GB2095235A - Preparation of chrysanthemic acid - Google Patents

Abstract

Chrysanthemic acid of high purity is prepared by alkaline hydrolysis of the corresponding alkyl esters containing 1 to 6 carbon atoms in the alkyl moiety, with an aqueous solution of an alkali metal hydroxide in the absence of organic solvents but in the presence of a catalyst. Chrysanthemic acid is a useful intermediate in the preparation of pyrethroids having insecticide activity.

Description

SPECIFICATION Preparation of chrysanthemic acid The invention relates to a process for the preparation of a chrysanthemic acid in high purity.

More particularly, the invention concerns a process for preparing a cyclopropanecarboxylic acid of the formula (I)

by hydrolysis of the corresponding esters.

According to another aspect of the invention there is provided a process for the separation of cis- and trans-isomers of the acid of formula (I) by selective hydrolysis of the cis and trans-esters of the formula (II)

wherein R is alkyl having 1 to 6 carbon atoms.

Insecticidally active pyrethroids, eg. cinerin, allethrin, phthaltrin, permetrin, cypermetrin, decametrin, etc. are generally synthetized starting from a lower aliphatic ester of a corresponding cyclopropaneca rboxylic acid. Pyrethroids can conveniently be prepared by hydrolysing those esters into the corresponding free acids and coupling the acids obtained with the alcohol components of the desired active compounds by conventional techniques of esterification.

The conventionally employed hydrolysis methods have numerous disadvantages which prohibit their application on a large scale. When hydrolysis is performed in a solvent large volumes are involved, the solvent has to be eliminated from the reaction mixture when the hydrolysis is complete, and the cyclopropanecarboxylic acid has to be isolated from the residual solution by means of a solvent which has to be removed again to afford the desired pure acid.

By the known methods mixtures of cis- and trans-cyclopropanecarboxylic acid esters are produced. The activity of pyrethroid compounds prepared from the two isomers is considerably different. It can therefore be desirable to separate the isomers. The methods known in the art generally relate to the separation of the cis- and trans-free acids, preferably by recrystallization from suitable solvents (J. Chem. Soc. 283, 1945).

Our object was to provide a simple, industrial process for the preparation of cyclopropanecarboxylic acids by a total hydrolysis of the corresponding esters in the absence of solvents, and for the selective hydrolysis of the trans-isomer in a mixture of cis- and trans-cyclopropanecarboxylic acid esters.

It has surprisingly been found that cyclopropanecarboxylic acid esters of formula (II), in which R is C16 alkyl and preferably C14 alkyl can be hydrolysed in the absence of organic solvents, if a small amount of a catalyst is present to facilitate the heterogenous reaction with the aqueous alkali metal hydroxide solutions used for hydrolysis.

Quaternary amines or other phase transfer catalysts, anionic, cationic or non-ionic emulsifying agents can be used as catalysts. It has further been found that the velocity of the hydrolysis of trans-isomers exceeds that of the cis-isomers, and accordingly the two isomers can be separated with a good selectivity when suitable reaction conditions are employed.

The method is equally suitable for total and a selective hydrolysis, for the preparation of pure acid from esters containing unhydrolysable impurities, for the separation of impurities and for the recovery of chrysanthemic acids from synthesis by-products.

Hydrolysis is carried out with an aqueous solution of alkali metal hydroxides. Generally 2 to 50, preferably 4 to 30%, by weight solutions of alkali metal hydroxides are used. As alkali metal hydroxide sodium hydroxide and/or potassium hydroxide is preferably employed.

The hydrolysis is generally accomplished at a temperature between 200C and 1 200C, preferably 500C and 1000C.

When complete hydrolysis of the starting ester is desired (the cis/trans ratio in the product being essentially preserved), the alkali metal hydroxide may be used in an amount of 100 to 150% by weight (preferably 1 02-130%) of the equivalent relative to the ester.

Lower alkali concentration, lower temperature and portionwise addition of the alkali metal hydroxide solution improve the selectivity of the hydrolysis. In case of partial hydrolysis (in which the transester is selectively hydrolysed and the cis/trans isomer ratio is changed) the alkali metal hydroxide solution is added in an amount corresponding to 70 to 1 10% by weight, preferably 80 to 95% by weight, of the amount equivalent to the trans-ester isomer preferably in small portions.

The catalyst should be alkali resistant. Its concentration in the reaction mixture is 0.01 to 5%, preferably 0.1 to 1%, based on the weight of the ester. Preferred phase transfer catalysts are anionic or cationic or non-ionic emulsifiers.

After the partial hydrolysis the unreacted ester enriched in the cis-isomers in the un-hydrolysed residue may be separated from the aqueous phase on the basis of their different specific weights, may be dissolved by a suitable solvent, e.g. chlorinated solvents, aliphatic or aromatic hydrocarbons, or may be evaporated by steam distilation.

The acid can be liberated from the aqueous solutions containing salts of cyclo-propanecarboxylic acid with a mineral acid. The chrysanthemic acid which is insoluble in the aqueous phase can be eliminated in a melt of a solid, crystalline substance, can be dissolved in an organic solvent or if desired can be recrystallized from a solvent, preferably an aliphatic hydrocarbon.

Chrysanthemic acid obtained has a high purity and can be used for the preparation of pyrethroid insecticides without further purification.

Further details of the invention are illustrated by the following examples, which are not intended to limit the scope of the invention in any way.

Example 1 20 g. of chrysanthemic acid ethyl ester (2,2 dimethyl-3-)2,2-dimethylvinyl(-cyclopropanel - carboxylic acid ethyl ester) (purity: 98%, cis/trans ratio: 14:86), 20 g. of a 25% aqueous sodium hydroxide solution and 0.02 g. of p-nonylphenylpolyglycol ether are stirred at 950C for 180 minutes. After about 30 to 40 minutes a homogenous mixture is obtained, whereupon a total hydrolysis takes place and ethanol formed in the reaction is distilled off. From the sodium chrysanthemate solution obtained chrysanthemic acid is precipitated by adding 50 g. of a 10% aqueous hydrochloric acid solution at 500C, with stirring. The upper oily phase turns crystalline upon cooling and can be separated as a solid.

Yield: 16.2 g. (96%).

Example 2 20 g. of chrysanthemic acid ethyl ester (2,2-di methyl-3-)2,2-dim ethylvinyl(-cyclopropane- 1- carboxylic acid ethyl ester) (purity: 98%, cis/trans ratio: 38: 62) are stirred with 6 ml. of water and 0.01 g. of sodium dodecylsulfate and subsequently 4 g. of a 20% aqueous sodium hydroxide solution. By adding two 4 g. portions of the sodium hydroxide solution every 40 minutes, hydrolysis is performed at a temperature of 700C for altogether 240 minutes. After cooling the reaction mixture the unreacted ester is separated.

Weight: 8.3%, purity: 95%, cis/trans ratio: 78: 22.

The aqueous phase is admixed with 6.4 g. of a 35% aqueous hydrochloric acid solution and the precipitated chrysanthemic acid is isolated as described in Example 1. Yield: 9.6 g., cis/trans ratio: 10: 90.

Claims (17)

Claims

1. A process for the preparation of a cyclopropanecarboxylic acid of the formula (I)

by complete or selective hydrolysis of cis- and trans-esters of the formula (II)

(wherein R is Cie alkyl) in which the hydrolysis is carried out with an aqueous solution of an alkali metal hydroxide in the presence of a catalyst but in the absence of an organic solvent.

2. A process as claimed in claim 1 wherein the catalyst is a phase transfer catalyst or a cationic, anionic or non-ionic emulsifying agent.

3. A process as claimed in claim 1 or claim 2 wherein the catalyst is present in an amount of 0.01 to 5% by weight based on the weight of the ester.

4. A process as claimed in claim 3 wherein 0.1 to 1% of the catalyst is present.

5. A process as claimed in any one of the preceding claims wherein a 2 to 50% by weight aqueous solution of the alkali metal hydroxide is used.

6. A process as claimed in any one of the preceding claims wherein a 4 to 30% by weight solution of the alkali metal hydroxide is used.

7. A process as claimed in any one of the preceding claims wherein the alkali metal hydroxide is sodium or potassium hydroxide.

8. A process as claimed in any one of the preceding claims wherein the reaction temperature is 20 to 1 200C.

9. A process as claimed in any one of the preceding claims wherein the reaction temperature is 50 to 1000C.

10. A process as claimed in any one of the preceding claims for the complete hydrolysis of the ester, wherein the alkali metal hydroxide is used in an amount of 100 to 150% by weight of the equivalent relative to the ester.

11. A process as claimed in claim 10 wherein said amount is 102 to 130%.

1 2. A process as claimed in any one of claims 1 to 9 for the selective hydrolysis of the ester, wherein the alkali metal hydroxide is used in an amount of 70 to 110% by weight of the equivalent relative to the trans-ester.

13. A process as claimed in claim 12 wherein the said amount is 80 to 95%.

14. A process as claimed in any one of the preceding claims which includes the step of liberating the free acid of formula (I) from the initial reaction product.

1 5. A process as claimed in any one of the preceding claims wherein un-hydrolysed ester remaining after the reaction is recovered.

16. A process as claimed in claim 1, substantially as described herein in Example 1 or Example 2.

17. Pyrethroids when prepared from an acid of formula (I) when produced by a process as claimed in any one of the preceding claims.