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.