Metal‐ and Additive‐Free Aerobic Dehydrogenation of N‐Heterocycles and Hydrocarbons by N‐Doped Carbon

Abstract

Over metal catalysts, N-heteroarenes, as one of the most versatile building blocks in pharmaceuticals, bioactive molecules and natural products, can be easily accessed by the oxidation of N-heterocycles. Herein, we describe N-doped hierarchical pore carbons (N-HPCs) derived from readily available biomass (flour) efficiently catalyzed aerobic dehydrogenation of N-heterocycles without any additives under mild conditions, owing to N dopant and high surface area indicated by characterization analysis. A wide range of Nheteroarenes including quinolines, quinolinol, dihydroisoqui-nolines, isoquinolines, indoles, quinoxaline and acridine were obtained in good to excellent yields (36 examples, up to 99% yield). Mechanistic studies revealed a strikingly different nonfree radical process from metal catalysts, for which an apparent activation energy of 66.4 kJ mol À 1 was found. Moreover, the N-doped carbon exhibited robust stability and yielded 74.5% quinoline at the sixth reaction. The use of metal-free catalysts in organic synthesis is attractive but practically challenging. This work provides a promising example for metal-free catalysis.

Revisiting the phosphonium salt chemistry for P-doped carbon synthesis: toward high phosphorus contents and beyond the phosphate environment

“…23 The limited variation of yield in the presence of 2 equivalents of BHT (67%), TEMPO (80%) or i PrOH (61%) is within the reproducibility of the experiments, indicating the main pathway does not imply free radical species (entries 3-5). Such a result is in line with the works of Beller et al (N/P-doped carbon) 25 and Dai et al (N-doped carbon), 28 but contrasts with the one of Liu et al (N-doped carbon). 26 Recently, Patel et al reported the ODH of benzyl alcohol to benzaldehyde catalysed by P-doped carbons and suggested a shuttle of the active site from P (III) to P (V) .…”

“…The only substrate obtained with a low yield (6%) was 29 isoquinoline, for which mainly the half-dehydrogenated product was obtained, a limitation also reported by Mata et al 29 A total conversion was recorded for both the quinoxaline and the acridine, in line with previous works. 26,28 All of these results point toward the possible use of the developed material for aerobic ODH of N-heterocycles in N-heteroarenes under mild conditions. The mechanism of the ODH was assessed by control experiments changing the gas phase, or in the presence of radical scavengers (Table 3).…”

“…N-doped carbo-catalysts in particular proved to efficiently activate O 2 as primary oxidant for the ODH of indole and quinoline derivatives. 25,26,28,29,61 On the basis of these works, we evaluated the performance of the synthesized P/N co-doped carbonaceous material for the oxidation of indoline to indole, as a model reaction. As these reactions are usually run on N-heterocycles bearing other reactive groups, we targeted mild conditions in terms of temperature (o100 1C) and oxidant (air).…”

Aromatization of cyclic hydrocarbons via thioether elimination reaction

“…Currently, the direct dehydrogenation of cyclic hydrocarbons relies on several approaches: (i) catalytic approaches involving transition-metal-catalyzed acceptorless dehydrogenation 7,8 and stoichiometric oxidants (Scheme 1b) 9 and (ii) noncatalytic approaches directly employing stoichiometric oxidizing chemicals. The noncatalytic approaches for aromatization of cyclic hydrocarbons involves TEMPO, 10 polyoxometalate (H 5 PV 2 Mo 10 O 40 ), 11 AIBX, 12 I 2 , 13 Selectfluor, 14 graphite oxide, 15 N-doped hierarchical pore carbons (N-HPCs), 16 and H 2 SO 4 . 17 Alkenes are obtained through the dehydrosulfenylation of sulfide, sulfoxide and sulfone derivatives by releasing sulfur-free radicals or sulfur-negative ions.…”

NBS-mediated bromination and dehydrogenation of tetrahydro-quinoline in one pot: scope and mechanistic study

“…Numerous examples include metal-complex catalysed acceptor less dehydrogenation, [19][20][21] along with aerobic oxidation catalyzed by Ru(phd) 3 , 22 Mn(Pc), 23 Co 3 O 4 -NGr/C, 24 and CuI/DEAD 25 etc. In addition, metal free catalytic systems, 26,27 photochemical methods, [28][29][30] and electrochemical methods 31 have also been reported. However, there are few reports on the simultaneous dehydrogenation and functionalization or halogenation of tetrahydroquinolines.…”