Quantum mechanical calculations were carried out with the M06-2X density functional and have revealed the role of nitrogen-substituent effects on the torquoselectivity of electrocyclic ring-opening reactions of 2-azetines. The two diastereomeric transition structures differ in that the lone pair orbital on nitrogen rotates inward or outward. A wide scope of nitrogen substituents were tested and the inward rotation of the lone pair on nitrogen is favored for all substituents. The favored conrotatory transition state is stabilized by the favorable interactions of the nitrogen lone pair with the vacant p* LUMO orbital. Electron-withdrawing groups dramatically alter the ground state geometry by rehybridizing nitrogen from sp3 to sp2. The Taft sR� parameter of N-substituents correlate with the calculated activation barriers for the favored conrotatory transition states. However, there is no correlation of sR� with the calculated activation barriers for the disfavored conrotatory transition states. Mechanistic implications will be discussed.

Torquoselectivity of electrocyclic ring opening reactions of 2-azetines

247th National Meeting (2014)

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