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 SO2

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 d[2]/dt = k[1] SO2 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 + SO2 did not occur. Ab initio and DFT quantum calculations confirmed that the cheletropic addition 1 + SO2 --> 2 follows two parallel mechanisms, one involving two molecules of SO2 and the transition structure with DeltaG(double dagger) = 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 SO2 with DeltaG(double dagger) = 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 SO2 in the transition structure requires a higher activation energy, DeltaG(double dagger) = 25.2 kcal/mol [B3LYP/6-31G(d,p)]. Comparison of the geometries and energetics of the structures involved into the 1 + SO2 --> 2, 3 and 1 + 2SO(2) --> 2, 3 + SO2 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 + SO2 --> 3 also prefers a mechanism in which three molecules of SO2 are involved in the cycloaddition transition structure. At 198 K and in SO2 solutions, the entropy cost (TDeltaS(double dagger)) is overcompensated by the specific solvation by SO2 in the transition structures of both the cheletropic and hetero-Diels-Alder reactions of (E)-1-methoxybutadiene with SO2.