Substituent Effects on Rates and Torquoselectivities of Electrocyclic Ring-Openings of N?Substituted 2?Azetines

摘要

Transition structures for the conrotatory electrocyclic ring-opening reactions of N-substituted 2-azetines were computed with the density functional M06-2X/6- 31+G(d,p). A wide range of substituents from π acceptors (e.g., CHO, CN) to π donors (NMe_2, OMe) was explored. Acceptor substituents delocalize the nitrogen lone pair and stabilize the reactant state of 2-azetines, while donors destabilize the 2-azetine reactant state. The conrotatory ring-opening is torquoselective, and the transition state for the outward rotation of the N-substituent and inward rotation of the nitrogen lone pair is preferred. This transition structure is stabilized by an interaction between the nitrogen lone pair and the vacant π* orbital. The activation free energies are linearly related to the reaction free energies and the Taft σ_R~0 parameter.