Process for preparation of
substituted pyridinyl phosphorothioate
FIELD OF INVENTION
Present invention relates to a novel process for preparation of substituted pyridinyl phosphorothioate. More particularly, present invention relates to a solvent-free process for preparation of 0,0- dimethyl-0-(3,5,6-trichloro-2-pyridinyl)phosphorothioate by condensation of alkali metal salt of 3,5,6-trichloro-2-pyridinol with 0,0- dimethyl pho sphorochloridothioate.
BACKGROUND AND PRIOR ART
A method for preparation of O, O - dimethyl-O- (3,5,6-trichloro - 2 - pyridinyl) phosphorothioate is disclosed in GB 2148898, wherein a 2-phase system comprising water and a water-immiscible organic solvent is employed for carrying out the reaction for synthesis of said compound.
As per method disclosed in GB1490138, the title compound is produced in 93.7% yield by reaction of sodium 3,5,6-trichloro-2-pyridinate and 0,0-dimethyl phosphorochloridothioate in presence of a catalyst. However, in this process also a two-phase solvent system is employed comprising methylene chloride and water. US3917621 and US3686191 disclose a similar process.
US4814451 discloses a process for preparation of phosphorothioate compounds, wherein use is made of a non-ionic surfactant and ortho- boric acid.
Thus, all processes reported in the prior art for preparation of 0,0- dimethyl-0-(3,5,6-trichloro-2-pyridinyl)phosphorothioate make use of either an organic solvent or surfactants and ortho boric acid. Although organic solvents can be recycled, practically there is always some loss which adds to the cost. Moreover, it is not desirable from environment point of view. Hence there is a need for process of preparation of title compound which obviates the need of carrying out the reaction in an organic solvent or using surfactants and ortho boric acid.
OBJECTS OF INVENTION
The object of present invention is to provide a process for preparation of O, O-dimethyl-O- (3,5,6 -trichloro-2 -pyridinyl)phosphorothioate, which obviates the need of carrying out the reaction in an organic solvent or using surfactants and ortho boric acid.
SUMMARY OF THE INVENTION
The invented process for preparation of 0,0-dimethyl-0-(3,5,6-trichloro- 2-pyridinyl)phosphorothioate involves condensation of alkali metal salt (preferably sodium salt) of 3,5,6-trichloro-2-pyridinol with 0,0-dimethyl phosphorochloridothioate in presence of one or more catalyst(s), without use of any organic solvent or surfactant.
DETAILED DESCRIPTION OF THE INVENTION
As per the invented process for preparation of title compound, alkali metal salt {preferably sodium salt) of 3,5,6-trichloro-2-pyridinol (Na-TCP) is reacted with 0,0-dimethyl phosphorochloridothioate (DMTC1) in presence of a tertiary amine such as 4-(N,N-dimethylamino)pyridine (DMAP); a quaternary ammonium salt such as triethylbenzylammonium chloride (TEBAC), tetrabutylammonium bromide (TBAB) or cetyl trimethylammonium bromide (CTAB) as a catalyst; an alkali metal salt, preferably sodium chloride (NaCl).
The reaction is carried out in a two-phase system, one phase of which is an aqueous phase. The reactant 0,0-dimethyl phosphorochloridothioate serves as the organic phase initially. As this reactant gets consumed during the course of the reaction, the title product is formed which is in a molten state under reaction conditions and it serves as organic phase. No other organic solvent is used in this process.
pH of the reaction mixture drops during the course of the reaction and hence it is maintained to 7-8 by addition of aqueous solution of Na2CO3.
The invented process includes steps comprising (1) preparing an aqueous slurry containing sodium salt of 3,5,6-trichloro-2-pyridinol, NaCl, tertiary amine and a quaternary ammonium salt as catalysts; (2) heating said slurry to a temperature in the range 40-45°C.; (3) gradual addition of 0,0-dimethyl phosphorochloridothioate with constant stirring, maintaining temperature in the range 40-45°C, and maintaining the pH of reaction mixture between 7-8 with aqueous solution of Na2CO3; (4) cooking the reaction mass at 40-45°C; (5) cooling the reaction mixture to 20-22°C; (6) separation of solid product obtained by filtration; (7) washing the product with water and drying.
EXAMPLE - 1
40 gms (0.160 mol) of 88% sodium 3,5,6-trichloro-2-pyridinate (NaTCP) (Containing 9% water), 110 ml of water, 6 g (0.102 mol) of sodium chloride (NaCl), 0.8 g (0.0035 mole) of triethylbenzylammonium chloride (TEBAC), 0.4 gm (0.0032 mole) of 4-N,N-dimethylamino pyridine (DMAP) were taken in a round bottom flask equipped with a mechanical stirrer, thermometer pocket, pH meter and reflux condenser to obtain an aqueous slurry. The mixture was heated to a temperature of 40°C. To this slurry, 27.5 gm (0.166 mol) of 98% 0,0-dimethyl phosphorochloridothioate was added over a period of 1.5 hrs. During addition, pH of the reaction mass was maintained at 7-7.5 by addition of 18% aqueous sodium carbonate (Na2C03) solution (approximately 14-15 ml). The reaction mass was cooked for 3 hrs. at 40°C. It was then cooled to 20-22°C. The solid product obtained was separated by filtration. It was then washed with water and dried to obtain 98.8% 0,0- dimethyl-0-(3,5,6-trichloro-2-pyridinyl)phosphorothioate with 87.7 % yield.
Examples 2-7
Examples 2-7 were carried out in an identical manner as Example - 1. The results are presented in Table-1.
Examples 8-13
Examples 8-13 were carried out in an identical manner as Example - 1, except that the catalyst TBAB was used in place of TEBAC. The results are presented in Table-2.
Examples 14 - 20
Examples 14-20 were carried out in an identical manner as Example - 1, except that catalyst CTAB was used in place of TEBAC. The results are presented in Table-3.

CLAIMS:
1. A process for preparation of 0,0-dimethyl-0-(3,5,6-trichloro-2- pyridinyl)phosphorothioate comprising steps of:
(1) heating a mixture comprising alkali metal salt of 3,5,6- trichloro-2-pyridinol? water, NaCl, a tertiary amine and a quaternary ammonium salt;
(2) gradual addition of 0,0-dimethyl phosphorochloridothioate to the above mixture with constant stirring, controlling temperature and controlling the pH of reaction mixture during addition;
(3) cooling the reaction mixture; and
(4) separating the solid product.
2. A process as claimed in claim-1, wherein in step (1), the mixture comprising alkali metal salt of 3,5,6-trichloro-2-pyridinol, water, NaCl, a tertiary amine and a quaternary ammonium salt, is heated to 40-46°C.
3. A process as claimed in claim-1, wherein in step (2), the temperature is controlled to 40-46°C.
4. A process as claimed in claim-1, wherein in step (2), the pH is controlled to 7-8.
5. A process as claimed in any of the claims 1 or 4, wherein the pH is controlled by adding aqueous solution of Na2CO3.
6. A process as claimed in claim-1, wherein in step (3), the reaction mixture is cooled to 20-22°C.
7. A process as claimed in claim-1, wherein said alkali metal salt of 3,5,6-trichloro-2-pyridinol is sodium salt.
8. A process as claimed in claim-1, wherein said tertiary amine is 4- N,N-dimethylamino pyridine.
9. A process as claimed in claim-1, wherein said quaternary ammonium salt is selected from triethylbenzylammonium chloride, tetrabutylammonium bromide or cetyl trimethylammonium bromide.
10. A process as claimed in claim-1, wherein the product obtained is given washing(s) with water to remove excess alkali.
11. A process as claimed in any of the claims 1 or 10, wherein the product obtained is dried.
12. A process as claimed in claim-1, wherein steps (3) and (4) are omitted.