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

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–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 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–Alder reaction.将对取代苯乙烯（XSty，X=氢、甲基、甲氧基或氯）加入二氯甲烷中的四氯邻苯醌（o-chloranil，简称oCA）溶液中，会瞬态生成电荷转移配合物[XSty, oCA]，随后发生狄尔斯-阿尔德（Diels–Alder）反应。在低温环境下，该反应主要生成高纯度的环内加合物。对于反电子需求狄尔斯-阿尔德（inverse-electron-demand Diels–Alder）反应而言，其环加成速率常数随给体强度的提升而升高，这与预期相符。然而，尽管在低温条件下，四氯邻苯醌与对甲基、对氯或未取代苯乙烯的纯混合物可顺利发生环加成，但涉及最强给体甲氧基取代苯乙烯（MeOSty）的类似体系却意外呈现出反应迟滞现象。X射线结构测定与量子力学计算结果表明，这一反常现象源于：电荷转移配合物[MeOSty, oCA]中双烯体/亲双烯体的取向，与环内加合物中的取向以及生成该异构体的最低能量过渡态中的取向恰好相反。因此，该环加成反应的进行需要先解离该“死端”复合物。对于以四氯邻苯醌作为双烯体，分别以氢取代苯乙烯（HSty）、氯取代苯乙烯（ClSty）或甲基取代苯乙烯（MeSty）作为亲双烯体的体系，电荷转移配合物中的给体/受体排布与对应产物中的排布一致，此类配合物的形成并不会阻碍狄尔斯-阿尔德反应的进行。