Carbocatalytic Oxidative Dehydrogenative Couplings of (Hetero)Aryls by Oxidized Multi‐Walled Carbon Nanotubes in Liquid Phase

Abstract

HNO 3 -oxidized carbon nanotubes catalyze oxidative dehydrogenative (ODH) carbon-carbon bond formation between electron-rich (hetero)aryls with O 2 as a terminal oxidant. The recyclable carbocatalytic method provides ac onvenient and an operationally easy synthetic protocol for accessing various benzofused homodimers, biaryls, triphenylenes, and related benzofused heteroaryls that are highly useful frameworks for material chemistry applications.C arbonyls/quinones are the catalytically active site of the carbocatalyst as indicated by model compounds and titration experiments.F urther investigations of the reaction mechanism with ac ombination of experimental and DFT methods support the competing nature of acid-catalyzed and radical cationic ODHs, and indicate that both mechanisms operate with the current material.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.

Divergent Carbocatalytic Routes in Oxidative Coupling of Benzofused Heteroaryl Dimers: A Mechanistic Update

“…In many reported carbocatalysed C−C bond forming reactions the net reaction, i.e., oxidative dehydrogenative (ODH) conversion, has been reported to proceed via acid catalysed, Friedel–Crafts (FC) type mechanism [2] . Furthermore, we [3a,b] and others [4] have demonstrated that carbocatalytic aryl–aryl couplings can proceed through a SET‐based process as well [3a] . We concluded in our recent experimental—computational study that in carbocatalysis both mechanisms can operate depending on the substrate and used catalyst [3b] .…”

“…Furthermore, we [3a,b] and others [4] have demonstrated that carbocatalytic aryl–aryl couplings can proceed through a SET‐based process as well [3a] . We concluded in our recent experimental—computational study that in carbocatalysis both mechanisms can operate depending on the substrate and used catalyst [3b] . The reaction mechanisms, however, cannot always be easily distinguished.…”

“…Subsequently, the activation free energy barriers for the C−C bond formations were lowered by 16.3 and 15.5 kcal mol −1 compared to TS3 for TS1 and TS2 , respectively. This led us to the conclusion that it is possible to improve the previous carbocatalytic methodology for COT synthesis [3c] with oAC and molecular acid additives since our recent studies have indicated that acid additives increase the oxidative power of quinones [5, 19] and protonate the substrate [3b] . This could lead to a significant broadening of the substrate scope and new COT substrates.…”

“…In addition, we studied several different carbon‐based heterogeneous catalysts and homogeneous oxidants that we and others have found highly suitable for similar reactions (Table 1). A catalytic trial with graphene oxide (GO) [3b] delivered only the migratory product 4 a in a 16 % yield (entry 6). Characteristically, GO has a higher amount of carboxylic groups than carbonyl groups and it is often utilized as an acid catalyst in, e.g., FC type reactions [2, 22] …”

“…Recently, we have demonstrated that HNO 3 ‐oxidized carbon nanotubes (oCNTs) can successfully catalyse various intra‐ and intermolecular ODH couplings [3b] . However, for substrate 1 a , oCNT with MsOH additive delivered a mixture of 2 a , 3 a , and 4 a , with 24 %, 9 %, and 7 % yields, respectively.…”