Geminal bond participation and torquoselectivity in cheletropic reactions

Predominance of the participation of the geminal over vicinal bonds: torquoselectivity of retro-Nazarov reactions

Torquoselectivities of electrocyclic ring-opening reactions have been proposed to be controlled by the participation of electron-donating σ-bonds (σ-EDB) geminal to the breaking σ-bond and/or vicinal electron-accepting σ-bonds (σ-EAB) or lone pair(s) of the substituent(s). We designed reactions in which the effects of the vicinal bonds should be significantly diminished by the cationic nature of the reactants. The calculated enthalpies of activation of the retro-Nazarov reactions of some 3-substituted cyclopentenyl cations show inward rotation of the geminal σ-EDB, which is consistent with the theory of geminal bond participation.

A Torquoselective Extrusion of Isoxazoline N-Oxides. Application to the Synthesis of Aryl Vinyl and Divinyl Ketones for Nazarov Cyclization

A mild, convenient reaction sequence for the synthesis of Nazarov cyclization substrates is described. The [3+2] dipolar cycloaddition of a nitrone and an electron-deficient alkyne gives an isolable isoxazoline intermediate, which upon oxidation undergoes stereoselective extrusion of nitrosomethane to give aryl vinyl or divinyl ketones.

An Orbital Phase Theory for the Torquoselectivity of the Ring-Opening Reactions of 3-Substituted Cyclobutenes: Geminal Bond Participation

We apply an orbital phase theory to the torquoselectivity of the electrocyclic reactions of 3-substituted (X) cyclobutenes. The torquoselectivity is shown to be controlled by the orbital-phase relation of the reacting pi(CC) and sigma(CC) bonds with the sigma(CX) bond geminal to the sigma(CC) bond to be cleaved. The inward rotation of electron-donating sigma(CX) bonds and outward rotation of electron-withdrawing sigma(CX) bonds have been deduced from the orbital-phase theory. Enhancement of the inward rotation by the electron-donating capability of the sigma(CX) bonds is confirmed by the correlation between the torquoselectivity and sigma(CX) orbital energy. The orbital overlaps between the geminal sigma(CX) (sigma(CH)) and sigma(CC) bonds are found to be important as well. Unsaturated substituents with low-lying unoccupied pi orbitals also promote the inward rotation.