Effects of Substituents in the 3-Position on the [2 + 2] Pentadienyl Cation Electrocyclization¹

Ab initio molecular orbital calculations at the RHF/3-21G(*), MP2/6-31G**//RHF/3-21G(*), and MP2/6-31G* levels of theory were used to examine the conformational preferences, transition structures, and products of the [2 + 2] electrocyclization reactions of 1,4-pentadienyl systems bearing substituents on carbon 3. The susbstituents examined include resonance-donating and -withdrawing groups: NH 2, OH, SH, H, PH 2, BH 2, and AlH 2 . The activation energy for the cyclization (the difference in energy between the most stable acyclic conformation of the various 3-substituted pentadienyl systems and the transition state) decreases in the order shown, as does the energy of cyclization (with the exception of the PH 2 system). These results, as well as the conformational preferences, are explained by a combination of steric and electronic interactions.