Absence Of External Reductant Research Articles

Electrochemically Driven Nickel-Catalyzed Enantioselective Hydro-Arylation/Alkenylation of Enones.

Herein, the study reports the first electrochemical nickel-catalyzed enantioselective hydro-arylation/alkenylation of enones in an undivided cell with low-cost electrodes in the absence of external reductants and supporting electrolytes. Aryl bromides/iodides/triflates or alkenyl bromides are employed as electrophiles for the efficient preparation of more than 56 valuable β-arylated/alkenylated ketones in a simple manner (up to 97% yield, 97% ee). With the advantages of electrochemistry, excellent functional group tolerance and late-stage modification of complex natural products and pharmaceuticals made the established protocol greener and more economic. Mechanism investigation suggests that a NiI/NiIII cycle may be involved in this electro-reductive reaction rather than metal reductant driven Ni0/NiII cycle. Overall, the efficient electrochemical activation and turnover of the nickel catalyst avoid the drawbacks posed by the employment of stoichiometric amount of sensitive metal powder reductants.

The impact of chlorides on NMC leaching in hydrometallurgical battery recycling

The current paper studies the impact of chlorides on NMC111 (LiNi1/3Mn1/3Co1/3O2) leaching mechanism. Experiments were conducted in chloride (HCl) and mixed sulfate–chloride (H2SO4–NaCl) solutions as well as in sulfate solution (H2SO4, reference), in the absence of external reductants. The results suggest no major differences in the leaching performance between studied lixiviants at 30 °C (∼75% for Li, ∼35% for Co, Ni, Mn), and increasing the temperature to 80 °C mainly improved the dissolution of lithium (>90% in all lixiviants). However, the leaching behavior of manganese was found to be different – in sulfate solutions, Mn precipitated as MnO2 with only 6% Mn left in solution after 2 h (at 80 °C), whereas in chloride-containing solutions, dissolved Mn remained soluble (30–40% extraction). The presence of chlorides also supported cobalt and nickel extraction, the mixed H2SO4–NaCl system resulting in extractions of 55% for Co and 58% for Ni at 80 °C. The increase in the dissolutions is suggested to be explained by the experimentally verified reaction between Cl- ions and NMC111, resulting in Cl2 formation and simultaneous NMC dissolution. The results indicate that hydrometallurgical battery recycling processes may face challenges with manganese extraction to solution when operating at increased temperatures, and that the presence of chloride ions can help to mitigate such difficulties by inhibiting unwanted precipitation of manganese compounds, producing Cl2 gas as a result. Nevertheless, as the final extractions were low even in the most effective lixiviant (55% Co, 58% Ni, 39% Mn at 80 °C), the use of additional reductants is necessary to ensure high metal leaching efficiencies also in such chloride-containing systems.

3D-Hydrogel Based Polymeric Nanoreactors for Silver Nano-Antimicrobial Composites Generation.

This study underscores the development of Ag hydrogel nanocomposites, as smart substrates for antibacterial uses, via innovative in situ reactive and reduction pathways. To this end, two different synthetic strategies were used. Firstly thiol-acrylate (PSA) based hydrogels were attained via thiol-ene and radical polymerization of polyethylene glycol (PEG) and polycaprolactone (PCL). As a second approach, polyurethane (PU) based hydrogels were achieved by condensation polymerization from diisocyanates and PCL and PEG diols. In fact, these syntheses rendered active three-dimensional (3D) hydrogel matrices which were used as nanoreactors for in situ reduction of AgNO3 to silver nanoparticles. A redox chemistry of stannous catalyst in PU hydrogel yielded spherical AgNPs formation, even at 4 °C in the absence of external reductant; and an appropriate thiol-functionalized polymeric network promoted spherical AgNPs well dispersed through PSA hydrogel network, after heating up the swollen hydrogel at 103 °C in the presence of citrate-reductant. Optical and swelling behaviors of both series of hydrogel nanocomposites were investigated as key factors involved in their antimicrobial efficacy over time. Lastly, in vitro antibacterial activity of Ag loaded hydrogels exposed to Pseudomona aeruginosa and Escherichia coli strains indicated a noticeable sustained inhibitory effect, especially for Ag–PU hydrogel nanocomposites with bacterial inhibition growth capabilities up to 120 h cultivation.

Confined Pool-Buried Water-Soluble Nanoparticles from Reverse Micelles.

With the special nature of confined water pools, reverse micelles (RMs) have shown potential for a wide range of applications. However, the inherent water insolubility of RMs hinders their further application prospect especially for applications related to biology. We present herein the first successful transformation of water-insoluble RMs into water-soluble nanoparticles without changing the confined aqueous interiors by hydrolysis/aminolysis of arm-cleavable interfacial cross-linked reverse micelles formed from diester surfactant 1. The unique properties exhibited by the aqueous interiors of the resulting pool-buried water-soluble nanoparticles (PWNPs) were demonstrated both by the template synthesis of gold nanoparticles in the absence of external reductants and by the fluorescence enhancement of encapsulated thioflavin T (ThT). Importantly, the unique potential for PWNPs in biological applications was exemplified by the use of ThT@PWNPs and "cell targeted" ThT@PWNPs as effective optical imaging agents of living cells. This work conceptually overcomes the application bottleneck of RMs and opens an entry to a new class of functional materials.

The recent achievements of redox-neutral radical C-C cross-coupling enabled by visible-light.

Visible-light photoredox catalysis has been esteemed as one sustainable and attractive synthetic tool. In the past four years, a new yet challenging trend, visible-light-driven redox-neutral radical C-C cross-coupling involving putative radical intermediates, has been booming rapidly. Its advent brings a powerful platform to achieve non-classical C-C connections, and should lead to fundamental changes in retrosynthetic analysis. In this tutorial review, we highlight the recent achievements of visible-light-mediated redox-neutral radical C(sp3)-C(sp2), C(sp3)-C(sp), and C(sp3)-C(sp3) bond formation, opening a new window for C-C cross-coupling through the photoredox electron shuttling cycle between two coupling partners. While radical-radical coupling steered by the persistent radical effect was proposed as a rational explanation for the redox-neutral photoredox events, alternative kinetically driven chain propagation and radical addition pathways cannot be ruled out. This tutorial review aims to highlight the recent achievements of photoredox-neutral radical C-C coupling in synthetic chemistry.

ChemInform Abstract: Ligand‐Free Palladium Catalyzed Phosphorylation of Aryl Iodides.

A reusable ligand-free palladium catalyzed phosphorylation of aryl iodides using PdCl2/tetrabutylammonium bromide is described. Functionalized triarylphosphines were obtained in good to excellent yields in the absence of external reductants and supporting ligands.

Ligand-free palladium catalyzed phosphorylation of aryl iodides

A reusable ligand-free palladium catalyzed phosphorylation of aryl iodides using PdCl2/tetrabutylammonium bromide is described. Functionalized triarylphosphines were obtained in good to excellent yields in the absence of external reductants and supporting ligands.

ChemInform Abstract: Nickel‐Catalyzed Reductive Cleavage of Aryl Alkyl Ethers to Arenes in Absence of External Reductant.

AbstractA novel procedure for the nickel‐catalyzed deoxygenation of aryl ethers in the absence of an external reductant is developed.

Nickel-catalyzed reductive cleavage of aryl alkyl ethers to arenes in absence of external reductant.

The reductive cleavage of the C-O bonds of aryl ethers has great potential in organic synthesis. Although several catalysts that can promote the reductive cleavage of aryl ethers have been reported, all such systems require the use of an external reductant, e.g., hydrosilane or hydrogen. Here, we report the development of a new nickel-based catalytic system that can cleave the C-O bonds of ethers in the absence of an external reductant. The hydrogen atom required in this new reductive cleavage reaction is provided by the alkoxy group of the substrate, which serves as an internal reductant. The absence of an external reductant enables the unique chemoselectivity, i.e., the selective reduction of an alkoxy group over alkenes and ketones.

Ribonucleotide reductase inhibition by metal complexes of Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone): A combined experimental and theoretical study

Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone, 3-AP) is currently the most promising chemotherapeutic compound among the class of α-N-heterocyclic thiosemicarbazones. Here we report further insights into the mechanism(s) of anticancer drug activity and inhibition of mouse ribonucleotide reductase (RNR) by Triapine. In addition to the metal-free ligand, its iron(III), gallium(III), zinc(II) and copper(II) complexes were studied, aiming to correlate their cytotoxic activities with their effects on the diferric/tyrosyl radical center of the RNR enzyme in vitro. In this study we propose for the first time a potential specific binding pocket for Triapine on the surface of the mouse R2 RNR protein. In our mechanistic model, interaction with Triapine results in the labilization of the diferric center in the R2 protein. Subsequently the Triapine molecules act as iron chelators. In the absence of external reductants, and in presence of the mouse R2 RNR protein, catalytic amounts of the iron(III)–Triapine are reduced to the iron(II)–Triapine complex. In the presence of an external reductant (dithiothreitol), stoichiometric amounts of the potently reactive iron(II)–Triapine complex are formed. Formation of the iron(II)–Triapine complex, as the essential part of the reaction outcome, promotes further reactions with molecular oxygen, which give rise to reactive oxygen species (ROS) and thereby damage the RNR enzyme. Triapine affects the diferric center of the mouse R2 protein and, unlike hydroxyurea, is not a potent reductant, not likely to act directly on the tyrosyl radical.

Enzyme Ribonucleotide Reductase: Unraveling an Enigmatic Paradigm of Enzyme Inhibition by Furanone Derivatives

Several 2'-substituted-2'-deoxyribonucleotides are potent inactivators of the enzyme ribonucleotide reductase (RNR), by destroying the essential tyrosyl radical located in subunit R2 or/and covalently alkylating the subunit R1. In the absence of external reductants, the inactivation is achieved by alkylation of subunit R1 by a methylene-3(2H)-furanone. The furanone is generated in solution through degradation of a keto-deoxyribonucleotide intermediate, produced during the inhibitory mechanism of a wide group of 2'-substituted inhibitors, and is easily detected experimentally by UV spectroscopy. Interestingly, the same keto-deoxyribonucleotide is also a proposed intermediate of the normal substrate pathway, but by some unknown reason, it does not dissociate from the active site and does not inactivate the enzyme. Therefore, if the currently accepted mechanism for substrate reduction is correct, there must be some specific reason that makes such a reactive intermediate behave differently, not dissociating from the active site during substrate reduction. In this article, we propose to validate the current substrate mechanism by showing that the keto-deoxyribonucleotide dissociates from the active site only in the case of the inhibitors, and therefore, it corresponds to a viable intermediate in the substrate mechanism. Furthermore, we answer unexplained experimental observations that concern the predomination of the normal reduction mechanism over the abnormal ketone formation in the FdNDP and the release of F(-), either in the normal or in the abnormal turnover. For that purpose, we have investigated the interaction between the enzyme and this keto-deoxyribonucleotide generated from the normal substrate and from two widely studied representative inhibitors. A model containing 140 atoms was used to represent the desired structures. The results allowed us to conclude that the solvation free energy of the 2'-substituents, its influence inside the active site, and the charge transfer mechanism from a protein side chain to solution are the thermodynamic driving forces for the intermediate dissociation and subsequent RNR inhibition. Such charge transfer cannot be accomplished by the natural substrate, preventing its dissociation. These results elucidate a paradox which has been unexplained for more than 20 years and further validates both the proposed substrate and inhibition chemical mechanisms.

Light-induced Electron Transfer in Arabidopsis Cryptochrome-1 Correlates with in Vivo Function

Cryptochromes are blue light-activated photoreceptors found in multiple organisms with significant similarity to photolyases, a class of light-dependent DNA repair enzymes. Unlike photolyases, cryptochromes do not repair DNA and instead mediate blue light-dependent developmental, growth, and/or circadian responses by an as yet unknown mechanism of action. It has recently been shown that Arabidopsis cryptochrome-1 retains photolyase-like photoreduction of its flavin cofactor FAD by intraprotein electron transfer from tryptophan and tyrosine residues. Here we demonstrate that substitution of two conserved tryptophans that are constituents of the flavin-reducing electron transfer chain in Escherichia coli photolyase impairs light-induced electron transfer in the Arabidopsis cryptochrome-1 photoreceptor in vitro. Furthermore, we show that these substitutions result in marked reduction of light-activated autophosphorylation of cryptochrome-1 in vitro and of its photoreceptor function in vivo, consistent with biological relevance of the electron transfer reaction. These data support the possibility that light-induced flavin reduction via the tryptophan chain is the primary step in the signaling pathway of plant cryptochrome.

Reduction pathway of end-on coordinated dinitrogen. 3. Electronic structure and spectroscopic properties of molybdenum/tungsten hydrazidium complexes.

The spectroscopic properties and electronic structure of the hydrazidium complexes [MF(NNH(3))(depe)(2)](BF(4))(2), M = Mo and W, are investigated (depe = 1,2-bis(diethylphosphino)ethane). Vibrational spectroscopic data for both compounds are evaluated with a quantum-chemistry-assisted normal coordinate analysis, giving an N-N force constant of 6.03 mdyn/A and metal-N force constants of 8.01 (Mo-N) and 7.31 mdyn/A (W-N), respectively. On the basis of these results and DFT calculations on a [MoF(NNH(3))(PH(3))(4)](2+) model system, the N-N bond order in these systems is 1 (single sigma bond) and metal-N bonding corresponds to a triple bond. The metal centers are assigned a +IV oxidation state (d(2) configuration) and the NNH(3) ligand is assigned a -1 formal charge which by sigma- and pi-donation to the metal is reduced to +0.48. The two metal-d electrons are located in the nonbonding (n) d(xy)() orbital. This bonding description is supported by the results of optical absorption spectroscopy showing the n --> (metal-ligand)pi transition at 536 nm (not observed in the tungsten compound) and the (metal-ligand)pi --> (metal-ligand)pi transition at 251 nm for the MoNNH(3) and at 237 nm for the WNNH(3) complex. The activation enthalpy for splitting of the N-N bond in these systems to generate NH(4)(+) is estimated to be larger than 40 kcal/mol. Hydrazidium complexes with diphosphine coligands are therefore inert with respect to N-N cleavage and thus represent the ultimate stage of N(2) reduction at six-coordinate d(6) metal centers in the absence of external reductants.