited to, chemical groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl, and t-butyl. In the present invention, the term “C 1 -C 6 alkoxy”, employed alone or in combination with other terms, refers to a group of formula -O-alkyl, wherein the alkyl group has from 1 to 6 carbons. Example of the alkoxy groups include methoxy, ethoxy, and propoxy (e.g., n-propoxy and iso-propoxy). In some embodiments, the alkyl group has from 1 to 3 carbon atoms. In the present invention, the term “aryl”, “aryl ring” or “aryl group”, employed alone or in combination with other terms, refers to a monocyclic or polycyclic (e.g., having 2, 3 or 4 fused rings) aromatic hydrocarbon, such as, but not limited to, phenyl, 1-naphthyl, 2-naphthyl, anthracenyl, phenanthrenyl, and the like. In some embodiments, the aryl is C 6 -C 10 aryl. In some embodiments, the aryl group is a naphthalene ring or phenyl ring. In some embodiments, the aryl group is phenyl. In the present invention, the term “C3-C8 cycloalkyl” or “C3-C8 cycloalkyl ring”, employed alone or in combination with other terms, refers to a non-aromatic cyclic hydrocarbon moiety having 3 to 8 ring- forming carbon atoms, which may optionally contain one or more alkenylene groups as part of the ring structure. Cycloalkyl groups can include mono- or polycyclic (e.g., having 2, 3 or 4 fused rings) ring systems. Also included in the definition of cycloalkyl are moieties that have one or more aromatic rings fused (i.e., having a bond in common with) to the cycloalkyl ring, for example, benzo derivatives of cyclopentane, cyclopentene, cyclohexane, and the like. A cycloalkyl group that includes a fused aromatic ring can be attached to the core or scaffold via any ring-forming atom, including a ring- forming atom of the fused aromatic group. One or more ring-forming carbon atoms of a cycloalkyl group can be oxidized to form carbonyl linkages. In some embodiments, cycloalkyl is C3-C8 cycloalkyl, C 3 -C 7 cycloalkyl, or C 5 -C 6 cycloalkyl. Exemplary cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclopentenyl, cyclohexenyl, cyclohexadienyl, cycloheptatrienyl, norbornyl, norpinyl, norcarnyl, and the like. Further exemplary cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl. In the present invention, the term “heterocycloalkyl” or “heterocycloalkyl ring”, employed alone or in combination with other terms, refers to non-aromatic heterocyclic ring system, which may optionally contain one or more unsaturation as part of the ring structure, and which has at least one heteroatom ring member independently selected from nitrogen, sulphur and oxygen. In some embodiments, the heterocycloalkyl group has 1, 2, 3, or 4 heteroatom ring members. In some embodiments, the heterocycloalkyl group has 1, 2, or 3 heteroatom ring members. In some embodiments, the heterocycloalkyl group has 1 or 2 heteroatom ring members. In some embodiments, the heterocycloalkyl group has 1 heteroatom ring member. When the heterocycloalkyl group contains more than one heteroatom in the ring, the heteroatoms may be the same or different. Example ring- forming members include CH, CH2, C(O), N, NH, O, S, S(O), and S(O)2. Heterocycloalkyl groups can include mono- or polycyclic (e.g., having 2, 3 or 4 fused rings) ring systems, including spiro systems. Also included in the definition of heterocycloalkyl are moieties that have one or more aromatic rings fused to (i.e., having a bond in common with) the non-aromatic ring, for example, 1,2,3,4-tetrahydro- quinoline, dihydrobenzofuran, and the like. A heterocycloalkyl group including a fused aromatic ring can be attached to the core or scaffold via any ring-forming atom, including a ring-forming atom of the fused aromatic group. The S or N ring-forming atoms can be optionally “oxidized” to include one or two oxo groups as valency permits (e.g., sulfonyl or sulfinyl or N-oxide). One or more ring-forming carbon atoms of the heterocycloalkyl group can include an oxo moiety to form a ring-forming carbonyl. In some embodiments, a ring-forming nitrogen atom can be quaternized. In some embodiments, the heterocycloalkyl is 5- to 10-membered, 4- to 10-membered, 4- to 7-membered, 5- membered, or 6-membered. Examples of heterocycloalkyl groups include 1,2,3,4-tetrahydro- quinolinyl, dihydrobenzofuranyl, azetidinyl, azepanyl, pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl, thiomorpholinyl, dihydrofuranyl, tetrahydrofuranyl, 2-oxopyrrolidinyl, 3-oxomorpholinyl, 2-oxooxazolidinyl, and pyranyl. Further examples of heterocycloalkyl groups include 2,3-dihydro-1H- pyrrolyl; 2-oxo-2,3-dihydro-1H-pyrrolyl; 2,3-dihydro-oxazolyl; 2-oxo-2,3-dihydro-oxazolyl; 3,4- dihydro-2H-1,4-oxazinyl; 3-oxo-3,4-dihydro-2H-1,4-oxazinyl; or 2,3-dihydro-furanyl. In further embodiments, the heterocycloalkyl group is azetidinyl, piperidinyl, pyrrolidinyl, diazapanyl, or diazaspirononanyl. In yet further embodiments, the heterocycloalkyl group is 2,3-dihydro-1H-indolyl; 2,3-dihydro-1,3-benzoxazolyl; 3,4-dihydro-2H-pyrido[3,2-b][1,4]oxazinyl; 3,4-dihydro-2H-1,4- benzoxazinyl; or 2,3-dihydro-1-benzofuran. In the present invention, the term “heteroaryl” or “heteroaryl ring”, employed alone or in combination with other terms, refers to a monocyclic or polycyclic (e.g., having 2, 3 or 4 fused rings) aromatic heterocylic moiety, having one or more heteroatom ring members selected from nitrogen, sulphur and oxygen. In some embodiments, the heteroaryl group has 1, 2, 3, or 4 heteroatom ring members. In some embodiments, the heteroaryl group has 1, 2, or 3 heteroatom ring members. In some embodiments, the heteroaryl group has 1 or 2 heteroatom ring members. In some embodiments, the heteroaryl group has 1 heteroatom ring member. In some embodiments, the heteroaryl group is 5- membered. In some embodiments, the heteroaryl group is 6-membered. When the heteroaryl group contains more than one heteroatom ring member, the heteroatoms may be the same or different. The nitrogen atoms in the ring(s) of the heteroaryl group can be oxidized to form N-oxides. Exemplary heteroaryl groups include, but are not limited to, pyridyl, pyrimidinyl, pyrazinyl, pyridazinyl, pyrrolyl, pyrazolyl, azolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, imidazolyl, furanyl, thienyl, triazolyl, tetrazolyl, thiadiazolyl, quinolinyl, isoquinolinyl, indolyl, benzothienyl, benzofuranyl, benzisoxazolyl, imidazo[1,2-b]thiazolyl, purinyl, triazinyl, and the like. In some embodiments, the heteroaryl group is pyridyl, 1H-indazolyl, 1H-pyrrolo[2,3-b]pyridinyl, or 1H-benzo[d]imidazolyl. A 5-membered heteroaryl is a heteroaryl group having five ring-forming atoms comprising wherein one or more of the ring-forming atoms are independently selected from nitrogen, sulphur and oxygen. In some embodiments, the 5-membered heteroaryl group has 1, 2, 3, or 4 heteroatom ring members. In some embodiments, the 5-membered heteroaryl group has 1, 2, or 3 heteroatom ring members. In some embodiments, the 5-membered heteroaryl group has 1 or 2 heteroatom ring members. In some embodiments, the 5-membered heteroaryl group has 1 heteroatom ring member. Example ring-forming members include CH, N, NH, O, and S. Examples of 5-membered heteroaryls are thienyl, furyl, pyrrolyl, imidazolyl, thiazolyl, oxazolyl, pyrazolyl, isothiazolyl, isoxazolyl, 1,2,3-triazolyl, tetrazolyl, 1,2,3-thiadiazolyl, 1,2,3-oxadiazolyl, 1,2,4-triazolyl, 1,2,4-thiadiazolyl, 1,2,4-oxadiazolyl, 1,3,4-triazolyl, 1,3,4-thiadiazolyl, and 1,3,4-oxadiazolyl. A 6-membered heteroaryl is a heteroaryl group having six ring-forming atoms wherein one or more of the ring-forming atoms are independently selected from nitrogen, sulphur and oxygen. In some embodiments, the 6-membered heteroaryl group has 1, 2, or 3 heteroatom ring members. In some embodiments, the 6-membered heteroaryl group has 1 or 2 heteroatom ring members. In some embodiments, the 6-membered heteroaryl group has 1 heteroatom ring member. Example ring- forming members include CH, N, NH, O, and S. Example six-membered ring heteroaryls are pyridyl, pyrazinyl, pyrimidinyl, triazinyl, and pyridazinyl. In the present invention, the term “halo” or “halogen”, employed alone or in combination with other terms, refers to a halogen atom selected from F, Cl, I or Br. In some embodiments, “halo” refers to a halogen atom selected from F, Cl, or Br. In some embodiments, the halo is I. In the present invention, the term “substituent” or “substituents” as used refers to C1-C6 alkyl, aryl, C1-C6 alkoxyl, OH, halo, -NH2, -NO2, carbaldehyde, carboxylic acid, esters, sulfonic acid, cyano and/or isocyano. Particularly, the present invention provides a process for producing a compound of formula (I), comprising electrochemical oxidation of a compound of formula (II) to obtain the compound of formula (I): Wherein: R is H or C1-C6 alkyl, optionally substituted with at least one substituent; X is H, OH, halogen or C1-C6 alkoxy, optionally substituted with at least one substituent; and R', at each occurrence, is independently C1-C6 alkyl, aryl, C1-C6 alkoxy, cyano or halogen, optionally substituted with at least one substituent, or two adjacent R’, together with the atoms to which they are attached, form a fused C3-C8 cycloalkyl ring, a fused 5- or 6-membered heterocycloalkyl ring, a fused aryl ring or a fused 5- or 6-membered heteroaryl ring, optionally substituted with at least one substituent; and the subscript n is 0, 1, 2, 3 or 4. Preferably, R is H, methyl or ethyl; more preferably, R is H or methyl; and the most preferably, R is H. Preferably, X is H, OH, halogen or methoxy; more preferably X is H, OH, Cl, Br, I or methoxy; and the most preferably X is H, OH or methoxy. Preferably, R’, at each occurrence, is independently C1-C6 alkyl, aryl, C1-C6 alkoxy, cyano or halogen. More preferably, R’, at each occurrence, is independently methyl, ethyl, propyl, butyl, methoxy, phenyl, cyano and halogen. More preferably, two adjacent R’, together with the atoms to which they are attached, form a fused C3-C8 cycloalkyl ring such as cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl; a fused 5- or 6- membered heterocycloalkyl ring such as azetidinyl, piperidinyl and pyrrolidinyl; a fused aryl ring such as phenyl, 1-naphthyl, 2-naphthyl, anthracenyl and phenanthrenyl; or a fused 5- or 6-membered heteroaryl ring such as pyridyl, pyrimidinyl, pyrazinyl, pyridazinyl, pyrrolyl, pyrazolyl, azolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, imidazolyl, furanyl, thienyl, triazolyl, tetrazolyl, thiadiazolyl, quinolinyl, isoquinolinyl, indolyl, benzothienyl, benzofuranyl, benzisoxazolyl, imidazo[1,2-b]thiazolyl, purinyl and triazinyl. The most preferably, two adjacent R’, together with the atoms to which they are attached, form a fused C3-C8 cycloalkyl ring such as cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl; or a fused aryl ring such as phenyl, 1-naphthyl, 2-naphthyl, anthracenyl and phenanthrenyl. The most preferably, the compound of formula (I) is any one of the following compounds: In the present invention, the anode used for the electrochemical oxidation is made of the material selected from the group consisting of dimensional stable anodes (DSA), boron-doped diamond (BDD), and carbon-based materials such as graphite and glassy carbon. Preferably, the anode used in the present invention is a dimensional stable anode. The dimensional stable anode may contain, on a substrate such as titanium or tantalum, a coating of at least two metal oxides selected from the group consisting of RuO2, IrO2, PtO2, TiO2 and TaOx (x is 1 or 2), preferably selected from the group consisting of RuO2, IrO2 and TiO2, more preferably the dimensional stable anode contains a coating of RuO2 and IrO2. In the present invention, the material of the cathode is not critical and can be any suitable materials known in the art, such as steel, glassy carbon and platinum. The form and size (which also means the surface area) of the electrodes in the present invention are also not critical. They may be in any size and in any form, such as in a form of a wire, a rod, a cell, a mesh, a grid, a sponge, or any other design suitable for the electrochemical reactor (cell) used in the process of the present invention. The cell, also known as voltaic cells or galvanic cells, used in the process according to the present invention can be any one of those known by a person skilled in the art. Usually and preferably it is a two-compartments electrochemical flow-cell. In the present invention, the electrochemical oxidation is carried out in a medium which may be water or a non-aqueous solvent or mixture thereof. The examples of the non-aqueous solvent include but are not limited to alkylene carbonates (such as propylene carbonate and butylene carbonate), polyethylene glycol, N-methyl-2-pyrrolidone, N-butylpyrrolidone (NBP), alcohols (such as methanol, ethanol and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)), acetone, acetic acid, sulpholane, dimethylsulphoxide (DMSO), tetrahydrofuran (THF), 2-methyl-tetrahydrofuran (MeTHF), dimethylformamide (DMF), hexa-methylphosphoramide, acetonitrile (MeCN), dichloromethane (DCM), dimethoxyethane (DME), pyridine and hexafluoro-2-propanol, and mixture thereof. Preferably, the medium used in the present invention is a non-aqueous solvent or mixture thereof. More preferably, the medium used in the present invention is acetic acid, MeCN, DME or HFIP, or mixture thereof. The more preferably, the medium is MeCN. In the context of the present invention, the “non-aqueous solvent” means that no water is included in the solvent on purpose. However, it might be possible that the solvent comprises traces of water (usually below 5 wt%, based on the total weight of the solvent). In the present invention, the medium essentially comprises at least one supporting electrolyte, which may be added to in the form of a salt and/or in form of an acid. Any commonly known and commonly used supporting electrolyte can be used. Examples of the suitable supporting electrolytes include but are not limited to HCl, H 2 SO 4 , Na 2 SO 4 , NaCl, NaHSO 4 , alkyl- or arylsulfonic acids (such as methanesulfonic acid and p-toluenesulfonic acid), phosphoric acid, phosphates, and tetraalkylammonium salts (such as tetrabutylammonium acetate (NBu4OAc), tetrabutylammonium tetrafluoroborate and tetrabutylammonium hexafluorophosphate). Preferably, the supporting electrolyte used in the present invention is an acid such as HCl, H2SO4, phosphoric acid or mixture thereof. More preferably, the supporting electrolyte used in the present invention is H2SO4. In the present invention, the concentration of the supporting electrolyte in the medium is up to 5 mol/L (M), preferably from 0.01 M to 4 M, more preferably from 0.05 M to 3 M, the most preferably from 0.5 M to 1.5 M. Preferably, an oxygen source is added into the reaction of the present invention. Any material which can donate oxygen in the electro-oxidation environment of the present invention is suitable. Examples of the oxygen source includes but are not limited to water, methanol, acetic acid, tert- butanol, isopropanol, ethanol, hydrogen peroxide, tert-butyl hydroperoxide and oxygen. Preferably, the oxygen source is selected from the group consisting of water, methanol, acetic acid, tert-butanol, isopropanol and ethanol. More preferably, the oxygen source is water and/or methanol. The most preferably, the oxygen source is water and methanol. In the present invention, the oxygen source is included in the medium in an amount of from 0.1 vol% to 20 vol%, preferably from 0.5 vol% to 15.0 vol%, more preferably from 1.0 vol% to 10 vol%, such as 1 vol%, 2 vol%, 3 vol%, 3.5 vol%, 4 vol%, 4.5 vol%, 5 vol%, 5.5 vol%, 6 vol%, 6.5 vol%, 7 vol%, 7.5 vol%, 8 vol%, 8.5 vol%, 9 vol%, 9.5 vol% and 10 vol%. In one preferable embodiment of the present invention, the oxygen source is water, and it is included in the medium in an amount of from 1.0 vol% to 10 vol%, preferably from 2.0 vol% to 7.5 vol%, more preferably from 4.0 vol% to 6.0 vol% such as 4.0 vol%, 4.5 vol% and 5.0 vol%. In another preferable embodiment of the present invention, the oxygen source is methanol, and it is included in the medium in an amount of from 0.1 vol% to 10 vol%, preferably from 0.5 vol% to 7.5 vol%, more preferably from 1.0 vol% to 6.0 vol% such as 1.0 vol%, 2.0 vol%, 3.0 vol%, 4.0 vol% and 5.0 vol%. In a further preferable embodiment of the present invention, the oxygen source is water and methanol, and water is included in the medium in an amount of from 1.0 vol% to 10 vol%, preferably from 2.0 vol% to 7.5 vol%, more preferably from 4.0 vol% to 6.0 vol% such as 4.0 vol%, 4.5 vol% and 5.0 vol% and methanol is included in the medium in an amount of from 0.1 vol% to 10 vol%, preferably from 0.5 vol% to 7.5 vol%, more preferably from 1.0 vol% to 6.0 vol% such as 1.0 vol%, 2.0 vol%, 3.0 vol%, 4.0 vol% and 5.0 vol%. As anticipated by any person skilled in the art, electric current is used oxidant in the process of the present application. Accordingly, no additional terminal oxidant is preferably used in the present invention. In the present invention, the compound of formula (II) may be added into the medium in an amount of from 10 mmol/L (mM) to 100 mM, preferably from 20 mM to 75 mM, more preferably from 25 mM to 50 mM. In the present invention, the transferred charge for the electrochemical oxidation may be 10 Faraday (F) or less. A suitable range is 1-15 F, preferred is 2-10 F; more preferred is 3-9 F; most preferred is 4-8 F such as 4, 5, 6, 7 and 8 F. In the present invention, the current density used in the electrochemical oxidation may be from 0.5 mA/cm 2 to 100 mA/cm 2 , preferably from 1 mA/cm 2 to 50 mA/cm 2 , preferably from 2 mA/cm 2 to 20 mA/cm 2 . The electrochemical oxidation according to the present invention can be carried out in galvanostatic or potentiostatic mode. Depending on the cell, the electrochemical oxidation according to the present invention can be carried out batch-wise, semi-batch, or in a continuous way, preferably in a continuous way. The electrochemical oxidation according to the present invention is carried out at a temperature range of from 10 °C to 75 °C, preferably from 15 °C to 60 °C, more preferably at ambient temperature. The electrochemical oxidation according to the present invention is usually carried out at ambient pressure. The obtained compound of formula (I) according to the present invention can be isolated from the reaction medium using commonly methods. The process of the present invention can convert a broad range of phenols and analogues, especially hydroxyarenes and methoxyarenes, into the desired compound of formula (I) in an attractive yield without the need to modify the protocol. The process uses electricity as oxidant which is safe and inexpensive, avoids cost-intensive catalysts and oxidants, and does not provide reagent waste, so it is cost effective and environment friendly. The present invention is further illustrated by the following examples. In all the examples, the electrodes were obtained from commercial suppliers: DSA electrodes (De Nora, Italy), boron- doped diamond (DIACHEM®, CONDIAS GmbH, Germany), platinum (Ögussa, Austria), graphite (Sigrafine® V2100, Bonn-Bad Godesberg, Germany). Examples Example 1: Electrochemical synthesis of 2,6-dimethoxycyclohexa-2,5-diene-1,4-dione (2a) under alteration of the anode material The electrochemical oxidation was performed in a divided Teflon ® electrolysis cell equipped with magnetic stir bars (VWR brand, Germany). A glass frit was used as a separator. A platinum foil on a PTFE support was used as cathode and various materials (2.5 cm 2 ) as listed in Table 1 were used as anode. The electrolyte was prepared, by adding 5 vol% H2O and 1 M concentrated sulfuric acid (98%) to acetonitrile at 0 °C. Afterwards the electrolyte was allowed to reach room temperature. The 2,6- dimethoxyphenol (0.123 mmol, 25 mM) was added to the anodic compartment of the cell and 5 mL electrolyte was added to each compartment. The mixture was stirred with 300 rpm. The electrolysis was performed at 22 o C and at a constant current of 25 mA, a charge of 6 F and a current density of 6 mA/cm 2 . The reaction mixture was transferred to an Erlenmeyer flask afterwards, and the cell was washed with 20 mL dichloromethane. The pH value was adjusted to pH 7 by the addition of saturated aqueous sodium carbonate solution. The mixture was transferred to a separatory funnel and extracted with dichloromethane three times (3×20 mL). The combined organic fractions were dried over magnesium sulfate and filtered. The solvent was removed in vacuo. 1,3,5- Trimethoxybenzene (0.123 mmol, 21 mg, 1 eq.) was added as internal standard and the mixture was dissolved in deuterated chloroform. Afterwards 0.7 mL of the solution were transferred into an NMR tube, and the yield of the title compound is listed in Table 1. Table 1 Following the same process as above expect that a constant current of 15 mA and a charge of 4 F were used and additional 1 vol% methanol was added into the electrolyte solution, the materials listed in Table 2 were tested as anode. The yield of the title compound is listed in Table 2. Table 2 Example 2: Electrochemical synthesis of 2,6-dimethoxycyclohexa-2,5-diene-1,4-dione (2a) under various contents of oxygen source The electrochemical oxidation was performed in a flow electrolysis cell ElectraSyn flow (IKA ® - Werke GmbH & Co. KG, Germany) with ruthenium-iridium oxide on titanium support (12 cm 2 ) (Industrie De Nora S.p.A., Italy) as anode and platinum (12 cm 2 ) as cathode. A Nafion TM N324 membrane (DuPont, United States) was used as a separator and a 0.25 mm Teflon® spacer (IKA ® -Werke GmbH & Co. KG, Germany) was placed on each side of the membrane. The electrolyte solutions were prepared by dissolving 1 M concentrated sulfuric acid (98%), 4.5 vol% H2O and various amount of methanol as shown in Table 3 in acetonitrile at 0 °C. To the anolyte, appropriate amounts of the substrate 2,6-dimethoxyphenol, to maintain a concentration of 25 mM, were added. The anolyte solution was stirred during the course of the electrolysis. A multi-channel peristaltic pump was used to simultaneously pump both solutions through the half cells. The electrolysis was performed as single-pass electrolysis at 22 o C and at a constant current of 24 mA, a charge of 4 F and a current density of 2 mA/cm 2 . The first 5 mL of electrolyzed solutions were discarded. Afterwards the solutions were collected 30 in two graduate cylinders. After appropriate amounts of solutions were collected, 5 mL of each graduate cylinder were transferred to an Erlenmeyer flask using a bulb pipette. 20 mL dichloromethane were added, and the pH value was adjusted to 7 by the addition of saturated aqueous sodium carbonate solution. The solution was transferred to a separatory funnel and extracted two more times with 20 mL (3×20 mL) dichloromethane. The combined organic fractions were dried over magnesium sulfate and filtered. The solvent was removed in vacuo. 1,3,5- Trimethoxybenzene (0,123 mmol, 21 mg, 1 eq.) was added as an internal standard and the mixture was dissolved in deuterated chloroform. Afterwards 0.7 mL of the solution were transferred to an NMR tube, and the yield of the title compound is listed in Table 3. Table 3 Example 4: Electrochemical synthesis of 1,4-quinones with various substrates The electrochemical oxidation was performed in a flow electrolysis cell ElectraSyn flow (IKA ® - Werke GmbH & Co. KG, Germany) with ruthenium-iridium oxide on titanium support (12 cm 2 ) (Industrie De Nora S.p.A., Italy) as anode and platinum (12 cm 2 ) as cathode. A Nafion TM N324 membrane (DuPont, United States) was used as a separator and a 0.25 mm Teflon® spacer (IKA ® -Werke GmbH & Co. KG, Germany) was placed on each side of the membrane. The electrolyte solutions were prepared by dissolving 1 M concentrated sulfuric acid (98%), 4.5 vol% H2O and 5 vol% methanol in acetonitrile at 0 °C. To the anolyte, appropriate amounts of the substrates as listed in Table 4, to maintain a concentration of 25 mM, were added. The anolyte solution was stirred during the course of the electrolysis. A multi-channel peristaltic pump was used to simultaneously pump both solutions through the half cells. The electrolysis was performed as single-pass electrolysis at 22 o C and at a constant current of 24 mA, a charge of 2-12 F(Faraday) and a current density of 2 mA/cm 2 . The first 5 mL of electrolyzed solutions were discarded. Afterwards, the solutions were collected into two graduate cylinders. After appropriate amounts of solutions were collected, a transfer to an Erlenmeyer flask was performed. A two-fold volume of dichloromethane was added, and the pH value was adjusted to 7 by the addition of saturated aqueous sodium carbonate solution. The solution was transferred to a separation funnel and extracted two more times with 50 mL of dichloromethane. The combined organic fractions were dried over magnesium sulfate and filtered. The solvent was removed in vacuo. The crude product was purified by flash column chromatography unless otherwise stated. The results are listed in Table 4 Table 4 [a] Deviation: c(substrate): 17.1 mM.