Ab initio and experimental studies on the hetero-Diels-Alder and cheletropic additions of sulfur dioxide to (E)-1-methoxybutadiene: a mechanism involving three molecules of SO(2)

Kinetics on the cheletropic addition of sulfur dioxide to (E)-1-methoxybutadiene (1) to give the corresponding sulfolene 2 (2-methoxy-2,5-dihydrothiophene-1,1-dioxide) gave the rate law d2]/dt = k1]SO(2)](x)() with x = 2.6 +/- 0.2 at 198 K. Under these conditions, no sultine 3 (2RS,6RS)-6-methoxy-3,6-dihydro-1,2-oxathiin-2-oxide] resulting from a hetero-Diels-Alder addition was observed, and the cheletropic elimination 2 --> 1 + SO(2) did not occur. Ab initio and DFT quantum calculations confirmed that the cheletropic addition 1 + SO(2) --> 2 follows two parallel mechanisms, one involving two molecules of SO(2) and the transition structure with DeltaG(++) = 18.2 +/- 0.2 kcal/mol at 198 K (exptl); 22.5-22.7 kcal/mol B3LYP/6-31G(d,p)], the other one involving three molecules of SO(2) with DeltaG(++) = 18.9 +/- 0.1 kcal/mol at 198 K (exptl); 19.7 kcal/mol B3LYP/6-31G(d,p)]. The mechanism involving only one molecule of SO(2) in the transition structure requires a higher activation energy, DeltaG(++) = 25.2 kcal/mol B3LYP/6-31G(d,p)]. Comparison of the geometries and energetics of the structures involved into the 1 + SO(2) --> 2, 3 and 1 + 2SO(2) --> 2, 3 + SO(2) reactions obtained by ab initio and DFT methods suggest that the latter calculation techniques can be used to study the cycloadditions of sulfur dioxide. The calculations predict that the hetero-Diels-Alder addition 1 + SO(2) --> 3 also prefers a mechanism in which three molecules of SO(2) are involved in the cycloaddition transition structure. At 198 K and in SO(2) solutions, the entropy cost (TDeltaS(++)) is overcompensated by the specific solvation by SO(2) in the transition structures of both the cheletropic and hetero-Diels-Alder reactions of (E)-1-methoxybutadiene with SO(2).