Torquoselectivity of the Ring-Opening Reaction of 3,3-Dihalosubstituted Cyclobutenes: Lone Pair Repulsion and Cyclic Orbital Interaction

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 s_CC 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.