FIELD OF THE INVENTION
The present invention provides an improved process for preparation difluorocyclopropane derivatives of Formula I,

Formula I
wherein, R1 and R2 are hydrogen, substituted or unsubstituted alkyl or aryl group, halogen, or RCOO group; wherein R is a substituted or unsubstituted alkyl group.

BACKGROUND OF THE INVENTION
The difluorocyclopropane derivatives are useful as an intermediate for pharmaceuticals or pesticides.
There are several methods are known for the preparation of difluorocyclopropane derivatives.
Organic Letters, 2(4), 563-564; 2000, describes a process for preparation of alkyl-2,2-difluorocyclopropane carboxylate by reacting butyl acrylate with trimethylsilyl-2-fluorosulfonyl-2,2-difluoroacetate using sodium fluoride in presence of toluene.
Advanced Synthesis & Catalysis, 360 (21) 4104-4114, 2018 discloses a process for the preparation of alkyl-2,2-difluorocyclopropane carboxylate by reacting alkyl acrylate with trimethyl (trifluoromethyl)silane using sodium iodide in presence of tetrahydrofuran.
The reagents used in above processes suffer from several disadvantages such as they are very difficult to prepare at commercial scale and are not safe to health and environment. These reagents are very costly thereby makes the process less economic.
Therefore there is an urgent need to develop a cost effective, simple and safe process for the formation of difluorocyclopropane derivatives. Hence, the present invention provides a new and improved commercially needed process for the preparation of difluorocyclopropane derivatives using simple and cost effective reagent.

OBJECT OF THE INVENTION
The main object of the present invention is to provide a new and industrially applicable process for the preparation of difluorocyclopropane derivatives.

SUMMARY OF THE INVENTION
The present invention provides an improved process for preparation of a compound of Formula I,

Formula I
wherein, R1 and R2 are hydrogen, substituted or unsubstituted alkyl or aryl group, halogen, RCOO group; wherein R is a substituted or unsubstituted alkyl group,
comprising the step of reacting a compound of Formula II,

Formula II
with a difluorocarbene generating compound in presence of an alkali metal halide in a solvent.

DETAILED DESCRIPTION OF THE INVENTION
As used herein, the alkyl group refer to C1-C10 alkyl chain optionally substituted with a halogen atom namely, fluorine, chlorine, bromine or iodine.
As used herein, the aryl group refer to phenyl group optionally substituted with a halogen namely, fluorine, chlorine, bromine or iodine.
As used herein, RCOO group refer to alkyl carboxylate groups optionally substituted with a halogen namely, fluorine, chlorine, bromine or iodine.
As used herein, the “difluorocarbene generating compound” refer to a compound of formula III

Formula III
wherein L1 is a leaving group selected from a group consisting of alkyl or aryl carboxylate or sulfonate.
As used herein, the carboxylate are selected from a group consisting of acetate, propanoate and benzoate, or the like.
As used herein, the sulfonate are selected from a group consisting of mesylate, triflate, benzenesufonate and tosylate, or the like.
As used herein, the alkali metal halide refer to sodium or potassium chlorides, bromides or iodides.
As used herein, the solvent refer to aprotic polar solvents, preferably the high boiling aprotic polar solvents selected from a group consisting of diglyme, triglyme, dimethylsulfoxide, dimethylformamide, dimethylacetamide, sulfolane, anisole, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene ether glycol, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene and 1-4-dimethoxybenzene or the like.
In an embodiment, the present process is additionally carried out in the presence of a phase transfer catalyst selected from the group consisting of ethyltriphenylphosphonium chloride, benzyltriethylammonium chloride, methyltricaprylammonium chloride, methyltributylammonium chloride, methyltrioctylammonium chloride, tetraethylammonium bromide, tetraethylphosphonium bromide, tetraethylammonium chloride, tetraoctylphosphonium bromide, tetra-n-butylammonium bromide, tetra-n-butylammonium chloride, tetrabutylammonium hydrogen sulphate, methyltriphenylphosphonium iodide, butyltriphenylphosphonium bromide, butyltriphenylphosphonium chloride, benzyltriphenylphosphonium chloride, tetraphenylphosphonium bromide and tetrapropylammonium bromide or the like, most preferably tetrabutyl ammonium bromide.
In an embodiment, the process of the present invention is carried out at a temperature selected in the range of 120°C-160?.
In another embodiment, the present invention provides an improved process for preparation of a alkyl-2,2-difluorocyclopropane-1-caboxylate, comprising the step of reacting an alkyl acrylate with methyl chlorodifluoro acetate and potassium bromide in a solvent to obtain an alkyl-2,2-difluorocyclopropane-1-caboxylate.
In an another embodiment, the present invention provides an improved process for preparation of an alkyl-2,2-difluorocyclopropane-1-caboxylate, comprising the step of reacting an alkyl acrylate with methyl chlorodifluoro acetate and potassium bromide in a solvent in presence of a phase transfer catalyst to obtain an alkyl-2,2-difluorocyclopropane-1-caboxylate.
In another embodiment, the present invention provides an improved process for preparation of a alkyl-2,2-difluorocyclopropane-1-caboxylate, comprising the step of adding a mixture of alkyl acrylate and methyl chlorodifluoro acetate to a mixture of an alkali metal halide and a solvent to obtain an alkyl-2,2-difluorocyclopropane-1-caboxylate.
In an another embodiment, the present invention provides an improved process for preparation of an alkyl-2,2-difluorocyclopropane-1-caboxylate, comprising the step of adding a mixture of an alkyl acrylate and methyl chlorodifluoro acetate to a mixture of an alkali metal halide and a phase transfer catalyst in a solvent to obtain an alkyl-2,2-difluorocyclopropane-1-caboxylate.
In another embodiment of the present invention, the solvent is recovered, recycled and reused in the process.
In another embodiment of the present invention, the “difluorocarbene generating compound”, if used in excess is recovered and reused in the process.
The compound of Formula I is isolated by using techniques known in the art for example distillation, evaporation, column chromatography and layer separation or combination thereof.
The compound of Formula I so obtained by the present invention has a purity greater than 95%, more preferably greater than 98%, most preferably greater than 99.6% by gas chromatography.
The reactants used in the present invention i.e., compound of formula II and III along with reagents and solvents are commercially available.
Unless stated to the contrary, any of the words “comprising”, “comprises” and includes 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 following example is given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLES
Example 1: Process of preparation for butyl-2,2-difluorocyclopropane-1-carboxylate:
Potassium bromide (46.41g, 0.390mol), tetrabutylammonium bromide (2.51g, 0.0078mol) and dimethyl ether of diethylene glycol (418g, 3.12mol) were added to a reactor and heated to 130°C. A mixture of butyl acrylate (50g, 0.390mol) and methyl chlorodifluoro acetate (225g, 1.56mol) was added to it over a period of 4 hours. The progress of the reaction was monitored and after completion of the reaction, the reaction mass was cooked for 24 hours, cooled to room temperature and filtered to get crude product. The crude product was purified by hexane to get a pure titled compound having purity of 99.6%.
Example 2: Preparation of butyl-2,2-difluorocyclopropane-1-carboxylate
Sodium bromide (9.29g, 0.078mol), methyltributylammonium chloride (0.51g, 0.02mol) and anisole (83.6g, 0.624mol) were added to a reactor and heated to 130°C. A mixture of butyl acrylate (10g, 0.078mol) and methyl chlorodifluoro acetate (67.5g, 0.468mol) was added to it over a period of 4 hours. The progress of the reaction was monitored and after completion of the reaction, the reaction mass was cooked for 24 hours, cooled to room temperature and filtered to get crude product. The crude product was purified by hexane to get a pure titled compound having purity of 96.3%.
Example 3: Preparation of butyl-2,2-difluorocyclopropane-1-carboxylate
Potassium bromide (23.2g, 0195mol), tetra-n-butylammonium chloride (1.25, 0.003mol) and 1,2-dimethoxybenzene (209.04g, 1.56mol) were added to a reactor and heated to 130°C. A mixture of butyl acrylate (25g, 0.195mol) and methyl chlorodifluoro acetate (67.5g, 0.468mol) was added to it over a period of 4 hours. The progress of the reaction was monitored and after completion of the reaction, the reaction mass was cooked for 24 hours, cooled to room temperature and filtered to get a crude product. The crude product was purified by hexanes to get a pure titled compound having purity of 98.2%.
Example 4: Preparation of butyl-2,2-difluorocyclopropane-1-carboxylate
Potassium chloride (23.2g, 0.195078mol) and 1,3-dimethoxybenzene (209.3, 1.562mol) were added to a reactor and heated to 130°C. A mixture of butyl acrylate (25g, 0.195) and methyl chlorodifluoro acetate (112.8g, 0.781mol) was added to it over a period of 4 hours. The progress of the reaction was monitored and after completion of the reaction, the reaction mass was cooked for 24 hours, cooled to room temperature and filtered to get a crude product. The crude product was purified by hexane to get a pure titled compound having purity of 98.4%.
Example 5: Preparation of butyl-2,2-difluorocyclopropane-1-carboxylate
Sodium chloride (23.2g, 0.195078mol), and 1-4-dimethoxybenzene (209.3, 1.562mol) were to a reactor and heated to 130°C. A mixture of butyl acrylate (25g, 0.195mol) and methyl chlorodifluoro acetate (84.5g, 0.585mol) was added to it over a period of 4 hours. The progress of the reaction was monitored and after completion of the reaction, the reaction mass was cooked for 24 hours, cooled to room temperature and filtered to get a crude product. The crude product was purified by heptane to get a pure titled compound having purity of 94.6%.

CLAIMS:WE CLAIM:
1. A process for preparation of a compound of Formula I,

Formula I
comprising the step of reacting a compound of Formula II,

Formula II
wherein, R1 and R2 are hydrogen, substituted or unsubstituted alkyl or aryl group, halogen, RCOO group; wherein R is a substituted or unsubstituted alkyl group,
with a difluorocarbene generating compound in presence of an alkali metal halide and a solvent.
2. The process as claimed in claim 1, wherein the “difluorocarbene generating compound” refer to the compound of formula III

Formula III
wherein L1 is a leaving group selected from a group consisting of alkyl or aryl carboxylate or sulfonate.
3. The process as claimed in claim 1, wherein the alkali metal halide is selected from sodium or potassium chlorides, bromides or iodides.
4. The process as claimed in claim 1, wherein the solvent is an aprotic polar solvent selected from a group consisting of diglyme, triglyme, dimethylsulfoxide, dimethylformamide, dimethylacetamide, sulfolane, anisole, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene ether glycol, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene and 1-4-dimethoxybenzene or a mixture thereof.
5. The process as claimed in claim 1, wherein the process is carried out in presence of a phase transfer catalyst.
6. The process as claimed in claim 5, wherein the phase transfer catalyst is selected from the group consisting of ethyltriphenylphosphonium chloride, benzyltriethylammonium chloride, methyltricaprylammonium chloride, methyltributylammonium chloride, methyltrioctylammonium chloride, tetraethylammonium bromide, tetraethylphosphonium bromide, tetraethylammonium chloride, tetraoctylphosphonium bromide, tetra-n-butylammonium bromide, tetra-n-butylammonium chloride, tetrabutylammonium hydrogen sulphate, methyltriphenylphosphonium iodide, butyltriphenylphosphonium bromide, butyltriphenylphosphonium chloride, benzyltriphenylphosphonium chloride, tetraphenylphosphonium bromide and tetrapropylammonium bromide.
7. The process as claimed in claim 1, wherein the reaction is carried out at a temperature range of 120°C-160?.