A robust iron catalyst for the selective hydrogenation of substituted (iso)quinolones

Transfer Hydrogenation of N- and O-Containing Heterocycles Including Pyridines with H3N-BH3 Under the Catalysis of the Homogeneous Ruthenium Precatalyst

In this study, we report a transfer hydrogenation protocol that utilizes borane-ammonia (H3N-BH3) as the hydrogen source and a commercially available RuCl3·xH2O precatalyst for the selective aromatic reduction of quinolines, quinoxalines, pyridines, pyrazines, indoles, benzofurans, and furan derivatives to form the corresponding alicyclic heterocycles in good to excellent isolated yields. Applications of this straightforward protocol include the efficient preparation of useful key pharmaceutical intermediates, such as donepezil and flumequine, including a biologically active compound.

Strategically Designed Pd-Induced Changes in Alkaline Hydrogen Evolution Reaction and Oxygen Evolution Reaction Performances of Electrochemical Water Oxidation by the Galvanically Synthesized MoO2/MoO3 Composite Thin Film

Electrochemical water oxidation is believed to be an effective pathway to produce clean, carbon-free, and environmentally sustainable green energy. In this work, we report a simple, easy-to-construct, facile, low-cost, and single-step galvanic technique to synthesize a Pd-supported temperature-assisted MoOx thin film nanocomposite for effective water oxidation. The most suitable nanocomposite exhibits very low overpotential at 10 mA/cm2 with smaller Tafel slope values for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) processes in an alkaline medium. The formation of a metal oxide-metal junction accelerates the growth of more active sites, promoting induced electronic synergism at the MoOx-Pd interface. This endows higher electrical conductivity and faster electron transfer kinetics, thus accelerating the faster water dissociation reaction following the Tafel-Volmer mechanism to boost the HER process in an alkaline medium. The excellent electrochemical HER and OER performances of our electrocatalyst even supersede the accomplishments of the benchmark catalysts Pt/C and RuO2. Moreover, neither of these two catalysts demonstrates both catalytic reactions, i.e., HER and OER at the same time, which have been observed for our synthesized catalyst. Our findings illustrate the potential of a thin-film MoOx-Pd nanocomposite to be an exceedingly effective electrocatalyst developed by interface engineering strategies. This also provides insight into designing several other semiconductor composite catalysts using simple synthesis techniques for highly efficient HER/OER processes that could be alternatives to benchmark electrocatalysts for water electrolysis.

Colloidal ruthenium catalysts for selective quinaldine hydrogenation: Ligand and solvent effects

Colloidal Ru nanoparticles (NP) display interesting catalytic properties for the hydrogenation of (hetero)arenes as they proceed efficiently in mild reaction conditions. In this work, a series of Ru based materials was used in order to selectively hydrogenate quinaldine and assess the impact of the stabilizing agent on their catalytic performances. Ru nanoparticles stabilized with polyvinylpyrrolidone (PVP) and 1-adamantanecarboxylic acid (AdCOOH) allowed to obtain 5,6,7,8-tetrahydroquinaldine with a remarkable selectivity in mild reaction conditions by choosing the suitable solvent. The presence of a carboxylate ligand on the surface of the Ru NP led to an increase in the activity when compared to Ru/PVP catalyst. The stabilizing agent had also an impact on the selectivity, as carboxylate ligand modified catalysts promoted the selectivity towards 1,2,3,4-tetrahydroquinaldine, with bulky carboxylate displaying the highest ones.

A general and robust Ni-based nanocatalyst for selective hydrogenation reactions at low temperature and pressure

Catalytic hydrogenations are important and widely applied processes for the reduction of organic compounds both in academic laboratories and in industry. To perform these reactions in sustainable and practical manner, the development and applicability of non-noble metal-based heterogeneous catalysts is crucial. Here, we report highly active and air-stable nickel nanoparticles supported on mesoporous silica (MCM-41) as a general and selective hydrogenation catalyst. This catalytic system allows for the hydrogenation of carbonyl compounds, nitroarenes, N-heterocycles, and unsaturated carbon─carbon bonds in good to excellent selectivity under very mild conditions (room temperature to 80°C, 2 to 10 bar H2). Furthermore, the optimal nickel/meso-silicon dioxide catalyst is reusable (4 cycles) without loss of its catalytic activity.

Usnic Acid-Loaded Magnetite Nanoparticles-A Comparative Study between Synthesis Methods

Since cancer is a continuously increasing concern for the general population, more efficient treatment alternatives ought to be developed. In this regard, a promising direction is represented by the use of magnetite nanoparticles (MNPs) to act both as a nanocarrier for the targeted release of antitumoral drugs and as hyperthermia agents. Thus, the present study focused on improving the control upon the outcome properties of MNPs by using two synthesis methods, namely the co-precipitation and microwave-assisted hydrothermal method, for the incorporation of usnic acid (UA), a natural lichen-derived metabolite with proven anticancer activity. The obtained UA-loaded MNPs were thoroughly characterized regarding their morpho-structural and physicochemical properties through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS) and zeta potential, scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). Results demonstrated the formation of magnetite as the unique mineralogical phase through both types of synthesis, with increased uniformity regarding the drug loading efficiency, size, stability, and magnetic properties obtained through the microwave-assisted hydrothermal method. Furthermore, the cytotoxicity of the nanostructures against the HEK 293T cell line was investigated through the XTT assay, which further proved their potential for anticancer treatment applications.

Zinc/Cerium-Substituted Magnetite Nanoparticles for Biomedical Applications

Numerous studies have reported the possibility of enhancing the properties of materials by incorporating foreign elements within their crystal lattice. In this context, while magnetite has widely known properties that have been used for various biomedical applications, the introduction of other metals within its structure could prospectively enhance its effectiveness. Specifically, zinc and cerium have demonstrated their biomedical potential through significant antioxidant, anticancer, and antimicrobial features. Therefore, the aim of the present study was to develop a series of zinc and/or cerium-substituted magnetite nanoparticles that could further be used in the medical sector. The nanostructures were synthesized through the co-precipitation method and their morpho-structural characteristics were evaluated through X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) analyses. Furthermore, the nanostructures were subjected to a ROS-Glo H2O2 assay for assessing their antioxidant potential, MTT assay for determining their anticancer effects, and antimicrobial testing against S. aureus, P. aeruginosa, and C. albicans strains. Results have proven promising for future biomedical applications, as the nanostructures inhibit oxidative stress in normal cells, with between two- and three-fold reduction and cell proliferation in tumor cells; a two-fold decrease in cell viability and microbial growth; an inhibition zone diameter of 4–6 mm and minimum inhibitory concentration (MIC) of 1–2 mg/mL.

FePO4 supported Rh subnano clusters with dual active sites for efficient hydrogenation of quinoline under mild conditions

Chemoselective hydrogenation of quinoline and its derivatives under mild reaction conditions still remains a challenging topic, which requires a suitable interaction between reactants and a catalyst to achieve high performance and stability. Herein, FePO₄-supported Rh single atoms, subnano clusters and nanoparticle catalysts were synthesized and evaluated in the chemoselective hydrogenation of quinoline. The results show that the Rh subnano cluster catalyst with a size of ∼1 nm gives a specific reaction rate of 353 mol_quinoline mol_Rh^-1 h^-1 and a selectivity of >99% for 1,2,3,4-tetrahydroquinoline under mild conditions of 50 °C and 5 bar H₂, presenting better performance compared with the Rh single atoms and nanoparticle counterparts. Moreover, the Rh subnano cluster catalyst exhibits good stability and substrate universality for the hydrogenation of various functionalized quinolines. A series of characterization studies demonstrate that the acidic properties of the FePO₄ support favors the adsorption of quinoline while the Rh subnano clusters promote the dissociation of H₂ molecules, and then contribute to the enhanced hydrogenation performance. This work provides an important implication to design efficient Rh-based catalysts for chemoselective hydrogenation under mild conditions.

Core-Shell Nano-Cobalt Catalyzed Chemoselective Reduction of N-Heteroarenes with Ammonia Borane

An easily prepared core-shell heterogeneous nanocobalt catalyst was reported, which could achieve selective reduction of N-heteroarenes with ammonia borane under mild conditions and ambient atmosphere. Various quinoline, quinoxaline, naphthyridine, isoquinoline, acridine, and phenanthroline derivatives were hydrogenated with high selectivity and efficiency. Notably, substrates bearing sensitive functional groups under molecular hydrogen reduction conditions, such as cyano, ester, and halogens were well tolerated by the catalytic system. Moreover, with our novel method several bioactive molecules were prepared. Also, this catalyst could be applied in the liquid organic hydrogen storage system by reversible hydrogenation and dehydrogenation of heteroarene in high efficiencies.

Electrocatalytic hydrogenation of quinolines with water over a fluorine-modified cobalt catalyst

Room temperature and selective hydrogenation of quinolines to 1,2,3,4-tetrahydroquinolines using a safe and clean hydrogen donor catalyzed by cost-effective materials is significant yet challenging because of the difficult activation of quinolines and H₂. Here, a fluorine-modified cobalt catalyst is synthesized via electroreduction of a Co(OH)F precursor that exhibits high activity for electrocatalytic hydrogenation of quinolines by using H₂O as the hydrogen source to produce 1,2,3,4-tetrahydroquinolines with up to 99% selectivity and 94% isolated yield under ambient conditions. Fluorine surface-sites are shown to enhance the adsorption of quinolines and promote water activation to produce active atomic hydrogen (H*) by forming F⁻-K⁺(H₂O)₇ networks. A 1,4/2,3-addition pathway involving H* is proposed through combining experimental and theoretical results. Wide substrate scopes, scalable synthesis of bioactive precursors, facile preparation of deuterated analogues, and the paired synthesis of 1,2,3,4-tetrahydroquinoline and industrially important adiponitrile at a low voltage highlight the promising applications of this methodology.

Selective hydrogenation of quinolines with easy-to-handle hydrogen donors and cost-effective catalysts is desirable. Here electrocatalytic quinoline hydrogenation to 1,2,3,4-tetrahydroquinolines is reported with water over a fluorine-modified cobalt.

Heterogeneous Hydrogenation of Quinoline Derivatives Effected by a Granular Cobalt Catalyst

We communicate a convenient method for the pressure hydrogenation of quinolines in aqueous solution by using a particulate cobalt-based catalyst that is prepared in situ from simple Co(OAc)2·4H2O through reduction with abundant zinc powder. This catalytic protocol permits a brisk and atom-efficient access to a variety of 1,2,3,4-tetra­hydroquinolines thereby relying solely on easy-to-handle reagents that are all readily obtained from commercial sources. Both the reaction setup assembly and the autoclave charging procedure are conducted on the bench outside an inert-gas-operated containment system, thus rendering the overall synthesis time-saving and operationally very simple.

Unique catalytic properties of Ni–Ir alloy for the hydrogenation of N-heteroaromatics

SiO2-supported Ni–Ir alloy catalysts showed much higher catalytic activity for the hydrogenation of N-heteroaromatics including pyridines and quinolines than monometallic Ir/SiO2 and Ni/SiO2.

Silica-supported Fe/Fe–O nanoparticles for the catalytic hydrogenation of nitriles to amines in the presence of aluminium additives

The hydrogenation of nitriles to amines represents an important and frequently used industrial process due to the broad applicability of the resulting products in chemistry and life sciences. Despite the existing portfolio of catalysts reported for the hydrogenation of nitriles, the development of iron-based heterogeneous catalysts for this process is still a challenge. Here, we show that the impregnation and pyrolysis of iron(II) acetate on commercial silica produces a reusable Fe/Fe–O@SiO2 catalyst with a well-defined structure comprising the fayalite phase at the Si–Fe interface and α-Fe nanoparticles, covered by an ultrathin amorphous iron(III) oxide layer, growing from the silica matrix. These Fe/Fe–O core–shell nanoparticles, in the presence of catalytic amounts of aluminium additives, promote the hydrogenation of all kinds of nitriles, including structurally challenging and functionally diverse aromatic, heterocyclic, aliphatic and fatty nitriles, to produce primary amines under scalable and industrially viable conditions.

Chemoselective and Tandem Reduction of Arenes Using a Metal-Organic Framework-Supported Single-Site Cobalt Catalyst

The development of heterogeneous, chemoselective, and tandem catalytic systems using abundant metals is vital for the sustainable synthesis of fine and commodity chemicals. We report a robust and recyclable single-site cobalt-hydride catalyst based on a porous aluminum metal-organic framework (DUT-5 MOF) for chemoselective hydrogenation of arenes. The DUT-5 node-supported cobalt(II) hydride (DUT-5-CoH) is a versatile solid catalyst for chemoselective hydrogenation of a range of nonpolar and polar arenes, including heteroarenes such as pyridines, quinolines, isoquinolines, indoles, and furans to afford cycloalkanes and saturated heterocycles in excellent yields. DUT-5-CoH exhibited excellent functional group tolerance and could be reusable at least five times without decreased activity. The same MOF-Co catalyst was also efficient for tandem hydrogenation-hydrodeoxygenation of aryl carbonyl compounds, including biomass-derived platform molecules such as furfural and hydroxymethylfurfural to cycloalkanes. In the case of hydrogenation of cumene, our spectroscopic, kinetic, and density functional theory (DFT) studies suggest the insertion of a trisubstituted alkene intermediate into the Co-H bond occurring in the turnover limiting step. Our work highlights the potential of MOF-supported single-site base-metal catalysts for sustainable and environment-friendly industrial production of chemicals and biofuels.

Iridium-Catalyzed Asymmetric Transfer Hydrogenation of Quinolines in Biphasic Systems or Water

An asymmetric transfer hydrogenation (ATH) of quinolines in water or biphasic systems was developed. This ATH reaction proceeds smoothly without the need for inert atmosphere protection in the presence of a water-soluble iridium catalyst, which bears an easily available aminobenzimidazole ligand. This ATH system can work at a catalyst loading of 0.001 mol % (S/C = 100 000, turnover number (TON) of up to 33 000) under mild reaction conditions. The turnover frequency (TOF) value can reach as high as 90 000 h-1. A variety of quinoline and N-heteroaryl compounds are transformed into the desired products in high yield and up to 99% enantiomeric excess (ee).

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.

Iridium Complex-Catalyzed Transfer Hydrogenation of N-Heteroarenes and Tentative Asymmetric Synthesis

An iridium-catalyzed transfer hydrogenation of N-heteroarenes to access a series of substituted 1,2,3,4-tetrahydroquinoline derivatives in excellent yields is disclosed. This transformation is distinguished with water-soluble and air-stable iridium complexes as the catalyst, formic acid as the hydrogen source, mild reaction conditions, and broad functional group compatibility. Most importantly, a tentative chiral N,N-chelated Cp*Ir(III) complex-catalyzed enantioselective transfer hydrogenation is also presented, affording chiral products in excellent yields and good enantioselectivities.

Atomically dispersed Ir/α-MoC catalyst with high metal loading and thermal stability for water-promoted hydrogenation reaction

Synthesis of atomically dispersed catalysts with high metal loading and thermal stability is challenging but particularly valuable for industrial application in heterogeneous catalysis. Here, we report a facile synthesis of a thermally stable atomically dispersed Ir/α-MoC catalyst with metal loading as high as 4 wt%, an unusually high value for carbide supported metal catalysts. The strong interaction between Ir and the α-MoC substrate enables high dispersion of Ir on the α-MoC surface, and modulates the electronic structure of the supported Ir species. Using quinoline hydrogenation as a model reaction, we demonstrate that this atomically dispersed Ir/α-MoC catalyst exhibits remarkable reactivity, selectivity and stability, for which the presence of high-density isolated Ir atoms is the key to achieving high metal-normalized activity and mass-specific activity. We also show that the water-promoted quinoline hydrogenation mechanism is preferred over the Ir/α-MoC, and contributes to high selectivity towards 1,2,3,4-tetrahydroquinoline. The present work demonstrates a new strategy in constructing a high-loading atomically dispersed catalyst for the hydrogenation reaction.

Electronic Effect in a Ruthenium Catalyst Designed in Nanoporous N-Functionalized Carbon for Efficient Hydrogenation of Heteroarenes

Active metal catalysts are the key in chemical industry for sustainable production of multitude of chemical resources. Here, we report a new ruthenium (Ru) composite with a synergistically controlled nanostructure and electronic properties as a highly efficient hydrogenation catalyst which comprises stable small Ru nanoparticles (mean particle size, ca. 0.9 nm) in situ generated into a nanoporous N-functionalized carbon with high surface area (ca. 650 m² g^-1) and has strong electron-donating power of Ru sites of nanoparticles. The scalable and highly reusable catalyst, prepared from a self-assembled Ru complex, performs actively with low per metal usage under mild conditions (60-80 °C and 0.5-1.0 MPa H₂) for selective hydrogenation of various quinolines and pyridines. The role of electron-donating properties of the new Ru nanohybrid for highly efficient catalysis was characterized by both experiments and computational studies. Density functional theory calculations reveal that weak adsorption energies of quinoline at the electron-rich Ru surface prevents poisoning caused by its strong coordination and provides excellent reusability of the catalyst, while low activation barriers for the hydrogenation steps of the N-heterocyclic ring correlate with high catalytic activity. Our catalyst exhibits 5-24-fold higher turnover frequency up to ca. 167 h^-1 among the efficient noble metal catalysts reported for selective hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline.

Synthesis of Tetrahydroquinolines via Borrowing Hydrogen Methodology Using a Manganese PN3 Pincer Catalyst

A straightforward and selective synthesis of 1,2,3,4-tetrahydroquinolines starting from 2-aminobenzyl alcohols and simple secondary alcohols is reported. This one-pot cascade reaction is based on the borrowing hydrogen methodology promoted by a manganese(I) PN³ pincer complex. The reaction selectively leads to 1,2,3,4-tetrahydroquinolines thanks to a targeted choice of base. This strategy provides an atom-efficient pathway with water as the only byproduct. In addition, no further reducing agents are required.

A General Catalyst Based on Cobalt Core-Shell Nanoparticles for the Hydrogenation of N-Heteroarenes Including Pyridines

Herein, we report the synthesis of specific silica-supported Co/Co3 O4 core-shell based nanoparticles prepared by template synthesis of cobalt-pyromellitic acid on silica and subsequent pyrolysis. The optimal catalyst material allows for general and selective hydrogenation of pyridines, quinolines, and other heteroarenes including acridine, phenanthroline, naphthyridine, quinoxaline, imidazo[1,2-a]pyridine, and indole under comparably mild reaction conditions. In addition, recycling of these Co nanoparticles and their ability for dehydrogenation catalysis are showcased.

Boric acid catalyzed chemoselective reduction of quinolines

Boric acid promoted transfer hydrogenation of substituted quinolines to synthetically versatile 1,2,3,4-tetrahydroquinolines (1,2,3,4-THQs) was described under mild reaction conditions using a Hantzsch ester as a mild organic hydrogen source. This methodology is practical and efficient, where isolated yields are excellent and reducible functional groups are well tolerated in the N-heteroarene moiety. The reaction parameters and tentative mechanistic pathways are demonstrated by various control experiments and NMR studies. The present work can also be scaled up to obtain gram quantities and the utility of the developed process is illustrated by the transformation of 1,2,3,4-THQs into a series of biologically important molecules including the antiarrhythmic drug nicainoprol.

Convenient synthesis of cobalt nanoparticles for the hydrogenation of quinolines in water

Easily accessible cobalt nanoparticles are prepared by hydrolysis of NaBH₄ in the presence of inexpensive Co(ii) salts.

Biomass-derived Fe-NC hybrid for hydrogenation with formic acid: control of Fe-based nanoparticle distribution

A series of Fe-NC catalysts were synthesized by pyrolyzing an Fe complex and wheat flour at 500 °C. All of them were characterized and applied in the catalytic transfer hydrogenation of nitroarenes with formic acid. It was found that the catalytic activity was significantly affected by the size and distribution of Fe-based nanoparticles (NPs), which could be easily regulated by altering the Fe source. Meanwhile, more basic nitrogen sites were preserved on the catalyst so that the reaction ran smoothly without base additives. Among all catalysts, Fe-NC-FeCl₂ exhibited the best catalytic performance due to smaller Fe₃O₄ NPs and greater N doping. Moreover, it showed excellent applicability for diverse nitroarenes. Obviously, this work demonstrates the importance of the metallic NPs' size and distribution, providing a new insight into the design of M-NC catalysts. The catalyst is economical and eco-friendly, and shows potential application value in industry.

A series of iron-based, N-doped carbon catalysts exhibits excellent activity in catalytic transfer hydrogenation of nitroarenes without base additive.

Liberating N-CNTs Confined Highly Dispersed CoNx Sites for Selective Hydrogenation of Quinolines

Selective hydrogenation of quinoline and its derivatives is an important means to produce corresponding 1,2,3,4-tetrahydroquinolines for a wide spectrum of applications. A facile and efficient "laser irradiation in liquid" technique to liberate the inaccessible highly dispersed CoN_x active sites confined inside N-doped carbon nanotubes is demonstrated. The liberated CoN_x sites possess generic catalytic activities toward selective hydrogenation of quinoline and its hydroxyl, methyl, and halogen substituted derivatives into corresponding 1,2,3,4-tetrahydroquinolines with almost 100% conversion efficiency and selectivity. This laser irradiation treatment approach should be widely applicable to unlock the catalytic powers of inaccessible catalytic active sites confined by other materials.

Hydrogenation of Functionalized Nitroarenes Catalyzed by Single-Phase Pyrite FeS2 Nanoparticles on N,S-Codoped Porous Carbon

Catalytic hydrogenation of nitroarenes is an industrially very important and environmentally friendly process for the production of anilines; however, highly chemoselective reduction of nitroarenes decorated with one or more reducible groups in a nitroarene molecule remains a challenge. Herein, a novel hybrid non-noble iron-based nanocatalyst (named as FeS₂ /NSC) was developed, which was prepared from biomass as C and N source together with inexpensive Fe(NO₃ )₃ as Fe source through high-temperature pyrolysis in a straightforward and cost-effective procedure. Comprehensive characterization revealed that single-phase pyrite FeS₂ nanoparticles with precisely defined composition and uniform size were homogeneously dispersed on N,S-codoped porous carbon with large specific surface area, hierarchical porous channels, and high pore volume. The resultant catalyst FeS₂ /NSC demonstrated good catalytic activity for hydrogenation of functionalized nitroarenes with good tolerance of various functional groups in water as a sustainable and green solvent. Compared with bulk pyrite FeS₂ and other non-noble metal-based heterogeneous catalysts reported in the literature, a remarkably enhanced activity was observed under mild reaction conditions. More importantly, FeS₂ /NSC displayed exclusive chemoselectivity for the reduction of nitro groups for nitroarenes bearing varying readily reducible groups.

Synthesis of aryl-substituted quinolines and tetrahydroquinolines through Suzuki–Miyaura coupling reactions

The synthesis and characterization of substituted (trifluoromethoxy, thiomethyl, and methoxy) phenyl quinolines is described. Dichlorobis(triphenylphosphine)palladium(II)-catalyzed Suzuki–Miyaura cross-coupling of 6-bromo- and 6,8-dibromo-1,2,3,4-tetrahydroquinolines, 5-bromo-8-methoxyquinoline, and 5,7-dibromo-8-methoxyquinoline with substituted phenylboronic acids affords the corresponding 6-aryl- (13a–d), 6,8-diaryl- (14a–c), 5-aryl- (15), and 5,7-diaryl- (16b, c) tetrahydroquinolines and quinolines in high yields (68%–82%). The structures of all the products are characterized by 1H NMR, 13C NMR,19F NMR, and Fourier transform infrared spectroscopy and by elemental analysis.

Copper-Catalyzed Transfer Hydrogenation of N-Heteroaromatics with an Oxazaborolidine Complex

The first-time use of the oxazaborolidine complex in transfer hydrogenation was accomplished. It was prepared without difficulty from cheap materials: ethanolamine and BH₃·THF. A general and efficient method for copper-catalyzed transfer hydrogenation of a variety of N-heteroaromatics with an oxazaborolidine-BH₃ complex under mild reaction conditions afforded the corresponding hydrogenated products in up to 96% yield. Mechanistic studies indicate that the hydrogen source originated from water and borane that coordinate with the nitrogen atom of oxazaborolidine. Accordingly, a plausible mechanism for this reaction was proposed. This method was successfully used in the key step synthesis of natural products (±)-angustureine and (±)-galipinine in three steps.