GB2104518A - Stereospecific decarboxylation of dihalovinyl cyclopropane carboxylic acids - Google Patents

Abstract

A process for the stereospecific preparation of a compound of the general formula <IMAGE> in which each Hal independently represents a fluorine, chlorine or bromine atom, comprises stereospecific decarboxylation of a compound of the general formula <IMAGE> or a salt thereof, in which each Hal has the meaning given above, without addition of copper salts and in the presence of water.

Description

SPECIFICATION Stereospecificdecarboxylation of dihalovinyl cyclopropane carboxylic acids This invention relates to the stereospecific decarboxylation of dihalovinyl cyclopropane carboxylic acids.

Synthetic pyrethroid insecticides are esters which consist of an acid portion and an alcohol portion. In one group of pyrethroids, the acid portion is derived from a 2,2-dihalovinylcyclopropane carboxylic acid.

Such an acid exists in the form of geometric isomers, in which the 2,2-dihalovinyl and the corboxyl groups may be cis or trans to each other. Synthetic pyrethroids in which the acid portion is in the cis form often have greater insecticidal activity than the corresponding trans compounds, and a great deal of research has been directed towards the preparation of the distinct geometric isomers of 2,2-dihalovinyl cyclopropane carboxylic acids.

US Patent Specification No. 4,228,299 and UK Patent Specification No. 1,580,203 disclose that 1 cyano - 2 - (2,2 - dihalovinyl) - 3,3 - dimethylcyclopropanes can be prepared by decarboxylation of the corresponding 1 - cyano - 1 - carboxylic acid or a salt thereof by heating in a polar aprotic solvent. The resulting cyano compound can of course be converted into the corresponding acid or an ester thereof by hydrolysis or alcholysis.

This process proceeds in high chemical yield, and is perfectly satisfactory for the preparation of compounds in which the porportion of the two possible geometric isomers obtained is not of crucial importance. However, it is often desirable to carry out the reaction with retention of steric configuration, expecially when using a starting material containing a major proportion of one geometric isomer e.g. as shown below for one of the two possible isomers:

Unfortunately, in practice the decarboxylation always proceeds with some degree of inversion of sterochemistry, and so for the case illustrated above the other isomer is also formed::

Thus the use of a starting material containing a major proportion of a desired steric configuration usually results in a product containing a considerably lower proportion of the corresponding decarboxylated compound in that stearic configuration.

For example, when carrying out the preferred embodiment of the process of US 4,228,299 (i.e.

decarboxylating in the presence of a copper salt and water), using a starting material predominently in a specific steric configuration, it is found that partial racemisation occurs resulting in a product which contains a significantly lower proportion of that specific configuration.

Most surprisingly,it has now been found that the retention of steric configuration in the decarboxylation process is much improved by carrying out the reaction in the presence of water but in the absence of a copper salt.

The invention therefore provides a process for the preparation of a nitrile of the general formula

in which each Hal independently represents a fluorine, chlorine or bromine atom, which comprises decarboxylating a carboxylic acid of the general formula

or a salt thereof, in which Hal has the meaning given above, characterised in that the steric configuration of the carboxylic acid II is substantially retained in the nitrile I by effecting the decarboxylation without addition of copper salts and in the presence of water.

Preferably, each Hal represents the same halogen atom, especially a chlorine atom.

If the starting material of the general formula 11 is used in the form of a salt, it may for example be an alkali metal salt or an optionally alkyl-substituted ammonium salt.

The process of the invention is of particular value in the decarboxylation of carboxylic acids containing predominentlythetrans configuration, namely:

into the corresponding stereoisomer of the nitrile, which is designated the cis isomer:

It should be noted that, although the nomenclature changes from trans to cis, the actual steric relationship of all the substituent groups remains the same.

this apparent inconsistency derives from the application ofthe IUPAC nomenclature rules, which provide that geometric isomers should be designated as cis or trans by reference to the telative positions of the largest substituents at the relevant locations. Thus, in the acid of formula Ill the largest substituents are the-CH=CHal2 and the -COOH groups, which are in a trans relationship in the isomer illustrated. However, decarboxylation removes the -COOH substituent and leaves -CN as the substituent whose relationship to -CH =CHal2 determines the nomenc nature.

The relative proportions ofthe geometric isomers of the compound I in which the -CN group iscis or trans to the dihalovinyl group, depends on the precise reaction conditions and, of course, on the proportion of the corresponding isomer in the carboxylic acid of formula II in which the -COOH and dihalovinyl groups are, respectively, trans orcis to each other. Generally, there will be some decrease in the proportion of the major isomer throughout the process of the present invention, this decrease however being much less severe than in the prior art processes. Preferably, the carboxylic acid II contains at least 70%, and preferably at least 80%, of the desired isomer.Especially preferred is the use of a carboxylic acid II containing a major proportion of the isomer in which the-COOH and dihalovinyl groups are trans to each other, i.e. the isomer of formula Ill above.

The process according to the invention is preferably carried out in the presence of an additional polar organic solvent. Suitable solvents include amides, for example dimethylformamide, dimethylacetamide, N-methylpyrrolidone and hexamethylphosphortriamide; sulphur-containing compounds, for example dimethylsulphoxide and sulpholane; amides, for example dimethylaniline, pyridine or picoline; and nitriles, for example acetonitrile. Amides are especially useful solvents.

The quantity of water present in the in the system is not very critical, though the use of higher proportions of water can lead to the formation of larger amounts of by-products. It is therefore preferable to use a molar ratio of water to the compound of the general formula II in the range of from 0.5:1 to 15:1, especially 1:1to 10:1.

The temperature of the reaction may for example be in the range of from 100 to 200"C, especially 120 to 1 600C, and when using an organic solvent is conveniently at the reflux temperature of the reaction mixture. In some cases, particularly when using relatively large quantities of water, the boiling point of the reaction mixture at atmospheric pressure may be less than the desired reaction temperature. In this case, the reaction is advantageously carried out under pressure, for example a pressure of up to about 16 bar.

The process according to the invention is preferably carried out in the presence of a base. Suitable bases are weak organic or inorganic bases, for example salts of carboxylic acids, especially alkanoic acids, such as sodium acetate; ammonia or amines such as triethylamine; alkali metal fluorides, for example potassium fluoride; and carbonates and bicarbonates, such as sodium carbonate. The amount of base added is not critical, but the number of equivalents of base per mole of the compound of the general formula II is preferably in the range of from 0.5 to 10, expecially 1 to 5. It may be desirable to carry out the reaction in the presence of a buffer, since very highly basic conditions may lead to some by-product formation by dehydrohalogenation of the dihalovinyl group, giving the corresponding acetylene group, -C=-CHal.

The water may be added to the reaction mixture as such, or it may be generated in situ, for example by the reaction of an acid with a base. For example, as discussed above, the reaction may be carried out in the presence of a salt of a carboxylic acid. This salt may be generated, along with water, by the reaction of a carboxylic acid with a base. Thus for example, the example, the addition to the reaction mixture of acetic acid plus sodium hydroxide generates the essential water and the preferred base, sodium acetate.

The carboxylic acid of the general formula II may be added to the reaction mixture as such, or it may be generated in situ, suitably by the dehydrohalogenation in the presence of a base of a compound of the general formula

or a salt thereof in which each Hal independently represents a fluorine, chlorine or bromine atom.

Suitable bases include those described above as being useful inthe decarboxylation process according to the invention, and also strong bases such as alkali metal hydroxides or alkoxides, for example sodium hydroxide. In a preferred embodimentofthe process according to the invention, the carboxylic acid of the general formula II is generated in situ by dehydrohalogenation of a compound ofthe general formula IV in the presence of a weak base, the number of equivalents of base per mole of the compound ofthe general formula IV being in the range of from 1.5 to 11, especially 2 to 6. In this way, the subsequent decarboxylation occurs in the presence of the preferred quantities of weak base as discussed above.

The nitrile compound of the general formula I prepared by the process according to the invention may be converted into the corresponding acid or a salt, ester or amide thereof by known methods of hydrolysis or alcoholysis. Depending upon the precise reaction conditions employed in the process according to the invention, some or all of the resulting compound of formula I may be hydrolysed to the corresponding amide or acid of salt thereofin situ, especially in the presence of relatively large concentrations of water. The production of such hydrolysis products in situ should be understood to be within the scope of the present invention, and may in some cases be a preferred embodiment of the process according to the invention.Generally, however, maximum yields are obtained by conducting the process according to the invention under conditions such that hydrolysis does not occur to an appreciable extent, and then if desired hydrolysing the resulting product after a suitable work-up procedure under conditions optimum for the hydrolysis.

The following Examples illustrate the invention. In the Examples, the following abbreviations are used.

CompoundA: 1 - cyano - 2,2 - dimethyl - 3 - (2,2,2 trichloroethyl)cyclopropane carboxylic acid, trans isomer; i.e. the CO2H group trans to the -CH2CCI3 group.

Compound B: 1 - cyano - 2,2 - dimethyl - 3 - (2,2 dichlorovinyl)cyclopropane carboxylic acid, trans isomer; i.e. the CO2H group trans to the -CH =CCl2 group.

Compound C: 1 - cyano - 2,2 - dimethyl - 3 - (2,2 dichlorovinyl)cyclopropane cis isomer; i.e. the CN group cis to the-CH=CCI2 group.

Example 1 A 1-litre glass reactor equipped with a reflux condenser, was charged with sodium acetate (90.29, 1.1 mol), acetic acid (6.0g, 0.1 mol), dimethylformamide (41 5g), water (24.0g, 1.33 mol) and compound A (90.1g, 0.33 mol) having a trans:cis ratio of 87:13.

The stirred mixture was heated to refluxtemperature (136"C) for 18 hours, after which time the reaction was shown by gas-liquid chromatography to be complete. The mixture was then cooled to 250C and 77.79 of 36%w aqueous hydrochloric acid were introduced. The resulting precipitate of sodium chloride was filtered off and washed with dimethyl formamide. The combined filtrates were subjected to a flash distillation under reduced pressure to remove volatiles. The solution remaining was treated with 1 00g dichloroethane, the organic phase was washed twice with sodium carbonate solution and once with water, and then flash distilled under reduced pressure to isolate the desired product.

Compound C was obtained (0.28 mol, corresponding to a yield of 85%) with a cis:trans ratio of 80:20.

Examples 2 to 4 The general procedure described in Example 1 was repeated exceptthatthe quantityofwater added was varied. This resuited in a variation in the reflux temperature, and in the time taken to complete the reaction. These parameters and the results of the experiments are given in Table I.

TABLEI

Example H2O added Reflux- Reaction Compound Cproduced No. (moles per temperature, time, cis:trans yield, mole compound A) C hours ratio 2 1.0 148 6 75:25 80 3 2.0 145 8.5 78:22 80 4 6.0 133 30 82:18 80 Example 5 The procedure of Example 1 was repeated except that no dimethylformamide was added and the reaction was carried out using water (25 moles) as solvent. The reaction was carried out under a pressure of 4 bar at a reflux temperature of 140"C for 100 hours.At the end of this time, the cis:trans ratio of compound C obtained was 84:16. However, in addition to compound C, a considerable amount of other products had also been obtained, the yield of compound C being about 40%.

Example 6 The procedure of Example 1 was repeated except that compound B was used instead of compound A (trans:cis ratio 87:13) and no sodium acetate or acetic acid was added. The cis:trans ratio of compound C was 74:26.

Example 7 Following the general procedure of Example 6, compound B was decarboxylated in the presence of varying amounts of water and at different reaction (reflux) temperatures. The results of these experiments are set out in Table 2 below.

TABLE2

Water added Reaction Reaction Compound C Produced (moles/mole Temp "C Time (hrs) ofcomp. B cis:trans ration % Yield 0 150-155 6 65:35 78 0 135 < 28 70:30 77 1.0 140 6 75:25 80 2.0 145 8.5 78:22 80 4.0 136 18 80:20 81 6.0 133 30 82:18 80 20.0 120 260 84:16 57.5 Comparative Examples Comparison A The procedure of Example 1 was followed except that no water was added. Reflux temperature was 150-1 55"C, and the reaction time was 6 hours. The yield of compound C was 78%, and the cis:trans ratio was 65:35.

Comparison B The procedure of A was followed except that the reaction mixture was heated only to 135"C-i.e. less than reflux temperature. The yield of compound C was 77%, and the cis:trans ratio was 70:30.

Comparison C The procedure of Example 1 was followed except that 0.033 mol of CuSO4.5 H20 were also added. The reaction proceeded very rapidly, being complete in about 3 hours, but the cis:trans ratio of the compound C obtained was 40:60.

Comparison D The procedure of Example 6 was followed except that 0.033 mol of CuSO4.5H20 was also added. After a rapid reaction, the cis:trans ratio of compound C was 53:47.

Claims (11)

1. A process for the preparation of a nitrile of the general formula

in which each Hal independently represents a fluorine, chlorine or bromine atom, which comprises decarboxylating a carboxylic acid of the general formula

or a salt thereof, in which each Hal has the meaning given above, characterised in that the steric configuration of the carboxylic acid Il is substantially retained in the nitrile I by effecting the decarboxylation without addition of copper salts and in the presence of water.

2. Process as claimed in claim 1 wherein the carboxylic acid Il is predominently in the trans config duration

3. Process as claimed in claim 2 wherein the carboxylic acid II contains at least 70% of the defined trans isomer.

4. Process as claimed in claim 1,2 or3 wherein each Hal represents a chlorine atom.

5. Process as claimed in any one of the preceding claims wherein the decarboxylation is effected by heating in the presence of a polar organic solvent.

6. A process as claimed in claim 5, in which the polar organic solvent is an amide.

7. A process as claimed in any one of the preceding claims in which the molar ratio of water to the carboxylic acid II or salt thereof is in the range of from 1:1 to 10:1.

8. A process as claimed in any one ofthe preceding claims, wherein the decarboxylation is effected by heating at a temperature in the range offrom 100 to 200"C.

9. A process as claimed in any one of the preceding claims, carried out in the presence of a base.

10. A process as claimed in any one of the pre- ceding claims, in which the starting material is generated in situ by the dehydrohalogenation in the pre senceofa base of a compound of the general formula

or a salt thereof, in which Hal is as defined in claim 1.

11. A compound ofthe general formula I given in claim 1, whenever prepared by a process as claimed in any one of claims 1 to 10.