Mechanism and Enantioselectivity of Dirhodium-Catalyzed Intramolecular C–H Amination of Sulfamate

Abstract

The mechanisms and enantioselectivities of the dirhodium (Rh2L4, L = formate, N-methylformamide, S-nap)-catalyzed intramolecular C-H aminations of 3-phenylpropylsulfamate ester have been investigated in detail with BPW91 density functional theory computations. The reactions catalyzed by the Rh2(II,II) catalysts start from the oxidation of the Rh2(II,II) dimer to a triplet mixed-valent Rh2(II,III)-nitrene radical, which should facilitate radical H-atom abstraction. However, in the Rh2(formate)4-promoted reaction, as a result of a minimum-energy crossing point (MECP) between the singlet and triplet profiles, a direct C-H bond insertion is postulated. The Rh2(N-methylformamide)4 reaction exhibits quite different mechanistic characteristics, taking place via a two-step process involving (i) intramolecular H-abstraction on the triplet profile to generate a diradical intermediate and (ii) C-N formation by intersystem crossing from the triplet state to the open-shell singlet state. The stepwise mechanism was found to hold also in the reaction of 3-phenylpropylsulfamate ester catalyzed by Rh2(S-nap)4. Furthermore, the diradical intermediate also constitutes the starting point for competition steps involving enantioselectivity, which is determined by the C-N formation open-shell singlet transition state. This mechanistic proposal is supported by the calculated enantiomeric excess (94.2% ee) with the absolute stereochemistry of the product as R, in good agreement with the experimental results (92.0% ee).

Rhodium(ii)-catalyzed C–H aminations using N-mesyloxycarbamates: reaction pathway and by-product formation

“…With rhodium( ii ) dimer complexes, such as Rh 2 (OAc) 4 , using carbamate- and sulfamate-derived iminoiodinane reagents, a concerted asynchronous insertion of a singlet Rh( ii )–nitrene has been found in previous computational studies,59,60 in agreement with what was observed experimentally 33,58,61. For Rh 2 (esp) 2 , an alternative one-electron mechanism involving Rh( ii )–Rh( iii ) intermediates has been proposed with a concerted C–H insertion step 62–64. Conversely, computational and experimental data have established a stepwise mechanism for the C–H amination catalyzed by a diruthenium complex involving a short-lived diradical species 23,65…”

Mechanistic Insight into the Intramolecular Benzylic C−H Nitrene Insertion Catalyzed by Bimetallic Paddlewheel Complexes: Influence of the Metal Centers

“…Conversely, the S−O(R) bond inclines parallel to the metal plane in the Lie‐mode. Our previous work and an investigation of Du Bois and co‐workers found that the Stand‐mode of the transition states of the hydrogen abstraction appears preferable and thereby, only the Stand‐mode will be further considered.…”

Mechanism and Diastereoselectivity of [3+3] Cycloaddition between Enol Diazoacetate and Azomethine Imine Catalyzed by Dirhodium Tetracarboxylate: A Theoretical Study

“…With the development of theoretical methods and the diversity of chemical means, DFT has become an important tool for comprehending the mechanisms of chemical reactions, and numerous theoretical studies on rhodium‐catalyzed reactions have come to our attention. Zhao and co‐workers explained that the preferential mechanism of [Rh 2 ( S‐ nap) 4 ]‐catalyzed [nap = N‐(2‐hydroxypiperidin‐3‐yl)‐4‐methylbenzenesulfonamide] C–H amination reactions mainly relies on the electronic effect of catalyst ligands. Qu and Cramer reported C–H activation and illustrated the importance of the catalyst ligand in Rh‐based systems.…”