Polarization of the carbonyl double

These results point out an important difference between intramolecular ene reactions of dienes and unsaturated aldehydes. Intramolecular ene reactions of 1,6-dienes to give vinylcyclopentanes are much faster than ene reactions of 1,7-dienes to give vinylcyclohexanes due to tihe less negative ASt for formation of five-membered rings. Both reactions are thermodynamically favored even at elevated temperatures, Ene reactions of unsaturated aldehydes are much faster than those of dienes due to the polarity of the carbonyl double bond. However, the ene reactions of aldehydes are less... 

---