Nitrogen‐Doped Carbon as a Highly Active Metal‐Free Catalyst for the Selective Oxidative Dehydrogenation of N‐Heterocycles

Abstract

N‐heteroarenes represents one of the most important chemicals in pharmaceuticals and other bio‐active molecules, which can be easily accessed from the oxidation of N‐heterocycles over metal catalysts. Herein, the metal‐free oxidative dehydrogenation of N‐heterocycles into N‐heteroarenes was developed using molecular oxygen as the terminal oxidant. The nitrogen‐doped carbon materials were facilely prepared via the simple pyrolysis process using biomass (carboxymethyl cellulose sodium) and dicyandiamide as the carbon and nitrogen source, respectively, and they were discovered to be robust for the oxidative dehydrogenation of N‐heterocycles into N‐heteroarenes under mild conditions (80 °C under 1 bar O2) with water as the green solvent. Diverse N‐heterocycles including 1,2,3,4‐tetrahydroisoquinolines, indolines and 1,2,3,4‐tetrahydroquinoxalines were smoothly converted into N‐heteroarenes with high to excellent yields (76–>99 %). Superoxide radical (⋅O2−) and hydroxyl radical (⋅OH) were probed as the reactive oxygen species for the oxidation of N‐heterocycles into N‐heteroarenes. More importantly, the nitrogen‐doped carbon catalyst can be reused with a high stability. The method provides an environmentally friendly and economical route to access important N‐hetero‐aromatic commodities.

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

“…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).…”

“…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) . 23 Such a mechanism, however, involves the formation of an alkoxide directly linked to the phosphorus atom, and can less easily be transposed to the ODH of N-heterocycles.…”

Charge Density Modulation of Pyrene-Related Small Molecules by Nitrogen Heteroatoms Precisely Regulates Photocatalytic Generation of Hydrogen

“…The photocatalytic process is commonly divided into three stageslight absorption, charge separation, and surface reactionwith the pivotal determinant of success residing in the development of highly efficient semiconductor photocatalysts. − To date, a multitude of researchers have made substantial contributions to the design, enhancement, and mechanistic elucidation of these photocatalysts. , In particular, heteroatom doping, encompassing elements such as carbon, nitrogen, and sulfur, among others, has emerged as a potent means of augmenting the photocatalytic performance of semiconductors. This approach affords several advantages, including the capacity to modulate the conduction band position, narrow the band gap, and enhance the utilization of visible light, while concurrently serving as carrier traps to effectively curtail the recombination of photogenerated carriers. − Within the spectrum of heteroatom doping, the N atom holds particular prominence. Its utilization has demonstrated the capability to finely tune the light absorption characteristics of organic semiconductors, functioning not only as a light-absorbing moiety but also as active centers for the adsorption of reactant molecules. − …”

General synthesis of secondary imines via reductive coupling of carbonyl and nitro compounds employing a reusable cobalt catalyst

“…20 In addition, two peaks at 2 θ = 25.4–25.9° and 42.9° were assigned to the (002) and (100) planes of the graphitic carbon in these samples. 25 The structure of the nitrogen-doped carbon in the catalysts was further characterized by Raman spectroscopy (Fig. 1b).…”