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 p acceptors (e.g., CHO, CN) to p donors (NMe2, OMe) was explored. Acceptor substituents delocalize the nitrogen lone pair and stabilize the ground state of 2-azetines, while donors destabilize the 2-azetine ground state. The conrotatory ring opening is torquoselective, 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 p* orbital. The activation free energies are linearly related to the reaction free energies and the Taft sR0 parameter.

Substituent effects on rates and torquoselectivities of electrocyclic ring openings of N-substituted-2-azetines

248th National Meeting (2014)

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