1001-1009The
reaction between chiral selenium ylide and benzaldehyde leads to the formation
of (2S,3S)-trans-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 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