Field of the invention
The present invention provides a process for the preparation of pure compound of Formula I.

Formula I

Background of the invention
The Journal of the Electrochemical Society, 148(6) E262-E266 (2001) simply mentions perfluorination of hexafluorobutanol to form n-perfluorobutyryl fluoride in the presence of anhydrous hydrogen fluoride. According to the paper, perfluorobutyrl fluoride is the only product formed, as seen in the mass spectra.
While working on the present invention, the inventors have observed that electrochemical fluorination of hexafluorobutanol in the presence of anhydrous hydrogen fluoride produces heptafluorobutyryl fluoride, hydrogen fluoride, hexafluorobuyryl fluoride, carbonyl fluoride, perfluoro acetyl fluoride and a small amount of perfluoropropynyl fluoride. These acid fluorides readily react with methanol forming a mixture comprising methyl heptafluorobutyrate, hydrogen fluoride, dimethyl carbonate and esters. Dimethyl carbonate and methanol forms an azeotrope. Similarly, the product methylheptafluorobutyrate and methanol forms a binary azeotrope. Thus making the distillation of pure heptafluorobutyrate difficult. The inventors have surprisingly observed that by heating the mixture comprising methyl heptafluorobutyrate, hydrogen fluoride, dimethyl carbonate and esters tends to decompose dimethyl carbonate to methanol and dimethyl ether. The dimethyl ether having a low boiling point of -24°C, is readily separated from the product and the pure methylheptafluorobutyrate is obtained.

Object of the invention
The present invention provides a simple, cost effective and industrially applicable process for the preparation of pure compound of Formula I.

Formula I

Summary of the invention
The present invention provides a process for the preparation of pure compound of Formula I having purity of 95% or more, comprising:
a) contacting a compound of Formula III with anhydrous hydrogen fluoride in an electrochemical fluorination cell to obtain a first mixture comprising compound of Formula II and carbonyl fluoride,
b) contacting methanol with the first mixture to obtain a second mixture comprising compound of Formula I having purity of 85% or less, hydrogen fluoride, dimethyl carbonate and esters,
c) heating the second mixture at a temperature above 100°C to obtain compound of Formula I having purity of 95 % or more, and
d) isolating the compound of Formula I from step c) having purity of 95% or more.

Formula III

Formula II Formula I

Detailed description of the invention
The present invention provides a process for the preparation of pure compound of Formula I having purity of 95% or more, comprising:
a) contacting a compound of Formula III with anhydrous hydrogen fluoride in an electrochemical fluorination cell to obtain a first mixture comprising compound of Formula II and carbonyl fluoride,
b) contacting methanol with the first mixture to obtain a second mixture comprising compound of Formula I having purity of 85% or less, hydrogen fluoride, dimethyl carbonate and esters,
c) heating the second mixture at a temperature above 100°C to obtain compound of Formula I having purity of 95 % or more, and
d) isolating the compound of Formula I from step c) having purity of 95% or more.

Formula III

Formula II Formula I

As used hereinabove, and as will be used hereinafter and in the claims, the term "esters" is intended to include methyl trifluoroacetate and methyl pentafluoro propionate.
In an embodiment, electrochemical fluorination is carried out using an electrolytic cell made of stainless steel having a reflux cooler maintained at a temperature in the range of 0°C to 18°C. The reflux cooler is arranged in the upper portion of the reactor. The cathode and anode of the electrochemical cell are made of nickel plates of 1 mm thickness which are alternately arranged so that the distance between every two adjacent plates is in the range of 1.3mm to 5mm.
The contacting of the first mixture with methanol may takes place at a temperature in the range of 5oC to 50oC.
The second mixture is heated with hydrogen fluoride at a temperature in the range of 100oC to 125oC.
The compound of Formula I as obtained by the process of the present invention has a purity greater than 95%, preferably greater 97% by gas chromatography.

The compound of Formula I is isolated by any of the methods known in the art, for example, distillation, evaporation and layer separation or mixture thereof.
The term ‘about’ refers to a variation of 10% on the higher and lower side of specified parameter.

Unless stated to the contrary, any of the words “including,” “includes,” “comprising” and “comprises” mean “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 compound of Formula III may be obtained commercially or may be prepared by any method known in the art, for example, as disclosed in Indian Patent Application No. 201611028362.

Example
Preparation of pure methyl heptafluorobutanoate
2,2,3,4,4,4-hexafluorobutan-1-ol is continuously dosed into electrochemical cell at a rate of 9 to 20 g/hour . The cell has 1.8 kg of anhydrous hydrogen fluoride inside to obtain a mixture containing heptafluoro butanoyl fluoride as one of the component from the outlet of electrochemical fluorination cell.
The outlet mixture of the electrochemical fluorination cell was reacted with methanol. The outlet mixture of electrochemical cell is condensed into traps maintained at -78 ºC. A quantity of 350-400g of condensed material is then made to react with methanol to get crude mixture of methyl heptafluoro butyrate, methyl heptafluoro isobutyrate, methyl pentafluoro propionate, dimethyl carbonate, hydrogen fluoride and unreacted methanol. 300 g of crude mixture containing dimethyl carbonate (28g) was taken in an autoclave fitted with a magnetically coupled agitator, a pressure gauge and a band heater. The initial pressure of the autoclave was around 1 to 2 kg/cm2. The temperature of the autoclave was slowly raised to 110oC, a sudden change in the reactor pressure was observed. The maximum pressure observed was 25 kg/cm2 at 110oC. This pressure change is due to the decomposition of dimethyl carbonate with hydrogen fluoride. Small amount of sample was withdrawn and analysed by gas chromatography to check the complete decomposition of dimethyl carbonate. Once all the dimethyl carbonate got decomposed, the reactor was cooled and the gaseous by-products are vented off. The dimethyl carbonate free reaction mixture was subjected to fractional distillation to get pure methyl heptafluoro butyrate.
Purity (%): 97 (by gas chromatography)

WE CLAIM:
1. A process for the preparation of pure compound of Formula I having purity of 95% or more, comprising:
a) contacting a compound of Formula III with anhydrous hydrogen fluoride in an electrochemical fluorination cell to obtain a first mixture comprising compound of Formula II and carbonyl fluoride,
b) contacting methanol with the first mixture to obtain a second mixture comprising compound of Formula I having purity of 85% or less, hydrogen fluoride, dimethyl carbonate and esters,
c) heating the second mixture at a temperature above 100°C to obtain compound of Formula I having purity of 95 % or more, and
d) isolating the compound of Formula I from step c) having purity of 95% or more.

Formula III



Formula II Formula I

2. The process as claimed in claim 1 wherein, the compound of Formula III is contacted with anhydrous hydrogen fluoride at a temperature in the range of 0°C to 18°C.
3. The process as claimed in claim 1 wherein, the first mixture is contacted with methanol at a temperature in the range of 5oC to 50oC.

4. The process as claimed in claim 1 wherein, the second mixture is heated at a temperature in the range of 100oC to 125oC.

5. The process as claimed in claim 1 wherein, the esters are methyl trifluoroacetate and methyl pentafluoro propionate.

Dated this 8th day of December, 2016.

Kapil
Senior Research Associate (IPR)
SRF Limited