Patent Application
20080161597
The invention relates to a process for preparing thioacetic acid and its salts.
Thioacetic acid and its salts are important industrial starting materials and are used, for example, for the preparation of D-acetylthioisobutyric acid or active pharmaceutical ingredients such as Captopril (ACE inhibitor against hypertension, JP-A-09-295963) and for the synthesis of biotin (DE-A-2807200). Potassium thioacetate is used, for example, in the preparation of testosterone derivatives (DE-A-19860917).
Known processes for the synthesis of thioacetic acid start, for example, from acetic anhydride which is reacted with hydrogen sulfide (OA-A-2110) or metal hydrogensulfides (US-A-2568020) to give the desired product.
A further process starts from ketene which is reacted with hydrogen sulfide in the gas phase and in the presence of a solid aluminum-containing catalyst at from 10 to 204.4° C. (50 to 400° F.) , preferably at from 65.6 to 176.7° C. (150 to 350° F) to give thioacetic acid or diacetyl sulfide (US-A-2639293) .
A common process for preparing alkali metal thiocarboxylates starts from thiocarboxylic acids which, for example, are converted to the corresponding salts by means of alkali metal hydrides (Kato, S. et al. , Z. Naturforsch. 1983, 38B(12), 1585-1590).
The methods disclosed to date have the disadvantage that thioacetic acid and its salts have always been prepared in separate process steps.
It is therefore an object of the present invention to provide a simple process for directly preparing thioacetic acid and its salts, which can also be conducted in a “one-pot process”, works inexpensively, utilizes readily available starting materials and affords a small amount of waste products and by-products.
According to the invention, this object is achieved by the process as claimed in patent claim 1. It has been found that ketene can be reacted in the presence of a nitrogen base directly with hydrogen sulfide or a dissolved alkali metal hydrogensulfide. Depending on the requirement, the thioacetic acid can be obtained directly in good yield and high purity by subsequent distillation, or its corresponding salt by addition of ammonia, or of an alkali metal or alkaline earth metal, aluminum or titanium base. More preferably, the formation of the thioacetic acid with subsequent salt formation can be carried out as a “one-pot process”. This allows solvent to be saved and waste substances to be prevented to a high degree.
The inventive reaction of ketene with hydrogen sulfide or a dissolved alkali metal hydrogensulfide in the absence of an aluminum catalyst is surprising, since US-A-2639293 states that, in the reaction of ketene with hydrogen sulfide in the absence of a catalyst, no detectable amounts of thioacetic acid or diacetyl sulfide are obtained. The reaction of ketene with a dissolved alkali metal hydrogensulfide has not yet been described.
In the process according to the invention for preparing thioacetic acid and its salts of the formulae
in which Mn+is ammonium or an alkali metal, alkaline earth metal, aluminum or titanium cation, selected from the group consisting of lithium, sodium, potassium, magnesium, calcium, barium, aluminum and titanium, and in which n is the number of positive charges of the cation, ketene is reacted in the presence of a nitrogen base with hydrogen sulfide or a dissolved alkali metal hydrogensulfide, and the thus obtained thioacetic acid is optionally subsequently reacted with ammonia or an alkali metal, alkaline earth metal, aluminum or titanium base to give the corresponding salt.
Here and hereinbelow, alkali metal hydrogensulfides refer in particular to the hydrogensulfides of lithium, sodium and potassium.
Here and hereinbelow, alkali metal or alkaline earth metal, aluminum or titanium bases refer in particular to hydroxides, carbonates, hydrogencarbonates, alkoxides, phenoxides, carboxylates, oxides, hydrides, sulfonates, phosphates, sulfides, sulfinates, oxalates, hexafluorophosphates and tetrafluoroborates of lithium, sodium, potassium, magnesium, calcium, barium, aluminum and titanium.
Here and hereinbelow, alkoxides and phenoxides of alkali metals, alkaline earth metals, aluminum and titanium refer in particular to the compounds of the formula
where R1 is a C1-6-alkyl or phenyl radical, and Mn+is an alkali metal cation, preferably Li+, Na+or K+, an alkaline earth metal cation, preferably Mg2+, Ca2+or Ba2+, an aluminum cation or a titanium cation, and in which n is the number of positive charges of the cation.
Here and hereinbelow, carboxylates of alkali metals, alkaline earth metals, aluminum and titanium refer in particular to compounds of the formula
where R2 is an optionally partly or fully halogenated C1-6-alkyl or phenyl radical, and Mn+is an alkali metal cation, preferably Li+, Na+or K+, an alkaline earth metal cation, preferably Mg2+, Ca2+or Ba2+, an aluminum cation or a titanium cation, and in which n is the number of positive charges of the cation.
Here and hereinbelow, nitrogen bases refer in particular to nitrogen compounds which have a pKa of >7, for example primary, secondary and tertiary alkylamines, for example di-tert-butylamine, trimethylamine and triethylamine, primary, secondary and tertiary arylamines, cyclic amines, for example piperidine, pyrrolidine or morpholine, aromatic nitrogen hetero-cycles such as pyridine or substituted pyridines, for example 4-dimethylaminopyridine, ammonia, guanidines, bicyclic nitrogen compounds, for example 1,4-diaza-bicyclo[2.2.2]octane (DABCO), 1,5-diazabicyclo-[4.3.0]non-5-ene (DBN) or 1,8-diazabicyclo-[5.4.0]undecene (DBU), and also basic nitrogen-containing ion exchangers which are obtainable, for example, under the names Lewatit®, Duolite® and Dowex®. The task of the nitrogen base is to activate the H2S.
In a preferred embodiment, Mn+is selected from the group consisting of Li+, Na+, K+, Mg2+, Ca2+and Ba2+.
The nitrogen base used is preferably a tertiary amine, more preferably a trialkylamine, most preferably trimethyl- or triethylamine.
In an advantageous process variant, the reaction of ketene with hydrogen sulfide or a dissolved alkali metal hydrogensulfide is carried out in a polar solvent.
In a preferred embodiment, the polar solvent used is a C1-4 alcohol, water, diethyl ether, tetrahydrofuran, dichloromethane, pyridine, methylpyridine or dimethylformamide.
In a particularly preferred embodiment, the polar solvent used is a C1-4 alcohol or water.
In a further particularly preferred embodiment, potassium thioacetate or sodium thioacetate is prepared by using 1-butanol as a polar solvent, in which potassium thioacetate and sodium thioacetate crystallize out particularly efficiently.
In a particular process variant, ketene can be reacted in aqueous solution with an alkali metal hydrogensulfide solution, optionally with addition of gaseous hydrogen sulfide.
In the process according to the invention, hydrogen sulfide and ketene or alkali metal hydrogensulfide and ketene are used advantageously in a molar ratio between 0.5:and 2:. In a preferred embodiment, a molar hydrogen sulfide:ketene ratio or alkali metal hydrogensulfide:ketene ratio between 0.8:and 1.3:is used. In a particularly preferred embodiment, a molar hydrogen sulfide: ketene ratio or alkali metal hydrogensulfide:ketene ratio of 1:is used.
In a preferred embodiment of the process according to the invention, a molar ketene:nitrogen base ratio between 1:0.001 and 1:0.5 is used. In a particularly preferred embodiment, a molar ketene:nitrogen base ratio between 1:0.001 and 1:0.1 is used.
In a preferred embodiment, the reaction of ketene with hydrogen sulfide is carried out at temperatures between +60 and -40° C., more preferably between +10 and -20° C.
For the formation of the salts of thioacetic acid, alkali metal, alkaline earth metal, aluminum or titanium bases may be used in the process according to the invention. The alkali metal or alkaline earth metal, aluminum or titanium bases used are preferably hydroxides, carbonates, hydrogencarbonates, alkoxides, phenoxides, carboxylates, oxides, hydrides, sulfonates, phosphates, sulfides, sulfinates, oxalates, hexafluoro-phosphates or tetrafluoroborates of lithium, sodium, potassium, magnesium, calcium, barium, aluminum or titanium. The resulting metal thioacetate may optionally be removed by filtration from the corresponding acid of the metal base used, and it is possible depending on the solubility for the metal thioacetate or the corresponding acid to be dissolved.
In a particularly preferred embodiment, a hydroxide, alkoxide, phenoxide or carboxylate of lithium, sodium or potassium is used for salt formation.
In one process variant, the alkali metal, alkaline earth metal, aluminum or titanium base may be added as a solid or as an aqueous solution and/or suspension.
In a further process variant, sparingly soluble or insoluble alkaline earth metal, aluminum or titanium bases, for example aluminum hydroxide, calcium carbonate and titanium dioxide, preferably in fine granulation, more preferably in pulverized form, may also be reacted with the thioacetic acid for salt formation.
In a further process variant, the reaction with the alkali metal or alkaline earth metal, aluminum or titanium base may be carried out at temperatures of from −20 to +100° C., more preferably at temperatures of from −10 to +10° C.
In the process according to the invention, the salt formation may also be carried out as a “one-pot process” without preceding isolation of the thioacetic acid.
The examples which follow illustrate the performance of the process according to the invention without any implied restriction.
A mixture of 1-butanol (1300 ml) and triethylamine (17.7 g; 0.175 mol) was initially charged and cooled to −10° C. Subsequently, hydrogen sulfide was introduced with a flow rate of 34.4 g/h in a total amount of 60.2 g (1.75 mol). After 10 min, ketene was additionally introduced with a flow rate of 42 g/h in a total amount of 99.4 g (1.75 mol). During the introduction of hydrogen sulfide and ketene, the reaction temperature was kept below −5° C. On completion of the introduction, the thioacetic acid formed was distilled off. 86 g (65%) of thioacetic acid were obtained in a purity of >98% (GC).
1H NMR (400 MHz, DMSO) : δ=2.41 (s, 3H) ; 13C NMR (133 MHz, CDCl13) : δ=171.6 ppm.
A mixture of 1-butanol (1300 ml) and triethylamine (17.7 g; 0.175 mol) was initially charged and cooled to −10° C. Subsequently, hydrogen sulfide was introduced with a flow rate of 34.4 g/h in a total amount of 60.2 g (1.75 mol). After 10 min, ketene was additionally introduced with a flow rate of 42 g/h in a total amount of 99.4 g (1.75 mol). During the introduction of hydrogen sulfide and ketene, the reaction temperature was kept below −5° C. On completion of the introduction, a 45% aqueous potassium hydroxide solution (218 g; 1.75 mol) was metered in within 20 min and the mixture was heated under reflux for 1 h. In the course of this, white potassium thioacetate precipitated out, and, after cooling of the reaction mixture to −5° C., was filtered off, washed with 1-butanol and then dried. 98.5 g (63%) of potassium thioacetate were obtained in a purity of >98% (titrimetric).
| Number | Date | Country | Kind |
|---|---|---|---|
| 03004654.4 | Mar 2003 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2004/002089 | 3/2/2004 | WO | 00 | 3/10/2008 |
