A PROCESS FOR MANUFACTURING 5-AMINO-l-(2,6-DICHLORO-4 TRIFLUOROMETHYLPHENYL)-3-CYANO-4-TRIFLUOROMETHYL SULPHINYL PYRAZOLE

FIELD OF THE INVENTION

The present invention relates to a process for preparing 5-amino-l-(2,6- dichloro-4-trifluoromethylphenyl)-3-cyano-4-trifluoromethyl sulphinyl pyrazole.

BACKGROUND OF THE INVENTION

F ipron i 1 [5 - Amino- 1 -(2 ,6-dichloro-4-trifluoromethy lphenyl)-3 -cy ano-4- trifluoromethyl sulphinyl pyrazole] is one of the important fluorine bearing 1- Aryl pyrazole derivatives developed in the recent two decades. It is a novel pesticide characterized by high efficiency, low toxicity and especially low residue. There are various routes to synthesize fipronil by oxidation of thiopyrazole with various other oxidizing agents in suitable solvents. fhe known commercial processes for manufacture of fipronil using corrosive and expensive chemical such as trifluoracetic acid/hydrogen peroxide, m- chloroperbenzoic acid/dichloromethane/chloroform and the like have implication from commercial point of view namely, use of expensive tri fluoroacetic acid in large quantities as well as use of m-chloroperbenzoic acid

i which is difficult to handle at commercial scale due to it's un-stability and detonating effect.

Oxidation of sulfides is a very useful route for the preparation of sulfoxides. Literature is replete with the conversion of sulfides to sulfoxides and/or sulfones. However, most of the existing methods use expensive, toxic or rare oxidizing reagents, which are difficult to prepare and use them at commercial scale. Many of these processes suffer from poor selectivity. A number of oxidants like sodium vanadate, sodium tungstate, peracetic acid, performic acid and pertrifluoroacetic acid have been employed in an attempt to obtain regioselective oxidation to provide 5-amino-l-(2,6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl sulphinylpyrazole.

WO 01/30760 describes oxidation of 5-amino-l-(2,6-dichloro-4- trifluoromethy lphenyl)-3 -cyano-4-trifluoromethyl thio-pyrazole with trifluoroacetic. acid and hydrogen peroxide in presence of boric acid. The quantity of trifluoroacetic acid used is 1610 g/mole (14.08 m/m).

European Patent publication No.2951 17 describes the preparation of 5-amino-l- (2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-triiluorome thyl

sulphinylpyrazole by the oxidation of 5-amino-l(2,6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl thiopyrazole with 3- chloroperbenzoic acid in chloroform. The oxidation of electron deficient sulfides such as trifluoromethylsulfides, which are less readily oxidised than other sulfides, entails several difficulties. The molecule has to be stable under the desired conditions of oxidation and oxidation should proceed to the desired extent with minimum of by products such as formation of sulfonyl and amide derivatives. At the same time, level of starting material should be very low such so that product of acceptable quality can be achieved. Oxidants such as peracids, peroxides, persulfates have been widely used for effecting the oxidation.

Various organic solvents such as trifluroacetic acid, mixtures of trichloro/dichloroacetic acid, dichloromethane and chloroform are employed to bring about the oxidation of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)- 3-cyano-4-trifluoromethyl thio-pyrazole. Use of mineral acid as a medium is not useful due to instability of the compound towards strong mineral acids.

Trifluoroacetic acid and trichloroacetic acid were found to be very efficient and regioselective oxidation medium for oxidation of 5-amino-l-(2,6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl thio-pyrazole in the presence of hydrogen peroxide. However, trichloroacetic acid can not be used alone due to higher melting point and need to be used in presence of a melting point depressant. Trifluoroacetic acid on the other hand is very regioselective with respect to conversion and low by-products formation. However, it suffers with disadvantages such as it is expensive, water miscible, corrosive to metal as well as glass, comparatively lower boiling and it's recovery (in anhydrous form) is complex in nature.

Accordingly, it is desired to develop a substitute for corrosive and expensive solvent (trifluoro acetic acid) for the synthesis of 5-amino- 1 -(2, 6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl sulphinyl pyrazole and a process employing such inexpensive, easily available but effective alternative solvent.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a process for the preparation of 5-Amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-t rifluoromethyl sulphinyl pyrazole.

It is another object of the present invention to provide a process for the preparation of 5-amino- 1 -(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4- trifluoromethyl sulphinyl pyrazole which is simple, safe, convenient, easy to operate on commercial scale and cost-effective.

It is a further another object of the present invention to provide a process for the preparation of 5-amino- 1 -(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4- trifluoromethyl sulphinyl pyrazole which provides highly pure product with high yield. SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a process for the preparation of a compound of formula (I);

[I] said process comprising oxidizing a compound of formula (II)

[II]

in a medium comprising at least one oxidizing agent, a solvent system and a corrosion inhibiting compound.

Typically, the oxidizing agent is a peroxide compound selected from the group consisting of hydrogen peroxide, tert butyl hydrogen peroxide, benzoyl peroxide and sodium peroxide. Preferably, the oxidizing agent is hydrogen peroxide.

Typically, the solvent system comprises at least one solvent selected from the group consisting of trichloroacetic acid, monobromoacetic acid, dibromoacetic acid, tribromoacetic acid, chlorobenzene, diehloromethane and dichloroethane.

In accordance with one of the preferred embodiments of the present invention the solvent system comprises monobromoacetic acid, dibromoacetic acid or tribromoacetic acid and chlorobenzene in various ratios.

Typically, the proportion of hydrogen peroxide used is about 1.05 to about 1.2 moles per mole of the compound of formula II.

Typically, the concentration of hydrogen peroxide is in the range of about 45 to about 70% w/w.

Typically, at least one of the corrosion inhibiting compounds is boric acid.

Typically, the amount of corrosion inhibiting compound employed is about 2g/m to lOg/m of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4- trifluoromethyl thio pyrazole. Preferably, the amount of corrosion inhibiting compound employed is about 4g/m to 6g/m of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4- trifluoromethyl thio pyrazole.

Typically, the process is carried out at a temperature in the range of about 0 to 50°C.

Preferably, the process is carried out at a temperature in the range of about 10 to 30°C.

DETAILED DESCRIPTION OF THE INVENTION

Several process problems are encountered when trifluoro acetic acid (TFA) is used as a solvent for oxidation of 5-amino-l-(2,6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl thio pyrazole with 50 % aqueous hydrogen peroxide. The main problem is selection of material of construction of equipment for conducting the experiment. Due to corrosive environment under the oxidation condition, both MSGL and SS reactor can not be normally used.

WO 01/30760 discloses a process for the preparation of fipronil using trifluoracetic acid in an amount of about 14.4 m/m of thio pyrazole. It further discloses addition of corrosion inhibiting compounds such as boric acid to the reaction mixture to inhibit the corrosion and reduces the speed of corrosion to a level, which permits the use of glass reactor. However, the process disclosed in WO 01 /30760 suffers with several disadvantages such as trifluoroacetic acid is a costly reagent and its process economy depends on its recovery and recycles. Further, trifluoroacetic acid is wet because it forms azeotrope with water, (water from aqueous hydrogen peroxide) and to get anhydrous trifluoroacetic acid from this requires azeotropic distillation in presence of concentrated H ₂ S0 ₄ . Moreover, residual TFA is converted to its alkyl ester (methyl, ethyl and others), which needs to be converted back to TFA. Thus, use of TFA as a solvent for oxidation of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3- cyano-4-trifluoromethyl thio pyrazole is difficult and expensive from commercial point of view.

Therefore, the present invention is centered on finding a solution to this problem by selecting a solvent or a mixture of solvent which provides selective degree of oxidation as that of trifluoroacetic acid.

In accordance with the present invention there is provided a process for the preparation of a compound of formula (I).

[I] The process in accordance with the present invention comprising oxidizing a compound of formula (II)

[II]

in a medium comprising at least one oxidizing agent, a solvent system and a corrosion inhibiting compound to obtain a compound of formula (I).

The oxidizing agent employed in the reaction is a peroxide compound selected from the group consisting of hydrogen peroxide, tert butyl hydrogen peroxide, benzoyl peroxide and sodium peroxide.

In accordance with the preferred embodiment of the present invention the oxidizing agent employed is hydrogen peroxide.

In accordance with the present invention the solvent system comprises at least one solvent selected from the group consisting of trichloroacetic acid, monobromoacetic acid, dibromoacetic acid, tribromoacetic acid, chlorobenzene, dichloromethane and dichloroethane. In accordance with one of the preferred embodiments of the present invention the solvent system comprises monobromoacetic acid, dibromoacetic acid or tribromoacetic acid and chlorobenzene in various ratios.

The quantity of peroxide used depends on required optimal conversion with minimum by product formation such as sulfone and amide derivative. In accordance with the preferred embodiment of the present invention the proportion of hydrogen peroxide used is about 1.05 to about 1.2 moles per mole of the compound of formula II. The sulfone derivative formed under this condition is less than 1.8 to 2.5 % w/w.

In accordance with the present invention the concentration of hydrogen peroxide used in the reaction is in the range of about 45 to about 70% w/w.

In accordance with the present invention one of the corrosion inhibiting compounds employed in the reaction is boric acid.

Typically, the amount of corrosion inhibiting compound employed is about 2g/m to lOg/m of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4- trifluoromethyl thio pyrazole. Preferably, the amount of corrosion inhibiting compound employed is about 4g/m to 6g/m of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4- trifluoromethyl thio pyrazole.

The reaction temperature is chosen so as to have selective kinetics of oxidation as well as the stability of the peracid under the reaction condition. Therefore, in accordance with the present invention the process is carried out at a temperature in the range of about 0 to 50°C.

In accordance with the preferred embodiment of the present invention the process is carried out at a temperature in the range of about 10 to 30°C.

The following examples are merely illustrative of the invention and should not be construed as limiting.

Example 1

In a mixture of 1200 gms of monobromoacetic acid, 300 gms of chlorobenzene, 2 gms of boric acid, was added 421 gms of 5-amino-l-(2,6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl thiopyrazole and cooled to 15- 20°C. Aqueous H ₂ 0 ₂ (68 g, 50 %) was added and mass was stirred for 20 hours. After work up 34% fipronil was isolated and 5-amino-l-(2,6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl thiopyrazole from various streams was recycled. Fipronil thus obtained was purified using chlorobenzene to get 95% pure product.

Example 2

A mixture of 570 g of dibromoacetic acid and 30 gm tribromoacetic acid was taken along with 150 g of chlorobenzene, 1 g of boric acid and 211 g of 5- amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-tri fluoromethyl thiopyrazole. The mixture was cooled to 15 - 20°C. Aqueous H ₂ 0 ₂ (34 g, 50%) was added and mass was stirred for 23 hours. After work up 50% fipronil was isolated and 5 -amino- 1 -(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4- trifluoromethyl thiopyrazole from various streams was recycled. The Fipronil thus obtained was purified using chlorobenzene to get 96 % pure product.

Example 3

A mixture of 120 g of tribromoacetic acid and 30 gm chlorobenzene, 0.2 g of boric acid and 42.1 g of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3- cyano-4-trifluoromefhyl thiopyrazole was cooled to 20 - 25 °C. Aqueous H ₂ 0 ₂ (6.8 g, 50%) was added and mass was stirred for 23 hours. After work up 30% fipronil was isolated and 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3- cyano-4-trifluoromethyl thiopyrazole from various streams was recycled. The Fipronil thus obtained was purified using chlorobenzene to get 95.7 % pure product. Example 4

A mixture of 86 g of dibromoacetic acid was taken along with 22 g of chlorobenzene, 0.15 g of boric acid and 30 g of 5-amino-l-(2,6-dichloro-4- trifluoromethylphenyl)-3-cyano-4-trifluoromethyl thiopyrazole. The mixture was cooled to 15 - 17°C. Aqueous H ₂ 0 ₂ (4.84 g , 50%) was added and mass was stirred for 23 hours. After work up 55.5% fipronil was isolated and 5- amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-tri fluoromethyl thiopyrazole from various streams was recycled.

Example 5

Methane sulfonic acid (17.3 g) was charged in a reactor followed by addition of 9.4 g of m-chlorobenzoic acid and 25 ml dichloromethane to obtain a mixture. The mixture was then cooled to 25 °C and to this 2.6 g of 78 % hydrogen peroxide was added. The mixture was stirred at 25 °C for 3 hours. Additional dichloromethane (50 ml) was added to the above mixture of m- chloroperbenzoic acid to get stirrable mass.

Dichloromethane (84 ml) was charged in another reactor and to this 21.05 g of 5-amino-l-(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-t rifluoromethyl thiopyrazole was added. The resultant mixture was cooled to 13-15 °C. To this, slurry of m-chloroperbenzoic acid as prepared above was added in lots over a period of 6 hours. The mixture was stirred at 15°C. HPLC analysis showed 27 % Fipronil conversion after 5 hours and 29.9 % conversion after 10 hours of maintenance at 15°C. To this was added 1.1 g of 78 % hydrogen peroxide in lots and the reaction temperature was raised to 25°C. HPLC analysis showed 46% Fipronil conversion after further 12 hours of maintenance at 25 °C.

Example 6

A solvent mixture (50 g) containing trichloroacetic acid, dichloroacetic acid and monochloroacetic acid (33: 57: 10 % w/w) was charged in a reactor. To this 13g of 5-amino- 1 -(2,6-dichloro-4-trifluoromethylphenyl)-3-cyano-4-trifluorom ethyl thiopyrazole was added and the mixture was cooled to 8 to 10 °C. To this mixture 2.8 g of 81 % tert-butyl hydrogen peroxide was added in lots over a period of 2 hours. The mixture was then maintained for 15 hours at 10 °C and 8 hours at ^« 30 °C to obtain 2.9% Fipronil.

TECHNICAL ADVANCE:

The process disclosed in the present invention employs inexpensive, easily available and effective solvent as a substitute for corrosive and expensive solvent for the synthesis of fipronil.

Further, the process disclosed in the present invention particularly, employs a mixture of solvents in particular proportion, which in turn provides effective degree of oxidation and results in high yield of the fipronil. Still further, the process disclosed in the present invention is simple, safe, convenient, easy to operate on commercial scale and cost-effective. While considerable emphasis has been placed herein on the specific steps of the preferred process, it will be appreciated that many steps can be made and that many changes can be made in the preferred steps without departing from the principles of the invention. These and other changes in the preferred steps of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.