Diels鈥揂lder Reactivities of Strained and Unstrained Cycloalkenes with Normal and Inverse-Electron-Demand Dienes: Activation Barriers and Distortion/Interaction Analysis

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• 作者：Fang Liu ; Robert S. Paton ; Seonah Kim ; Yong Liang ; K. N. Houk
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：October 16, 2013
• 年：2013
• 卷：135
• 期：41
• 页码：15642-15649
• 全文大小：524K
• 年卷期：v.135,no.41(October 16, 2013)
• ISSN：1520-5126

文摘

The Diels鈥揂lder reactions of the cycloalkenes, cyclohexene through cyclopropene, with a series of dienes鈥?,3-dimethoxybutadiene, cyclopentadiene, 3,6-dimethyltetrazine, and 3,6-bis(trifluoromethyl)tetrazine鈥攚ere studied with quantum mechanical calculations and compared with experimental values when available. The reactivities of cycloalkenes as dienophiles were found by a distortion/interaction analysis to be distortion controlled. The energies required for cycloalkenes to be distorted into the Diels鈥揂lder transition states increase as the ring size of cycloalkenes increases from cyclopropene to cyclohexene, resulting in an increase in activation barriers. The reactivities of the dienes are controlled by both distortion and interaction energies. In normal Diels鈥揂lder reactions with cycloalkenes, the electron-rich 1,3-dimethoxybutadiene exhibits stronger interaction energies than cyclopentadiene, but the high distortion energies required for 1,3-dimethoxybutadiene to achieve transition-state geometries overtake the favorable interaction, resulting in higher activation barriers. In inverse-electron-demand Diels鈥揂lder reactions of 3,6-dimethyltetrazine and 3,6-bis(trifluoromethyl)tetrazine, the reactivities are mainly controlled by interaction energies.