Organic heterocyclothiazenes. Part 5. Cycloaddition reactions of tetrasulphur tetranitride with highly electron deficient alkynes

In contrast with previous, complex, S4N4-alkyne reactions, treatment of butynedinitrile and S4N4 is found to give 1,3,5,2,4-trithiadiazepine-6,7-dicarbonitrile (8) and 1,2,5-thiadiazole-3,4-dicarbonitrile (9) very cleanly and in high yield. Hexafluorobut-2-yne and 4,4,4-trifluorobutynonitrile (4) similarly give the trithiadiazepines (10) and (12), though more slowly and in slightly lower yields. Structures (8) and (10) correct literature assignments. The analogous reactions of ethyl 3-formylpropynoate (6), hex-3-yne-2,5-dione (7), and butynedial, like those of acetylenedicarboxylate esters, give complex mixtures from which trithiadiazepines, trithiatriazepines, and thiadiazoles are variously isolated. A mechanistic rationalization of these results is suggested. 1,2,5-Thiadiazole-3,4-dicarbaldehyde (18b) is prepared in high yield from S4N4 and 1,1,4,4-tetraethoxybutyne (20), followed by hydrolysis. This dialdehyde and other 3,4-dioxo-1,2,5-thiadiazoles are used to prepare 1,2,5-thiadiazolo[3,4-d]pyridazine and its derivatives (22) for the first time. Improved preparations for ethyl 3-formylpropynoate (6) and hex-3-yne-2,5-dione (7) are reported.