FIELD OF THE INVENTION
The present invention provides a process for the preparation of fluorobenzenesulfonyl fluorides of formula II.

Formula II
where n ranges from 1 to 5.

BACKGROUND OF THE INVENTION
The exchange fluorination reactions, that is, the exchange of fluorine atom for at least one chlorine atom, are well known in the art. The U.S Patent No. 4,369,145 describes a process for preparation of 2, 4-difluorobenzenesulfonyl fluoride by exchange fluorination of chlorine atom of 2, 4-dichlorobenzenesulfonyl chloride. The 2, 4-dichlorobenzenesulfonyl chloride is contacted with potassium fluoride in the presence of anhydrous sulfolane at a temperature ranging from 100oC to 170oC for 12 hours. The percentage yield of desired 2, 4-difluorobenzenesulfonyl fluoride, so obtained, is 34%. The ‘145 Patent further describes the use of tertiary amine sequestering agent in exchange fluorination, which form complex with alkali metal fluoride, thereby activates the reaction and improves the yield.

The present inventors, while working on present invention, observed that such low yield, as described in prior art, is mainly due to two reasons. The exchange fluorination of dichlorobenzenesulfonyl chloride of formula I takes place in two steps that is, the chlorine attached to sulfur is exchanged with fluorine and only thereafter the ring chlorine(s) get exchanged with fluorine(s). The present inventors observed that high temperature of more than 100oC during fluorine exchange of chlorine attached to sulfur of dichlorobenzenesulfonyl chloride results in decomposition to trichlorobenzene. Further,it has observed that as the fluorine exchange reaction proceeds, it results in the formation of potassium chloride. The potassium chloride, so formed, forms a coating on potassium fluoride, which results in less availability of potassium fluoride for the fluorine exchange reaction.

Thus, the present inventors have found an inexpensive, easy and industrially favorable method for the preparation of fluorobenzenesulfonyl fluorides via exchange fluorination reactions without the aid of any sequestering agent.

SUMMARY OF THE INVENTION
The present invention provides a process for the preparation of a compound of Formula II, comprising;
a) contacting a compound of Formula I with an alkali metal fluoride at a temperature in the range of 0oC to 50oCto obtain a compound of Formula III,

Formula III Formula I

b) contacting the compound of Formula III with an alkali metal fluoride to obtain the compound of Formula II, and

Formula II
c) isolating the compound of Formula II obtained from step b).
where n ranges from 1 to 5.

DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a process for the preparation of a compound of Formula II, comprising;
a) contacting a compound of Formula I with an alkali metal fluoride at a temperature in the range of 0oC to 50oC to obtain a compound of Formula III,

Formula III Formula I

b) contacting the compound of Formula III with an alkali metal fluoride to obtain the compound of Formula II, and

Formula II
c) isolating the compound of Formula II obtained from step b).
where n ranges from 1 to 5.

The alkali metal fluoride used in the step a) and the step b) is selected from the group consisting of potassium fluoride, sodium fluoride and cesium fluoride.
The preferred alkali metal fluoride used in the step a) and the step b) consists of potassium fluoride.

According to the present invention, the step a)takes place at a temperature selected in the range of 0oC to 50oC, preferably in the range of 5oC to 35oC, more preferably in the range of 25oC to 35oC.
The fluorination reaction of step a) is completed in a time period of 3 to 14 hours, preferably in a time period of 10 to 14 hours.

According to the present invention, the step a)may also take place in the presence ofa solvent. The solvent may be acetonitrile.
The amount of alkali metal fluoride may range from 1 to 4 molar equivalent, preferably from1 to 2 molar equivalents.

According to the present invention, the step b) may take place at a temperature selected in the range of 50oC to 250oC, preferably in the range selected from 120oC to 180oC.
The fluorination reaction of step b) is completed in a time period of 10 to 14 hours.
The step b) may take place in the presence of solvent selected from sulfolane.

The compound of Formula II is isolated by any method known in the art, for example, acid-base neutralization, distillation, evaporation, crystallization, re-crystallization, column chromatography and filtration or mixture thereof.

EXAMPLES
Preparation of 2, 6-difluorobenzenesulfonyl fluoride from 2, 6-
dichlorobenzenesulfonyl chloride

Step a): A 5L round bottom flask was fitted with a mechanical stirrer, thermopocket, stopper and boil-off set up. 2, 6-Dichlorobenzenesulfonyl chloride (2.19 mol) was charged followed by 780mL of acetonitrile and stirring at 25oC to 35oC. Potassium fluoride (4.38mol) slurry in acetonitrile was added in portions for about 2 hours. After stirring the suspension for 2 hours, the reaction mass was filtered to remove potassium fluoride and potassium chloride solid from the mixture.

Step b):The filtrate was added in portions to an anhydrous suspension of potassium fluoride (8 moles) in 1800mL of sulfolane at 120oC and slowly increasing the mass temperature to 180oC. The reaction mass was stirred for about 10-14 hours and filtered. The filtrate was distilled under vacuum to get the title compound.
Yield: 70%

COMPARATIVE EXAMPLE
Preparation of 2, 6-difluorobenzenesulfonyl fluoride from 2, 6-
dichlorobenzenesulfonyl chloride
A 2L round bottom flask was fitted with mechanical stirrer, thermopocket, stopper and boil-off set up. A mixture of 2, 6-dichlorobenzenesulfonyl chloride (1.1 mol), potassium fluoride (6.6 mol) and 1080mL sulfolane was heated and stirred at 100oC for 4 hours and then to 170oC for 8 hours and filtered. The filtrate was distilled under vacuum to get the title compound.
Yield: 26%

WE CLAIM:
1. A process for the preparation of a compound of Formula II, comprising;
a) contacting a compound of Formula I with an alkali metal fluoride at a temperature in the range of 0oC to 50oCto obtain a compound of Formula III,

Formula III Formula I

b) contacting the compound of Formula III with an alkali metal fluoride to obtain the compound of Formula II, and

Formula II
c) isolating the compound of Formula II obtained from step b).
where n ranges from 1 to 5

2. The process as claimed in claim 1, wherein the alkali metal fluoride used in the step a) and step b) is selected from the group consisting of potassium fluoride, sodium fluoride and cesium fluoride.

3. The process as claimed in claim 2, wherein the alkali metal fluoride is potassium fluoride.

4. The process as claimed in claim 1, wherein the step a)is carried out in presence or absence of a solvent.

5. The process as claimed in claim 4, wherein the solvent is acetonitrile.

6. The process as claimed in claim 1, wherein the step b) is carried out in presence or absence of a solvent.

7. The process as claimed in claim 6, wherein the solvent is sulfolane.

8. The process as claimed in claim 1, wherein the step a)is carried out in a time period of 3 to 14 hours, preferably in a time period of 10 to 14 hours.

9. The process as claimed in claim 1 wherein the alkali metal fluoride is used in an amount in the range of 1 to 4 molar equivalent, preferably in the range from 1 to 2 molar equivalent.

10. The process as claimed in claim 1, wherein the step b) is carried out at a temperature selected in the range of 50oC to 250oC for about 10 to 14 hours.