Uncatalysed Oxidative C‒H Amination of 9,10-Dihydro-9-Heteroanthracenes: A Mechanistic Study

A new method for the one-step C‒H amination of xanthene and thioxanthene with sulfonamides is reported, without the need for any metal catalyst. A benzoquinone is employed as a hydride (or two-electron and one-proton) acceptor. Moreover, a previously unknown and uncatalysed reaction between iminoiodanes and xanthene, thioxanthene and dihydroacridines (9,10-dihydro-9-hetero-anthracenes or dihydroheteroanthracenes) is disclosed. The reactions proceed via hydride transfer from the heteroarene substrate to the iminoiodane or benzoquinone, followed by conjugate addition of the sulfonamide to the oxidized heteroaromatic compounds. These findings may have important mechanistic implications for metal-catalysed C‒H amination processes involving nitrene transfer from iminoiodanes to dihydroheteroanthracenes. Due to the weak C‒H bond, xanthene is an often-employed substrate in mechanistic studies of C‒H amination reactions, which are generally proposed to proceed via metal-catalysed nitrene insertion, especially for reactions involving nitrene or imido complexes that are less reactive (i.e. less strongly oxidizing). However, these substrates clearly undergo non-catalysed (proton-coupled) redox coupling with amines, thus providing alternative pathways to the widely assumed metal-catalysed pathways.