Vanadium in Modern Organic Synthesis

Photocatalytic Hantzsch Ester-Mediated Pinacol-Type Coupling of Aldehydes and Ketones

We have developed a photocatalyst-free reaction system that uses Hantzsch esters as photoreducing agents to promote the coupling of carbonyl compounds to 1,2-diols. The system fully utilizes the single electron transfer and proton donor roles of Hantzsch esters. The system shows a wide range of substrate application. Aromatic ketones, aliphatic ketones, and aldehydes can be applied to the catalytic system. Both self-coupling and cross-coupling can achieve ideal results.

Charge and Solvent Effects on the Redox Behavior of Vanadyl Salen-Crown Complexes

The incorporation of charged groups proximal to a redox active transition metal center can impact the local electric field, altering redox behavior and enhancing catalysis. Vanadyl salen (salen = N,N'-ethylenebis(salicylideneaminato)) complexes functionalized with a crown ether containing a nonredox active metal cation (V-Na, V-K, V-Ba, V-La, V-Ce, and V-Nd) were synthesized. The electrochemical behavior of this series of complexes was investigated by cyclic voltammetry in solvents with varying polarity and dielectric constant (ε) (acetonitrile, ε = 37.5; N,N-dimethylformamide, ε = 36.7; and dichloromethane, ε = 8.93). The vanadium(V/IV) reduction potential shifted anodically with increasing cation charge compared to a complex lacking a proximal cation (ΔE_1/2 > 900 mV in acetonitrile and >700 mV in dichloromethane). In contrast, the reduction potential for all vanadyl salen-crown complexes measured in N,N-dimethylformamide was insensitive to the magnitude of the cationic charge, regardless of the electrolyte or counteranion used. Titration studies of N,N-dimethylformamide into acetonitrile resulted in cathodic shifting of the vanadium(V/IV) reduction potential with increasing concentration of N,N-dimethylformamide. Binding constants of N,N-dimethylformamide (log(K_DMF)) for the series of crown complexes show increased binding affinity in the order of V-La > V-Ba > V-K > (salen)V(O), indicating an enhancement of Lewis acid/base interaction with increasing cationic charge. The redox behavior of (salen)V(O) and (salen-OMe)V(O) (salen-OMe = N,N'-ethylenebis(3-methoxysalicylideneamine) was also investigated and compared to the crown-containing complexes. For (salen-OMe)V(O), a weak association of triflate salt at the vanadium(IV) oxidation state was observed through cyclic voltammetry titration experiments, and cation dissociation upon oxidation to vanadium(V) was identified. These studies demonstrate the noninnocent role of solvent coordination and cation/anion effects on redox behavior and, by extension, the local electric field.

A Combined Experimental and Theoretical Investigation of Oxidation Catalysis by cis-[VIV(O)(Cl/F)(N4)]+ Species Mimicking the Active Center of Metal-Enzymes

Reaction of V^IVOCl₂ with the nonplanar tetradentate N₄ bis-quinoline ligands yielded four oxidovanadium(IV) compounds of the general formula cis-[V^IV(O)(Cl)(N₄)]Cl. Sequential treatment of the two nonmethylated N₄ oxidovanadium(IV) compounds with KF and NaClO₄ resulted in the isolation of the species with the general formula cis-[V^IV(O)(F)(N₄)]ClO₄. In marked contrast, the methylated N₄ oxidovanadium(IV) derivatives are inert toward KF reaction due to steric hindrance, as evidenced by EPR and theoretical calculations. The oxidovanadium(IV) compounds were characterized by single-crystal X-ray structure analysis, cw EPR spectroscopy, and magnetic susceptibility. The crystallographic characterization showed that the vanadium compounds have a highly distorted octahedral coordination environment and the d(V^IV–F) = 1.834(1) Å is the shortest to be reported for (oxido)(fluorido)vanadium(IV) compounds. The experimental EPR parameters of the V^IVO²⁺ species deviate from the ones calculated by the empirical additivity relationship and can be attributed to the axial donor atom trans to the oxido group and the distorted V^IV coordination environment. The vanadium compounds act as catalysts toward alkane oxidation by aqueous H₂O₂ with moderate ΤΟΝ up to 293 and product yields of up to 29% (based on alkane); the vanadium(IV) is oxidized to vanadium(V), and the ligands remain bound to the vanadium atom during the catalysis, as determined by ⁵¹V and ¹H NMR spectroscopies. The cw X-band EPR studies proved that the mechanism of the catalytic reaction is through hydroxyl radicals. The chloride substitution reaction in the cis-[V^IV(O)(Cl)(N₄)]⁺ species by fluoride and the mechanism of the alkane oxidation were studied by DFT calculations.

Highly distorted N₄−oxidovanadium(IV), cis-[V^IV(=O)(Cl/F)(N₄)]⁺, compounds mimicking the distorted coordination environment of the metal-ion in metal-enzymes’ active center catalyze oxidation of alkanes. The structural distortion results in significant deviation of the cw EPR parameters from the expected ones and equilibrium between six- and five- coordinate species in solution. DFT studies show that distortion, steric hindrance, and the coordinated halogen define the catalytic mechanism of oxidation of alkanes by cis-[V^IV(=O)(Cl/F)(N₄)]⁺ compounds.

Epoxidation vs. dehydrogenation of allylic alcohols: heterogenization of the VO(acac)2 catalyst in a metal-organic framework

Allylic alcohol epoxidation and dehydrogenation reactivity is distinguished when VO(acac)₂ is used in solution or anchored in a metal-organic framework (MOF). The chemical mechanism depends on the electronic profile of alkene substituents when the vanadyl complex is used in the homogenous phase. However, confinement effects imparted by MOF channels allow gaining control of the chemoselectivity toward the dehydrogenation product.

Synthetic utility of biomimicking vanadium bromoperoxidase and n-tetrabutylammonium tribromide (TBATB) in organic synthesis

Nature has established a broad spectrum of methods to introduce halogen atoms in organic compounds. Recent developments have revealed that haloperoxidases are one of the major sources responsible for incorporating bromines to produce bromoorganics in nature. Pioneering studies of numerous researchers have unravelled the details of haloperoxidases, mainly vanadium dependent enzyme bromo- and iodo-peroxidases, including reaction mechanism, kinetics and especially biomimicking studies. In this review, we initially have described the scope of biomimicking vanadium bromoperoxidase in producing the bromonium ion and its further utilisation in conducting oxidative bromination or cleavage of various organic molecules. Moreover, by biomicmicking, the synthesis of OATB and the synthetic utility of various organic ammonium tribromides (OATBs) have been discussed. Among such OATBs, n-tetrabutylammonium tribromide (TBATB) has been explored for bromination of organic molecules as well as in the facile removal of several protecting groups and as a potential catalyst in various synthetic transformations. This review attempts to compile a myriad of reactions concerning the catalytic activity of vanadium bromoperoxidases and the usefulness of various OATBs, particularly with special emphasis on TBATB in various organic transformations.

Efficient One-pot Synthesis of 3,3-di(indolyl)indolin-2-ones from Isatin and Indole Catalyzed by VOSO4 as Non-Sulfonamide Carbonic Anhydrase Inhibitors

Background A high yielding green protocol has been developed and delineated for the synthesis of 3,3-di(indolyl)indolin-2-ones, potentially bioactive compounds, involving one pot aqueous medium condensation of isatin with indole in the presence of VOSO4. The synthesized compounds were screened for their carbonic anhydrase inhibitory activity against human (h) isoforms hCA I, hCA II, hCA IX and hCA XII. These non-sulfonamide derivatives selectively inhibited hCA II in the micromolar range. Objective To develop a high yielding green protocol to synthesize 3,3-diindolyl oxindole derivatives using water as solvent media and screening of the synthesized molecules for their carbonic anhydrase inhibitory activity. Methods The target compound is obtained by taking isatin, indole, VOSO4and H2O in one-pot at 70 oC. Results The designed molecules were synthesized by using the new method. The molecules were screened for their CA inhibitory activity which shows selective inhibition toward hCA II.The result shown an excellent yield without any loss or decrease in catalytic activity, hence proved the performance and recyclability of the catalyst. Conclusion An efficient, simple and green protocol was established that provides a facile and straightforward approach for the preparation of 3,3-diindolyl oxindole derivatives (3a-r) from Isatin and Indole by using 10 mol % VOSO4 in high yields in a short period of time by a one-pot coupling reaction. Furthermore, the catalyst can also be recovered and reused for three consecutive catalytic cycle without any loss of its efficiency which was confirmed by performing the experiment with 3a. The newly synthesized molecules (3a-r) were screened for their carbonic anhydrase inhibition potency against four isoforms, hCA I, II, IX and XII and most of the compounds were found potent against hCA II with potency low to submicromolar range.

Catalytic Generation of Carbanions through Carbonyl Umpolung

Carbonyl Umpolung is a powerful strategy in organic chemistry to construct complex molecules. Over the last few years, versatile catalytic approaches for the generation of acyl anion equivalents from carbonyl compounds have been developed, but methods to obtain alkyl carbanions from carbonyl compounds in a catalytic fashion are still at an early stage. This Minireview summarizes recent progress in the generation of alkyl carbanions through catalytic carbonyl Umpolung. Two different catalytic approaches can be utilized to enable the generation of alkyl carbanions from carbonyl compounds: the catalytic Wolff–Kishner reaction and the catalytic single‐electron reduction of carbonyl compounds and imines. We discuss the reaction scope, mechanistic insights, and synthetic applications of the methods as well as potential future developments.

A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2via decarboxylation

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Novel Oxovanadium Complex VO(hntdtsc)(NPIP): Anticancer Activity and Mechanism of Action on HeLa Cells

Oxovanadium complexes, particularly vanadyl (IV) derivatives with hybrid ligands of Schiff base and polypyridyl, have been demonstrated to possess great anticancerous therapeutic efficacy. However, most of the studies on the activity of these oxovanadium complexes have mainly focused on in vitro studies, and animal studies in vivo are extremely scarce. Based on the antitumor test results of four novel oxovanadium complexes in our previous work, this work further conducted a comprehensive antitumor activity study in vitro and in vivo on VO(hntdtsc)(NPIP), which owned the strongest inhibitory activity in vitro on multiple tumor cell proliferation. The cellular mechanism study suggested that VO(hntdtsc)(NPIP) inhibited the cell proliferation via arresting the cell cycle at G0/G1 phase through the p16-cyclin D1-CDK4-p-Rb pathway and inducing cell apoptosis through mitochondrial-dependent apoptosis pathway on HeLa cells. Inconsistent with the effects in vitro, VO(hntdtsc)(NPIP) significantly inhibited the growth of tumor and induced the apoptosis of cancer cells in mice xenograft models according to the results of nude mice in vivo image detection, H&E pathological examination, and immunohistochemical detection of p16/Ki-67 protein expression. Collectively, all the results, particularly studies in vivo, demonstrated that VO(hntdtsc)(NPIP) hold a potential to be the lead compound and further to be an anticervical cancer drug.

Oxovanadium(v)-catalyzed amination of carbon dioxide under ambient pressure for the synthesis of ureas

Carbon dioxide is regarded as a reliable C1 building block in organic synthesis because of the nontoxic, abundant, and economical characteristics of carbon dioxide. In this manuscript, a commercially available oxovanadium(v) compound was demonstrated to serve as an efficient catalyst for the catalytic amination of carbon dioxide under ambient pressure in the synthesis of ureas. The catalytic transformation of chiral amines into the corresponding chiral ureas without loss of chirality was also performed. Furthermore, a gram-scale catalytic urea synthesis under ambient pressure was successfully achieved to validate the scalability of this catalytic activation of carbon dioxide.

Catalytic activation of carbon dioxide as a C1 building block under ambient pressure was performed by using a commercially available oxovanadium(v) catalyst for the synthesis of ureas.

Novel Effect of Zinc Nitrate/Vanadyl Oxalate for Selective Catalytic Oxidation of α-Hydroxy Esters to α-Keto Esters with Molecular Oxygen: An In Situ ATR-IR Study

Selective oxidation of α-hydroxy esters is one of the most important methods to prepare high value-added α-keto esters. An efficient catalytic system consisting of Zn(NO₃)₂/VOC₂O₄ is reported for catalytic oxidation of α-hydroxy esters with molecular oxygen. Up to 99% conversion of methyl DL-mandelate or methyl lactate could be facilely obtained with high selectivity for its corresponding α-keto ester under mild reaction conditions. Zn(NO₃)₂ exhibited higher catalytic activity in combination with VOC₂O₄ compared with Fe(NO₃)₃ and different nitric oxidative gases were detected by situ attenuated total reflection infrared (ATR-IR) spectroscopy. UV-vis and ATR-IR results indicated that coordination complex formed in Zn(NO₃)₂ in CH₃CN solution was quite different from Fe(NO₃)₃; it is proposed that the charge-transfer from Zn²⁺ to coordinated nitrate groups might account for the generation of different nitric oxidative gases. The XPS result indicate that nitric oxidative gas derived from the interaction of Zn(NO₃)₂ with VOC₂O₄ could be in favor of oxidizing VOC₂O₄ to generate active vanadium (V) species. It might account for different catalytic activity of Zn(NO₃)₂ or Fe(NO₃)₃ combined with VOC₂O₄. This work contributes to further development of efficient aerobic oxidation under mild reaction conditions.

Enhancing catalytic aerobic oxidation performance of cyclohexaneviasize regulation of mixed-valence {V16} cluster-based metal–organic frameworks

Size-controllable synthesis of mixed-valence {V₁₆} cluster-based metal–organic frameworks used for highly efficient catalytic oxidation of cyclohexane.

Oxovanadium(v)-catalyzed deoxygenative homocoupling reaction of alcohols

Catalytic direct hydrazination of allyl alcohol and deoxygenative homocoupling reaction of alcohols depending on hydrazine derivatives were performed by utilizing oxovanadium(v) catalysts.

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.

Our Contributions in Synthesis of Diverse Heterocyclic Scaffolds by Using Mixed Oxides as Heterogeneous Catalysts

This personal account mainly introduces and reviews our recent contributions in developing different catalyst materials involving mixed oxides and their scope as renewable catalysts in multicomponent reactions to synthesize various novel heterocyclic scaffolds under green conditions. The application of various mixed oxides and their composites in the organic synthesis is emphasized through this review, in order to reveal the versatility, scope and importance of mixed oxides and their interactions during the reaction. We have also briefed the limitations of mixed oxides as catalysts, to put forward the broader prospective in the field.

Synthesis, structure, solution behavior, reactivity and biological evaluation of oxidovanadium(iv/v) thiosemicarbazone complexes

The synthesis and characterization of an oxidovanadium(iv) [VIVO(L)(acac)] (1) and of two dioxidovanadium(v) [VVO2(L')] (2) and [VVO2(L)] (2a) complexes of the Schiff base formed from the reaction of 4-(p-fluorophenyl) thiosemicarbazone with pyridine-2-aldehyde (HL) are described. The oxidovanadium(iv) species [VIVO(L)(acac)] (1) was synthesized by the reaction of VIVO(acac)2 with the thiosemicarbazone HL in refluxing ethanol. The recrystallization of [VIVO(L)(acac)] (1) in DMF, CH3CN or EtOH gave the same product i.e. the dioxidovanadium(v) complex [VVO2(L)] (2a); however, upon recrystallization of 1 in DMSO a distinct compound [VVO2(L')] (2) was formed, wherein the original ligand L- is transformed to a rearranged one, L'-. In the presence of DMSO the ligand in complex 1 is found to undergo methylation at the carbon centre attached to imine nitrogen (aldimine) and transformed to the corresponding VVO2-species through in situ reaction. The synthesized HL and the metal complexes were characterized by elemental analysis, IR, UV-Vis, NMR and EPR spectroscopy. The molecular structure of [VVO2(L')] (2) was determined by single crystal X-ray crystallography. The methylation of various other ligands and complexes prepared from different vanadium precursors under similar reaction conditions was also attempted and it was confirmed that the imine methylation observed is both ligand and metal precursor specific. Complexes 1 and 2 show in vitro insulin-like activity against insulin responsive L6 myoblast cells, higher than VIVO(acac)2, with complex 1 being more potent. In addition, the in vitro cytotoxicity studies of HL, and of complexes 1 and 2 against the MCF-7 and Vero cell lines were also done. The ligand is not cytotoxic and complex 2 is significantly more cytotoxic than 1. DAPI staining experiments indicate that an increase in the time of incubation and an increase of concentration of the complexes lead to the increase in cell death.

Asymmetric Synthesis of Gonytolide A: Strategic Use of an Aryl Halide Blocking Group for Oxidative Coupling

The first synthesis of the chromanone lactone dimer gonytolide A has been achieved employing vanadium(V)-mediated oxidative coupling of the monomer gonytolide C. An o-bromine blocking group strategy was employed to favor para- para coupling and to enable kinetic resolution of (±)-gonytolide C. Asymmetric conjugate reduction enabled practical kinetic resolution of a chiral, racemic precursor and the asymmetric synthesis of (+)-gonytolide A and its atropisomer.

Non-innocent probes in direct sonication: Metal assistance in oxidative radical CH functionalization

Direct sonication by means of ultrasound horns constitutes a widely used technique in chemical process technology. However, the direct contact between the metal probe and the reaction mixture does not always leave the chemical system unaffected. In this report, we study the tert-butyl hydroperoxide-mediated trifluoromethylation of heterocyclic structures, and the influence of sonication thereon. Metal leaching is observed during the process and further examined, showing that several metals can interfere significantly with the chemical reaction under study. Notably, vanadium metal was found to increase the reaction rate exceptionally well, rendering it a useful additive for this type of reactions. Ultimately, some mechanistic considerations are offered, to provide more insight into the nature of the catalytic effect of leached metals.

Catalytic dehydrogenative aromatization of cyclohexanones and cyclohexenones

Prompted by the scant attention paid by published literature reviews to the comprehensive catalytic dehydrogenative aromatization of cyclohexa(e)nones, this review describes recent methods developed to-date involving transition-metal-catalyzed oxidative aromatization and metal-free strategies for the transformation of cyclohexa(e)nones to substituted phenols.

Toward a Unified Total Synthesis of the Xiamycin and Oridamycin Families of Indolosesquiterpenes

A unified synthetic strategy toward the oridamycin and xiamycin families of natural products was designed, aiming to access several natural products from a common synthetic intermediate readily prepared from geranyl acetate. Part of this strategy was successfully realized, culminating in the synthesis of oridamycin A and oridamycin B. Key steps include a Mn(III)-mediated oxidative radical cyclization to construct the trans-decalin ring, and a 6π-electrocyclization/aromatization sequence to produce the 2,3-fused carbazole. Oridamycin B was accessed through a late-stage, C-H oxidation that converted the C16 methyl to a hydroxymethyl. A variety of strategies were explored to form a chelated radical intermediate en route to xiamycin A, including enolate SET oxidation, oxo-vanadium oxidation, and atom-transfer cyclization. Unfortunately, none of these strategies provided the desired C16-epimeric trans-decalin. Exploratory studies on photoredox-catalyzed radical cyclizations yielded interesting results, including the formation of a bicyclic lactone arising from oxidative termination of the photoredox-catalyzed radical cyclization, and a double 6-endo cyclization with catalyst loadings as low as 0.01 mol%.

Interaction of [VIV O(acac)2 ] with Human Serum Transferrin and Albumin

[VO(acac)₂ ] is a remarkable vanadium compound and has potential as a therapeutic drug. It is important to clarify how it is transported in blood, but the reports addressing its binding to serum proteins have been contradictory. We use several spectroscopic and mass spectrometric techniques (ESI and MALDI-TOF), small-angle X-ray scattering and size exclusion chromatography (SEC) to characterize solutions containing [VO(acac)₂ ] and either human serum apotransferrin (apoHTF) or albumin (HSA). DFT and modeling protein calculations are carried out to disclose the type of binding to apoHTF. The measured circular dichroism spectra, SEC and MALDI-TOF data clearly prove that at least two VO-acac moieties may bind to apoHTF, most probably forming [V^IV O(acac)(apoHTF)] complexes with residues of the HTF binding sites. No indication of binding of [VO(acac)₂ ] to HSA is obtained. We conclude that V^IV O-acac species may be transported in blood by transferrin. At very low complex concentrations speciation calculations suggest that [(VO)(apoHTF)] species form.

Vapor-phase dehydrogenation of ethylbenzene to styrene over a V2O5/TiO2–Al2O3 catalyst with CO2

A stable V₂O₅/TiO₂–Al₂O₃ catalyst was synthesized for the oxidative dehydrogenation of ethylbenzene to styrene using CO₂ as a mild oxidant.