Cu(I) Complexes Catalyzed the Dehydrogenation of N-Heterocycles

A copper-catalyzed method for the dehydrogenation of various nitrogen-containing heterocycles to furnish quinolines and indoles has been developed. A range of 1,2,3,4-tetrahydroquinolines underwent dehydrogenation by employing 2 mol % of copper complex Cat 3 as a catalyst and using O2 as an oxidant at 120 °C in 1,2-dichlorobenzene to afford the desired quinolines. The method enables the dehydrogenation of a variety of indolines in the presence of 2 mol % of copper complex Cat 2, using 10 mol % of TEMPO as an additive and O2 as an oxidant under room temperature in tetrahydrofuran to furnish indoles in high yields. Mechanistic studies suggested that the dehydrogenative activity is ascribed to the formation of a copper(II) active species from copper(I) complexes oxidized by O2, which was proved by high-resolution mass spectrometry (HRMS). The copper-catalyzed dehydrogenation reaction proceeds via a superoxide radical anion (·O2-) as proved by electron paramagnetic resonance (EPR) spectrometry. In situ infrared spectroscopy revealed that the dihydroquinoline intermediate was formed in the dehydrogenation of 1,2,3,4-tetrahydroquinolines.

Recent Advances in Reversible Liquid Organic Hydrogen Carrier Systems: From Hydrogen Carriers to Catalysts

Liquid organic hydrogen carriers (LOHCs) have gained significant attention for large-scale hydrogen storage due to their remarkable gravimetric hydrogen storage capacity (HSC) and compatibility with existing oil and gas transportation networks for long-distance transport. However, the practical application of reversible LOHC systems has been constrained by the intrinsic thermodynamic properties of hydrogen carriers and the performances of associated catalysts in the (de)hydrogenation cycles. To overcome these challenges, thermodynamically favored carriers, high-performance catalysts, and catalytic procedures need to be developed. Here, significant advances in recent years have been summarized, primarily centered on regular LOHC systems catalyzed by homogeneous and heterogeneous catalysts, including dehydrogenative aromatization of cycloalkanes to arenes and N-heterocyclics to N-heteroarenes, as well as reverse hydrogenation processes. Furthermore, with the development of metal complexes for dehydrogenative coupling, a new family of reversible LOHC systems based on alcohols is described that can release H2 under relatively mild conditions. Finally, views on the next steps and challenges in the field of LOHC technology are provided, emphasizing new resources for low-cost hydrogen carriers, high-performance catalysts, catalytic technologies, and application scenarios.

Role of ancillary ligands in selectivity towards acceptorless dehydrogenation versus dehydrogenative coupling of alcohols and amines catalyzed by cationic ruthenium(II)-CNC pincer complexes

An unexpected reversal in catalytic activity for acceptorless dehydrogenative coupling compared to acceptorless alcohol dehydrogenation has been observed using a series of cationic Ru(II)-CNC pincer complexes with different ancillary ligands. In continuation of our study of cationic Ru(II)-CNC pincer complexes 1a-6a, new complexes with bulky N-wingtips [Ru(CNCiPr)(CO)(PPh3)Br]PF6 (1b), [Ru(CNCCy)(CO)(PPh3)Cl]PF6 (1c), [Ru(CNCCy)(CO)(PPh3)H]PF6 (2c), [Ru(CNCiPr)(PPh3)2Cl]PF6 (3b), [Ru(CNCCy)(PPh3)2Cl]PF6 (3c), [Ru(CNCiPr)(PPh3)2H]PF6 (4b), [Ru(CNCCy)(PPh3)2H]PF6 (4c), [Ru(CNCiPr)(DMSO)2Cl]PF6 (6b), and [Ru(CNCCy)(DMSO)2Cl]PF6 (6c) [CNCR = 2,6-bis(1-alkylimidazol-2-ylidene)-pyridine] have been synthesized and the catalytic activities of the new complexes have been compared with their N-methyl analogues for transfer hydrogenation of cyclohexanone and acceptorless dehydrogenation of benzyl alcohol. Furthermore, all complexes have been utilized as catalysts in the dehydrogenative coupling reaction of benzyl alcohol with amines. While the catalytic activities of the new complexes for transfer hydrogenation and acceptorless alcohol dehydrogenation were found to be in line with the previously observed trend based on the ancillary ligands (CO > COD > DMSO > PPh3), for the acceptorless dehydrogenative coupling reaction, complexes containing PPh3 and DMSO ligands performed better compared to complexes containing CO and COD ligands. Based on NMR and mass investigation of catalytic reactions, a plausible mechanism has been suggested to explain the difference in catalytic activity and its reversal during the dehydrogenative coupling reaction. Furthermore, the substrate scope for the dehydrogenative coupling reaction of benzyl alcohol with a wide range of amines has been explored, including synthesizing some pharmaceutically important imines. All new complexes have been characterized by various spectroscopic techniques, and the structures of 4b and 6b have been confirmed by the single-crystal X-ray diffraction technique.

A Convenient and Stable Heterogeneous Nickel Catalyst for Hydrodehalogenation of Aryl Halides Using Molecular Hydrogen

Hydrodehalogenation is an effective strategy for transforming persistent and potentially toxic organohalides into their more benign congeners. Common methods utilize Pd/C or Raney-nickel as catalysts, which are either expensive or have safety concerns. In this study, a nickel-based catalyst supported on titania (Ni-phen@TiO₂ -800) is used as a safe alternative to pyrophoric Raney-nickel. The catalyst is prepared in a straightforward fashion by deposition of nickel(II)/1,10-phenanthroline on titania, followed by pyrolysis. The catalytic material, which was characterized by SEM, TEM, XRD, and XPS, consists of nickel nanoparticles covered with N-doped carbon layers. By using design of experiments (DoE), this nanostructured catalyst is found to be proficient for the facile and selective hydrodehalogenation of a diverse range of substrates bearing C-I, C-Br, or C-Cl bonds (>30 examples). The practicality of this catalyst system is demonstrated by the dehalogenation of environmentally hazardous and polyhalogenated substrates atrazine, tetrabromobisphenol A, tetrachlorobenzene, and a polybrominated diphenyl ether (PBDE).

Non-metal-mediated N-oxyl radical (TEMPO)-induced acceptorless dehydrogenation of N-heterocycles via electrocatalysis

The development of protocols for direct catalytic acceptorless dehydrogenation of N-heterocycles with metal-free catalysts holds the key to difficulties in green and sustainable chemistry. Herein, an N-oxyl radical (TEMPO) acting as an oxidant in combination with electrochemistry is used as a synthesis system under neutral conditions to produce N-heterocycles such as benzimidazole and quinazolinone. The key feature of this protocol is the utilization of the TEMPO system as an inexpensive and easy to handle radical surrogate that can effectively promote the dehydrogenation reaction. Mechanistic studies also suggest that oxidative TEMPOs redox catalytic cycle participates in the dehydrogenation of 2,3-dihydro heteroarenes.

The development of protocols for direct catalytic acceptorless dehydrogenation of N-heterocycles with metal-free catalysts holds the key to difficulties in green and sustainable chemistry.

Recent advances in the synthesis of N-heteroarenes via catalytic dehydrogenation of N-heterocycles

Bio-active molecules having N-heteroarene core are widely used for numerous medicinal applications and as lifesaving drugs. In this direction, dehydrogenation of partially saturated aromatic N-heterocycles shows utmost importance for the synthesis of heterocycles. This feature article highlights the recent advances, from 2009 to April 2021, on the dehydrogenation of N-heteroaromatics. Notable features considering the development of newer catalysis for dehydrogenations are: (i) approaches based on precious metal catalysis, (ii) newer strategies and catalyst development technology using non-precious metal-catalysts for N-heterocycles having one or more heteroatoms, (iii) Synthesis of five or six-membered N-heterocycles using photocatalysis, electrocatalytic, and organo-catalytic approaches using different homogeneous and heterogeneous conditions' (iv) metal free (base and acid-promoted) dehydrogenation along with I₂, N-hydroxyphthalimide (NHPI) and bio catalyzed miscellaneous examples have also been discussed, (v) mechanistic studies for various dehydrogenation reactions and (vi) synthetic applications of various bio-active molecules including post-drug derivatization are discussed.

Tuning the Catalytic Performance of Cobalt Nanoparticles by Tungsten Doping for Efficient and Selective Hydrogenation of Quinolines under Mild Conditions

Non-noble bimetallic CoW nanoparticles (NPs) partially embedded in a carbon matrix (CoW@C) have been prepared by a facile hydrothermal carbon-coating methodology followed by pyrolysis under an inert atmosphere. The bimetallic NPs, constituted by a multishell core–shell structure with a metallic Co core, a W-enriched shell involving Co₇W₆ alloyed structures, and small WO₃ patches partially covering the surface of these NPs, have been established as excellent catalysts for the selective hydrogenation of quinolines to their corresponding 1,2,3,4-tetrahydroquinolines under mild conditions of pressure and temperature. It has been found that this bimetallic catalyst displays superior catalytic performance toward the formation of the target products than the monometallic Co@C, which can be attributed to the presence of the CoW alloyed structures.

Understanding of TEMPO-electrocatalyzed acceptorless dehydrogenation of tetrahydroquinoline by in situ extractive electrospray ionization mass spectrometry

The TEMPO-electrocatalyzed acceptorless dehydrogenation of tetrahydroquinoline, a transformation in green synthesis and hydrogen storage, has been investigated by coupling of an electrocatalytic system with in situ extraction electrospray ionization mass spectrometry. Dynamic changes in important species and intermediates were monitored, which evoked an updated AD understanding.

An effective strategy for hydrogen supply: catalytic acceptorless dehydrogenation of N-heterocycles

Catalytic acceptorless dehydrogenation of N-heterocycles will offer great hope to solve numerous existing complex scientific and technological problems with simple, efficient, stable and controllable energy output, especially facilitating development in the field of PEMFC.

Monomeric vanadium oxide: a very efficient species for promoting aerobic oxidative dehydrogenation of N-heterocycles

The isolated monomeric VO₄ species, controlled by natural ligand tartaric acid, in the VO_x/NbO_y@C catalysts exhibited excellent performances and good recyclability in the dehydrogenation of N-heterocycles.

Cascade Synthesis of Pyrroles from Nitroarenes with Benign Reductants Using a Heterogeneous Cobalt Catalyst

A bifunctional 3d-metal catalyst for the cascade synthesis of diverse pyrroles from nitroarenes is presented. The optimal catalytic system Co/NGr-C@SiO2 -L is obtained by pyrolysis of a cobalt-impregnated composite followed by subsequent selective leaching. In the presence of this material, (transfer) hydrogenation of easily available nitroarenes and subsequent Paal-Knorr/Clauson-Kass condensation provides >40 pyrroles in good to high yields using dihydrogen, formic acid, or a CO/H2 O mixture (WGSR conditions) as reductant. In addition to the favorable step economy, this straightforward domino process does not require any solvents or external co-catalysts. The general synthetic utility of this methodology was demonstrated on a variety of functionalized substrates including the preparation of biologically active and pharmaceutically relevant compounds, for example, (+)-Isamoltane.

Understanding the mechanism of the competitive adsorption in 8-methylquinoline hydrogenation over a Ru catalyst

The competitive adsorption of 8-methylquinoline (8-MQL) and partially hydrogenated product, 4H-8-MQL, was studied by performing a combination of experiments and first-principles calculations over a selected Ru catalyst. A series of hydrogenation reactions were conducted with 8-MQL and 4H-8-MQL as initial reactants, respectively. 8-MQL exhibits stronger adsorption on catalyst surface active sites compared with 4H-8-MQL and the massive adsorption of 8-MQL hampers the further adsorption of 4H-8-MQL. The effects of temperature, pressure and solvent on the selectivity in 8-MQL hydrogenation were investigated as well. Full hydrogenation of 8-MQL to 10H-8-MQL was achieved within 120 min when the catalyst dosage increased from 5 wt% to 7 wt% under 160 °C and a hydrogen pressure of 7 MPa. The electronic charge of the N-heteroatom in 8-MQL and 4H-8-MQL was analyzed and the adsorption geometries of 8-MQL and 4H-8-MQL on the Ru(001) surface were optimized by DFT calculations to explain the competitive adsorption behaviors of 8-MQL and 4H-8-MQL.

A rare earth hydride supported ruthenium catalyst for the hydrogenation of N-heterocycles: boosting the activity via a new hydrogen transfer path and controlling the stereoselectivity

Hydrogenation of N-heterocycles is of great significance for their wide range of applications such as building blocks in drug and agrochemical syntheses and liquid organic hydrogen carriers (LOHCs). Pursuing a better hydrogenation performance and stereoselectivity, we successfully developed a rare earth hydride supported ruthenium catalyst Ru/YH3 for the hydrogenation of N-heterocycles, especially N-ethylcarbazole (NEC), the most promising LOHC. Full hydrogenation of NEC on Ru/YH3 can be achieved at 363 K and 1 MPa hydrogen pressure, which is currently the lowest compared to previous reported catalysts. Furthermore, Ru/YH3 shows the highest turnover number, namely the highest catalytic activity among the existing catalysts for hydrogenation of NEC. Most importantly, Ru/YH3 shows remarkable stereoselectivity for all-cis products, which is very favorable for the subsequent dehydrogenation. The excellent performance of Ru/YH3 originates from the new hydrogen transfer path from H2 to NEC via YH3. Ru/LaH3 and Ru/GdH3 also reveal good activity for hydrogenation of NEC and Ru/YH3 also possesses good activity for hydrogenation of 2-methylindole, indicating that the use of rare earth hydride supported catalysts is a highly effective strategy for developing better hydrogenation catalysts for N-heterocycles.

Synergistic catalysis on Fe-N x sites and Fe nanoparticles for efficient synthesis of quinolines and quinazolinones via oxidative coupling of amines and aldehydes

In this paper, we developed a reusable heterogeneous non-precious iron nanocomposite comprising metallic Fe-Fe3C nanoparticles and Fe-N x sites on N-doped porous carbon, which allows for highly efficient synthesis of quinolines and quinazolinones via oxidative coupling of amines and aldehydes using H2O2 as the oxidant in aqueous solution under mild conditions. A set of quinazolines and quinazolinones were synthesized in high yields with a broad substrate scope and good tolerance of functional groups. Characterization and control experiments disclose that a synergistic effect between the metallic Fe nanoparticles and built-in Fe-N x sites is primarily responsible for the outstanding catalytic performance. Furthermore, the iron nanocomposite could be readily recovered for successive use without appreciable loss in catalytic activity and selectivity. This work provides an expedient and sustainable method to access pharmaceutically relevant N-heterocycles.

Nitrogen-thermal modification of the bifunctional interfaces of transition metal/carbon dyads for the reversible hydrogenation and dehydrogenation of heteroarenes

A nitrogen-thermal approach via the reaction between transition metal species and N dopants affords us the ability to optimize the tradeoff between the number of exposed transition metal/carbon (exemplified by cobalt in this work) boundaries and the most pronounced interfacial rectifying contact to achieve the highly efficient and selective hydrogenation and dehydrogenation of N-heterocycle compounds in a reversible manner.

Transition metal silicides: fundamentals, preparation and catalytic applications

Transition metal silicides as low-cost and earth-abundant inorganic materials are becoming indispensable constituents in catalytic systems for a variety of applications and exhibit excellent properties for sustainable industrial process.