Transition Structures of the Lewis Acid Catalyzed Diels-Alder Reaction of Butadiene with Acrolein. The Origins of Selectivity

Four transition structures for the Diels-Alder reaction of butadiene and acrolein complexed with BH₃ have been located with ab initio calculations at the RHF/3-21G level, and activation energies have been calculated with the 6-31G^⋆ basis set. These calculations reproduce both the decreased activation energy and the enhanced endo selectivity experimentally observed upon catalysis. The transition structures show significant zwitterionic character, which is related to the greater calculated asynchronicity in the catalyzed reaction as compared to the uncatalyzed one.