DFT Study of the Molecular Mechanism of Lewis Acid Induced [4+3] Cycloadditions of 2-Alkylacroleins with Cyclopentadiene

The mechanism of the Lewis acid (AlCl3) induced [4 + 3] cycloaddition of 2-methylacrolein with cyclopentadiene (Cp) [J. Am. Chem. Soc. 2004, 126, 2692] has been examined here through DFT calculations at the MPWIK(DCM)/6-31+G** level. Formation of these seven-membered carbocycles is a domino process that comprises three consecutive reactions. The first one is a polar Diels-Alder reaction that is initialized by the nucleophilic attack of Cp to the beta-conjugated position of acrolein, yielding the formation of the endo and exo [4 + 2] cycloadducts. The corresponding LA-[4 + 2] cycloadduct complexes equilibrate through a skeleton rearrangement with a low free activation energy with two seven-membered zwitterionic intermediates, which undergo a rapid intramolecular hydride shift to yield irreversibly the formally endo and exo [4 + 3] cycloadducts. A comparative analysis of this mechanism with that tor the Lewis acid induced [4 + 3] cycloadditions of 2-silyloxyacroleins allows establishment of the requirements for the formation of the seven-membered arbocycles.