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
  • C25B3/01—Products
  • C25B3/07—Oxygen containing compounds

Definitions

• This invention relates to an improved process for the preparation of benzaldehyde dialkyl acetals of the general formula I in which R1 is C1-C6-alkyl and R2 is C1-C6-alkyl, C1-C6-alkoxy, halogen, cyano or carboxyalkyl having 1 to 6 carbon atoms in the alkyl group and n is an integer from 1 to 3, the radicals R2 in the case of n> 1 may be the same or different, by electrochemical oxidation of substituted toluene compounds of the general formula II Process products I serve as intermediates for the production of crop protection agents and pharmaceuticals.
• DE-A 41 06 661 relates to a process for the preparation of substituted 2-methylbenzaldehyde dialkyl acetals. According to the teaching of this document, the process can be carried out continuously or discontinuously at normal pressure or elevated pressure. However, the selectivities to be achieved according to the teaching of this publication are not sufficient in all cases for large-scale practice of the process.
• EP-12 240 relates to the electrochemical oxidation of optionally substituted toluene compounds to the corresponding benzaldehyde dialkyl acetals in the presence of alkanols and of conductive salts which are derived from sulfuric acid or phosphoric acid.
• the reaction mixture can be worked up by distillation to give the process product and the by-products isolated in this way the oxidation level can be returned.
• By-products that can interfere with the oxidation reaction are subjected to hydrogenation before being recycled.
• the object was therefore to provide a process which allows the continuous electrochemical production of benzaldehyde dialkyl acetals both with high conversions and with high selectivities.
• gas released during the expansion of the reaction solution after the electrolysis which is predominantly hydrogen discharged from the electrolysis cell, is separated off.
• the reaction solution is then returned to the electrolysis cell, electrolyzed and then relaxed.
• This sequence of process steps is referred to below as cycles. It has proven to be advantageous to expose the reaction solution to a large number of cycles, as a result of which a higher yield can be achieved economically than in only two cycles. 20 to 1000 cycles are preferred, particularly preferably 100 to 800.
• the oxidation of the starting compound II in one cycle is generally not carried out until complete conversion. Depending on the number of cycles, the turnover is generally 0.1 to 5% of the theoretical turnover.
• the reaction solution is worked up onto the product. This is done in a manner known per se, predominantly by distillation. If a solvent is present in the reaction solution, it is distilled off. If neutral salts are used as auxiliary electrolyte, these can then be filtered off before the acetal I is distilled. Solvents, electrolyte and unreacted starting compound can be used again in further process approaches.
• the continuous embodiment of the present invention is preferred.
• a partial stream of the reaction solution is separated off and worked up.
• This partial stream is generally less than 5% by weight, preferably 0.01 to 1% by weight, of the total stream.
• Part of the gas dissolved in the reaction solution is discharged from the electrolysis circuit through this partial flow.
• a separate degassing of the entire reaction solution is not necessary, but can be advantageous in the case of small partial flows and relatively large amounts of gas.
• the partial flow is worked up as described above.
• Solvents, auxiliary electrolyte, starting compounds and, if appropriate, incompletely oxidized intermediates can be added to the reaction solution which is returned to the electrolytic cell.
• the recycled reaction solution is further replaced by the amount of starting compounds which corresponds to the amount of the separated product. After recycling and oxidation, the cycle described is repeated as often as required.
• the reaction is carried out in the presence of an auxiliary electrolyte.
• an auxiliary electrolyte This is generally in a concentration of 0.1 to 6% by weight, based on the reaction mixture.
• Protonic acids such as organic acids, e.g. Methylsulfonic acid, benzenesulfonic acid or toluenesulfonic acid, but also mineral acids such as sulfuric acid and phosphoric acid.
• Neutral salts can also be used as auxiliary electrolytes.
• Metal cations of lithium, sodium, potassium but also tetraalkylammonium compounds such as tetramethylammonium, tetraethylammonium, tetrabutylammonium and dibutyldimethylammonium are possible as cations.
• anions fluoride, tetrafluoroborate, sulfonates such as methyl sulfonate, benzenesulfonate, toluenesulfonate, sulfates such as sulfate, methyl sulfate, ethyl sulfate, phosphates such as methyl phosphate, ethyl phosphate, dimethyl phosphate, diphenyl phosphate, hexafluorophosphate and phosphonyl methylphosphate, phosphonyl methylphosphate such as phosphonate methyl phosphonate.
• fluoride tetrafluoroborate
• sulfonates such as methyl sulfonate, benzenesulfonate, toluenesulfonate
• sulfates such as sulfate, methyl sulfate, ethyl sulfate
• phosphates such
• Unbranched C1-C6 alkanols are preferably used as alkanols; methanol and ethanol are particularly preferred.
• the concentration of the alkanol in the feed to the electrolysis cell is generally 50 to 98% by weight, preferably 70 to 95% by weight.
• the reaction mixture can contain one or more additional inert solvents.
• additional inert solvents Compounds such as methylene chloride, acetonitrile, methyl tert-butyl ether, butyrolactone or dimethyl carbonate are suitable for this.
• the concentration of these solvents can be 0 to 30% by weight, based on the reaction mixture.
• the current density in the process according to the invention is generally 2 to 10 A / dm2, preferably 3 to 8 A / dm2.
• the total amount of charge transferred to the starting compound II in the process according to the invention is generally 3 to 9 F / mol II, preferably 4 to 8 F / mol II.
• Precious metals such as platinum and oxides such as chromium or ruthenium oxide and mixed oxides such as Ti / RuO x are suitable as anode materials.
• platinum and oxides such as chromium or ruthenium oxide and mixed oxides such as Ti / RuO x are suitable as anode materials.
• graphite is the preferred anode material.
• the electrochemical oxidations can be carried out in divided, but preferably in undivided, flow cells.
• the oxidation is generally carried out at temperatures from 0 to 120 ° C., preferably at 20 to 80 ° C.
• Process products I can be hydrolyzed to the corresponding aldehydes in a manner known per se.
• the compounds I thus represent storage-stable depot compounds for the much more sensitive aldehydes.
• the process according to the invention allows the starting compounds II to be converted into the products I with high conversion. Remarkably, the electrochemical oxidations proceed with high selectivity under these conditions.
• the by-products possibly formed in the reaction can be returned to the reaction without any special work-up steps. No interfering side reactions from such by-products were found.
• the electrolyte had the following composition: 450 g (15% by weight) of p-tert-butyltoluene 10 g (0.3 wt%) sulfuric acid 2450 g (84.7% by weight) of methanol
• the electrolyte was neutralized with sodium methylate, the methanol was distilled off and the precipitated salt was filtered off.
• the stated products were obtained after vacuum distillation.
• Example 1.1 Carried out as in Example 1.1, but without excess pressure in the electrolysis cell The following were isolated (data in mol%, based on the TBT used): 1% p-tert-butyltoluene 18% p-tert-butylbenzyl methyl ether 61% p-tert-butylbenzaldehyde dimethyl acetal
• the electrolyte had the following composition: 450 g (15% by weight) of p-tert-butyltoluene 10 g (1% by weight) of sodium benzene sulfonate 2510 g (84% by weight) of methanol
• the electrooxidation was carried out in a cell as described in Example 1 at 55 ° C.
• the current density was 3.4 A / dm2.
• the different charge quantities can be found in the following table.
• the procedure was analogous to Example 1. For working up, the reaction solution was freed from methanol by distillation, the salt which precipitated was filtered off and the acetal was purified by distillation.
• the electrolyte was pumped around at 200 l / h.
• Refurbished partial stream 0.15% by weight of the total current Inlet quantity: 307 g electrolyte / h
• Example 2.1 Carried out as in Example 2.1, but without excess pressure in the electrolysis cell The following were isolated (data in mol%, based on the TBT used): 1% p-tert-butyltoluene 12% p-tert-butylbenzyl methyl ether 64% p-tert-butyl aldehyde dimethyl acetal example Amount of charge F / mol TBT Turnover related to TBT + TBE Selectivity for acetal 2.1 erf. disk. 7.5 98 85 2.2 req. cont. 6.0 79 91 2.3 comparison disk. 7.5 87 74
• the electrolyte had the following composition: 450 g (15% by weight) xylene 30 g (1% by weight) of potassium benzene sulfonate 2540 g (84% by weight) of methanol After carrying out and working up in analogy to Example 1.1, the following were isolated: 4% p-methylbenzyl methyl ether 81% p-tolylaldehyde dimethyl acetal The conversion, based on the starting compound and p-methylbenzyl methyl ether, was 96% and the selectivity (with respect to acetal) was 84%.
• the electrolyte had the following composition: 15% by weight of p-methoxytoluene 0.4% by weight sodium benzene sulfonate 83.1% by weight of methanol 3.5 wt% recycle stream Execution and processing as in Example 1.3
• the recycle stream contained the components which boiled more easily than the product when the substream was worked up by distillation.

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• Chemical & Material Sciences (AREA)
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• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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Citations (3)

• 1993
  • 1993-08-14 DE DE4327361A patent/DE4327361A1/de not_active Withdrawn
• 1994
  • 1994-08-08 EP EP94112334A patent/EP0638665B1/fr not_active Expired - Lifetime
  • 1994-08-08 DE DE59400935T patent/DE59400935D1/de not_active Expired - Lifetime
  • 1994-08-11 US US08/289,277 patent/US5507922A/en not_active Expired - Lifetime
  • 1994-08-12 JP JP6190546A patent/JPH0776545A/ja active Pending

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