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Substituent-Induced Switch of the Role of Charge-Transfer Complexes in the Diels–Alder Reactions of <i>o</i>-Chloranil and Styrenes

By Sergiy V. Rosokha (1614049), Vasiliy Korotchenko (1613236), Charlotte L. Stern (1375188), Vladimir Zaitsev (2062513) and Jeremy T. Ritzert (2062510)


Addition of <i>p</i>-substituted styrenes, XSty (X = H, Me, MeO, or Cl) to the solutions of <i>o</i>-chloranil, oCA, in dichloromethane resulted in the transient formation of the charge-transfer complexes, [XSty, oCA], followed by the Diels–Alder reaction. At low temperatures, these reactions led to formation of essentially pure endocycloadducts. As expected for the inverse-electron-demand Diels–Alder reaction, the rate constants of the cycloaddition rose with the increase of the donor strength. However, while facile cycloaddition took place in the neat mixtures of the <i>o</i>-chloranil with <i>p</i>-methyl, <i>p</i>-chloro-, or unsubstituted styrenes at low temperatures, a similar system involving the strongest MeOSty donor was surprisingly persistent. X-ray structural measurements and quantum-mechanical computations indicated that this anomaly is related to the fact that the diene/dienophile orientation in the charge-transfer [MeOSty, oCA] complex is opposite to that in the endocycloadduct and in the lowest-energy transition state leading to this isomer. Thus, the proceeding of the cycloaddition requires dissociation of the (dead-end) complex. For the systems involving the oCA diene and either the HSty, ClSty, or MeSty dienophile, the donor/acceptor arrangements in the charge-transfer complexes apparently are consistent with that in the corresponding products, and the formation of these complexes does not hinder the Diels–Alder reaction

Topics: Biophysics, Biochemistry, Cell Biology, Genetics, Marine Biology, Inorganic Chemistry, Biological Sciences not elsewhere classified, Chemical Sciences not elsewhere classified, Physical Sciences not elsewhere classified, unsubstituted styrenes, oCA diene, complex, cycloaddition, rate constants, Diel, XSty, MeOSty donor, formation, MeSty dienophile, endocycloadduct, donor strength
Year: 2012
DOI identifier: 10.1021/jo300614k.s001
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Provided by: FigShare
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