A COMPARATIVE-ANALYSIS OF THE MECHANISMS OF CHELETROPIC AND DIELS-ALDER REACTIONS OF 1,3-DIENES WITH SULFUR-DIOXIDE - KINETIC AND THERMODYNAMIC CONTROLS

Ab initio calculations at the MP2/6-31G*//HF/6-31G* level have been carried out to study both thermal and Lewis acid-catalyzed [(omega)2(s) + (pi)4(s)]-cheletropic and [(pi)4(s) + (pi)2(s)]-cycloaddition reactions of sulfur dioxide with 1,3-dienes (1,3-butadiene and isoprene). The ab initio results have been analyzed by means of a theoretical method based on the expansion of the MOs of the supermolecule in terms of those of the fragments and the performance of the configuration analysis. While the thermal cheletropic reaction proceeds in a way which closely resembles a sigma donation/pi back-donation mechanism, the alternative Diels-Alder cycloaddition mostly involves charge transfer in one direction, HOMO(diene) --> LUMO(dienophile). Although a Diels-Alder adduct is the kinetic product of reactions of sulfur dioxide with conjugated dienes, the cheletropic reaction gives rise to a thermodynamically more stable five-membered ring adduct, in agreement with the experimental facts.