A process for preparation of pyroxasulfone

Abstract

The present invention relates to a continuous flow process for preparation of pyroxasulfone.

Description

[0001] A PROCESS FOR PREPARATION OF PYROXASULFONE

[0002] FIELD OF THE INVENTION:

[0003] The present invention relates to a process for preparation of pyroxasulfone, substantially free of undesired impurity. More particularly, the invention relates to a continuous flow process for preparation of pyroxasulfone, substantially free of undesired impurity.

[0004] BACKGROUND OF THE INVENTION:

[0005] Pyroxasulfone is a herbicide belonging to the group of pyrazolium. Pyroxasulfone is chemically known as 3-[5-(difluoromethoxy)-l-methyl-3- (trifluoromethyl)pyrazol-4-ylmethylsulfonyl]-4,5-dihydro-5,5-dimethyl-l,2- oxazole and is represented by compound of formula (I):

[0006] Formula (I)

[0007] Pyroxasulfone is a pre-emergence herbicide that inhibits the biosynthesis of very long chain fatty acids (VLCFA). It can be used to effectively control grass and broad-leaved weeds in corn, soybean and wheat fields.

[0008] Pyroxasulfone was first disclosed in US patent No. 7, 238, 689. Currently, few processes for preparation of pyroxasulfone are known in the art. One such method is described in US patent application number 20120264947, which involves oxidation of compound of formula (II) in presence of sodium tungstate dihydrate, hydrogen peroxide and acetic acid at room temperature for 16 hours. The reaction can be represented as follows:

[0009] 1

[0010] Internal

[0011]

[0012] However, the yield of pyroxasulfone obtained using said process is very low and hence, the process is not acceptable at an industrial scale. Another drawback of said process is that it leads to incomplete oxidation and generation of undesired monoxide compound of formula (III):

[0013] This compound of formula (III) forms as a by-product in the final reaction mixture and is very difficult to remove due to structural similarities. However, if compound of formula (III) is not separated, it may lead to quality deterioration of pyroxasulfone formulation and may give rise to the possibility of phytotoxicity to crops. Also, if such impurity is not removed / controlled, it would raise regulatory concerns. Hence, it is important to develop a production method in which oxidation reaction proceeds sufficiently and the compound of formula (III) does not substantially remain in the product.

[0014] Although various processes for preparation of pyroxasulfone have been disclosed and claimed previously, the processes disclosed do not solve the aforementioned problems of prior art. One of the reason being that the methods disclosed in the prior art are performed in batch process mode.

[0015] Batch process refers to a manufacturing method where specific quantity of goods are made in a single production run. The batch process can be a single or a multistep process. In the batch process, the concentration of reagents and solvents varies as the reaction progresses. After completion of reaction, the reaction mixture is required to be subjected to steps such as isolation and purification to obtain desired quantity / quality of product. Each of these steps needs human intervention; the reactor needs to be cleaned between the run and requires workforce for several days depending on batch size. The inventors of the present invention provide a continuous flow process for the preparation of pyroxasulfone, which is substantially free of monoxide compound of formula (III), and wherein all the problems associated with the prior art are overcome.

[0016] OBJECTIVES OF THE INVENTION:

[0017] It is an objective of the present invention to provide a process for preparation of pyroxasulfone via a continuous flow reaction.

[0018] Another objective of the present invention to provide a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III).

[0019] Another objective of the present invention is to provide a process for preparation of pyroxasulfone of formula (I) having high yield.

[0020] Yet another objective of the present invention is to provide a simple, cost effective and industrially viable process for preparation of pyroxasulfone of formula (I).

[0021] SUMMARY OF THE INVENTION: According to an aspect of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid.

[0022] According to an aspect of the present invention, there is provided a process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III); wherein the process comprises preparing pyroxasulfone via a continuous flow reaction being carried out in a continuous flow reactor.

[0023] According to another aspect of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III).

[0024] According to another aspect of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III), said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid as a solvent. Formula (II)

[0025] According to another aspect of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III), said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid as a solvent at a temperature of more than about 40°C; wherein amount of acetic acid used is in a range from about 0.1 moles to about 50 moles with respect to the compound of formula (II).

[0026] According to yet another aspect of the present invention, there is provided a continuous flow process for preparation of 4-[[(4,5-dihydro-5,5-dimethyl-3- isoxazolyl)thio]methyl]- 1 -methyl-3 -(trifluoromethyl)- l / f-Pyrazol-5-ol or salts thereof, the process comprising: a) producing stream 1 comprising 4,5-Dihydro-5,5-dimethyl-3-isoxazolyl carbamimidothioate or its salt, by reacting 3-halo-4,5-dihydro-5,5- dimethylisoxazole with acid addition salt of thiourea; b) producing stream 2 comprising 5,5-Dimethyl-3-isoxazolidinethione or its salt by reacting a stream 1 of step a) and a base; and c) reacting a stream 2 of step b) with 5 -Hydroxy- 1 -methyl -3 -(trifluorom ethyl)- H- pyrazole-4-methanol or its salt, to obtain stream 3 comprising 4-[[(4,5-dihydro- 5,5-dimethyl-3-isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- 1H- Pyrazol-5-ol or salts thereof.

[0027] BRIEF DESCRIPTION OF DRAWINGS:

[0028] Figure 1 illustrates in a schematic view, a continuous flow process for preparation of pyroxasulfone of formula (I), in accordance with Example 1 :

[0029] Reactant A: Solution of 3-({[5-(difluoromethoxy)-l-methyl-3-(trifluoromethyl)- lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-l,2-oxazole in acetic acid;

[0030] Reactant B: Aqueous solution of oxidizing agent;

[0031] Reactant C: Aqueous solution of metal catalyst;

[0032] Product: Pyroxasulfone.

[0033] Figure 2 illustrates in a schematic view, a continuous flow process for preparation of pyroxasulfone of formula (I), in accordance with Example 2:

[0034] Reactant A: Solution of 3-({[5-(difluoromethoxy)-l-methyl-3-(trifluoromethyl)- lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-l,2-oxazole in acetic acid;

[0035] Reactant B: Aqueous solution of oxidizing agent;

[0036] Reactant C: Aqueous solution of metal catalyst;

[0037] Product: Pyroxasulfone.

[0038] Figure 3 illustrates in a schematic view, a continuous flow process for preparation of pyroxasulfone of formula (I), in accordance with Example 3:

[0039] Reactant A: Solution of 3-({[5-(difluoromethoxy)-l-methyl-3-(trifluoromethyl)- lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-l,2-oxazole in acetic acid;

[0040] Reactant B: Aqueous solution of oxidizing agent;

[0041] Reactant C: Aqueous solution of metal catalyst;

[0042] Product: Pyroxasulfone. Figure 4 illustrates in a schematic view, a continuous flow process for preparation of 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l-methyl-3-

[0043] (trifluoromethyl)- U7-Pyrazol-5-ol or its salt, in accordance with Example 4: DL A: Dosing line A DL B: Dosing line B DL C: Dosing line C DL D: Dosing line D

[0044] CSRL Continuous reactor 1

[0045] CSR2: Continuous reactor 2

[0046] CSR3: Continuous reactor 3

[0047] DETAILED DESCRIPTION OF THE INVENTION:

[0048] Those skilled in art will be aware that the invention described herein is subject to variations and modifications other than those specifically described. It is to be understood that the invention described herein includes all such variations and modifications. The invention also includes all such steps, features, compositions and methods referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more said steps or features.

[0049] For convenience, before further description of the present invention, certain terms employed in the specification, examples are described here. These definitions should be read in light of the remainder of the disclosure and understood as by a person of skill in the art. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by a person of ordinary skill in the art. The terms used throughout this specification are defined as follows, unless otherwise limited in specific instances. The terms used herein are defined as follows.

[0050] As used in the specification and the claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. The term “about” shall be interpreted to mean “approximately” or “reasonably close to” and any statistically insignificant variations therefrom. For example, “about” can mean within one or more standard deviations, or within ± 10 or ± 5 of the stated value.

[0051] As used herein, the terms “comprising”, “including”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

[0052] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. In an embodiment, the aspects and embodiments described herein shall also be interpreted to replace the clause “comprising” with either “consisting of’ or with “consisting essentially of’ or with “consisting substantially of’.

[0053] The term “room temperature” unless stated otherwise, essentially means temperature in a range from about 20°C to about 35°C.

[0054] The term “purity” means purity as determined by High Pressure Liquid Chromatography (HPLC).

[0055] The term “pyroxasulfone” as used herein, includes pyroxasulfone free base or its salt and is used interchangeably throughout the disclosure.

[0056] The term “monoxide impurity” as used herein, includes “monoxide compound of formula (III)” and is used interchangeably throughout the disclosure.

[0057] As used herein, the term “substantially free of monoxide compound of formula (III)” refers to pyroxasulfone containing less than or equal to 1% w / w of monoxide compound of formula (III) or containing less than 0.8% w / w of monoxide compound of formula (III). Preferably less than 0.5% w / w of monoxide compound of formula (III).

[0058] The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only.

[0059] According to an embodiment of the present invention, there is provided a process for preparation of pyroxasulfone of formula (I); wherein the process comprises preparing pyroxasulfone via a continuous flow reaction being carried out in a continuous flow reactor.

[0060] According to an aspect of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid.

[0061] In an embodiment, said continuous flow process for preparation of pyroxasulfone is carried out at temperature ranging from about 0°C to about 120°C.

[0062] In one embodiment, said continuous flow process for preparation of pyroxasulfone is carried out without application of external heat. In another embodiment, said continuous flow process for preparation of pyroxasulfone is carried out at temperature ranging from about 10°C to about 50°C, preferably at room temperature.

[0063] In yet another embodiment, said continuous flow process for preparation of pyroxasulfone is carried out at temperature ranging from about 90°C to about 110°C.

[0064] According to an embodiment of the present invention, there is provided a process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III); wherein the process comprises preparing pyroxasulfone via a continuous flow reaction being carried out in a continuous flow reactor.

[0065] In another embodiment, the continuous flow reactor is selected from the group comprising agitated cell reactor, agitated tube reactor, rotary tube reactor, static mixer flow reactor, or combinations thereof.

[0066] According to an embodiment of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III):

[0067]

[0068] In another embodiment, the continuous flow process comprises oxidation of a compound of formula (II).

[0069] In an embodiment, the process comprises oxidation of a compound of formula (II) in presence of an oxidizing agent and a metal catalyst.

[0070] In an embodiment, the process comprises oxidation of a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and an organic solvent.

[0071] In an embodiment, the organic solvent is an organic acid solvent. In an embodiment, the compound of formula (II) is oxidized in presence of a single organic acid solvent. The organic acid solvent used is selected from, but not limited to, formic acid, acetic acid, or combinations thereof. In a preferred embodiment, said organic acid solvent is acetic acid. In an embodiment, acetic acid is single organic solvent used for the reaction. In an embodiment, the process further comprises additional solvents selected from, but not limited to, water, alcohols, aromatic hydrocarbons, halogenated aliphatic hydrocarbons, esters, ethers, nitriles, or combinations thereof.

[0072] According to an embodiment of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III), said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid.

[0073] Formula (II)

[0074] In an embodiment, the oxidizing agent and the metal catalyst are fed to a continuous flow reactor as their respective aqueous solutions.

[0075] In an embodiment, the oxidizing agent is selected from an organic peroxide or an inorganic peroxide. The oxidizing agent used is selected from, but not limited to, an organic peroxide such as m-chloroperbenzoic acid, performic acid, peracetic acid; or an inorganic peroxide selected from hydrogen peroxide, potassium permanganate, sodium periodate, potassium peroxymonosulfate, and the like. In a preferred embodiment, the oxidizing agent used is hydrogen peroxide. In a preferred embodiment, the oxidizing agent used is aqueous solution of hydrogen peroxide.

[0076] In an embodiment, the amount of oxidizing agent used is in a range from about 2 moles to about 10 moles with respect to the compound of formula (II), preferably from about 3 moles to about 8 moles with respect to the compound of formula (II).

[0077] In another embodiment, the compound of formula (II) is oxidised in presence of a metal catalyst. In an embodiment, the metal catalyst used is selected from, but not limited to, tungsten catalyst, molybdenum catalyst, titanium catalyst, zirconium catalyst, or combinations thereof. Preferably, the metal catalyst used is tungsten catalyst, tungstic acid, tungstic acid salt, metallic tungsten, tungsten oxide, tungsten carbide, or combinations thereof. More preferably, the tungsten catalyst used is sodium tungstate or its dihydrate.

[0078] In an embodiment, the amount of the metal catalyst used is in a range of catalytic amount to about 0.1 moles with respect to the compound of formula (II).

[0079] In an embodiment, the metal catalyst is fed to a continuous flow reactor as an aqueous solution of metal catalyst.

[0080] In an embodiment, the process further comprises an additional catalyst selected from an inorganic acid comprising hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, or combinations thereof.

[0081] In an embodiment, the compound of formula (II) is fed to a continuous flow reactor as a solution in acetic acid. In an embodiment, the amount of acetic acid used is in a range from about 0.1 moles to 50 moles, preferably from about 0.1 moles to 40 moles, preferably from about 0.1 moles to 30 moles, preferably from about 0.1 moles to 20 moles, preferably from about 1 moles to 20 moles, preferably from about 1 moles to 10 moles, with respect to the compound of formula (II).

[0082] In a preferred embodiment, the amount of acetic acid used is in a range from about 1 mole to 30 moles with respect to the compound of formula (II).

[0083] In another preferred embodiment, the amount of acetic acid used is in a range from about 5 moles to 10 moles with respect to the compound of formula (II).

[0084] In an embodiment, the flow rate of the compound of formula (II), the oxidizing agent and the metal catalyst is adjusted such that the stoichiometric ratio is in a range of about 0.5-2 : 2-6 : 0.01-0.1 respectively, and preferably in a range of about 0.5-1 : 3-5 : 0.03-0.07 respectively, in the continuous flow reactor.

[0085] In an embodiment, the compound of formula (II) is reacted with the oxidizing agent, the metal catalyst and acetic acid in the continuous flow reactor to yield pyroxasulfone of formula (I).

[0086] It is understood that the flow of different reactants in the reactor(s) may vary according to different parameters, such as, and in non-limiting manner, the volume (diameter / length) of the reactor(s), mixing profile, and the time of residence into the reactor(s). The flow may be set by a skilled man through different routine calculation and / or experiments.

[0087] In an embodiment, the flow rate of aqueous solution of oxidizing agent in the reactor is in the range of 1 to 15 ml / min. In an embodiment, the flow rate of the solution of compound of formula (II) in acetic acid is in the reactor is in the range of 1 to 60 ml / min.

[0088] In an embodiment, the flow rate of the aqueous solution of metal catalyst in the reactor is in the range of 0.5 to 10 ml / min. The time of residence in the reactor is set so as to allow a complete reaction between the reactants. The time of residence may vary according to different parameters, such as, and in non-limiting manner, the volume (diameter / length) of the reactor and the flow of the compounds into the chamber. The time of residence may be set by a skilled man through different routine experiments.

[0089] In an embodiment, the residence time of the reaction mixture in the continuous flow reactor is in a range from about 5 seconds to about 30 minutes, preferably from about 10 seconds to about 20 minutes, and more preferably from about 10 seconds to about 15 minutes.

[0090] In an embodiment, the compound of formula (II) as a solution in acetic acid is introduced to the continuous flow reactor together, with an aqueous solution of the oxidizing agent and an aqueous solution of the metal catalyst.

[0091] In an embodiment, the compound of formula (II) as a solution in acetic acid, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are introduced to the reactor in any sequence.

[0092] In another embodiment, the compound of formula (II) as a solution in acetic acid, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are introduced simultaneously to the continuous flow reactor.

[0093] In another embodiment, the compound of formula (II) as a solution in acetic acid and an aqueous solution of the oxidizing agent are first introduced to the continuous flow reactor and allowed to react; followed by introduction of the aqueous solution of the metal catalyst.

[0094] In another embodiment, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are first introduced to the continuous flow reactor and allowed to react; followed by introduction of the compound of formula (II) as a solution in acetic acid.

[0095] In an embodiment, the residence time of the reaction mixture in the continuous flow reactor is in a range from about 10 seconds to about 10 minutes, preferably from about 6 minutes to about 8 minutes.

[0096] In another embodiment, the compound of formula (II) as a solution in acetic acid and the aqueous solution of the oxidizing agent are introduced to the continuous flow reactor and allowed to react with a residence time from about 10 seconds to about 5 minutes; followed by introducing the aqueous solution of the metal catalyst to the reaction mixture with a residence time from about 10 seconds to about 10 minutes.

[0097] In another embodiment, the compound of formula (II) as a solution in acetic acid and the aqueous solution of the oxidizing agent are introduced to the continuous flow reactor and allowed to react with a residence time from about 30 seconds to about 3 minutes; followed by introducing the aqueous solution of the metal catalyst to the reaction mixture with a residence time from about 30 seconds to about 8 minutes.

[0098] In another embodiment, the compound of formula (II) as a solution in acetic acid and the aqueous solution of the oxidizing agent are introduced to the continuous flow reactor and allowed to react with a residence time from about 1 minute to about 3 minutes; followed by introducing the aqueous solution of the metal catalyst to the reaction mixture with a residence time from about 4 minutes to about 6 minutes. In another embodiment, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are introduced to the continuous flow reactor and allowed to react with a residence time from about 0.1 minute to about 1 minute; followed by introducing the compound of formula (II) as a solution in acetic acid to the reaction mixture with a residence time from about 1 minutes to about 10 minutes.

[0099] In an embodiment, the compound of formula (II) as a solution in acetic acid, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are introduced to the reactor at room temperature.

[0100] In an embodiment, the compound of formula (II) as a solution in acetic acid, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are reacted without external heating.

[0101] In another embodiment, the compound of formula (II) as a solution in acetic acid, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are reacted at the temperature above 40°C, preferably above 50°C, preferably above 60°C, preferably above 70°C, preferably above 75°C, preferably above 80°C, and more preferably above 85°C.

[0102] In an embodiment, the compound of formula (II) as a solution in acetic acid, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are reacted at the temperature ranging from about 75°C to about 120°C, preferably from about 80°C to about 120°C, preferably from about 85°C to about 120°C, preferably from about 85°C to about 110°C, and most preferably from about 90°C to about 110°C. According to an embodiment, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III), said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid as a solvent at a temperature in a range from about 90°C to about 110°C.

[0103] In an embodiment, the amount of acetic acid used is in a range from about 5 mole to 10 moles with respect to the compound of formula (II).

[0104] In an embodiment, the continuous flow process for preparation of pyroxasulfone of formula (I) according to present invention is carried out without application of external heat. According to an embodiment, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III), said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid, without application of external heat.

[0105] In a preferred embodiment, the amount of acetic acid used is in a range from about 1 mole to 20 moles with respect to the compound of formula (II).

[0106] After completion of reaction, the reaction mass obtained by the continuous flow reaction, may be optionally subjected to purification in said continuous flow reactor or in a batch process reactor, to obtain the desired quality of pyroxasulfone. In an embodiment, the reaction mass obtained by the continuous flow reaction is subjected to treatment with water to obtain the desired quality of pyroxasulfone. In an embodiment, pyroxasulfone obtained by the present process is subjected to washing with water and an organic solvent, preferably with a non-polar organic solvent. The non-polar organic solvent used is selected from, but not limited to, heptane, hexane, petroleum ether, or combinations thereof.

[0107] In an embodiment, the yield of pyroxasulfone obtained is more than about 50%, preferably more than about 70%. In an embodiment, the present invention provides pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III).

[0108] In an embodiment, the present invention provides pyroxasulfone of formula (I) comprising monoxide compound of formula (III) in an amount less than or equal to about 0.5% w / w.

[0109] In an embodiment, the present invention provides pyroxasulfone of formula (I) having purity of more than about 95%, preferably more than about 98%.

[0110] According to an embodiment of the present invention, there is provided a continuous flow process for preparation of pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III),

[0111] said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid at a temperature of more than about 40°C; wherein amount of acetic acid used is in a range from about 0.1 moles to about 50 moles with respect to the compound of formula (II).

[0112] In an embodiment, the continuous flow process comprises: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid as a solvent at a temperature of more than about 40°C; wherein amount of acetic acid used is in a range from about 0.1 moles to about 30 moles with respect to the compound of formula (II).

[0113] In an embodiment, the continuous flow process comprises: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid as a solvent; wherein amount of acetic acid used is in a range from about 1 moles to about 30 moles with respect to the compound of formula (II) and wherein the process is carried out without application of external heat.

[0114] In another embodiment, the advantages of the present invention comprise:

[0115] • minimized handling of intermediates

[0116] • reduced solvent load, minimized effluent and waste generation and thereby a greener chemistry approach;

[0117] • reduced reaction time and increased process efficiency; and • high output and high yield of pyroxasulfone, which is substantially free of monoxide impurity.

[0118] In another embodiment, the inventors of the present invention have selected the range of temperature which ensures completion of the reaction for obtaining pyroxasulfone of formula (I), substantially free of monoxide compound of formula (III). This finding of the present invention is based on multiple experimentation and extensive research by the inventors. A major issue known in the prior art was the presence of monoxide impurity in pyroxasulfone obtained from similar processes. The process of present invention not only solves this problem of the prior art but also improves the yield of the process substantially.

[0119] In another aspect of the present invention there is provided a continuous process for preparation of 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l-methyl- 3 -(trifluoromethyl)- lJT-Pyrazol-5-ol or salts thereof, the process comprising: a) producing a stream 1 comprising 4,5-Dihydro-5,5-dimethyl-3-isoxazolyl carbamimidothioate or its salt, by reacting 3-halo-4,5-dihydro-5,5- dimethylisoxazole with acid addition salt of thiourea; b) producing a stream 2 comprising 5,5-Dimethyl-3-isoxazolidinethione or its salt by reacting the stream 1 of step a) and a base; and c) reacting the stream 2 of step b) with 5-Hydroxy-l-methyl-3-(trifluoromethyl)- l / Z-pyrazole-4-methanol or its salt, to obtain stream 3 comprising 4-[[(4,5- dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- lJT-Pyrazol-5-ol or salts thereof.

[0120] In an embodiment, step a) is carried out in the presence of water, an organic solvent or mixture thereof.

[0121] The organic solvent used is selected from ester solvents such as methyl acetate, ethyl acetate, n-butyl acetate, and the likes. In preferred embodiment, step a) is carried out in presence of water and n-butyl acetate.

[0122] In said step a) 3-halo-4,5-dihydro-5,5-dimethylisoxazole is introduced to the reactor as a solution in n-butyl acetate; and acid addition salt of thiourea is introduced to the reactor as an aqueous solution.

[0123] In an embodiment, content of 3-halo-4,5-dihydro-5,5-dimethylisoxazole is ranging from 0.5 to 2 equivalents; and the content of acid addition salt of thiourea is ranging from 0.5 to 2 equivalents.

[0124] In an embodiment, 3-halo-4,5-dihydro-5,5-dimethylisoxazole is introduced to the reactor as a solution in n-butyl acetate at a concentration ranging from 10% to 90%, preferably from 50% to 90%, more preferably 60% to 80%.

[0125] In an embodiment, the content of 3-halo-4,5-dihydro-5,5-dimethylisoxazole is ranging from 0.5 to 2 equivalents, preferably 0.8 to 1.5 equivalents.

[0126] In an embodiment, the flow rate of solution of 3-halo-4,5-dihydro-5,5- dimethylisoxazole in the reactor is in the range of 0.5 to 3 ml / min.

[0127] In an embodiment, 3-halo-4,5-dihydro-5,5-dimethylisoxazole is selected from, but not limited to, 3-bromo-4,5-dihydro-5,5-dimethylisoxazole, 3-chloro-4,5-dihydro- 5,5-dimethylisoxazole and the like; preferably 3-bromo-4,5-dihydro-5,5- dimethylisoxazole is used.

[0128] In an embodiment, acid addition salt of thiourea is introduced in step a) is selected from, but not limited to, thiourea hydrochloride, thiourea sulfate, thiourea nitrate or the like; preferably thiourea hydrochloride is used.

[0129] In an embodiment, acid addition salt of thiourea is introduced to the reactor as an aqueous solution at a concentration ranging from 5% to 50%, preferably from 20% to 30%. In an embodiment, the content of acid addition salt of thiourea is ranging from 0.5 to 2 equivalents, preferably 0.8 to 1.5 equivalents.

[0130] In an embodiment, the flow rate of solution of aqueous solution of acid addition salt of thiourea in the reactor is in the range of 1 to 5 ml / min.

[0131] In another embodiment, step a) of the process is carried out at temperature ranging from about 10°C to about 100°C, preferably from about 60°C to about 80°C.

[0132] In another embodiment, step a) of the process is carried out at pressure ranging from about 1 bar to about 10 bar, preferably from about 1 bar to 3 bar.

[0133] In an embodiment, the residence time for step a) is anywhere between 2 to 8 minutes.

[0134] The phrase “stream 1 comprising 4,5-Dihydro-5,5-dimethyl-3-isoxazolyl carbamimidothioate or its salt” is used interchangeably as outstream / outlet of the step a) throughout the specification.

[0135] In an embodiment, the stream 1 obtain in step a) comprises 4, 5 -Dihydro-5, 5- dimethyl-3-isoxazolyl carbamimidothioate hydrobromide salt.

[0136] In an embodiment, the stream 1 obtained in step a) is cooled to room temperature before directing it to step b) reaction.

[0137] In an embodiment, the flow rate of stream 1 to the reactor of step b) is in the range of 2 to 8 ml / min.

[0138] In an embodiment, in step b) base is introduced to the reactor as an aqueous base.

[0139] In an embodiment, the concentration of aqueous base is ranging from about 20% to about 30%.

[0140] The base used is selected from an inorganic base such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, cesium carbonate, and the like. In an embodiment, the content of inorganic base is from 1 to 10 equivalents, preferably 4 to 8 equivalent.

[0141] In an embodiment, the flow rate of solution of aqueous base in the reactor is in the range of 4 to 10 ml / min.

[0142] In another embodiment, step b) of the process is carried out at temperature ranging from about 10°C to about 50°C, preferably from about 20°C to about 40°C.

[0143] In another embodiment, step b) of the process is carried out at pressure ranging from about 1 bar to about 10 bar, preferably from about 1 bar to 3 bar.

[0144] In an embodiment, the residence time for step b) is anywhere between 1 to 3 minutes.

[0145] The phrase “stream 2 comprising 5,5-Dimethyl-3-isoxazolidinethione or its salt” is used interchangeably as outstream / outlet of the step b) throughout the specification.

[0146] In an embodiment, the stream 2 obtain in step b) comprises sodium salt of 5,5- Dimethyl-3-isoxazolidinethione.

[0147] In an embodiment, the flow rate of stream 2 to the reactor of step c) is in the range of 5 to 15 ml / min, preferably 8 to 12 ml / min.

[0148] In an embodiment, 5-Hydroxy- l -methyl-3-(trifluoromethyl)- l / / -pyrazole-4- methanol or its salt is introduced to the reactor as an aqueous solution at a concentration ranging from 5% to 40%, preferably 15% to 30%.

[0149] In an embodiment, the content of 5-Hydroxy- l -methyl-3-(trifluoromethyl)- l / / - pyrazole-4-methanol or its salt is ranging from 0.5 to 2 equivalents, preferably 0.8 to 1.5 equivalents.

[0150] In an embodiment, the flow rate of solution of 5 -Hydroxy- l-methyl-3- (trifluoromethyl)-U / -pyrazole-4-methanol or its salt in the reactor is in the range of 1 to 10 ml / min, preferably 5 to 8 ml / min.

[0151] In an embodiment, in step c) sodium salt of 5-Hydroxy-l-methyl-3- (trifluoromethyl)-U / -pyrazole-4-methanol is used. In another embodiment, step c) of the process is carried out at temperature ranging from about 10°C to about 50°C, preferably from about 20°C to about 40°C.

[0152] In another embodiment, step c) of the process is carried out at pressure ranging from about 1 bar to about 10 bar, preferably from about 1 bar to 3 bar.

[0153] In an embodiment, the residence time for step c) is anywhere between 1 to 10 minutes, preferably 4 to 8 minutes.

[0154] The phrase “stream 3 comprising 4-[[(4,5-dihydro-5,5-dimethyl-3- isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- UT-Pyrazol-5-ol or its salt” is used interchangeably as outstream / outlet of the step c) throughout the specification.

[0155] In an embodiment, the stream 3 comprises sodium salt of 4-[[(4,5-dihydro-5,5- dimethyl-3-isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- lJT-Pyrazol-5- ol.

[0156] The stream 3 of step c) was collected and acidified with an acid to obtain 4-[[(4,5- dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l-methyl-3-(tri fluoromethyl)- lJT-Pyrazol-5-ol or its salt.

[0157] In an embodiment the continuous reactor used in step a) to c) is selected from plug flow reactor, tube reactor, continuous stir tank reactor, micro-reactor, packed bed reactor, coiled reactor, Spinning Disk Reactors, Oscillatory Flow Reactors, Aspirator Reactors, Multiphase Reactors.

[0158] In an embodiment, the continuous reactor is plug flow reactor.

[0159] It is understood that the flow of the different reactants in the reactor(s) may vary according to different parameters, such as, and in non-limiting manner, the volume (diameter / length) of the reactor(s), mixing profile, and the time of residence into the reactor(s). The flow may be set by a skilled man through different routine calculation and / or experiments The time of residence in the reactor is set so as to allow a complete reaction between the reactants. The time of residence may vary according to different parameters, such as, and in non-limiting manner, the volume (diameter / length) of the reactor and the flow of the compounds into the chamber. The time of residence may be set by a skilled man through different routine experiments.

[0160] According to an embodiment, there is provided a continuous process for preparation of sodium salt of 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l- methyl-3 -(trifluoromethyl)- lJT-Pyrazol-5-ol, the process comprising: a) producing stream 1 comprising 4,5-Dihydro-5,5-dimethyl-3-isoxazolyl carbamimidothioate or its hydrobromide salt, by reacting 3-Bromo-4,5- dihydro-5,5-dimethylisoxazole solution in n-butyl acetate with aqueous solution of thiourea hydrochloride; b) producing stream 2 comprising sodium salt of 5,5-Dimethyl-3- isoxazolidinethione by reacting to stream 1 of step a) and an aqueous base; and c) reacting the stream 2 of step b) with aqueous solution of sodium salt of 5- Hydroxy- 1 -methyl-3 -(trifluoromethyl)- 1 JT-pyrazole-4-m ethanol to obtain stream 3 comprising sodium salt of 4-[[(4,5-dihydro-5,5-dimethyl-3- isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- lJ / -Pyrazol-5-ol.

[0161] In an embodiment, 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l- methyl-3 -(trifluoromethyl)- 1 JT-Pyrazol-5-ol or salts thereof, prepared according to present invention is converted to compound of formula (II) by batch or continuous flow process.

[0162] In an embodiment, 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l- methyl-3-(trifluorom ethyl)- lJT-Pyrazol-5-ol or salts thereof is converted to compound of formula (II) by batch processes known in prior art by reaction with freon gas. In an embodiment, 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l- methyl-3 -(trifluoromethyl)- l / / -Pyrazol-5-ol or salts thereof is converted to compound of formula (II) by continuous flow processes by reaction with freon gas.

[0163] Figures 1, 2 & 3 depict a schematic representation of a process for the continuous production of a pyroxasulfone in accordance with a particularly preferred embodiment of the invention, which Figure and description serve to further describe the invention.

[0164] With reference to Figure 1 :

[0165] Solution in acetic acid of 3-({[5-(difluoromethoxy)-l-methyl-3-(trifluoromethyl)- lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-l,2-oxazole, aqueous solution of hydrogen peroxide and aqueous solution of sodium tungstate namely, reactant A, B and C respectively; from three different dosing lines namely, first, second and third are mixed and then fed to continuous flow reactor. In this continuous flow reactor the three reactants react to produce a reaction mixture comprising Pyroxasulfone which is directed to an outlet and is collected.

[0166] With reference to Figure 2:

[0167] Solution in acetic acid of 3-({[5-(difluoromethoxy)-l-methyl-3-(trifluoromethyl)- lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-l,2-oxazole and aqueous solution of hydrogen peroxide namely, reactant A and reactant B respectively; coming from first and second dosing lines are mixed and then fed to continuous flow reactor. In the continuous reactor through third dosing line is fed aqueous solution of sodium tungstate namely, reactant C. In this continuous flow reactor the three reactants react to produce a reaction mixture comprising Pyroxasulfone which is directed to an outlet and is collected.

[0168] With reference to Figure 3 :

[0169] An aqueous solution of hydrogen peroxide and aqueous solution of sodium tungstate namely, reactant B and reactant C respectively; coming from first and second dosing lines are mixed and then fed to continuous flow reactor. In the continuous reactor through third dosing line is fed a solution in acetic acid of 3- ({ [5-(difluorom ethoxy)- 1 -methyl-3-(trifluoromethyl)-lH-pyrazol-4- yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-l,2-oxazole namely, reactant A. In this continuous flow reactor the three reactants react to produce a reaction mixture comprising Pyroxasulfone which is directed to an outlet and is collected.

[0170] Figure 4 depicts a schematic representation of a process for the continuous production of a sodium salt of 4-[[(4,5-dihydro-5,5-dimethyl-3- isoxazolyl)thio]methyl]- 1 -methyl-3 -(trifluoromethyl)- 1 JT-Pyrazol-5-ol (an intermediate of Pyroxasulfone) in accordance with a particularly preferred embodiment of the invention, which Figure and description serve to further describe the invention.

[0171] With reference to Figure 4:

[0172] In step a) 3-Bromo-4,5-dihydro-5,5-dimethylisoxazole solution in n-butyl acetate (from dosing line DL A, through pump) and aqueous solution of thiourea hydrochloride (from dosing line DL B, through pump) are are feed to continuous flow reactor (CSR1) in which these components react to produce stream 1 comprising 4,5-Dihydro-5,5-dimethyl-3-isoxazolyl carbamimidothioate or its hydrobromide salt. In step b) the stream 1 was then fed to continuous flow reactor (CSR2) along with aqueous sodium hydroxide (from dosing line DL C, through pump) in which these components react to produce stream 2 comprising sodium salt of 5,5-Dimethyl-3-isoxazolidinethione or its salt. In step c) the stream 2 was then fed to continuous flow reactor (CSR3) along with aqueous solution of sodium salt of 5-Hydroxy-l-methyl-3-(trifluoromethyl)-U / -pyrazole-4-methanol (from dosing line DL D, through pump) in which these components react to produce stream 3 comprising sodium salt of 4-[[(4,5-dihydro-5,5-dimethyl-3- isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- lZf-Pyrazol-5-ol.

[0173] The following examples are presented to provide what is believed to be the most useful and readily understood description of procedures and conceptual aspects of this invention. The examples provided below are merely illustrative of the present invention and are not intended to limit the same to disclosed embodiments. Variations and changes obvious to one skilled in the art are intended to be within the scope and nature of the invention.

[0174] EXAMPLES:

[0175] Example 1 : Preparation of pyroxasulfone

[0176] A plug flow reactor was used to perform a continuous reaction, to the said reactor was fed 40% solution in acetic acid of 3-({[5-(difhioromethoxy)-l-methyl-3- (trifluoromethyl)-lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-

[0177] 1,2-oxazole (wherein molar ratio of 3-({[5-(difhioromethoxy)-l-methyl-3- (trifluoromethyl)-lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-

[0178] 1.2-oxazole to acetic acid is 1 :8 to 9) by first dosing line at the rate of 8 ml / min, 50% aqueous solution of hydrogen peroxide by second dosing line at the rate of 2 ml / min and 20% aqueous solution of sodium tungstate by third dosing line at the rate of 0.7 ml / min. The stoichiometric ratio of 3-({[5-(difluoromethoxy)-l-methyl- 3-(tri fluoromethyl)- lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-

[0179] 1.2-oxazole, hydrogen peroxide and sodium tungstate was maintained to 1 : 4.75: 0.05. The residence time of the reactor based on the total feed rate was 7 minutes and operating temperature was maintained between 95°C to 100°C. At the outlet of the reactor, a set point was maintained at 50°C to 60°C and the reaction mixture was collected. The reaction mixture was diluted with water and then cooled to about 8°C to 10°C. The product was filtered out by washing with water and petroleum ether; and dried to obtain Pyroxasulfone.

[0180] (Yield: 83.37%; Purity: 98.7% W / W; Monoxide Impurity: less than 0.2% w / w in HPLC)

[0181] Example 2: Preparation of Pyroxasulfone

[0182] A plug flow reactor was used to perform a continuous reaction, to the said reactor was fed 40% solution in acetic acid of 3-({[5-(difhioromethoxy)-l-methyl-3- (trifluoromethyl)-lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro- 1,2-oxazole by first dosing line at the rate of 11 ml / min and 50% aqueous solution of hydrogen peroxide by second dosing line at the rate of 3 ml / min. The reaction mixture was allowed to react in the continuous flow reactor for a residence time of 2 minutes at a temperature ranging from 95°C to 100°C. After 2 minutes of residence time, to the reactor was fed 20% aqueous solution of sodium tungstate by third dosing line at the rate of 1 ml / min and the reaction mixture was allowed to react in the continuous flow reactor for a residence time of 5 minutes by maintaining the operating temperature between 95°C to 100°C. At the outlet of the reactor, a set point was maintained at 50°C to 60°C and the reaction mixture was collected. The reaction mixture was diluted with water and then cooled to about 8°C to 10°C. The product was filtered out by washing with water and petroleum ether; and dried to obtain pyroxasulfone.

[0183] Example 3: Preparation of Pyroxasulfone

[0184] A plug flow reactor was used to perform a continuous reaction, to the said reactor was simultaneously fed 50% aqueous solution of hydrogen peroxide by first dosing line at the rate of 11.45ml / min and 10% aqueous solution of sodium tungstate by second dosing line at the rate of 5.96 ml / min for initial 30 secs. Then 26% solution in acetic acid of 3-({[5-(difluoromethoxy)-l-methyl-3- (trifluoromethyl)-lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-

[0185] 1,2-oxazole (wherein molar ratio of 3-({[5-(difluoromethoxy)-l-methyl-3- (trifluoromethyl)-lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-

[0186] 1.2-oxazole to acetic acid is 1 :17 to 18) was fed by third dosing line at the rate of 49.24 ml / min. The stoichiometric ratio of 3-({[5-(difluoromethoxy)-l-methyl-3- (trifluoromethyl)-lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro-

[0187] 1.2-oxazole, hydrogen peroxide and sodium tungstate was maintained to 1 : 5: 0.05. The reaction mixture was allowed to react in the continuous flow reactor for a residence time of 3 minutes without any external heating. The stream comprising Pyroxasulfone was collected at the outlet of reactor. The product was filtered out by washing with water and petroleum ether; and dried to obtain Pyroxasulfone with yield of 92-94%; Monoxide Impurity: 0.01area%. (Yield: 92-94%; Monoxide Impurity: 0.01 area%)

[0188] Example 4: Process for preparation of sodium salt of 4-[[(4,5-dihydro-5,5- dimethyl-3-isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- IT / -Pyrazol-5- ol

[0189] A plug flow reactor was used to perform a continuous reaction, to the said reactor was fed 70% of 3-Bromo-4,5-dihydro-5,5-dimethylisoxazole solution in n-butyl acetate by dosing line A at the rate of 1.68 ml / min (1 eq.) and 24% aqueous solution of thiourea hydrochloride by dosing line B at the rate of 2.31 ml / min (1 eq.). The reaction mixture was allowed to react in the continuous flow reactor 1 (CSR1) for a residence time of 5 minutes at a temperature ranging from 70°C. The mixture was then cooled to 25°C at residence time of 0.5 minutes to obtain a stream 1 comprising 4,5-Dihydro-5,5-dimethyl-3-isoxazolyl carbamimidothioate or its hydrobromide salt. The stream 1 was then fed to second reactor at flow rate of 4 ml / min along with 25% aqueous sodium hydroxide by dosing line C at the rate of 6.26 ml / min (6 eq.). The reaction mixture was allowed to react in the continuous flow reactor 2 (CSR2) for a residence time of 2 minutes at a temperature ranging from 25°C to 30°C to obtain stream 2 comprising sodium salt of 5,5-Dimethyl-3-isoxazolidinethione. Said stream 2 is fed to third reactor at flow rate of 10 ml / min along with 18% aqueous solution of sodium salt of 5 -Hydroxy- 1 -methyl -3 -(trifluoromethyl)- H- pyrazole-4-methanol by dosing line D at the rate of 5.42 ml / min (1 eq.). The reaction mixture obtained was allowed to react in the continuous flow reactor 3 (CSR3) for a residence time of 6 minutes at a temperature ranging from 25°C to 30°C to obtain stream 3 comprising sodium salt of 4-[[(4,5-dihydro-5,5-dimethyl- 3-isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)- UT-Pyrazol-5-olat 20. The stream 3 was collected and acidified with 30% aqueous sulfuric acid 20°C to 30°C to obtain a solid product which was dried to obtain 25.3 g sodium salt of 4- [[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l-methyl-3- (trifluoromethyl)- IT / -Pyrazol-5-ol. Example 5: Process for preparation of 3-({[5-(difhroromethoxy)-l-methyl-3- (trifluoromethyl)-lH-pyrazol-4-yl]methyl}sulfanyl)-5,5-dimethyl-4,5-dihydro- 1,2-oxazole

[0190] 1 mole of sodium salt of 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]- l-methyl-3 -(trifluoromethyl)- lJT-Pyrazol-5-ol obtained in example 4, was taken in 32.91 mols of acetonitrile and stirred at 5°C to 8°C. To this mixture, was purged 1.1 mol of freon gas for 1 to 1.5 hours maintaining the reaction temperature at 5 °C -15°C. After completion of reaction, the solvent was evaporated, and to the mixture was added methylene di chloride, water and 30% hydrochloric acid. The mixture was stirred followed by separation of organic layer and aqueous layers. The organic layer was washed with brine and the layer was distilled to yield 95% of 3- ({[5- (difluoromethoxy)-l-methyl-3-(trifluoromethyl)-lH-pyrazol-4- yljmethyl} sulfanyl)-5, 5-dimethyl-4, 5-dihydro-l,2-oxazole.

Claims

We claim:

1. A continuous flow process for preparation of pyroxasulfone, said process comprising: oxidizing a compound of formula (II) in presence of an oxidizing agent, a metal catalyst and acetic acid.Formula (II)2. The process as claimed in claim 1, wherein the compound of formula (II), the oxidizing agent and the metal catalyst are present in a stoichiometric ratio in a range of about 0.5-2:2-6:0.01-0.1 respectively.

3. The process as claimed in claim 1, wherein the oxidizing agent and the metal catalyst are fed to a continuous flow reactor as aqueous solutions.

4. The process as claimed in claim 1, wherein the compound of formula (II) is fed to a continuous flow reactor as a solution in acetic acid.

5. The process as claimed in claim 4, wherein the amount of acetic acid used is in a range from about 1 mole to about 30 moles with respect to the compound of formula (II).

6. The process as claimed in claim 1, wherein said process is carried out at a temperature ranging from about 0°C to about 120°C.

7. The process as claimed in claim 6, wherein the process is carried out at a temperature ranging from about 90°C to about 110°C.

8. The process as claimed in claim 1, wherein said process is carried out without application of external heat.

9. The process as claimed in claim 1, wherein the compound of formula (II) as a solution in acetic acid, the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are introduced simultaneously to the continuous flow reactor.

10. The process as claimed in claim 1, wherein the compound of formula (II) as a solution in acetic acid and the aqueous solution of the oxidizing agent are first introduced to the continuous flow reactor and allowed to react; followed by introduction of the aqueous solution of the metal catalyst.

11. The process as claimed in claim 1, wherein the aqueous solution of the oxidizing agent and the aqueous solution of the metal catalyst are first introduced to the continuous flow reactor and allowed to react; followed by introduction of the compound of formula (II) as a solution in acetic acid.

12. The process as claimed in claim 1, wherein pyroxasulfone obtained contains monoxide compound of formula (III)Formula (III) in an amount less than or equal to about 0.5% w / w.

13. A continuous process for preparation of 4-[[(4,5-dihydro-5,5-dimethyl-3- isoxazolyl)thio]methyl]-l-methyl-3-(trifluoromethyl)-lJ / -Pyrazol-5-ol or salts thereof, the process comprising:a) producing stream 1 comprising 4,5-Dihydro-5,5-dimethyl-3-isoxazolyl carbamimidothioate or its salt, by reacting 3-halo-4,5-dihydro-5,5- dimethylisoxazole with acid addition salt of thiourea; b) producing stream 2 comprising 5,5-Dimethyl-3-isoxazolidinethione or its salt by reacting the stream 1 of step a) and a base; and c) reacting the stream 2 of step b) with 5-Hydroxy-l-methyl-3- (trifluorom ethyl)- IT / -pyrazole-4-methanol or its salt, to obtain stream 3 comprising 4-[[(4,5-dihydro-5,5-dimethyl-3-isoxazolyl)thio]methyl]-l- methyl-3 -(trifluoromethyl)- IT / -Pyrazol-5-ol or salts thereof.

14. The process as claimed in claim 13, wherein in said step a) 3-halo-4,5-dihydro- 5,5-dimethylisoxazole is introduced to a reactor as a solution in n-butyl acetate; and acid addition salt of thiourea is introduced to the reactor as an aqueous solution.

15. The process as claimed in claim 13, wherein content of 3-halo-4,5-dihydro- 5,5-dimethylisoxazole is ranging from 0.5 to 2 equivalents; and content of acid addition salt of thiourea is ranging from 0.5 to 2 equivalents.

16. The process as claimed in claim 13, wherein said step a) of the process is carried out at a temperature ranging from about 10°C to about 100°C.

17. The process as claimed in claim 13, wherein said stream 1 obtained in step a) is cooled to room temperature before directing it to step b) reaction.

18. The process as claimed in claim 13, wherein in said step b) the base is introduced to the reactor as an aqueous base.

19. The process as claimed in claim 18, wherein said base is selected from the group comprising sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and cesium carbonate.

20. The process as claimed in claim 13, wherein content of the base is about 1 to 10 equivalents.

21. The process as claimed in claim 13, wherein said step b) of the process is carried out at a temperature ranging from about 10°C to about 50°C.

22. The process as claimed in claim 13, wherein said step c) 5-Hydroxy-l-methyl-3 -(trifluoromethyl)- l / / -pyrazole-4-m ethanol or its salt, is introduced to the reactor as an aqueous solution.

23. The process as claimed in claim 22, wherein the content of 5-Hydroxy-l- methyl-3 -(trifluoromethyl)- l / / -pyrazole-4-methanol or its salt, is ranging from about 0.5 to 2 equivalents.

24. The process as claimed in claim 13, wherein said step c) of the process is carried out at a temperature ranging from about 10°C to about 50°C.

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