Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 74, Issue 16, Pages 5934-5940Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jo900889q
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- Ministerio de Ciencia c Innovacion of the Spanish Government [CTQ2006-14-297/BQU]
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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.
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