Geminal bond participation in Alder ene reaction

We applied the geminal bond participation theory to Alder ene reactions. The interaction between the σ-orbital at the Z-position of the double bond of propene and the π∗-orbital of ethylene was predicted to be bonding, while that at the E-position should be antibonding. This prediction was confirmed by the bond model analysis of the model compound. These results suggest that the reactivity could be enhanced by substituting a more electron-donating σ-bond at the Z-position. To examine our prediction, theoretical calculations were performed for the systematically substituted substrates. The prediction was confirmed except in the case where the inductive effect of oxygen could affect the reactivity. The same trend in reactivity was observed for formaldehyde as an enophile.