Process for preparing dithiine-tetracarboxy-diimides

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

in which

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

R 3 is hydrogen, C 1 -C 4 -alkyl or C 1 -C 4 -alkylcarbonyl or is aryl which is optionally substituted one or more times by halogen, C 1 -C 4 -alkyl or C 1 -C 4 -haloalkyl, and

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

said process comprises reacting

(A) a compound of formula (VI)

in which R is R 1 or R 2 are defined above

with an inorganic sulphide or hydrogen sulphide in a solvent or solvent mixture in a molar ratio between 0.2 and 0.95 mol of sulphide or hydrogen sulphide per mole of the compound of formula (VI) to form a reaction product, and

(B) heating the reaction product, which is composed of a mixture of at least on compound of formula (I) and at least one compound of formula (VII)

in which

R independently at each occurrence is R 1 or R 2 as defined above, and

n is an integer between 1 and 50,

in a diluent.

2. A compound of formula (VII)

in which

R independently at each occurrence is R 1 or R 2 ,

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

R 3 is hydrogen, C 1 -C 4 -alkyl or C 1 -C 4 -alkylcarbonyl or is aryl which is optionally substituted one or more times by 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,

n is an integer between 1 and 50.

3. The process according to claim 1 , wherein the sulphide or hydrogen sulphide is lithium sulphide, sodium sulphide, sodium hydrogen sulphide, potassium sulphide, calcium sulphide, calcium hydrogen sulphide, magnesium sulphide or ammonium sulphide or mixtures thereof or hydrates thereof.

4. The process according to claim 1 , wherein a polar solvent is used as diluent in process step (B).

5. The process according to claim 4 , wherein the solvent is an amide, a nitrile, a ketone, dimethyl sulphoxide, sulpholane or water, either used alone or in mixtures with water.

6. The process according to claim 5 , wherein the amide is selected from the group consisting of formamide, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

7. The process according to claim 5 , wherein the alcohol is selected from the group consisting of propanol, isobutanol, pentanol, and ethylene glycol.

8. The process according to claim 5 , wherein the ester is selected from the group consisting of methyl acetate, ethyl acetate, and butyl acetate.

9. The process according to claim 5 , wherein the nitrile is selected from the group consisting of acetonitrile, and butyronitrile.

10. The process according to claim 5 , wherein the ketone is selected from the group consisting of pinacolone, and methyl isobutyl ketone.