Aerobic Oxidation Reactions Catalyzed by Vanadium Complexes of Bis(Phenolate) Ligands

Mononuclear Oxidovanadium(IV) Complexes with BIAN Ligands: Synthesis and Catalytic Activity in the Oxidation of Hydrocarbons and Alcohols with Peroxides

Reactions of VCl3 with 1,2-Bis[(4-methylphenyl)imino]acenaphthene (4-Me-C6H4-bian) or 1,2-Bis[(2-methylphenyl)imino]acenaphthene (2-Me-C6H4-bian) in air lead to the formation of [VOCl2(R-bian)(H2O)] (R = 4-Me-C6H4 (1), 2-Me-C6H4 (2)). Thes complexes were characterized by IR and EPR spectroscopy as well as elemental analysis. Complexes 1 and 2 have high catalytic activity in the oxidation of hydrocarbons with hydrogen peroxide and alcohols with tert-butyl hydroperoxide in acetonitrile at 50 °С. The product yields are up to 40% for cyclohexane. Of particular importance is the addition of 2-pyrazinecarboxylic acid (PCA) as a co-catalyst. Oxidation proceeds mainly with the participation of free hydroxyl radicals, as evidenced by taking into account the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, as well as the dependence of the reaction rate on the initial concentration of cyclohexane.

Vanadium(IV) Complexes with Methyl-Substituted 8-Hydroxyquinolines: Catalytic Potential in the Oxidation of Hydrocarbons and Alcohols with Peroxides and Biological Activity

Methyl-substituted 8-hydroxyquinolines (Hquin) were successfully used to synthetize five-coordinated oxovanadium(IV) complexes: [VO(2,6-(Me)₂-quin)₂] (1), [VO(2,5-(Me)₂-quin)₂] (2) and [VO(2-Me-quin)₂] (3). Complexes 1-3 demonstrated high catalytic activity in the oxidation of hydrocarbons with H₂O₂ in acetonitrile at 50 °C, in the presence of 2-pyrazinecarboxylic acid (PCA) as a cocatalyst. The maximum yield of cyclohexane oxidation products attained was 48%, which is high in the case of the oxidation of saturated hydrocarbons. The reaction leads to the formation of a mixture of cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone. When triphenylphosphine is added, cyclohexyl hydroperoxide is completely converted to cyclohexanol. Consideration of the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, respectively, indicates that the oxidation proceeds with the participation of free hydroxyl radicals. The complexes show moderate activity in the oxidation of alcohols. Complexes 1 and 2 reduce the viability of colorectal (HCT116) and ovarian (A2780) carcinoma cell lines and of normal dermal fibroblasts without showing a specific selectivity for cancer cell lines. Complex 3 on the other hand, shows a higher cytotoxicity in a colorectal carcinoma cell line (HCT116), a lower cytotoxicity towards normal dermal fibroblasts and no effect in an ovarian carcinoma cell line (order of magnitude HCT116 > fibroblasts > A2780).

Protein binding and cytotoxic activities of monomeric and dimeric oxido-vanadium(V) salan complexes: Exploring the solution behavior of monoalkoxido-bound oxido-vanadium(V) complex

Three ONNO donor tetradentate diamino bis(phenolato) "salan" ligands, N, N'-dimethyl-N, N'-bis-(5-chloro-2-hydroxy-3-methyl-benzyl)-1,2-diaminoethane (H₂L¹), N, N'-dimethyl-N, N'-bis-(5-chloro-2-hydroxy-3-isopropyl-6-methyl-benzyl)-1,2-diamino-ethane (H₂L²) and N, N'-bis-(5-chloro-2-hydroxy-3-isopropyl-6-methyl-benzyl)-1,2-diaminocyclohexane (H₂L³) have been synthesized by following Mannich condensation reaction. Reaction of these ligands with their corresponding vanadium metal precursors gave one oxidomethoxidovanadium(V) [V^VOL¹(OCH₃)] (1) and two monooxido-bridged divanadium (V, V) complexes [V^VOL^2-3]₂(μ-O) (2-3). The complexes were characterized by IR, UV-vis, NMR and ESI mass spectrometry. Also, the structure of all the complexes (1-3) was confirmed by the Single-Crystal X-ray diffraction analysis, which revealed a distorted octahedral geometry around the metal centres. The solution behavior of the [V^VOL¹(OCH₃)] (1) reveals the formation of two different types of V(V) species in solution, the structurally characterized compound 1 and its corresponding monooxido-bridged divanadium (V, V) complex [V^VOL¹]₂(μ-O), which was further studied by IR, and NMR spectroscopy. The electrochemical behavior of all the complexes was evaluated through cyclic voltammetry. Interaction of the salan-V(V) complexes with human serum albumin (HSA) and bovine serum albumin (BSA) were analysed through fluorescence quenching, UV-vis absorption titration, synchronous fluorescence, circular dichroism studies, and förster resonance energy transfer (FRET). Finally, the in vitro cytotoxicity of the complexes was investigated against MCF-7 and HT-29 and NIH-3T3 cell lines. Cytotoxicity value of complexes in both MCF-7 and HT-29 follows the same trend that is 3 > 1 > 2 which is in line with protein binding affinity of the complexes.

Catalytic Cleavage of the C-O Bond in 2,6-dimethoxyphenol Without External Hydrogen or Organic Solvent Using Catalytic Vanadium Metal

Hydrogenolysis of the C-O bonds in lignin, which promises to be able to generate fuels and chemical feedstocks from biomass, is a particularly challenging and important area of investigation. Herein, we demonstrate a vanadium-catalyzed cleavage of a lignin model compound (2,6-dimethoxyphenol). The impact of the catalyst in the context of the temperature, reaction time, and the solvent, was examined for the cleavage of the methyl ethers in 2,6-dimethoxyphenol. In contrast to traditional catalytic transfer hydrogenolysis, which requires high pressure hydrogen gas or reductive organic molecules, such as an alcohol and formic acid, the vanadium catalyst demonstrates superior catalytic activity on the cleavage of the C-O bonds using water as a solvent. For example, the conversion of 2,6-dimethoxyphenol is 89.5% at 280°C after 48 h using distilled water. Notably, the vanadium-catalyzed cleavage of the C-O bond linkage in 2,6-dimethoxyphenol affords 3-methoxycatechol, which undergoes further cleavage to afford pyrogallol. This work is expected to provide an alternative method for the hydrogenolysis of lignin and related compounds into valuable chemicals in the absence of external hydrogen and organic solvents.

Vanadium-Substituted Phosphomolybdic Acids for the Aerobic Cleavage of Lignin Models-Mechanistic Aspect and Extension to Lignin

This work deals with the aerobic oxidative cleavage of C-C and C-O bonds catalyzed by the Keggin-type phosphovanadomolybdic acid (H₆[PMo₉V₃O₄₀], noted H₆PV₃). The latter was synthesized by an adapted hydrothermal procedure classically used for lower vanadium content and was tested as a catalyst for the aerobic cleavage of 2-phenoxyacetophenone (noted K1_HH) and 1-phenyl-2-phenoxyethanol (A1_HH) used as two lignin models. The operative conditions (solvent, catalytic loading, etc.) were adjusted on K1_HH and extrapolated to A1_HH. The cleavage of the alcohol model required more drastic conditions and therefore further optimization. Preliminary attempts on an Organosolv wheat straw lignin were performed too. From the kinetic study, high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) data, a mechanism of the cleavage of both models was proposed.

Trinuclear vanadium(iv) and vanadium(v) complexes derived from 2,4,6-triacetylphloroglucinol and study of their peroxidase mimicking activity

Solid state and solution studies of trinuclear V^IVO- and V^VO₂-complexes and their peroxidase mimicking activity, through oxidation of dopamine to aminochrome, are reported.

Ru-Catalyzed Oxidative Cleavage of Guaiacyl Glycerol--Guaiacyl Ether-a Representative -O-4 Lignin Model Compound

The introduction of efficient and selective catalytic methods for aerobic oxidation of lignin and lignin model compounds to aromatics can extend the role of lignin applications in biorefineries. The current study focussed on the catalytic oxidative transformation of guaiacyl glycerol--guaiacyl ether (GGGE)–a -O-4 lignin model compound to produce basic aromatic compounds (guaiacol, vanillin and vanillic acid) using metal-supported catalysts. Ru/Al2O3, prepared with ruthenium(IV) oxide hydrate, showed the highest yields of the desired products (60%) in acetonitrile in a batch reactor at 160 C and 5-bar of 20% oxygen in argon. Alternative catalysts containing other transition metals (Ag, Fe, Mn, Co and Cu) supported on alumina, and ruthenium catalysts based on alternative supports (silica, spinel, HY zeolite and zirconia) gave significantly lower activities compared to Ru/Al2O3 at identical reaction conditions. Moreover, the Ru/Al2O3 catalyst was successfully reused in five consecutive reaction runs with only a minor decrease in catalytic performance.

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

A comprehensive account of recent advances in the synthesis of imidazopyridines, assisted through transition-metal-catalyzed multicomponent reactions, C-H activation/functionalization and coupling reactions are highlighted in this review article. The basic illustration of this review comprises of schemes with concise account of explanatory text. The schemes depict the reaction conditions along with a quick look into the mechanism involved to render a deep understanding of the catalytic role. At some instances optimizations of certain features have been illustrated through tables, i.e., selectivity of catalyst, loading of the catalyst and percentage yield with different substrates. Each of the reported examples has been rigorously analyzed for reacting substrates, reaction conditions and transition metals used as the catalyst. This review will be helpful to the chemists in understanding the challenges associated with the reported methods as well as the future possibilities, both in the choice of substrates and catalysts. This review would be quite appealing to a wider range of organic chemists in academia and industrial R&D sectors working in the field of heterocyclic syntheses. In a nutshell, this review will be a guiding torch to envisage: (i) the role of various transition metals in the domain dedicated towards method development and (ii) for the modifications needed thereof in the R&D sector.

New Oxidovanadium(IV) Complexes with 2,2′-bipyridine and 1,10-phenathroline Ligands: Synthesis, Structure and High Catalytic Activity in Oxidations of Alkanes and Alcohols with Peroxides

Reactions of [VCl3(thf)3] or VBr3 with 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) in a 1:1 molar ratio in air under solventothermal conditions has afforded polymeric oxidovanadium(IV) four complexes 1–4 of a general formula [VO(L)X2]n (L = bpy, phen and X = Cl, Br). Monomeric complex [VO(DMF)(phen)Br2] (4a) has been obtained by the treatment of compound 4 with DMF. The complexes were characterized by IR spectroscopy and elemental analysis. The crystal structures of 3 and 4a were determined by an X-ray diffraction (XRD) analysis. The {VOBr2(bpy)} fragments in 3 form infinite chains due to the V = O…V interactions. The vanadium atom has a distorted octahedral coordination environment. Complexes 1–4 have been tested as catalysts in the homogeneous oxidation of alkanes (to produce corresponding alkyl hydroperoxides which can be easily reduced to alcohols by PPh3) and alcohols (to corresponding ketones) with H2O2 or tert-butyl hydroperoxide in MeCN. Compound 1 exhibited the highest activity. The mechanism of alkane oxidation was established using experimental selectivity and kinetic data and theoretical DFT calculations. The mechanism is of the Fenton type involving the generation of HO• radicals.

Bio-Based Chemicals from Renewable Biomass for Integrated Biorefineries

The production of chemicals from biomass, a renewable feedstock, is highly desirable in replacing petrochemicals to make biorefineries more economical. The best approach to compete with fossil-based refineries is the upgradation of biomass in integrated biorefineries. The integrated biorefineries employed various biomass feedstocks and conversion technologies to produce biofuels and bio-based chemicals. Bio-based chemicals can help to replace a large fraction of industrial chemicals and materials from fossil resources. Biomass-derived chemicals, such as 5-hydroxymethylfurfural (5-HMF), levulinic acid, furfurals, sugar alcohols, lactic acid, succinic acid, and phenols, are considered platform chemicals. These platform chemicals can be further used for the production of a variety of important chemicals on an industrial scale. However, current industrial production relies on relatively old and inefficient strategies and low production yields, which have decreased their competitiveness with fossil-based alternatives. The aim of the presented review is to provide a survey of past and current strategies used to achieve a sustainable conversion of biomass to platform chemicals. This review provides an overview of the chemicals obtained, based on the major components of lignocellulosic biomass, sugars, and lignin. First, important platform chemicals derived from the catalytic conversion of biomass were outlined. Later, the targeted chemicals that can be potentially manufactured from the starting or platform materials were discussed in detail. Despite significant advances, however, low yields, complex multistep synthesis processes, difficulties in purification, high costs, and the deactivation of catalysts are still hurdles for large-scale competitive biorefineries. These challenges could be overcome by single-step catalytic conversions using highly efficient and selective catalysts and exploring purification and separation technologies.

Probing the chirality of oxidovanadium(v) centers in complexes with tridentate sugar Schiff-base ligands: solid-state and solution behavior

Configurations of oxidovanadium centers in diastereomeric complexes with chiral sugar ligands are assigned and in the solid state triggered by the coordination number at the vanadium center through the steric requirements of the chelate ligand.

In vitrocytotoxicity and catalytic evaluation of dioxidovanadium(v) complexes in an azohydrazone ligand environment

Synthesis, characterization,in vitrocytotoxicity and catalytic potential of the dioxidovanadium(v) complexes of azohydrazones.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Vinylation of Aryl Ether (Lignin β-O-4 Linkage) and Epoxides with Calcium Carbide through C-O Bond Cleavage

Calcium carbide has been increasingly used as a sustainable, easy-to-handle, and low-cost feedstock in organic synthesis. Currently, methodologies of using calcium carbide as "solid acetylene" in synthesis are strictly limited to activation and reaction with X-H (X=C, N, O, S) bonds. Herein, a mild and transition-metal-free protocol was developed for the vinylation of epoxides and aryl ether linkage (β-O-4 lignin model compound) with calcium carbide through C-O bond cleavage, forming valuable vinyl ether products. Calcium carbide plays a vital role in the C-O bond activation and cleavage, and in providing acetylide source for the formation of vinylated products. These exciting results may provide new methodologies for organic synthesis and new insights toward lignin- or biomassrelated degradation to useful products.

Coupling Reactions of Carbon Dioxide with Epoxides Catalyzed by Vanadium Aminophenolate Complexes

A series of vanadium compounds supported by tetradentate aminobis(phenolate) ligands were screened for catalytic reactivity in the reaction of propylene oxide (PO) with CO₂ : [VO(OMe)(O₂ NO^BuMeMeth )], [VO(OMe)(ON₂ O^BuMe )], [VO(OMe)(O₂ NN^BuBuPy )], and [VO(OMe)(O₂ NO^BuBuFurf )]. They showed similar reactivities, but reaction rates were higher for [VO(OMe)(ON₂ O^BuMe )], which was studied in more detail. Turnover frequencies for conversion of PO over 500 h^-1 were observed. Activation energies were determined experimentally through in situ IR spectroscopy for propylene carbonate (48.2 kJ mol^-1 ), styrene carbonate (45.6 kJ mol^-1 ), and cyclohexene carbonate (54.7 kJ mol^-1 ) formation.

Transition-metal catalyzed valorization of lignin: the key to a sustainable carbon-neutral future

The development of a sustainable, carbon-neutral biorefinery has emerged as a prominent scientific and engineering goal of the 21st century. As petroleum has become less accessible, biomass-based carbon sources have been investigated for utility in fuel production and commodity chemical manufacturing. One underutilized biomaterial is lignin; however, its highly crosslinked and randomly polymerized composition have rendered this biopolymer recalcitrant to existing chemical processing. More recently, insight into lignin's molecular structure has reinvigorated chemists to develop catalytic methods for lignin depolymerization. This review examines the development of transition-metal catalyzed reactions and the insights shared between the homogeneous and heterogeneous catalytic systems towards the ultimate goal of valorizing lignin to produce value-added products.