TORQUOSELECTIVITIES OF ELECTROCYCLIC RING OPENING REACTIONS OF 2-AZETINES
Steven Lopez, K. N. Houk.
University of California, Los Angeles, Los Angeles, CA.
Quantum mechanical calculations were carried out with the M06-2X density functional and have revealed the role of nitrogen-substituent effects on the torquoselectivities of electrocyclic ring-opening reactions of 2-azetines. The 2 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 Ï€* LUMO orbital. Electron-withdrawing groups dramatically alter the ground state geometry by rehybridizing nitrogen from sp3 to sp2. The Taft ÏƒR parameter of N-substituents correlates with the calculated activation barriers for the favored conrotatory transition states. However, there is no correlation of ÏƒR with the calculated activation barriers for the disfavored conrotatory transition states. Mechanistic implications will be discussed.