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 orthotrans 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 orthotrans cycloadducts into useful synthons, which makes the approach hold great potential for organic synthesis.