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

Directarene C-H functionalization by amidyl radicals forarylamides synthesis has made significant advances. While photocatalyticprotocols can offer easy generation of amidyl radicals under mildconditions, designing catalysts involving earth-abundant Fe complexesfor C-H amidation is an attractive approach to bring aboutligand-enabled selectivity control. However, due to preference forhigh-spin configuration, designing robust Fe catalysts for C-Hamidation remains a substantial challenge. Taking the advantage ofthe strong Cp-Fe linkages, here we developed the 17-electronferrocenium complexes as effective catalysts for facile intramoleculararyl C-H amidation with 1,4,2-dioxazol-5-one as amidyl radicalprecursors. The ferrocenium-catalyzed reaction affords 3,4-dihydroquinolin-2(1H)-ones in excellent yields and selectivity. Our experimentaland computational studies revealed that the intramolecular arene amidationis brought about by electrophilic arene addition by reactive Fe(IV)-amidylradical species. Consistent with the experimental findings, the regioselectivity(ipso- versus ortho-amidyl radicaladdition) is influenced by electronic factors. The ipso amidyl radical addition should form an azaspirocyclohexadienyl radicalintermediate. Subsequent radical-polar crossover and 1,2-alkyl migrationfollowed by rearomatization afforded the skeletal rearranged dihydroquinolinonelactams. A recyclable heterogeneous amidation catalyst has been preparedby grafting ferrocene onto commercially available silica, and comparablecatalytic activities with the homogeneous system were observed.