Geminal Bond Participation at the Transition States

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We analyzed the interactions of bonds at the trasition states of the Diels-Alder reaction between butadiene and ethylene, the Cope rearrangement of 1,5-hexadiene, and the electrocyclic reaction of 1,3-cyclohexadiiene. The electron delocalization from two σ bonds between a reacting center and substituents was found to have the bonding-antibonding properties opposite to each other. The delocalization from the geminal bonds at the Z-positions is bonding to the πbonds of the dienophile in Diels-Alder reaction and to the other reacting π bonds in the Cope rearrangement and the electrocyclic reaction. Electron donating bonds at the Z-positions were then predicted to enhance the reactivities of the dienes in the Diels-Alder reactions and the Cope rearrangements than those at the E-positions. Electropsitive substituents was predicted to prefer the inward rotation in the electrocyclic reactions. These predictions have been confirmed by ab initio molecular orbital and density functional theory calculations of the activation energies of the reactions.In summary, the geminal bond participation is significant in organic reactions, and electron donating bonds prefer the positions inside the cyclic structures of the trasition states.