Title of Invention

A PROCESS FOR PREPARING α - α DIMETHYL PHENYL ACETIC ACID

Abstract

Process for the preparation of α - α dimethylphenylacetic acid from α - α dimethylbenzyl cyanide under normal pressure A process for obtaining α-α dimethylphenylacetic acid wherein α - α dimethylbenzyl cyanide is reacted in the presence of sodium hydroxide, water and a C4- and/or C5-alcohol at temperatures above 100˚c and the α - α dimethylphenylacetic acid is obtained by acidification is described.

Full Text

Description: Process for the preparation of a,a-dimethylphenylacetic acid from a,a-dimethylbenzyl cyanide under normal pressure The preparation of a,a-dimethylphenylacetic acid from a,a-dimethylbenzyl cyanide is known. The preparation is described for instance in the Journal of Medical Chemistry, 1977, volume 20, No. 8, page 1063. In this process, 2-phenyl-2-methylpropionic acid- nitrile (a,a-dimethylbenzyl cyanide) is stirred in a solution of KOH (fivefold molar excess) and methanol in an autoclave at 140 to 150X under pressure for 20 hours, and the reaction mixture is then concentrated under reduced pressure, diluted with water and acidified. The a,a-dimethylphenylacetic acid obtained as white crystals is then purified by an expensive purification process - filtration and recrystallization from ethanol. An a,a-dimethylphenylacetic acid with a melting point of SO^C is obtained in this process. The theoretical yield is 90%. Our own experiments in which 2-phenyl-2-methyipropionic acid nitrile (a,a-dimethylbenzyl cyanide) was reacted in mixtures of n-butanol, water and KOH (threefold molar excess) at about 125°C for 22 hours also did not lead to a complete hydrolysis of the nitrile employed. The corresponding amide intermediate was, furthermore, also found in amounts of 2%. Depletion of this intermediate would necessitate an additional purification step in order to obtain an a,a-dimethylphenylacetic acid of high purity. Our own experiments in mixtures of n-butanol, water and lithium hydroxide indicated only the formation of about 1% of the amide intermediate after a hydrolysis time of 4 hours at about 100°C (reflux). No hydrolysis up to the formation of the carboxylic acid was detected. Further experiments of our own in 50% strength aqueous potassium or sodium hydroxide solution (threefold molar excess) at 125X with vigorous stirring showed the following analytical results after a reaction time of 20 hours: Potassium hydroxide solution: Sodium hydroxide solution; nitrile 0,4% amide 51% nitrile 62% amide 34% carboxylic acid 47% carboxylic acid 27% The hydrolysis reaction with the stronger base, potassium hydroxide, proceeded much more rapidly than that with the weaker base. The following rates of hydrolysis therefore resulted: LiOH It has now been found that the nitrile is hydrolyzed to the carboxylic acid in a very short time under normal pressure in a mixture of a C4- or Cs-alcohol, water and NaOH. . The object of the invention is therefore to provide in high yields, by modification of the process conditions, an a,a-dimethylphenylacetic acid which is substantially free from 2-phenyl-2-methylpropionic acid nitrile (a,a-dimethylbenzyl cyanide) and 2-phenyl-2-methylpropionic acid amide. The object is achieved by carrying out the reaction of a,a-dimethylbenzyl cyanide in the presence of sodium hydroxide, water and a C4'0r Cs-alcohol at temperatures above 100X and then obtaining the a,a-dimethyl-phenylacetic acid by acidification. Additional purification steps are no longer necessary. The invention therefore relates to a process for obtaining the compound of the formula I COOH (I) which comprises reacting a compound of the formula in the presence of water, sodium hydroxide and a C4-alcohol or Cs-alcohol at temperatures of more than 100°C to give the corresponding carboxylic acid salt and then obtaining the compound of the formula I by addition of an acid. In the preparation of the compound of the formula I, a procedure is followed in which the C4- or Cs-alcohol, water, the sodium hydroxide and the compound of the formula II (a.a-dimethyibenzyl cyanide) are first mixed and the mixture is heated to more than 100°, while stirring. A mixture of C^. and Cs-alcohol can also be employed. After an appropriate reaction time, the reaction mixture is cooled and a,a-dimethylphenylacetic acid is precipitated out with an acid. The a,a-dimethylphenylacetic acid is isolated, for example, by crystallization or extraction. The crystallization is promoted by cooling of the suspension or further evaporation of the solvent. The extraction takes place by addition of organic solvents to the a,a-dimethylphenylacetic acid, for example toluene The term C4- and Cs-alcohol is understood as meaning, for example. n-butanol, 2-methyl-1-propanol, n-pentanol, 2-pentanol, 3-pentanol, 2-methyl-2-butanol, 3-methyl-2-butanol, 2-methyl-3-butanol, 3-methyl-1"butanol or 2-methyl-1-butanol. The term sodium hydroxide is understood as meaning caustic soda in solid form or in the form of alkaline solutions of venous concentrations. The water present in the alkaline solution is then included in the calculation when.prepanng the hydrolysis mixture. Suitable acids are, for example, hydrochloric acid, sulfuric acid, nitric acid or phosphoric acid or mixtures of the acids. Preferably, 120 mol to 250 mol of water and 150 mol to 300 mol of sodium hydroxide are used per 100 mol of the compound of the formula II for the hydrolysis reaction. The amount of C4- or Cs-alcohol used is in general 0.5 kg to 1.5 kg per of the compound of the formula II, preferably 0.6 kg to 1.0 kg. The reaction time is in general 2 to 6 hours, depending on the composition of the mixture and the temperature range chosen. The reaction temperature is 100X to 14QX, preferably 110°C to 130°C, depending on the C4- or Cs-alcohol used. The by-product 2-phenyl-2-methylpropionamide is present in the isolated a,a-dimethylphenylacetic acid in an amount of less than 0.1%, based on the compound of the formula I. The residual content of the starting substrate of the compound of the formula II (a,a-dimethylbenzyl cyanide) could no longer be detected in the isolated a,a-dimethylphenylacetic acid. The starting substances for the hydrolysis reaction according to the invention can be prepared by processes known from the literature. ■ The process product is suitable for the preparation of a large number of secondary products, for example for the preparation of medicaments having an antiallergic action, such as 4-[4-[4-(hydroxydiphenyl)-1-piperidinyl]-1-hydroxybutyl]-a,a-dimethylphenyl acetic acid (US 4 254 129). The very short reaction times, the omission of additional purification steps, the high yields and the high purity of the product prepared are advantageous. The substantially complete conversion into the compound of the formula I and a total content of by-products of less than 0.1% are an advantage of the process according to the invention. Example 1 Preparation of a.a-dimethylphenylacetic acid 150 g of n-butanol, 65 g of water, 150 g of caustic soda and 221 g of dimethylbenzyl cyanide (about 98.2% pure) were initially introduced into a stirred apparatus. The mixture was heated up to 120X to 126X, with thorough stirring, and allowed to after-react at this temperature for about 6 hours. 250 ml of water were then added and the n-butanol was removed from the reaction mixture by distillation. After addition of a further 500 ml of water, the mixture was cooled to SOX to 50X. The free dimethylphenylacetic acid was precipitated out by addition of about 433 g of hydrochloric acid, about 30% strength. This free acid could be either isolated, or extracted from the crystal suspension. During the isolation, the suspension was cooled further, and when the crystallization process had ended, the crystals were filtered off with suction and washed with cold water until free from chloride ions. After drying in a vacuum drying cabinet, a yield of 238.4 g (97.3% of theory) was obtained. The content of dimethylphenylacetic acid according to HPLC was more than 99.9% (amide content less than 0.1%). The melting point of the a,a-dimethylphenylacetic acid prepared in this way was 81.5°C, as the minimum of the differential scanning calorimetric method recorded. Example 2 Alternatively, the a,a-dimethylphenylacetic add can be extracted with an organic solvent (for example toluene). In this procedure, the a,a-dimethylphenylacetic acid formed is converted into the organic phase to the extent of more than 99%; the aqueous phase is separated off. The organic phase can then be employed directly for secondary reactions (for example esterification). HPLC determination Sample preparation Dissolve 10 mg of substance in 10 ml of a mixture of acetonitrile and water (4:6). Column: 0.15 m length 4.6 mm diameter Stationary phase: SB-phenyl on silica gel, 5 pm, Surtax ® Mobile phase: 30% of acetonitrile 70% of phosphate buffer, pH 2.5 Injection volume: 10 Flow rate: 1 ml/minute Detection: UV, 210 nm Migration time: 30 min 1. A process for obtaining the compound of the formula I in the presence of water, sodium hydroxide and a C4-alcohol and/or C5-alcohol at temperatures of more than 10OX to give the corresponding carboxylic acid salt and then obtaining the compound of the formula I by addition of an acid. 2. The process as claimed in claim 1, wherein a compound from the group consisting of n-butanol, 2-methyl-1-propanol, n-pentanol, 2-pentanol, 3-pentanol, 2-methyl-2-butanol, 3-methyl-2-butanol, 2-methyl-3-butanol, 3-methyl-1-butanol and 2-methyl-1-butanol, preferably n-butanol, is employed as the C4- or Cs-alcohol. 3. The process as claimed in claim 1 or 2, wherein a mixture of C4- and C5-alcohol is employed. 4. The process as claimed in one or more of claims 1 to 3, wherein the sodium hydroxide is employed in the form of a solid or as an aqueous solution. 5. The process as claimed in one or more of claims 1 to 4, wherein hydrochloric acid, sulfuric acid, nitric acid or phosphoric acid or a mixture of the acids, preferably hydrochloric acid, is employed as the acid. 6. The process as claimed in one or more of claims 1 to 5, wherein 120 mol to 250 mol of water and 150 mol to 300 mol of sodium hydroxide are employed per 100 mol of the compound of the formula IL 7. The process as claimed in one or more of claims 1 to 6, wherein 0.5 kg to 1,5 kg of the C4- and/or Cs-alcohol, preferably 0.6 kg to 1.0 kg, are employed per 1 kg of the compound of the formula II. 8. The process as claimed-in one or more of claims 1 to 7, wherein the reaction of the compound of the formula II is carried out at a temperature of 100°C to 140X, preferably 110X to 130°C. A process for obtaining a compound, substantially as herein described, and exemplified.

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