American Chemical Society
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Catalytic Synthesis of N‑Heterocycles via Direct C(sp3)–H Amination Using an Air-Stable Iron(III) Species with a Redox-Active Ligand

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posted on 2017-03-15, 00:00 authored by Bidraha Bagh, Daniël L. J. Broere, Vivek Sinha, Petrus F. Kuijpers, Nicolaas P. van Leest, Bas de Bruin, Serhiy Demeshko, Maxime A. Siegler, Jarl Ivar van der Vlugt
Coordination of FeCl3 to the redox-active pyridine–aminophenol ligand NNOH2 in the presence of base and under aerobic conditions generates FeCl2(NNOISQ) (1), featuring high-spin FeIII and an NNOISQ radical ligand. The complex has an overall S = 2 spin state, as deduced from experimental and computational data. The ligand-centered radical couples antiferromagnetically with the Fe center. Readily available, well-defined, and air-stable 1 catalyzes the challenging intramolecular direct C­(sp3)–H amination of unactivated organic azides to generate a range of saturated N-heterocycles with the highest turnover number (TON) (1 mol% of 1, 12 h, TON = 62; 0.1 mol% of 1, 7 days, TON = 620) reported to date. The catalyst is easily recycled without noticeable loss of catalytic activity. A detailed kinetic study for C­(sp3)–H amination of 1-azido-4-phenylbutane (S1) revealed zero order in the azide substrate and first order in both the catalyst and Boc2O. A cationic iron complex, generated from the neutral precatalyst upon reaction with Boc2O, is proposed as the catalytically active species.