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| - Characterization of the Switch in the Mechanism of anIntramolecular Diels−Alder Reaction
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| - Changing the dienophile moiety of an intramolecular Diels−Alder (IMDA) cycloaddition from an allylether to an allenyl ether can dramatically change the regioselectivity. We hereby show by density functionaltheory computations that such unprecedented divergence is produced by an underlying change in themechanism of the reaction. The allyl ether yields a fused tetrahydrofuran through a classical Diels−Alder reaction, whereas the allenyl ether yields a (methylidene)tetrahydropyran through a stepwise process.The latter reaction involves an extreme asynchronism in the bond-forming events with a diradicaloidintermediate that is stabilized by conjugation and synergistic (captodative) effects. Comparison withintermolecular model D−A reactions, which are concerted processes with various degrees of asynchrony,helps explain the change in regioselectivity for the IMDA reaction of allyl systems and the shift inmechanism for the IMDA reaction of the allenyl derivatives studied.
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