Mechanistic Understanding of the Aryl-Dependent Ring Formations in Rh(III)-Catalyzed C鈥揌 Activation/Cycloaddition of Benzamides and Methylenecyclopropanes by DFT Calculations

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• 作者：Wei Guo ; Tao Zhou ; Yuanzhi Xia
• 刊名：Organometallics
• 出版年：2015
• 出版时间：June 22, 2015
• 年：2015
• 卷：34
• 期：12
• 页码：3012-3020
• 全文大小：737K
• ISSN：1520-6041

文摘

The divergence between Rh(III)-catalyzed C鈥揌 activation/cycloaddition of phenyl- and 2-furanyl-containing benzamides with methylenecyclopropanes (MCP) was studied by DFT calculations. Calculations found that the C鈥揌 activation via a CMD mechanism is the most difficult step of the reaction involving phenyl. In contrast, the C鈥揌 activation of the 2-furanyl-containing substrate is kinetically easier but the formed five-membered rhodacycle is relatively unstable, making the following MCP insertion more difficult. Thus, different KIE data was obtained in experiments. The MCP insertion forms a seven-membered-ring rhodacycle intermediate, from which the chemoselectivity of the whole reaction is determined by the competitive pivalate migration (path I) and 尾-C elimination (path II). While the 尾-C elimination is lower in energy when a furanylene is contained in the intermediate, a reversed preference of pivalate migration was predicted for the phenylene counterpart. Structural analysis suggested that the unfavorable 尾-C elimination in the phenylene case should be attributed to the obviously increased ring strain in the corresponding transition state, instead of the difference in electronic properties between the aryl groups. This accounts for why aryl-dependent chemoselectivity was observed. In addition, the results indicated that for both paths I and II the generation of a Rh(V)鈥搉itrenoid intermediate from pivalate migration is crucial for the final C鈥揘 bond formation. This explains why no reaction occurred when the N鈥揙Piv moiety was replaced with an N鈥揙Me group, as no Rh(V) intermediate could be formed in this system.