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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
Published on: July 17, 2020
Fe(II)-Porphyrin Catalysts Emulate Enzymatic Structural Features for C-H Amination Reactions
Mayank Mahajan1, Bhaskar Mondal1
1School of Chemical Sciences, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh 175075, India.
Abstract:
Heme-bearing enzymes and synthetic Fe-porphyrin-based catalysts share an inherent structural and functional similarity; both necessitate a one-electron reduction of their resting Fe(III) state to the catalytically active Fe(II) form for facilitating nitrene transfer reactivity. However, the precise role of this Fe(III)-to-Fe(II) transformation in nitrene transfer chemistry remains unclear to date. Herein, we unravel the origin of the reactivity differences in nitrene transfer between Fe(III)- and Fe(II)-porphyrin catalysts using a prototypical C-H amination reaction involving 1,2,3,4-tetrazole and ethylbenzene. The stepwise reaction mechanism has been thoroughly investigated using density functional theory (DFT), followed by an in-depth electronic structure analysis of nitrene formation transition states using DFT and Fe-porphyrin-imido intermediates using multiconfigurational complete active-space self-consistent field (CASSCF) calculations. The catalytic inactivity of Fe(III)-porphyrin is attributed to the exceptionally high nitrene formation barrier (35.9 kcal/mol), stemming from the out-of-plane displacement of the Fe(III) center. Contrastingly, the catalytically active Fe(II)-porphyrin features an in-plane Fe(II) center and a significantly lower nitrene formation barrier (21.4 kcal/mol). Additionally, unlike Fe(II)-porphyrin catalyst, the Fe(III) counterpart exhibits a low thermodynamic driving force for the nitrene formation step due to a weaker Fe-N bond in the Fe-porphyrin-imido intermediate, a consequence of the multiconfigurational nature of the Fe-N core.
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