[2+3] Cycloadditions of ‘Thiocarbonyl Ylides’ (= (Alkylidenesulfonio)methanides) and diazoalkanes with N , N ′‐(thiocarbonyl)diimidazole (= 1,1′‐(carbonothioyl)bis[1 H ‐imidazole])

Heating of a mixture of N , N ′‐(thiocarbonyl)diimidazole (= 1,1′‐(carbonothioyl)bis[1 H ‐imidazole]; 1 ) and 2,5‐dihydro‐1,3,4‐thiadiazole 2a or 2b gave the 1,3‐dithiolanes 4a and 4b , respectively, via a regiospecific 1,3‐dipolar cycloaddition of the corresponding ‘thiocarbonyl methanides’ 3a , b onto the CS group of 1 ( Schemes 1 and 2 ). The adamantane derivative 4b was not stable in the presence of 1 H ‐imidazole and during chromatographic workup. The isolated 1,3‐dithiole 5 is the product of a base‐catalyzed elimination of 1 H ‐imidazole from the initial cycloadduct 4b . The formation of the S,N‐acetal 6 can be rationalized by a protonation of the ‘thiocarbonyl ylide’ 3b followed by a nucleophilic addition of 1 H ‐imidazole. With the diazo compounds 8a–e ( Scheme 3 ) 1 underwent a regiospecific 1,3‐dipolar cycloaddition to give the corresponding 2,5‐dihydro‐1,3,4‐thiadiazole derivatives 9 , which spontaneously eliminated 1 H ‐imidazole to yield (1 H ‐imidazol‐1‐yl)‐1,3,4‐thiadiazoles 10 . The structures of 10a and 10d were established by X‐ray crystallography. In the case of diazodiphenylmethane ( 8f ), the initial cycloadduct 9f decomposed via a ‘twofold extrusion’ of N 2 and S to give 1,1′‐(2,2‐diphenylethenylidene)bis[1 H ‐imidazole] ( 11 ; Scheme 3 ).