Mechanistic twist of the [8+2] cycloadditions of dienylisobenzofurans and dimethyl acetylenedicarboxylate: Stepwise [8+2] versus [4+2]/[1,5]-vinyl shift mechanisms revealed through a theoretical and experimental study
Mechanistic twist of the [8+2] cycloadditions of dienylisobenzofurans and dimethyl acetylenedicarboxylate: Stepwise [8+2] versus [4+2]/[1,5]-vinyl shift mechanisms revealed through a theoretical and experimental study
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DOI：
10.1021/ja072203u
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发表时间：
2007-09-05
影响因子：
15
通讯作者：
Yu, Zhi-Xiang
中科院分区：
文献类型：
--
作者：
Chen, Yuanyuan;Ye, Siyu;Yu, Zhi-Xiang
近年来，研究发现二烯基呋喃和二烯基异苯并呋喃都能与炔二甲酸二甲酯（DMAD）反应生成[8+2]环加合物。理解这些[8+2]反应将有助于设计其他的[8+2]反应，这些反应具有合成10元和更大碳环的潜力。本文旨在通过计算和实验的结合，从分子水平上研究二烯基异苯并呋喃与炔的[8+2]环加成反应的详细机理。在（U）B3 LYP/6-31+G（d）水平上的密度泛函理论计算表明，二烯基异苯并呋喃与炔之间的协同[8+2]途径并不有利.一个逐步的反应途径，涉及形成一个两性离子中间体的[8+2]反应之间的二烯基异苯并呋喃，含有给电子的甲氧基存在于其二烯部分和DMAD的已计算预测。该途径与二烯基异苯并呋喃的呋喃部分和DMAD之间的Diels-Alder [4+2]反应竞争。当在二烯基异苯并呋喃的二烯部分中不存在给电子基团时，[8+2]反应通过涉及四烯的呋喃部分与DMAD之间的[4+2]反应的替代机制发生，随后是[1，5]-乙烯基位移。这种计算预测的新机制得到了实验的支持。
Recently, it was reported that both dienylfurans and dienylisobenzofurans could react with dimethyl acetylenedicarboxylate (DMAD) to give [8+2] cycloadducts. Understanding these [8+2] reactions will aid the design of additional [8+2] reactions, which have the potential for the synthesis of 10-membered and larger carbocycles. The present Article is aimed to understand the detailed mechanisms of the originally reported [8+2] cycloaddition reaction between dienylisobenzofurans and alkynes at the molecular level through the joint forces of computation and experiment. Density functional theory calculations at the (U)B3LYP/6-31+G(d) level suggest that the concerted [8+2] pathway between dienylisobenzofurans and alkynes is not favored. A stepwise reaction pathway involving formation of a zwitterionic intermediate for the [8+2] reactions between dienylisobenzofurans that contain electron-donating methoxy groups present in their diene moieties and DMAD has been predicted computationally. This pathway is in competition with a Diels-Alder [4+2] reaction between the furan moieties of dienylisobenzofurans and DMAD. When there is no electron-donating group present in the diene moieties of dienylisobenzofurans, the [8+2] reaction occurs through an alternative mechanism involving a [4+2] reaction between the furan moiety of the tetraene and DMAD, followed by a [1,5]-vinyl shift. This computationally predicted novel mechanism was supported experimentally.