GB2116174A - Process for the production of dithiazole disulphides - Google Patents

Abstract

A process for the production of compounds of the formula: <IMAGE> wherein R and R' independently represent a hydrogen atom, a halogen atom, a nitro group, a hydroxyl group or a C1-C6 alkyl or alkoxy radical optionally substituted one or more times or a C6-C12 cycloalkyl or aryl radical, the substituents being a halogen atom, a nitro group or a C1-C5 alkyl or alkoxy radical, or R and R' together represent <IMAGE> Wherein R'', R''', R'''' and R''''', which may be the same or different, each have the same meaning as R and R'; comprises catalytically oxidizing a 2-mercapto-thiazole corresponding to the following general formula: <IMAGE> wherein R and R' are as defined above; with oxygen or a free oxygen-containing gas in water as a solvent in the presence of ammonia as a catalyst and, optionally, a heavy metal co- catalyst and at a temperature of from 0 to 150 DEG C is disclosed. The use of an inexpensive, safe and environmentally acceptable solvent is a particular advantage of the present improved process. Such compounds are commonly used as vulcanising agents for rubber.

Description

SPECIFICATION Process for the production of dithiazole disulphides This invention relates to a process for the production of dithiazole disulphides, more particularly it relates to a process for the production of dithiazolyl-(2,2')-disulphides by the oxidation of 2mercaptothiazoles with oxygen and, in particular, to a process for the production of dibenzothiazolyl disulphide from 2-mercaptobenzthiazole.

Various oxidising agents have hitherto been used for the production of dibenzothiazolyl disulphides by the oxidation of 2-mercaptobenthiazoles. However, most of the known processes are attended at the outset by the disadvantage that they use expensive oxidising agents compared with oxygen, for example chiorate/nitrite (Fiat Final Report 101 8, page 22), nitrite/oxygen (US Patent No. 1,908,935), nitrite (US Patent Nos. 2,119,131 and 3,062,825), chlorine (Kirk-Othmer, Encyclopedia of Polymer Science and Technology (1970), Vol. 12, page 262 and also DE-OS No. 2,309,584), hydroperoxides (DE-OS No. 2,349,314) and ozone (SU-PS No. 420,247).Most of the processes described in the above-mentioned publications also have other disadvantages, such as the need to use relatively large quantities of acids, bases or other auxiliaries, the accumulation, of in some cases, relatively large quantities of by-products or secondary products or complicated and critical reaction conditions with the need for careful control (as is the case, for example, when chlorine is used as the oxidising agent).

There is also a process for the electrolytic oxidation of 2-mercaptobenzthiazole to dibenzothiazolyl disulphide (DE-OS No. 2,743,629).

Now, attempts have also been made to carry out the oxidation of 2-mercaptobenzthiazole to dibenzothiazolyl disulphide using oxygen as the sole oxidising agent.

According to US-PS No. 3,654,297, this is possible if a cobalt phthalocyanine sulphate, disulphonate, trisulphonate or tetrasulphonate or a mixture thereof is used as the catalyst and if oxidation is carried out in an organic solvent containing less than 15%, by weight, of water at temperatures of from 50 to 80"C (cf. also US Patent No. 575,348; Chem. Abstr. 88 (1978), 89657 g). However, problems are involved in the production and industrial application of this catalyst.

In addition, it is known from DE-OS No. 2,335,897 that the oxidation of 2-mercaptobenzthiazoles to dibenzothiazyldisulphides may be carried out using oxygen or an oxygen-containing gas and iron chloride, particularly iron (Ill) chloride, in a saturated aliphatic (C,-CaO alcohol at temperatures of from 0 to 150"C. However, this catalyst only leads to a satisfactory reaction velocity when used in a relatively large quantity namely in a ratio of from 0.8 to 1.5 moles per mole of 2-mercaptobenzthiazole. However, the major disadvantage of this known process resides in the fact that the iron is precipitated in the form of basic salts during the reaction and the dibenzothiazolyl disulphide obtained is heavily contaminated with iron.A product obtained in this way cannot be used as a vulcanising agent, for example, without elaborate purification.

Also, an economically interesting process for the catalytic oxidation of 2-mercaptothiazoles with oxygen is described in DE-OS No, 2,944,225. In this process, a mercaptothiazole is reacted with oxygen or an oxygen containing gas in the presence of organic solvents and tertiary amines as catalyst and optionally in the presence of heavy metals or heavy metal compounds as co-catalysts, preferably at temperatures of from 20 to 90"C. The dithiazolyl-(2,2')-disulphides obtainable in this way are distinguished by the high purity thereof and, for example may be directly used without further purification as vulcanising agents for rubber.Compared with the two above-mentioned processes in which a 2-mercaptobenzthiazole is also oxidised with oxygen, this process is distinguished by the fact that simple and inexpensive catalysts are used in very small quantities and further by the fact that these catalysts may be recycled with the mother liquor without a significant reduction in the activity thereof.

An Addition (P 31 13 298.7) to the above German reference DE-OS No. 2.944,225 describes the production of dithiazolyl-(2,2')-disulphides using ammonia instead of tertiary amines as catalyst. In all these processes for the oxidation of 2-mercaptobenzthiazole to dibenzothiazolyl disulphide using oxygen as the sole oxidising agent as described in US Patent No. 3,654,297, DE-OS No. 2,355,897 and DE-OS No. 2,944,225 (and its Addition P 31 13 298.7), it is considered necessary to use organic solvents. In contrast, water appeared to be completely unsuitable for use as the solvent.Thus, US Patent No. 3,654,297, for example, discourages the use of a solvent containing more than 15%, by weight, of water, while DE-OS No. 2,944,255 (cf. page 16, lines 23 to 27) advises against allowing the percentage of water in the solvent to rise above 10%, by weight. Further experiments with the process according to DE-OS No. 2,944,225 have shown that, where water is used as the sole solvent, there is no reaction whatever of the 2-mercaptobenzthiazole to form dibenzthiazolyl-(2,2')-disulphide. If, for example, Examples 18, 24 and 28 of DE-OS No. 2,944,225 are carried out under otherwise the same conditions in water and not in the solvents mentioned therein, no oxidation takes place, as shown in Table 1 below.

Table 1: Tests on the oxidation of 2-mercaptobenzthiazole (MBT) to dibenzthiazolyl disulphide (MBTS) Reaction conditions: 60 mMoles of MBT, 108 mMoles of triethylamine, reaction temperature 70"C, oxygen pressure 20 bars, reaction time 6.5 h Solvent Copper cat. MBT-con- MBTS 120 g 0.02 mMole version yield % % DE-OS No.

2,944,225 Example 18 isopropanol copper (Il)- 82.7 80.5 acetate DE-OS No.

2,944,225 Example 24 toluene copper (Il)- 90.6 88.6 acetate Comparison test water copper (II)- 0 acetate DE-OS No.

2,944,225 Example 28 isopropanol copper (Il)- 82.0 80.2 oleate Comparison test water copper (II)- 0 oleate Further, prejudice must have existed against the use of water as solvent in the process according to the above-mentioned Addition P 31 13 298.8 because it was known that, in contrast to tertiary amines, ammonia and also primary and secondary amines in solution in water are oxidised by oxygen in the presence of metal catalysts.For example, it could be seen from the section entitled "Chemical behaviour of the ammonium ion and of ammonia in aqueous solution" in "Gmelins Handbuch der anorganischem Chemie", 8th Edition, Vol. 23, pages 1 2 et seq. (behaviour with respect to oxygen in the presence of metals and metal ions is reported in detail on page 13, line 21 to page 15, line 4 with particular reference to copper) that ammonia in aqueous solution is catalytically oxidised to the nitrite.

However, for reasons of economy and safety, there was a need for a process for the production of dithiazolyl-(2,2')-disulphides by the oxidation of 2-mercapto-benzthiazoles with oxygen which may be carried out not in organic solvents, but in the less expensive, safe and environmentally acceptable solvent, water.

Accordingly, an object of the present invention is to provide an economic process for the production of dithiazolyl-(2,2')-disulphides by the oxidation of 2-mercaptothiazoles with oxygen in the presence of water as solvent.

It have now suprisingly been found that given suitable reaction conditions, the oxidation of 2mercapto-thiazoles to dithiazolyl-(2,2')-disulphides by means of oxygen may also be successfully carried out in water as solvent providing ammonia, and optionally, a heavy metal derivative are used as catalyst (cf. Addition P 31 13 298.7). In this way, it was possible in the oxidation of 2mercaptobenzthiazole to obtain highly pure dibenzothiazolyl-(2,2')-disulphide in high yields (up to 99%) and with a high degree of selectivity (up to > 99%). None of the secondary reactions which had been expected on the strength of literature reports (oxidation of ammonia or formation of sulphenamide -cf. DE-OS No. 2,349.934, page 5, lines 22 to 24) were observed.

According to the present invention, the object is achieved by a process for the production of dithiazolyl-(2,2')-disulphides corresponding to the following general formula:

wherein R and R' independently represent a hydrogen atom, a halogen atom, a nitro group, a hydroxyl group or an organic radical optionally substituted one or more times such as C1 -C8 alkyl or alkoxy radical or a C6-C,2-cycloalkyl or aryl radical, the substituents being a halogen atom, a nitro group, a hydroxyl group or a C,-C5 alkyl or alkoxy radical, or R and R' together represent the radical:

wherein R", R"', R""and R""', which may be the same or different, each have the same meaning as R and R'; by the catalytic oxidation of a 2-mercaptothiazole corresponding to the following general formula:

wherein R and R' are as defined above; with oxygen or an oxygen-containing gas in a solvent in the presence of a catalyst and, optionally, a heavy metal co-catalyst and at temperatures of from 0 to 150"C, characterised in that water is used as the solvent and ammonia as the catalyst.

In general, the quantity of water used amounts to from 1.5 to 12 times the quantity, by weight, of 2-mercaptothiazole, used. Larger quantities of water should be avoided for economic reasons.

In some cases, it may be advantageous to use water containing additions of water-soluble organic solvents (for example alcohols) as the reaction medium.

The quantity of ammonia used may be varied within wide limits. However, the amonia is preferably used in quantities of from 0.5 to 25%, by weight, based on the reaction mixture.

The substituents R to R""' in general formulae (I), (II), and (III) preferably represent a chlorine or bromine atom, a hydroxy group, a nitro group, a straight or branched alkyl radical containing from 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl or t-butyl, an alkoxy radical containing from 1 to 4 carbon atoms, such as methoxy, ethoxy, propoxy or butoxy, or a phenyl, tolyl, ethylphenyl, nitrophenyl, chlorophenyl, bromophenyl or naphthyl radical.

Dithiazolyl-(2,2')-disulphides may be used as vulcanising agents for rubber.

The process according to the present invention is particularly significant for the production of dibenzothiazolyl-(2,2')-disulphide, the most important representative of this class of compounds.

However, it may also be successfully used in the production of other compounds of this type. 2mercaptobenzthiazole is used as starting material for the preferred production of dibenzothiazolyl-(2,2')-disulphide. The following compounds are examples of other mercapthothiazoles which are suitable for used as starting materials in the production of other dithiazolyl-(2,2')-disulphides corresponding to general formula (I): 2-mercapto-thiazole, 2-mercapto-4-methyl-thiazole, 2-mercapto-4-ethyl-thiazole, 2-mercapto-4- n propyl-thiazole, 2-mercapto-4, 5-dimethyl-thiazole, 2-mercapto-4,5-di-rabutyl-thiazole, 2-mercapto-4-phenyl-thiazole, 2-mercapto-5-chloro-4-phenyl-thiazole, 2-mercapto-4-*bromophenyl-thi- azole, 2-mercapto-4-nenitrophenyl-thiazole, 2-mercapto-4- mchlorophenyl-th iazole, 2-mercapto-4methyl-benzothiazole, 2-mercapto-5-methyl-benzothiazole, 2-mercapto-5-methyl-benzothiazole, 2mercapto-5, 5-dimethyl- benzothiazole, 2-mercapto-4-phenyl-benzothiazole, 2-mercapto-4-methoxy-benzothiazole, 2-mercapto-6-methoxy-benzothiazole, 2-mercapto-5, 6-dimethoxy-benzothia- zole, 2-mercapto-6-methoxy-4-nitro-benzothiazole, 2-mercapto-6-ethoxy-benzothiazole, 2-mercapto-4-chloro-benzothiazole, 2-mercapto-5-chlorobenzothiazole, 2-mercapto-6-chloro-benzothiazole, 2-mercapto-7-chloro-benzothiazole, 2-mercapto-5-chloro-6-methoxy-benthozthiazole, 2-mercapto-5-chloro-4-nitro-benzothiazole, 2-mercapto-5-chloro-6-nitro-benzothiazole, 2-mercapto-4,5dichloro-benzothiazole, 2-mercapto-4, 7-dichlorobenzothiazole, 2-mercapto-5-nitro-benzothiazole, 2-mercapto-6-nitrobenzothiazole, 2-mercapto-4-phenyl-benzothiazole, 2-mercapto-napthothiazole and 2-mercapto-6-hydroxy-benzothiazole.

The 2-mercaptothiazole may be present in the reaction mixture in a quantity of up to 40%, by weight.

The ammonia used in accordance with the present invention may be added as catalyst both on its own and also together with a heavy metal or heavy metal compound.

Examples of suitable co-catalysts are the metals iron, cobalt, nickel, copper, chromium, zinc manganese, silver, vanadium, molybdenum and cerium, oxides thereof, and also inorganic or organic salts or complexes thereof.

The heavy metal or heavy metal compound is generally used in quantities of less than 0.1%, by weight, based on the 2-mercaptothiazole.

Even traces of these co-catalysts, particularly in the case of copper, still develop considerable catalytic activity and lead to high yields of dithiazolyl disulphides. Accordingly, it is possible to use extremely small quantities of catalyst and repeatedly to recycle the mother liquor.

Particularly suitable heavy metal co-catalysts are copper and compounds thereof. Suitable copper compounds are monovalent or divalent inorganic, organic, simple or complex copper salts.

Examples of suitable monovalent copper salts are copper-(l)-chloride, bromide and iodide, addition compounds of these copper-(l)-halides with carbon monoxide, complex copper-(l)-salts, such as alkali metal chlorocuprates, complex ammoniates of copper-(l)-cyanide, for example cyanocuprates, such as potassium tricyanocuprate-(l), double salts with copper-(l)-thiocyanate, copper-(l)-acetate, copper-(l)-sulphide and complex double sulphides of copper-(l)-acetate, copper-(l)-sulphide and complex double sulphides of copper-(l)-sulphide and alkali metal polysulphides.

Examples of suitable copper-(ll)-salts are copper-(ll)-chloride, bormide, sulphide, sulphate, nitrate, nitrite, thiocyanate, cyanide, Cu-(ll)-salts of carboxylic acids, such as copper-(ll)-acetate, and the complex ammoniates of copper-(ll)-salts. Metallic copper and copper-(l)-oxide are also suitable for use as co-catalyst.

It is preferred to use copper-(l)-chloride, copper-(ll)-acetate, copper-(Il)-sulphate, copper-(ll)oleate, copper-(l 1)-acetylacetonate, copper-(l 1)-sulphide or copper-(l)-oxide.

Since the heavy metal compound is used in small quantities, heavy metal compounds of the type which are only sparingly soluble or are only soluble in traces in the water may be used in addition to the readily soluble heavy metal compounds.

Oxygen or an oxygen-containing gas, preferably air, is used as the oxidising agent. The conversion and reaction velocity increase with increasing oxygen pressure or partial pressure. In general, the oxygen pressure or partial pressure is from 0.1 to 1 50 bars. For economic reasons, it is preferred to apply oxygen pressure or partial pressures of from 0.2 to 10 bars.

The reaction temperature is from 0 to 150"C, preferably from 20 to 90"C, more preferably from 30 to 70"C. The reaction velocity decreases at lower temperatures, while the selectivity of the reaction decreases at higher temperatures.

The reaction time is generally from 0.25 to 6.5 hours. Under the preferred temperature and pressure conditions mentioned above and in the presence of a copper co-catalyst, it amounts to less than 1 hour for a conversion of 80%.

The process according to the present invention is carried out without difficulty by introducing the oxygen or oxygen-containing gas under pressure onto the reaction solution under the pressure and temperature conditions specified and/or by passing it into and/or through the solution comprising water, 2-mercaptothiazole, heavy metal catalyst and ammonia. The reaction product precipitated is removed by filtration or centrifuging. Fresh 2-mercapto-thiazole may be added to the mother liquor for recycling. Depending on the starting concentration of the heavy metal catalyst, fresh catalyst has to be added after a certain number of reaction cycles. The process is also suitable for continuous operation.

High yields (up to 99%) and high degrees of selectivity (up to > 99%) are obtained in the process according to the present invention. The dithiazolyl-(2,2')-disulphides obtainable by the process according to the present invention are distinguished by the high purity thereof.

Accordingly, they may be directly used without further purification, for example as vulcanisation aids. By virtue of the fact that the mother liquor may be recycled, the operation of the process according to the present invention involves a few effluent problems. Another advantage of the process according to the present invention over the processes described above, apart from the use of inexpensive, safe and environmentally acceptable solvent, residues in the elimination of the need for a separate operation to remove the water produced by the reaction from the solvent.

The following Examples illustrate the invention: EXAMPLE I 50 g (0.3mole) of 2-mercaptobenzthiazole (MBT), 12 mg (0.6 x 10-3 mole) of Cu (OAc)2.

H2O, 20.4 9 (1.2 moles) of ammonia and 320 g of water are introduced into a glass autoclave equipped with a double jacket for the circulation of a heating fluid, a thermometer, a pressure gauge and a stirrer. The reaction mixture is heated to 50"C, resulting in the formation of a clear solution, intensively stirred and placed under an oxygen pressure of 4 bars. An uptake of oxygen is immediately recorded and a deposit is formed by the formation of dibenzothiazyldisulphide (MBTS).After 1 hour, the test is terminated, the deposit is filtered off, washed with aqueous ammonia and with water and then dried in vacuo at 70"C, giving 47.7 g of a product which, in pure analytical data (elemental analysis, IR, 1H-NMR and MS), is identical with dibenzothiazolyl disulphide and which is shown by chromatographic analysis to be 100% pure (Mp 178"C).

According to analysis by gas chromatography, the mother liquor contains 1.8 g of unreacted MBT. Accordingly the MBT conversion amounts to 96.4% and the MBTS yield to 96.0% of the theoretical yield (selectivity 99.6%).

EXAMPLE 2 In the apparatus described in Example 1, a solution of 50 9 (0.3 mole) of MBT, 10.2 g (0.6 mole) of ammonia and 3 mg (0.015 x 10-3 mole) of Cu(OAc)2.H2O is dissolved in 100 9 of water and the resulting solution reacted with oxygen in the same way as described in Example 1. The oxygen pressure amounts to 4 bars and the reaction temperature to 50"C.

After 4.5 h, dibenzothiazolyl disulphide is obtained in a yield of 49.1 g, corresponding to 98.8% of the theorectical yield. The conversion is calculated at 99.4% (selectivity 99.4%).

EXAMPLE 3 The procedure is as in Example 1, except that no Cu(OAc)2.H20 is added. In this case, the mercaptobenzthiazole conversion after 20 hours amounts to 90.8% and the yield of MBTS to 44.9 g, corresponding to 90.3 % of the theoretical yield (selectivity 99.5%).

EXAMPLES 4 to 8 In these Examples, the quantity of ammonia is varied. In each Example, 50 g (0.3 mole) of 2mercaptobenzthiazole are reacted in the same way as in Example 1. The reaction conditions and the conversions, yields and selectivities obtained are set out in Table 2 below.

EXAMPLES 9 to 13 In these Examples, different heavy metal catalysts are used in different concentrations. In each Example, 50 9 (0.3 mole) of 2-mercaptobenzthiazole are reacted in the same way as in Example 1 under the reaction conditions shown in Table 2 (reaction temperatures 50"C; oxygen pressure 4 bars). The conversions, yields and selectivities obtained are shown in Table 3 below.

EXAMPLE 14 In the apparatus described in Example 1, a solution of 50 g (0.3 mole) of 2-mercaptobenzthiazole and 12 mg of Cu(OAc)2.H2O (0.06 x 10-3 mole) in 360 9 of aqueous ammonia (ammonia content: 50 9 (2.9 moles )) is oxidised with oxygen (oxygen pressure 4 bars) at room temperature. After a reaction time of 6.5 hours, the 2-mercaptobenzthiazole conversion amounts to 95.6% and the yield of dibenzothiazolyl disulphide to 47.3 9, corresponding to 95.1% of the theoretical yield (selectivity 99.5%).

EXAMPLE 15 In this Example, air is used as the oxygen-containing gas. In the apparatus described in Example 1, a solution of 50 g (0.3 mole) of MBT, 10.2 9 (0.6 mole) of ammonia and 6 mg (0.03 x 10-3 mole) of Cu(OAc)2.H20 is dissolved in 160 9 of water and oxidised with air (8 bars) at 50on. After 8 hours, the MBT conversion amounts to 94.3% and the yield of MBTS to 46.6 9, corresponding to 93.7% of the theoretical yield (selectivity 99.4%).

Table 2 (Examples 4 to 8) Example NH3 NO. (moles) (% of the CU(OAc)2 Water Temper- O2 pres- Reac MBT con- MBTS Selec reaction (10-3 mole) (g) ature sure time version yield tivity mixture) ( C) (bars) (h) (%) (% of (%) the theor retical yield) 4 2.3 (19.1) 0.02 90 70 2.5 1.5 86.1 83.4 96.9 5 2.4 (10.5) 0.04 300 50 4.0 2.5 95.4 94.1 98.6 6 0.4 (3.1) 0.015 163 50 4.0 6.0 94.9 94. 99.3 7 0.1 (0.9) 0.06 168 70 4.0 6.5 94.1 92.6 98.4 8 - - 0.06 170 70 4.0 8.0 - - Table 3 (Example 9 to 13) Example NH3 Co-catalyst Water Reaction MBT conversion MBTS yield Selectivity No. (moles (10-3 moles) (g) time (h) (%) (% of the (%) theoretical yield) 9 0.6 Cuso4 (0.015) 160 4.5 98.2 97.5 99.3 10 0.6 CU(OAc)2 (0.005) 160 8.5 93.1 92.1 99.0 11 1.0 Mn(OAc)2 (0.06) 300 7.5 91.1 90.5 99.3 12 0.8 FeSO4 (0.06) 160 9.0 90.4 88.9 98.3 13 1.2 none (0) 320 20.0 90.8 90.3 99.5

Claims (3)

1. A process for the production of a dithiazolyl-(2,2')-disulphide corresponding to the following general formula:

wherein R and R' independently represent a hydrogen atom, a halogen atom, a nitro group, a hydroxyl group or a C,-C6 alkyl or alkoxy radical optionally substituted one or more times or a C6-C,2 cycloalkyl or aryl radical, the substituents being a halogen aton, a nitro group, a hydroxyl group or a C1 -C8 alkyl or alkoxy radical, or R and R' together represent

wherein R", R"', R"" and R""', which may be the same or different, each have the same meaning as R and R'; which comprises catalytically oxdising a 2-mercaptothiazole corresponding to the following general formula:

wherein R and R' are as defined above; withoxygen or a free oxygen-containing gas in water as a solvent in the presence of ammonia as a catalyst and, optionally, a heavy metal co-catalyst and at a temperature from 0 to 150"C.

2. A process as claimed in claim 1 wherein the ammonia catalyst is used in quantities of from 1 to 25%, by weight, based on the reaction mixture.

3. A process as claimed in claim 1 substantially as herein described with particular reference to the Examples.