DECARBOXYLATION PROCESS

Abstract: A process for the monodecarboxylation of a halogenated aromatic dicarboxylic acid or derivative thereof in which the halogenated aromatic dicarboxylic acid or derivative thereof in a liquid medium is heated characterised in that the liquid medium is a polar aprotic liquid medium.

Description: DECAREOXYIATiαST PROCESS This invention relates to a process for decar oxylation particularly for the monodecarboxylation of halogenated aromatic dicarboxylic acids and derivatives thereof. JP 0125,737 discloses a process for onodecarboxylating 3,4,5,6- tetrafluorophthalic acid in a tertiary amine at temperatures at 130°C. A decarboxylation process is disclosed in GB 2122190 in which both carboxylic acid groups in 2,3,5,6-tetrafluoroterephthalic acid or both ester groups in 2,3,5,6-tetrachloroterephthalic acid dimethyl ester are removed to form 1,2,4,5-tetrafluoro- or 1,2,4,5- tetrachlorobenzene respectively. The present invention seeks to provide a process for monodecarboxylation of certain halogenated aromatic dicarboxylic acids and derivatives thereof under mild process conditions and in high yields. According to the present invention there is provided a process for the monodecarboxylation of a halogenated aromatic dicarboxylic acid or derivative thereof in which the halogenated aromatic dicarboxylic acid or derivative thereof in a liquid medium is heated characterised in that the liquid medium is a polar aprotic liquid medium. This process has the advantage that it may be readily controlled such that little or no doubly decarboxylated product is formed and gives high yields of monodecarboxylated product. The process has the further advantage that no catalysts such as metal or metal salts are required to effect decarboxylation. The halogenated aromatic dicarboxylic acid is preferably a halogenated benzene dicarboxylic acid of the Formula 1: F o rmu l a 1 in which each X independently is halogen; and n is an integer from 1 to 4. Where X is halogen it is preferably -F or -CI. n is preferably 4. Preferred halogenated benzene dicarboxylic acids of Formula 1 are preferably those in which the -COOH groups are in the 1- and 3-, or the 1- and 4- positions more preferably in the 1- and 4- positions. Preferred 1,4- (COOH) 2 compounds of Formula 1 are those in which X is -F or -CI and n = 4, especially those in which X is -F and n = 4. An especially preferred compound of Formula 1 is 2,3,5,6- tetrafluoroterephthalic acid. The polar aprotic medium may be selected from amides such as dimethylformamide or dimethylacetamide; N-heterocyclics such as N- methylpyrollidinone; S-heterocyclics such as sulpholane; and ethers such as dimethoxyethane. Preferred polar aprotic media are amides, p.articularly dimethylformamide and dimethylacetamide. The process may be performed at a temperature from 40°C to 80°C, preferably at a teπperature from 50°C to 75°C and especially at 55°C to 75°C. The product may be isolated from the reaction mixture by conventional techniques such as cooling the reaction mixture, adding water and collecting precipitated product by filtration. The halogenated aromatic dicarboxylic acids may be conveniently prepared by the acid hydrolysis of the corresponding halogenated aromatic dinitriles for example 2,3,5,6- tetrafluoroterephthalic acid may be preppared by hydrolysis of 2,3,5,6- tetrafluoro-terophthalodinitrile in a mixture of sulphuric and glacial acetic acids at 175°C. The invention is illustrated by the following example: EXAMPLE 1 Preparation of 2,3,5,6-tetrafluorobenzoic acid N,N-Dimethylacetamide (14.0 parts) and 2,3,5,6- tetrafluoroterephthalic acid (2.91 parts) was heated at 70°C for 4 hours before cooling to 20°C adding water (7 parts) and cooling to 5°C. The precipitated solid was collected by filtration and washed with cold water and dried, the aqueous filtrates were evaporated under vacuum and the resultant solids combined to give 2,3,5,6-tetrafluorobenzoic acid (98%). 1,2,4,5-tetrafluorobenzene (2%) was also formed.

Claims: CLAIMS 1. A process for the monodecarboxylation of a halogenated aromatic dicarboxylic acid or derivative thereof in which the halogenated aromatic dicarboxylic acid or derivative thereof in a liquid medium is heated characterised in that the liquid medium is a polar aprotic liquid medium. 2. A process according to Claim 1 in which the halogenated aromatic dicarboxylic acid is a halogenated benzene dicarboxylic acid of Formula 1: F o r m u l a 1 in which each X independently is halogen; and n is an integer from 1 to 4. 3. A process according to Claim 2 in which X in the coipound of Formula 1 is -F or -CI and n is 4. 4. A process according to Claim 2 in which the -COOH groups are in the 1- and 4- positions in the compound of Formula 1. 5. A process according to Claim 2 in which the compound of Formula 1 is 2,3,5,6-tetrafluoroterephthalic acid.

Inventors: HOWARTH MICHAEL SCOTT, JAMIESON GORDON, KORN STEWART REID, HOWARTH, MICHAEL, SCOTT, JAMIESON, GORDON, KORN, STEWART, REID
Applicants: HOWARTH MICHAEL SCOTT, JAMIESON GORDON, KORN STEWART REID, ZENECA LTD, HOWARTH, MICHAEL, SCOTT, JAMIESON, GORDON, KORN, STEWART, REID, ZENECA LIMITED
Application year: 1995
Main classification: C07C63
IPC codes: C07C51, C07C63