Field of the invention

The present invention provides a process for the preparation of substantially pure 2-cyanoimino-l,3-thiazolidine of Formula -I,

by acclimation of dimethyl N-cyanoiminodithiocarbonate ester and 2-aminoethanethiol or salts thereof in presence of aqueous ammonia.

Background and prior art

The methods for preparation of 2-cyanoimino-l, 3-thiazoIidine from the reaction between dimethyl N-cyanoiminodithiocarbonate ester and 2-aminoethanethiol, are those described in Arch, pharm. (withier, Ger.), 305(10), P731 (1972), Japanese unexamined patent publication (Kokai) No. 48-91064, Gazz.Chim.Ital., 110 (5-6), P345, and W092-17462 (1992). This reaction is considered to follow the following scheme:

J. Heterocyclic. Chem., 24(1), P275 (1987) describes a method for preparing 2-cyanoimino-1, 3-thiazolidine from the reaction between biphenyl N-cyanoiminodithiocarbonate and 2-aminoethanethiol. The drawback of this process is using the relatively expensive biphenyl N-cyanoiminodithiocarbonate compound. This reaction is depicted in the following scheme.


Org. Prep. Procedure Int. 23, (6), 721-728 (1991) describes a method for preparing cyanoimino-l,3-thiazolidine in 48% yield which comprises stirring dimethyl N-cyanoimidocarbonate for a prolonged period at a pH of 10-11 with cysteamine in aqueous sodium hydroxide solution. The melting point of the product thus obtained (m.p. 168°-170°C), however, differs considerably from that of pure 2-cyanoimino-l,3-thiazolidine (m.p. 154°-156°C), since the former is probably contaminated by secondary products. Therefore, further purification would reduce still further the yield from the reported 48%, so that this process is unsuitable for industrial production.
US 5574165 discloses a two-step process for preparing 2-cyanoimino-l, 3-thiazolidine, said process comprising; reacting 2-aminoethanethiol or salt thereof and daily N-cyanoimidocarbonate in a diluent, in the presence of a base such as sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, sodium methylate, potassium methylate, sodium ethylated or potassium methylate, in the presence of a protective-gas atmosphere to obtain an intermediate which is subsequently cyclized at a pH of from 8 to 9.5 to obtain 2-cyanoimino-l, 3-thiazolidine. In this process the purity of the product is only 95.8%. This reaction is depicted in the following scheme:


Step 2

US 5591859 discloses a process for preparation of 2-cyanoimino-l, 3-thiazolidine, said process comprising; reacting 2-aminoethanethiol or salt thereof (wherein 2-aminoethanethiol is released by addition of a base) and dialkyl N-cyanoimidocarbonate in water and/or an organic solvent in a pH range from 7 to 12, in the presence of a protective-gas atmosphere and completing the cyclization at a pH of > 8 by addition of a base such as alkali hydroxides, alkali carbonates and strongly basic amines in aqueous solution such as diethylamine and diethylamine to obtain 2-cyanoimino-l, 3-thiazolidine.

US 6858737 discloses a process for the preparation of 2-cyanoimino-l,3-thiazolidine comprising the stylization reaction of dimethyl N-cyanoiminodithiocarbonate with 2-aminoethanethiol or the salt thereof in the presence of an alkali metal hydroxide. This process is limited to the scope of using alkali metal hydroxide as a base and water as the solvent. Summary of the invention

The present invention provides a process for the preparation of substantially pure 2-cyanoimino-1, 3-thiazolidine of Formula-I, said process comprising: dissolving dimethyl N-cyanoiminodithiocarbonate ester of Formula-II in a solvent to obtain a solution of dimethyl N-cyanoiminodithiocarbonate ester of Formula-II; adding 2-aminoethanethiol or salt thereof of Formula-III and aqueous ammonia to said solution of dimethyl N-cyanoiminodithiocarbonate ester of Formula-II and performing the above reaction at 5°C to ambient temperature for 2 hours to obtain substantially pure 2-cyanoimino-l, 3-thiazolidine of Formula-I.

These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This Summary is provided to introduce a selection of concepts in a simplified form. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

Detailed description of the invention

Accordingly, the present invention provides a process for the preparation of substantially pure 2-cyanoimino-l, 3-thiazolidine of Formula-I,


said process comprising:

a. Dissolving dimethyl N-cyanoiminodithiocarbonate ester of formula II in a solvent ,
to obtain a solution of dimethyl N-cyanoiminodithiocarbonate ester of Formula-II;
N S
Formula-II

b. Adding 2-aminoethanethiol or shahs thereof of formula III to a mixture of water and aqueous ammonia and reacting the solution of ester of formula II and solution thiol of formula III at 0-35 ° C for 2 hours to obtain a precipitate which is then recrystallised from hot water to obtain substantially pure of 2-cyanoimino -1,3- thiazolidine ,wherein the molar ratio of dimethyl N-cyanoiminodithiocarbonate ester to 2-aminoethanethiol acid salt is from 1:0.95 to 1:1.15.
The reaction of the present invention is depicted in the form of the following scheme


In the present disclosure dimethyl N-cyanoiminodithiocarbonate of Formula-II is first dissolved in a solvent such as water and the reaction mixture is stirred at 0° to 10°C. Then acid salt of 2-aminoethanethiol dissolved in water is added to the reaction mixture in one lot followed by the addition of aqueous ammonia solution. The cyclisation reaction commences at 5°C, which is evident from the evolution of the equivocal quantity of methylmercaptan gas and slight exothermic during the reaction.

In an embodiment of the present disclosure performing the above reaction at 5°C to ambient temperature for 2 hours to obtain substantially pure 2-cyanoimino-1, 3-thiazolidine of Formula-I.
In another embodiment of the present disclosure the strength of aqueous ammonia is 18-25%.
In an embodiment of the present disclosure the solvent is water.
In yet embodiment of the present disclosure the quantity of aqueous ammonia and water in the said mixture is equal.

In an embodiment of the present disclosure time of reaction is 10 min to 5 hrs.
In another embodiment of the present disclosure the pH of the reaction mixture is 3-10.
It is an embodiment of the present disclosure the reaction ratio of the dimethyl N-cyanoiminodithiocarbonate and acid salt of 2-aminoethanethiol, by molar ratio is, preferably 1:1. If the amount of the 2-aminoethanethiol is too large, an undesirable polymerization reaction occurs resulting in lower yield.

Another embodiment of the present disclosure the methyl mercaptan gas can be trapped in sodium ethoxide solution or 10% caustic lye solution to avoid abnoxious, penetrating odour of methyl mercaptan and isolated as sodium salt as a valuable by product for other agrochemicals.
Yet another embodiment of the present disclosure the acid salt of 2-aminoethanethiol is hydrochloride, which is most preferable from the viewpoint of commercial availability.
The time of the cyclisation reaction is not particularly limited, but for example is 10 mins. to 5 hours, preferably 1 to 3 hours. If the cyclisation reaction time is too short, the

cyclisation minutes reaction does not proceed sufficiently, while if too long, the reaction will not proceed further, and therefore this is not wise economically.
Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.

EXAMPLE

The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure.

EXAMPLE 1:

Preparation of 2-Cyanoimino-l, 3-Thiazolidine

A 3 liter four-necked flask provided with a thermometer and a stirrer is charged with 238g of dimethyl N-cyanoiminodithiocarbonate (1.62moles) and 500g of water (27.77 moles), and stirred at 0° to 10°C. To this reaction mixture, 184g of 2-aminoethane thiol hydrochloride (1.62 moles) in equal quantity of water and 25% of aqueous ammonia (110.6g, 1.62moles) are charged in one lot. The reaction starts instantaneously with evolution of methyl mercaptan, which is traped in equivocal aqueous caustic lye, or methanol solution of caustic trap. The reaction is carried out preferably at 5°C and continued at ambient temperature for 2 hours. The reaction mixture cooled to room temperature, followed by nitrogen purging for complete evacuation of methyl mercaptan for 2 hours. The solid compound is precipitated as faint grayish in colour, which is recrystallised from hot water. The solid is filtered and dried at 80°C for 6-8 hours till moisture is below 0.5% by K.F. to obtain 170.76g of 2-cyanoimino-l, 3-thiazolidine (yield 83%) and having 98 to 99 % (w/w) HPLC purity.

Advantages of the present invention

a) The process of the present invention does not employ corrosive reagents like alkali metal hydroxides.
b) The methyl mercaptan formed as byproduct in the process is isolated as sodium salt which can be used in the preparation of agrochemicals.

We claim:

1. A process for the preparation of 2-cyanoimino -1,3- thiazolidine, said process
comprising dissolving dimethyl N-cyanoiminodithiocarbonate ester of formula
II in a solvent, adding 2-aminoethanethiol or salts thereof of formula III

to a mixture of water and aqueous ammonia and reacting the solution of ester of formula II and solution of formula III at 0-35 ° C for 2 hours to obtain a precipitate which is then recrystallised from hot water to obtain substantially pure of 2-cyanoimino -1, 3- thiazolidine , wherein the molar ratio of dimethyl N-cyanoiminodithiocarbonate ester to 2-aminoethanethiol acid salt is from 1:0.95 to 1:1.15.

2. A process as claimed in claim 1, wherein the molar ratio of dimethyl N-cyanoiminodithiocarbonate ester to 2-aminoethanethiol acid salt is 1:1.

3. A process as claimed in claim 1 wherein the strength of aqueous ammonia is 18-25%.

4. A process as claimed in claim 1 wherein the acid salt is a hydrochloride salt.

5. A process as claimed in claim 1 wherein the solvent is water.

6. A process as claimed in claim 1 wherein the quantity of aqueous ammonia and water in the said mixture is equal.

7. A process as claimed in claim 1 wherein time of reaction is 10 min to 5 hrs.