Process for preparing dithiine-tetracarboxy-diimides

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

in which

R 1 and R 2 are identical or different and are selected from the group consisting of hydrogen; C 1 -C 8 -alkyl, which is optionally substituted one or more times by halogen, —OR 3 , or —COR 4 ; 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; and 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, 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,

wherein

a compound of formula (VI)

in which R is R 1 or R 2

is reacted with thiourea in water, optionally in the presence of a phase transfer catalyst, in the absence of an organic solvent to give the compound of formula (I).

2. A process according to claim 1 , wherein the compound of formula (VI) is reacted with thiourea in the presence of a phase transfer catalyst of formula (VII)

in which

A is N or P,

R 5 , R 6 , R 7 and R 8 are identical or different and are independently branched or unbranched C 1 -C 28 -alkyl, C 6 -C 10 -aryl or benzyl,

X is halogen, hydrogen sulphate, sulphate, dihydrogen phosphate, hydrogen phosphate, phosphate or acetate.

3. A process according to claim 1 , wherein between 0.8 and 2 mol of thiourea are used per mole of the compound of formula (VI).

4. A process according to claim 1 , wherein between 0.9 and 1.7 mol of thiourea are used per mole of the compound of formula (VI).

5. A process according to claim 1 , wherein between 1.0 and 1.3 mol of thiourea are used per mole of the compound of formula (VI).

6. A process according to claim 1 , wherein the thiourea is initially introduced in water.

7. A process according to claim 2 , wherein between 0.8 and 2 mol of thiourea are used per mole of the compound of formula (VI).

8. A process according to claim 2 , wherein between 0.9 and 1.7 mol of thiourea are used per mole of the compound of formula (VI).

9. A process according to claim 2 , wherein between 1.0 and 1.3 mol of thiourea are used per mole of the compound of formula (VI).

10. A process according to claim 2 , wherein the thiourea is initially introduced in water.

11. A process according to claim 3 , wherein the thiourea is initially introduced in water.

12. A process according to claim 4 , wherein the thiourea is initially introduced in water.

13. A process according to claim 5 , wherein the thiourea is initially introduced in water.