DFT Study on the Rhodium(II)-Catalyzed C鈥揌 Functionalization of Indoles: Enol versus Oxocarbenium Ylide

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• 作者：Qiao Xie ; Xian-Shuang Song ; Deyu Qu ; Li-Ping Guo ; Zhi-Zhong Xie
• 刊名：Organometallics
• 出版年：2015
• 出版时间：July 13, 2015
• 年：2015
• 卷：34
• 期：13
• 页码：3112-3119
• 全文大小：562K
• ISSN：1520-6041

文摘

The reaction of carbene insertion into the C鈥揌 bond of indole catalyzed by the Rh₂(HCO₂)₄ complex has been investigated in detail through DFT calculations. Results indicate that the indole can proceed via a nucleophilic attack at the carbene first to generate a carbonium ylide intermediate followed by a [1,4]-proton transfer to give a free enol. In this case, the final C鈥揌 insertion product can be obtained through the self-catalyzed [1,3]-proton shift of enol. Alternatively, the nucleophilic attack can also involve the concerted formations of C鈥揙 and C鈥揅 bonds to produce an oxocarbonium ylide intermediate. The subsequent [1,2]-proton shift catalyzed by a molecule of enol is also energetically feasible to give the C鈥揌 insertion product. It indicates the coexistence of two distinct pathways for the C鈥揌 insertion reaction of indole. However, the ratio between them can be regulated by the substituents on both carbenoid and indole. For instance, the enol pathway is always dominant for the phenyl-substituted carbenoid. However, the ratio of the two pathways becomes comparable for the ethyl-substituted carbenoid. The reason is mainly associated with the flexibility of the Rh鈺怌 bond of the carbenoid, which plays an important role in determining the approach of indole to the carbenoid. This finding is also useful to understand the reaction mechanisms for the related [3 + 2] annulation and the three-component reactions of indole.