Geminal Bond Participation in the Cope Rearrangements of Z- and E-Substituted 1,5Hexadienes and in the Reverse Reactions

We analyzed the bond interactions at the transition state of the Cope rearrangement of 1,5-hexadiene to disclose the significant participation of the geminal a bonds at the reacting centers. The electron-donating (sigma) bonds at the Z-positions and the electron-accepting (sigma) bonds at the E-positions in the 1substituted 1,5-hexadienes were predicted from the geminal bond participation to enhance the reactivities. For the reverse reactions, the 3-substituted 1,5-hexadienes were similarly predicted to rearrange more readily into the I-substituted ones with the electron-donating a bonds at the Z-positions. The predictions were confirmed by the density functional theory calculations.