Process for synthesis of fipronil

1. A process for the preparation of a trifluoromethylsulfinylpyrazole compound of formula I,

said process comprising:

a) halogenating an aromatic compound of formula II:

wherein, R is a halogen and R 3 is a perhaloalkyl, to obtain a polyhalogenated perhaloalkyl benzene compound of formula III;

wherein, R and R 2 are halogen independently selected from the group consisting of F, Cl, Br, and I; and R 3 is a perhaloalkyl;

b) reacting in a polar solvent selected from the group consisting of N-methyl pyrrolidone, dimethylsulfone, N,N′-dimethyl imidazolidinone and diphenylsulfone; the compound of formula III with anhydrous ammonia in the presence of an alkali halide; at a temperature of about 200° C. to 300° C.; and a pressure of about 20 kg/cm 2 to about 50 kg/cm 2 to obtain a halo-perhaloalkyl aniline compound of formula IV,

wherein R 2 is halogen and R 3 is a perhaloalkyl;

c) reacting halo-perhaloalkylaniline compound of formula IV with at least one halogenating agent selected from the group consisting of chlorine, sulfuryl chloride, thionyl chloride and phosphorus pentachloride, in a chlorinated solvent selected from the group consisting of chloroform, dichloroethane, dichloromethane, chlorobenzene and o-dichlorobenzene to obtain a product mixture containing polyhalogenated perhaloalkylaniline compound of formula V,

wherein, R 1 and R 2 are halogen independently selected from the group consisting of F, Cl, Br, and I; and R 3 is a perhaloalkyl;

d) isolating polyhalogenated perhaloalkylaniline compound of formula V from the product mixture;

e) diazotizing the isolated polyhalogenated aniline compound of formula V with nitrosyl sulfuric acid in the presence of an acid solvent to form a diazotized derivative of compound V;

f) reacting the diazotized derivative of compound V with cyanoalkyl propionate derivative and ammonia at a temperature range of about 0° C. to 25° C. to obtain a reacted mixture containing pyrazole compound of formula VII,

wherein, R 1 and R 2 are halogen independently selected from the group consisting of F, Cl, Br, and I; and R 3 is a perhaloalkyl;

g) isolating the pyrazole compound of formula VII from the reacted mixture with a halogenated aliphatic hydrocarbon solvent followed by evaporating the solvent to yield crude pyrazole compound VII;

h) sulfenylating the crude pyrazole compound VII with a sulfenylating agent in a halogenated aliphatic hydrocarbon solvent to obtain a product mixture containing thiopyrazole compound of formula IX,

wherein, R 1 and R 2 are halogen independently selected from the group consisting of F, Cl, Br, and I; and R 3 is a perhaloalkyl; followed by isolating the thiopyrazole compound from the product mixture;

i) oxidizing the isolated thiopyrazole compound of formula IX at a temperature 10° C. to 30° C. in a reaction medium comprising an oxidizing agent which is a peroxide compound selected from the group consisting of hydrogen peroxide, tert-butyl hydrogen peroxide, benzoyl peroxide and sodium peroxide; a solvent system comprising in various ratios at least two solvents selected from a group of halogenated solvents consisting of trichloroacetic acid, di chloroacetic acid, mono bromoacetic acid, di bromoacetic acid, tri bromoacetic acid, chlorobenzene, dichloromethane and dichloroethane; and a corrosion inhibitor, to yield a product mixture containing trifluoromethyl sulfinyl pyrazole compound of formula I; and

j) optionally purifying trifluoromethyl sulfinyl pyrazole compound of formula I by at least one process selected from the process consisting of crystallization which is carried out using at least one solvent selected from the group consisting of toluene, chlorobenzene and ethyl acetate; and leaching which is carried out using at least one solvent selected from the group consisting of toluene, chlorobenzene and ethyl acetate.

2. The process as claimed in claim 1 , wherein R, R 1 and R 2 are chlorine and R 3 is trifluoromethyl.

3. The process as claimed in claim 1 , wherein the solvent system in step (i) is a mixture of trichloroacetic acid, dichloroacetic acid and chlorobenzene.

4. The process as claimed in claim 3 , wherein the chlorobenzene content in the mixture is about 20% to 30% w/w.

5. The process as claimed in claim 1 , wherein the solvent system in step (i) is a mixture of trichloroacetic acid and chlorobenzene in a ratio of 80:20 w/w.

6. The process as claimed in claim 4 , wherein the amount of the trichloroacetic acid is 5 to 14 molar equivalents.

7. The process as claimed in claim 1 , wherein the amount of hydrogen peroxide used is about 0.5 moles to about 1.2 moles per mole of the compound of formula IX.

8. The process as claimed in claim 1 , wherein the compound of formula II is 4-chlorobenzotriflouride.

9. The process as claimed in claim 1 , wherein the compound of formula III is 3,4-dichlorobenzotriflouride.

10. The process as claimed in claim 1 , wherein the compound of formula IV is 2-chloro-4-trifluoromethylaniline.

11. The process as claimed as claimed in claim 1 , wherein the compound of formula V is 2,6-dichloro-4-trifluoromethylaniline.

12. The process as claimed in claim 1 , wherein the cyanoalkyl propionate derivative is ethyl-2,3-dicyanopropionate.

13. The process as claimed in claim 1 , wherein the compound of formula VII is 5-amino-3-cyano-1-(2,6-dichloro-4-trifluoromethyl phenyl)-pyrazole.

14. The process as claimed in claim 1 , wherein the compound of formula IX is 5-amino-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-[trifluoromethyl sulfenyl]-1H-pyrazole-3-carbonitrile.

15. The process as claimed in claim 1 , comprising step (j), wherein the chlorobenzene content in the mixture used for leaching ranges from 5% to 100% v/v.

16. The process as claimed in claim 1 , wherein the chlorobenzene used for leaching is in the ratio of 2 to 10 parts per part of trifluoromethylsulfinyl pyrazole compound I.

17. The process as claimed in claim 1 , wherein the purity of 5-amino-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-[-(1(R,S)-trifluoro methyl)sulfinyl]-1H-pyrazole-3-carbonitrile is greater than 98%.