title: Mechanism and enantioselectivity of dirhodium-catalyzed intramolecular C-H amination of sulfamate.
abstract: The mechanisms and enantioselectivities of the dirhodium (Rh2L4, L = formate, N-methylformamide, S-nap)-catalyzed intramolecular C-H 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)-nitrene 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-H 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-N 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-N 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).
authors.affiliation: MOE Key Laboratory of Bioinorganic and Synthetic Chemistry/KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University , Guangzhou 510275, P. R. China.
authors.forename: Xiting
authors.initials: X
authors.lastname: Zhang
authors.forename: Zhuofeng
authors.initials: Z
authors.lastname: Ke
authors.forename: Nathan J
authors.initials: NJ
authors.lastname: DeYonker
authors.forename: Huiying
authors.initials: H
authors.lastname: Xu
authors.forename: Zhi-Feng
authors.initials: ZF
authors.lastname: Li
authors.forename: Xianyan
authors.initials: X
authors.lastname: Xu
authors.forename: Xuepeng
authors.initials: X
authors.lastname: Zhang
authors.forename: Cheng-Yong
authors.initials: CY
authors.lastname: Su
authors.forename: David Lee
authors.initials: DL
authors.lastname: Phillips
authors.forename: Cunyuan
authors.initials: C
authors.lastname: Zhao