Photooxidation of phenolic compounds

Abstract

The present invention relates to the photooxidation of phenolic compounds to the corresponding quinoid compounds, wherein light having a high wavelength range of the visible spectrum is used, methylene blue is used as photosensitizer in a solvent mixture of water and alcohol.

Description

TECHNICAL FIELD

[0001] The present invention relates to the preparation of quinoid compounds and, in particular, to the field of photooxidation of phenolic compounds. BACKGROUND

[0002] Quinones are a class of important chemicals which can be used widely for the synthesis of molecules in the fields of pharmaceuticals, fragrance ingredients, food and feed additives, and colorants and dyes.

[0003] It has been proposed by Murtinho D. et al., J. Chem. Soc. Perkin Trans. 2, 2000, 2441-2447 to photooxidize naphthalene-1,5-diol using oxygen in the presence of a photosensitizer to obtain 5-hydroxynaphthalene-1,4-dione. In particular, it is disclosed that methylene blue is used as photosensitizer in a mixture of acetonitrile and dichloromethane. However, as a result of moderate yields, it has been suggested to use a porphyrin-type photosensitizer instead.

[0004] McQuade D.T. et al. RSC Adv., 2016, 6, 12717-12725 disclose the photooxidation of different 1-naphthol derivatives to the corresponding 1,4-naphthoquinone derivatives using oxygen in the presence of a porphyrin-type photosensitizer in a mixture of acetonitrile and dichloromethane and with a white LED as light source in a flow reactor.

[0005] However, such porphyrin compounds as disclosed above are rather expensive and not readily commercially available. On the other hand, acetonitrile and dichloromethane are solvents which have significant ecological and ecotoxicological drawbacks.

[0006] G. Wurm et al., Arch. Pharm. 319, 97-101 (1986) disclose that 2-alkyl-5-methoxynaphthalen-1-ols have a low yield (<20%) of the corresponding 1,4-naphthoquinone derivatives when oxidized in a photo reaction. In particular, it is disclosed that 5-methoxy-2-(2-methylpropyl)-naphthalen-1-ol in methanol using methylene blue as sensitizer shows a yield of less than 3% of the desired 2-isobutyl-5-methoxynaphthalene-1,4-dione.

[0007] Methylene blue is often used as photosensitizer in photo reactions and is readily commercially available from various suppliers. SUMMARY

[0008] The process for obtaining a specific quinone of formula (I) from the corresponding phenolic compound is of great interest as the phenols are generally widely available and can be produced by known chemical transformations.

[0009] It has surprisingly been found that photooxidation according to claim 1 provides an effective way of solving this problem.

[0010] It has been found that this photooxidation of the compound of formula (II) leads to the desired product (I) with an exceptionally high conversion, yield and selectivity.

[0011] In the present application, methylene blue can be used, which is a very attractive photosensitizer which is readily available and cost-effective and which leads to the desired product not only with very high yield and high conversion but also with very high selectivity. It is particularly advantageous that this method can be performed without any chlorinated solvents. The method is thus very attractive for industrial applications.

[0012] Further aspects of the present application are subject matter of further independent claims. Particularly preferred embodiments are subject matter of dependent claims. DETAILED DESCRIPTION

[0013] The present application relates to a process for the preparation of a compound of formula (I) from a compound of formula (II) by photooxidation, which is carried out using oxygen and a photosensitizer of formula (III) in a solvent mixture of water and at least one C 1-8 alkanol or at least one C 2-4 alkylene glycol and using light having a peak wavelength (λ max ) in the range between 580 nm and 780 nm in its light spectrum,

[0014]

[0015] wherein R 1 and R 2 independently of one another represent H or C 1-4 alkyl or halogenated C 1-4 alkyl, OR 9 group or a -CH2-OR 9 group;

[0016] and wherein R 3 and R 4

[0017] independently of one another represent H or C 1-4 alkyl or halogenated C 1-4 alkyl or OR 9 group or a -CH2-OR 9 group, in particular H or CH3;

[0018] or together form a group having the following formula

[0019] or

[0020] wherein R 5 , R 6 and R 7 independently of one another represent H or OH

[0021] or C 1-4 alkyl or OR 9 ;

[0022] with the proviso that at least two of the residues R 5 , R 6 and R 7 are different from OH;

[0023] the dotted line represents a bond which binds the substituent of the formula to the remainder of the compound of the formula (I) or (II);

[0024] wherein R 9 is C 1-4 alkyl, in particular CH3,

[0025]

[0026] wherein R 8 , R 8 , R 8 and R 8 independently of one another represent H or C 1-4 alkyl;

[0027] or

[0028] wherein R 8 and R 8 and / or R 8 and R 8 together with N form a five- or six-membered ring; and X - represents an anion;

[0029] with the proviso that

[0030] if R 1 = R 2 = R 3 = CH3, then R 4 is different from H; and

[0031] if R 1 = R 2 = R 4 = CH3, then R 3 is different from H; and

[0032] the residues R 1 , R 2 , R 3 and R 4at least one of R

[0033] residue R 8 , R 8 , R 8 and R 8 is different from H.

[0034] For the sake of clarity, some of the terms used in this document are defined as follows:

[0035] In this document, a "C x-y alkyl" group is an alkyl group comprising x to y carbon atoms, i.e. for example, a C 1-3 alkyl group is an alkyl group comprising 1 to 3 carbon atoms. The alkyl group can be straight chained or branched. For example -CH(CH3)-CH2-CH3is considered as a C4alkyl group.

[0036] Similarly, a C x-y alkanol or C x-y alkylene glycol is an alcohol having one or two OH groups, wherein the alcohol has an alkyl or alkylene group comprising x to y carbon atoms.

[0037] In case there is a same label for a symbol or group present in several formulae, in this document, the definition of the group or symbol made in the context of one particular formula also applies to the other formulae comprising the same label.

[0038] The peak wavelength is the wavelength at which the spectrum reaches the highest intensity.

[0039] In the method, the compound of formula (II) is photo-oxidized to obtain the compound of formula (I). The compound of formula (II) is readily available and / or commercially available by synthesis known to the person skilled in the art.

[0040] In one embodiment, R 3 and R 4 independently from each other represent H or a C 1-4 alkyl group or a halogenated C 1-4 alkyl group, preferably a CF3group or an OR 9 group, preferably an OCH3group or a -CH2-OR 9 group, preferably a -CH2-OCH3group, in particular H or CH3

[0041] However, in case R 1 = R 2 = R 3 = CH3, R 4 is different from H. Furthermore, in case R 1 = R 2 = R 4 = CH3, R3 at least one of the residues R 1 , R 2 , R 3 and R 4 is different from H.

[0042] In other words, the compound of formula (II) is neither 2,3,6-trimethylphenol, nor 2,3,5-trimethylphenol or phenol.

[0043] In this embodiment, it is preferred that at least one of the residues R 1 , R 2 , R 3 and R 4 is an OR 9 group, preferably at least two are OR 9 groups, in particular methoxy groups.

[0044] The compound of formula (II) of this embodiment is preferably selected from the group consisting of o-cresol, m-cresol, 2-methoxyphenol, 3-methoxyphenol, 2,3-dimethylphenol, 2,5-dimethylphenol, 2,6-dimethylphenol, 3,5-dimethylphenol, 2-methyl-3-methoxyphenol, 2-methyl-5-methoxyphenol, 2-methoxy-6-methylphenol, 2-methoxy-3-methylphenol, 3-methoxy-5-methylphenol, 2-methoxy-5-methylphenol, 2,3-dimethoxyphenol, 2,5-dimethoxyphenol, 3,5-dimethoxyphenol, 2,6-dimethoxyphenol, 2,3-dimethoxy-6-methylphenol, 2,3-dimethoxy-5-methylphenol, 2,5-dimethoxy-3-methylphenol, 2,6-dimethoxy-3-methylphenol, 3,5-dimethoxy-2-methylphenol, 2-ethoxyphenol, 3-ethoxyphenol, 2,3-diethoxyphenol, 2,5-diethoxyphenol, 2,6-diethoxyphenol, 3,5-diethoxyphenol and 5-isopropyl-2-methylphenol.

[0045] In another embodiment, R3and R4together form a group of the following formula

[0046] or

[0047] wherein R 5 , R 6 and R 7 independently of one another represent H or OH or a C 1-4 alkyl or OR 9 group;

[0048] wherein R 9 is a C 1-4Alkyl groups, especially CH3,

[0049] The prerequisite is residue R. 5 R 6 and R 7 At least two residues in it are different from OH.

[0050] The compound of formula (II) in this embodiment is preferably selected from the group consisting of: 1-naphthol, 2-methylnaphthol-1-phenol, 3-methylnaphthol-1-phenol, 5-methylnaphthol-1-phenol, 6-methylnaphthol-1-phenol, 7-methylnaphthol-1-phenol, 8-methylnaphthol-1-phenol, 2,3-dimethylnaphthol-1-phenol, 2,5-dimethylnaphthol-1-phenol, 2,6-dimethylnaphthol-1-phenol, 2,7-dimethylnaphthol-1-phenol, 2,8-dimethylnaphthol-1-phenol, 3,5-dimethylnaphthol-1-phenol, 3,6-dimethylnaphthol-1-phenol, 3,7-dimethylnaphthol-1-phenol, 3,8-dimethylnaphthol-1-phenol, 5,6-dimethylnaphthol-1-phenol, 5,7-dimethylnaphthol-1-phenol, 5,7-dimethylnaphthol-1-phenol, 2,5-dimethylnaphthol-1-phenol, 2,6-dimethylnaphthol-1-phenol, 2,7-dimethylnaphthol-1-phenol, 2,8-dimethylnaphthol-1-phenol, 3,5-dimethylnaphthol-1-phenol, 3,6-dimethylnaphthol-1-phenol, 3,7-dimethylnaphthol-1-phenol, 3,8-dimethylnaphthol-1-phenol, 5,6-dimethylnaphthol-1-phenol, 5,7-dimethylnaphthol-1-phenol, 3,6-dimethylnaphthol-1-phenol, 3,7-dimethylnaphthol-1-phenol, 3,6-dimethylnaphthol-1-phenol, 3,7-di 1-Phenol, 5,8-dimethylnaphthol-1-phenol, 6,7-dimethylnaphthol-1-phenol, 6,8-dimethylnaphthol-1-phenol, 7,8-dimethylnaphthol-1-phenol, 2,3,5-trimethylnaphthol-1-phenol, 2,3,6-trimethylnaphthol-1-phenol, 2,3,7-trimethylnaphthol-1-phenol, 2,3,8-trimethylnaphthol-1-phenol, 5,6,7,8-tetramethylnaphthol-1-phenol, naphth-1,5-diol, naphth-1,6-diol, naphth-1,7-diol, 2-methoxynaphthol-1-phenol, 3-methoxynaphthol-1-phenol, 5-methoxynaphthol-1-phenol, 6-methoxynaphthol-1-phenol and 7-methoxynaphthol, especially 1-naphthol or 2-methylnaphthol-1-phenol.

[0051] R is preferred. 1 and R 2 and R 3 and R 4 Each can represent H or C independently. 1-4 Alkyl group, preferably H or CH3, provided that the substituent R is present. 1 and R 2 and R 3 and R 4 At least one of them does not represent H.

[0052] R is further preferred. 5 =R 6 =R 7 =H.

[0053] R is further preferred. 2 =H and R 1 =H or CH3.

[0054] The compound of formula (II) is particularly preferred.

[0055] In the photooxidation, a photosensitizer of the formula (III) is used,

[0056]

[0057] wherein R 8 , R 8 , R 8 and R 8 , independently of one another, represent H or C 1-4 alkyl;

[0058] or

[0059] wherein R 8 and R 8 and / or R 8 and R 8 together with N form a five- or six-membered ring;

[0060] and X - represents an anion;

[0061] with the proviso that

[0062] at least one of the residues R 8 , R 8 , R 8 and R 8 is different from H;

[0063] In one embodiment, R 8 and R 8 and / or R 8 and R 8 together form -(CH2)5- or -(CH2)2-NH-(CH2)2- or -(CH2)2-N(C 1-4 alkyl)-(CH2)2- or -(CH2)2-S-(CH2)2- or -(CH2)2-O-(CH2)2-.

[0064] Further preferred is R 8 = R 8 and / or R 8 = R 8 . More preferred is R 8 = R 8 = R 8 = R 8 .

[0065] More preferably, the substituents R 8 , R 8 , R 8 and R 8 represent C 1-4alkyl, even more preferably R 8 = R 8 = R 8 = R 8 = methyl or ethyl.

[0066] Most preferably, R 8 = R 8 = R 8 = R 8 = CH3.

[0067] In formula (III), X - represents an anion. The role of the anion is to balance the charge of the cation, which is represented in the above formula by the part within brackets ([)(]). Thus, in principle any anion can be used.

[0068] Preferably, X - represents a halide, most preferably a chloride.

[0069] Preferably, the compound of formula (III) is methylene blue. Further preferred is the compound of formula (III) in the form of a double salt with zinc chloride, in particular the double salt of methylene blue with zinc chloride, or the compound of formula (III) in the form of a hydrate, preferably methylene blue hydrate (CAS: 122965-43-9).

[0070] It has been found that the photosensitizer of formula (III) is particularly suitable for the photooxidation of the compound of formula (II).

[0071] It is important for the photooxidation described above that the light used has a peak wavelength (λ max ) in the range between 580 nm and 780 nm in its spectrum.

[0072] In a preferred embodiment, light is used which has a peak wavelength (λ max ) in the range between 625 nm and 740 nm in its spectrum. This corresponds to light which is perceived as red.

[0073] In another more preferred embodiment, light is used which has a peak wavelength (λ max ) in the range between 585 nm and 625 nm in its spectrum. This corresponds to light which is perceived as orange.

[0074] Such light has predominantly the high wavelength range of the visible spectrum.

[0075] In another preferred embodiment, the light used is characterized in that more than 80% of the light has a wavelength between 525 nm and 780 nm, preferably more than 80% of the light has a wavelength between 525 nm and 700 nm, more preferably more than 65% of the emitted light has a wavelength between 550 nm and 650 nm.

[0076] It is therefore important that the light used does not have a large amount of light with wavelengths below 580 nm in its spectrum. It has been found to be important that green, blue and violet light or light having a significant amount of green, blue and violet in its spectrum is not suitable for the above-mentioned photooxidation.

[0077] In one embodiment, the light used for photooxidation can be achieved by filtering out undesired wavelengths of light from the light source. For example, a light source having a polychromatic or white light emission can be filtered by a filter which blocks off the undesired wavelengths.

[0078] There are different possibilities for such known and commercially available filters using different physical methods for filtering light, such as absorption filters, dichroic filters, monochromatic filters, bandpass filters, shortpass filters or wedge filters.

[0079] Particularly useful are absorption filters or cut-off filters.

[0080] Figure 1a A schematic representation of this embodiment. The light source (1) emits radiation of different wavelengths, which have desired wavelengths (2a) and undesired wavelengths (2b). The light source is preferably white light, more preferably white LED. The filter (6) is located between the light source (1) and the photoreactor with transparent walls (4). The filter (6) filters out the light of the undesired wavelengths to provide light having a peak wavelength (λ max ) in its spectrum between 580 nm and 780 nm. The filter (6) is preferably an "orange filter" or a "red filter", i.e. a filter which only allows light with a wavelength between 585 nm and 625 nm or between 625 nm and 740 nm to pass through. The reaction mixture (3) contains at least oxygen and a compound of the formula (II) and water and at least one C 1-8 alkyl alcohol or at least one C 2-4 alkylene glycol. The reaction mixture (3) is inside the photoreactor (5).

[0081] By photo reaction, the compound of the formula (I) is produced from the compound of the formula (II) and oxygen, in particular in a gas mixture comprising at least 20% by volume of oxygen, by a photochemical reaction.

[0082] One particular preferred example of this embodiment is a white LED. Its light is filtered in such a way that all light not having the desired wavelength is blocked or at least significantly absorbed (e.g. using an "orange filter" (only light between 585 nm and 625 nm is transmitted) or a "red filter" (only light between 625 nm and 740 nm is transmitted)).

[0083] Thus, the light source for the light is preferably a combination of a white LED lamp and a filter blocking wavelengths below 500 nm, in particular 625 nm.

[0084] In another embodiment, the light for the photooxidation can be generated by a corresponding light source emitting light having the desired wavelength.

[0085] Figure 1b A schematic diagram representing this embodiment. The light source (1) emits radiation of a desired wavelength (2a) to provide light having a peak wavelength (λ max ) in the range between 580 nm and 780 nm in its spectrum. The light source is preferably an orange or red light, more preferably an orange or red LED to provide light using light having a peak wavelength (λ max ) in the range between 580 nm and 780 nm in its spectrum.

[0086] The reaction mixture (3) comprising at least oxygen and a compound of formula (II) and water and at least one C 1-8 alkyl alcohol or at least one C 2-4 alkylene glycol is inside a photo reactor (5). By photo reaction, the compound of formula (I) is generated from the compound of formula (II) and oxygen by a photochemical reaction.

[0087] A particular example of a light source of this embodiment is a red LED or a red or orange laser, preferably a red or orange LED lamp. Red and orange LED lamps are widely commercially available. Red and orange LEDs can provide a high intensity of red or orange light. In a preferred embodiment, the flexible band has a plurality of individual LEDs incorporated in the band. This allows to ensure the radial orientation of the LEDs around a curved surface (e.g. a transparent tube), for example by simply winding the band around the tube, preferably in a spiral manner.

[0088] The photooxidation is performed in a solvent mixture of water and at least one C 1-8 alkyl alcohol or at least one C 2-4 alkylene glycol.

[0089] C 1-8The alkanol is preferably selected from the group consisting of methanol, ethanol, propanol, isopropanol, butanol, heptanol and hexanol, more preferably from the group consisting of methanol, ethanol and isopropanol.

[0090] C 2-4 The alkylene glycol 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 from the group consisting of ethane-1,2-diol, propane-1,2-diol and propane-1,3-diol.

[0091] It is preferred that the solvent mixture is a mixture of water and at least one C 1-8 alkanols or at least one C 2-4 alkylene glycol forms a homogeneous phase.

[0092] It is preferred that the solvent mixture is a mixture of water and at least one C 1-8 alkanols or at least one C 2-4 alkylene glycol. More preferably, the solvent mixture is a mixture of water and C 1-8 alkanols.

[0093] More preferably, the solvent mixture is a mixture of water and methanol and / or ethanol and / or isopropanol.

[0094] It is preferred that the solvent mixture is a mixture of water and C 1-8 alkanols and C 2-4 alkylene glycols in a volume ratio in the range between 1 :10 and 1 :1, in particular between 1 :5 and 1 :2

[0095] In a very preferred embodiment, the solvent mixture is a mixture of water and methanol, preferably in a volume ratio of water to methanol in the range of 1 :20 to 1 :2, preferably 1 :10 to 1 :2, more preferably 1 :6 to 1 :3, most preferably 1 :4.

[0096] A key advantage of the present application is that the photooxidation is carried out in a solvent mixture consisting of water and at least one C 1-8 alkanols or at least one C 2-4 alkylene glycol, which is a very ecologically and ecotoxicologically suitable solvent and is also economically advantageous. It is therefore very advantageous that the above-mentioned process is carried out in the absence or presence of any chlorinated solvents.

[0097] It is preferred that the concentration of the compound of formula (II) at the beginning of the photooxidation is in the range between 0.002 mol / l and 2.0 mol / l, preferably between 0.01 mol / l and 0.2 mol / l.

[0098] It is further preferred that the ratio of the compound of formula (III) to the compound of formula (II) is in the range between 0.005 mol% and 20 mol%, preferably between 0.05 mol% and 20 mol%, more preferably between 0.2 mol% and 10 mol%.

[0099] In one embodiment, oxygen is used in the form of a mixture comprising oxygen and an inert gas. It is preferred that in such a mixture comprising oxygen and an inert gas, the amount of oxygen is at least 15 vol.%, in particular at least 20 vol.%. Such a mixture can for example be a binary mixture, such as an oxygen / nitrogen or oxygen / argon mixture, etc. The mixture can consist of or comprise two or more inert gases. It is particularly preferred to use air as such a mixture comprising oxygen and an inert gas.

[0100] In a preferred embodiment, oxygen is used in substantially pure form, i.e. the amount of oxygen in the gas is 90-100%, more preferably 95-100%, even more preferably 99-100%.

[0101] The photooxidation can be carried out at ambient pressure or under pressure. It is preferred that the oxidation is carried out under pressure, in particular at a pressure of more than 2 bar, preferably more than 3 bar, more preferably between 2 bar and 20 bar.

[0102] The photooxidation is carried out in a suitable photoreactor. A preferred photoreactor is a flow reactor, in particular a helical flow reactor.

[0103] 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.

[0104] In one preferred embodiment of the preferred embodiments, the oxygen-containing solvent mixture is admixed to the compound of formula (II) prior to entering the photoreactor.

[0105] In another preferred embodiment, the solvent mixture is admixed to the compound of formula (II) which already contains oxygen prior to entering the photoreactor.

[0106] In the most preferred embodiment, oxygen is added to a premix comprising at least the compound of formula (II) and the solvent mixture.

[0107] The reaction is preferably handled in a way that the pressure of oxygen is controlled by suitable valves and mass flow controllers. Such process control equipment and methods for photo reactions using liquids and gases are known to the person skilled in the art. BRIEF DESCRIPTION OF DRAWINGS

[0108] Figure 1a A schematic diagram of photo-oxidation using a light source and a filter is shown, wherein the light source and filter produce a peak wavelength (λ) in their spectrum between 580 nm and 780 nm. max (The light)

[0109] Figure 1b This demonstrates the use of peak wavelengths (λ) in its spectrum that are between 580 nm and 780 nm. max A schematic diagram of photo-oxidation using a light source.

[0110] Figure 2a A schematic diagram of one of the experimental layouts is shown.

[0111] Figure 2b A schematic diagram of a different experimental layout is shown.

[0112] Figure 2c A schematic diagram of another different experimental layout is shown.

[0113] Figure 3 This indicates the normalized emission spectrum of light used in the experiment for photo-oxidation using an orange filter, as well as white light.

[0114] exist Figure 2a A preferred experimental layout is shown in the figure. A container containing a premix (10) is pumped into a photoreactor (5) by a pump (7). The premix contains at least a compound of formula (II) and a photosensitizer of formula (III), as well as water and at least one C 1-8 Alkyl alcohol or at least one C 2-4 A solvent mixture of alkylene glycols. Oxygen (11) is incorporated into the premix before entering the photoreactor (5) to form a photooxidation reaction mixture (3). The amount of oxygen incorporated is controlled by a mass flow controller (8). A light source (1), particularly in a helical arrangement of LEDs, is arranged around the transparent wall (4) of the linear tubular photoreactor (5). The light source (1) is preferably a white LED. A filter (6) is positioned between the transparent wall (4) and the light source (1), thereby allowing the provision of a peak wavelength (λ) in its spectrum within the range of 580 nm to 780 nm. max The light (2a) of the filter (6) is specifically an orange filter or a red filter, respectively, to specifically provide a peak wavelength (λ) in its spectrum in the range between 585 nm and 625 nm or between 625 nm and 740 nm. max The photoreactor (5) is preferably a helical flow reactor. A back pressure regulator (9) is positioned at the outlet of the photoreactor before the product is collected in the collection container (12).

[0115] This experimental setup, in particular the combination of light source and photoreactor, is preferred for larger volumes of photoreactions.

[0116] In Figure 2b , another preferred experimental setup is shown. A container containing a premix (10) comprising at least the compound of formula (II) and the photosensitizer of formula (III) and a solvent mixture of water and at least one C 1-8 alkanol or at least one C 2-4 alkylene glycol is pumped by a pump (7) into the photoreactor (5). Oxygen (11) is admixed to the premix before entering the photoreactor (5) to form a photooxidation reaction mixture (3). The amount of admixed oxygen is controlled by a mass flow controller (8).

[0117] Between the transparent wall (4) of the photoreactor (5) and the light source (1), preferably white LEDs, a filter (6) is positioned, allowing to provide light (2a) having a peak wavelength (λ max ) in the range between 580 nm and 780 nm in its spectrum. In this illustration only one light source (1) and one filter (6) are shown. Of course, it is possible that several such combinations of light sources (1) and filters (6) are positioned around the photoreactor (5) in the form of a helical flow reactor, which can be positioned to allow a uniform illumination of the whole photoreactor (5). The filter (6) is in particular an orange filter or a red filter, respectively, to provide light having a peak wavelength (λ max ) in the range between 585 nm and 625 nm or between 625 nm and 740 nm in its spectrum, respectively. Light (2b) having an undesired wavelength is filtered out by the filter (6). At the outlet of the photoreactor a back pressure regulator (9) is positioned before the product is finally collected in a collection vessel (12).

[0118] This experimental setup, in particular the combination of light source and photoreactor, is preferred for smaller volumes of photoreactions.

[0119] In Figure 2c , another preferred experimental setup is shown. A container containing a premix (10) comprising at least the compound of formula (II) and the photosensitizer of formula (III) and a solvent mixture of water and at least one C 1-8 alkanol or at least one C 2-4 alkylene glycol is pumped by a pump (7) into the photoreactor (5). Oxygen (11) is admixed to the premix before entering the photoreactor (5) to form a photooxidation reaction mixture (3). The amount of admixed oxygen is controlled by a mass flow controller (8).

[0120] In this embodiment, a light source (1), preferably a white LED, is arranged in the hollow space formed by the helical winding of the helical flow reactor (5). Around the light source (1), i.e. between the transparent wall (4) of the photoreactor (5) and the light source (1), a filter (6) is positioned, allowing to provide light (2a) having a peak wavelength (l max ) in the range between 580 nm and 780 nm in its spectrum. The filter (6) is in particular an orange filter or a red filter, respectively, to provide light having a peak wavelength (l max ) in the range between 585 nm and 625 nm or between 625 nm and 740 nm in its spectrum, respectively. Light (2b) having an undesired wavelength is filtered out by the filter (6). A back pressure regulator (9) is positioned at the outlet of the photoreactor before the product is finally collected in a collection vessel (12).

[0121] This experimental setup, in particular the combination of light source and photoreactor, is preferably used for a photoreaction of smaller volume.

[0122] In a further embodiment, Figure 2b ) and Figure 2c ) the light source (1) and the filter (6) are combined. In other words, the filter and the light source can be arranged outside the photoreactor wall which is arranged inside and outside the space formed by the helical winding of the helical flow photoreactor (5).

[0123] List of element symbols

[0124] 1 light source

[0125] 2a light having a desired wavelength

[0126] 2b light having an undesired wavelength

[0127] 3 photooxidation reaction mixture

[0128] 4 transparent wall of the photoreactor

[0129] 5 photoreactor

[0130] 6 filter

[0131] 7 pump

[0132] 8 mass flow controller

[0133] 9 back pressure regulator

[0134] 10 premix

[0135] 11 oxygen

[0136] 12 collection vessel

[0137] Examples

[0138] The present invention is further illustrated by the following experiments.

[0139] Experimental layout

[0140] In the following experiments, an experimental setup as schematically represented in Figure 2c has been used.

[0141] A container containing a premix (10) of solvent or solvent mixture and the substance to be photooxidized as well as a photosensitizer was pumped by a pump (7) into a photo reactor (5), which was a spiral flow reactor (4.6 ml coil reactor). Oxygen (11) was admixed to the premix before entering the photo reactor (5), forming a photooxidation reaction mixture (3). The amount of admixed oxygen was controlled by a mass flow controller (8). The light of the light source (1), i.e. white LED (4000 lm, 32 W, 4100 K) (LSo; see emission spectrum in Figure 3 ), was filtered by an orange filter (6) to filter out light (2b) having an undesired wavelength, such that light (2a) having a desired wavelength fell on the transparent wall (4) of the photo reactor (5). The photo reactor (5) was wound around an inner glass cylinder around the LED lamp (1), which was cooled by a fan and equipped with a corresponding filter (6) between the LED lamp and the wall of the photo reactor (5). A back pressure regulator (9) was positioned at the outlet of the photo reactor before the product was finally collected in a collection container (12). The pressure and flow rate of the oxygen as well as the residence time (t R ) were indicated in the respective experiments.

[0142] The amount of formed product was determined by GC-FID and 1 H-NMR using Durene as internal standard.

[0143] The light used for the photooxidation experiments was

[0144] - white LED light (LSw)

[0145] - white LED light filtered by using an orange filter (LSo).

[0146] In Figure 3 , the spectrum of the light used for the photooxidation experiments between 325 nm and 700 nm is shown using the normalized relative intensity (I e,norm ) of the light entering the transparent wall of the photo reactor.

[0147] Experiment series 1

[0148] In the first series, different substrates were photo-oxidized at 35 °C using air (10 bar, 1.35 mL / min) and methylene blue (0.9 mol%) (0.02 mol / L in a 4 / 1 (v / v) methanol / water solvent mixture). (Flow rate 0.25 mL / min, t R =23min). The results are summarized in Table 1:

[0149]

[0150] Table 1. Photoreaction of different substrates using air and LSo as a light source.

[0151] Experiment series 2

[0152] In the second series, different substrates were photo-oxidized at 35 °C using air (10 bar, 0.25–1.35 mL / min) and methylene blue (0.9 mol%) (0.02 mol / L in a 4 / 1 (volume / volume) methanol / water solvent mixture). Residence times were selected to achieve conversions greater than 99%.

[0153] The results are summarized in Table 2:

[0154]

[0155] Table 2. Photoreactions of different substrates using air and LSo as a light source.

[0156] Experiment series 3

[0157] In the third series, different substrates were photo-oxidized at 35 °C using air (10 bar, 0.5–1.5 mL / min) and methylene blue (0.9 mol%) (0.02 mol / L in a 4 / 1 (volume / volume) methanol / water solvent mixture). Residence times were selected to achieve conversions greater than 99%.

[0158] The results are summarized in Table 3:

[0159]

[0160] Table 3. Photoreactions of different substrates using air and LSo as a light source.

[0161] Tables 1 to 3 show that, in photooxidation, compounds of formula (II) exhibit significantly higher yields of conversion to the corresponding quinones (products) compared to other phenols with similar structures as starting materials (substrates).

Claims

1. A method for preparing a compound of formula (I) from a compound of formula (II) by photo-oxidation, said method being carried out under the following conditions: using oxygen and a photosensitizer of formula (III), in water with at least one C 1-8 Alkyl alcohol or at least one C 2-4 In a solvent mixture of alkylene glycols, and using light, its spectrum has a peak wavelength λ in the range between 580 nm and 740 nm. max light, Where R 1 and R 2 Each can represent H or C independently. 1-4 Alkyl or OR 9 Group; And R 3 and R 4 Each can represent H or C independently. 1-4 alkyl; Or together they form a group having the following formula Where R 5 R 6 and R 7 They can represent H or OH independently; The prerequisite is that residue R 5 R 6 and R 7 At least two residues in it are different from OH; The dashed lines represent bonds that bind the substituents of the formula to the remainder of the compound of formula (I) or (II); Where R 9 It is C 1-4 alkyl; The prerequisite is that the substituent R 1 and R 2 and R 3 and R 4 At least one of them does not represent H; Where R 8 =R 8' =R 8” =R 8”' =CH3; And X - Represents chloride ions; among which, Water and C 1-8 Alkyl alcohols and C 2-4 The volume ratio of the sum of alkylene glycols is in the range between 1:10 and 1:

1.

2. The method according to claim 1, characterized in that, The light used has a peak wavelength λ in its spectrum in the range between 585 nm and 625 nm. max .

3. The method according to claim 1, characterized in that, The light used has a peak wavelength λ in its spectrum in the range between 625 nm and 740 nm. max .

4. The method according to any one of the preceding claims, characterized in that, More than 80% of the light has a wavelength between 525nm and 780nm.

5. The method according to claim 1, characterized in that, The solvent mixture is a mixture of water with methanol and / or ethanol and / or isopropanol.

6. The method according to claim 1, characterized in that, The light source used for the light is a red or orange LED.

7. The method according to claim 1, characterized in that, The light source used for the light is a combination of a white LED lamp and a filter that blocks wavelengths below 500nm.

8. The method according to claim 1, characterized in that, R 1 and R 2 and R 3 and R 4 Each can represent H or C independently. 1-4 Alkyl, provided that the substituent R is alkyl. 1 and R 2 and R 3 and R 4 At least one of them does not represent H.

9. The method according to claim 1, characterized in that, R 5 =R 6 =R 7 =H。 10. The method according to claim 8, characterized in that, R 2 =H and R 1 =H or CH3.

11. The method according to claim 1, characterized in that, At the start of the photo-oxidation, the concentration of the compound of formula (II) is in the range of 0.002 mol / L to 2.0 mol / L.

12. The method according to claim 1, characterized in that, The ratio of the compound of formula (III) to the compound of formula (II) is in the range of 0.005 mol% to 20 mol%.

13. The method according to claim 1, characterized in that, The photo-oxidation is performed in a flow reactor.

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