Regioselektive 1,3‐Dipolare Cycloadditionen eines ‘Thiocarbonyl‐methanids’ ((Alkylidensulfonio)methanids) mit aromatischen Sulfinen

Regioselective 1,3‐Dipolar Cycloadditions of a ‘Thiocarbonyl‐methanide’ ((Alkylidenesulfonio)methanide) with Aromatic Sulfines Reaction of the spirocyclic 2,5‐dihydro‐1,3,4‐thiadiazole 7 and thiobenzophenone S‐oxide (6a) in THF at 45° yielded the spirocyclic 1,3‐dithiolane 1‐oxide 8, thiirane 9, and the diazane derivative 10 in a ratio of 61:15:23 (Scheme 2). The formation of 8 is rationalized by a 1,3‐dipolar cycloaddition of ‘thiocarbonyl‐methanide’ 1, generated from 7 by thermal elimination of N2, and the CS bond of sulfine 6a. Cyclization of intermediate 1 leads to thiirane 9. Under the same conditions, 7 and adamantane‐2‐thione S‐oxide (6b) or 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone S‐oxide (4) reacted to give only 9 and 10 but no cycloadduct of type 8 (Scheme 4). With the aim to favor the formation of 8, a mixture of 6a and 1.1 equiv. of 7 was heated to 45° without any solvent in a sealed tube. The ratio of products was only slightly different from that of the thermolysis in THF. An analogous experiment with 7 and 9H‐fluorene‐9‐thione S‐oxide (6c) yielded cycloadduct 13 and 9 (Scheme 5). It is most interesting that the 1,3‐dipolar cycloadditions of 1 and the sulfines 6a and 6c proceeded with different regioselectivity. A reaction mechanism for the unexpected formation of 10 is proposed in Scheme 7. The key step is the base‐catalyzed ring opening of 7 and the nucleophilic addition of the thereby formed thiolate 21 onto the sulfonium ion 19.