Computational Insights into the Rhodium(III)-Catalyzed Coupling of Benzamides and 1,6-Enynes via a Tunable Arylative Cyclization

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• 作者：Lijuan Du ; Yilu Xu ; Shengwen Yang ; Juan Li ; Xionghui Fu
• 刊名：Journal of Organic Chemistry
• 出版年：2016
• 出版时间：March 4, 2016
• 年：2016
• 卷：81
• 期：5
• 页码：1921-1929
• 全文大小：713K
• ISSN：1520-6904

文摘

A density functional theory (DFT) study has been conducted to elucidate the mechanism of the rhodium(III)-catalyzed C–H activation of O-substituted N-hydroxybenzamides and cyclohexadienone-containing 1,6-enynes. The impact of different O-substituted internal oxidants (OPiv versus OMe) on the arylative cyclization (i.e., Ⓝ-Michael addition versus Ⓒ-Michael addition) has been evaluated in detail. The Ⓝ-Michael addition pathway proceeded via a Rh(I) species, while Rh(III) remained unchanged throughout the Ⓒ-Michael addition pathway. The Rh(III)/Rh(I) catalytic cycle in the Ⓝ-Michael addition pathway was different from those reported previously where the Rh(III)/Rh(V) catalytic cycle was favored for the Rh(III)-catalyzed C–H activation of O-substituted N-hydroxybenzamides. The first three steps were similar for the OPiv- and OMe-substituted substrates, which involved sequential N–H deprotonation, C–H activation (a concerted metalation–deprotonation process), and 1,6-enyne insertion steps. Starting from a seven-membered rhodacycle, the alternative mechanism would be controlled by the OR substituent. When the substituent was OMe, the unstable seven-membered rhodacycle was readily coordinated by a double bond of the cyclohexadienone which enabled the Ⓒ-Michael addition reaction. However, the presence of an N-OPiv moiety stabilized the seven-membered rhodacycle through a bidentate coordination which facilitated the Ⓝ-Michael addition process.