Substituent-Induced Switch of the Role of Charge-Transfer Complexes in the Diels鈥揂lder Reactions of o-Chloranil and Styrenes

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• 作者：Sergiy V. Rosokha ; Vasiliy Korotchenko ; Charlotte L. Stern ; Vladimir Zaitsev ; Jeremy T. Ritzert
• 刊名：The Journal of Organic Chemistry
• 出版年：2012
• 出版时间：July 20, 2012
• 年：2012
• 卷：77
• 期：14
• 页码：5971-5981
• 全文大小：389K
• 年卷期：v.77,no.14(July 20, 2012)
• ISSN：1520-6904

文摘

Addition of p-substituted styrenes, XSty (X = H, Me, MeO, or Cl) to the solutions of o-chloranil, oCA, in dichloromethane resulted in the transient formation of the charge-transfer complexes, [XSty, oCA], followed by the Diels鈥揂lder reaction. At low temperatures, these reactions led to formation of essentially pure endocycloadducts. As expected for the inverse-electron-demand Diels鈥揂lder 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 o-chloranil with p-methyl, p-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鈥揂lder reaction.