Search

A bond model for the transition states of organic reactions was proposed to investigate the interactions of bonds at the transition states. The application to the Diels−Alder reaction between butadiene and ethylene disclosed the significant participation of geminal σ bonds at the reacting centers of the diene. It was found that the electron delocalization from the geminal σ bonds at the Z-positions to the π bond in the dienophile is bonding while that from the σ bonds at the E-positions is antibonding. The finding led to the prediction that electron-donating σ bonds at the Z-positions should enhance the reactivity more than those at the E-positions. The prediction of the relative reactivities of Z- vs E-1-substituted butadienes was confirmed by the calculated activation energies of the 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 σ bonds at the reacting centers. The electron-donating σ bonds at the Z-positions and the electron-accepting σ bonds at the E-positions in the 1-substituted 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 1-substituted ones with the electron-donating σ bonds at the Z-positions. The predictions were confirmed by the density functional theory calculations.