Unique Steric Effect of Geminal Bis(silane) To Control the High Exo-selectivity in Intermolecular Diels–Alder Reaction

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.