Process for preparing dithiine-tetracarboxy-diimides

1. A process for preparing a compound of formula (I)

in which

R 1 and R 2 are identical or different and are hydrogen; C 1 -C 8 -alkyl which is optionally substituted with one or more halogen, —OR 3 , or —COR 4 ; C 3 -C 7 -cycloalkyl which is optionally substituted with one or more halogen, C 1 -C 4 -alkyl, or C 1 -C 4 -haloalkyl; or aryl or aryl-(C 1 -C 4 -alkyl) each of which is optionally substituted with one or more halogen, C 1 -C 4 -alkyl, C 1 -C 4 -haloalkyl, —COR 4 , or sulphonylamino,

R 3 is hydrogen, C 1 -C 4 -alkyl, C 1 -C 4 -alkylcarbonyl, or aryl, wherein the aryl is optionally substituted with one or more halogen, C 1 -C 4 -alkyl, or C 1 -C 4 -haloalkyl,

R 4 is hydroxyl, C 1 -C 4 -alkyl, or C 1 -C 4 -alkoxy,

comprising reacting

a compound of formula (VI)

in which R is R 1 or R 2

with an inorganic thiosulphate in a solvent or solvent mixture, wherein between 1.3 and 1.6 moles of thiosulphate are used per mole of dichloromaleimide of formula (VI).

2. The process of claim 1 , wherein the thiosulphate is soluble and is selected from lithium thiosulphate, sodium thiosulphate, potassium thiosulphate, cesium thiosulphate, magnesium thiosulphate, ammonium thiosulphate, or mixtures thereof.

3. The process of claim 1 , wherein R 1 is hydrogen, methyl, ethyl, n-propyl, isopropyl, 2,2-difluoroethyl, 2,2,2-trifluoroethyl, cyclopropyl, or cyclohexyl.

4. The process of claim 1 , wherein R 1 is hydrogen or methyl.

5. The process of claim 1 , wherein the thiosulphate is selected from sodium thiosulphate, potassium thiosulphate, ammonium thiosulphate, or mixtures thereof.

6. The process of claim 1 , wherein the thiosulfate is sodium thiosulfate.

7. The process of claim 1 , wherein the solvent is selected from water, dimethyl sulphoxide, sulpholane, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, cyclopentanol, cyclohexanol, ethylene glycol, ethylene glycol monomethyl ether, methyl acetate, ethyl acetate, formamide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, 1,4-dioxane, acetonitrile, propionitrile, butyronitrile, benzonitrile, acetone, methyl ethyl ketone, methyl isobutyl ketone, pinacolone, or mixtures thereof.

8. The process of claim 1 , wherein the solvent is selected from water, dimethyl sulphoxide, methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, cyclopentanol, cyclohexanol, ethylene glycol, methyl acetate, N,N-dimethylformamide, N,N-dimethylacetamide, tetrahydrofuran, 1,4-dioxane, acetonitrile, acetone, methyl ethyl ketone, methyl isobutyl ketone, or mixtures thereof.

9. The process of claim 1 , wherein the solvent is selected from water, methanol, ethanol, propanol, isopropanol, methyl acetate, tetrahydrofuran, 1,4-dioxane, acetonitrile, acetone, or mixtures thereof.

10. The process of claim 1 , wherein the compound of formula (I) is 2,6-dimethyl-1H,5H-[1,4]dithiino[2,3-c:5,6-c′]dipyrrole-1,3,5,7(2H,6H)tetrone or 1H,5H-[1,4]dithiino[2,3-c:5,6-c′]dipyrrole-1,3,5,7(2H,6H)tetrone.