TY - JOUR T1 - Rh2(II,III) Catalysts with Chelating Carboxylate and Carboxamidate Supports: Electronic Structure and Nitrene Transfer Reactivity JF - Journal of the American Chemical SocietyJournal of the American Chemical Society Y1 - 2016 A1 - Varela-Álvarez, Adrián A1 - Yang, Tzuhsiung A1 - Jennings, Heather A1 - Kornecki, Katherine P. A1 - Macmillan, Samantha N. A1 - Lancaster, Kyle M. A1 - Mack, James B. C. A1 - Du Bois, J. A1 - Berry, John F. A1 - Musaev, Djamaladdin G. AB -

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*)21*)22*)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*)1nitrene,1)1nitrene,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.

 

 

 

PB - American Chemical Society VL - 138 SN - 0002-7863 UR - http://dx.doi.org/10.1021/jacs.5b12790 IS - 7 N1 - doi: 10.1021/jacs.5b12790 JO - J. Am. Chem. Soc. ER -