Substituent effect on the competition between hetero-Diels-Alder and cheletropic additions of sulfur dioxide to 1-substituted buta-1,3-dienes

The reactivity of sulfur dioxide toward variously substituted butadienes was explored in an effort to define the factors affecting the competition between the hetero-Diels-Alder and cheletropic additions. At low temperature (<-70°), 1-alkyl-substituted 1,3-dienes 1 that can adopt s-cis-conformations add to SO2 in the hetero-Diels-Alder mode in the presence of CF3COOH as promoter. In the case of (E)-1-ethylidene-2-methylidenecyclohexane ((E)-4a), the [4+2] cycloaddition of SO2 is fast at -90° without acid catalyst. (E)-1-(Acyloxy)buta-1,3-dienes (E)-1c, (E)-1y, and (E)-1z with AcO, BzO, and naphthalene-2-(carbonyloxy). substituents, respectively also undergo the hetero-Diels-Alder addition with SO2+CF3COOH at low temperatures, giving a 1:10 mixture of the corresponding cis- and trans-6-(acyloxy)sultines c-2c,y,z and t-2c,y,z, respectively). Above -50°, the sultines undergo complete cycloreversion to the corresponding dienes and SO2, which that add in the cheletropic mode at higher temperature to give the corresponding 2-substituted sulfolenes (=2,5-dihydrothiophene 1,1-dioxides) 3. The hetero-Diels-Alder additions of SO2 follow the Alder endo rule, giving first the 6-substituted cis-sultines that equilibrate then with the more stable trans-isomers. This statement is based on the assumption that the S=O group in the sultine prefers a pseudo-axial rather than a pseudo-equatorial position, as predicted by quantum calculations. The most striking observation is that electron-rich dienes such as 1-cyclopropyl-, 1-phenyl-, 1-(4-methoxyphenyl)-, 1-(trimethylsilyl)-, 1-phenoxy-, 1-(4-chlorophenoxy)-, 1-(4-methoxyphenoxy)-, 1-(4-nitrophenoxy)-, 1-(naphthalen-2-yloxy)-, 1-(methylthio)-, 1-(phenylthio)-, 1-[(4-chlorophenyl)thio]-, 1-[(4-methoxyphenyl)thio]-, 1-[(4-nitrophenyl)thio]-, and 1-(phenylseleno)buta-1,3-diene, as well as 1-(methoxymethylidene)-2-methylidenecyclohexane (4f) do not equilibrate with the corresponding sultines between -100 and -10°, in the presence of a large excess of SO2, with or without acidic promoter. The hetero-Diels-Alder additions of SO2 to 1-substituted (E)-buta-1,3-dienes are highly regioselective, giving exclusively the corresponding 6-substituted sultines. The 1-substituted (Z)-buta-1,3-dienes do not undergo the hetero-Diels-Alder additions with sulfur dioxide.