Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/20—Processes
  • C25B3/23—Oxidation
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/20—Processes
  • C25B3/29—Coupling reactions

Definitions

• the invention relates to a process for the preparation of biaryl alcohols, which is carried out by anodic dehydrodimerization of substituted phenols in the presence of partially and / or perfluorinated mediators and a conductive salt on a graphite electrode.
• the method according to the invention makes it possible to use very inexpensive electrode materials, undivided cell structures and solvent-free methods.
• mediators e.g. 1, 1, 1, 3,3,3-hexafluoroisopropanol or the much cheaper trifluoroacetic acid are used.
• the workup and recovery of the desired biphenols is very simple.
• Biaryls are known as such and are industrially produced and used. Compounds of this class of compounds are i.a. as backbones for ligands of great interest for stereoselective transformations.
• One possible approach to this class of substance is the electrochemical oxidative dimerization of phenols, which, however, is nonselective in electrolytes known to those skilled in the art.
• iron (III) salts or other strong oxidizing agents are used as an alternative to the electrochemical dimerization of phenols.
• Particularly favored and therefore frequently used substrates have annellated benzene rings or sterically demanding alkyl groups.
• the 2,2'-dihydroxy-1,1'-binaphthyl (BINOL) prepared from 2-naphthol can be used here.
• the main product is usually not the desired ortho, ortho-linked product 2 but a derivative of the Pummerer ketone (3).
• the formation of the tricyclic skeleton 3 is known for para-alkyl-substituted phenols and is also found in the synthesis of many natural products.
• BDD boron-doped diamond electrodes
• the object of the present invention is to provide a method with which the selective and efficient oxidative coupling of substituted phenols takes place without having to work in the presence of expensive electrode material.
• the coupling of substituted phenols should take place in the ortho position.
• This object is achieved by a process for the preparation of biaryl alcohols, wherein substituted aryl alcohols are anodically dehydro-dimerized in the presence of partially and / or perfluorinated mediators and at least one conducting salt with the aid of a graphite electrode.
• the process according to the invention is advantageous if the OH group of the substituted aryl alcohols used is seated directly on the aromatic compound.
• the process according to the invention is advantageous if the substituted aryl alcohols used are identical.
• the process according to the invention is advantageous if the substituted aryl alcohols used can be mononuclear or polynuclear.
• the process according to the invention is advantageous if the dimerization takes place ortho to the alcohol group of the substituted aryl alcohols.
• the process according to the invention is advantageous if the mediators used are partially and / or perfluorinated alcohols and / or acids.
• the process according to the invention is advantageous if the mediators used are 1, 1, 1, 3,3,3-hexafluoroisopropanol or trifluoroacetic acid.
• the process according to the invention is advantageous if the conductive salts used are those selected from the group consisting of alkali metal, alkaline earth metal, tetra (C 1 to C 6 alkyl) ammonium salts.
• the counterions of the conducting salts are selected from the group consisting of sulfate, hydrogensulfate, alkyl sulfates, aryl sulfates, halides, phosphates, carbonates, alkyl phosphates, alkyl carbonates, nitrate, alcoholates, tetrafluoroborate, hexafluorophosphate and perchlorate.
• the process according to the invention is advantageous if no further solvent is used for the electrolysis.
• the process according to the invention is advantageous if a flow cell is used for the electrolysis.
• the process according to the invention is advantageous when current densities of 1 to 1000 mA / cm 2 are used.
• the process according to the invention is advantageous if the electrolysis is carried out at temperatures in the range from -20 to 60 ° C. and normal pressure.
• the process according to the invention is advantageous if 2,4-dimethylphenol is used as the aryl alcohol.
• aryl alcohol is understood as meaning aromatic alcohols in which the hydroxyl group is bonded directly to the aromatic nucleus.
• the aromatic which is based on the aryl alcohol, may be mononuclear or polynuclear.
• the aromatic is preferably mononuclear (phenol derivatives) or binuclear (naphthol derivatives), in particular mononuclear.
• the aryl alcohols may also carry further substituents.
• substituents are independently selected from the group of C 1 -C 10 -alkyl groups, halogens, C 1 -C 10 -alkoxy groups, alkylene or arylene radicals interrupted by oxygen or sulfur, C 1 -C 10 -alkoxycarboxyl, nitrile, nitro and C 1 -C 10 -alkoxycarbamoyl, particularly preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, trifluoromethyl, fluorine, chlorine, bromine, iodine, methoxy, ethoxy, methylene, ethylene, propylene, isopropylene, benzylidene, nitrile, nitro, very particularly preferably methyl, Me - thoxy, methylene, ethylene, trifluoromethyl, fluorine and bromine.
• aryl alcohols can be used.
• electron-rich arenes such as phenol and mono- or polysubstituted phenols and naphthol ( ⁇ - and ß-) and substituted derivatives thereof, very particularly preferred are phenols, and particularly particularly preferred are 4-alkyl and 2,4-dialkyl-substituted phenols.
• Suitable substrates for the electrodimerization according to the present invention are in principle all aryl alcohols, provided that they are capable of dimerization due to their spatial structure and steric requirements.
• the aryl alcohols may be mononuclear, dinuclear, trinuclear or higher nuclear. Preferably, they are mononuclear or dinuclear, in particular mononuclear.
• the aryl alcohols preferably have an OH function.
• Suitable aryl alcohols include phenol and mono- and polysubstituted substituted phenols represented by the following formula (I) wherein R1 to R4 are independently the same or different and are selected from the following substituents: H, Ci-Cio-alkyl , C 1 -C 10 -alkoxy, halogen, C 1 -C 10 -alkoxycarboxyl, nitrile and also mono- and di-C 1 -C 10 -alkoxycarbamoyl.
• R1 to R4 are independently the same or different and are selected from the following substituents: H, Ci-Cio-alkyl , C 1 -C 10 -alkoxy, halogen, C 1 -C 10 -alkoxycarboxyl, nitrile and also mono- and di-C 1 -C 10 -alkoxycarbamoyl.
• naphthol ⁇ - and ⁇ -
• substituted derivatives thereof according to the following formulas (II) and (III), in which the radicals R 1 to R 7, identical or different and selected from the following substituents: H, Ci-Cio Alkyl, Ci-Cio-Alkoxy, halogen, Ci-Cio-Alkoxycarboxyl, nitrile as well as mono- and di-C1-C10-Alkoxycarbamoyl.
• the electrolyte solution is worked up by general separation methods.
• the electrolyte solution is generally first distilled and recovered the individual compounds in the form of different fractions separately. Further purification can be carried out, for example, by crystallization, distillation, sublimation or chromatographic.
• the preparation of the biaryl alcohol is carried out electrolytically, with the corresponding aryl alcohol being oxidized anodically.
• the process according to the invention is referred to below as electrodimerization. It has surprisingly been found that arise by the process according to the invention using mediators, the biaryl alcohols selectively and in high yield. Furthermore, it has been found that very inexpensive electrode materials, undivided cell structures and solvent-free methods can be used by the method according to the invention.
• the workup and recovery of the desired biphenols is very simple.
• the electrolyte solution is worked up by general separation methods. For this purpose, the electrolyte solution is generally first distilled and recovered the individual compounds in the form of different fractions separately.
• Further purification can be carried out, for example, by crystallization, distillation, sublimation or chromatographic.
• Partially and / or perfluorinated alcohols and / or acids preferably perfluorinated alcohols and carboxylic acids, very particularly preferably 1, 1, 1, 3, 3, 3-hexafluoroisopropanol or trifluoroacetic acid are used as mediators in the process according to the invention. No additional solvents are required in the electrolyte.
• electrolysis is carried out in the usual, known in the art electrolysis cells. Suitable electrolysis cells are known to the person skilled in the art. Preferably, one works continuously in undivided flow cells or discontinuously in beaker cells.
• bipolar switched capillary gap cells or Plattenstapelzellen, in which the electrodes are designed as plates and are arranged plane-parallel as in Ullmann's Encyclopedia of Industrial Chemistry, 1999 electronic release, Sixth Edition, VCH-Weinheim, Volumne and in Electrochemistry , Chapter 3.5. special cell designs as well as Chapter 5, Organic Electrochemistry, Subchapter 5.4.3.2 Cell Design.
• the current densities at which the process is carried out are generally 1 to 1000, preferably 5 to 100 mA / cm 2 .
• the temperatures are usually from -20 to 60 ° C., preferably from 10 to 60 ° C.
• the reaction is generally carried out under atmospheric pressure. Higher pressures are preferably used when operating at higher temperatures to avoid boiling of the co-solvents or mediators.
• Suitable anode materials are, for example, noble metals such as platinum or metal oxides such as ruthenium or chromium oxide or mixed oxides of the type RuO x TiO x and diamond electrodes. Preference is given to graphite or carbon electrodes.
• the cathode material for example, iron, steel, stainless steel, nickel or E- delmetalle such as platinum and graphite or carbon materials and diamond electrodes into consideration.
• the system is preferably graphite as the anode and cathode, graphite as the anode and nickel, stainless steel or steel as the cathode and platinum as the anode and cathode.
• the aryl alcohol compound is dissolved in a suitable solvent.
• a suitable solvent preferably solvents from the group of polar protic and polar aprotic solvents, are suitable.
• the aryl alcohol compound itself serves as a solvent and reagent.
• polar aprotic solvents examples include nitriles, amides, carbonates, ethers, ureas, chlorinated hydrocarbons.
• polar aprotic solvents examples include Actonitrile, dimethylformamide, dimethyl sulfoxide, propylene carbonate and dichloromethane.
• polar protic solvents examples include alcohols, carboxylic acids and amides.
• polar protic solvents examples include methanol, ethanol, propanol, butanol, pentanol and hexanol. These may also be partially or completely halogenated, e.g. 1, 1, 1, 3,3,3-hexafluoroisopropanol (HFIP) or trifluoroacetic acid (TFA).
• HFIP 1, 1, 1, 3,3,3-hexafluoroisopropanol
• TFA trifluoroacetic acid
• the electrolysis solution is added to customary cosolvents.
• these are the inert solvents customary in organic chemistry and have a high oxidation potential. Examples include its dimethyl carbonate, propylene carbonate, tetrahydrofuran, dimethoxyethane, acetonitrile or dimethylformamide.
• Conducting salts which are contained in the electrolysis solution are generally alkali metal, alkaline earth metal, tetra (C 1 -C 6 -alkyl) ammonium, preferably tri (cis-bisalkyl) -methylammonium salts.
• Suitable counterions are sulfate, bisulfate, alkyl sulfates, aryl sulfates, halides, phosphates, carbonates, alkyl phosphates, alkyl carbonates, nitrate, alcoholates, tetrafluoroborate, hexafluorophosphate or perchlorate.
• the acids derived from the abovementioned anions are suitable as conductive salts.
• MTBS methyltributylammonium methylsulfates
• MTES methyltriethylammonium methylsulfate
• TABF tetrabutylammonium, tetrafluoroborate
• TFA trifluoroacetic acid
• AcOH acetic acid
• Phenol 2,4-dimethylphenol
• MTES methyltriethylammonium methyl sulfate
• Phenol 2-bromo-4-methylphenol
• a N, N-dimethylpyrrolidinium methylsulfate
• b yield considering the recovered phenol
• c isolation by crystallization from toluene and chromatographic
• d isolation by crystallization from 'PrOH: water and chromatographic.
• Example 1 Anodic oxidation of 2,4-dimethylphenol on graphite electrodes with trifluoroacetic acid
• the solvent is first removed and then excess phenol recovered by short path distillation.
• the reaction residue is taken up in 50 mL water and 30 mL TBME, the phases are separated and the aqueous phase extracted again with 3x30 mL TBME.
• the combined organic phases are washed with 50 ml of water and saturated sodium chloride solution, dried over magnesium sulfate and the solvent removed under reduced pressure.
• the crude product is dissolved in 10 ml of toluene at 50 ° C.
• the slow addition of n-heptane succeeds in crystallizing the product, which is obtained by filtration and washing with a little cold n-heptane.

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