Unraveling the reaction mechanism, enantio and diastereoselectivities of selenium ylide promoted epoxidation

Abstract

The reaction between chiral selenium ylide and benzaldehyde leads to the formation of <i style="mso-bidi-font-style:normal">(2S,3S)-trans</i>-epoxide with high enantio- and diastereoselectivity. Density functional theory and Hartree-Fock calculations using 6-31G(d) basis set have been performed to understand the reaction mechanism and factors associated with enantio- and diastereoselectivities. Conformation of chiral selenium ylide has been found to have a strong influence on the stability of the initial addition transition state between ylide and benzaldehyde. Calculated enantio- and diastereoselectivities from the energy differences between B3LYP/6-31G(d) addition TSs are in good agreement with the experimental data. The rate and diastereoselectivity are controlled by the <i style="mso-bidi-font-style: normal">cisoid-transoid</i> rotational transition state. Analysis of transition state geometries clearly reveals that unfavorable eclipsing interaction between phenyl groups of the benzaldehyde and ylidic substituents mainly governs the energy differences between the enantio and diastereomeric transition states. The favourable reactivity is also explained through Fukui function calculations.