DE2851732A1 - Substd. benzaldehyde di:alkyl acetal cpds. prodn. - by electrochemical oxidn. of substd. toluene in alcohol contg. tetra:alkyl-ammonium aryl-sulphonate - Google Patents

Abstract

In the prodn. of substd. benzaldehyde dialkyl acetals (I) by electrochemical oxidn. of substd. toluenes of formula (II) (in which R1 is (ar)alkoxy or aryloxy; R2 and R3 are H, (ar)alkyl or aryl) in alcoholic soln. in the presence of a conductive salt of formula (III) at oxidn.-stable anodes. In (III) R4 is alkyl; R5 and R6 are H, alkyl or halogen or together form a condensed aromatic ring. The alcohol used in the soln. has the formula R7-OH (in which R7 is alkyl,). The intermediates (IV) formed are returned to the starting material for electrochemical oxidn. (I) are useful intermediates, e.g. giving the corresp. substd. benzaldehydes, such as anisaldehyde, which are valuable perfume components, on hydrolysis with dil. acids. (I) are obtd. in high material and current yield, selectivity and purity. Under the conditions used, there is no introduction of an alkoxy gp. into the aromatic ring and no further oxidn. to the corresp. or tho-benzoic ester. Also, oxidative dimerisation of aromatics contg. side chains, as occurs esp. at graphite anodes, is completely suprressed.

Description

Process for the production of substituted benzaldehyde

dialkyl acetals The present invention relates to a method for Production of substituted benzaldehyde dialkyl acetals.

Substituted benzaldehyde dialkyl acetals are valuable intermediates and can for example in the presence of dilute acids to the corresponding substituted Benzaldehydes are hydrolyzed. The substituted benzaldehydes, for example Anisaldehyde, are valuable fragrance components (Ullmann's encyclopedia of technical Chemistry, 3rd edition, volume 14, page 738, Urban and Schwarzenberg, Munich-Berlin, 1963).

It is known to treat p-methoxytoluene with inorganic oxidizing agents, such as permanganate or dichromate, to be oxidized to anisaldehyde (Ullmann, loc. cit.).

This method has some disadvantages: it has to be partially Further oxidation to anisic acid can be expected, and more watery due to the inevitable attack Metal salt solutions, the process is a burden on the environment.

It is known to use methylene chloride as a solvent and in toluene Presence of methanol, a conductive salt and sulfuric acid electrochemically to form anisaldehyde to oxidize with 20% of the theoretical yield (Tetrahedron Letters 1978, p 3723).

It is also known to use p-methoxytoluene in methanol as a solvent and in the presence of sodium methylate, electrochemically to form anisaldehyde dimethylacetal to oxidize (J. Chemie. Soc., Perkin trans. act. I, 1978, page 708).

In this procedure there is a methoxylation in both the side chain as well as in the aromatic nucleus of the starting material (ibid. page 709).

Furthermore, the electrochemical oxidation of hydroquinone dimethyl ether in methanol as a solvent and in the presence of potassium hydroxide to form benzoquinone tetramethyl ketal known (J. Am. Chem. Soc. 85, 2525 (1963)), with methylation in the aromatic nucleus of the hydroquinone dimethyl ether is observed.

There was a process for the preparation of substituted benzaldehyde dialkyl acetals by electrochemical oxidation of substituted toluenes of the formula in which R1 is alkoxy, aryloxy or aralkoxy and R2 and R3, R2 and R3 are identical or different and are hydrogen, alkyl, aryl or aralkyl, found in alcoholic solution in the presence of a conductive salt on oxidation-stable anodes, which is characterized in that as a conductive salt a tetraalkylammonium arylsulfonate of the formula in which 4 R4 is alkyl and 5 6 R and R6 are identical or different are hydrogen, alkyl or halogen or together, if they are adjacent, can denote a condensed aromatic ring, and an alcohol of the formula R7-OH for the alcoholic solution (III), in which R7 denotes alkyl, is used, the intermediate products formed being able to be returned to the starting material for the electrochemical oxidation. Examples of alkyl are straight-chain or branched hydrocarbon radicals with 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl , Butyl or isobutyl. Preferred alkyl is methyl.

Aryl can be, for example, aromatic hydrocarbon radicals 6 to 14 carbon atoms mentioned, such as phenyl, naphthyl or anthryl. Preferred Aryl is phenyl.

Aralkyl are, for example, hydrocarbon radicals with 1 or 2 carbon atoms in the aliphatic part and 6 to 14 carbon atoms in the aromatic part Part called, such as benzyl, 2-phenylethyl, naphthylmethyl, 2-naphthylethyl, anthrylmethyl or 2-anthryl-ethyl. The preferred aralkyl is benzyl.

As alkoxy radicals may be mentioned1 which differ from a straight-chain or branched alcohol with 1 to 4 carbon atoms, such as methoxy, propoxy, Isopropyloxy, butoxy or isobutyloxy. The preferred alkoxy is methoxy.

As aryloxy, radicals may be mentioned, for example, which differ from one derive aromatic hydroxy compounds with 6 to 14 carbon atoms, such as phenoxy, Naphthyloxy or anthryloxy. The preferred aryloxy is phenoxy.

As aralkoxy, radicals may be mentioned, for example, which differ from one araliphatic alcohol with 1 or 2 carbon atoms in the aliphatic part and Derive 6 to 14 carbon atoms in the aromatic part, such as benzyloxy, 1-phenyl-ethyloxy, 2-phenyl-ethyloxy, naphthyl-methoxy, naphthyl-ethoxy, anthryl-methoxy or anthryl-ethoxy.

The preferred aralkoxy is benzyloxy.

Examples of halogen that may be mentioned are fluorine, chlorine or bromine. Preferred Halogen is chlorine.

As substituted toluenes used in the process according to the invention can be mentioned, for example: p-methoxy-toluene, p-ethoxy-toluene, p-propoxy-toluene, p-isopropyloxy-toluene, p-butoxy-toluene, p-isobutyloxytoluene, p-phenoxy-toluene, p-Benzyloxy-toluene, 2-methyl-4-methoxy-toluene, 3-methyl-4-methoxy-toluene, 2,3-dimethyl-3-methoxy-toluene, 2,5-dimethyl-4-methoxy-toluene, 2,6-dimethyl-4-methoxy-toluene, 2-phenyl-4-methoxy-toluene, 3-phenyl-4-methoxy-toluene, 2-benzyl-4-methoxy-toluene, 3-benzyl-4-methoxy-toluene, 2-phenyl-3-methyl-4-methoxytoluene, 2-methyl-3-phenyl-4-methoxy-toluene, 2-ethyl-3-benzyl-4-phenoxy-toluene, 2-benzyl-4-benzyloxy-5-isopropyl-toluene, 2,6-dibutyl-4-benzyloxy-toluene.

Examples of tetraalkylammonium aryl sulfonate that may be mentioned are: tetramethylammonium benzenesulfonate, Tetraethylammonium-p-toluenesulfonate, tetrabutylammonium-p-toluenesulfonate, trimethyl-butyl-ammonium-o-chloro-p-toluenesulfonate, Tetraethylammonium-2,4-dichlorobenzenesulfonate, dimethyl-dipropyl-ammonium-o-toluenesulfonate, Tetramethylammonium-2-bromo-4-methyl-benzenesulfonate, Tetraethylammonium naphthyl-1-sulfonate, tetrabutylammonium-naphthyl-2-sulfonate.

Preferred tetraalkylammonium aryl sulfonate for the invention The process is tetraethylammonium p-toluenesulfonate.

The inventive method can be in alcoholic solution, for example in methanol, ethanol, propanol, isopropanol, butanol or isobutanol, is preferred be carried out in methanol.

The concentration of the tetraalkylammonium aryl sulfonate as a conductive salt in the process according to the invention is, for example, 0.01 to 2 mol per liter, preferably 0.05 to 0.5 mol per liter.

The method according to the invention can be carried out on oxidation-stable anodes are, for example, on anodes made of graphite, lead dioxide, a metal of the platinum group, an alloy of metals from the platinum group or metals that have a coating wear made of a platinum metal. Anodes made from graphite, platinum or platinum-plated titanium are preferred.

The cathode material is, for example, copper, nickel, steel, platinum or graphite, preferably steel, called.

The electrochemical reaction according to the invention can be in an undivided or in one through a conventional diaphragm into an anode and a cathode compartment divided electrolytic cell can be carried out. Here one is under construction and undivided electrolysis cell that is less complex in operation without any disadvantage for the procedure.

In the process according to the invention, pure hydrogen is used as the electrolysis waste gas formed, so that apart from a flushing of the apparatus with inert gas before start-up no additional measures are necessary to protect against the formation of oxyhydrogen. The cell construction is not critical to the method according to the invention as long as it is it is ensured that the heat of reaction and the hydrogen formed are dissipated can be.

For example, plate cells are with an electrode spacing of 1 to 5 mm, through which the electrolyte is pumped parallel to the electrodes to name.

The concentration of the substituted toluene in the alcoholic Solution in the process according to the invention can, for example, be 1 to 50% by weight, preferably 10 to 30% by weight.

The inventive method can in the temperature range from -20 to + 600C, preferably from +10 to + 400C, can be carried out.

It may be useful to use a base that is difficult to oxidize to be used in the method according to the invention in order to reduce the pH to values less than 7 to prevent. Examples of this are collidine or 2,6-luthidine in an amount of 0.01 to 2% by weight, preferably 0.05 to 1% by weight, mentioned.

The current densities in the method according to the invention can be A / dm2, for example from 0.1 to 50 A / dm2, preferably from 1 A / dm2 to 20 A / dm2.

The cell voltage depends on the composition the electrolyte, the temperature and the geometry of the individual cell. she can for example, assume values of 4 to 12 V per single cell.

With the aid of the process according to the invention, substituted benzaldehyde dialkyl acetals of the formula in which R1 to R4 have the meaning given above.

Examples which may be mentioned as compounds of the formula (IV): p-methoxy-benzaldehyde-dimethylacetal, p-methoxy-benzaldehyde-diethylacetal, p-methoxy-benzaldehyde-dipropylacetal, p-methoxy-benzaldehyde-dibutyl-acetal, p-ethoxy-benzal- dehyde dimethylacetal, p-0thoxybenzaldehyde diethylacetal, p-propoxy-benzaldehyde-dimethylacetal, p-butoxybenzaldehyde-dimethylacetal, p-phenoxyl-benzaldehyde-dimethylacetal, p-phenoxy-benzaldehyde diisopropyl acetal, p-benzyloxy-benzaldehyde dimethyl acetal, p-Benzyloxybenzaldehyde diisobutyl acetal, 2-methyl-4-methoxybenzaldehyde dimethyl acetal, 3-methyl-4-methoxy-benzaldehyde diethyl acetal, 2,5-dimethyl-4-methoxy-benzaldehyde dimethyl acetal, 2,6-dimethyl-4-phenoxy-benzaldehyde dimethyl acetal, 2-methyl-4-methoxy-5-phenyl-benzaldehyde dimethyl acetal, 2-Benzyl-4-phenoxy-5-isopropyl-benzaldehyl-diethylacetal, 2-ethyl-3-benzyl-4-methoxybenzaldehyde-dimethylacetal.

The process according to the invention can be explained using the example of the conversion of p-methoxytoluene (p-methyl-anisole) in methanolic solution to p-methoxy-benzaldehyde-dimethylacetal (anisaldehyde-dimethylacetal) by the following equation: In the reaction given as an example of the process according to the invention, p-methoxymethyl anisole can be used as an intermediate according to the following equation appear.

It is possible to use the substituted alkoxymethylbenzene of the formula which is optionally formed as an intermediate in the process according to the invention in which R1, R2, R3 and R4 have the abovementioned meaning, which represents a generalization of the above example, to be returned to the starting material for the electrochemical oxidation of the process according to the invention.

To return it to the starting material, it is not necessary to subject the intermediate of the formula (V) to a purification. For example it can be mixed with the associated substituted benzaldehyde dialkyl acetal of formula (IV) can be returned to the starting material.

The process according to the invention can be both batchwise and be carried out continuously. When using the continuous working method For example, a side stream can be branched off from the electrolysis circuit and worked up while at other points of the electrolysis circuit output connection and the recycled electrolysis intermediate can be added.

The method according to the invention can also be connected in series Electrolytic cells can be carried out in the manner of a cascade.

It has also been found that by lowering the current density the selectivity of the method according to the invention can be increased at the electrodes can. The high space-time yield, a high current density and the high selectivity A low current density can therefore be combined in a further variant of the method according to the invention, the current density in the course of the electrochemical Decrease oxidation. With the cascade connection of several electrolysis cells for example, the current density in the first electrolytic cell be the highest and be lowered a little in the next cell.

The method according to the invention can be carried out, for example, as follows The electrolyte is made by dissolving the substituted toluene, the tetraalkylammonium aryl sulfonate and optionally prepared the basic compound in the desired alcohol and up to the desired degree of conversion, for example 60 to 90% of the theoretical Degree of conversion electrolyzed. After the electrolysis, the excess alcohol becomes distilled off and the crystalline-precipitated tetraalkylammonium arylsulfonate filtered off. The electrolysis products can then by customary work-up methods, for example by distillation. Any intermediate electrolysis product obtained can, as described above, be fed back into the electrolysis.

The substituted benzaldehyde dialkyl acetals which can be prepared in the process according to the invention are obtained in high selectivity and in high purity. At the same time will be high Material and current yields, based on the theoretically possible yields, achieved.

It is extremely surprising that under the conditions of Process according to the invention, the substitution of the aromatic nucleus as the starting material substituted toluenes used by an alkoxy group does not enter and a Further oxidation to the substituted benzoic acid orthoester does not occur like this was to be expected according to the state of the art (J. Am.

Chem. Soc. 85, 2525 (1963)). It is also surprising that the Oxidative dimerization of side chains observed especially in graphite anodes containing aromatics (Fritz Beck, Elektroorganische Chemie, pages 242 and 249, Verlag Chemie, Weinheim 1974) is almost completely suppressed.

Example 20 g (164 mmol) of p-methoxytoluene, dissolved in 70 ml of methanol, which contains 2 ml of collidine and 5 g of tetraethylammonium p-toluenesulfonate dissolved on a graphite anode 2 at a current density of 10 A / dm2 and a temperature Electrolyzed from 250C to a consumption of 0.6 F.

The electrolyte is concentrated in a water jet vacuum and the distilled Residue and receives 23 g of crude product with a boiling range of 70-1200C, which after gas chromatographic analysis 81.5% = 18.7 g = 103 mmol of anisaldehyde dimethyl acetal, 4.9% = 1.13 g of p-methoxymethyl anisole and 1.5% = 0.33 g of starting product.

This results in a material yield of 67% and a current yield of 71% based on the theoretical yields.

Claims (3)

Claims I 1J Process for the preparation of substituted benzaldehyde dialkyl acetals by electrochemical oxidation of substituted toluenes of the formula in which R1 is alkoxy, aryloxy or aralkoxy and R2 and R3 are identical or different and are hydrogen, alkyl, aryl or aralkyl, in alcoholic solution in the presence of a conductive salt on oxidation-stable anodes, characterized in that a tetraalkylammonium aryl sulfonate is used as the conductive salt the formula in which R4 denotes alkyl and R5 and R6 are identical or different and denote hydrogen, alkyl or halogen or together, if they are adjacent, can denote a condensed aromatic ring and an alcohol of the formula R7-OH in R7 for the alcoholic solution Alkyl means used, it being possible for the intermediate products formed to be returned to the starting material for the electrochemical oxidation. 2. The method according to claim 1, characterized in that the optionally formed as an intermediate substituted alkoxymethyl-benzene of the formula in which R1, R21R3 and R4 have the meaning given in claim 1, optionally without further purification, is returned to the starting material for the electrochemical oxidation. 3. The method according to claim 1, characterized in that the Current density decreased in the course of the electrochemical oxidation.