GB2122190A - Decarboxylation of halogenated benzoic acids or esters - Google Patents

Abstract

Compounds of the formula (I): <IMAGE> in which n is 1 or 2, R is hydrogen or alkyl, preferably lower alkyl, and the several substituents represented by X are independently fluorine, chlorine or bromine, preferably fluorine or chlorine, are decarboxylated in a solvent, preferably a polar aprotic solvent, at a temperature above 20 DEG C, preferably from 100 to 200 DEG C. The process provides a new route to 1,2,4,5-tetrafluorobenzene which is useful as an intermediate in the synthesis of certain pesticides.

Description

SPECIFICATION Decarboxylation process This invention relates to a process for the decarboxylation of certain halogenated benzene carboxylic acids.

The decarboxylation of a halogenated benzoic acid of the formula:

in which Xis fluorine or chlorine, is described in the Journal of the Chemical Society (C) (1971) 1341.

Decarboxylation is achieved under stringent conditions by heating the acid with freshly calcined soda lime (twice the weight of the acid) at 3600C for 30 mintues.

Tetrafluoroterephthalic acid has a high thermal stability. It is reported to be stable in fuming sulphuric acid at 1 500C (Journal of the Chemical Society (1961)1574) and has been found stable in a mixture of concentrated sulphuric acid and glacial acetic acid at 1 750C.

By the present invention certain halogenated benzene carboxylic acids can be decarboxylated under moderate process conditions.

According to the present invention there is provided a process for the decarboxylation of a halogenated benzene carboxylic acid of the formula (I):

in which n is 1 or 2, R is hydrogen or alkyl, preferably lower alkyl having from 1 to 4 carbon atoms, and the several substituents represented by X are independently fluorine, chlorine or bromine, preferably fluorine or chlorine, the process comprising heating the acid of formula (I) in a solvent at a temperature above 200C until the desired degree of decarboxylation is achieved. When n is 2, the carboxyl groups are preferablyin the m- orp- positions to each other.

The process is of particular interest for the decarboxylation of a halogenated benzene dicarboxylic acid of the formula (ll):

in which R and X have the meanings given hereinbefore, and especially for the decarboxylation of 2,3,5,6-tetrafluoro or tetrachloro terephthalic acid or an ester thereof.

The solvent used in the process is preferably a polar aprotic solvent, examples of which are dimethylformamide, dimethylacetamide, dimethylsulphoxide and suipholane, but other solvents may also be used. The temperature at which the process is carried out is usefully from 100 to 2000 C.

It is an advantage of the present process that decarboxylation can be achieved at a temperature of 1 500C or less under atmospheric pressure. In particular, 2,3,5,6-tetrafluoro terephthalic acid is readily decarboxylated in dimethylformamide at 1 000C in about an hour to give 1 2,4,5-tetrafluorobenzene.

This latter compound is useful as an intermediate in the synthesis of certain pesticides. Hitherto the tetrafluorobenzene has been made by the reduction of hexafluoro benzene with inter alia lithium aluminium hydride or by the treatment of pentafluoromonochlorobenzene with hydrazine. The starting fluorobenzenes are difficult to synthesize and expensive. The present process provides a more convenient and cheaper route.

2,3,5,6-Tetrafluoroterephthalic acid may be obtained by the acid hydrolysis of 2,3,5,6tetrafluorophthalodinitrile which, in turn, may be obtained by the fluorination of the corresponding tetrachlorinated compound with potassium fluoride in a polar aprotic solvent.

The invention is illustrated by the following Examples 1 to 1 5, 1 8 and 1 9 in which percentages are by weight unless otherwise stated. Examples 16, 1 7 and 20 are included for comparative purposes only.

Example 1 Preparation of 2,3,5,6-tetrafluoroterephthalic acid 2,3,5,6-Tetrafluoroterephthalodinitrile (2.0 g) was mixed with glacial acetic acid (10 ml) and concentrated sulphuric acid (10 ml) and heated under reflux at 1 750C for 3 hours. The mixture was allowed to cooi, poured into water (150 ml) and extracted with ether (4x 150 ml). The extracts were dried over anhydrous magnesium sulphate, filtered and the solvent removed by rotary evaporation to leave a white solid (2.35 g). The solid decomposed at 248 to 2520C and mass spectrometry confirmed the molecular formula C8H2F404.

An infra red spectrum of the solid was identical to a standard spectrum of 2,3,5,6-tetrafluoro- terephthalic acid.

The yield of product was 98.7% theory.

The high thermal stability of the tetrafluoroterephthalic acid is apparent.

Preparation of 1.2,4.Btetraf 1,2,4,5-tetrafluorobenzene The 2,3,5,6-tetrafluoroterephthalic acid obtained above (0.5 g) was stirred with dimethylformamide (10 ml) at 1000C for 75 minutes. The mixture was examined periodically by gle (5ft column of 10% E301 at 550) and the reaction was found to have reached completion within 1 hour. The yield of 1 ,2,4,5-tetrafluorobenzene, as indicated by glc analysis, was 81%.

Glc-mass spectrometry confirmed that the product had the molecular formula C6F4H2.

On distilling the reaction mixture, at 20 mm Hg pressure, the first 2 ml fraction was found to contain tetrafluorobenzene in more concentrated form in dimethylformamide. This fraction was examined by 19F NMR spectroscopy and a single fluorine resonance observed. Isomers of 1,2,4,5-tetrafluorobenzene, viz.

would have shown 3 and 2 types of fluorine nuclei, respectively. Thus, the positions of the fluorine substituents had been unaltered on decarboxylation.

Similar results were obtained when dimethylacetamide was used instead of dimethylformamide as solvent for the decarboxylation.

Example 2 Potassium fluoride (14.5 g; 0.25 mol) was added to a stirred mixture of dimethylformamide (100 ml) and cetrimide (1.5 g) and dimethylformamide (40 ml) was distilled off at atmospheric pressure to remove any traces of moisture. Tetrachloroterephthalic acid dimethyl ester (16:6 g; 0.05 mol) was then added and the reaction mixture stirred for 5 hours at 1 500 C. After cooling to ambient temperature, methylene chloride (250 ml) was added and the mixture screened to remove any solids. The methylene chloride solution was washed with water (6x250 ml), dried over anhydrous sodium sulphate and evaporated. The residue (10.7 g) was distilled under reduced pressure to give 4.4 g of a white solid product on cooling.

This product was identified as tetrachlorobenzene by mass spectrometry, and by glc against an authentic sample of 1 ,2,4,5-tetrachlorobenzene.

Similar results were also obtained when the dimethyl formamide solvent was replaced, in turn, by dimethylacetamide, dimethylsulphoxide and sulpholane. Also a similar result was obtained when the potassium fluoride was omitted.

Examples 3-14 2,3,5,6-Tetrafluoroterephthalic acid (1 g) was heated in the solvent (20 ml) at the temperature and for the time indicated in Table 1. The yield of 1 ,2,4,5-tetrafluorobenzene was analysed by glc (5 ft column of E301, 5%; 50 to 2500 at 1 60/rriin).

Table 1 Yield Example Temperature Time C6F4H2 no. Solvent OC (oil bath) (hr) (mol %) 3 Dimethylacetamide 100 1 85 4 Quinoline 100 1 9 5 Quinoline 200 1 84 6 Nitrobenzene 100 1 4 7 Nitrobenzene 200 1 4 8 Quinoline+O.1gCuO 100 1 14 9 Quinoline+O.1gCuO 200 1 85 10 Ethylene glycol 100 1 2 11 Ethylene glycol 200 1 32 12 Ethylene glycol 200 3 62 13 Sulpholane 100 1 None 14 Sulpholane 200 1 95 Examples 15-17 Examples 1 6 and 1 7 are not illustrative of the invention but included for comparative purposes only.

The carboxylic acid (1 g) listed in Table 2 was heated in dimethylacetamide (20 ml) at 1 500C for 2 hours. The result is indicated in Table 2.

Table 2 Example no. Carboxylic acid Result 1 5 Pentafluorobenzoic acid afforded pentafluorobenzene* 16 Benzoic acid did not decarboxylate 17 Terephthalic acid did not decarboxylate *Glc mass spec. m/e 168, 149, 137, 118, 99.

Examples 18-20 Example 20 is not illustrative of the invention but included for comparative purposes only.

A sample of the carboxylic acid listed in Table 3 was heated in an oil bath held at a temperature of 2000 C. After 2 hours, the samples were analysed by glc with the results indicated in Table 3.

Table 3 Example no. Carboxylic acid Result 18 2,4,5,6-tetrachloro- afforded 1 ,2,3,5-tetra- isophthalic acid chlorobenzene 19 3,4,5,6-tetrafluoro- gave 0.5% yield of phthalic acid 1 ,2,3,4-tetrafluorobenzene* 20 2,5-dimethyl-3,6-dichloro- did not decarboxylate terephthalic acid *There was formed a compound having a glc mass spec. pattern of m/e 221/220, 202, 177, 149, which is consistent with the structure:

Claims (9)

Claims

1. A process for the decarboxylation of a halogenated benzene carboxylic acid of the formula (I):

in which n is 1 or 2, R is hydrogen or alkyl, and the several substituents represented by X are independently fluorine, chlorine or bromine, the process comprising heating the acid of formula (I) in a solvent at a temperature above 200C until the desired degree of decarboxylation is achieved.

2. A process according to claim 1 in which the solvent is a polar aprotic solvent.

3. A process according to -claim 1 in which the solvent is selected from the group comprising dimethylformamide, dimethylacetamide, sulpholane, quinoline and ethylene glycol.

4. A process according to any one of the preceding claims in which the temperature is from 100 to 2000 C.

5. A process according to any one of the preceding claims in which X is fluorine or chlorine.

6. A process according to any one of the preceding claims in which when n is 2, the carboxyl groups are in the m- or p- positions to each other.

7. A process according to any one of the preceding claims in which the halogenated benzene carboxylic acid is 2,3,5,6-tetrafluoro- or tetrachloro-terephthalic acid or an ester thereof.

8. A process for the decarboxylation of a compound of the formula (ill):

in which X is fluorine or chlorine and R is hydrogen or alkyl, which comprises heating the compound of formula (I) in a polar aprotic solvent at a temperature of from 100 to 2000C.

9. A process substantially as described with reference to any one of Examples 1 to 1 5, 1 8 and 1 9.