Characterization of the switch in the mechanism of an intramolecular Diels-Alder reaction.
Changing the dienophile moiety of an intramolecular Diels-Alder (IMDA) cycloaddition from an allyl ether to an allenyl ether can dramatically change the regioselectivity. We hereby show by density functional theory computations that such unprecedented divergence is produced by an underlying change in the mechanism 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 diradicaloid intermediate that is stabilized by conjugation and synergistic (captodative) effects. Comparison with intermolecular 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 in mechanism for the IMDA reaction of the allenyl derivatives studied.
López
Carlos Silva
CS
Departamento de Quimica Organica, Universidade de Vigo, Lagoas Marcosende, 36310 Vigo, Galicia, Spain. csilval@uvigo.es
Faza
Olalla Nieto
ON
Lera
Angel R de
AR
D000498:Allyl Compounds / Q000737:chemistry; D003198:Computer Simulation; D003500:Cyclization; D004987:Ethers / Q000138:chemical synthesis* / Q000737:chemistry; D005663:Furans / Q000138:chemical synthesis / Q000737:chemistry; D008956:Models, Chemical; D015394:Molecular Structure; D013237:Stereoisomerism
D000498:Allyl Compounds; D004987:Ethers; D005663:Furans