The Hetero-Diels-Alder Addition of Sulfur Dioxide: The Pseudo-Chair Conformation of a 4,5-Dialkylsultine

Even unsubstituted butadiene adds to sulfur dioxide in the hetero-Diels-Alder mode more rapidly than in the chelotropic mode. The sultine can be observed in equilibrium with the diene and the sulfur dioxide only at low temperature and in the presence of CF₃COOH. Crystals of 4,5-dialkyl-sultine resulting from the SO₂ addition to 1,2-dimethylidenecyclohexane have been obtained at -100°C and analyzed by X-ray diffraction. Quantum chemical calculations have shown that hyperconjugative interactions within the sulfinyl moiety are responsible for the anomeric effects observed in sultines that prefer pseudo-chair conformations with pseudo-axial S=O bonds.