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Competition between Hetero-Diels−Alder and Cheletropic Addition of Sulfur Dioxide. Theoretical and Experimental Substituent Effects on the Relative Stability of 3,6-Dihydro-1,2-oxathiin-2-oxides (Sultines) and 2,5-Dihydrothiophene-1,1-dioxides (Sulfolenes). Anomeric Effects in Sultine and 6-Substituted Derivatives

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journal contribution
posted on 13.11.1998, 00:00 by Tino Fernández, Dimas Suárez, José A. Sordo, Frédéric Monnat, Elena Roversi, Antonio Estrella de Castro, Kurt Schenk, Pierre Vogel
At low temperature and in the presence of CF3COOH, SO2 undergoes Diels−Alder additions with (E)-1-acetoxybutadiene (8d) giving a 1:10 mixture of diastereomeric 6-acetoxysultines (9d + 10d). The Van't Hoff plot for equilibria 8d + SO29d + 10d led to ΔHr = −7.0 ± 0.3 kcal/mol, ΔSr = −42 ± 3 cal·mol-1·K-1. At 20 °C, 8d underwent a slow cheletropic addition with SO2 giving 2-acetoxysulfolene (11d, ΔHr ≅ −11.5 kcal/mol), the structure of which was established by single-crystal X-ray diffraction studies. (E)-Chloro (8e) and (E)-bromobutadiene (8f) did not undergo Diels−Alder additions with SO2, even in the presence of protic or Lewis acid promoters. Low yields of 2-chloro- (11e) and 2-bromosulfolene (11f) were obtained at 20 °C. The structure of 11e was confirmed by single-crystal X-ray diffraction. The potential energy hypersurfaces of the Diels−Alder and cheletropic additions of SO2 to butadiene (8a), (E)-piperilene (8b), (E)-1-methoxy- (8c), (E)-1-acetoxy- (8d), and (E)-1-chlorobutadiene (8e) were studied by ab initio quantum calculations at the MP2/6-31G* level. In agreement with the experiment, 6-substituted sultines 9X and 10X were less stable than the corresponding 2-substituted sulfolenes 11X for X = Me, OAc, Cl. With X = OMe, the two diastereomeric 6-methoxysultines (9c, 10c) and 2-methoxysulfolene (11c) were calculated to have similar stabilities. This is attributed to a stabilizing thermodynamic anomeric effect or gem-sulfinate/methoxy disubstitution effect in 9c, 10c. Such effects were not detected for sulfinate/acetoxy (9d, 10d) and sulfinate/chloro (9e, 10e) disubstitutions. The relative instability of 2-acetoxy- (11d) and 2-chlorosulfolene (11e) compared with their cycloaddents is attributed to repulsive interactions between the SO2 moieties and the 2-substituents. The Alder endo mode of [4 + 2] cycloaddition of SO2 is predicted to be faster than the “anti-Alder mode” of additions for dienes 8X, X = Me, OMe, OAc, Cl. The resulting diastereomeric sultines 9X and 10X, respectively, exist as equilibria (energy barrier: ca. 5−6 kcal/mol) of two conformers 9X9X, 10X10X. In general, the conformers 9X, 10X with pseudoaxial SO group are preferred (conformational anomeric effect of the sulfinate moiety). Repulsive interactions between pseudoaxial SO and polar cis-6-substituents (e.g.:  X = OMe, OAc) in 9X may render conformers 9X (with the SO and 6-X groups in pseudoequatorial positions) as stable as conformers 9X. The calculations predict the existence of conformational anomeric effects of 2−3 kcal/mol for the gem-sulfinate/methoxy (9c, 10c) and gem-sulfinate/acetoxy disubstitution (9d, 10d).