Sustainable vanadium-catalyzed oxidation of organic substrates with H2O2

Modulation of ODH Propane Selectivity by Zeolite Support Desilication: Vanadium Species Anchored to Al-Rich Shell as Crucial Active Sites

The commercially available zeolite HY and its desilicated analogue were subjected to a classical wet impregnation procedure with NH₄VO₃ to produce catalysts differentiated in acidic and redox properties. Various spectroscopic techniques (in situ probe molecules adsorption and time-resolved propane transformation FT-IR studies, XAS, ⁵¹V MAS NMR, and 2D COS UV-vis) were employed to study speciation, local coordination, and reducibility of the vanadium species introduced into the hierarchical faujasite zeolite. The acid-based redox properties of V centres were linked to catalytic activity in the oxidative dehydrogenation of propane. The modification of zeolite via caustic treatment is an effective method of adjusting its basicity—a parameter that plays an important role in the ODH process. The developed mesopore surface ensured the attachment of vanadium species to silanol groups and formation of isolated (SiO)₂(HO)V=O and (SiO)₃V=O sites or polymeric, highly dispersed forms located in the zeolite micropores. The higher basicity of HY_deSi, due to the presence of the Al-rich shell, aided the activation of the C−H bond leading to a higher selectivity to propene. Its polymerisation and coke formation were inhibited by the lower acid strength of the protonic sites in desilicated zeolite. The Al-rich shell was also beneficial for anchoring V species and thus their reducibility. The operando UV-vis experiments revealed higher reactivity of the bridging oxygens V-O-V over the oxo-group V=O. The (SiO)₃V=O species were found to be ineffective in propane oxidation when temperature does not exceed 400 °C.

A Stoichiometric Solvent-Free Protocol for Acetylation Reactions

Considering the remarkable relevance of acetylated derivatives of phenols, alcohols, and aryl and alkyl thiols in different areas of biology, as well as in synthetic organic chemistry, a sustainable solvent-free approach to perform acetylation reactions is proposed here. Acetylation reactions are classically performed using excess of acetic anhydride (Ac₂O) in solvent-free conditions or by eventually working with stoichiometric amounts of Ac₂O in organic solvents; both methods require the addition of basic or acid catalysts to promote the esterification. Therefore, they usually lead to the generation of high amounts of wastes, which sensibly raise the E-factor of the process. With the aim to develop a more sustainable system, a solvent-free, stoichiometric acetylation protocol is, thus, proposed. The naturally occurring phenol, thymol, can be converted to the corresponding—biologically active—ester with good yields, in the presence of 1% of VOSO₄. Interestingly, the process can be efficiently adopted to synthesize other thymyl esters, as well as to perform acetylation of alcohols and aryl and alkyl thiols. Remarkably, a further improvement has been achieved replacing Ac₂O with its greener alternative, isopropenyl acetate (IPA).

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).

Tailored Functionalization of Natural Phenols to Improve Biological Activity

Phenols are widespread in nature, being the major components of several plants and essential oils. Natural phenols' anti-microbial, anti-bacterial, anti-oxidant, pharmacological and nutritional properties are, nowadays, well established. Hence, given their peculiar biological role, numerous studies are currently ongoing to overcome their limitations, as well as to enhance their activity. In this review, the functionalization of selected natural phenols is critically examined, mainly highlighting their improved bioactivity after the proper chemical transformations. In particular, functionalization of the most abundant naturally occurring monophenols, diphenols, lipidic phenols, phenolic acids, polyphenols and curcumin derivatives is explored.

A Dual Role of Vanadium in Environmental Systems-Beneficial and Detrimental Effects on Terrestrial Plants and Humans

The importance of vanadium (V) in the functioning of land systems is extremely diverse, as this element may exert both positive and harmful effects on terrestrial organisms. It recently become considered an element of beneficial character with a range of applications for human welfare. The health-ameliorative properties of this transition element depend on its degree of oxidation and on optimal concentration in the target cells. It was found that a similar relationship applies to vascular plants. However, excessive amounts of vanadium in the environment contaminate the soil and negatively affect the majority of living organisms. A significantly elevated level of V results in the destabilization of plant physiological balance, slowing down the growth of biomass which significantly reduces yield. In turn, low doses of the appropriate vanadium ions can stimulate plant growth and development, exert cytoprotective effects, and effectively enhance the synthesis of some biologically active compounds. We present the scientific achievements of research teams dealing with such topics. The issues discussed concern the role of vanadium in the environment, particular organisms, and highlight its dualistic influence on plants. Achievements in the field of V bioremediation, with the use of appropriately selected microorganisms and plant species, are emphasized.

Sustainable Highly Selective Toluene Oxidation to Benzaldehyde

Thanks to the well-recognized role of benzaldehyde in industry, nowadays the research of new and sustainable approaches to selectively synthesize such an interesting product is receiving great attention from the chemists’ community. In this paper, a V-based catalytic biphasic system is adopted to perform toluene oxidation to benzaldehyde. Importantly, to pursue sustainability, organic solvents have been avoided, so toluene is used as substrate and co-solvent, together with water. Also, the use of hydrophobic ionic liquids has been explored. To perform oxidation, NH4VO3 catalyst, H2O2, and a safe and inexpensive co-catalyst are used. Among the tested co-catalysts, KF and O2 were found to be the best choice, to guarantee good yields, in mild reaction conditions. In fact, with such a sustainable method, up to 30% of benzaldehyde can be obtained at 60 °C and, more interestingly, the oxidative system can be recharged, raising-up the yield. The entire process results highly selective, since no traces of benzyl alcohol or benzoic acid are detected. Hence, it constitutes a very appealing synthetic route, even suitable to be easily scaled-up at an industrial level.

Stereochemistry of Vanadium Peroxido Complexes: The Case of the Quinoline-2-carboxylato Ligand

A new mononuclear vanadium peroxido complex [VO(O₂)(phen)(quin)]·H₂O (1) exhibiting an unprecedented isomerism of its ligands was isolated from a two-component water-acetonitrile solvent system. DFT computations aimed at inspecting the stability of all possible isomers of complexes [VO(O₂)(L¹)(L²)], where L¹ and L² are NN+ON, OO+ON, NN+OO, and ON+ON donor atom set ligands, suggested that every complex characterized so far was the one preferred thermodynamically. However, the particular case of complex [VO(O₂)(phen)(quin)] reported herein poses a notable exception to this rule, as this complex yielded single crystals of the isomer with total energy above the anticipated isomer, although both of these isomers could be observed concurrently in solution and also in the solid state. ⁵¹V NMR spectroscopy suggested these isomers to be present both in the crystallization solution and in the acetonitrile solution of 1. The coexistence of two isomers is a consequence of their small computed energy difference of 2.68 kJ mol^-1, while the preferential crystallization favoring the unexpected isomer is likely to be triggered by solvent effects and the effects of different solubility and/or crystal packing. The coordination geometry of the unusual isomer also manifests itself in FT-IR and Raman spectra, which were corroborated with DFT computations targeted at band assignments.

Biomimetic Vanadate and Molybdate Systems for Oxidative Upgrading of Iono- and Organosolv Hard- and Softwood Lignins

Recently reported acetosolv soft- and hardwood lignins as well as ionosolv soft- and hardwood lignins were transformed into monomeric aromatic compounds using either a vanadate or a molybdate-based catalyst system. Monomers were generated with remarkable, catalyst-dependent selectivity and high depolymerisation yields via oxidative exo- and endo-depolymerisation processes. Using the vanadate–hydrogen peroxide system on acetosolv pine lignin, vanillin and isovanillin were produced as main products with depolymerisation yields of 31%. Using the molybdate system on acetosolv and ionosolv lignin, vanillic acid was the practically exclusive product, with depolymerisation yields of up to 72%. Similar selectivities, albeit with lower depolymerisation yields of around 50% under standardised conditions, were obtained for eucalyptus acetosolv lignin, producing vanillin and syringaldehyde or vanillic acid as products, by using the vanadate- or the molybdate-based systems respectively.

DNA binding, cytotoxic effects and probable targets of an oxindolimine–vanadyl complex as an antitumor agent

The vanadyl–oxindolimine complex as an antitumor agent.

Selective catalytic oxidation of benzene to phenol by a vanadium oxide nanorod (Vnr) catalyst in CH3CN using H2O2(aq) and pyrazine-2-carboxylic acid (PCA)

A vanadium oxide nanorod (V_nr) catalyst has been synthesized without using surfactants through crystallization, which is highly active for benzene to phenol oxidation.

Activation of sodium percarbonate by vanadium for the degradation of aniline in water: Mechanism and identification of reactive species

Sodium percarbonate (SPC)-based advanced oxidation process has been applied to the wastewater treatment in recent years. In the present study, a novel catalyst utilization of vanadium(V) was investigated for the activation of SPC for aniline degradation. The mechanism of SPC activation by V(IV) was demonstrated, and the major free radicals were identified through scavenging tests and electron paramagnetic resonance (EPR) analysis. The performance of aniline degradation was evaluated in the V(IV)/H₂O₂ and V(IV)/H₂O₂/Na₂CO₃ systems and compared with that of the V(IV)/SPC system. The influences of initial pH and effects of inorganic anions were also studied. The results show that aniline could be decomposed efficiently by SPC activated with V(IV) and the degradation efficiency increased with the increase in V(IV) and SPC dosage. O₂^-, CO₃^-, and OH were found to participate in aniline degradation, and O₂^- and CO₃^- were confirmed to be the predominant species. The decomposition of aniline was enhanced when equal amount of H₂O₂ was utilized instead of SPC under acidic condition while a comparative behavior was achieved in the V(IV)/H₂O₂/Na₂CO₃ system. Furthermore, aniline degradation was not impacted significantly by the initial pH and addition of Cl^-, SO₄^2- and NO₃^-, while HCO₃^- led to a betterment of aniline removal. These results indicate that the V(IV)/SPC system is an effective and promising approach for the removal of aniline from water for its feasibility and stability, which achieves eliminating contaminants by another waste.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

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.

The Beneficial Sinergy of MW Irradiation and Ionic Liquids in Catalysis of Organic Reactions

The quest for sustainable processes is becoming more and more important, with catalysis playing a major role in improving atom economy and reducing waste. Organic syntheses with less need of protecting/de-protecting steps are highly desirable. The combination of microwave irradiation, as energy source, with ionic liquids, as both solvents and catalysts, offered interesting solutions in recent years. The literature data of the last 15 years concerning selected reactions are presented, highlighting the importance of microwave (MW) technology coupled with ionic liquids.