Uncatalyzed Oxidative C−H Amination of 9,10‐Dihydro‐9‐Heteroanthracenes: A Mechanistic Study

Abstract

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 was employed as a hydride (or two‐electron and one‐proton) acceptor. Moreover, a previously unknown and uncatalyzed reaction between iminoiodanes and xanthene, thioxanthene and dihydroacridines (9,10‐dihydro‐9‐heteroanthracenes or dihydroheteroanthracenes) is disclosed. The reactions proceed through 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‐catalyzed 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‐catalyzed 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‐catalyzed (proton‐coupled) redox coupling with amines, thus providing alternative pathways to the widely assumed metal‐catalyzed pathways.

Evaluation of the Synthetic Scope and the Reaction Pathways of Proton‐Coupled Electron Transfer with Redox‐Active Guanidines in C−H Activation Processes

“…The slow reaction rate could be explained by a substantial barrier for the initial electron‐transfer step. A similar strong correlation between the potential ( E ox or E 1/2 value) of the PCET reagent and the reaction yield was recently reported for oxidative C−H amination reactions of 9,10‐dihydro‐9‐heteroanthracenes with quinones as PCET reagents [41] …”

“…As imilar strong correlation between the potential (E ox or E 1/2 value) of the PCET reagent and the reaction yield was recently reported for oxidative CÀHa mination reactions of 9,10-dihydro-9-heteroanthracenes with quinones as PCET reagents. [41] Next, we tested the performance of 2 2 + in an aryl-aryl coupling reaction. Reaction of 2 2 + with 3,3''-4,4''-tetramethoxy-oterphenyl (TMTP) at room temperature did not produce the product in the absence of an acid.…”

Electronically Asynchronous Transition States for C–N Bond Formation by Electrophilic [CoIII(TAML)]-Nitrene Radical Complexes Involving Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered Spin Shuttle

“…Under the optimal reaction conditions, PPh 4 [Co III (TAML red )] already reached the maximum yield of 1 after 20 min, whereas this process takes 2 h for [Co III (TAML sq )] ( Figure 5 ). A control reaction without a cobalt catalyst, to test for possible background reactions with PhINNs, 19 did not yield any aziridine 1 ( Table 1 , entry 10).…”

Gold‐Catalyzed Direct C(sp3)−H Acetoxylation of Saturated Hydrocarbons

“…This observation is in line with the calculations shown in Table 3 and corroborates our assignment of hydride transfer. This may be further substantiated by the observation of the group of de Bruin that amination of the C(sp 3 )−H bonds in 9,10‐dihydro‐9‐heteroanthracenes is possible with PhINTs via initial hydride transfer [20] . Notably, this reaction does not require the addition of a catalyst for activation of the hypervalent iodine reagent.…”