摘要
Dirhodium-catalyzed C-H amination is hypothesized to proceed via Rh 2 -nitrene intermediates in either the Rh 2 (II,II) or Rh 2 (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 [Rh 2 II,III (O 2 CCH 3 ) 4 (L) n ] + (1-L) (L = none, Cl - , and H 2 O), [Rh 2 (esp) 2 ] + (2), and Rh 2 (espn) 2 Cl (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 (π 1 ∗) 2 (π 2 ∗) 1 and (δ∗) 2 (π 1 ∗) 1 (π 2 ∗) 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 NSO 3 R 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 Rh 2 (II,II) with a coordinated nitrene radical cation, (π∗) 4 (δ∗) 2 (π nitrene,1 ) 1 (π nitrene,2 ) 0 . Conversely, the GES of 3N is Rh 2 (II,III) with a coordinated triplet nitrene, (π∗) 4 (δ∗) 1 (π nitrene,1 ) 1 (π nitrene,2 ) 1 . Quartet transition states ( 4 TSs) are found to react via a stepwise radical mechanism, whereas 2 TSs are found to react via a concerted mechanism that is lower in energy compared to 4 TSs for both 2N-L and 3N-L. The experimental (determined by intramolecular competition) and 2 TS-calculated kinetic isotopic effect (KIE) shows a KIE ∼ 3 for both 2N and 3N, which is consistent with a concerted mechanism.