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 sigma-bonds (sigma-EDB) geminal to the breaking sigma-bond and/or vicinal electron-accepting sigma-bonds (sigma-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 sigma-EDB, which is consistent with the theory of geminal bond participation.