Mechanistic Insight into Asymmetric N鈥揌 Insertion Cooperatively Catalyzed by a Dirhodium Compound and a Spiro Chiral Phosphoric Acid

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• 作者：Xu-Chao Wang ; Xian-Shuang Song ; Li-Ping Guo ; Deyu Qu ; Zhi-Zhong Xie ; Francis Verpoort ; Jun Cao
• 刊名：Organometallics
• 出版年：2014
• 出版时间：August 11, 2014
• 年：2014
• 卷：33
• 期：15
• 页码：4042-4050
• 全文大小：614K
• ISSN：1520-6041

文摘

The insertion of a carbenoid into an N鈥揌 bond of an amine cooperatively catalyzed by a dirhodium catalyst and a spiro chiral phosphoric acid has been investigated in detail using density functional theory methods. Calculations indicate that the reaction begins with the nucleophilic amine attacking at the carbenoid, forming a metal-associated ammonium ylide first followed by a rapid proton transfer to afford a metal-associated enamine intermediate. Subsequently, the enamine intermediate dissociates from the metal and yield a more stable seven-membered-ring conformation via an intramolecular hydrogen-bond exchange. Formation of the enamine intermediate requires an overall barrier of 5.7 kcal/mol and is exergonic by 5.1 kcal/mol. Calculations demonstrated that, although the conversion of the achiral enamine into the N鈥揌 insertion product can be facilitated efficiently by the dirhodium catalyst through a two-step process, it can be compressed to a large extent. This is due to the more competitive decomposition of the diazoacetate catalyzed by the dirhodium catalyst, which can give a carbenoid for the next catalytic cycle. Meanwhile, formation of the carbenoid is considerably exergonic, which can promote the direct [1,3]-proton shift of enamine. However, in the presence of the spiro chiral phosphoric acid, the asymmetric proton induction of enamine is greatly favored, requiring an activation free energy of 6.0 kcal/mol to afford the major R product. This agrees well with the experimental observation.