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Unique Steric Effect of Geminal Bis(silane) To Control the High Exo-selectivity in Intermolecular Diels–Alder Reaction
- Liu, Zengjin, Lin, Xinglong, Yang, Na, Su, Zhishan, Hu, Changwei, Xiao, Peihong, He, Yanyang, Song, Zhenlei
- Journal of the American Chemical Society 2016 v.138 no.6 pp. 1877-1883
- carbonyl compounds, cyclohexenes, enantiomers, molecular conformation, silane
- The unique steric effect of geminal bis(silane) [(R₃Si)₂CH] allows an exo-selective intermolecular Diels–Alder reaction of geminal bis(silyl) dienes with α,β-unsaturated carbonyl compounds. The approach shows good generality to form ortho–trans cyclohexenes in good yields with high exo-selectivity and high enantioselectivity in some asymmetric cases. The excellent exo-stereocontrol aptitude of (R₃Si)₂CH group is highlighted by comparing with R₃SiCH₂ and R₃Si groups, which leads to endo-selectivity predominantly. The conformational analysis of dienes suggests that (R₃Si)₂CH group effectively shields both sides of the diene moiety, ensuring the desired exo-selectivity. Moreover, the geminal bis(silane) can be further functionalized to transform the resulting ortho–trans cycloadducts into useful synthons, which makes the approach hold great potential for organic synthesis.