FIELD OF THE INVENTION
The present invention relates to a process for preparation of 2,3,6-trihalophenylacetic acid of Formula I and intermediates thereof. These compounds are used in wide variety of applications.

Formula I
wherein X independently represents a chloro or fluoro group.

BACKGROUND OF THE INVENTION
2,3,6-Trihalophenylacetic acid, especially 2,3,6-trifluorophenylacetic acid and intermediates thereof are valuable intermediates in synthesis of active agrochemical and pharmaceutical ingredients.
U.S. Patent No. 4,874,764 discloses a process for preparation of 2,3,6-trifluorophenylacetic acid by reacting ethyl 2-(2,3,6-trifluorophenyl)acetate with sodium hydroxide in presence of ethanol at 70?.
U.S. Patent No. 5047553 provides a process for preparation of tetrafluorophthalic acid involving a step of fluorination of tetrachloro-N-phenylphthalimide using potassium fluoride in presence of tributylhexadecylphosphonium bromide in sulfolane solvent to form tetrafluoro-N-phenylphthalimide, which upon hydrolysis in presence of an acid gives tetrafluorophthalic acid. However, the process is very time consuming and involves multiple purification steps resulting in low yield of tetrafluorophthalic acid.
Chinese Patent Application No. 110437056 provides a process of decarboxylating 2,3,4,5-tetrafluorophthalic in presence of cupric sulfate at high temperature to give 2,3,4,5-tetrafluorobenzoic acid. However, the process is carried out at high temperature.
The present invention aims to provide an alternate process for preparation of 2,3,6-trihalophenylacetic acid and intermediates thereof.

OBJECT OF THE INVENTION
The main object of present invention is to provide an economical and industrially applicable process for preparation of 2,3,6-trihalophenylacetic acid of Formula I and intermediates thereof.

SUMMARY OF THE INVENTION
In first aspect, the present invention provides a process for preparation of a compound of Formula I,

Formula I
wherein X independently represents a chloro or fluoro group,
comprising the steps of:
a) alkylating 2,3,4,5-tetrahalobenzoic acid of Formula V,

Formula V
to obtain an alkyl ester of 2,3,4,5-tetrahaobenzoic acid of Formula IV;

Formula IV
wherein X independently represents a chloro or fluoro,
and R represents an alkyl group;
b) reacting the alkyl ester of 2,3,4,5-tetrahalobenzoic acid with a nucleophile of Formula VII,

Formula VII
wherein R1 and R2 independently represent H, CN, CONH2, COOH, COOR and R represents C1-C4 alkyl, wherein both R1 and R2 cannot be H,
in presence of a base to obtain a compound of Formula III;

Formula III
wherein X, R, R1 and R2 are as defined above,
c) hydrolyzing the compound of Formula III to obtain a compound of Formula II, and

Formula II
wherein X, R1 and R2 are as defined above
d) de-carboxylating the compound of Formula II to obtain the compound of Formula I.
In second aspect, the present invention provides a process for preparation of 2,3,4,5-tetrafluorobenzoic acid, comprising the steps of:
a) fluorinating N-phenyl-3,4,5,6-tetrachlorophthalimide with a fluorinating agent in presence of a phase transfer catalyst to give N-phenyl-3,4,5,6-tetrafluorophthalimide;
b) hydrolysing N-phenyl-3,4,5,6-tetrafluorophthalimide using an acid to obtain 3,4,5,6-phthalic acid; and
c) decarboxylating 3,4,5,6-phthalic acid to obtain 2,3,4,5-tetrafluorobenzoic acid.
In third aspect, the present invention provides a process for preparation of 2,3,4,5-tetrahalobenzoic acid of Formula V,

Formula V
wherein X independently represents a chloro or fluoro,
comprising the steps of:
a) de-halogenating 2,3,4,5,6-pentahalobenzoic acid of Formula VI,

Formula VI
wherein X independently represents a chloro or fluoro,
with zinc in the presence of a catalyst to obtain a compound of Formula V;
In a fourth, the present invention provides a compound of Formula II,

Formula II
wherein X independently represents a chloro or fluoro,
In a fifth aspect, the present invention provides a compound of Formula III,

Formula III
wherein X independently represents a chloro or fluoro and R represents an alkyl group; R1 and R2 independently represent H, CN, CONH2, COOH, COOR, wherein both R1 and R2 cannot be H.
DETAILED DESCRIPTION OF THE INVENTION
A few examples of some of the compounds of Formula III are as provided below:

As used herein, “alkyl” may be selected from a group comprising methyl, ethyl, propyl and isopropyl or the like.
In an embodiment, the step of alkylating the compound of V to obtain a compound of Formula IV is carried out in presence of a base and an alkylating agent in a solvent. The alkylating agent is selected from dimethyl sulphate, dimethyl carbonate and methyl iodide, or generated in-situ by reacting methanol with sulphuric acid.
As used herein, “nucleophile of Formula VII” may be selected from a group comprising diethyl malonate, ethylmethyl malonate, dimethylmalonate, methylcyanoacetate, ethylcyanoacetate, propanedinitrile, ethyl acetate, methyl acetate, acetonitrile or the like.
In another embodiment, the step of reacting a compound of Formula IV with a nucleophile is carried out at a temperature range of 40 to 70?.
In another embodiment, the molar ratio of the compound of Formula IV with respect to a nucleophile is in the range of 0.9- 1.0.
In another embodiment, the step of alkylation and step of reacting a compound of Formula IV with a nucleophile are performed in-situ, without isolation of the compounds of Formula IV.
In another embodiment, the ‘base’ used in step of alkylation and step of reacting a compound of Formula IV with a nucleophile is selected from a group consisting of a metal hydride, metal borohydride, and alkali metal carbonate or the like. The example of the base includes sodium hydride, potassium hydride, sodium butoxide, potassium butoxide, lithium (hexamethylsilyl)amide, potassium (hexamethylsilyl)amide, butyl lithium, sodium metal, potassium metal, sodium ethoxide and sodium methoxide, alkali carbonate such as sodium carbonate, potassium carbonate, cesium carbonate and magnesium carbonate or the like.
The ‘solvent’ used in the step of alkylation and step of reacting a compound of Formula IV with a nucleophile is selected from a group consisting of a polar aprotic solvent selected from a group consisting of acetone, ethylacetate, dimethylsulfoxide (DMSO), dimethyl sulfide (DMS), N,N-dimethylformamide (DMF), acetonitrile, N-methylpyrrolidone (NMP), tetrahydrofuran, hexamethylphosphoric triamide (HMPT), dichloroethane N,N-dimethylacetamide, sulfolane, methyl tetrahydrofuran, 1,4-dioxane and1,3-dimethyl-2-imidazolidinone or a mixture thereof.
In an embodiment, the hydrolysis of the compound of Formula III or N-phenyl-3,4,5,6-tetrafluorophthalimide is carried out using an acid selected from sulphuric acid, hydrochloric acid, hydrobromic acid, acetic acid, trifluoroacetic acid and p-toluenesulphonic acid or the like.
In another embodiment, the step of hydrolysis of Formula III or N-phenyl-3,4,5,6-tetrafluorophthalimide is carried out at a temperature of 80-160ºC and preferably at a temperature of 100-150ºC.
In another embodiment, the hydrolysis of a compound of Formula III or N-phenyl-3,4,5,6-tetrafluorophthalimide is carried out at a temperature range of 120 to 140? using 5 to 10 equivalents of an acid.
In another embodiment, the hydrolysis of the compound of Formula III is further accompanied by decarboxylation of the hydrolysed intermediate to obtain compound of formula II.
The metal salt for de-carboxylating the compound of Formula II is selected from metal oxide, metal hydroxide or metal carbonate. The metal salt includes copper oxide, silver oxide and calcium oxide or calcium hydroxide or the like. The de-carboxylation is preferably carried out in presence of cupric oxide.
In an embodiment, the step of de-carboxylating the compound of Formula II or 3,4,5,6-tetrafluorophthalic acid is carried out in an organic solvent.
The ‘organic solvent’ used in the step of de-carboxylation of the compound of Formula II or 3,4,5,6-tetrafluorophthalic acid is selected from a group consisting of a polar aprotic organic solvent such as, sulpholane, N-methyl-2pyrrolidone, benzonitrile, N-vinylpyrrolidone, hexamethylphosphoramide, dimethylsulfoxide, N,N-dimethyl formamide and N,N-dimethyl acetamide or a mixture thereof. N-methyl-2-pyrrolidone is preferred as a solvent for the step of decarboxylation.
In another embodiment the step of de-carboxylation of compound of Formula II or 3,4,5,6-tetrafluorophthalic acid involves recycling and reusing the solvent.
In an embodiment, the step of de-carboxylation of a compound of Formula II is carried out at a temperature range of 120 to 140°C.
In an embodiment, the mole equivalent of metal oxide used for the step of de-carboxylation of a compound of Formula II is 0.05-0.5 molar equivalent.
In an embodiment, the present invention provides a process for preparation of 2,3,6-trifluorophenylacetic acid, comprising the steps of:
a) de-halogenating pentafluorobenzoic acid using zinc powder in presence of nickel chloride complex in water and an organic solvent to obtain 2,3,4,5tetrafluorobenzoic acid;
b) reacting 2,3,4,5-tetrafluorobenzoic acid with an alkylating agent followed by reaction with a nucleophile in presence of a base;
c) hydrolysing the reaction mixture of step b) using sulphuric acid; and
d) de-carboxylating the reaction mixture of step c) using a metal oxide or metal hydroxide to obtain 2,3,6-trifluorophenylacetic acid.
In another embodiment, the compounds of Formula II, Formula III, and Formula IV are isolated and are used in the next step.
In another embodiment, the compounds of Formula II, Formula III, and Formula IV can be purified by crystalizing in a solvent selected from hexane, heptane, diethylether, and acetone or the mixture thereof.
In another embodiment, the compounds of Formula II, Formula III, and Formula IV are not isolated and are used in situ for the next step.
The compound of Formula III of present invention is isolated in a purity of greater than 95%. The compound of Formula II of present invention is isolated in a purity of greater than 90%.
In an embodiment, the present invention provides a process for preparation of compound of Formula I, having selectivity greater than 75%, having yield greater than 80% and purity greater than 95%.
The 2,3,6-trifluorophenylacetic acid of present invention is isolated in a purity of 95% to 99.8%.
The isolation of compound of Formula I of the present invention may be carried out by any method selected from evaporation, distillation, column chromatography, and filtration or the like.
As used herein, “fluorinating agent” refers to a compound selected from sodium fluoride, potassium fluoride, caesium fluoride or the mixture thereof.
In an embodiment, the fluorination of N-phenyl-3,4,5,6-tetrachlorophthalimide is carried out using potassium fluoride in presence of phase transfer catalyst and a solvent.
In another embodiment, the fluorination of N-phenyl-3,4,5,6-tetrachlorophthalimide is carried out at 100-150ºC.
In another embodiment, the fluorination of N-phenyl-3,4,5,6-tetrachlorophthalimide is carried out in a solvent selected from a group consisting of dimethylformamide, sulfolane, dimethylsulfoxide, N-methylpyrrolidone, acetonitrile, dimethylsulfone, dimethyl formamide or dimethyl acetamide or the like.
In another embodiment, the fluorination of N-phenyl-3,4,5,6-tetrachlorophthalimide is carried out in presence of a phase transfer catalyst, a quaternary ammonium halide selected from a group consisting of tetrabutylammonium bromide, tetrabutylammonium chloride, benzyltrimethylammonium fluoride, dodecyldimethyl ammonium chloride, tetraethylammonium bromide or like. The present invention does not use phosphonium salt as phase transfer catalyst for fluorination process as they are very slow and require long reaction hours.
In another embodiment of the present invention, the step of fluorination is carried out using potassium fluoride in a range of 1: 8 equivalents with respect to N-phenyl-3,4,5,6-tetrachlorophthalimide.
In another embodiment of the present invention, the step of fluorination is carried out at a temperature of 80-150ºC and preferably at a temperature of 90-140ºC.
The yield of fluorination process to obtain N-phenyl-3,4,5,6-tetrafluorophthalimide is greater than 80 % and preferably greater than 90% and having purity greater than 99%.
In an embodiment, present invention provides a process for hydrolysis of N-phenyl-3,4,5,6-tetrafluorophthalimide using an acid to obtain 3,4,5,6-tetrafluorophthalic acid.
The concentration of acid may vary from 30-90% and preferably between 50-85%.
In an embodiment, present invention provides a process for hydrolysis of N-phenyl-3,4,5,6-tetrafluorophthalimide using 50-80% sulfuric acid to obtain 3,4,5,6-tetrafluorophthalic acid.
The hydrolysis of N-phenyl-3,4,5,6-tetrafluorophthalimide using 80% sulfuric acid gives 3,4,5,6-tetrafluorophthalic acid, having purity greater than 95% and yield greater than 90%.
In another embodiment, the step of decarboxylation of 3,4,5,6-tetrafluorophthalic acid is carried out in a mixture of solvent and water.
In another embodiment, the step of decarboxylation of 3,4,5,6-tetrafluorophthalic acid is carried out in a mixture of sulfolane and water.
In another embodiment, the step of decarboxylation of 3,4,5,6-tetrafluorophthalic acid is carried out in a mixture of sulfolane and water in a ratio of 70-90:10-30.
In another embodiment, the step of decarboxylation of 3,4,5,6-tetrafluorophthalic acid is carried out in a batch process.
In another embodiment, the step of decarboxylation of 3,4,5,6-tetrafluorophthalic acid is carried out in a pinched coil reactor.
In a preferred embodiment, present invention provides a process for decarboxylation of 3,4,5,6-phthalic acid in a mixture of sulfolane and water in absence of a metal catalyst to obtain 2,3,4,5-tetrafluorobenzoic acid.
In another preferred embodiment, the present invention provides a process for decarboxylation of 3,4,5,6-tetrafluorophthalic acid in a solvent in absence of a metal catalyst to give 2,3,4,5-tetrafluorobenzoic acid. The absence of metal catalyst in the process reduces the problems of effluent treatment and disposal.
In preferred embodiment, present invention provides a process for decarboxylation of 3,4,5,6-phthalic acid in a mixture of sulfolane and water in absence of metal catalyst to obtain 2,3,4,5-tetrafluorobenzoic acid having a purity of greater than 99%.
In an embodiment of the present invention, N-phenyl-3,4,5,6-tetrafluorophthalimide, 3,4,5,6-tetrafluorophthalic acid, and 2,3,4,5-tetrafluorobenzoic acid were recrystalized in a solvent selected from dichloromethane, N,N-dimethylformamide, tetrahydrofuran, hexane, heptane or the mixture thereof to obtain pure product, having purity greater than 99%.
In a particular embodiment, the present invention provides a process for preparation of 2,3,4,5-tetrafluorobenzoic acid, comprising the steps of:
a) fluorinating N-phenyl-3,4,5,6-tetrachlorophthalimide with potassium fluoride in sulfolane and in presence of tetrabutylammonium bromide to N-phenyl-3,4,5,6-tetrafluorophthalimide at 120ºC-140ºC;
b) hydrolysing N-phenyl-3,4,5,6-tetrafluorophthalimide using 50-80% sulfuric acid at 90 ºC -140ºC to obtain 3,4,5,6-phthalic acid; and
c) decarboxylating 3,4,5,6-phthalic acid in a mixture of sulfolane and water in absence of metal catalyst to obtain 2,3,4,5-tetrafluorobenzoic acid.
In an embodiment, the compound of formula V includes 2,3,4,5-tetrafluorobenzoic acid.
The present invention provides a process for obtaining 2,3,4,5-tetrafluorobenzoic acid, having purity greater than 90% and preferably 95%.
In an embodiment of the present invention, the step of de-halogenation is carried out using zinc in presence of a catalyst in a solvent. The catalyst comprises a metal complex selected from a group consisting of a nickel chloride hexahydrate with a ligand selected from 2,2´-bipyridine and 1,10-phenanthroline and derivatives thereof.
The solvent for de-halogenation is selected from a group consisting of water and an organic solvent. The examples of organic solvent include ethyl acetate, methyl acetate, butyl acetate, ethyl pentonoate, propyl acetate, pentyl acetate, methyl butyrate, isobutyl acetate, methyl benzoate, ethyl benzoate, methyl tetrahydrofuran, toluene and benzonitrile or a mixture thereof. The organic solvent also includes polar aprotic solvents selected from a group consisting of dimethylsulfoxide, N,N-dimethyl formamide, N-methylpyrrolidone, N,N-dimethyl acetamide, N-vinylpyrrolidone, hexamethyl phosphoramide, tetrahydrofuran, acetonitrile and sulfolane or a mixture thereof.
In another embodiment of the present invention, the solvent used is recovered, recycled and reused.
In another embodiment, the step of de-halogenation is carried out in presence of nickel salt and the ligand in the ratio of 1 to 3.
In another embodiment, the mole equivalents of zinc used in the step of dehalogenation is in the range of 2 to 3 molar equivalents.
In another embodiment, the step of de-halogenation is carried out at a temperature range of 30 to 60?.
In an embodiment, the present invention provides a process of preparation of a compound of Formula IV by de-halogenating pentahalobenzoic acid.
In another embodiment, the present invention provides a process for preparation of 2,3,4,5-tetrafluorobenzoic acid, comprising a step of de-halogenating pentafluorobenzoic acid using zinc powder in presence of nickel chloride complex in water and an organic solvent.
The compound as a starting material pentahalobenzoic acid, N-phenyl-3,4,5,6-tetrachlorophthalimide and reagents used in the present invention is commercially available and can be easily obtained.
Unless stated to the contrary, any of the words “comprising”, “comprises” mean 20 “including without limitation” and shall not be construed to limit any general statement that it follows to the specific or similar items or matters immediately following it. Embodiments of the invention are not mutually exclusive, but may be implemented in various combinations. The described embodiments of the invention and the disclosed examples are given for the purpose of illustration rather than limitation of the invention as set forth in the appended claims.
The completion of the reaction can be monitored by any one of chromatographic techniques such as thin layer chromatography (TLC), high pressure liquid chromatography (HPLC), ultra-pressure liquid chromatography (UPLC), Gas chromatography (GC) and alike.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLES
Example 1: Preparation of 2,3,4,5-tetrafluorobenzoic acid.
Nickel chloride hexahydrate (0.2 M), 2,2´-bipyridine (0.2 M), N,N-dimethylformamide (200 ml) and water (100 ml) were added to a reactor equipped with a condenser and stirred at 60-70°C for 2-3 hours. A solution of pentafluorobenzoic acid (200 g) in N,N-dimethylformamide (200 ml) was added to the reaction mixture in 30 minutes. Zinc dust (2 M) was added lot-wise to the reaction mixture and the reaction temperature was maintained at 60-70°C for 12-15 hours. The progress of the reaction was monitored by HPLC. After completion of the reaction, pH of the reaction mass was adjusted using hydrochloric acid (6 M) at 10-20°C, the reaction mass was extracted using ethyl acetate (300 ml X 2) and washed with brine solution to obtain solid residue. The residue was crystallised using heptane at 60-70°C to obtain 2,3,4,5-tetrafluorobenzoic acid (yield: 80% and purity: 90%).
Example 2: Preparation of diethyl [2,3,6-trifluoro-4-(methoxycarbonyl)phenyl] propanedioate.
2,3,4,5-Tetrafluorobenzoic acid (1.0 M), dimethylsulfoxide (600 ml) and potassium carbonate (1.15 M) were added to a reactor equipped with condenser and gradually dimethyl sulphate (1.1 M) was added to it and heated for 2-3 hours at 60-70°C. Cooled the mixture to 40-50°C and potassium carbonate (2.35 M) was added to the reaction mixture. Then diethylmalonate (1.1 M) was added to the reaction mixture and maintained the reaction at 60-70°C for 4-5 hours Thereafter, the progress of the reaction was monitored using HPLC. After the completion of the reaction, pH of the reaction mass was adjusted using hydrochloric acid (6 M) at 10-20°C, extracted the reaction mass using toluene (300 ml X 2) and washed with brine solution to obtain a solid residue. The residue was crystallised using a mixture of heptane and acetone at 10-70°C to obtained titled compound (yield: 90%; purity: 95%).
Example 3: Preparation of 4-(carboxymethyl)-2,3,5-trifluorobenzoic acid.
Diethyl [2,3,6-trifluoro-4-(methoxycarbonyl)phenyl] propanedioate (200 g; obtained in above example) was slowly added to a solution of sulphuric acid (30%, 5 M) in a reactor equipped with condenser at a room temperature and heated at 120-130°C for 20-24 hours. After completion of the reaction, the mixture was cooled to 10-15°C and then filtered to get wet cake. 2-Methyltetrahydrofuran (300 g) was added to the wet cake and washed twice with water (150 ml X 2). The excess 2-methyltetrahydrofuran was distilled to get titled compound (yield: 85%; purity 95%).
Example 4: Preparation of 2,3,6-trifluorophenylacetic acid
4-(Carboxymethyl)-2,3,5-trifluorobenzoic acid (100 g, obtained in above example) and N-methyl-2-pyrrolidone (100 g) was taken in a reactor equipped with a condenser.
Copper oxide (0.1 M) was added to the reaction mixture and heated mixture to 150-160°C. Thereafter, the progress of the reaction was monitored by HPLC. After completion of the reaction, the reaction mass was cooled to 20-25°C and an aqueous solution of potassium carbonate (20%) was added dropwise to it in an hour to adjust the pH 8-9. Activated carbon (10 g) was added to the reaction mixture and filtered. The filtrate was extracted thrice with toluene (100 g x 3). An aqueous solution of sodium metabisulfite (40%, 1.5 M) was added into toluene layer and stirred at a temperature of 50-60°C for an hour and thereafter, gradually cooled to 20-30°C for 8-10 hours. The reaction mass was filtered under reduced pressure, wet caked was washed twice with dichloromethane (100 ml X 2) and dried for an hour. The dried cake was to a reactor and heptane (100 g) and an aqueous solution of sodium hydroxide (20-30%, 2 M) was added to it at 40-50°C for 4-5 hours. The layers were separated, the organic layer was washed with water (100 g X 2) and heptane was completely removed under reduced pressure to get titled compound (yield: 80%; purity 99.2%)
Example 5: Preparation of N-phenyl-3,4,5,6-tetrafluoro phthalimide
N-phenyl-3,4,5,6-tetrachloro (100g), potassium fluoride (129g), tetrabutylammonium bromide (10.0g), and dimethylformamide (700g) were charged in a reactor. The reaction mass was heated to 100?C to 120?C under vigorous stirring for 3-4 hours. The reaction mass was analysed for reaction completion. The reaction mass was filtered to remove spent potassium fluoride and potassium chloride. The filtrate was concentrated to obtain N-phenyl-3,4,5,6-tetrafluoro phthalimide as product. Purity (HPLC): 99.0%, Yield: 80%
Example 6: Preparation of 3,4,5,6-tetrafluoro phthalic acid
N-phenyl-3,4,5,6-tetrafluoro phthalimide (50 g) was charged in a reactor and slowly added aqueous sulfuric acid (80%, 500 g) in the reactor. The reaction mass was heated to 140 ?C under vigorous stirring and stirred for 8 hours. The reaction sample was analysed on HPLC. The reaction mass was cooled to 0-5?C and filtered to get solid. Further, product was isolated by acid-base work up and purification. Purity (HPLC): 99%, Yield: 92%
Example 7: Preparation of 2,3,4,5-tetrafluorobenzoic acid
3,4,5,6-tetrafluoro phthalic acid (5 g), water (1g) and sulpholane (16g) were charged in a reactor. The reaction mass was heated to 130 ?C under vigorous stirring and stirred for 3-4 hours. Purity (HPLC): 99%, Yield: 95%

CLAIMS:

WE CLAIM:
1. A process for preparation of a compound of Formula I,

Formula I
wherein X independently represents a chloro or fluoro group,
comprising the steps of:
a) alkylating 2,3,4,5-tetrahalobenzoic acid of Formula V,

Formula V
to obtain an alkyl ester of 2,3,4,5-tetrahalobenzoic acid of Formula IV;

Formula IV
wherein X is as defined above and R represents an alkyl group,
b) reacting the alkyl ester of 2,3,4,5-tetrahalobenzoic acid of Formula IV with a nucleophile of Formula VII,

Formula VII
wherein R1 and R2 independently represent H, CN, CONH2, COOH, COOR and R represents C1-C4 alkyl, wherein both R1 and R2 cannot be H,
in presence of a base to obtain a compound of Formula III;

Formula III
wherein X, R, R1 and R2 are as defined above,
c) hydrolyzing the compound of Formula III to obtain a compound of Formula II, and

Formula II
wherein X, R1 and R2 are as defined above
d) de-carboxylating the compound of Formula II to obtain the compound of Formula I.
2. The process as claimed in claim 1, wherein the step of alkylating the compound of V to obtain a compound of Formula IV is carried out in presence of an alkylating agent selected from a group consisting of dimethyl sulphate, dimethyl carbonate and methyl iodide, or generated in-situ by reacting methanol with sulphuric acid.
3. The process as claimed in claim 1, wherein the step of alkylating the compound of V and reacting the compound of Formula IV with the nucleophile of Formula VII is carried out in presence of a base selected from a group consisting of a metal hydride, metal borohydride and alkali metal carbonate.
4. The process as claimed in claim 1, wherein the step of de-carboxylation of the compound of Formula II is carried out using a metal salt selected from metal oxide, metal hydroxide and metal carbonate.
5. A process for preparation of 2,3,4,5-tetrafluorobenzoic acid, as defined under 2,3,4,5-tetrahalobenzoic acid of Formula V of claim 1, comprising the steps of:
a) fluorinating N-phenyl-3,4,5,6-tetrachlorophthalimide with a fluorinating agent in presence of a phase transfer catalyst to give N-phenyl-3,4,5,6-tetrafluorophthalimide;
b) hydrolysing N-phenyl-3,4,5,6-tetrafluorophthalimide using an acid to obtain 3,4,5,6-phthalic acid; and
c) decarboxylating 3,4,5,6-phthalic acid to obtain 2,3,4,5-tetrafluorobenzoic acid.
6. The process as claimed in claim 5, wherein fluorination of N-phenyl-3,4,5,6-tetrachlorophthalimide is carried out using a “fluorinating agent” selected from a group consisting of sodium fluoride, potassium fluoride and caesium fluoride.
7. The process as claimed in claim 1 and 5, wherein the hydrolysis of the compound of Formula III or N-phenyl-3,4,5,6-tetrafluorophthalimide is carried out using an acid selected from a group consisting of sulphuric acid, hydrochloric acid, hydrobromic acid, trifluoroacetic acid, acetic acid and p-toluenesulphonic acid.

8. A process for preparation of 2,3,4,5-tetrahalobenzoic acid of Formula V,

Formula V
wherein X independently represents a chloro or fluoro,
comprising the steps of:
a) de-halogenating 2,3,4,5,6-pentahalobenzoic acid of Formula VI,

Formula VI
wherein X independently represents a chloro or fluoro,
with zinc in the presence of a catalyst to obtain a compound of Formula V.
9. A compound of Formula II,

Formula II
wherein X independently represents a chloro or fluoro,
10. A compound of Formula III,

Formula III
wherein X independently represents a chloro or fluoro and R represents an alkyl group; R1 and R2 independently represent H, CN, CONH2, COOH, COOR, wherein both R1 and R2 cannot be H.