Mechanism of alcohol oxidation mediated by copper(II) and nitroxyl radicals

Integration of mesopores and crystal defects in metal-organic frameworks via templated electrosynthesis

Incorporation of mesopores and active sites into metal-organic framework (MOF) materials to uncover new efficient catalysts is a highly desirable but challenging task. We report the first example of a mesoporous MOF obtained by templated electrosynthesis using an ionic liquid as both electrolyte and template. The mesoporous Cu(II)-MOF MFM-100 has been synthesised in 100 seconds at room temperature, and this material incorporates crystal defects with uncoupled Cu(II) centres as evidenced by confocal fluorescence microscopy and electron paramagnetic resonance spectroscopy. MFM-100 prepared in this way shows exceptional catalytic activity for the aerobic oxidation of alcohols to produce aldehydes in near quantitative yield and selectivity under mild conditions, as well as having excellent stability and reusability over repeated cycles. The catalyst-substrate binding interactions have been probed by inelastic neutron scattering. This study offers a simple strategy to create mesopores and active sites simultaneously via electrochemical formation of crystal defects to promote efficient catalysis using MOFs.

Incorporating mesopores and active sites into metal-organic framework materials has proven advantageous for their catalytic application, but remains challenging to achieve. Here the authors obtain mesoporous, defect-rich metal-organic frameworks through templated electrosynthesis using ionic liquids as both electrolyte and template.

Copper-catalyzed dehydrogenative γ-C(sp3)-H amination of saturated ketones for synthesis of polysubstituted anilines

Metal-catalyzed β-C-H functionalization of saturated carbonyls via dehydrogenative desaturation proved to be a powerful tool for simplifying synthesis of valuable β-substituted carbonyls. Here, we report a copper-catalyzed dehydrogenative γ-C(sp3)-H amination of saturated ketones that initiates the three-component coupling of saturated ketones, amines and N-substituted maleimides to construct polysubstituted anilines. The protocol presented herein enables both linear and α-branched butanones to couple a wide spectrum of amines and various N-substituted maleimides to produce diverse tetra- or penta-substituted anilines in fair-to-excellent yields with good functional group tolerance. The mechanism studies support that this ketone dehydrogenative γ-C(sp3)-H amination was triggered by the ketone α,β-dehydrogenation desaturation that activates the adjacent γ-C(sp3)-H bond towards functionalization. This α,β-dehydrogenation desaturation-triggered cascade sequence opens up a new avenue to the remote C(sp3)-H functionalization of saturated ketones and has the potential to enable the rapid syntheses of complex compounds from simple starting materials.

Oxazoline-/Copper-Catalyzed Alkoxyl Radical Generation: Solvent-Switched to Access 3a,3a'-Bisfuroindoline and 3-Alkoxyl Furoindoline

We report the first example of oxazoline-/copper-catalyzed alcohol oxidation to generate the alkoxyl radical under additive-free conditions. The resulting alkoxyl radical addition to alkene enables useful C-O bond-forming and selective C(sp³)-C(sp³) radical-radical dimerization/radical-trapping reactions, providing direct access to the 3a,3a'-bisfuro[2,3-b]indoline scaffold for the first time and a wide range of 3-alkoxyl furoindolines with high efficiency.

Oxidative Amide Coupling from Functionally Diverse Alcohols and Amines Using Aerobic Copper/Nitroxyl Catalysis

The aerobic Cu/ABNO catalyzed oxidative coupling of alcohols and amines is highlighted in the synthesis of amide bonds in diverse drug-like molecules (ABNO=9-azabicyclo[3.3.1]nonane N-oxyl). The robust method leverages the privileged reactivity of alcohols bearing electronegative hetero- atoms (O, F, N, Cl) in the β-position. The reaction tolerates over 20 unique functional groups and is demonstrated on a 15 mmol scale under air. Steric constraints of the catalyst allow for chemoselective amidation of primary amines in the presence of secondary amines. All catalyst components are commercially available, and the reaction proceeds under mild conditions with retention of stereocenters in both reaction partners, while producing only water as a by-product.

Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I)

Copper salts and organic aminoxyls, such as TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), are versatile catalysts for aerobic alcohol oxidation. Previous reports in the literature contain conflicting proposals concerning the redox interactions that take place between copper(I) and copper(II) salts with the aminoxyl and hydroxylamine species, TEMPO and TEMPOH, respectively. Here, we reinvestigate these reactions in an effort to resolve the conflicting claims in the literature. Under anaerobic conditions, Cu^IIX₂ salts [X = acetate (OAc), trifluoroacetate (TFA), and triflate (OTf)] are shown to promote the rapid proton-coupled oxidation of TEMPOH to TEMPO: Cu^IIX₂ + TEMPOH → Cu^IX + TEMPO + HX. In the reaction with acetate, however, slow reoxidation of Cu^IOAc occurs. This process requires both TEMPO and HOAc and coincides with the reduction of TEMPO to 2,2,6,6-tetramethylpiperidine. Analogous reactivity is not observed with trifluoroacetate and triflate species. Overall, the facility of the proton-coupled oxidation of TEMPOH by Cu^II salts suggests that this process could contribute to catalyst regeneration under aerobic oxidation conditions.

The winding road of the uvaretin class of natural products: from total synthesis to bioactive agent discovery

Herein, we disclose the development of a synthetic route to gain access to the uvaretin class of chalcone natural products. In this, the construction of a small library was achieved, and the collection was evaluated for cytotoxicity and other biological properties. Uvaretin (1) was accessed via a seven-step route in an overall yield of 15.1%. Within this route, the unsaturated enone variant of uvaretin (2), also a natural product, was accessed in a 16.7% yield over six steps. This route provides a nearly three-fold increase in yields of 1 and 2 in comparison to the previous synthetic route accessing them in 5.8% and 3.0% overall yields, respectively. Evaluation of 1 and 2 revealed IC₅₀ values between 2.0 and 5.1 μM in the cancerous cell lines HeLa, U937, A549, and MIA PaCa-2. Screening of the whole chalcone library set led to the discovery of over 30 compounds, within six cancerous cell lines, possessing single digit μM IC₅₀ activity as sole agents. Furthermore, multiple library members were found to possess promising potentiating properties with known chemotherapeutic agents.

Biomimetic Cu/Nitroxyl Catalyst Systems for Selective Alcohol Oxidation

The oxidation of alcohols to the corresponding carbonyl products is an important organic transformation and the products are used in a variety of applications. The development of catalytic methods for selective alcohol oxidation have garnered significant attention in an attempt to find a more sustainable method without any limitations. Copper, in combination with 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) and supported by organic ligands, have emerged as the most effective catalysts for selective alcohol oxidation and these catalyst systems are frequently compared to galactose oxidase (GOase). The efficiency of GOase has led to extensive research to mimic the active sites of these enzymes, leading to a variety of Cu/TEMPO· catalyst systems being reported over the years. The mechanistic pathway by which Cu/TEMPO· catalyst systems operate has been investigated by several research groups, which led to partially contradicting mechanistic description. Due to the disadvantages and limitations of employing TEMPO· as co-catalyst, alternative nitroxyl radicals or in situ formed radicals, as co-catalysts, have been successfully evaluated in alcohol oxidation. Herein we discuss the development and mechanistic elucidation of Cu/TEMPO· catalyst systems as biomimetic alcohol oxidation catalysts.

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

Synthesis of discrete catalytic oligomers and their potential in silica-supported cooperative catalysis

The synthesis of discrete catalytic oligomers and their potential in supported cooperative catalysis are presented.

Electrochemical Performance of ABNO for Oxidation of Secondary Alcohols in Acetonitrile Solution

The ketones was successfully prepared from secondary alcohols using 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) as the catalyst and 2,6-lutidine as the base in acetonitrile solution. The electrochemical activity of ABNO for oxidation of 1-phenylethanol was investigated by cyclic voltammetry, in situ Fourier transform infrared spectroscopy (FTIR) and constant current electrolysis experiments. The resulting cyclic voltammetry indicated that ABNO exhibited much higher electrochemical activity when compared with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) under the similar conditions. A reasonable reaction mechanism of the electrocatalytic oxidation of 1-phenylethanol to acetophenone was proposed. In addition, a series of secondary alcohols could be converted to the corresponding ketones at room temperature in 80⁻95% isolated yields.

Catalytic Aerobic Oxidation of Alcohols by Copper Complexes Bearing Redox-Active Ligands with Tunable H-Bonding Groups

In this research article, we describe the structure, spectroscopy, and reactivity of a family of copper complexes bearing bidentate redox-active ligands that contain H-bonding donor groups. Single-crystal X-ray crystallography shows that these tetracoordinate complexes are stabilized by intramolecular H-bonding interactions between the two ligand scaffolds. Interestingly, the Cu complexes undergo multiple reversible oxidation-reduction processes associated with the metal ion (Cu^I, Cu^II, Cu^III) and/or the o-phenyldiamido ligand (L^2-, L^•-, L). Moreover, some of the Cu^II complexes catalyze the aerobic oxidation of alcohols to aldehydes (or ketones) at room temperature. Our extensive mechanistic analysis suggests that the dehydrogenation of alcohols occurs via an unusual reaction pathway for galactose oxidase model systems, in which O₂ reduction occurs concurrently with substrate oxidation.

NH3⋅H2O: The Simplest Nitrogen-Containing Ligand for Selective Aerobic Alcohol Oxidation to Aldehydes or Nitriles in Neat Water

Aqueous ammonia (NH3⋅H2O) has been shown to serve as the simplest nitrogen-containing ligand to effectively promote copper-catalyzed selective alcohol oxidation under air in water. A series of alcohols with varying electronic and steric properties were selectively oxidized to aldehydes with up to 95 % yield. Notably, by increasing the amount of aqueous ammonia in neat water, the exclusive formation of aryl nitriles was also accomplished with good-to-excellent yields. Additionally, the catalytic system exhibits a high level of functional group tolerance with -OH, -NO2, esters, and heteroaryl groups all being amenable to the reaction conditions. This one-pot and green oxidation protocol provides an important synthetic route for the selective preparation of either aldehydes or nitriles from commercially available alcohols.

CuCl/TMEDA/nor-AZADO-catalyzed aerobic oxidative acylation of amides with alcohols to produce imides

Although aerobic oxidative acylation of amides with alcohols would be a good complement to classical synthetic methods for imides (e.g., acylation of amides with activated forms of carboxylic acids), to date, there have been no reports on oxidative acylation to produce imides. In this study, we successfully developed, for the first time, an efficient method for the synthesis of imides through aerobic oxidative acylation of amides with alcohols by employing a CuCl/TMEDA/nor-AZADO catalyst system (TMEDA = teramethylethylendiamine; nor-AZADO = 9-azanoradamantane N-oxyl). The proposed acylation proceeds through the following sequential reactions: aerobic oxidation of alcohols to aldehydes, nucleophilic addition of amides to the aldehydes to form hemiamidal intermediates, and aerobic oxidation of the hemiamidal intermediates to give the corresponding imides. This catalytic system utilizes O2 as the terminal oxidant and produces water as the sole by-product. An important point for realizing this efficient acylation system is the utilization of a TMEDA ligand, which, to the best of our knowledge, has not been employed in previously reported Cu/ligand/N-oxyl systems. Based on experimental evidence, we consider that plausible roles of TMEDA involve the promotion of both hemiamidal oxidation and regeneration of an active CuII-OH species from a CuI species. Here promotion of hemiamidal oxidation is particularly important. Employing the proposed system, various types of structurally diverse imides could be synthesized from various combinations of alcohols and amides, and gram-scale acylation was also successful. In addition, the proposed system was further applicable to the synthesis of α-ketocarbonyl compounds (i.e., α-ketoimides, α-ketoamides, and α-ketoesters) from 1,2-diols and nucleophiles (i.e., amides, amines, and alcohols).

Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions

N-Oxyl compounds represent a diverse group of reagents that find widespread use as catalysts for the selective oxidation of organic molecules in both laboratory and industrial applications. While turnover of N-oxyl catalysts in oxidation reactions may be accomplished with a variety of stoichiometric oxidants, N-oxyl reagents have also been extensively used as catalysts under electrochemical conditions in the absence of chemical oxidants. Several classes of N-oxyl compounds undergo facile redox reactions at electrode surfaces, enabling them to mediate a wide range of electrosynthetic reactions. Electrochemical studies also provide insights into the structural properties and mechanisms of chemical and electrochemical catalysis by N-oxyl compounds. This review provides a comprehensive survey of the electrochemical properties and electrocatalytic applications of aminoxyls, imidoxyls, and related reagents, of which the two prototypical and widely used examples are 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) and phthalimide N-oxyl (PINO).

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O₂. The rates and selectivities (for production of H₂O₂ vs H₂O) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Distinguishing ionic and radical mechanisms of hydroxylamine mediated electrocatalytic alcohol oxidation using NO–H bond dissociation energies

An idea is proposed to sort N-oxyl radicals with respect to their mechanisms of electrocatalytic alcohol oxidation by knowing the NO–H bond dissociation energies of their precursors.

Effects of appended hydroxyl groups and ligand chain length on copper coordination and oxidation activity

The effects of appended hydroxyl groups and the chain length of a series of (imino)pyridine ligands on copper coordination and copper-catalyzed aerobic oxidation of alcohols were investigated.

Electronic Structural Analysis of Copper(II)-TEMPO/ABNO Complexes Provides Evidence for Copper(I)-Oxoammonium Character

Copper/aminoxyl species are proposed as key intermediates in aerobic alcohol oxidation. Several possible electronic structural descriptions of these species are possible, and the present study probes this issue by examining four crystallographically characterized Cu/aminoxyl halide complexes by Cu K-edge, Cu L_2,3-edge, and Cl K-edge X-ray absorption spectroscopy. The mixing coefficients between Cu, aminoxyl, and halide orbitals are determined via these techniques with support from density functional theory. The emergent electronic structure picture reveals that Cu coordination confers appreciable oxoammonium character to the aminoxyl ligand. The computational methodology is extended to one of the putative intermediates invoked in catalytic Cu/aminoxyl-driven alcohol oxidation reactions, with similar findings. Collectively, the results have important implications for the mechanism of alcohol oxidation and the underlying basis for cooperativity in this co-catalyst system.

Photocatalytic Dehydrogenation of Primary Alcohols: Selectivity Goes against Adsorptivity

Solid/liquid heterogeneous photocatalysis was often considered to occur on the active sites of a solid catalyst surface. Herein, we report that the selectivity of photocatalytic dehydrogenative oxidations of aliphatic primary alcohols in acetonitrile solution into corresponding aldehydes exhibits an anomalous relationship with adsorption behavior of the alcohols. By using Pt-loaded TiO₂ photocatalyst in an inert atmosphere under UV light illumination, primary short-chain alcohols (SCAs) with strong adsorption were dehydrogenated into aldehydes in very poor selectivity, whereas weak-adsorbable long-chain alcohols (LCAs) were transformed into corresponding aldehydes with much higher selectivity. More than 20 examples of primary LCAs (C₄-C₁₀) were successfully transformed into their corresponding aldehydes with satisfactory selectivity and yield. Both solid-state magic-angle-spinning ¹³C NMR and attenuated total reflectance-Fourier transform infrared spectroscopy studies provided concrete differences in adsorption behaviors on the Pt-TiO₂ photocatalyst surface between SCA ethanol and LCA n-octanol. To further uncover the mechanism for different selectivities of SCAs and LCAs in photodehydrogenation, in situ electron paramagnetic resonance (EPR) experiments (at 8 K temperature) were employed to observe the oxidation features of photogenerated hole in the valance band of Pt-TiO₂ (h_vb ⁺). The EPR experimental studies exhibited that unlike ethanol, either n-octanol or solvent acetonitrile alone all could not scavenge photogenerated h_vb ⁺ on Pt-P25 photocatalyst and only n-octanol dissolved in acetonitrile solvent could smoothly react with photoinduced hole. This indicated that selective oxidations of LCAs were achieved by solvent-delivered oxidation rather than directly destructive oxidation of photogenerated h_vb ⁺. Our results may open an alternative way in selective dehydrogenative oxidation of various substrates sensitive to both dioxygen and high-temperature treatments.

A bis(pyridyl)-N-alkylamine/Cu(i) catalyst system for aerobic alcohol oxidation

Herein a bis(pyridyl)-N-alkylamine/Cu^I/TEMPO/NMI catalyst system is reported for aerobic oxidation of a variety of primary alcohols to the corresponding aldehydes using readily available reagents, at room temperature and ambient air as the oxidant. ESI-MS analysis of the reaction showed the formation of a [(L1)(NMI)Cu^II-OOH]⁺ species, which is a key intermediate in the alcohol oxidation reaction. Evaluation of the effect of reaction parameters on the initial rate of the reaction allowed us to obtain the optimum conditions for catalytic activity. The careful choice of reaction solvent allowed for the oxidation of 4-hydroxybenzyl alcohol, a substrate which proved problematic in previous studies. In the case of 2-pyridinemethanol as substrate, experimental evidence shows that catalytic activity is diminished due to competitive inhibition of the catalyst by the alcohol substrate.

Second-Order Biomimicry: In Situ Oxidative Self-Processing Converts Copper(I)/Diamine Precursor into a Highly Active Aerobic Oxidation Catalyst

A homogeneous Cu-based catalyst system consisting of [Cu(MeCN)₄]PF₆, N,N'-di-tert-butylethylenediamine (DBED), and p-(N,N-dimethylamino)pyridine (DMAP) mediates efficient aerobic oxidation of alcohols. Mechanistic study of this reaction shows that the catalyst undergoes an in situ oxidative self-processing step, resulting in conversion of DBED into a nitroxyl that serves as an efficient cocatalyst for aerobic alcohol oxidation. Insights into this behavior are gained from kinetic studies, which reveal an induction period at the beginning of the reaction that correlates with the oxidative self-processing step, EPR spectroscopic analysis of the catalytic reaction mixture, which shows the buildup of the organic nitroxyl species during steady state turnover, and independent synthesis of oxygenated DBED derivatives, which are shown to serve as effective cocatalysts and eliminate the induction period in the reaction. The overall mechanism bears considerable resemblance to enzymatic reactivity. Most notable is the "oxygenase"-type self-processing step that mirrors generation of catalytic cofactors in enzymes via post-translational modification of amino acid side chains. This higher-order function within a synthetic catalyst system presents new opportunities for the discovery and development of biomimetic catalysts.