Copper Co-catalyst Research Articles

Solution-processable polymers of intrinsic microporosity for gas-phase carbon dioxide photoreduction

Four solution-processable, linear conjugated polymers of intrinsic porosity are synthesised and tested for gas phase carbon dioxide photoreduction. The polymers’ photoreduction efficiency is investigated as a function of their porosity, optical properties, energy levels and photoluminescence. All polymers successfully form carbon monoxide as the main product, without the addition of metal co-catalysts. The best performing single component polymer yields a rate of 66 μmol h−1 m−2, which we attribute to the polymer exhibiting macroporosity and the longest exciton lifetimes. The addition of copper iodide, as a source of a copper co-catalyst in the polymers shows an increase in rate, with the best performing polymer achieving a rate of 175 μmol h−1 m−2. The polymers are active for over 100 h under operating conditions. This work shows the potential of processable polymers of intrinsic porosity for use in the gas phase photoreduction of carbon dioxide towards solar fuels.

Copper Cocatalyst Modulated Radical Generation for Selective Heterogeneous Photosynthesis of α-Haloketones.

The α-haloketones are important precursors for synthetic chemistry and pharmaceutical applications; however, their production relies heavily on traditional synthetic methods via halogenation of ketones that are toxic and environmentally risky. Here, we report a heterogeneous photosynthetic strategy of α-haloketone production from aromatic olefins using copper-modified graphitic carbon nitride (Cu-C3N4) under mild reaction conditions. By employing NiX2 (X = Cl, Br) as the halogen source, a series of α-haloketones can be synthesized using atmospheric air as the oxidant under visible-light irradiation. In comparison with pristine carbon nitride, the addition of Cu as a cocatalyst provides a moderate generation rate of halogen radicals and selective reduction of molecular oxygen into •OOH radicals, thus leading to a high selectivity to α-haloketones. The Cu-C3N4 also exhibits high stability and versatility, rendering it a promising candidate for solar-driven synthetic applications.

Sustainable Wacker-Type Oxidations.

The Wacker reaction is the oxidation of olefins to ketones and typically requires expensive and scarce palladium catalysts in the presence of an additional copper co-catalyst under harsh conditions (acidic media, high pressure of air/dioxygen, elevated temperatures). Such a transformation is relevant for industry, as shown by the synthesis of acetaldehyde from ethylene as well as for fine-chemicals, because of the versatility of a carbonyl group placed at specific positions. In this regard, many contributions have focused on controlling the chemo- and regioselectivity of the olefin oxidation by means of well-defined palladium catalysts under different sets of reaction conditions. However, the development of Wacker-type processes that avoid the use of palladium catalysts has just emerged in the last few years, thereby paving the way for the generation of more sustainable procedures, including milder reaction conditions and green chemistry technologies. In this Minireview, we discuss the development of new catalytic processes that utilize more benign catalysts and sustainable reaction conditions.

Copper- and Chiral Nitroxide-Catalyzed Oxidative Kinetic Resolution of Axially Chiral N-Arylpyrroles.

A readily prepared C2-symmetric, α-hydrogen-substituted chiral hydroxylamine serves as a precatalyst to generate a chiral nitroxide in situ. This chiral nitroxide catalyst in combination with a copper co-catalyst functions as an oxidant for an unprecedented enantioselective oxidative kinetic resolution (OKR) of racemic axially chiral N-arylpyrrole alcohols using atmospheric oxygen as an environmentally friendly terminal oxidant. The OKR process provides the axially chiral N-arylpyrroles in er up to 3.5:96.5 and with s factors up to 24.

Intrinsic Cu Nanoparticle Decoration of TiO2 Nanotubes: A Platform for Efficient Noble Metal Free Photocatalytic H2 Production

The use of titanium dioxide (TiO2) for photocatalytic applications has been intensively investigated for years due to appropriate electronic properties, chemical and photocorrosion resistance, as well as the low cost of the material [1]. Although TiO2 has a suitable conduction band position for the photocatalytic H2 formation reaction, the hydrogen evolution reaction from water on a TiO2 surface is kinetically hampered. To tackle this problem, typically decoration with noble metals such as Pt, Pd, Au that act as a co-catalyst is employed. However, these noble metals are costly and a variety of pathways for replacing these elements have been explored. One of the promising substitutes for noble metals are copper and copper-compounds. The fabrication of copper co-catalyst loaded TiO2 nanotubes is conventionally carried out by chemical methods such as hydrothermal, impregnation, photo-deposition and electrochemical deposition [2]. However, a most straightforward and unique method to modify anodic TiO2 nanotube properties is employing, instead of pure Ti metallic substrate, a Ti‐X alloy in the anodization process, were X is another metal. During anodizing the second metal can be simultaneously oxidized and doped into the TiO2 lattice (in substitutional or interstitial position), giving the nanotubes a specific functionality [3–5]. A number of such intrinsically doped nanotube arrays have been reported, namely using Nb, Ta, Ru and etc., alloyed in Ti [3–5].In this work, we grow intrinsically Cu doped TiO2 nanotubes (TiNTs) by self-organizing anodization of Ti-Cu binary alloys. We demonstrate that up to a copper concentration of 1.5 at.% in the alloy, self-ordered Cu2+-doped nanotubes can be grown. Under UV illumination the Cu2+ ion-doped oxide structures can be converted to nanotubes that carry metallic nanoparticles (NPs) uniformly decorated TiNTs. We investigate the formation of these metallic nanoparticles under UV illumination by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The resulting intrinsic copper doped and decorated TiNTs provide a strongly enhanced photocatalytic activity for H2 evolution in comparison to pristine TiNTs. Key is the light induced conversion of the intrinsic Cu doping to metallic copper nanoparticles that act as a stable co-catalyst for H2 generation.[1] K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 2014, 114, 9385–9454.[2] M. Janczarek, E. Kowalska, Catalysts. 2017, 7, 317-324.[3] P. Roy, C. Das, K. Lee, R. Hahn, T. Ruff, M. Moll, P. Schmuki, J. Am. Chem. Soc. 2011, 133, 5629–5631.[4] M. Altomare, K. Lee, M.S. Killian, E. Selli, P. Schmuki, Chem. - A Eur. J. 2013, 19, 5841–5844.[5] C. Das, P. Roy, M. Yang, H. Jha, P. Schmuki, Nanoscale. 2011, 3, 3094–3096. Figure 1

DNA-Scaffolded Synergistic Catalysis.

We report DNA-scaffolded synergistic catalysis, a concept that combines the diverse reaction scope of synergistic catalysis with the ability of DNA to precisely preorganize abiotic groups and undergo stimuli-triggered conformational changes. As an initial demonstration of this concept, we focus on Cu-TEMPO-catalyzed aerobic alcohol oxidation, using DNA as a scaffold to hold a copper cocatalyst and an organic radical cocatalyst (TEMPO) in proximity. The DNA-scaffolded catalyst maintained a high turnover number upon dilution and exhibited 190-fold improvement in catalyst turnover number relative to the unscaffolded cocatalysts. By incorporating the cocatalysts into a DNA hairpin-containing scaffold, we demonstrate that the rate of the synergistic catalytic reaction can be controlled through a reversible DNA conformational change that alters the distance between the cocatalysts. This work demonstrates the compatibility of synergistic catalytic reactions with DNA scaffolding, opening future avenues in reaction discovery, sensing, responsive materials, and chemical biology.

A facile protocol for copper‐free palladium‐catalyzed Sonogashira coupling in aqueous media

AbstractThe combination of a readily available palladium catalyst and an eco‐friendly basic aqueous solution of room‐temperature ionic liquid, choline hydroxide (ChOH), was used in a facile protocol alternative to the Sonogashira coupling reaction, alkynylation of aryl halides in the absence of a copper cocatalyst and an external base. The dual nature of ChOH to act as a base and a green solvent played a crucial role in the catalytic cycle. The coupling reaction progressed efficiently to form a CspCsp2 bond under the identified conditions although the reaction outcome depended significantly on the substrates.

Synthesis, Analytical Characterization and Anti-Diabetic Activity of Some Heterocycles of Quinazolin-4(3H)one

Aim: Novel quinazolin-4(3H)-one heterocycles were synthesized and assessed for their anti-diabetic activity. Non-enzymatic glycosylation of haemoglobin assay was carried out to identify their potential as anti-diabetic. The cyclization of quinazolinone-4(3H)-one heterocycles was achieved, whereas carbon-carbon cross coupling reactions were carried out using Sonogashira and Suzuki-Miyaura reaction conditions and characterized with analysis. This synthesis method afforded corresponding 2, 3 and 6 substituted quinazolin-4(3H)-ones (3a to 3m) with excellent yields.
 Methods: 2-Amino-6-bromobenzoic acid was used as a substrate which was converted to corresponding benzamide derivatives (1a-1b) by reaction with benzylamine or cyclohexylamine using acid-amine reaction, followed by cyclization and oxidation using suitable aldehyde in DMSO under microwave condition to give bromo substituted quinazolin-4(3H)-ones (2a-2c), which were cross coupled to suitable terminal alkyne with palladium catalyst as well as copper co-catalyst using Sonogashira condition to obtain desired (3a-3h) and suitable boronic acid with palladium catalyst using Suzuki-Miyaura condition to obtain desired (3i-3m). All synthesized compounds were characterized by FTIR, proton NMR, LC-MS analysis and evaluated for in vitro anti-diabetic activity using non-enzymatic glycosylation of haemoglobin assay.
 Results: Compounds 3m showed good inhibition of glycosylation of haemoglobin which in turn suggest good anti-oxidant potential on metabolism of glucose and hence lower glucose concentration. It showed IC50 value of 35.91±0.82 µg/mL which was comparable to the standard alpha tocopherol (34.47±0.87µg/mL).
 Conclusion: In-vitro non-enzymatic glycosylation of haemoglobin method is one of important assays to judge the control of diabetes. The haemoglobin present in RBCs has an affinity to bind with glucose. The greater the glucose level in blood, more amount of glucose-bound (called glycosylated) haemoglobin will be formed. Accordingly, presence of lower concentration of glycosylated haemoglobin is a sure guide to the lower concentration of glucose in the blood. Synthesized compounds (3a-3m) lower the blood glucose level and 3m has highest potential among those which can be further developed as potent anti-diabetic.

Copper-Nickel-Oxide Nanomaterial for Photoelectrochemical Hydrogen Evolution and Photocatalytic Degradation of Volatile Organic Compounds

Cocatalysts play an important role in photocatalysis towards achieving enhanced efficiency and stability. The coupling of metal cocatalysts with semiconductor photocatalysts is the most proficient path to facilitate the separation of photoexcited electron-hole pairs and to suppress their recombination. Herein, a strategy for the preparation of copper-nickel-oxide photocatalyst, based on the metal organic framework (Cu-MOF-74), has been proposed. Hydrothermal treatment of copper-MOF and nickel salt resulted in coupling of a nickel oxide with a copper cocatalyst. As prepared nanomaterial absorbs light in a broad wavelength range, especially in the visible light range. The material exhibits an impressive activity towards photocatalytic and photoelectrochemical water splitting with hydrogen evolution (more than 1.5 mmol h−1 g−1 without platinum cocatalyst) as well as towards photocatalytic degradation of volatile organic compounds. Its activity has been confirmed also in the analysis of generated cathodic photocurrents, that reached almost 1 mA cm−2 in alkaline solution.

Synthesis of Alkynyl Ketones by Sonogashira Cross-Coupling of Acyl Chlorides with Terminal Alkynes Mediated by Palladium Catalysts Deposited over Donor-Functionalized Silica Gel

Palladium catalysts deposited over silica gel bearing simple amine (≡Si(CH2)3NH2) and composite functional amide pendants equipped with various donor groups in the terminal position (≡Si(CH2)3NHC(O)CH2Y, Y = SMe, NMe2 and PPh2) were prepared and evaluated in Sonogashira-type cross-coupling of acyl chlorides with terminal alkynes to give 1,3-disubstituted prop-2-yn-1-ones. Generally, the catalysts showed good catalytic activity in the reactions of aroyl chlorides with aryl alkynes under relatively mild reaction conditions even without adding a copper co-catalyst. However, their repeated use was compromised by a significant loss of activity after the first catalytic run.

Synergistic Ni/Cu catalyzed migratory arylsilylation of terminal olefins

Synthesis of organosilanes from alkenes is a very important topic owing to their wide applications. A Ni/Cu dual metal-catalyzed arylsilylation of terminal alkenes, featuring migratory selectivity, has been developed. A wide diversity of aliphatic silanes have been prepared from terminal alkenes, aryl halides and Suginome’s reagent. This protocol is highlighted by excellent regioselectivity, mild reaction conditions and good functional group tolerance. In addition to benzylic positions, carbon–carbon bonds can also be constructed at allylic positions. Preliminary mechanistic studies suggest that the copper cocatalyst promotes the transmetalation of Suginome’s reagent, and the addition of a PyrOx ligand inhibits the formation of side-products from the carbon-Heck pathway. Moreover, studies toward the nature of the PyrOx ligand revealed that the steric hindrance of the oxazoline moiety greatly affects the chain–walking process, but not the arylation step.

Highly Efficient Ethenolysis and Propenolysis of Methyl Oleate Catalyzed by Abnormal N-Heterocyclic Carbene Ruthenium Complexes in Combination with a Phosphine–Copper Cocatalyst

Fluorinated imidazo[1,5-a]pyridine abnormal carbene ruthenium complexes (F-aImPy–Ru) with a tricyclohexylphosphine copper chloride (Cy3P–CuCl) cocatalyst were developed for the ethenolysis/propenolysis of methyl oleate. These catalysts showed remarkable catalytic efficiencies (turnover numbers of 100,000 for ethenolysis and 200,000 for propenolysis) and excellent α-olefin selectivity (up to 99%). A computational study indicated that the energy barrier of the β-hydride elimination pathway in the presence of a phosphine–copper cocatalyst is higher than that in the absence of the cocatalyst, which might contribute to the improved longevity of the ruthenium catalyst in the reaction.

Iron/copper co-catalyzed highly selective arylation of sulfinamides with aryl iodides

In the present study, an inexpensive but efficient Iron(III) and Copper(II) co-catalyst without ligands catalyzed arylation of sulfinamides with aryl iodide was primarily reported. In brief, in the presence of Fe(NO3)3·9H2O, CuO and K3PO4, the highly selective C–N cross coupling of several sulfinamides and aryl iodides was achieved in high chemical yield, while the aryl chlorides and bromides could not yield coupling products. It is noteworthy that through the arylation of chiral tert-butanesulfinamide with aryl iodides, N-aryl tert-butanesulfinamides are provided without racemization, even in gram-scale. The possible mechanism was that This oxidative addition process between copper catalyst and aryl-iodides might be significantly accelerated by active Fe(III) species. Moreover, using this synthetic method, a facile and efficient access was developed for the derivatives of N-phenyl sulfinamides, which might help to develop new drug molecules and material chemicals.

Unexpected formation of 2-alkynylpyridines from iodo-selenopyridines by selective activation of C[sbnd]Se over C[sbnd]I bonds

The synthesis of bicyclic 2-alkynylpyridines is performed in the presence of iodo-selenopyridines and terminal alkynes, using a palladium(II) catalyst, a copper(I) co-catalyst, and triethylamine as a base. This facile cross-coupling reaction leads to a wide variety of 2-alkynylpyridines in good yields. Mechanistic investigations provided by DFT calculations reveal that the reaction should involve a key selective activation of the CSe bond of the 2-selenopyridyl moiety by the in situ generated Pd(0) catalyst, preferentially to the CI bond activation. A transmetalation step with copper acetylide, followed by a reductive elimination, affords the 2-alkynylpyridine product.

Electrostatically assembled construction of ternary TiO2-Cu@C hybrid with enhanced solar-to-hydrogen evolution employing amorphous carbon dots as electronic mediator

The huge demand for renewable hydrogen produced by water splitting has prompted people to conduct in-depth research on the hydrogen evolution reaction for the development of earth-abundant, non-precious, and multi-functional metal catalysts. Herein, a noble-metal-free ternary composite of TiO2-Cu@C was prepared by electrostatic self-assembly loaded copper nanoparticles and amorphous carbon dots (CDs) on porous TiO2 microrods. The good conductivity of the CDs was beneficial to promoting the charge transfer and separation, generating an enhanced solar-to-hydrogen performance on TiO2-Cu@C. The optimized TiO2-Cu@C reveals a stable and notable hydrogen evolution rate of 3911 μmol g−1h−1, which is 1.6 times that of TiO2-Cu and many times higher than that of TiO2. Instead of providing active sites for hydrogen production, the CDs act as an electronic mediator and provide another electron pathway to further enhance the activity of TiO2-Cu, where the photogenerated electrons on TiO2 could pass through the CDs to the copper cocatalyst and reduce water to hydrogen.

Visible light-induced diastereoselective semihydrogenation of alkynes to cis-alkenes over an organically modified titanium(IV) oxide photocatalyst having a metal co-catalyst

Hydrogen (H2)-free and poison (lead and quinoline)-free semihydrogenation of alkynes to cis-alkenes under gentle conditions is one of the challenges to be solved. In this study, a titanium(IV) oxide photocatalyst having two functions (visible light responsiveness and semihydrogenation activity) was prepared by modification with 2,3-dihydroxynaphthalene (DHN) and a copper (Cu) co-catalyst, respectively. The photocatalyst (DHN/TiO2-Cu) showed high performance for diastereoselective semihydrogenation of alkynes to cis-alkenes in water-acetonitrile solution under visible light irradiation without the use of H2 and poisons. Alkynes having reducible functional groups were converted to the corresponding alkenes with the functional groups being preserved. The addition of water to acetonitrile changed the amount of alkynes adsorbed on the photocatalyst, which was a decisive factor determining the rate of hydrogenation. A relatively large apparent activation energy, 27 kJ mol−1, was obtained by a kinetic study, indicating that the rate-determining step of this reaction was not an electron production process but a thermal catalytic semihydrogenation process over the Cu co-catalyst. Semihydrogenation and hydrogen evolution occurred competitively on Cu metals and the former became predominant at slightly elevated temperatures, which is discussed on the basis of the kinetic parameters of two reactions.

Tandem copper and photoredox catalysis in photocatalytic alkene difunctionalization reactions.

Oxidative alkene difunctionalization reactions are important in synthetic organic chemistry because they can install polar functional groups onto simple non-polar alkene moieties. Many of the most common methods for these reactions rely upon the reactivity of pre-oxidized electrophilic heteroatom donors that can often be unstable, explosive, or difficult to handle. Herein, we describe a method for alkene oxyamination and diamination that utilizes simple carbamate and urea groups as nucleophilic heteroatom donors. This method uses a tandem copper–photoredox catalyst system that is operationally convenient. The identity of the terminal oxidant is critical in these studies. Ag(I) salts proved to be unique in their ability to turn over the copper cocatalyst without deleteriously impacting the reactivity of the organoradical intermediates.

A regioselectivity switch in Pd-catalyzed hydroallylation of alkynes.

By exploiting the reactivity of a vinyl-Pd species, we control the regioselectivity in hydroallylation of alkynes under Pd-hydride catalysis. A monophosphine ligand and carboxylic acid combination promotes 1,5-dienes through a pathway involving isomerization of alkynes to allenes. In contrast, a bisphosphine ligand and copper cocatalyst favor 1,4-dienes via a mechanism that involves transmetalation. Our study highlights how to access different isomers by diverting a common organometallic intermediate.

Titanium(iv) oxide having a copper co-catalyst: a new type of semihydrogenation photocatalyst working efficiently at an elevated temperature under hydrogen-free and poison-free conditions.

A copper-loaded titanium(iv) oxide photocatalyst exhibited perfect selectivity in hydrogenation of alkynes to alkenes in an alcohol solution at 298 K under hydrogen-free and poison-free conditions. A slight elevation in the reaction temperature to 323 K greatly increased the reaction rate with the selectivity being preserved and the formation of an H2 by-product being suppressed. The apparent activation energy of 4-octyne semihydrogenation was determined to be 54 kJ mol-1, indicating that the rate determining step of this photocatalytic reaction was not an electron production process but a thermocatalytic hydrogenation process under light irradiation.

Reversing the Regioselectivity of Halofunctionalization Reactions through Cooperative Photoredox and Copper Catalysis

AbstractHalofunctionalization of alkenes is a classical method for olefin difunctionalization. It gives rise to adducts which are found in many natural products and biologically active molecules, and offers a synthetic handle for further manipulation. Classically, this reaction is performed with an electrophilic halogen source and leads to regioselective formation of the halofunctionalized adducts. Herein, we demonstrate a reversal of the native regioselectivity for alkene halofunctionalization through the use of an acridinium photooxidant in conjunction with a copper cocatalyst.