Ab Initio Molecular Orbital Calculations on Allylic 1,3‐Strain of Electron‐Donor‐ and Electron‐Acceptor‐Substituted Alkenes

Ab initio calculations on the conformations of several electron‐rich and electron‐poor alkenes 2, 8–15 were performed up to the MP2/6–31G*/RHF/6–31G* level. It was proven that allylic 1,3‐strain can be traced back to steric interactions between the allylic center and the (Z) substituent of the double bond. The electron density of the double bond seems to have no effect on the difference in energy between the global and the local minimum. The local minimum (180° conformation) of (Z)‐methyl‐substituted alkenes 2, 14 and 15 carrying a methyl, carboxyl and methoxy group, respectively, at the double bond in (E) position, lies 3.37–3.52 kcal/mol above the 0° conformation (global minimum); thus, the (E) substituent has no significant influence on this energy difference and therefore on the equilibrium distribution.