Process for preparing dithiine-tetracarboximides

1. A process for preparing a 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 with one or more halogen, —OR 3 , and/or —COR 4 , are 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 are aryl or aryl-(C 1 -C 4 -alkyl) each of which is optionally substituted with one or more halogen, C 1 -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 with one or more halogen, C 1 -C 4 -alkyl or C i -C 4 -haloalkyl,

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

(1) reacting a succinic monoamide of formula (VI)

in which R is R 1 or R 2 with an excess of thionyl chloride, optionally in the presence of a diluent,

(2) removing thionyl chloride to form a first product mixture; and

(3) adding an organic solvent, water and a phase transfer catalyst to the first product mixture and after a reaction time of between 5 minutes and 24 hours converting, into a second product mixture comprising a compound of formula (I), wherein the phase transfer catalyst is selected from the group consisting of tetrabutylammonium bromide, tetrabutylammonium hydrogen sulphate, benzyldodecyldimethylammonium chloride, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, methyltrioctylammonium chloride, methyltridecylammonium chloride, a mixture of methyltrioctylammonium chloride and methyltridecylammonium chloride, tetraoctylammonium bromide, and didecyldimethylammonium chloride.

2. The process according to claim 1 , wherein the phase transfer catalyst is selected from the group consisting of tetrabutylammonium hydrogen sulphate, benzyldodecyldimethylammonium chloride, dodecyltrimethylammonium chloride, tetradecyltrimethylammonium bromide, a mixture of methyltrioctylammonium chloride and methyltridecylammonium chloride, tetraoctylammonium bromide, and didecyldimethylammonium chloride.

3. The process according to claim 1 , wherein the organic solvent in (3) is selected from the group consisting of hexane, heptane, cyclohexane, methylcyclohexane, octane, isooctane, toluene, xylenes, mesitylene, ethylbenzene, chlorobenzene, dichlorobenzene, nitrobenzene, and mixtures thereof.

4. The process of claim 1 , further comprising:

(4) filtering the second product mixture of (3) to separate the compound of formula (I) from filtrate.

5. The process of claim 4 , wherein an organic solvent, water, and a phase transfer catalyst is added to the filtrate obtained in (4) and after a reaction time of between 5 minutes and 24 hours is converted into a compound of formula (I).

6. The process according to claim 5 , wherein the filtrate is used again up to ten times.

7. The process according to claim 5 or 6 , wherein only the organic phase, following removal of the desired product, is re-used.

8. The process of claim 1 , wherein in (1), between 4 and 50 moles of thionyl chloride are used per mole of the succinic monoamide of formula (VI).

9. The process of claim 1 , wherein in (1), between 10 and 40 moles of thionyl chloride are used per mole of the succinic monoamide of formula (VI).

10. The process of claim 1 , wherein the succinic monoamide is N-methylsuccinamide, N-butylsuccinamide, N-cyclohexylsuccinamide, or N-propylsuccinamide.

11. The process of claim 1 , wherein the compound of formula (I) is selected from:

2,6-dimethyl-1H,5H-[1,4]dithiino[2,3-c:5,6-c′]dipyrrole-1,3,5,7(2H,6H)-tetrone;

2,6-di-tert-butyl-1H,5H-[1,4]dithiino[2,3-c:5,6-c′]dipyrrole-1,3,5,7(2H,6H)-tetrone;

2,6-dicyclohexyl-1H,5H-[1,4]dithiino[2,3-c:5,6-c′]dipyrrole-1,3,5,7(2H,6H)-tetrone; or 2,6-dipropyl-1H,5H-[1,4]dithiino[2,3-c:5,6-c′]dipyrrole-1,3,5,7(2H,6H)-tetrone.

12. The process of claim 1 , wherein the thionyl chloride is removed by distillation.

13. The process of claim 1 , wherein (3) is carried out at a temperature between 30 ° C. and 130 ° C.

14. The process of claim 1 , wherein in (3) the reaction time is between 30 minutes and 6 hours.