Catalytic Synthesis of N-Heterocycles via Direct C(sp ³ )–H Amination Using an Air-Stable Iron(III) Species with a Redox-Active Ligand

Abstract

Coordination of FeCl₃ to the redox-active pyridine-aminophenol ligand NNO^H2 in the presence of base and under aerobic conditions generates FeCl₂(NNO^ISQ) (1), featuring high-spin Fe^III and an NNO^ISQ 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.