A process for producing N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide

Abstract: The present invention relates to a process for making N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide, which process includes the steps of reacting 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4,-thiadiazole with N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide in an aprotic, aromatic solvent with aqueous alkali to form an aqueous phase and an organic phase, separating the aqueous and organic phases and recovering the N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide from the organic phase. A preferred solvent and aqueous alkali are toluene and aqueous sodium hydroxide, respectively.

Description: Technical Field of the Invention The field of this invention is the synthesis of acetamide herbicides. More particularly, this invention relates to processes for producing N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide. BACKGROUND OF THE INVENTION Certain heteroaryloxy-carboxylic acid amides and heteroaryloxyacetamides of the general formula R-O-CH(R 1 )-CO-N(R 2 )(R 3 ) are known to have herbicidal activity (See, e.g., United States Patent Nos. 4,756,741 and 5,101,034). United States Patent No. 5,101,034 discloses a particular class of heteroaryloxyacetamides, namely thiadiazole acetamides as having herbicidal activity. The thiadiazole acetamides are made by reacting a thiadiazole sulfone with a hydroxyacetanilide in acetone. Of particular relevance to the present invention is the disclosure of a synthetic scheme for making N-methyl-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetanilide. In accordance with that synthetic scheme, 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole is reacted with N-methyl-2-hydroxyacetanilide, potassium carbonate, and tetraethylammonium bromide. Acetone is used as the solvent for the reaction. The reaction is carried out at a temperature of 20°C - 25°C for 20 hours. Undissolved salts are filtered off and washed with acetone. The filtrate is freed of solvent in vacuo and the resulting residue taken up in diethyl ether, washed with dilute hydrochloric acid, dried and filtered. After freeing the filtrate of solvent, the end product is crystallized from the oily residue. Reported yields are about 90%. United States Patent Nos. 4,756,741 and 4,645,525 also disclose a synthetic method for making N-methyl-2-[(5-(trifluoromethyl)-1,3,4,-thiadiazol-2-yl)-oxy]-acetanilide. In accordance with that method, N-methyl-2-hydroxyacetanilide is reacted with dimethylsulfoxide and calcium oxide at 50°C for 1 hour. 5-Bromo-2-(trifluoromethyl)-1,3,4-thiadiazole is then added to the reaction mixture and the mixture stirred at 50°C for 40 hours. The mixture is then poured into water and the oil that precipitates is extracted with methylene chloride. The end product is obtained in about 90% yield by distilling off the methylene chloride. United States Patent No. 4,585,471 discloses a synthetic scheme and mrthods for making 2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetic acid 2-ethylpiperidide and 2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetic acid 2-methylpiperidide. In accordance with those synthetic methods, the ethylpiperidide compound is made by reacting hydroxyacetic acid-2-ethylpiperidide with 2-chloro-5-(trifluoromethyl)-1,3,4-thiadiazole in the presence of potassium tert -butanolate in tert -butanol at a temperature of 20°C to 30°C for 3 hours and the methylpiperidide compound is made by reacting 2-chloro-5-(trifluoromethyl)-1,3,4-thiadiazole in toluene with hydroxyacetic acid 2-methylpiperidide in the presence of sodium hydroxide. The end product, in both cases, is recovered from the reaction mixture by acidification with hydrochloric acid, drying, removal of solvent and crystallization. The reported yields of the end-products were 66% (2-ethylpiperidide) and 54% (2-methylpiperidide). United States Patent Nos. 4,968,342 and 5,090,991 disclose a synthetic method for making N-(3-chlorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4,-thiadiazol-2-yl)-oxy]-acetamide. In accordance with that method, 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole, dissolved in acetone, is reacted with N-(3-chlorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide in the presence of sodium hydroxide and water for 3 hours at -20°C. Water is added to the reaction mixture and the crystalline end-product is obtained by crystallization in 85% yield. It can be seen from the above, that existing methods for making acetamide herbicides suffer from low yields (54% to 85%), prolonged reaction times (20 to 40 hours) or the use of problematic solvents (acetone). There continues to be a need in the art, therefore, for a practical method of making these herbicides, which method avoids the problems of the existing art. EP-A 300 344 disclose the preparation of compounds similar to N(4-(fluorophenyl)-N-(1-methylethyl)-2-[5-(trifluoromethyl)-1,3,4-thiadiazol)-2-yl)-oxy-acetamide in a water/toluene two-phase reaction system. However, this process cannot be successfully employed for the preparation of the title compound in high yields. BRIEF SUMMARY OF THE INVENTION The present invention provides a process for making N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide. The process includes the steps of reacting 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole ("TDA-sulfone") with N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxy-acetamide in the presence of an aprotic, aromatic solvent and an aqueous alkali to form an aqueous phase and an organic phase, separating the aqueous and organic phases, acidifying the organic phase and recovering the N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide from the organic phase. Exemplary and preferred aprotic, aromatic solvents are toluene, cumene, xylene or mesitylene. Toluene is most preferred. The alkali is preferably an alkali metal hydroxide or alkali metal carbonate. A preferred alkali metal is sodium, potassium or lithium. A preferred alkali metal hydroxide is sodium hydroxide. The pH of the aqueous phase is from about 11 to about 14, preferably from about 12 to 14. N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl-1,3,4-thiadiazol-2-yl)-oxy]-acetamide is recovered from the organic phase by acidifying the organic phase after separation from the aqueous phase and removing the solvent from the acidified organic phase. DETAILED DESCRIPTION OF THE INVENTION I. The Invention The present invention relates to process for the synthesis, recovery and isolation of the herbicide, N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy)-acetamide. The synthetic process includes the step of reacting 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole with N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide in an aprotic, aromatic solvent and in the presence of an aqueous alkali to form two phases followed by acidification of the organic phase after separation from the aqueous phase. Formed N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide can be recovered via phase separation followed by acidification of the organic phase. By following such a protocol it is also possible to use a lower quality TDA-sulfone because the impurities therein are retained in the alkaline aqueous phase. Also, because most of the by-products of the reaction are acidic in nature, they remain in the alkaline aqueous phase. Isolation is accomplished by removal of the solvent and flaking of the molten residue. II. Method of Making N-(4-fluorophenyl)-N-(1-methylethyl)-2 - [ (5 -(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide Using an Aprotic Aromatic Solvent The process for making N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide includes the step of reacting 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole with N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide in an aprotic, aromatic solvent. The solvent is preferably toluene, xylene, cumene or mesitylene and, most preferably toluene. The 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole ("TDA-sulfone") and the N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide ("FOE-hydroxy") used in the present process can be made using any method. Preferably the two reactants being dissolved in the aprotic, aromatic solvent used in the present process. Some processes for making TDA-sulfone can be found in United States Patents Application with Serial Number 08/989,568 published as EP-A 926 143 and U.S. Patent No. 5,856,499. Preferred means for making FOE-hydroxy can be found in United States Patents No. 5,808,153 and 5,808,152. The molar ratio of 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole to N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide is from about 1.5:1 to about 1:1.5 and more preferably about 1:1. Where the TDA-sulfone and FOE-hydroxy are not provided in the solvent, the solvent is added to the reaction mixture. The molar ratio of solvent (e.g., toluene) to either TDA-sulfone or FOE-hydroxy is from about 1:1 to about 5:1. The solvent is preferably present in a molar excess relative to TDA-sulfone and FOE-hydroxy. The reaction of TDA-sulfone and FOE-hydroxy typically occurs at a relatively cool temperature. Preferably the reaction temperature is from about -10°C to about 30°C. More preferably, the temperature is from about -5°C to about 15°C and, most preferably from about 0°C to about 10°C The reaction occurs in the presence of an aqueous alkali. Exemplary and preferred aqueous alkalis are aqueous alkali metal hydroxides or carbonates. Alkali metal hydroxides and carbonates are well-known in the art. Exemplary and preferred alkali metals are potassium, sodium and lithium. Sodium is most preferred. The molar ratio of the aqueous alkali (e.g., sodium hydroxide) to the primary reactants (e.g., TDA-sulfone and FOE-hydroxy) is from about 1:1 to about 2:1 and, preferably from about 1.25:1 to about 1.75:1. The aqueous alkali is added to the reaction mixture as an aqueous solution of the hydroxide or carbonate. Preferably the alkali metal hydroxide or carbonate concentration of the solution is from about 15 weight percent to about 60 weight percent. More preferably, the concentration is from about 15 weight percent to about 50 weight percent. The aqueous alkali can be added at a single time or added in portions over a prolonged period of time. The mixture of TDA-sulfone and FOE-hydroxy is agitated during the entire time over which the alkali is added. A sufficient amount of alkali is used so as to maintain the pH of the reaction mixture at a pH value of from about 10 to about 14. In an especially preferred embodiment, the process includes the steps of adding 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole in toluene and N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide in toluene to a reaction vessel to form a reaction mixture, cooling the reaction mixture to about 0°C-5°C, adding an aqueous solution of sodium hydroxide to the reaction mixture slowly over 0 to 3 hours and maintaining the reaction mixture at a temperature of from about 0°C to about 5°C for a period of time ranging from about 0 hour to about 3 hours. The process can further include the step of recovering the N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide. Recovery can be accomplished using any well known recovery method. Recovery is accomplished as set forth below. III. Recovery of N - (4-fluorophenyl)-N-(1-methylethyl) -2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide via Acidification After Phase Separation In another aspect, the present invention provides a process for recovering N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide from a reaction mixture. The recovery process can be used to recover N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide produced by any means. However formed, the present recovery process begins with N-(4-fluorophenyl)-N-( 1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide dissolved or suspended in a reaction mixture having an organic phase ( e.g., an aprotic, aromatic solvent such as toluene) and an alkaline aqueous phase. In a preferred embodiment, N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide is made by the process of Section II, above). The aqueous and organic phases are separated and the N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide is recovered from the organic phase. The organic phase containing the N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide is acidified prior to removal of solvent. Acidification is accomplished by adding an acid to the organic phase. Suitable acids, for example, are hydrochloric acid, nitric acid and sulfuric acid. Sulfuric acid is preferred. N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide can be recovered from the acidified organic phase using any means well known in the art. Exemplary such means include crystallization, flaking and filtration after removal of solvent. A preferred means of isolation is flaking as described below. As set forth hereinafter in the Examples, recovery using acidification results in yields of greater than about 90%. In an especially preferred embodiment, N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide is made and recovered according to a process that includes the steps of adding 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole in toluene and N-(4-fluorophenyl)-N-(1-methylethyl)-2-hydroxyacetamide in toluene to a reaction vessel to form a reaction mixture, cooling the reaction mixture to about 0°C-5°C, adding an aqueous solution of sodium hydroxide to the reaction mixture and maintaining the reaction mixture at a temperature of from about 0°C to about 15°C for a period of time ranging from about 0 to about 3 hours to form a reaction product having an aqueous and an organic phase, separating the aqueous and organic phases, adding sulfuric acid to the organic phase and recovering the N-(4-fluorophenyl)-N-(1 -methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide from the organic phase. IV. Isolation of N - (4-fluorophanyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazole-2-yl)-oxy]-acetamide via Flaking In another aspect, the present invention provides a process for isolating N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide. In accordance with this process, N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide, obtained by any means, is in a solvent such as toluene. The solvent is removed via evaporation and the solid product isolated from its molten form via a flaking operation. As used herein, the term "flaking" means, as is well known in the art, solidification or crystallization. Solvent can be removed from the solution using any means well known in the art. A preferred means of solvent removal is vacuum or steam distillation. The molten material is then spread out in a thin layer on a solid surface, cooled to a temperature of from about 20°C to about 30°C (room temperature) and maintained at that temperature until crystallization. The product crystallizes as aggregates of flakes on the solid surface. The Examples to follow illustrate preferred embodiments of the present invention and are not limiting of the claims and specification in any way. Example 1 Synthesis of N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide (Fluthiamide). FOE-hydroxy ("FOEH"), TDA-sulfone ("TDAS") and toluene were added to a reaction vessel. NaOH was slowly added over time to the vessel. The reaction mixture was agitated over a period of time. The reaction conditions are set forth below. Toluene/FOEH Mole Ratio 5.90-6.60 TDAS/FOEH Mole Ratio 1,00-1.15 NaOH/FOEH Mole Ratio 1.00-1.75 NaOH Concentration (wt. percent) 15-60 NaOH Add Time 1-3 hours Reaction Temperature -10 to 30°C Reaction Time 0-3 hours Example 2 Recovery of Fluthiamide After the primary reaction (see Example 1), the temperature was adjusted to the desired level. Three phases were formed: a bottom phase - NaOH aqueous (discarded); a middle phase - rag (retained for treatment); and a top phase - Fluthiamide in toluene. The phases were separated in accordance with the conditions summarized as follows. The rag phase was retained for further treatment as set forth below. Process water and sulfuric acid were charged to this phase to reduce the pH. The resulting acidified mixture was heated to the desired temperature and filtered at that temperature. The phase separation conditions are summarized as follows. H 2 O/Rag Wt. Ratio 0.50-1.50 Phase Separation pH 1.0-4.0 Phase Separation Temperature 50-70°C Sulfuric acid (70% weight) was added to the Fluthiamide/Toluene, phase to reduce the pH of that phase to 1.5. This acidified phase was retained for Fluthiamide isolation. Fluthiamide was isolated using two different methods (Examples 3 and 4). The amount of the rag can be decreased by using lower temperatures. Example 3 Isolation of Fluthiamide via Solidification The organic phase from Example 2 was used for isolation of Fluthiamide. Toluene was removed by an atmospheric steam strip. After removal of the toluene, the Fluthiamide was left in a molten Fluthiamide/aqueous phase mixture. The aqueous phase was removed and the Fluthiamide was obtained by solidification of the organic phase. When this isolation method was used, the Fluthiamide was produced with an average purity of 96.2%. Example 4 Synthesis of Fluthiamide. A 2 liter, 4-necked round bottom flask containing 632.2 g of a TDA-sulfone suspension in toluene (1.0 M of ca. 99% A.I. TDA-sulfone, on a solvent-free basis) was fitted with an overhead stirrer, thermometer, 250 ml barostatic addition funnel and a dry ice acetone bath. The toluene TDA-sulfone slurry was cooled to 0°C with agitation. 415.7 g of a FOE-hydroxy solution in toluene (1.0 M of ca. 98% A.I. FOE-hydroxy on a solvent-free basis) was added to the TDA-sulfone slurry in toluene with agitation at 0-5°C. The amount of toluene required for the reaction can be reduced by about 50% if solid TDA-sulfone is used instead of a TDA-sulfone suspension in toluene. A 25% NaOH solution was added to the reaction mixture over a period of 1 to 5 hours at 0 to 5°C under constant agitation. The addition of NaOH can be done over various times without adverse results. The entire mixture was cooked for 1 hr at 0-5°C. The mixture was transferred to a separatory funnel and the phases were separated at 0-10°C. The aqueous phase was extracted with two portions of toluene (about 50 g each portion). The organic phases were all combined and treated with about 1 g of conc. HCI or 1 g of conc. H 2 SO 4 (pH 4.5). The solvent toluene was stripped by means of a rotary evaporator under water aspirator vacuum (ca. 20mm Hg) using a maximum bath temperature of 80°C. The molten product was poured onto an enamel pan and allowed to flake. The flaked material was crushed and dried under vacuum at ambient temperature. Fluthiamide was produced with a purity of 98.2% and a net yield (N.Y.) of 93.2%, based on TDA-Sulfone. In additional studies, the effects of reaction temperature and FOE-hydroxy purity on fluthiamide production were tested. The procedures used were the same as set forth above. The results of these studies are summarized in the tables, below: Exper # Rxn Temp (°C) Fluthiamide A.I. (%) Net Yield based on FOE-Chloride (%) 1 0-5 98.2 95.5 2 5-10 98 95.1 3 10-15 98.1 94.7 4 15-20 97.8 94 4 20-30 96.8 93.6 Exper#" = Experiment No.; "Rxn Temp" = Reaction Temperature; FOE-Chloride" = N-(4-fluorophenyl)-N-(1-methylethyl)-2-chloroacetamide (the precursor to FOE-hydroxy) It can be seen from the data in the table, above, that fluthiamide purity and net yield decreased with increasing reaction temperatures over the range of 0°C to 30°C. Exper # FOE-Hydroxy A.I. (%) Fluthiamide A.I. (%) Net Yield based on FOE-Chloride (%) 1 98.2 98.2 95.5 2 97.3 96.8 94.8 3 96.4 95.5 93.5 4 95.2 95.1 93.4 5 94.8 94.3 93.2 The data in the table above shows that fluthiamide purity and yield are directly proportional to the purity of FOE-hydroxy used in the reaction.

Claims: A process for making N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide comprising reacting 2-(methylsulfonyl)-5-(trifluoromethyl)-1,3,4-thiadiazole with N-(4-fluorophenyl)-N(1-methylethyl)-2-hydroxyacetamide in the presence of such an amount of an aqueous alkali that a pH of the reaction mixture of from about 10 to about 14 is maintained and in the presence of an aprotic, aromatic solvent to form an aqueous phase and an organic phase, separating the aqueous and organic phases, acidifying the organic phase and isolating the N-(4-fluorophenyl)-N-(1-methylethyl)-2-[(5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)-oxy]-acetamide from the organic phase. The process of Claim 1, wherein the aprotic, aromatic solvent is toluene, cumene, xylene or mesitylene. The process of Claim 2, wherein the solvent is toluene. The process of Claim 1, wherein the aqueous alkali is an aqueous alkali metal hydroxide or an aqueous alkali metal carbonate. The process of Claim 4, wherein the alkali metal is sodium, potassium of lithium. The process of Claim 5, wherein the alkali metal hydroxide is sodium hydroxide. The process of Claim 1, wherein the aqueous alkali is an aqueous solution of from about 15 weight percent to about 50 weight percent sodium hydroxide. The process of Claim 1, wherein recovery is performed by removing the solvent from the acidified organic phase followed by solidification of the molten product.

Inventors: APPLEGATE JACQUELINE M DR, JELICH KLAUS DR, PRASAD VIDYANATHA A, WASLESKI DANIEL M, APPLEGATE, JACQUELINE M. DR., JELICH, KLAUS DR., PRASAD, VIDYANATHA A., WASLESKI, DANIEL M.
Applicants: BAYER AG, BAYER CORPORATION
Application year: 1998
Main classification: C07D285
IPC codes: C07D285, A01N43