Efficiently Aerobic Dehydrogenation of N-Heterocycles and Hydrocarbons over MnAl Oxides

Dehydrogenation of saturated N-heterocycles and hydrocarbons provides an important route to corresponding aromatic compounds. Herein, an efficient and selective MnAlO-catalyzed aerobic oxidative dehydrogenation of five- and six-membered N-heterocycles, imines and hydrocarbons is presented. The reaction features excellent yields of the products, mild reaction conditions, readily available and reusable catalyst. Various N-heterocycles and hydrocarbons can be tolerated by the reaction system and transformed to the corresponding aromatic counterparts. Mechanistic studies revealed that the reaction proceeds mainly through an electron transfer mechanism.     

Gold(0) catalyzed dehydrogenation of N-heterocycles

Gold nanoclusters are good catalyst precursors for the catalytic dehydrogenation of indolines, tetrahydroquinazolines, and related N-heterocycles. The catalytically active species is presumably Au(0) nanoparticles.     

Syntheses of 1H-Indoles,Quinolines, and 6-Membered Aromatic N-Heterocycle-Fused Scaffolds via Palladium(II)-Catalyzed Aerobic Dehydrogenation under Alkoxide-Free Conditions

Aromatic N-heterocycle-fused scaffolds such as indoles and quinolines are important core structures found in various bioactive natural products and synthetic compounds. Recently, various dehydrogenation methods with the help of alkoxides, known to significantly promote dihydro- or tetrahydro-heterocycles to be oxidized, were developed for the heterocycle synthesis. However, these approaches are sometimes unsuitable due to resulting undesired side reactions such as reductive dehalogenation. Herein, expedient syntheses of 1H-indoles, quinolines, and 6-membered N-heterocycle-fused scaffolds from their hydrogenated forms through palladium(II)-catalyzed aerobic dehydrogenation under alkoxide-free conditions are reported. A total of 48 compounds were successfully synthesized with a wide range of functional groups including halogens (up to 99% yield). These methodologies provide facile routes for various privileged structures possessing aromatic N-heterocycles without the help of alkoxides, in highly efficient manners.     

A fast and highly efficient protocol for Michael addition of N-heterocycles to α,β-unsaturated compound using basic ionic liquid [bmIm]OH as catalyst and green solvent

A fast and green protocol for the Michael addition of N-heterocycles to α,β-unsaturated compounds at room temperature was developed using a basic ionic liquid, 1-methyl-3-butylimidazolium hydroxide, [bmIm]OH, as a catalyst and a reaction medium. The reactions were performed at room temperature with good yields in short reaction times (0.5-3 h). This strategy is quite general and it works with a broad range of N-heterocycles, including five-membered N-heterocycles, pyrimidines and purines. The recovered ionic liquid could be reused for several cycles with consistent activity.     

Diverse catalytic systems for nitrogen-heterocycle formation from O-acyl ketoximes

O-Acyl ketoximes has been proven to be versatile building blocks for practical construction of N-heterocycles. In the last few years, diverse catalytic systems have been discovered to enable efficient transformations of O-acyl ketoximes to a range of nitrogen-heterocycles. Herein, we summarized our recent examples of novel nitrogen-heterocycle formation with new function findings of O-acyl ketoximes through facile aerobic copper catalysis, metal-free NO bond activation, multi-component assembly, and bis-annulations. From the green chemistry perspective, these works represent efficient methods with high atom economy, high selectivity, and minimized chemical waste. These findings also complement well to the previous mainly copper-based catalytic systems and more importantly enrich the oxime chemistry in organic synthesis.     

Mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes as precatalysts for direct arylation of azoles with aryl bromides

A series of mixed N-heterocycles/N-heterocyclic carbene palladium(II) allyl complexes with general formula [(NHC)Pd(η³-allyl)]₂(μ₂-N-heterocycles)(BF₄)₂ were prepared in one pot based on anion metathesis of (NHC)Pd(η³-allyl)Cl complexes and then ligand replacement with N-heterocycles [N-heterocycles?=?pyrazine (pyz), 4,4′-bipyridine (bpy) and trans-4,4′-bipyridylethylene (bpe)]. The solid-state structures shown dinuclear structures with two palladium(II) centers holding together by bridged N-heterocycles. Initially investigation of the obtained complexes as precatalysts for direct CH bond arylation of azoles with aryl bromides was carried out.     

Transition-metal-switchable divergent synthesis of nitrile-containing pyrazolo[1,5-a]pyridines and indolizines

Palladium-catalyzed oxidative formal [4 + 1] annulation of pyridine-substituted acrylonitriles toward divergent fused N-heterocycles synthesis is reported. The heterodifunctionalization reaction with Cu(OAc)₂ and urea as the nitrogen source accesses to nitrile-substituted pyrazolo[1,5-a]pyridines in moderate to good yields, while the homodifunctionalization reaction with FeBr₃ leads to synthesis of nitrile-substituted indolizines in excellent yields.     

Synthesis of a new ordered mesoporous NiMoO4 complex oxide and its efficient catalytic performance for oxidative dehydrogenation of propane

Highly ordered mesoporous NiMoO4material was successfully synthesized using mesoporous silica KIT-6 as hard template via vacuum nanocasting method. The structure was characterized by means of XRD, TEM, N2adsorption-desorption, Raman and FT-IR. The mesoporous NiMoO4with the coexistence of α-NiMoO4and β-NiMoO4showed well-ordered mesoporous structure, a bimodal pore size distribution and crystalline framework. The catalytic performance of NiMoO4was investigated for oxidative dehydrogenation of propane. It is demonstrated that the mesoporous NiMoO4catalyst with more surface active oxygen species showed better catalytic performance in oxidative dehydrogenation of propane in comparison with bulk NiMoO4.     

Electrochemical utilization of methanol and methanol-d4 as a C1 source to access（deuterated） 2,3-dihydroquinazolin-4（1H）-one

Herein, an electrocatalytic protocol for the synthesis of 2,3-dihydroquinazolin-4(1H)-one has been disclosed. Methanol is activated and utilized as the C1 source to cyclize with 2-aminobenzamides.This cyclization reaction proceeds conveniently(room temperature and air atmosphere) without any homogeneous metal catalysts, external oxidants, or bases. A wide variety of N,N-disubstituted 2,3-dihydroquinazolin-4(1H)-ones are obtained via this approach. Moreover, when methanol-d4is used, a deuterated ...     

Synthesis of N-aryl amines enabled by photocatalytic dehydrogenation

Catalytic dehydrogenation (CD) via visible-light photoredox catalysis provides an efficient route for the synthesis of aromatic compounds. However, access to N-aryl amines, which are widely utilized synthetic moieties, via visible-light-induced CD remains a significant challenge, because of the difficulty in controlling the reactivity of amines under photocatalytic conditions. Here, the visible-light-induced photocatalytic synthesis of N-aryl amines was achieved by the CD of allylic amines. The unusual strategy using C₆F₅I as an hydrogen-atom acceptor enables the mild and controlled CD of amines bearing various functional groups and activated C–H bonds, suppressing side-reaction of the reactive N-aryl amine products. Thorough mechanistic studies suggest the involvement of single-electron and hydrogen-atom transfers in a well-defined order to provide a synergistic effect in the control of the reactivity. Notably, the back-electron transfer process prevents the desired product from further reacting under oxidative conditions.

The synergy of SET, HAT, and BET enables a visible-light induced catalytic dehydrogenation for the synthesis of N-aryl amines.     

Synthesis of modified binol-phosphoramidites

A number of chiral phosphoramidite ligands containing electronically different N-heterocycles at the 3,3′-positions of the binol scaffold were synthesized. The nucleophilicity of the pendant heterocycles correlated with the propensity of the P(III) centre to undergo aerobic oxidation to P(V). Due to an unexpected Staudinger-type reaction between the product phosphoramidites, the order in which the individual synthetic transformations were conducted was found to be important. The synthesis of a phosphoramidite ligand containing flanking groups at the 3,3′-positions of the binol scaffold in addition to a stereogenic phosphorus atom was also undertaken.     

The catalytic activity of a cyclometalated ruthenium(III) complex for aerobic oxidative dehydrogenation of benzylamines

The ruthenium(III) complex bearing benzo[h]quinoline as a cyclometalated ligand was synthesized and characterized by ESI-MS, elemental analysis, cyclic voltammetry and crystallography. The complex serves as an efficient catalyst for the aerobic oxidative dehydrogenation of benzylamines to the corresponding benzonitriles under mild conditions.     

Oxidative C-arylation of free (NH)-heterocycles via direct (sp3) C-H bond functionalization

The development of a new chemical transformation, namely oxidative C-arylation of saturated (NH)-heterocycles, is described. This reaction combines dehydrogenation and arylation in one process, leading to cross-coupling of (NH)-heterocycles and haloarenes. Typical reaction conditions involve heating the reaction partners in anhydrous dioxane at 120-150 degrees C in the presence of RhCl(CO)[P(Fur)3]2 as the catalyst and Cs2CO3 as the base. Addition of tert-butylethylene as the hydrogen acceptor increases the chemical yield by diminishing the dehalogenation pathway. This method demonstrated a good substrate scope, allowing for cross-coupling of a variety of (NH)-heterocycles (e.g., pyrrolidine, piperidine, piperazine, morpholine) and halo(hetero)arenes to afford valuable heterocyclic products in one step. The preliminary mechanistic studies provided some insight regarding the key events in the proposed catalytic cycle, including beta-hydride elimination of an amido rhodium complex and carbometalation of the resulting imine. A large kinetic isotope effect [KIE (kC-H/kC-D) = 4.3] suggests that one or both beta-hydride elimination steps are rate determining. The central role for the phosphine ligand was established in controlling the partitioning between the oxidative C-arylation and N-arylation pathways.     

Iridium-catalyzed enantioselective allylation of sodium 2-aminobenzenethiolate: an access to chiral benzo-fused N,S-heterocycles

The use of sodium 2-aminobenzenethiolate in the enantioselective iridium catalyzed allylic substitution with a range of methyl allyl carbonates allows the concise synthesis of the branch-type products with both excellent regio- and enantioselectivities, which are functionalized N,S-containing allylic intermediates for the formation of chiral benzo-fused N,S-heterocycles.     

Copper‐Catalyzed Aerobic Autoxidation of N‐Hydroxycarbamates Probed by Mass Spectrometry

We present herein a mechanistic investigation by nanoelectrospray ionization mass spectrometry of copper‐catalyzed aerobic oxidative processes involved in the N‐nitrosocarbonyl aldol reaction of N‐hydroxycarbamates. Protonated amine and copper as charge‐tags aided the detection of reaction intermediates, which verified the enamine mechanism together with a competing enol process. Our experimental results reveal that the copper‐catalyzed aerobic oxidation of N‐hydroxycarbamates may proceed through an autoxidation catalytic mechanism in which a CbzNHO^. radical abstracts a hydrogen from the bound N‐hydroxycarbamate to release the nitroso intermediate through a bimolecular hydrogen‐atom transfer. In this process, the chiral diamine also works as a ligand for copper to facilitate the aerobic oxidative step. The dual role of the chiral vicinal diamine as both an aminocatalyst and a bidentate ligand was finally uncovered.     

Gallium-containing catalysts for oxidative dehydrogenation of organic compounds

A method was developed for introducing gallium into Mg-Al hydrotalcites—precursors of oxide catalysts for oxidative dehydrogenation of alkanes. Samples of oxide catalysts were synthesized that contained gallium oxide and also oxides of magnesium, aluminum, chromium, vanadium, molybdenum, and niobium in various combinations. The catalytic properties of the produced catalysts were studied in the oxidative dehydrogenation of ethane, propane, isobutane, and hexane. It was established that the addition of gallium to catalysts increases the ethylene and propylene yields in the oxidative dehydrogenation of ethane and propane. New hydroxo salts with a layered structure of the hydrotalcite type were synthesized: ternary magnesium gallium aluminum hydroxonitrate of variable composition [Al_{1 ? n} Ga _n Mg _m (OH)_{3 + 2m ? 1}][NO₃ · nH₂O] and quaternary magnesium gallium chromium aluminum hydroxonitrate of the composition [AlGaCrMg_1.8(OH)_11.6][NO₃ · nH₂O]; these salts were found to be isostructural.     

Co-Catalyzed Metal-Ligand Cooperative Approach for N-alkylation of Amines and Synthesis of Quinolines via Dehydrogenative Alcohol Functionalization

Herein we report a cobalt-catalyzed sustainable approach for C−N cross-coupling reaction between amines and alcohols. Using a well-defined Co-catalyst 1 a bearing 2-(phenyldiazenyl)-1,10-phenanthroline ligand, various N-alkylated amines were synthesized in good yields. 1 a efficiently alkylates diamines producing N, N′-dialkylated amines in good yields and showed excellent chemoselectivity when oleyl alcohol and β-citronellol, containing internal carbon-carbon double bond were used as alkylating agents. 1 a is equally compatible with synthesizing N-heterocycles via dehydrogenative coupling of amines and alcohols. 1H-Indole was synthesized via an intramolecular dehydrogenative N-alkylation reaction, and various substituted quinolines were synthesized by coupling of 2-aminobenzyl alcohol and secondary alcohols. A few control reactions and spectroscopic experiments were conducted to illuminate the plausible reaction mechanism, indicating that the 1 a -catalyzed N-alkylation proceeds through the borrowing hydrogen pathway. The coordinated arylazo ligand participates actively throughout the reaction; the hydrogen eliminated during dehydrogenation of alcohols was set aside in the ligand backbone and subsequently gets transferred in the reductive amination step to imine intermediates yielding N-alkylated amines. On the other hand, 1 a -catalyzed quinoline synthesis proceeds through dehydrogenation followed by successive C−C and C−N coupling steps forming H₂O₂ as a by-product under air.     

Porphyrin covalent organic framework for photocatalytic synthesis of tetrahydroquinolines

A metal-free porphyrin covalent organic framework was employed as the heterogeneous photocatalyst for the synthesis of tetrahydroquinolines under aerobic conditions. With visible light irradiation of a catalytic amount of H₂P-Bph-COF at room temperature, various substituted N,N-dimethylanilines and N-aryl maleimides were transformed to tetrahydroquinoline derivatives in moderate to good yields. This was the first example of the synthesis of tetrahydroquinolines via the photocatalytic aerobic annulation reaction employing the metal-free COF as the heterogeneous photocatalyst.     

Selective C-C bonds formation,N-alkylation and benzo[d]imidazoles synthesis by a recyclable zinc composite

Earth abundant metals are much less expensive, promising, valuable metals and could be served as catalysts for the borrowing hydrogen reaction, dehydrogenation and heterocycles synthesis, instead of noble metals. The uniformly dispersed zinc composites were designed, synthesized and carefully characterized by means of XPS, EDS, TEM and XRD. The resulting zinc composite showed good catalytic activity for the N-alkylation of amines with amines, ketones with alcohols in water under base-free conditions, while unsaturated carbonyl compounds could also be synthesized by tuning the reaction conditions. Importantly, it was the first time to realize the synthesis of 2-aryl-1H-benzo[d]imidazole derivatives by using this zinc composite under green conditions. Meanwhile, this zinc catalyst could be easily recovered and reused for at least five times.     

X-ray structure of (N-heterocyclic)(η4-diene)-dicarbonyliron(0) compounds

(N-Heterocyclic)(η⁴-diene)dicarbonyliron(0) compounds, where the N-heterocycles are quinoline and pyrazine and the diene is the diethyl ester of the hexa-2,4-dien-1,6-dioic acid (diethyl muconate) were studied by X-ray diffraction. The measurements demonstrated that the N-heterocycle occupies different positions of a tetragonal pyramid in these compounds. Quinoline occupies a basal position and pyrazine an apical position. This is discussed in terms of the π-bonding ability of the ligands.