Abstract
Three major factors determine
torquoselectivity, which is the diastereoselectivity in electrocyclic
ring-opening reactions to produce E/Z-double bond(s). One is the interaction
between the decomposing sCC bond and
low-lying vacant orbital(s), such as a p*- or s*-orbital
on the substituent, which promotes the reaction, resulting in inward rotation
of the substituent. Second, for a substituent with a lone pair(s), repulsive
interaction between the decomposing s-bond and the lone pair(s) hinders inward rotation,
so that the products of outward rotation should be preferred. Finally, a more
strongly donating s-electron-donating group (sEDG) rotates
inwardly due to stabilization by phase-continuous cyclic orbital interaction. We
compared the latter two interactions, repulsion between the lone pairs on the
substituent and stabilization from phase-continuous cyclic orbital interaction,
to determine which has a greater effect on the diastereoselectivity. We considered
a series of model reactions with halogen substituents, and concluded that the
diastereoselectivity is mainly controlled by cyclic orbital interaction.