Field of the invention
The present invention provides an improved process for preparation of difluoropropionic acid and derivatives thereof.

Background of the invention
The difluoropropionic acid and derivatives thereof have widespread use, for example as building blocks for agrochemicals, as monomers for fluoropolymers. Fluoropolymers have good optical and mechanical properties and so are suitable for the production of optical devices (for example lenses, compact discs) and of fibres and membranes. These compounds are also useful as monomers for medical polymers.
Various methods for preparing difluoropropionic acid and derivatives thereof have been described in the prior art.
The U.S. Patent No. 5,072,030 discloses a process for the preparation of ethyl 2, 3-difluoropropionate by introducing a mixture of fluorine and nitrogen to a solution of ethyl acrylate in the presence of trichlorofluoromethane as a solvent at -78°C.
The main disadvantage of this process is that the fluorination using hydrofluorocarbon solvent results in low yield as hydrofluorocarbon solvent also participates in fluorination reaction.
The U.S. Patent No. 9,061,990 discloses a process for the preparation of methyl 2, 3-difluoropropanoate from methyl acrylate in presence of 2H, 3H-decafluoropentane as a solvent and fluorine in helium as a fluorinating agent under cryogenic comditions. The process results in different undesired products such as methyl 2, 3, 3-trifluoropropanoate, fluoromethyl 2, 3, 3-trifluoropropanoate and fluoromethyl 2, 3-difluoropropanoate.
Thus all the above processes suffers several disadvantages, for example, low selectivity and formation of several undesired side products which makes the process tedious and uneconomic.

So there is a need to develop a cost effective, simple and highly selective process for formation of difluoropropionic acid and derivatives thereof.

Object of the invention
The object of the present invention is to provide an economic and commercially viable process for the preparation of difluoropropionic acid and derivatives thereof.

Summary of the invention
The present invention provides a process for the preparation of a compound of formula 1, comprising:
a) contacting a compound of formula 2 with fluorine in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1; and
b) isolating the compound of formula 1 obtained from step a)

wherein R represents halogen, hydroxyl, methoxy, ethoxy, propoxy, 2-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentoxy, iso-pentoxy, sec-pentoxy, or tert-pentoxy.

The present invention also provides a process for the preparation of a compound of formula 1, comprising:
a) contacting a compound of formula 2 with a mixture of fluorine and air or oxygen or inert gas in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1; and
b) isolating the compound of formula 1 obtained from step a)

wherein R is as defined above.

Detailed description of the invention
In one aspect, the present invention provides a process for the preparation of a compound of formula 1, comprising:
a) contacting a compound of formula 2 with fluorine in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1; and
b) isolating the compound of formula 1 obtained from step a)

wherein R represents halogen, hydroxyl, methoxy, ethoxy, propoxy, 2-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentoxy, iso-pentoxy, sec-pentoxy, or tert-pentoxy.
Preferably, R represents methoxy, ethoxy, hydroxyl or chloro.
In particular, the present invention provides a process for the preparation of a compound of formula 1a, comprising:
a) contacting a compound of formula 2a with fluorine in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1a; and
b) isolating the compound of formula 1a obtained from step a)

In second aspect, the present invention provides a process for the preparation of a compound of formula 1, comprising:
a) contacting a compound of formula 2 with a mixture of fluorine and air or oxygen or inert gas in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1; and
b) isolating the compound of formula 1 obtained from step a)

wherein R is as defined above.

In particular, the present invention provides a process for the preparation of a compound of formula 1a, comprising:
a) contacting a compound of formula 2a with a mixture of fluorine and air or oxygen or inert gas in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1a; and
b) isolating the compound of formula 1a obtained from step a)

wherein R is as defined above.
The compounds of Formula-2 may be prepared by any method known in the prior art or may be obtained commercially.
The fluorination reaction of the present invention is carried out using fluorine gas or mixture of fluorine gas with air, oxygen or inert gas.
The mixture of fluorine with air, oxygen or inert gas consists of 2 to 50% concentration of fluorine.
The fluorination reaction of the step a) is carried out at a temperatures in the range of -80°C to 10°C.
The inert gas used in the step a) is selected from the group consisting of argon and nitrogen.
In a preferred embodiment, methyl acrylate was charged in a reaction vessel. Anhydrous hydrofluoric acid was then added and cooled to -70°C. The mixture of fluorine gas diluted with nitrogen (20%) was added to the reaction mass until completion of reaction. The progress of the reaction was monitored by gas chromatography. After completion of the reaction, the crude product obtained was distilled to get the title compound i.e., methyl 2, 3-difluoropropionate.

The compound of formula-1 is isolated by using techniques known in the art for example distillation, evaporation, column chromatography and layer separation or combination thereof.
The inventors of the present invention have found that the preparation of ethyl 2, 3-difluoropropionate using anhydrous hydrofluoric acid as a solvent results in very good yield as anhydrous hydrofluoric acid solvent do not participates in fluorination reaction.
The use of anhydrous hydrofluoric acid as a solvent also results in easy isolation of the product due to significant difference in boiling point of anhydrous hydrofluoric acid and the title compound i.e., methyl 2, 3-difluoropropionate.
In a preferred embodiment, liquid anhydrous hydrofluoric acid is used.

Further the inventors of the present invention has found that the fluorination using fluorine when mixed with air or oxygen or inert gas in presence of anhydrous hydrofluoric acid prevents polymerization.
While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLE
Preparation of methyl 2, 3-difluoropropionate
143g of Methyl acrylate was charged in a reaction vessel. 1350ml of anhydrous hydrofluoric acid was then added and cooled to -70°C. The mixture of fluorine gas diluted with nitrogen (20%) was added to the reaction mass until completion of reaction. The reaction was monitored by gas chromatography. After completion of the reaction, the crude product obtained was distilled to get the title compound i.e., methyl 2, 3-difluoropropionate.
Yield: 58.5%
Purity: 99.1% (by gas chromatography)

WE CLAIM:
1. A process for preparation of a compound of formula 1, comprising:
a) contacting a compound of formula 2 with fluorine in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1; and
b) isolating the compound of formula 1 obtained from step a)

wherein R represents halogen, hydroxyl, methoxy, ethoxy, propoxy, 2-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentoxy, iso-pentoxy, sec-pentoxy, or tert-pentoxy.
2. The process as claimed in claim 1, wherein, fluorination reaction of step a) is carried out using fluorine gas or mixture of fluorine gas with air, oxygen or inert gas.
3. A process for preparation of a compound of formula 1, comprising:
a) contacting a compound of formula 2 with a mixture of fluorine and air or oxygen or inert gas in presence of anhydrous hydrofluoric acid to obtain the compound of formula 1; and
b) isolating the compound of formula 1 obtained from step a)

wherein R represents halogen, hydroxyl, methoxy, ethoxy, propoxy, 2-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentoxy, iso-pentoxy, sec-pentoxy, or tert-pentoxy.
4. The process as claimed in claim 3, wherein, mixture of fluorine with air, oxygen or inert gas consists of 2 to 50% concentration of fluorine.
5. The process as claimed in claim 1, wherein, fluorination reaction of step a) is carried out at a temperatures selected in the range of -80°C to 10°C.
6. The process as claimed in claim 1, wherein, anhydrous hydrofluoric acid is in liquid form.