Description:FORM 2
THE PATENT ACT 1970
(39 of 1970)
&
THE PATENT RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)

“AN IMPROVED PROCESS FOR PREPARATION OF 5-AMINO SUBSTITUTED PHENYLTRIAZOLONE”
This patent application is a modification of the invention filed in Indian patent application No. 202111043830 filed on 28 September 2021.

SRF LIMITED, AN INDIAN COMPANY,
SECTOR 45, BLOCK-C, UNICREST BUILDING,
GURGAON – 122003,
HARYANA (INDIA)

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention provides a process for preparation of 5-amino substituted phenyltriazolone of formula I.

Formula I
wherein X is selected from halogens.

BACKGROUND OF THE INVENTION
The present invention provides a process for preparing 5-amino substituted phenyltriazolone, specifically 2-(5-amino-2,4-dichlorophenyl)-4-(difluoromethyl)-5-methyl-2,4-dihydro-3H-1,2,4-triazol-3-one, a key intermediate for preparing herbicide like sulfentrazone.
There are several methods known in the art for preparation of 5-amino substituted phenyltriazolone derivatives.
Chinese patent application No. 109748878 discloses a process for preparation of 2-(2,4-dichloro-5-aminophenyl)-4-(difluoromethyl)-2,4-dihydro-5-methyl-3H-1,2,4-triazole-3-one by reductive hydrolysis of 1-(2,4-dichloro-5-azidobenzene)-4-difluoromethyl-2,4-dihydro-5-methyl-3H-1,2,4-triazol-3-one using methanol and Pd/C catalyst.
Chinese patent application No. 104326992 discloses a preparation of 1-(5-amino-2,4-dichlorophenyl)-4-difluoromethyl-3-methyl-1H-1,2,4-triazolin-5-one by reacting 1-(5-nitro-2,4-dichlorophenyl)-4-difluoromethyl-3-methyl-1H-1,2,4-triazolin-5-one with Pd/C and methanol at 0.5MPa and 60? for 2 hours. The patent application condemns the use of Raney Nickel as a hydrogenation catalyst for safety reasons.
Indian Patent Application No. 202111043830 filed by the applicant discloses a preparation of 1-(5-amino-2,4-dichlorophenyl)-4-difluoromethyl-3-methyl-1H-1,2,4-triazolin-5-one by reacting 1-(5-nitro-2,4-dichlorophenyl)-4-difluoromethyl-3-methyl-1H-1,2,4-triazolin-5-one using transition metal catalyst selected from palladium on carbon, platinum on carbon and Raney nickel or the like in a continuous flow reactor.
The present invention provides an improved process for preparation of 5-amino substituted phenyltriazolone derivatives with higher selectivity and cost effectiveness.

OBJECT OF THE INVENTION
The main object of the present invention is to provide a simple, safe, and commercially viable process for preparation of 5-amino substituted phenyltriazolone derivatives in good yield and selectivity.

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

Formula I
wherein X is selected from hydrogen or halogens,
comprising a step of hydrogenating 5-nitro substituted phenyltriazolone of formula II,

Formula II
wherein X is selected from hydrogen or halogens,
using doped metal catalyst in a solvent to obtain the compound of formula I.

DETAILED DESCRIPTION OF THE INVENTION
In another embodiment of the present invention, the doped metal catalyst refers to Raney nickel with a metal oxide selected from iron oxide, copper oxide, zinc oxide, manganese oxide, and aluminum oxide or the like.
In another embodiment, the hydrogenation is performed using doped metal catalyst that contains nickel in the range from 70-95%.
In another embodiment of the present invention, the molar ratio of Raney nickel catalyst w.r.t. formula II is in the range from 0.1-0.2 and preferably between 0.01-0.1.
In another embodiment of the present invention, the metal oxide is used in the range from 0.01-0.5 equivalent w.r.t. formula II, preferably 0.01-0.2 equivalent.
In a preferred embodiment, the hydrogenation is performed using Raney nickel that contains nickel and a metal oxide selected from a group consisting of iron oxide, copper oxide, zinc oxide, manganese oxide, and aluminum oxide or like, in the ratio of 0.2:1 to 1: 5, more preferably 1:1 to 1:3.
In another embodiment of the present invention, the solvent is selected from a group consisting of water, an alcohol such as methanol, ethanol, propanol, isopropyl alcohol, butanol, and tert-butanol or the like, an ester such as methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, iso propyl acetate, n-propyl acetate, n-butyl acetate, propyl formate, tetrahydrofuran, 1,4-dioxane, cyclohexane and a mixture thereof.
In another embodiment, the present invention provides a compound of formula I with 0 to 0.1% of phenolic impurity.
In an embodiment, the solution of compound of formula II in a solvent is contacted with hydrogen gas pressure of 0.1 bar to 10 bar, preferably 5 to 8 bar.
In an embodiment of the present invention, the reduction is carried out either in batch mode or in continuous mode.
In an embodiment of the present invention, the reduction in continuous mode is carried out by contacting a solution of compound of formula II in solvent and hydrogen gas over the bed of catalyst.
The solution of compound of formula II in a solvent and hydrogen gas are purged continuously through catalytic fixed bed.
In an embodiment of the present invention, the step of reduction is carried out at a temperature of 10-80°C, preferably at a temperature of 30-60°C.
In preferred embodiment of the present invention, the step of reduction is carried out at a temperature of 30-60°C.
The present invention for preparation of the compound of formula I has following advantages over the known methods:
1. The use of mild reaction condition in form of using low temperature range prevents degradation of product and improves yield significantly.
2. The process provides better selectivity, reduces the time of reaction, and eliminates the formation of phenolic impurity.
3. The process enables efficient recovery and recycling of the metal catalyst, thereby making the process cost effective at commercial scale.
In another embodiment, the solvent used in the reaction are recovered, recycled, and reused.
In another embodiment, hydrogenation of formula II using doped metal in presence of solvent to obtain a compound of formula I is carried out in a batch mode.
The purity of isolated compound of formula I is greater than 90% and preferably greater than 95% and more preferably in the range of 99-99.9%.
The yield of compound of formula I is greater than 85% and preferably in the range of 90-98%.
As used herein, the compound of formula II may be prepared by any method known in the art or may be obtained commercially.
The compound of formula I is isolated by any method known in the art, for example, chemical separation, extraction, acid-base neutralization, distillation, evaporation, column chromatography and filtration or a mixture thereof.
The completion of the reaction may be monitored by any one of chromatographic techniques such as thin layer chromatography (TLC), high pressure liquid chromatography (HPLC), ultra-pressure liquid chromatography (UPLC), Gas chromatography (GC), liquid chromatography (LC) and alike.
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: Preparation of 2-(5-amino-2,4-dichlorophenyl)-4-(difluoromethyl)-5-methyl-2,4-dihydro-3H-1,2,4-triazol-3-one via reduction using Nickel catalyst in methanol solvent
2-(5-Nitro-2,4-dichlorophenyl)-4-(difluoromethyl)-5-methyl-2,4-dihydro-3H-1,2,4-triazol-3-one (53g) in methanol (672g) was charged in a pressure reactor at 40-45°C and added with nickel catalyst (2.38g). The reactor was flushed with nitrogen gas and followed by hydrogen gas. The mixture was heated to 50°C and pressurized the reactor with hydrogen gas to 10 bar for 8-10 hours. The progress of the reaction was monitored by HPLC. After completion of reaction, the reaction mixture was cooled to 30-35°C and filtered (using hi-flow bed) to remove the nickel catalyst. The spent catalyst was first washed with methanol, then with water to leave spent catalyst in wet form. The combined filtrate was distilled, where first 80% methanol was distilled out atmospherically at 60-70°C. Toluene (300g) was added to distillation bottom mass and distilled the mass to remove remaining methanol as toluene and methanol azeotrope (12: 88; Toluene: MeOH) at 63-64°C vapour temperature, followed by removal of toluene and water azeotrope at 83-84°C to obtain the titled product (95%).

Comparative Example: Preparation of 2-(5-amino-2,4-dichlorophenyl)-4-(difluoromethyl)-5-methyl-2,4-dihydro-3H-1,2,4-triazol-3-one via reduction using Nickel + Iron oxide catalyst in methanol solvent
2-(5-Nitro-2,4-dichlorophenyl)-4-(difluoromethyl)-5-methyl-2,4-dihydro-3H-1,2,4 -triazol-3-one (53g) in methanol (672g) was charged in a pressure reactor at 40-45°C and added with Doped Nickel catalyst (0.6g nickel catalyst and 1.8 g iron oxide). The reactor was flushed with nitrogen gas and followed by hydrogen gas. The mixture was heated to 50°C and pressurized the reactor with hydrogen gas up to 10 bar for 4 hours. The progress of reaction was monitored by HPLC. After completion of reaction, the reaction mixture was cooled to 30-35°C and filtered (using hi-flow bed) to remove the nickel catalyst. The spent catalyst was first washed with methanol, then with water to leave spent catalyst in wet form. The combined filtrate was distilled, where first 80% methanol was distilled out atmospherically at 60-70°C re-boiler temp. Toluene (300g) was added to distillation bottom mass and distilled to remove remaining methanol as toluene and methanol azeotrope (12: 88; Toluene: MeOH) at 63-64°C vapour temperature followed by removal of toluene and water azeotrope at 83-84°C to obtain the titled product (97-98%).
Advantage of the process:
1. In presence of doping metal salt decreased the nickel loading drastically. Nickel is costlier than iron. Using iron along with nickel will reduce cost impact.
2. The reaction is faster compared to the reaction run with just nickel, which will save time.
, Claims:WE CLAIM:
1. A process for preparation of a compound of formula I,

Formula I
wherein X is selected from halogens,
comprising a step of hydrogenating 5-nitro substituted phenyltriazolone of formula II,

Formula II
wherein X is selected from halogens,
using doped metal catalyst in a solvent to obtain the compound of formula I.
2. The process as claimed in claim 1, wherein the doped metal catalyst refers to Raney nickel with a metal oxide selected from iron oxide, copper oxide, zinc oxide, manganese oxide, and aluminium oxide.
3. The process as claimed in claim 1, wherein the hydrogenation is carried out using the catalyst containing nickel in the range from 70-95%.
4. The process as claimed in claim 1, wherein the solution of compound of formula II in a solvent and hydrogen gas are purged continuously through catalytic fixed bed.
5. The process as claimed in claim 1, wherein the solvent is selected from a group consisting of water, methanol, ethanol, propanol, isopropyl alcohol, butanol, tert-butanol, methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, isopropyl acetate, n-propyl acetate, n-butyl acetate, propyl formate, tetrahydrofuran, 1,4-dioxane, cyclohexane and a mixture thereof.
6. The process as claimed in claim 1, wherein the process is carried out in a batch mode.
7. The process as claimed in claim 1, wherein the solvent and catalyst are recovered, recycled, and reused.

Dated this 15th day of December 2022.