Process for making 2-(methylthio)-5(trifluoromethyl)-1,3,4,-thiadiazole using methyldithiocarbazinate with trifluoroacetic acid with selective removal of 2,5-bis(methylthio)-1,3,4-thiadiazole

Abstract

The present invention provides a process for making 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole. The process includes the steps of reacting methyldithiocarbazinate with trifluoroacetic acid to form a mixture of 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole and 2,5-bis-(methylthio)-1,3,4-thiadiazole and selectively removing the 2,5-bis-(methylthio)-1,3,4-thiadiazole by acidifying the mixture.

Description

TECHNICAL FIELD OF THE INVENTION

The field of this invention is the synthesis of thiadiazoles. More particularly, this invention pertains to improved processes for making 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole using trifluoroacetic acid and methyldithiocarbazinate.

BACKGROUND OF THE INVENTION

Existing methods for preparing 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole are limited by the excessive cost of commercial scale application of laboratory procedures. Many existing reports are based on laboratory studies and thus present little information on how reaction temperatures and particular reactants would affect product yield or purity. In addition, the use of procedures and reactions developed in the laboratory cannot be directly applied to commercial scale production because such laboratory procedures typically involve the use of expensive reactants and or expensive (e.g., separation and purification procedures) techniques.

U.S. Pat. No. 3,562,284 discloses a process for making certain 2-(alkylthio)-5-(halogenoalkyl)-1,3,4-thiadiazoles such as 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazoles wherein methyldithiocarbazinate is reacted with a carboxylic anhydride (e.g., trifluoroacetic anhydride) or with a carboxylic acid (e.g., trifluoroacetic acid) in a solvent (e.g., toluene). The reaction can occur in the presence of phosphorous trichloride and pyridine with added sulfuric acid (DE-A-3,422,861) or with carbonyl chlorides (e.g., trifluoroacetyl chloride) and diethylene glycol dimethyl ether, as well as with pyridine and sulfuric acid (DE-A-3,722,320). The first-mentioned method is poorly suited for commercial, large-scale production because the reactants (anhydrides) are expensive and they are used in excess. In addition, by using an anhydride, only half of the reaction moiety is utilized. The reaction with carboxylic acids, phosphorus trichloride, pyridine, sulfuric acid and carbonyl chlorides requires an extensive work-up process in which the pyridine is separated off and recovered. Further, phosphorus trichloride forms only sparingly soluble reaction products, which makes mixing difficult. Finally, the yields realized from such processes are unacceptably low.

Other procedures for making a 2-(substituted)-5-(trifluoromethyl)-1,3,4-thiadiazole involve the reaction of a carboxylic acid (e.g., trifluoroacetic acid) and a dithiocarbazic ester in the presence of a phosphorylchloride or polyphosphoric acid. (See, e.g., U.S. Pat. No. 5,162,539 and Gyoefi and Csavassy, Acta Chimica Academiae Scientiarum Hungaricae, Tomus 82 (1), (91-97, 1974). The use of such phosphorous compounds results in the formation of waste products containing unacceptably high levels of phosphates and, thus, creates an environmental hazard. Still further, this method requires the use of dry methyldithiocarbazinates (a toxic convulsant). In the dry state this material creates a severe industrial hygiene problem.

There is a need in the art, therefore, for an efficient, high yield, practical, safe method for the commercial, large-scale production of 2-(methylthio)-5- (trifluoromethyl)-1,3,4 thiadiazole. The present invention provides such a process.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process for preparing 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole. The process includes the steps of reacting methyldithiocarbazinate with trifluoroacetic acid to form a mixture of 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole and 2,5-bis-(methylthio)-1,3,4-thiadiazole, and then selectively removing the 2,5-bis-(methylthio)-1,3,4-thiadiazole (bis by-product) by acidifying the mixture subsequent to removal of excess trifluoroacetic acid.

The mixture is acidified with a concentrated inorganic acid such as hydrochloric acid, sulfuric acid or nitric acid. The use of sulfuric acid is preferred. The sulfuric acid has a concentration of from about 55% to about 95% and, preferably about 70%. Where 70% sulfuric acid is used, the amount of sulfuric acid added to the reaction mixture is from about 2 moles to about 10 moles of sulfuric acid per mole of the 2,5-bis-(methylthio)-1,3,4-thiadiazole and, preferably from about 4 moles to about 7 moles of sulfuric acid per mole of the 2,5-bis-(methylthio)-1,3,4-thiadiazole.

Acidification typically occurs at a temperature of from about 10.degree. C. to about 60.degree. C., preferably from about 20.degree. C. to about 40.degree. C. and, more preferably from about 25.degree. C. to about 30.degree. C.

The reaction of methyldithiocarbazinate with trifluoroacetic acid can occur in the presence of a solvent. The solvent can be the trifluoroacetic acid itself or an aromatic solvent such as toluene, xylene, cumene or mesitylene. Toluene is preferred.

Any suitable ratio of methyldithiocarbazinate and trifluoroacetic acid can be used. Either reactant can be present in a molar excess. Thus the molar ratio of methyldithiocarbazinate to trifluoroacetic acid can range from about 4:1 to about 1:5. Where methyldithiocarbazinate is present in a molar excess, a preferred molar ratio of methyldithiocarbazinate to trifluoroacetic acid is from about 2:1 to about 1.5:1. Where trifluoroacetic acid is present in a molar excess, a preferred molar ratio of methyldithiocarbazinate to trifluoroacetic acid is from about 1:1.25 to about 1:2.

DETAILED DESCRIPTION OF THE INVENTION

I. The Invention

The present invention provides novel processes for preparing 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole (TDA), an intermediate useful in the preparation of herbicides. The novel processes of this invention use methyldithiocarbazinate (MDTC) and trifluoroacetic acid (TFA) as the primary reactants. The processes allow for production of TDA in high yields with efficient means for removing by-products and recycling key reagents.

II. Process Using Excess Trifluoroacetic Acid

In one aspect, a process of the present invention for preparing 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole includes the steps of reacting methyldithiocarbazinate in a solvent with an excess of trifluoroacetic acid, and removing water and excess trifluoroacetic acid.

MDTC prepared by any means can be used in the present process. Especially preferred means for preparing MDTC are disclosed in U.S. Pat. application Ser. Nos. 08,743,763, 08,743,764 and 08,743,775, all filed in Nov. 7, 1996. The disclosures of all those patent applications are incorporated herein by reference. TFA is commercially available.

MDTC is reacted with a molar excess of TFA. As used herein, the phrase "molar excess" means that the moles of TFA exceed the moles of MDTC in the reaction. TFA is preferably present at a 10 to 500 percent molar excess relative to MDTC. That is, the molar ratio of TFA to MDTC (TFA:MDTC) is from about 1.1:1 to about 5:1. More preferably, the TFA:MDTC molar ratio is from about 1.25:1 to about 2.5:1 and, even more preferably from about 1.25:1 to about 2:1. As shown hereinafter in the Examples, increasing the molar excess of TFA relative to MDTC significantly and substantially increases the yield of TDA.

The reaction preferably occurs at a temperature of from about 30.degree. C. to about 150.degree. C. and, more preferably from about 30.degree. C. to about 140.degree. C. Where the temperature is from about 80.degree. C. to about 130.degree. C., reaction time is from about 1 to about 5 hours.

The MDTC used in the present process can contain water. The ability to use "wet" MDTC offers a substantial benefit over existing processes that use only dry MDTC. MDTC is a known toxic substance and its use in dry form is likely to result in contamination of the air in processing plants with MDTC dust. This environmental hazard is substantially reduced where wet MDTC can be used. For use in the present process, MDTC can contain up to about 10 weight percent water.

Further, unlike existing processes, water can be added as a separate reactant. The total amount of water in the reaction mixture is preferably less than about 30 grams of water per 0.5 moles of MDTC. As shown hereinafter in the Examples, the presence of 30 or less grams of water per 0.5 moles of MDTC has no deleterious effect on product formation. Increasing the amount of water to 40 grams or more resulted in reductions in product (TDA) yield.

The reaction of TFA and MDTC occurs in the presence of a solvent. In one embodiment, the trifluoroacetic acid itself serves as the solvent. Preferably, however, an aprotic, aromatic co-solvent is used. Such co-solvents are well known in the art. Exemplary and preferred such co-solvents are toluene, xylene, cumene and mesitylene. Toluene is especially preferred.

The amount of co-solvent used can vary over a wide range as readily determined by a skilled artisan. Where a co-solvent is used, it is present in an amount of from about 0.5 moles to about 3.5 moles of toluene per mole of MDTC. Preferably, toluene is present in an amount of from about 1.5 moles to about 3.0 moles per mole of MDTC and, more preferably in an amount of from about 2.5 to about 3.0 moles of toluene per mole of MDTC.

The reaction can proceed by mixing the entire desired amounts of MDTC and TFA. All other modes of addition are suitable as well.

The reaction mixture of MDTC and TFA can optionally include a catalyst. An exemplary and preferred catalyst is p-toluene sulfonic acid. Where p-toluene sulfonic acid is used, it is present in an amount of about 2.0 grams per mole of MDTC.

Water is formed as a reaction product of the TFA and MDTC reaction. Additional water may also be present because of recycle streams. Water is removed from the reaction mixture by an azeotropic distillation. The azeotropic removal of water is readily accomplished in the presence of the solvent, particularly where toluene is used as a co-solvent. The temperature required for the completion of the reaction is adequate for the azeotropic removal of the water and the excess trifluoroacetic acid. Therefore, no additional work-up is required.

III. Removal of Bis-Thiadiazole

In another aspect, the present invention provides a process for preparing TDA, which process includes the steps of reacting MDTC with TFA to form a mixture of TDA and 2,5-bis-(methylthio)-1,3,4-thiadiazole, (the bis-byproduct) and then selectively removing the bis-byproduct by acidification of the reaction mixture followed by phase separation. Reaction temperatures for the reaction of MDTC and TFA are the same as set forth above.

Any suitable ratio of MDTC and TFA can be used. Either reactant can be present in a molar excess. Thus the molar ratio of MDTC to TFA can range from about 4:1 to about 1:5. Where MDTC is present in a molar excess, a preferred molar ratio of MDTC to TFA to is from about 2:1 to about 1.5:1. Where TFA is present in a molar excess, a preferred molar ratio of MDTC to TFA is from about 1:1.25 to about 1:2.0. The amount of the bis-byproduct produced from the reaction of MDTC and TFA decreases as the ratio of MDTC to TFA decreases.

As set forth above, the reaction of MDTC with TFA preferably occurs in the presence of a co-solvent. Preferred co-solvents are the same as set forth above. Toluene is most preferred.

The mixture is acidified with a concentrated inorganic acid such as hydrochloric acid (HCl), sulfuric acid (H.sub.2 SO.sub.4) or nitric acid (HNO.sub.3). Preferably, the acid has a pKa of from about 1 to about 4. The use of sulfuric acid is preferred. The sulfuric acid has a concentration of from about 55% to about 95% and, preferably about 70%. Where 70% sulfuric acid is used, the amount of sulfuric acid added to the reaction mixture is from about 2 moles to about 10 moles of sulfuric acid per mole of the bis-byproduct and, preferably from about 4 moles to about 7 moles of sulfuric acid per mole of the bis-byproduct. Acidification typically occurs at a temperature of from about 10.degree. C. to about 60.degree. C., preferably from about 20.degree. C. to about 40.degree. C. and, more preferably from about 25.degree. C. to about 30.degree. C.

The following Examples illustrate preferred embodiments of the present invention and are not limiting of the specification or claims in any way.

EXAMPLES

EXAMPLE 1

Production of 2-(methylthio)-5-(trifluoromethyl)-1.3,4-thiadiazole (TDA) Using MDTC and Excess TFA

A. General Protocol- Toluene (125 grams) was charged to a flask. 67.9 grams (0.5 moles) of methyldithiocarbazinate (MDTC)(90% A.I. with 5% water and 5% impurities) was added to the flask to form a mixture. Trifluoroacetic acid (TFA) (114 grams, 1.0 mole) was added to the mixture with agitation over 10 to 15 minutes without cooling. The temperature of the mixture rose to about 38.degree. C. upon TFA addition.

The mixture was heated to about 70.degree. C. and maintained at that temperature for about 3 hours. The mixture was then heated to reflux (about 115.degree. C.-116.degree. C.) to remove water and any distillable TFA. This temperature was maintained for about 10 minutes until no aqueous phase separates from the condensate. The yield of TDA was about 90% to 93%.

B. Effects of Excess TFA-The reaction of MDTC and TFA was carried out as set forth above in (A) except that the amount of TFA relative to MDTC was varied. TDA yields were determined at each TFA level. The results are summarized below in Table 1.

TABLE 1______________________________________Effect of TFA Excess on TDA Yield (2.7 moles toluene/mole MDTC) TFA Excess, Net Yield, % Bis-by-product % % (Solvent Free)______________________________________ 0 70.4 9.8 10 81.5 9.4 20 88.2 6.2 30 90.2 5.5 40 91.0 4.3 50 91.1 3.8 100 92.2 1.9 200 92.8 1.2______________________________________

It can be seen from the data in Table 1 that increasing the molar excess of TFA increased the yield of TDA. The greatest increases in TDA yield were seen when the molar excess of TFA increased from 10% to about 100%. Increases in the molar excess of TFA from about 100% to about 200% resulted in only small gains in TDA yield.

C. Effects of Toluene as a Solvent-TDA was prepared in accordance with paragraph (A) above except that the level of toluene relative to the level of MDTC was varied. For these studies, 2 moles of TFA were reacted with one mole of MDTC. Summary data are shown in Table 2, below.

TABLE 2______________________________________Effect of Toluene on TDA Yield 2.0 moles TFA/mole MDTC) Moles Toluene/Moles MDTC % TDA net yield based on MDTC______________________________________2.70 92.2 2.05 89.6 1.35 87.8 0.67 86.2______________________________________

The data in Table 2 show that TDA yield increases with increasing levels of toluene. TDA yield did not improve substantially when toluene levels exceeded about 2.7 moles per mole of MDTC.

D. Effects of Water Levels-Water can be expected in the primary reaction from two main sources. First, the MDTC used in the reaction can contain up to about 10 weight percent water. Second, water can be added to enhance the recovery of TFA. Therefore, the effect of water on TDA recovery was studied. For these studies, 2.0 moles of TFA were reacted with one mole of MDTC. 2.1 moles of toluene per mole of MDTC were used. Results of these studies are shown below in Table 3.

TABLE 3______________________________________Effect of Water on the TDA Yield gms water added (0.5M batch) TDA net yield % based on MDTC______________________________________ 0 92.0 10 91.8 20 91.9 30 91.6 35 89.2 40 88.7 50 83.7______________________________________

The data in Table 3 show that the presence of up to 60 grams of water per mole of MDTC in the reaction medium did not adversely affect TDA net yields. When 1.5 moles of TFA were reacted with one mole of MDTC, however, a perceptible drop in TDA net yields was noticed at water levels of 30-40 grams of water per mole of MDTC (See Table 4).

TABLE 4______________________________________Effect of Water on TDA Yield gms water added (0.5M batch) TDA net yield % based on MDTC______________________________________ 0 91.1 10 90.6 15 90.1 20 89.3 30 87.5 35 84.2 40 83.1______________________________________

EXAMPLE 2

Reduction of Bis-byproduct Levels

The main by-product that results from the reaction of MDTC with TFA is 2,5-bis-(methylthio)-1,3,4-thiadiazole (bis-byproduct). That bis-byproduct can be removed via acidification followed by phase separation. The effects of acidification on bis-byproduct removal and TDA yield were examined as follows. Following the reaction of TFA with MDTC, the reaction mixture was cooled to a temperature of about 25.degree. C. to 30.degree. C. and mixed with either HCl or H.sub.2 SO.sub.4. The levels of the bis-byproduct were determined.

Data from Table 1, above, show that the production of the bis-byproduct is dependent upon the molar excess of TFA used in the initial reaction. The production of the bis-byproduct decreases as the molar excess of TFA relative to MDTC increases. For subsequent studies, the molar ratio of TFA to MDTC was 1.5:1.

Data from the following two tables (Tables 5 & 6) demonstrates the effect of temperature and agitation on the removal of bis by-product vis-a-vis recovery of TDA.

Studies showed that up to 10 moles of 70% H.sub.2 SO.sub.4 per mole of the bis-byproduct could be used at 25.degree. C. to 30.degree. C. for the selective removal of the bis-by-product. With a solution of about 40% TDA and 60% toluene, 5 moles of 70% H.sub.2 SO.sub.4 per mole of the bis-byproduct reduced bis-byproduct levels on a solvent-free basis to about 0.1%. With a solution of about 60% TDA and 40% toluene, 4 moles of 70% H.sub.2 SO.sub.4 per mole of bis-byproduct reduced bis-byproduct levels on a solvent-free basis to about 0.1%.

In both cases, an additional 2 to 3 moles of 70% H.sub.2 SO.sub.4 appeared to be necessary to reduce the bis-byproducts to undetectable levels. Such attempts, however, resulted in TDA losses ranging from 2-3%. It was difficult to reduce bis-byproduct levels to below about 0.1% even with 8 to 10 moles of 70% H.sub.2 SO.sub.4 per mole of bis-byproduct at 60.degree. C. to 70.degree. C. Under such conditions, TDA losses ranged from 5-8%.

TABLE 5______________________________________ Solvent-free Solvent-free "bis-sulfide" "bis-sulfide" Mol 70% before after % Net H.sub.2 SO.sub.4 /Mol H.sub.2 SO.sub.4 H.sub.2 SO.sub.4 TDA Temp .degree. C. "bis-sulfide" treatment treatment Recovery______________________________________Set I With PPP 6-25-0128 Ambient 6.0 2.21% 0.17% 99.5 (25.degree.) 40.degree. C. 6.0 2.21% 0.22% 99.1 50.degree. C. 6.5 2.21% 0.24% 98.9 60.degree. C. 6.5 2.21% 0.31% 98.6Set II with PPP 6-25-0130 30.degree. C. 6.0 2.16% 0.22% 99.6 50.degree. C. 6.0 2.16% 0.29% 99.0 70.degree. C. 6.0 2.16% 0.38% 98.3 Repeat 6.0 2.16% 0.39% 98.4 70.degree. C.______________________________________ TABLE 6__________________________________________________________________________"Bis" Removal from TDA/Tol (6 Moles H.sub.2 SO.sub.4 per Mole of Removable "Bis")Temp RPM % "BIS" % "BIS" % "BIS" % "BIS" % "BIS" % "BIS" % "BIS" % "BIS" % "BIS" .degree. C. (agitation) zero time 5 mts 10 mts 15 mts 20 mts 25 mts 30 mts 35 mts 40 mts__________________________________________________________________________50 100 0.543 0.367 0.333 0.265 0.210 -- 0.134 -- 0.085 50 300 0.543 0.103 0.039 0.015 0.017 0.017 0.014 0.025 0.019 30 300 0.543 0.053 0.017 -- zero -- zero -- zero__________________________________________________________________________

Thus, a practical approach is to reduce bis-byproduct levels to about 0.1%, on a solvent-free basis, by using about 4-5 moles of 70% H.sub.2 SO.sub.4 per mole of bis-byproduct at a temperature of about 25.degree. C. to 30.degree. C.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims

1. A process of making 2-(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole comprising the steps of:

a. reacting methyldithiocarbazinate with trifluoroacetic acid in the absence of phosphorus trichloride to form a mixture of -(methylthio)-5-(trifluoromethyl)-1,3,4-thiadiazole and 2,5-bis-(methylthio)-1,3,4-thiadiazole, wherein the molar ratio of methyldithiocarbazinate to trifluoroacetic acid is from about 4:1 to about 1:5; and

b. selectively removing the 2,5-bis-(methylthio)-1,3,4-thiadiazole by acidification of the reaction mixture followed by phase separation.

2. The process of claim 1 wherein the mixture is acidified with a concentrated inorganic acid.

3. The process of claim 2 wherein the concentrated inorganic acid is hydrochloric acid, sulfuric acid or nitric acid.

4. The process of claim 3 wherein the concentrated inorganic acid is sulfuric acid.

5. The process of claim 4 wherein the sulfuric acid has a concentration of from about 55% to about 95%.

6. The process of claim 5 wherein the sulfuric acid has a concentration of about 70%.

7. The process of claim 6 wherein the amount of sulfuric acid added to the reaction mixture is from about 2 moles to about 10 moles of sulfuric acid per mole of the 2,5-bis-(methylthio)-1,3,4-thiadiazole.

8. The process of claim 7 wherein the amount of sulfuric acid added to the reaction mixture is from about 4 moles to about 7 moles of sulfuric acid per mole of the 2,5-bis-(methylthio)-1,3,4-thiadiazole.

9. The process of claim 1 wherein acidification occurs at a temperature of from about 20.degree. C. to about 60.degree. C.

10. The process of claim 9 wherein acidification occurs at a temperature of from about 20.degree. C. to about 40.degree. C.

11. The process of claim 10 wherein acidification occurs at a temperature of from about 25.degree. C. to about 30.degree. C.

12. The process of claim 1 wherein the reaction occurs in the presence of an aprotic, aromatic solvent.

13. The process of claim 12 wherein the aromatic solvent is toluene, xylene, cumene or mesitylene.

14. The process of claim 13 wherein the aromatic solvent is toluene.

15. The process of claim 1 wherein the molar ratio of methyldithiocarbazinate to trifluoroacetic acid is from about 1:1.25 to about 1:2.5.