Method for producing 2,3,5-trimethyl benzoquinone by oxidation of 2,3,6-trimethyl phenol

Abstract

The present invention relates to a process of oxidation of 2,3,6-trimethyl phenol by oxygen or an oxygen-containing gas in the presence of in the presence of a copper (II) halide as catalyst and of water and a specific diol comprising at least on tertiary hydroxyl group as reaction medium.

Description

[0001] , , oquinone obtained by oxidation of 2,3,6-trimethyl phenol.

[0002] Background of the invention

[0003] 2,3,5-Trimethyl benzoquinone is an important intermediate for producing 2,3,5-trimethyl hydroquinone, which is an important precursor for the industrial manufacture of a-tocopherol and a-tocopheryl acetate.

[0004] For the synthesis of 2,3,5-trimethyl benzoquinone, several syntheses are known. Typically, molecular oxygen is used. It has been shown that copper (II) halides are the most effective catalysts for the oxidation of 2,3,6-trimethyl phenol. However, this process has several drawbacks: This process leads particularly to the formation of organochlorine by-products, such as 4-chloro-2,3,6-trimethyl phenol as pointed out in EP 3 016 928 B1 .

[0005] EP 3 016 928 B1 discloses the oxidation of 2,3,6-trimethyl phenol to 2,3,5- trimethyl benzoquinone with oxygen or an oxygen-containing gas in a two-phase or multi-phase reaction medium, which comprises water and at least one secondary aliphatic acyclic alcohol having six or more carbon atoms in the presence of a copper(ll) halide. In this process the formation of organochlorine by-products is said to be reduced. To get the desired product in acceptable yields, stoichiometric amounts of lithium chloride relative to the copper catalyst needs to be added as cocatalyst. Lithium chloride, however, is an ingredient which is very expensive and difficult to recycle. Due to a significant raise in the global demand and, consequently, the price for lithium, cost contribution of LiCI in the production cost based on LiCI will increase in the next future considerably as well. Furthermore, the use of lithium-based catalysts is critical in view of sustainability. Finally, the use of lithium compounds is also problematic in view of ecotoxicology as it has for example remarkable negative effect on aquatic life forms. Furthermore, the oxidation of 2,3,6-trimethyl phenol is carried out at a temperature between 323 and 338 K, particularly between 326 and 331 K. This temperature, however, is very near or even above the flash point (fp) of indicated secondary alcohols (3-heptanol (flash point: 60°C 1333 K) or 2-octanol (flash point: 71 °C / 344K)).

[0006] Summary of the invention

[0007] Therefore, the problem to be solved by the present invention is to offer a method for the oxidation of 2,3,6-trimethyl phenol, which is highly efficient, provides the product in high yield at temperatures which are significantly below the flash point of the solvent without using of lithium chloride as co-catalyst for copper (II) halides.

[0008] Surprisingly, it has been found that the process according to claim 1 offers a solution to this problem.

[0009] This can be achieved particularly by the using a diol of the formula (III) and water as reaction medium.

[0010] It has been found that the oxidation of 2,3,6-trimethyl phenol proceeds already at low temperatures. It has been observed that at mild reaction temperatures of 318 K or below, yields of more than 85 % can be achieved. Furthermore, the invention allows the production of 2,3,5-trimethyl benzoquinone at very high yields, e.g. > 97%, at higher temperatures, which are still significantly lower than the flash point of the diol used. It has been found that performing the reaction at temperatures even lower than 45 K relative to the flash point is possible and moderate yields of more than 80% can be obtained. All this allows reducing risks due to fire hazards of the alcohol used in the oxidation.

[0011] Furthermore, it has been surprisingly found that in 2,3,5-trimethyl benzoquinone as produced by the process, no or significantly low amounts of organochlorine compounds such as 4-chloro-2,3,6-trimethylphenol as well as of undesired dimerization products, such as 2,2',3,3',5,5'-hexamethyl-[1 ,T-biphenyl]- 4,4'-diol, can be found.

[0012] Furthermore, it has been found that in this process of oxidation of 2,3,6- trimethyl phenol there is no need for lithium chloride as co-catalyst, which is a remarkable step forward in improving cost situation and ecology, particularly in reducing lithium waste. Finally, this process also allows the production of 2,3,5- trimethyl benzoquinone under lithium-free conditions.

[0013] Further aspects of the invention are subject of further independent claims.

[0014] Particularly preferred embodiments are subject of dependent claims.

[0015] Detailed description of the invention

[0016] In a first aspect the present invention relates to a process for preparing 2,3,5-trimethyl benzoquinone of the formula (I) by oxidation of 2,3,6-trimethyl with oxygen or an oxygen-containing gas in a reaction medium comprising water and a diol, which comprises at least one tertiary hydroxyl group of the formula (III) in the presence of a copper (II) halide as catalyst or as part of a catalyst system, characterized in that

[0017] R’ and R”, independently from each other, represent a Ci-12-alkyl group or a

[0018] C5-i2-cycloalkyl group or a C5-12 aryl group;

[0019] R1and R2, independently from each other, represent either H or a methyl group, and n = 0 - 6, preferably 0 or 1 , more preferably n = 1 .

[0020] For sake of clarity, some terms used in the present document are defined as follows: In the present document, a “Cx-y-alkyl” group is an alkyl group comprising x to y carbon atoms, i.e., for example, a Ci-3-alkyl group is an alkyl group comprising 1 to 3 carbon atoms. The alkyl group can be linear or branched. For example -CH(CH3)-CH2-CH3 is considered as a C4-alkyl group.

[0021] In the present document, a “Cx-y-aryl” group is an aryl group comprising x to y carbon atoms, whereas not all carbon atoms need to be aromatic carbon atoms. For example, an o-tolyl group, is considered as a Cz-aryl and 1 -naphthyl as a Cw-aryl group. Such a Cx-y-aryl may also comprise also hetero atoms, particularly N, in the aromatic group. Therefore, a pyridyl group is regarded as a Cs-aryl group.

[0022] In case identical labels for symbols or groups are present in several formulae, in the present document, the definition of said group or symbol made in the context of one specific formula applies also to other formulae which comprises the same said label.

[0023] The term “independently from each other” in this document means, in the context of substituents, moieties, or groups, that identically designated substituents, moieties, or groups can occur simultaneously with a different meaning in the same molecule.

[0024] The term “tertiary hydroxyl group” in this document means a hydroxyl group which is bound to a carbon atom which is bound to three other carbon atoms.

[0025] A “secondary hydroxyl group” in this document means a hydroxyl group which is bound to a carbon atom which is bound to one hydrogen atoms and two other carbon atoms:

[0026] A “primary hydroxyl group” in this document means a hydroxyl group which is bound to a carbon atom which is bound to two hydrogen atoms and one other carbon atom.

[0027] A “diol” in this document means an alcohol comprising two hydroxyl groups, also known as dihydric alcohol.

[0028] The flash points as used in this document are measured using the closed cup method. In the present reaction, 2,3,6-trimethyl phenol of the formula (II) is oxidized to 2,3,5-trimethyl benzoquinone of the formula (I).

[0029] 2,3,6-trimethyl phenol of the formula (II) is broadly available and is typically prepared from m-cresol by catalytic alkylation with methanol or by condensation of diethyl ketone with crotonaldehyde or with methyl vinyl ketone to give 2,3,6-trimethylcyclohex-2-en-1-one, followed by catalytic dehydrogenation as described for example by Bonrath, W., Wyss, A., Litta, G., Baldenius, K.-U., von dem Bussche-Hiinnefeld, L., Hilgemann, E., Hoppe, P., Sturmer, R. and Netscher, T. (2021). Vitamins, 4. Vitamin E (Tocopherols, Tocotrienols). In Ullmann's Encyclopedia of Industrial Chemistry. htps: / / doi.org / 10.1002 / 143560Q7.o27 o07.pub2.

[0030] As given above, the oxidation process is performed with oxygen or an oxygen-containing gas.

[0031] In one of the embodiments the process is performed in the presence of oxygen, i.e. molecular oxygen (O2).

[0032] In another embodiment, the process is performed in the presence of an oxygen (O2) containing gas.

[0033] An oxygen containing gas is particularly a gas mixture of oxygen with at least one inert gas, such as argon, xenon, or nitrogen (N2).

[0034] Particularly preferred as oxygen (O2) containing gas is air.

[0035] The oxygen or the oxygen containing gas is introduced into a reaction medium comprising water and a diol of the formula (III).

[0036] Said reaction medium is a key element of the present invention. Diol having at least a tertiary hydroxyl group

[0037] The reaction medium comprises at least one tertiary hydroxyl group of the formula (III)

[0038] R’ and R”, independently from each other, represent a Ci-12-alkyl group or a C5-i2-cycloalkyl group or a C5-12 aryl group. Hence, the hydroxyl group near R’ and R” is a tertiary hydroxyl group.

[0039] For the present invention, it is essential that the alcohol of the formula (III) is a diol, i.e., that it has two hydroxyl groups, of which at least one is a tertiary hydroxyl group.

[0040] It is preferred that R’ and / or R” represent a Ci-12-alkyl group, particularly a Ci-3-alkyl group, preferably a methyl or ethyl group, most preferably a methyl group.

[0041] It is particularly preferred that R’ = R”.

[0042] Most preferably, R’ = R” = CH3.

[0043] R1and R2, independently from each other, represent either H or a methyl group.

[0044] In case R1= R2= methyl, the hydroxyl group near R1and R2is a tertiary hydroxyl group.

[0045] In case R1is H and R2is methyl, the hydroxyl group near R1and R2is a secondary hydroxyl group.

[0046] In case R1= R2= H, the hydroxyl group near R1and R2is a primary hydroxyl group.

[0047] In case n is 0, it is preferred that that R1= R2= R’ = R” = methyl. It is preferred that the diol of the formula (III) is selected from the group consisting of

[0048] 2-methyl-2,4-pentanediol (MPD)

[0049] 3-methyl-1 ,3-butanediol (MBD)

[0050] 2,4-dimethylpentane-2,4-diol It is particularly preferred that the diol of the formula (III) is selected from the group consisting of 2-methyl-2,4-pentanediol (MPD) (lll-A), 3-methyl-1 ,3- butanediol (MBD) (lll-B), 2,4-dimethylpentane-2,4-diol (lll-C), and 2,3-dimethyl- butane-2,3-diol (pinakol) (lll-D).

[0051] In a highly preferred embodiment, R1= H and R2= CH3 and n = 1.

[0052] In one of the most preferred embodiments, the diol of the formula (III) is 2-methyl- 2,4-pentanediol (MPD) (lll-A).

[0053] In another preferred embodiment, R1= R2= H and n = 1 .

[0054] In another of the most preferred embodiments, the diol of the formula (III) is 3- methyl-1 ,3-butanediol (MBD) (lll-B).

[0055] When using any organic solvent, the flash point of said organic solvent sets a critical limit for the highest temperature at which the oxidation reaction can be performed at safe conditions.

[0056] In general, it has been found for the oxidation with copper (II) halides, that the oxidation proceeds better at higher temperatures.

[0057] In other words, it is preferred that the diol of the formula (III) has a flash point as high as possible which allows to perform the oxidation at temperatures as high as possible.

[0058] However, it has been found that in the present process, the oxidation of 2,3,6-trimethyl phenol of the formula (II) to 2,3,5-trimethyl benzoquinone of the formula (I) can be performed at already 318 K in moderate (above 80%) yields. As already pointed out above, the yield raises if the operating temperature increases.

[0059] Whereas the state-of-the-art procedure typically requires reaction times of 4 to 8 hours, preferably 5 to 7 hours, for the oxidation reaction of 2 ,3, 6-trimethyl phenol, it has been found that even at low temperatures, the oxidation in the present process proceeds very fast, so that after already 1-2 hours reaction time of oxidation, high conversion and yields can be achieved.

[0060] Therefore, it has been found that the oxidation of 2, 3, 6-trimethyl phenol can be performed at a temperature of between 313 K and the flash point of the diol of the formula (III), particularly between 333 K and the flash point of the diol of the formula (III), preferably between 343 K and the flash point of the diol of the formula (III).

[0061] In other words, it is preferred if the diol of the formula has a flash point as high as possible.

[0062] However, the risk of any fire hazard can be reduced if the temperature of operating the oxidation reaction is significantly lower than, particularly at least 10 K lower, preferably 15 or even 25 K, than the flash point of the solvent.

[0063] Therefore, it is preferred that the oxidation is performed at a temperature of 10 K lower, particularly 15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III).

[0064] In certain cases, it has been found that the oxidation process can be performed at temperatures which are even more than 40 K below the flash point of the diol of formula (III) and still yields of more than 85% can be achieved. It is particularly preferred that the oxidation is performed at a temperature of between 313 K and a temperature of 10 K lower, particularly 15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III), particularly of between 333 K and a temperature of 10 K lower, particularly 15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III), preferably of between 343 K and a temperature of 10 K lower, particularly 15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III). As pointed out a high flash point is a big advantage of the diol of the formula (III) as part of the reaction medium in the oxidation reaction of the present invention. Table 1 showstheflashpointsforafewpreferreddiolsoftheformula(lll).

[0065] Table 1. Flash points of selected diols of formula (III)

[0066] 1Chemical Book (https: / / www.chemicalbook.com / )

[0067] 2determined by closed cap, SDS Sigma-Aldrich

[0068] 3Calculated using Advanced Chemistry Development (ACD / Labs) Software V11 .02 (© 1994-2023 ACD / Labs) It is particularly preferred that the diol of the formula (III) has a flash point of more than 353 K, more preferably more than 363 K, most preferably more than 366 K or above.

[0069] Next to the diol of formula (III), another important and required part of the reaction medium is water.

[0070] It is preferred that the weight ratio of water to the diol of the formula (III) is between 1 :1 and 1 :10, particularly between 1 :1 and 1 :6, preferably between 1 :2 and 1 :5.

[0071] It is, particularly preferred that the diol of the formula (III) is in excess relative to the weight of water in the reaction medium.

[0072] Copper (II) halide

[0073] As mentioned above the oxidation is performed in the presence of a copper (II) halide as catalyst or as part of a catalyst system.

[0074] The preferred halide is either chloride or bromide, particularly chloride.

[0075] The copper (II) halide is preferably CuC , Cu2CI(OH)3, or any hydrate of CuCI2, particularly CuC ’2 H2O.

[0076] A catalyst system of which copper (II) halide is part of, can be systems in which copper (II) halide is not a sole catalyst or is not its free form, such for example a mixture of copper (II) halide and another oxidation catalyst or a copper (II) halide with is fixed or adsorbed on the surface or inside a solid carrier material.

[0077] The most preferred copper (II) halide is CuCl2*2 H2O.

[0078] It is preferred that the molar ratio of 2 ,3 ,6-trimethyl phenol of the formula (II) to the copper (II) halide is between 1.5:1 and 50:1 , particularly between 2:1 and 20:1 , preferably between 2:1 and 10:1. Most preferred is said molar ratio between 2:1 and 3:1. It is preferred that the ratio of weight of Cu(ll) to the weight of the sum of water and diol of the formula (III) is preferably in the range of 1 :100 to 20:100, particularly 5:100 to 15:100.

[0079] It is further preferred that the mol-ratio of Cu(ll) and the diol of the formula (III) is between 15:100 and 30:100, particularly between 20:100 and 25:100.

[0080] It is further preferred that the mol-ratio of Cu(ll) to 2, 3,6-trimethyl phenol is between 25:100 and 60:100, particularly between 30:100 and 50:100, preferably between 35:100 and 45:100.

[0081] As a big advantage towards the known processes of the state of the art, it has been found that the oxidation reaction of the present invention can be performed in the absence of any alkali compound or alkaline earth metal compound.

[0082] Hence, it is preferred that said oxidation reaction is performed in the absence of any alkali compound or alkaline earth metal compound, particularly in the absence of any magnesium or lithium compound.

[0083] Particularly LiCI is very expensive. Due to the global increase of demand for lithium, particularly for batteries, the lithium price has tremendously increased the last decade. It is not to be expected that said trend of cost increase will stop in the near future.-Hence, a process which can be performed in the absence of any lithium compound is very advantages, particularly in view of cost savings.

[0084] Furthermore, the use of lithium is very disadvantageous in the aspect of sustainability. It is particularly difficult to remove and / or to recycle lithium from the waste water of the process. This is particularly relevant as lithium has significant negative effects on aquatic life forms. It is, therefore, very advantageous that the present process can offer the possibility to have a lithium-free oxidation process. In one of the preferred embodiments, the oxygen or oxygen-containing gas is added in a batch at a pressure of between 1.1 and 2 bara, preferably at about 1.3 bara during the addition of 2,3,6-trimethyl phenol, and a stirring time of typically 1 to 4 hours, preferably 1.5 to 2.5 hours, after the completion of 2,3,6- trimethyl phenol addition.

[0085] In another preferred embodiment, the oxygen or oxygen-containing gas is added in a purge mode whereby a constant pressure of between 1.1 and 2 bara, preferably at about 1.3 bara is applied during the dropwise addition of 2,3,6- trimethyl phenol under a purge stream of oxygen or oxygen-containing gas during the reaction time of typically 1 to 4 hours.

[0086] Depending on the diol used, the reaction mixture is at the end of the reaction homogeneous or heterogeneous (two-liquid phases). In case of two phases, these two phases can be separated and the 2,3,5-trimethyl benzoquinone can be isolated from the diol phase.

[0087] 2,3,5-Trimethyl benzoquinone can be further purified for example by extraction, e.g. with toluene.

[0088] In case the reaction mixture does not show macroscopic phase separation, which would allow a swift phase separation, either water can be added to trigger a phase separation, or the reaction mixture can be directly extracted with a suitable solvent.

[0089] As described above in great the detail, the oxidation process for the oxidation of is 2,3,6-trimethyl phenol of the formula (II) to 2,3,5-trimethyl benzoquinone of the formula (I) is very advantageous.

[0090] Hence, in a further aspect, the current invention relates to a composition suitable to be oxidized comprising

[0091] 2,3,6-trimethyl phenol of the formula (II) copper (II) halide water; and a diol of the formula (III) , y from each other, represent a Ci-12-alkyl group or a C5-i2-cycloalkyl group or a C5-12 aryl group;

[0092] R1and R2, independently from each other, represent either H or a methyl group, n = 0 - 6, preferably 0 or 1 , more preferably n = 1 .

[0093] It is known, for example from in EP 3 016 928 B1 , that in an oxidation process involving copper(ll) chloride typically organochlorine compounds are formed and found in the product, i.e. in 2,3,5-trimethyl benzoquinone, which reduces the yield in 2,3,5-trimethyl benzoquinone. Furthermore, it is very difficult to separate the organochlorine compounds from 2,3,5-trimethyl benzoquinone.

[0094] Therefore, it is very advantageous if the content of organochlorine compounds in 2,3,5-trimethyl benzoquinone is as little as possible.

[0095] This reduced formation of organochlorine compounds is important as it is known that, particularly under thermal load, the organochlorine compounds can give off hydrogen chloride, which might result in processing 2,3,5-trimethyl- benzoquinone, e.g. in further reaction steps in the production of 2,3,5-trimethyl hydroquinone. It has been found that the process as described above in great details produces 2 ,3, 5-trimethyl benzoquinone of the formula (I) which comprises no or at least only very small amounts of organochlorine products, such as 4-chloro-2,3,6- trimethylphenol (formula (IV)). It has been particularly found that, by said process, compositions of 2,3,5- trimethyl benzoquinone of the formula (I) can obtained which have a content of the compound of the formula (IV) of less than 2.2 % by weight, preferably of less than 1.1 %by weight, most preferably less than 0.1 % by weight.

[0096] Furthermore, it has been observed that said process produces 2,3,5- trimethyl benzoquinone of the formula (I) which comprises no or at least only very small amounts of undesired dimerization products, such as 2, 2', 3, 3', 5,5'- hexamethyl-[1 ,1'-biphenyl]-4,4'-diol (formula (V).

[0097] Dimerization products are also disadvantageous as their formation reduces the yield of the desired oxidation product, i.e. 2, 3, 5-trimethyl benzoquinone.

[0098] Particularly, it has been found that, by said process, compositions of 2, 3, 5-trimethyl benzoquinone of the formula (I) can be obtained with a content of the compound of the formula (V) of less than 3 % by weight, preferably of less than 0.3 % by weight, most preferably less than 0.1 % by weight. The formation of the undesired by-product reduces the yield of the desired 2,3,5-trimethyl benzoquinone.

[0099] As has been shown above, the diol of the formula (III) plays a key role in the present invention.

[0100] Hence, in a further aspect, the invention relates to the use of a diol of the formula (III) in the oxidation of 2,3,6-trimethyl phenol of the formula (II) to yield 2,3,5-trimethyl benzoquinone of the formula (I) wherein

[0101] R’ and R”, independently from each other, represent a Ci-12-alkyl group or a C5-i2-cycloalkyl group or a C5-12 aryl group;

[0102] R1and R2, independently from each other, represent either H or a methyl group, n = 0 - 6, preferably 0 or 1 , more preferably n = 1 .

[0103] As mentioned above, 2,3,5-trimethyl benzoquinone is an important intermediate to produce 2,3,5-trimethyl hydroquinone, which is a key building block for a-tocopherol and a-tocopheryl acetate. Hence, the present invention also relates to the use of the 2,3,5-trimethyl- benzoquinone prepared by the process of the invention, for the synthesis of 2,3,5- trimethyl hydroquinone and particularly for a-tocopherol, such as (all racj-a-toco- pherol or 2 / ?S-a-tocopherol, or a-tocopheryl acetate, such as (all-rac)-a-toco- pheryl acetate or 2 / ?S-a-tocopheryl acetate.

[0104] Examples

[0105] The present invention is further illustrated by the following experiments. General Example

[0106] In a 250 mL titanium autoclave with gas entrainment stirrer, a suspension of copper(ll) chloride dihydrate (concentration given in table 1) in the solvent mixture as indicated in table 1 was warmed to the temperature T as given in table 1 . With mechanical stirring (1400 rpm), molecular oxygen was introduced into the system (oxygen inlet below surface) at a pressure as indicated in table 1 at 20 mL / min purge stream. Then, a solution of 2,3,6-trimethyl phenol in the solvent mixture was added at the temperature T as given in table 1 via a dosing pump within the time t1 given in table 1. After addition of 2,3,6-trimethyl phenol was complete, stirring was continued for a time t2 (as given in tablei) at the temperature T. After that, the reaction mixture was cooled to 297 K within 30 min and unloaded from the autoclave. It was analyzed by quantitative HPLC measurement. 2,3,5-Trimethyl benzoquinone was obtained in yields and conversions as given in table 1.

[0107] benzoquinone.

[0108] 1W=H20; 3H = 3-heptanol; MPD= 2-methyl-2,4-pentanediol

[0109] 2vol / vol

[0110] 3concentration of 2,3,6-trimethylphenol

[0111] 4CuCh’2 H2O

[0112] 5Time of adding 2 ,3, 6-trim ethy I phenol ( / / ) + stirring time after addition (t2)

[0113] 6fp= flash point of alcohol used

[0114] 7AT = distance to flash point = fp - T

[0115] 8Yield in 3,5-trimethyl benzoquinone.

[0116] 9Conversion of 2,3,6-trimethyl phenol

[0117] 10Yield in 4-chloro-2,3,6-trimethyl phenol (determined by quant. HPLC)

[0118] 11Yield in 2,2',3,3',5,5'-hexamethyl-[1 ,1'-biphenyl]-4,4'-diol trimethylphenol (determined by quant. HPLC)

Claims

Claims1 . A process for preparing 2 ,3, 5-trimethyl benzoquinone of the formula (I) by oxidation of 2,3,6-trimethyl phenol of the formula (II)with oxygen or an oxygen-containing gas in a reaction medium comprising water and a diol, which comprises at least one tertiary hydroxyl group of the formula (III)in the presence of a copper (II) halide as catalyst or as part of a catalyst system, characterized in thatR’ and R”, independently from each other, represent a Ci-12-alkyl group or a C5-i2-cycloalkyl group or a C5-12 aryl group;R1and R2, independently from each other, represent either H or a methyl group, and n = 0 - 6, preferably 0 or 1 , more preferably n = 1 .

2. The process according to claim 1 , characterized in that R’ = R” = CH3.

3. The process according to claim 1 , characterized in that R1= H and R2= CH3 and n = 1 , preferably that the diol of formula (III) is 2-methyl-2,4-pentanediol.

4. The process according to claim 1 or 2, characterized in that R1= R2= H and n = 1 , preferably that the diol of formula (III) is 3-methyl-1 ,3-butanediol.

5. The process according to any of the preceding claims characterized in that the oxidation is performed at a temperature of between 313 K the flash point of the diol of the formula (III), particularly between 333 K and the flash point of the diol of the formula (III), preferably between 343 K and the flash point of the diol of the formula (III).

6. The process according to any of the preceding claims 1 to 3, characterized in that the oxidation is performed at a temperature of 10 K lower, particularly15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III).

7. The process according to any of the preceding claims 1 to 4, characterized in that the oxidation is performed at a temperature of between 313 K and a temperature of 10 K lower, particularly 15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III), particularly of between 333 K and a temperature of 10 K lower, particularly 15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III), preferably of between 343 K and a temperature of 10 K lower, particularly 15 K lower, preferably 19 K lower, more preferably 25 K lower, than the flash point of the diol of the formula (III).

8. The process according to any of the preceding claims characterized that the weight ratio of water to the diol of the formula (III) is between 1 :1 and 1 :10, particularly between 1 :1 and 1 :6, preferably between 1 :2 and 1 :5.

9. The process according to any of the preceding claims characterized in that the copper (II) halide is CuC , Cu2CI(OH)3, or any hydrate of CuC , particularly CuCl2*2 H2O.

10. The process according to any of the preceding claims characterized that the molar ratio of 2,3,6-trimethylphenol of the formula (II) to the copper (II) halide is between 1.5:1 and 50:1 , particularly between 2:1 and 20:1 , preferably between 2:1 and 10:1.11 . The process according to any of the preceding claims characterized that the oxidation is made by molecular oxygen gas.

12. The process according to any of the preceding claims 1 to 10 characterized the oxidation is made by an oxygen containing gas, particular air.

13. The process according to any of the preceding claims characterized in that the oxidation is performed in the absence of any alkali compound or alkaline earth metal compound, particularly in the absence of any magnesium or lithium compound.

14. A composition suitable to be oxidized comprising2 ,3,6-trimethyl phenol of the formula (II)a diol of the formula (III)whereinR’ and R”, independently from each other, represent a Ci-12-alkyl group or a C5-i2-cycloalkyl group or a C5-12 aryl group;R1and R2, independently from each other, represent either H or a methyl group, n = 0 - 6, preferably 0 or 1 , more preferably n = 1 .

15. Use of a diol of the formula (III) in the oxidation of 2,3,6-trimethyl phenol of the formula (II) to yield 2,3,5-trimethyl benzoquinone of the formula (I)whereinR’ and R”, independently from each other, represent a Ci-12-alkyl group or a C5-i2-cycloalkyl group or a C5-12 aryl group;R1and R2, independently from each other, represent either H or a methyl group, n = 0 - 6, preferably 0 or 1 , more preferably n = 1 .

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