In-situ treatment of pyridine 2,3-dicarboxylic acid esters with an oxidizing agent

1. A method for the in-situ removal of impurities from a saponified solution of a pyridine-2,3-dicarboxylic acid ester, said method comprising the steps of:

providing a saponified solution comprising the product produced by adding a base to a pyridine-2,3-dicarboxylic acid ester;

reacting said saponified solution with an amount of an oxidizing agent effective to remove impurities, thereby providing a purified saponified solution; and

collecting said purified saponified solution;

wherein said pyridine-2,3-dicarboxylic acid ester is a compound of the formula

wherein R 4 and R 6 are each independently H, C 1 -C 6 alkyl, C 1 -C 6 alkenyl, phenyl or substituted phenyl;

R 5 is H; halogen; C 1 -C 6 alkyl optionally substituted with one or more C 1 -C 4 alkoxy groups; C 1 -C 6 alkenyl; phenyl or substituted phenyl; and

R 2 and R 3 are each independently C 1 -C 6 alkyl, phenyl or substituted phenyl; and

wherein said oxidizing agent is selected from the group consisting of peroxides, peroxyacids, and hypohalite salts.

2. The method of claim 1 wherein said base is a hydroxide.

3. The method of claim 2 wherein said hydroxide is sodium hydroxide.

4. The method of claim 1 wherein said oxidizing agent is hydrogen peroxide.

5. The method of claim 1 wherein said oxidizing agent is sodium hypochlorite or sodium hypobromite.

6. The method of claim 4 wherein said amount of hydrogen peroxide effective to remove impurities is an amount in the range of about 0.1 to about 2.0 moles hydrogen peroxide per mole of pyridine-2,3-dicarboxylic acid ester.

7. The method of claim 4 wherein said amount of hydrogen peroxide effective to remove impurities is an amount in the range of about 0.2 to about 0.8 moles hydrogen peroxide per mole of pyridine-2,3-dicarboxylic acid ester.

8. The method of claim 1 further comprising adding oxidizing agent until the color of said saponified solution changes from a darker color to a lighter color.

9. The method of claim 8 wherein the color of said saponified solution is changed from black to light amber.

10. The method of claim 1 wherein said reaction is performed at a temperature of about 60° C. to about 110° C.

11. The method of claim 1 wherein said oxidizing agent is added over a time period of about 15 to about 120 minutes.

12. The method of claim 1 wherein said reaction further comprises stirring said saponified solution.

13. The method of claim 12 wherein said stirring is carried out for a time period of about 15 to about 120 minutes.

14. A method for the in-situ removal of impurities from a solution of pyridine-2,3-dicarboxylic acid ester, said method comprising the steps of:

providing a solution comprising a pyridine-2,3-dicarboxylic acid ester;

saponifying said solution by adding a base thereto, thereby forming a saponified solution comprising a pyridine-2,3-dicarboxylic acid salt;

reacting said saponified solution with an amount of an_oxidizing agent effective to remove impurities, to produce a purified saponified solution;

acidifying said purified saponified_solution and converting said pyridine-2,3-dicarboxylic acid salt into the corresponding pyridine-2,3-dicarboxylic acid by adding an acid to said purified saponified solution; and

collecting a purified solution comprising the pyridine-2,3-dicarboxylic acid;

wherein said pyridine-2,3-dicarboxylic acid ester is a compound of the formula

wherein R 4 and R 6 are each independently H, C 1 -C 6 alkyl, C 1 -C 6 alkenyl, phenyl or substituted phenyl;

R 5 is H; halogen; C 1 -C 6 alkyl optionally substituted with one or more C 1 -C 4 alkoxy groups; C 1 -C 6 alkenyl; phenyl or substituted phenyl; and

R 2 and R 3 are each independently C 1 -C 6 alkyl, phenyl or substituted phenyl; and

wherein said oxidizing agent is selected from the group consisting of peroxides, peroxyacids, and hypohalite salts.

15. The method of claim 14 wherein said base is a hydroxide.

16. The method of claim 15 wherein said hydroxide is sodium hydroxide.

17. The method of claim 14 wherein said oxidizing agent is hydrogen peroxide.

18. The method of claim 14 wherein said oxidizing agent is sodium hypochlorite or sodium hypobromite.

19. The method of claim 17 wherein said amount of hydrogen peroxide effective to remove impurities is an amount in the range of about 0.1 to about 2.0 moles hydrogen peroxide per mole of pyridine-2,3-dicarboxylic acid ester.

20. The method of claim 17 wherein said amount of hydrogen peroxide effective to remove impurities is an amount in the range of about 0.2 to about 0.8 moles hydrogen peroxide per mole of pyridine-2,3-dicarboxylic acid ester.

21. The method of claim 14 wherein said amount of oxidizing agent effective to remove impurities is an amount necessary to change the color of said saponified solution from a darker color to a lighter color.

22. The method of claim 21 wherein the color of said saponified solution is changed from black to light amber.

23. The method of claim 14 wherein said reaction is performed at a temperature of about 60° C. to about 110° C.

24. The method of claim 14 wherein said oxidizing agent is added over a time period of about 15 to about 120 minutes.

25. The method of claim 14 wherein said reaction further comprises stirring said saponified solution.

26. The method of claim 25 wherein said stirring is carried out for a time period of about 15 to about 120 minutes.

27. The method of claim 14 wherein said acid is sulfuric acid.

28. The method of claim 14 , wherein said pyridine-2,3-dicarboxylic acid is 5-methyl-pyridine-2,3-dicarboxylic acid or 5-ethyl-pyridine-2,3-dicarboxylic acid.

29. A method for the preparation of herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters, and salts, said method comprising the steps of:

providing a solution comprising a pyridine-2,3-dicarboxylic acid ester;

saponifying said solution by adding a base thereto, thereby forming a saponified solution comprising a pyridine-2,3-dicarboxylic acid salt;

reacting said saponified solution with an amount of an oxidizing agent effective to remove impurities to produce a purified saponified solution;

acidifying said purified saponified solution and converting said pyridine-2,3-dicarboxylic acid salt into the corresponding pyridine-2,3-dicarboxylic acid by adding an acid to said purified saponified solution;

using said pyridine-2,3-dicarboxylic acid as an intermediate in the preparation of herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters, and salts;

wherein said pyridine-2,3-dicarboxylic acid ester is a compound of the formula

and

converting said pyridine-2,3-dicarboxylic acid into a herbicidal 2-(2-imidazolin-2-yl)nicotinic acid, ester, or salt;

wherein R 4 and R 6 are each independently H, C 1 -C 6 alkyl, C 1 -C 6 alkenyl, phenyl or substituted phenyl;

R 5 is H; halogen; C 1 -C 6 alkyl optionally substituted with one or more C 1 -C 4 alkoxy groups; C 1 -C 6 alkenyl; phenyl or substituted phenyl; and

R 2 and R 3 are each independently C 1 -C 6 alkyl, phenyl or substituted phenyl; and

wherein said oxidizing agent is selected from the group consisting of peroxides, peroxyacids, and hypohalite salts.

30. The method of claim 29 wherein said base is a hydroxide.

31. The method of claim 30 wherein said hydroxide is sodium hydroxide.

32. The method of claim 29 wherein said oxidizing agent is hydrogen peroxide.

33. The method of claim 29 wherein said oxidizing agent is sodium hypochlorite or sodium hypobromite.

34. The method of claim 32 wherein said amount of hydrogen peroxide effective to remove impurities is an amount in the range of about 0.1 to about 2.0 moles hydrogen peroxide per mole of pyridine-2,3-dicarboxylic acid ester.

35. The method of claim 32 wherein said amount of hydrogen peroxide effective to remove impurities is an amount in the range of about 0.2 to about 0.8 moles hydrogen peroxide per mole of pyridine-2,3-dicarboxylic acid ester.

36. The method of claim 29 wherein said amount of oxidizing agent effective to remove impurities is an amount necessary to change the color of said saponified solution from a darker color to a lighter color.

37. The method of claim 36 wherein the color of said saponified solution is changed from black to light amber.

38. The method of claim 29 wherein said reaction is performed at a temperature of about 60° C. to about 110° C.

39. The method of claim 29 wherein said oxidizing agent is added over a time period of about 15 to about 120 minutes.

40. The method of claim 29 wherein said reaction further comprises stirring said saponified solution.

41. The method of claim 40 wherein said stirring is carried out for a time period of about 15 to about 120 minutes.

42. The method of claim 29 wherein said acid is sulfuric acid.

43. The method of claim 29 wherein said pyridine-2,3-dicarboxylic acid is 5-methyl-pyridine-2,3-dicarboxylic acid or 5-ethyl-pyridine-2,3-dicarboxylic acid.