Intramolecular Arene C(sp2)–H Amidation Enabled by Ferrocenium-Mediated Decomposition of 1,4,2-Dioxazol-5-ones as Amidyl Radical Precursors

Direct arene C–H functionalization by amidyl radicals for arylamides synthesis has made significant advances. While photocatalytic protocols can offer easy generation of amidyl radicals under mild conditions, designing catalysts involving earth-abundant Fe complexes for C–H amidation is an attractive approach to bring about ligand-enabled selectivity control. However, due to preference for high-spin configuration, designing robust Fe catalysts for C–H amidation remains a substantial challenge. Taking the advantage of the strong Cp–Fe linkages, here we developed the 17-electron ferrocenium complexes as effective catalysts for facile intramolecular aryl C–H amidation with 1,4,2-dioxazol-5-one as amidyl radical precursors. The ferrocenium-catalyzed reaction affords 3,4-dihydroquinolin-2(1H)-ones in excellent yields and selectivity. Our experimental and computational studies revealed that the intramolecular arene amidation is brought about by electrophilic arene addition by reactive Fe(IV)-amidyl radical species. Consistent with the experimental findings, the regioselectivity (ipso- versus ortho-amidyl radical addition) is influenced by electronic factors. The ipso amidyl radical addition should form an azaspirocyclohexadienyl radical intermediate. Subsequent radical-polar crossover and 1,2-alkyl migration followed by rearomatization afforded the skeletal rearranged dihydroquinolinone lactams. A recyclable heterogeneous amidation catalyst has been prepared by grafting ferrocene onto commercially available silica, and comparable catalytic activities with the homogeneous system were observed.