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 [Rh₂p>II,IIIp>(O₂CCH₃)₄(L)_n]p>+p> (1_L) (L = none, Clp>–p>, and H₂O), [Rh₂(esp)₂]p>+p> (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*)p>2p>(π₁*)p>2p>(π₂*)p>1p> and (δ*)p>2p>(π₁*)p>1p>(π₂*)p>2p>, consistent with reported EPR g values [ Chem. Phys. Lett.pan class="NLM_x">p://www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve"> pan>1986pan class="NLM_x">p://www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve">, pan>130pan class="NLM_x">p://www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve">, pan>20−23]. In contrast, EPR spectra of 2 show g values consistent with the DFT-computed (π*)p>4p>(δ*)p>1p> GES. EPR spectra and Cl K-edge XAS for 3 are consistent with a (π*)p>4p>(δ*)p>1p> 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*)p>4p>(δ*)p>2p>(π_nitrene,1)p>1p>(π_nitrene,2)p>0p>. Conversely, the GES of 3N is Rh₂(II,III) with a coordinated triplet nitrene, (π*)p>4p>(δ*)p>1p>(π_nitrene,1)p>1p>(π_nitrene,2)p>1p>. Quartet transition states (p>4p>TSs) are found to react via a stepwise radical mechanism, whereas p>2p>TSs are found to react via a concerted mechanism that is lower in energy compared to p>4p>TSs for both 2N_L and 3N_L. The experimental (determined by intramolecular competition) and p>2p>TS-calculated kinetic isotopic effect (KIE) shows a KIE ∼ 3 for both 2N and 3N, which is consistent with a concerted mechanism.