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 Rhb>2 b>-nitrene intermediates in either the Rhb>2 b>(II,II) or Rhb>2 b>(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 [Rhb>2 b>II,III(Ob>2 b>CCHb>3 b>)b>4 b>(L)b>n b>]+ (<b>1_Lb>) (<b>Lb> = none, Cl–, and Hb>2 b>O), [Rhb>2 b>(esp)b>2 b>]+ (<b>2b>), and Rhb>2 b>(espn)b>2 b>Cl (<b>3b>) (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate and espn = α,α,α′,α′-tetramethyl-1,3-benzenedipropanamidate). CASSCF calculations on <b>1_Lb> yield a wave function with two closely weighted configurations, (δ*)2(πb>1 b>*)2(πb>2 b>*)1 and (δ*)2(πb>1 b>*)1(πb>2 b>*)2, consistent with reported EPR g values [ Chem. Phys. Lett. 1986, 130, 20−23]. In contrast, EPR spectra of <b>2b> show g values consistent with the DFT-computed (π*)4(δ*)1 GES. EPR spectra and Cl K-edge XAS for <b>3b> are consistent with a (π*)4(δ*)1 GES, as supported by DFT. Nitrene intermediates <b>2N_Lb> and <b>3N_Lb> are also examined by DFT (the nitrene is an NSOb>3 b>R species). DFT calculations suggest a doublet GES for <b>2N_Lb> and a quartet GES for <b>3N_Lb>. CASSCF calculations describe the GES of <b>2Nb> as Rhb>2 b>(II,II) with a coordinated nitrene radical cation, (π*)4(δ*)2(πb>nitrene,1 b>)1(πb>nitrene,2 b>)0. Conversely, the GES of <b>3Nb> is Rhb>2 b>(II,III) with a coordinated triplet nitrene, (π*)4(δ*)1(πb>nitrene,1 b>)1(πb>nitrene,2 b>)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 <b>2N_Lb> and <b>3N_Lb>. The experimental (determined by intramolecular competition) and 2TS-calculated kinetic isotopic effect (KIE) shows a KIE ∼ 3 for both <b>2Nb> and <b>3Nb>, which is consistent with a concerted mechanism.