Exo/endo Selectivity Control in Diels–Alder Reactions of Geminal Bis(silyl) Dienes: Theoretical and Experimental Studies

Abstract

The factors controlling the reactivity and exo/ endo selectivity of Diels-Alder reactions of geminal bis(silyl) dienes catalyzed by AlEt2Cl are studied at the B3LYP-D3(BJ)(SMD,CH2Cl2)/6-31++G**//B3LYP-D3(BJ)(SMD,CH2Cl2)/6-31+G* theoretical level. The reaction proceeds via a two-stage one-step mechanism, and the AlEt2Cl as a Lewis acid catalyst enhances the electrophilicity of the carbonyl compound by coordination, consequently accelerating a cycloaddition reaction with a low energy barrier. A geminal bis(silyl) group of the diene and an α-substituent in α,β-unsaturated carbonyl compounds adjust the interaction energy (Δ Eint) as well as the deformation energy (Δ Estrain) of the diene and dienophile, affecting the barrier height and the diastereochemical outcome accordingly. The steric repulsion between the geminal bis(silyl) group and Al(III) catalyst increases the Pauli repulsion energy (Δ EPauli) and strain energy of dienophile fragment (Δ Estrain(dienophile-LA)) in the endo pathway, ensuring the exo selectivity. The introduction of a halogen atom (Cl or Br) or methyl group at the α-position of α,β-unsaturated carbonyl compounds increases the deformation energy of the diene fragment. Meanwhile, the noncovalent interactions (that is, dispersion and electrostatic interaction) stabilize the endo transition state, leading to predominant endo products. The theoretical predictions of the exo/ endo selectivity for Diels-Alder reactions of the substituted α,β-unsaturated carbonyl compound with Cl or Br atoms by the DFT method are also well confirmed by experiment.