Mechanism and Enantioselectivity of Dirhodium-Catalyzed Intramolecular C鈥揌 Amination of Sulfamate

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• 作者：Xiting Zhang ; Zhuofeng Ke ; Nathan J. DeYonker ; Huiying Xu ; Zhi-Feng Li ; Xianyan Xu ; Xuepeng Zhang ; Cheng-Yong Su ; David Lee Phillips ; Cunyuan Zhao
• 刊名：Journal of Organic Chemistry
• 出版年：2013
• 出版时间：December 20, 2013
• 年：2013
• 卷：78
• 期：24
• 页码：12460-12468
• 全文大小：587K
• 年卷期：v.78,no.24(December 20, 2013)
• ISSN：1520-6904

文摘

The mechanisms and enantioselectivities of the dirhodium (Rh₂L₄, L = formate, N-methylformamide, S-nap)-catalyzed intramolecular C鈥揌 aminations of 3-phenylpropylsulfamate ester have been investigated in detail with BPW91 density functional theory computations. The reactions catalyzed by the Rh₂^II,II catalysts start from the oxidation of the Rh₂^II,II dimer to a triplet mixed-valent Rh₂^II,III鈥搉itrene radical, which should facilitate radical H-atom abstraction. However, in the Rh₂(formate)₄-promoted reaction, as a result of a minimum-energy crossing point (MECP) between the singlet and triplet profiles, a direct C鈥揌 bond insertion is postulated. The Rh₂(N-methylformamide)₄ reaction exhibits quite different mechanistic characteristics, taking place via a two-step process involving (i) intramolecular H-abstraction on the triplet profile to generate a diradical intermediate and (ii) C鈥揘 formation by intersystem crossing from the triplet state to the open-shell singlet state. The stepwise mechanism was found to hold also in the reaction of 3-phenylpropylsulfamate ester catalyzed by Rh₂(S-nap)_4. Furthermore, the diradical intermediate also constitutes the starting point for competition steps involving enantioselectivity, which is determined by the C鈥揘 formation open-shell singlet transition state. This mechanistic proposal is supported by the calculated enantiomeric excess (94.2% ee) with the absolute stereochemistry of the product as R, in good agreement with the experimental results (92.0% ee).