EP1637628A2 - Process for the preparation of glyoxylic acid alkyl ester dialkyl acetal - Google Patents

Abstract

Preparation of glyoxalic acid alkyl ester dialkyl acetal (I) comprises electrochemical oxidation of glyoxaldialkylacetal in the presence of alkyl alcohol and ionogenic halogenide.

Description

The present invention relates to an electrochemical process for the preparation of glyoxalic acid alkyl ester dialkyl acetal.

The electrochemical preparation of aliphatic carboxylic acid esters from aliphatic aldehydes which carry a methylene group in the alpha position to the oxo group is known from the following publications:

J. Org. Chem. 50, 1985, 4967-4969 describes the use of KBr as a mediator for this reaction

From Bull. Chem. Soc. Jpn. 55, 1982, 335-336 discloses the use of NaCN as a mediator.

The electrochemical preparation of aliphatic carboxylic acid esters from aliphatic aldehydes bearing a hydroxyl group alpha to the oxo group is known from the following publications:

J. Org. Chem. 53, 1988, 218-219 describes the reaction mediated with KI starting with the addition of NaOMe.

DE-A-38 14 498 relates to the use of ionic bromides or chlorides as a mediator in this reaction.

DE-A-10209195 discloses the preparation of trialkyl orthoformates by electrochemical oxidation of glyoxal tetraalkyl acetals.

• Oxidation zu Oxalsäure(derivaten)
• Hydrolyse der Acetalfunktion
• Oxidative Spaltung der C-C-Funktion
• Acetalisierung der Carbonylfunktion
• Hydrodimerisierung der Carbonylfunktion

It is furthermore known that the electrochemical reaction of aliphatic ketones or aldehydes can lead to different reactions on the carbonyl group, depending on the reaction conditions and substituents. However, it is not possible to deduce from the literature how the reaction can be controlled to produce from glyoxal or a glyoxal derivative glyoxalic acid alkyl ester dialkyl acetal without, for example, the following reactions taking place:

• Oxidation to oxalic acid (derivatives)
• Hydrolysis of the acetal function
• Oxidative cleavage of the CC function
• Acetalization of the carbonyl function
• Hydrodimerization of the carbonyl function

It was therefore an object to provide an electochemical process for the preparation of glyoxalic acid alkyl ester dialkyl acetal with high yields and high selectivity.

Accordingly, a process has been found for the preparation of alkyl glyoxylate dialkyl acetal which electrochemically oxidizes a glyoxal dialkyl acetal in the presence of an alkyl alcohol and an ionic halide.

in der R¹ und R² eine C₁- bis C₄-Alkylgruppe bedeuten oder gemeinsam eine C₁- bis C₄-Alkylengruppe bilden.Preference is given to using a glyoxal dialkyl acetal of the general formula I,

in which R ¹ and R ² denote a C ₁ - to C ₄ -alkyl group or together form a C ₁ - to C ₄ -alkylene group.

Preferably, R ¹ and R ^{2 are} methyl or ethyl or together are ethylene.

Such compounds, if appropriate as a mixture with water and the alkyl alcohol, can be prepared in a particularly favorable manner by reacting a mixture of glyoxal and water and optionally glyoxaltraalkyl acetal in the presence of an alkyl alcohol until the reaction equilibrium is reached. The Glyoxaltraacetal possibly present carries alkoxy groups, which are derived from the alkyl alcohol used. From the reaction mixture, the Glyoxaltetraecetal and any remaining glyoxal and optionally water and the alkyl alcohol is then removed. Details of this method are described in DE-A-196 51 325.

The alkyl alcohol used is preferably a C ₁ - to C ₄ -alkyl alcohol or a C ₂ - to C ₄ -alkanediol. Preference is given to methanol, ethanol or ethylene glycol.

Particularly preferred is a method in which methanol is used in combination with glyoxal dimethyl acetal as the alkyl alcohol.

in der R¹ und R² die gleiche Bedeutung wie in der allgemeinen Formel I haben und R³ sich von der Alkylgruppe des eingesetzten Alkylalkohols ableitet, also C₁- bis C₄-Alkyl bedeutet.Preferred alkyl glyoxalate dialkyl acetals are thus those of the general formula II

in which R ¹ and R ^{2 have} the same meaning as in the general formula I and R ^{3 is} derived from the alkyl group of the alkyl alcohol used, ie C ₁ - to C ₄ alkyl.

The alkyl alcohol is generally used in at least stoichiometric amounts. The proportions of glyoxal dialkyl acetal to alkyl alcohol are therefore preferably 1: 1 to 100: 1, preferably 1: 1 to 50: 1. The amounts of alkyl alcohol used can be significantly greater than the stoichiometrically required amounts, namely in particular when the alkyl alcohol is used as the solvent for the Glyoxaldialkylacetal and the other constituents of the electrolyte is used.

The ionogenic halide used is preferably an iodide, bromide or chloride. The ionogenic halides are usually the salts of the hydrohalic acids or the hydrohalic acids themselves.

Particularly preferred is an ionic halide selected from the group of hydroiodic acid, hydrobromic acid, hydrochloric acid, alkali metal salts of the three aforementioned acids, alkaline earth metal salts of the three aforementioned acids and quaternary ammonium salts of the three aforementioned acids.

Suitable alkali metal salts are preferably the sodium or potassium salts.

Suitable quaternary ammonium salts are those with NH ₄ , or monodi-tri or C ₁ - to C ₆ -alkyl ammonium as cation or cations derived from pyridine, imidazole or methylimidazole.

The ionic halides are generally contained in the electrolyte in amounts of from 0.1 to 10% by weight, preferably from 0.1 to 1.0% by weight.

Optionally, the electrolysis solution is added to customary cosolvents. These are the inert solvents generally used in organic chemistry with a high oxidation potential. Examples include tetrahydrofuran, acetonitrile, dimethylformamide, dimethoxyethane, dimethyl carbonate or propylene carbonate. Possibly. is also used as a solvent, water in a concentration of up to 10 wt .-%, preferably 1 to 5 wt .-%, based on the electrolyte.

Conducting salts can be added to the electrolyte to improve the conductivity. Preferred conductive salts are alkali, tetra (C ₁ - to C ₆ -alkyl) ammonium, preferably tri (C ₁ - to C ₆ -alkyl) -methylammonium salts. Suitable counterions are sulfate, bisulfate, alkyl sulfates, aryl sulfates, halides, phosphates, carbonates, alkyl phosphates, alkyl carbonates, nitrate, alcoholates, tetrafluoroborate or perchlorate. Particularly preferred is (CF ₃ SO ₂ ) ₂ NLi.

In addition, suitable conductive salts are ionic liquids as conductive salts. Suitable ionic liquids are described in "Ionic Liquids in Synthesis", ed. Peter Wasserscheid, Tom Welton, Verlag Wiley VCH, 2003, Ch. 1 to 3 and DE-A-102004011427.

The process according to the invention can be carried out in all customary divided or undivided types of electrolytic cell. Preferably, one works continuously with undivided flow cells.

Particularly suitable are bipolar switched capillary gap cells or plate stacked cells, in which the electrodes are designed as plates and are arranged plane-parallel (see Ullmann's Encyclopedia of Industrial Chemistry, 1999 electronic release, Sixth Edition, VCH Verlag Weinheim, Volume Electrochemistry, Chapter 3.5 Designs and Chapter 5, Organic Electrochemistry, Subchapter 5.4.3.2 Cell Design). As the electrode material, graphite is preferable.

Suitable anode materials include, 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 . Preference is given to graphite or carbon electrodes.

As cathode materials are, for example, iron, steel, stainless steel, nickel or precious metals such as platinum and graphite or carbon materials into consideration. Preferably, the system is graphite as the anode and cathode and graphite as the anode and nickel, stainless steel or steel as the cathode.

The current densities at which the process is carried out are generally 1 to 1000, preferably 1 to 100, particularly preferably 25 to 50 mA / cm ² . The temperatures are usually -20 to 60, preferably -10 to 35 ° C. In general, working at atmospheric pressure. Higher pressures are preferably used when operating at higher temperatures to avoid boiling of the starting compounds or the solvent.

The pH of the electrolyte is preferably adjusted to 6 to 14, more preferably 6 to 8, which is generally done by adding a Broensted base or acid if necessary.

After completion of the reaction, the electrolyte solution is worked up by general separation methods. For this purpose, the halides are optionally removed, for example, first by means of extraction or filtration from the electrolysis solution. For further workup, the electrolysis solution is generally first distilled and recovered the individual compounds in the form of different fractions separately. Another cleaning can be done for example by crystallization, extraction, distillation or chromatographic.

Experimental part example 1

An undivided plate stack cell with graphite electrodes was used. 60 g of glyoxal dimethylacetal were reacted in 540 g of methanol and 6 g of NaBr at a temperature of 20 ° C. The electrolysis was carried out at 3.4 A / dm2 and it was a charge amount of 3 F based on the glyoxal dimethyl acetal passed through the cell. In the electrolysis effluent 94, GC area% methyl dimethoxyacetate was obtained. After removal of the solvent in a water-jet vacuum and fractional distillation, the product was isolated in a yield of 77%.

Example 2

An undivided plate stack cell with graphite electrodes was used. There were reacted 75 g of glyoxal dimethyl acetal in 660 g of methanol and 15 g of Nal at a temperature of 18-25 ° C. The electrolysis was carried out at 6.3 A / dm2 and it was a charge amount of 2.5 F based on the glyoxal dimethyl acetal passed through the cell. In Elektrolyseaustrag 78 wt .-% of dimethoxyacetate was obtained.

Example 3

An undivided plate stack cell with graphite electrodes was used. 280 g of glyoxal dimethylacetal were reacted in 2522 g of methanol and 57 g of Nal at a temperature of 20 ° C. The glyoxal dimethyl acetal used had a water content of 16.2%. The total electrolyte contained 1.7% water. The electrolysis was carried out at 6.3 A / dm2 and it was a charge amount of 2.5 F based on the glyoxal dimethyl acetal passed through the cell. 7.4% by weight of dimethoxyacetic acid methyl ester were obtained in the electrolysis discharge. At 700-250 mbar, methanol was distilled off and the residue was fractionally distilled at 16 mbar. Isolated yield 68% (purity> 97%).

Claims (10)

A process for the preparation of alkyl glyoxylate dialkyl acetal which comprises electrochemically oxidizing a glyoxal dialkyl acetal in the presence of an alkyl alcohol and an ionic halide. A process as claimed in claim 1, wherein a glyoxal dialkyl acetal of the general formula I is used,

in which R ¹ and R ² denote a C ₁ - to C ₄ -alkyl group or together form a C ₁ - to C ₄ -alkylene group. Method according to one of the preceding claims, wherein the alkyl alcohol used is a C ₁ - to C ₄ -alkyl alcohol or a C ₂ - to C ₄ -alkanediol. A process as claimed in any one of the preceding claims, wherein the alkyl alcohol used is methanol and glyoxal dimethyl acetal. Method according to one of the preceding claims, wherein the ionogenic halide used is an iodide, bromide or chloride. Process according to one of the preceding claims, wherein the ionogenic halide used is a compound selected from the group of hydroiodic acid, hydrobromic acid, hydrochloric acid, alkali metal salts of the three abovementioned acids, alkaline earth metal salts of the abovementioned three acids and quaternary ammonium salts of the abovementioned three acids. Method according to one of the preceding claims, wherein the method is carried out in an undivided electrolysis cell. Method according to one of the preceding claims, wherein the method is carried out in a bipolar switched capillary gap cell or plate stack cell. A process according to any one of the preceding claims wherein the glyoxyl dialkyl acetal is provided by reacting a mixture of glyoxal, water and, optionally, glyoxaltraalkyl acetal in the presence of an alkyl alcohol until the reaction equilibrium is reached. Method according to one of the preceding claims, wherein the electrodes used are graphite electrodes.