1,3‐Dipolar Cycloadditions, 96. – Cycloadditions of Alicyclic Thione S‐Oxides with Alicyclic Thiones

Thione S‐oxides (sulfines) harbor the allyl anion MO of 1,3‐dipoles, but fail to undergo 1,3‐cycloadditions to the usual dipolarophiles with a CC multiple bond. Thioketones stand the test as superdipolarophiles. The thione S‐oxides 3 and 9 combined with the thioketones 5 and 7 at 80–100°C, furnishing spiro‐1,2,4‐oxadithiolanes in high yields despite steric hindrance. The structures of the cycloadducts were clarified by their spectra and an X‐ray analysis of 11. Becke3LYP/6‐31G* calculations of transition structures and products for the cycloadditions of thioformaldehyde S‐oxide to thioformaldehyde support the observed regiochemistry. The activation energy amounts to 3.9 kcal mol−1, in contrast to EA = 17.4 kcal mol−1 for the corresponding addition to ethylene. The transition structures are in accordance with concerted cycloadditions.