Rh2(II,III) Catalysts with Chelating Carboxylate and Carboxamidate Supports: Electronic Structure and Nitrene Transfer Reactivity
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- 作者：Adrián Varela-Álvarez ; Tzuhsiung Yang ; Heather Jennings ; Katherine P. Kornecki ; Samantha N. Macmillan ; Kyle M. Lancaster ; James B. C. Mack ; J. Du Bois ; John F. Berry ; Djamaladdin G. Musaev
- 刊名：Journal of the American Chemical Society
- 出版年：2016
- 出版时间：February 24, 2016
- 年：2016
- 卷：138
- 期：7
- 页码：2327-2341
- 全文大小：804K
- ISSN：1520-5126
文摘
Dirhodium-catalyzed C–H amination is hypothesized to proceed via Rh2-nitrene intermediates in either the Rh2(II,II) or Rh2(II,III) redox state. Herein, we report joint theoretical and experimental studies of the ground electronic state (GES), redox potentials, and C–H amination of [Rh2II,III(O2CCH3)4(L)n]+ (1_L) (L = none, Cl–, and H2O), [Rh2(esp)2]+ (2), and Rh2(espn)2Cl (3) (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate and espn = α,α,α′,α′-tetramethyl-1,3-benzenedipropanamidate). CASSCF calculations on 1_L yield a wave function with two closely weighted configurations, (δ*)2(π1*)2(π2*)1 and (δ*)2(π1*)1(π2*)2, consistent with reported EPR g values [ Chem. Phys. Lett. 1986, 130, 20−23]. In contrast, EPR spectra of 2 show g values consistent with the DFT-computed (π*)4(δ*)1 GES. EPR spectra and Cl K-edge XAS for 3 are consistent with a (π*)4(δ*)1 GES, as supported by DFT. Nitrene intermediates 2N_L and 3N_L are also examined by DFT (the nitrene is an NSO3R species). DFT calculations suggest a doublet GES for 2N_L and a quartet GES for 3N_L. CASSCF calculations describe the GES of 2N as Rh2(II,II) with a coordinated nitrene radical cation, (π*)4(δ*)2(πnitrene,1)1(πnitrene,2)0. Conversely, the GES of 3N is Rh2(II,III) with a coordinated triplet nitrene, (π*)4(δ*)1(πnitrene,1)1(πnitrene,2)1. Quartet transition states (4TSs) are found to react via a stepwise radical mechanism, whereas 2TSs are found to react via a concerted mechanism that is lower in energy compared to 4TSs for both 2N_L and 3N_L. The experimental (determined by intramolecular competition) and 2TS-calculated kinetic isotopic effect (KIE) shows a KIE ∼ 3 for both 2N and 3N, which is consistent with a concerted mechanism.