Photooxidation of 2,3,6-trimethylphenol

Abstract

The present invention relates to the photooxidation of 2,3,6-trimethylphenol using light in the long wavelength region of the visible spectrum with methylene blue as a photosensitizer in a solvent mixture of water and alcohol to give 2,3,5-trimethylbenzoquinone.

Description

Detailed Description of the Invention

[0001] [Technical field] The present invention relates to the field of the preparation of 2,3,5-trimethylbenzoquinone, and in particular the photooxidation of 2,3,6-trimethylphenol.

[0002] [Background of the invention] 2,3,5-Trimethylbenzoquinone is a key intermediate in the production of 2,3,5-trimethylhydroquinone or α-tocopherol, respectively.

[0003] Murtinho D. et al., J. Chem. Soc. Perkin Trans. 2, 2000, 2441-2447, proposes the photooxidation of 2,3,5-trimethylphenol with oxygen in the presence of a photosensitizer to obtain 2,3,5-trimethylbenzoquinone. In particular, methylene blue is disclosed as a photosensitizer for obtaining 1,5-dihydroxynaphthalene in a mixture of acetonitrile and dichloromethane. However, since the yield is not very high at 78-82%, it has been proposed to use a porphyrin-type photosensitizer instead. This type of porphyrin compound is somewhat expensive and not easily available commercially. On the other hand, both acetonitrile and dichloromethane are solvents with significant environmental and ecotoxicological disadvantages.

[0004] Moreover, it is known that it is much more difficult to oxidize phenols than it is to oxidize naphthols.

[0005] In an alternative procedure, 2,3,5-trimethylbenzoquinone can essentially be obtained by oxidation of 2,3,6-trimethylphenol.

[0006] Methylene blue is commonly used as a photosensitizer in photoreactions and is readily available commercially from a variety of sources.

[0007] [Summary of the invention] Therefore, an object of the present invention is to provide a highly efficient method for synthesizing 2,3,5-trimethylbenzoquinone from 2,3,6-trimethylphenol in high yield and high selectivity.

[0008] Surprisingly, it has been found that photo-oxidation as defined in claim 1 provides a highly efficient way to solve this problem.

[0009] In the present invention, methylene blue, a very attractive photosensitizer, which is readily available and cost-effective, can be used, and the desired product can be obtained not only with high yields at high conversion rates, but also with very high selectivity. Particularly advantageously, the process can be carried out in the absence of chlorinated solvents, making the process very attractive for industrial applications.

[0010] Further aspects of the invention are the subject matter of the other independent claims. Particularly preferred embodiments are the subject matter of the dependent claims.

[0011] [Detailed Description of the Invention] The present invention relates to a process for preparing a compound of formula (I) from a compound of formula (II), comprising: [ka] Oxygen and formula (III): [ka] (In the formula, R 8 , R 8’ , R 8’’ , and R 8’’’ are each independently H or C 1~4 represents either an alkyl group; or R 8 and R 8’ and / or R 8’’ and R 8’’’ together with N form a 5- or 6-membered ring; However, R8 Group, R 8’ Group, R 8’’ Groups and R 8’’’ At least one of the groups is not H; X - represents an anion), Water and at least one C 1~8 Alkanol or at least one C 2~4 Used in a solvent mixture with an alkylene diol, The peak wavelength of the spectrum (λ max ) in the range of 580 to 780 nm to carry out photo-oxidation to produce a compound of formula (I) from a compound of formula (II).

[0012] For clarity, several terms used in this document are defined as follows:

[0013] In this document, "C x~y An "alkyl" group is an alkyl group containing x to y carbon atoms, i.e., for example, C 1~3 An alkyl group is an alkyl group containing 1 to 3 carbon atoms. An alkyl group may be straight or branched chain. For example, -CH(CH3)-CH2-CH3 is considered a C4 alkyl group.

[0014] Similarly, C x~y Alkanol or C x~y An alkylenediols is an alcohol containing x to y carbon atoms, each having one or two OH groups and each having an alkyl or alkylene group.

[0015] In this document, when the same subscript for a symbol or group occurs in several formulas, the definition of said group or symbol given in relation to one particular formula also applies to other formulas containing the same subscript.

[0016] The peak wavelength is the wavelength at which the intensity of the spectrum is greatest.

[0017] In the above method, 2,3,6-trimethylphenol (=the compound of formula (II)) is photooxidized to give 2,3,5-trimethylbenzoquinone (=the compound of formula (I)).

[0018] 2,3,6-Trimethylphenol is a known chemical and is available commercially in large quantities from a variety of sources.

[0019] This photo-oxidation involves the reaction of a compound represented by formula (III): [ka] (In the formula, R 8 , R 8’ , R 8’’ , and R 8’’’ are each independently H or C 1~4 represents either an alkyl group; or R 8 and R 8’ and / or R 8’’ and R 8’’’ together with N form a 5- or 6-membered ring; However, R 8 Group, R 8’ Group, R 8’’ Groups and R 8’’’ At least one of the groups is not H; X - A photosensitizer (wherein represents an anion) is used.

[0020] In one embodiment, R 8 and R 8 ' and / or R 8’’ and R 8’’’ taken together form -(CH2)5- or -(CH2)2-NH-(CH2)2- or -(CH2)2-N(C 1~4 The alkyl group forms -(CH2)2- or -(CH2)2-S-(CH2)2- or -(CH2)2-O-(CH2)2-.

[0021] More preferably, R 8 =R 8’’and / or R 8’ =R 8’’’ More preferably, R 8 =R 8’ =R 8’’ =R 8’’’ It is.

[0022] More preferably, the substituent R 8 , R 8’ , R 8’’ , and R 8’’’ is C 1~4 R represents an alkyl group, and more preferably R 8 =R 8’ =R 8’’ =R 8’’’ = methyl or ethyl.

[0023] Most preferably, R 8 =R 8’ =R 8’’ =R 8’’’ =CH3.

[0024] In formula (III), X - represents an anion. The role of the anion is to balance the charge of the cation, which is represented as the part inside the square brackets ([)(]) in the above formula. Therefore, in principle, any anion can be used.

[0025] In formula (III), X - represents an anion. The role of the anion is to balance the charge of the cation, which is represented as the part inside the square brackets ([)(]) in the above formula. Therefore, in principle, any anion can be used.

[0026] Preferably, X - represents a halide, most preferably a chloride.

[0027] Preferably, the compound of formula (III) is methylene blue. More preferably, the compound of formula (III) is in the form of a double salt with zinc chloride, in particular a double salt of methylene blue with zinc chloride, or in the form of a hydrate, preferably methylene blue hydrate (CAS: 122965-43-9).

[0028] It has been found that photosensitizers of formula (III) are particularly suitable for the photooxidation of compounds of formula (II).

[0029] In order to carry out the above-mentioned photooxidation, the peak wavelength (λ max It is essential to use light with an optical wavelength of 580 to 780 nm.

[0030] In a preferred embodiment, the peak wavelength (λ max ) in the range of 625 to 740 nm, which corresponds to the light perceived as red.

[0031] In another more preferred embodiment, the peak wavelength (λ max ) in the range of 585 to 625 nm, which corresponds to the light perceived as orange.

[0032] This light is primarily in the long wavelength region of the visible spectrum.

[0033] In a further preferred embodiment the light used is characterised in that more than 80% of the light has a wavelength between 525 and 780 nm, preferably more than 80% of the light has a wavelength between 525 and 700 nm, more preferably more than 65% of the emitted light has a wavelength between 550 and 650 nm.

[0034] It is therefore important to use light with a spectrum that does not contain much light at wavelengths below 580 nm. It is extremely important that green, blue and violet light or light with a high content of green, blue and violet in its spectrum has been found to be unsuitable for the photo-oxidation described above.

[0035] In one embodiment, the light used for photooxidation can be obtained by filtering out undesired wavelengths of light from a light source, for example, a polychromatic or white emitting light source can be screened with filters that block undesired light.

[0036] Known commercially available filters of this type include a variety of possibilities such as absorptive filters, dichroic filters, monochromatic filters, bandpass filters, shortpass filters, or wedge filters, using various physical methods to filter the light.

[0037] In particular, an absorption filter or a cut-off filter is useful.

[0038] FIG. 1a is a schematic diagram of this embodiment. A light source (1) emits light of various wavelengths, including desired wavelengths (2a) and undesired wavelengths (2b). The light source is preferably a white light source, more preferably a white LED. A filter (6) is placed between the light source (1) and a photoreactor having a transparent wall (4). The filter (6) filters out the light of the undesired wavelengths, thereby reducing the peak wavelength (λ) of the spectrum. max The filter (6) is preferably an "orange filter" or a "red filter", i.e., a filter that transmits only light having a wavelength between 585 and 625 nm or between 625 and 740 nm. In the photoreactor (5), at least oxygen, the compound of formula (II), water and at least one C 1~8 Alkanol or at least one C 2~4 A reaction mixture (3) is placed which comprises a mixture of a solvent with an alkylene diol.

[0039] In the photoreaction, the compound of formula (II) photochemically reacts with oxygen, particularly in a gas mixture containing at least 20% by volume of oxygen, to produce the compound of formula (I).

[0040] A particularly preferred example of this embodiment is a white LED, whose light is filtered to block, or at least absorb most, all light that does not have the desired wavelength (e.g., using an "orange filter" (which transmits only light between 585-625 nm) or a "red filter" (which transmits only light between 625-740 nm).

[0041] Therefore, the light source is preferably a white LED lamp in combination with a filter that blocks wavelengths below 500 nm, especially below 625 nm.

[0042] In a further embodiment, the light used for photo-oxidation can be generated by a separate light source that emits light of the desired wavelength.

[0043] FIG. 1b is a schematic diagram of this embodiment. A light source (1) emits light of a desired wavelength (2a) to produce a peak wavelength (λ max The light source preferably provides light having a peak wavelength (λ ) in the range of 580 to 780 nm. The light source is preferably an orange or red light source, more preferably a light source having a peak wavelength (λ ) in the range of 580 to 780 nm. max ) is an orange or red LED for providing light using a light source in the range of 580 to 780 nm.

[0044] In the photoreactor (5), at least oxygen and a compound of formula (II) and water and at least one C 1~8 Alkanol or at least one C 2~4 A reaction mixture (3) containing a solvent mixture with an alkylene diol is placed in. By photoreaction, the compound of formula (II) photochemically reacts with oxygen to produce the compound of formula (I).

[0045] Specific examples of light sources in this embodiment include red LEDs or red or orange lasers, preferably red or orange LED lamps. Red and orange LED lamps are widely available commercially. Red and orange LEDs can provide high intensity red or orange light. In a preferred embodiment, multiple individual LEDs are integrated into a flexible strip. This allows for a reliable radial arrangement of multiple LEDs around a curved surface, such as a transparent tubular body, by simply wrapping the strip around the curved surface, preferably in a spiral fashion.

[0046] Photooxidation is a process that involves the reaction of water with at least one C 1~8 Alkanol or at least one C 2~4 It is carried out in a solvent mixture with an alkylene diol.

[0047] C 1~8 The alkanol is preferably selected from the group consisting of methanol, ethanol, propanol, isopropanol, butanol, heptanol, and hexanol, more preferably selected from the group consisting of methanol, ethanol, and isopropanol.

[0048] C 2~4 The alkylene diol is preferably selected from the group consisting of ethane-1,2-diol, propane-1,2-diol, propane-1,3-diol, butane-1,3-diol, butane-1,4-diol, butane-1,2-diol, and butane-2,3-diol, preferably selected from the group consisting of ethane-1,2-diol, propane-1,2-diol, and propane-1,3-diol.

[0049] Preferably, the solvent mixture is a mixture of water and at least one C soluble benzene ring that forms a homogeneous phase. 1~8 Alkanol or at least one C 2~4 It is a mixture with alkylene diol.

[0050] Preferably, the solvent mixture is a mixture of water and at least one C1~8 Alkanol or at least one C 2~4 More preferably, the solvent mixture is a mixture of water and C 1~8 It is a mixture with alkanols.

[0051] More preferably, the solvent mixture is a mixture of water with methanol and / or ethanol and / or isopropanol.

[0052] Preferably, water C 1~8 Alkanols and C 2~4 The volume ratio of the alkylene diols to the sum of the alkylene diols is in the range of 1:10 to 1:1, particularly in the range of 1:5 to 1:2.

[0053] In a highly preferred embodiment, the solvent mixture is a mixture of water and methanol, preferably with a volume ratio of water to methanol in the range of 1:20 to 1:2, preferably in the range of 1:10 to 1:2, more preferably in the range of a ratio of 1:6 to 1:3, and most preferably 1:4.

[0054] An important advantage of the present invention is that it is possible to use a highly environmentally and ecotoxicologically favorable and economically advantageous solvent, water and at least one C 1~8 Alkanol or at least one C 2~4 The photo-oxidation is carried out in a solvent mixture with an alkylene diol. It is therefore highly preferred that the process be carried out in the absence of chlorinated solvents.

[0055] Preferably, the concentration of the compound of formula (II) at the start of the photo-oxidation is in the range of 0.002 to 2.0 mol / l, preferably in the range of 0.01 to 0.2 mol / l.

[0056] More preferably, the ratio of the compound of formula (III) to the compound of formula (II) is in the range of 0.005 to 20 mol %, preferably in the range of 0.05 to 20 mol %, and more preferably in the range of 0.2 to 10 mol %.

[0057] In one embodiment, oxygen is used in the form of a mixture comprising oxygen and an inert gas. Preferably, the amount of oxygen in such a mixture comprising oxygen and an inert gas is at least 15% by volume, in particular at least 20% by volume. Such a mixture can be, for example, a binary mixture, such as an oxygen / nitrogen mixture or an oxygen / argon mixture or the like. The mixture can consist of or contain two or more inert gases. Particularly preferably, air is used as such a mixture comprising oxygen and an inert gas.

[0058] In a preferred embodiment, oxygen is used in substantially pure form, i.e. the amount of oxygen in the gas is between 90% and 100%, more preferably between 95% and 100%, even more preferably between 99% and 100%.

[0059] The photo-oxidation can be carried out either under normal pressure or under increased pressure. Preferably, the oxidation is carried out under increased pressure, in particular at a pressure of more than 2 bar, preferably at a pressure of more than 3 bar, more preferably between 2 and 20 bar.

[0060] The photooxidation is carried out in a suitable photoreactor. Preferred photoreactors are flow type reactors, in particular spiral flow reactors.

[0061] The individual components can be introduced into the photoreactor separately or as a mixture. Preferably, the reaction mixture is prepared prior to entering the photoreactor.

[0062] In one preferred embodiment, the oxygen-containing solvent mixture is mixed with the compound of formula (II) prior to flow into the photoreactor.

[0063] In another preferred embodiment, the solvent mixture is mixed with the compound of formula (II) that already contains oxygen before it is flowed into the photoreactor.

[0064] In a most preferred embodiment, oxygen is added to a premix comprising at least the compound of formula (II) and the solvent mixture.

[0065] The reaction is preferably conducted in a manner in which the oxygen pressure is controlled by appropriate valves and mass flow controllers. Such process control equipment and methods for carrying out photoreactions using liquids and gases are known to those skilled in the art. [Brief description of the drawings]

[0066] [Figure 1a] FIG. 1 shows a schematic diagram of photo-oxidation using a light source and filters to generate light whose spectrum has a peak wavelength (λmax) in the range of 580-780 nm. [Figure 1b] A schematic diagram of photooxidation using a light source whose spectrum has a peak wavelength (λmax) in the range of 580 to 780 nm is shown. [Figure 2a] A schematic diagram of one experimental setup is shown. [Figure 2b] Schematic diagrams of different experimental setups are shown. [Figure 2c] Schematic diagrams of other different experimental setups are shown. [Diagram 3] Normalized emission spectra of the light used for photo-oxidation in the experiment using an orange filter and white light are shown.

[0067] One preferred experimental arrangement is shown in FIG. 2a. At least a compound of formula (II) and a photosensitizer of formula (III) and water and at least one C 1~8 Alkanol or at least one C 2~4A pump (7) is used to pump a premix (10) from a vessel containing the premix, which comprises a mixture of a solvent with an alkylene diol, into the photoreactor (5). Before the premix enters the photoreactor (5), oxygen (11) is mixed in to form the photooxidation reaction mixture (3). The amount of oxygen mixed in is regulated by a mass flow controller (8). Around the transparent wall (4) of the linear tubular photoreactor (5), the light source (1) is arranged, in particular with LEDs arranged in a spiral. The light source (1) is preferably a white LED. A filter (6) is arranged between the transparent wall (4) and the light source (1) to filter out the peak wavelength (λ) of its spectrum. max Specifically, the filter (6) can supply light (2a) having a spectrum peak wavelength (λ) in the range of 580 to 780 nm. max ) is an orange filter or a red filter for supplying specific light in the range of 585-625 nm or 625-740 nm, respectively. The photoreactor (5) is preferably a spiral flow reactor. At the outlet of the photoreactor, a back pressure regulator (9) is placed before the collection vessel (12) for collecting the product.

[0068] This experimental setup, and in particular this light source and photoreactor combination, is preferably used for larger scale photoreactions.

[0069] Another preferred experimental setup is shown in FIG. 2b. At least a compound of formula (II) and a photosensitizer of formula (III) and water and at least one C 1~8 Alkanol or at least one C 2~4 A premix (10) containing a mixture of alkylene diol and solvent is pumped from a vessel containing the premix to the photoreactor (5) using a pump (7). Before the premix enters the photoreactor (5), oxygen (11) is mixed to form the photooxidation reaction mixture (3). The amount of oxygen mixed is regulated by a mass flow controller (8).

[0070] A filter (6) is placed between the transparent wall (4) of the photoreactor (5) and the light source (1), preferably a white LED, to filter out the peak wavelength (λ max) in the range of 580 and 780 nm (2a). Only one light source (1) and one filter (6) are shown in the drawing. Of course, several such light sources (1) in combination with filters (6) can be arranged around the photoreactor (5) in the form of a spiral flow reactor, so that the entire photoreactor (5) can be evenly irradiated. The filters (6) are specifically orange or red filters for providing specific light whose spectrum has a peak wavelength (λmax) in the range of 585-625 nm or 625-740 nm, respectively. Light with undesired wavelengths (2b) is blocked by the filters (6). At the exit of the photoreactor, a back pressure regulator (9) is arranged before the final product collection vessel (12).

[0071] This experimental setup, and in particular this light source and photoreactor combination, is preferably used for smaller volume photoreactions.

[0072] Another preferred experimental setup is shown in FIG. 2c. At least a compound of formula (II) and a photosensitizer of formula (III) and water and at least one C 1~8 Alkanol or at least one C 2~4 A premix (10) containing a mixture of alkylene diol and solvent is pumped from a vessel containing the premix to the photoreactor (5) using a pump (7). Before the premix enters the photoreactor (5), oxygen (11) is mixed to form the photooxidation reaction mixture (3). The amount of oxygen mixed is regulated by a mass flow controller (8).

[0073] In this embodiment, the light source (1), preferably a white LED, is placed in the hollow space formed by the spiral winding of the spiral flow reactor (5). A filter (6) is placed around the light source (1), i.e., between the transparent wall (4) of the photoreactor (5) and the light source (1), to filter out the peak wavelength (λ) of its spectrum. maxSpecifically, the filter (6) can supply light (2a) having a spectrum peak wavelength (λ) between 580 and 780 nm. max ) are orange and red filters for providing specific light in the range of 585-625 nm and 625-740 nm, respectively. Light with undesired wavelengths (2b) is blocked by filter (6). At the exit of the photoreactor, a back pressure regulator (9) is placed before the collection vessel (12) for final product collection.

[0074] This experimental setup, and in particular this light source and photoreactor combination, is preferably used for smaller volume photoreactions.

[0075] In a further embodiment, the light source (1) and filter (6) of figures 2b) and 2c) are combined, in other words the filter and light source are placed on the outside of the photoreactor wall and can be placed both inside and outside the space formed by the spiral winding of the spiral flow photoreactor (5).

[0076] [Example] The invention is further illustrated by the following experiments.

[0077] [Experimental configuration] The experiments and experimental setup described below were as shown diagrammatically in FIG. 2c.

[0078] A premix (10) containing the solvent or solvent mixture and the substance to be photooxidized as well as the photosensitizer is pumped from a vessel to the photoreactor (5), which is a spiral flow reactor (4.6 mL coil reactor) using a pump (7). Before the premix enters the photoreactor (5), oxygen (11) is mixed in to form the photooxidation reaction mixture (3). The amount of oxygen mixed in is regulated by a mass flow controller (8). The light source (1) is a white LED (4000 lm, 32 W, 4100 K) and the light (LSo; see FIG. 3 for emission spectrum) is filtered by an orange filter (6) to block light of undesired wavelengths (2b) so that light with the desired wavelength (2a) hits the transparent wall (4) of the photoreactor (5). The LED lamp (1), cooled by a fan, is surrounded by a cylindrical glass, on which the photoreactor (5) is wound, with filters (6) between the LED lamp and the wall of the photoreactor (5). At the exit of the photoreactor, a back pressure regulator (9) is placed before the collection vessel (12) that finally collects the product. The oxygen pressure and flow rate as well as the residence time (t R ) should be specified for each experiment.

[0079] The amount of product produced was measured using GC-FID and duren as an internal standard. 1 Determined by H-NMR.

[0080] The light used in the photooxidation experiment is -White LED light (LSw) -White LED light with orange filter (LSo) Either of the following.

[0081] Figure 3 shows the spectrum of light used in the photooxidation experiments, with wavelengths between 325 and 700 nm, as a function of the normalized relative intensity (I e,norm ) is used.

[0082] [Experiment series 1] In the first series, various substrates (0.02 mol / L in a 4 / 1 (vol / vol) solvent mixture of methanol / water) were photooxidized using air (10 bar, 1.35 mL / min) and methylene blue (0.9 mol%) at 35 °C (flow rate 0.25 mL / min, t R =23 minutes).

[0083] The results are summarized in Table 1.

[0084] [Table 1]

[0085] The results in Table 1 show that under these conditions, 2,3,6-trimethylphenol is converted to much more of the desired products than the other phenolic compounds.

[0086] [Experiment series 2] In the second series, various substrates (0.02 mol / L in a 4 / 1 (vol / vol) solvent mixture of methanol / water) were photooxidized using air (10 bar, 0.25–1.35 mL / min) and methylene blue (0.9 mol%) at 35 °C. Residence times were selected to achieve conversions greater than 99%.

[0087] The results are summarized in Table 2.

[0088] [Table 2]

[0089] From Table 2, it can be seen that the use of 2,3,6-trimethylphenol as the starting material (substrate) for photooxidation leads to a much higher yield of the corresponding quinone (product) compared to the use of other phenols with similar structures. [Explanation of symbols]

[0090] 1 light source 2a Light of desired wavelength 2b Undesirable wavelengths of light 3. Photooxidation reaction mixture 4. Transparent walls of the photoreactor 5. Photoreactor 6 Filters 7. Pump 8 Mass Flow Controller 9 Backpressure Regulator 10 Premix 11. Oxygen 12 Collection container

Claims

1. A process for preparing a compound of formula (I) from a compound of formula (II), comprising: 【Chemistry 1】 Oxygen and formula (III): 【Chemistry 2】 (In the formula, R ８ , R ８’ , R ８’’ , and R ８’’’ are each independently H or C １～４ represents either an alkyl group; or R ８ and R ８’ and / or R ８’’ and R ８’’’ together with N form a 5- or 6-membered ring; However, R ８ group, R ８’ group, R ８’’ Groups, and R ８’’’ At least one of the groups is not H; X － represents an anion), Water and at least one C １～８ Alkanol or at least one C ２～４ Used in a solvent mixture with an alkylene diol, The peak wavelength of the spectrum (λ ｍａｘ ) in the range of 580-780 nm to carry out photo-oxidation.

2. The light used has a peak wavelength in its spectrum (λ ｍａｘ 2. The method of claim 1, wherein the wavelength of the incident light is in the range of 585 to 625 nm.

3. The light used has a peak wavelength in its spectrum (λ ｍａｘ 2. The method of claim 1, wherein the wavelength of the incident light is in the range of 625 to 740 nm.

4. 4. The method according to any one of claims 1 to 3, characterized in that more than 80% of the light has a wavelength between 525 and 780 nm, preferably more than 80% of the emitted light has a wavelength between 525 and 700 nm, more preferably more than 65% of the light has a wavelength between 550 and 650 nm.

5. 5. The method according to claim 1, wherein the solvent mixture is a mixture of water and methanol and / or ethanol and / or isopropanol.

6. Water C １～８ Alkanols and C ２～４ 6. The process according to claim 1, characterized in that the volume ratio of alkylene diol to the sum is in the range of 1:10 to 1:1, in particular in the range of 1:5 to 1:

2.

7. The method according to any one of claims 1 to 6, characterized in that the light source is a red or orange LED lamp.

8. Method according to any one of claims 1 to 7, characterized in that the light source is a white LED lamp combined with a filter cutting off wavelengths below 500 nm, in particular wavelengths below 625 nm.

9. R ８ = R ８’ = R ８’’ = R ８’’’ =CH ３ The method according to any one of claims 1 to 8, characterized in that

10. X － 10. The process according to claim 1, wherein represents a halide, in particular a chloride.

11. 11. The method according to any one of claims 1 to 10, characterized in that the concentration of compound of formula (II) at the start of the photo-oxidation is in the range of 0.002 to 2.0 mol / L, preferably in the range of 0.01 to 0.2 mol / L.

12. 12. The process according to any one of claims 1 to 11, characterized in that the ratio of the compound of formula (III) to the compound of formula (II) is in the range of 0.005 to 20 mol%, preferably in the range of 0.05 to 20 mol%, more preferably in the range of 0.2 to 10 mol%.

13. The method according to any one of claims 1 to 12, characterized in that the photooxidation is carried out in a flow type reactor, in particular a spiral flow reactor.

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