Vanadium in Modern Organic Synthesis

Aerobic oxidation of thiols to disulfides catalyzed by trichlorooxyvanadium

Thiols were converted into disulfide by the aerobic oxidation catalyzed by trichlorooxyvanadium in the presence of molecular sieves 3A.

Sterically Controlled Esterification on Bis(μ-hydroxo) Dioxovanadium Site in γ-H2SiV2W10O404-

The esterification of a bis(mu-hydroxo) dioxovanadium site in divanadium-substituted silicotungstate, gamma-H(2)SiV(2)W(10)O(40)(4)(-), with alcohols is sterically controlled: The secondary and tertiary alcohol esters are hardly formed (equilibrium constant < 0.01), and a large equilibrium constant of 75 is observed for the reaction with methanol.

Vanadium-catalyzed sulfenylation of indoles and 2-naphthols with thiols under molecular oxygen

Vanadium oxyacetylacetonate [VO(acac)(2)] works as a catalyst for the direct synthesis of 3-sulfanylindoles from indoles and thiols under an atmospheric pressure of molecular oxygen as a reoxidant. For example, the reaction of 2-phenylindole with benzenethiol in the presence of a catalytic amount of VO(acac)(2), potassium iodide, and 2,6-di-tert-butyl-p-cresol in chlorobenzene under molecular oxygen proceeds to afford 2-phenyl-3-(phenylsulfanyl)indole in 86% yield. This catalytic system can also be applied to 2-naphthols instead of indoles to give the corresponding 1-sulfanyl-2-naphthols in up to 57% yield.

Mononuclear and Dinuclear Monoperoxovanadium(V) Complexes with a Hetero Ligand. 1.1 Self-Decomposition Reaction, Detection of Reactive Oxygen Species, and Oxidizing Ability

A mononuclear peroxovanadium(V) complex with histamine-N,N-diacetate (histada), K[VO(O(2))(histada)], and a dinuclear peroxovanadium(V) complex with 2-oxo-1,3-diaminopropane-N,N,N',N'-tetraacetate (dpot), Cs(3)[(VO)(2)(O(2))(2)(dpot)], were prepared and characterized. The self-decomposition reaction was examined for these peroxovanadium(V) complexes as well as for K[VO(O(2))(cmhist)] (cmhist = N-carboxymethylhistidinate). The reaction profiles depicted by the absorbance change in the UV-vis spectrum show a sigmoid shape with an induction period. The induction period is reduced by the addition of acid, fluoride, thiocyanate, VO(2+), VO(2)(+), and trolox compared to the solution containing perchlorate. On the other hand, the induction period was elongated by the addition of chloride, bromide, and 2-tert-butyl-p-cresol. These behaviors are discussed on the basis of a radical chain mechanism. The self-decomposition reactions have also been followed by the (1)H and (51)V NMR and EPR spectra. These spectral studies as well as the UV-vis spectral study indicate that vanadium(V) is partly reduced to vanadium(IV) in the self-decomposition process. The histada complex yields a mixed-valence dinuclear complex in a concentrated solution, and the dpot complex yields a mixed-valence tetranuclear complex. The reduction of vanadium ion suggests that the peroxo ligand may act as a reducing agent. In order to know the fate of the peroxo ligand, we tried to detect superoxide anion and hydroxyl radical, which were anticipated to be produced in the self-decomposition process. The formation of superoxide anion was spectrophotometrically confirmed using two independent methods, including the reduction of cytochrome c and the reduction of sodium 4-[3-(iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate (WST-1). The formation of hydroxyl radical was confirmed by an EPR spin trapping technique. The oxidizing abilities of the peroxovanadium(V) complexes toward bovine serum albumin (BSA) were also evaluated. In the protein carbonyl assay, it was found that the total amount of protein carbonyl in BSA was increased by the reaction with the peroxovanadium complexes in the concentration-dependent manner. In addition, the oxidation of sulfhydryl group in BSA induced by the peroxovanadium complexes was confirmed.

Vanadium-catalyzed selenide oxidation with in situ[2,3] sigmatropic rearrangement (SOS reaction): scope and asymmetric applications

A vanadium-catalyzed method for the oxidation of prochiral aryl, allylic selenides with tandem [2,3] sigmatropic rearrangement has been developed. This protocol has been screened on a series of substrates to test for its generality and effectiveness. The applicability of this methodology to the synthesis of enantiomerically enriched allylic alcohols has been studied on a series of chiral oxazole-containing systems with a diastereomeric ratio (d.r.) of up to 85 : 15. The chiral transfer observed in the allyl alcohol products is the result of a net 1,9- and/or 1,10-induction. Finally, the first example of a selenium-oxygen nonbonding interaction in oxazole-containing selenide appears to have been observed via X-ray crystal analysis.

DNA modification by oxovanadium(IV) complexes of salen derivatives

Oxovanadium(IV) complexes of hydroxysalen derivatives have been prepared and tested as DNA reactive agents. The nuclease activity has been investigated under oxidative or reducing conditions, on the basis of the various oxidation states of vanadium: V(III), V(IV) and V(V). In the absence of an activating agent, none of the compounds tested was able to induce cleavage of DNA, whereas in the presence of mercaptopropionic acid (MPA) or Oxone the four complexes induced DNA modifications. Under both conditions, the para-hydroxy complex was found to be the most active compound. Reaction of these salen complexes with DNA occurs essentially at guanine residues and is more efficient in the presence of Oxone than under reducing conditions. The extent of Oxone-mediated DNA oxidation by the four vanadyl complexes was clearly superior to VOSO(4) and was observed without piperidine treatment. EPR studies provided information on the reactive metal-oxo species involved under each conditions and a mechanism of reaction with DNA is discussed.

New protocol for oxidation in water: micellar oxidation systems composed of novel amphiphilic hydroperoxides

A series of novel amphiphilic hydroperoxides, alpha-alkoxyalkyl hydroperoxides (alpha-AHPs) and their related hydroperoxides, were designed and prepared with the intention of developing new oxidizing agents bearing a micelle-forming character in water. After their fundamental physical and interfacial properties were elucidated, both oxidation of benzyl sulfide and epoxidation of geraniol promoted by these hydroperoxides were investigated in detail under various conditions. Effective oxidation ofbenzyl sulfide to the corresponding sulfoxide was achieved in aqueous micellar systems composed of alpha-AHPs la-d and a catalytic amount of MoO2(acac)2 at 30 degrees C. Up to 100% conversion was observed under the optimum conditions. In the case of epoxidation of geraniol in water, the corresponding 2,3-epoxide was selectively formed in good yields. Because the conversion of each substrate in the micellar systems was higher than that in nonmicellar media, the solubilization of substrates into micelles was certainly effective in promoting oxidations of insoluble substrates in aqueous media. Micellar oxidation systems composed of novel amphiphilic hydroperoxides afford a new protocol in order to derive safer conditions under which these reactions may be carried out.

Biphenyl derived Schiff-base vanadium(v) complexes with pendant OH-groups—structure, characterization and hydrogen peroxide mediated sulfide oxygenation

A series of mononuclear oxovanadium(v) complexes of tridentate Schiff bases HL(1)-HL(4) and H(2)L(5)-H(2)L(8) derived from 6-phenylsalicylaldehyde and 6-(2-hydroxyphenyl)salicylaldehyde and four different amines was synthesized. The systematically selected ligands contain aliphatic or aromatic nitrogen, or alkoxy- and phenoxy-oxygen as third donor atom. The complexes were characterised by spectroscopic methods in solution and the solid state. Single-crystal X-ray analyses were performed with VO(2)L(1)(), VO(2)L(3)x1/2EtOH (), VO(2)L(4)(), VO(OiPr)L(7)xiPrOH, VO(OiPr)L(8) and H(2)L(8). For all compounds the vanadium(v) cores contain distorted tetragonal pyramidal geometry around the dioxo- and oxovanadium site at which the N(2)O- and NO(2)-donor ligands bind equatorially. Complexes and display intramolecular hydrogen bonding of the pendant hydroxyphenyl group to a coordinated oxygen trans to a nitrogen atom and therefore serve as suitable models for the native site of vanadium dependent haloperoxidases. Variable-temperature (1)H NMR spectra revealed significant hydrogen bond interaction in acetonitrile solution. In situ prepared catalysts are active for hydrogen peroxide mediated oxygenation of ethyl phenyl sulfide and showed complete conversion of the substrate to ethyl phenyl sulfoxide, together with small amounts of the corresponding sulfone, as detected by GC/MS after 10 min. The complex of H(2)L(7) turned out to be most efficient while HL(1) and HL(2) were completely inactive. Catalysis is supported by the pendant OH group in the complex of HL(3), the catalyst is twice as active as the complex of HL(4).

Novel highly regioselective VO(acac)2/TBHP mediated oxidation of o-alkenyl phenols to o-hydroxybenzyl ketones

A novel mild methodology for the preparation of o-hydroxybenzyl ketones is described starting from o-alkenyl phenols and based on the VO(acac)(2)/TBHP (2 mol %/1.2 equiv) system. VO(acac)(2) first catalyzes the epoxidation of o-alkenyl phenols and then the rearrangement of the epoxyphenols to ketones via the selective benzylic C-O cleavage and 1,2 hydride migration. The protocol has also been applied to set up a useful and easy one-pot conversion of o-alkenyl phenols to benzo[b]furans by means of the sequential addition of TFA, after the generation of the intermediate o-hydroxybenzyl ketones.

Kinetic-catalytic determination of vanadium(IV) using methyl orange-bromate redox reaction

Determination of V(IV) based on its catalytic effect on the reaction between Methyl Orange and bromate in thepresence of citric acid was studied. The calibration curve obtained by fixed-time method was linear in the range of 2.5-300 ng ml(-1). By use of slope method, a calibration curve containing two linear portions were obtained. Using fixed-time and slope methods, we obtained detection limits of 0.8 and 1.5 ng ml(-1), respectively. Fe2+, As(III), V(V) and Hg2+ interfered. The method was successful for analysis of water samples.

Potent dual anti-HIV and spermicidal activities of novel oxovanadium(V) complexes with thiourea non-nucleoside inhibitors of HIV-1 reverse transcriptase

We have previously demonstrated that tetrahedral bis(cyclopentadienyl)vanadium(IV) complexes and square pyramidal oxovanadium(IV) complexes of vanadium are rapid and selective spermicidal agents at low micromolar concentrations. This study investigated the potential utility of oxovanadium in combination with thiourea non-nucleoside inhibitors (NNIs) of HIV-1 reverse transcriptase (RT) for the development of an effective dual-function anti-HIV spermicide. Two rationally designed substituted phenyl-ring containing pyridyl thiourea NNIs, N-[2-(2-chlorophenethyl)]-N(')-[2-(5-bromopyridyl)-thiourea) [1] and N-[2-(2-methoxyphenethyl)]-N(')-[2-(pyridyl)-thiourea [2] that exhibited subnanomolar IC(50) values against the drug-sensitive, drug-resistant, and multidrug-resistant strains of HIV-1, were complexed with oxovanadium. The oxovanadium-thiourea [OVT] NNIs, C(29)H(27)Br(2)Cl(2)N(6)O(2)S(2)V [3], and C(31)H(35)N(6)O(4)S(2)V [4], were synthesized by reacting VOSO(4), a V(IV) compound, with the corresponding deprotonated thiourea NNI compounds as ligands. Elemental analysis showed that each OVT-NNI used two thiourea molecules as ligands. The existence of the Vz.dbnd6;O bond (968cm(-1)) was confirmed by IR spectroscopy. No d-d bands were observed in the visible spectra of OVT-NNIs and their EPR spectra were featureless, indicating that the vanadium centers were oxidized to V(V). The new OVT-NNIs as well as their thiourea NNI ligands were evaluated for (i) anti-HIV activity using the cell-free recombinant RT inhibition assays, (ii) cellular HIV replication assays, (iii) spermicidal activity against human sperm by computer-assisted sperm analysis (CASA), and (iv) cytotoxicity against normal human female genital tract epithelial cell using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) dye-reduction assays. Similar to thiourea NNIs 1 and 2, the OVT-NNIs 3 and 4, exhibited potent anti-HIV activity with submicromolar IC(50[p24]) values (0.08 and 0.128 microM, respectively) and submicromolar IC(50[RT]) values (2.1 and 0.87 microM, respectively). Notably, OVT-NNIs were spermicidal against human sperm at low micromolar concentrations (IC(50)=34 and 55 microM, respectively) and induced rapid sperm immobilization (T(1/2)=12 and 240s) when compared with their respective thiourea NNI ligands (EC(50)=>400 microM and T(1/2)=>180min). Moreover, OVT-NNIs displayed high selectivity indices against normal female genital tract epithelial cells (IC(50) values >250 microM) when compared to the detergent-type spermicide, nonoxynol-9, which was cytotoxic at spermicidal concentrations (IC(50) values 32-64 microM). This is the first report on the dual anti-HIV and spermicidal activities of a vanadium/oxovanadium complex. Our discovery of potent anti-HIV and rapid spermicidal activities of OVT-NNIs may be useful for the development of an effective and safe vaginal anti-HIV spermicide for women who are at high risk for acquiring HIV/AIDS by heterosexual transmission.

VO(acac)(2)-catalyzed oxidative coupling reactions of phosphonium salts

A novel eco-safer protocol for the preparation of symmetric or asymmetric olefins directly from phosphonium salts under oxygen atmosphere in the presence of VO(acac)(2) (1.0 mol%) was developed.

Catalytic nucleophilic acyl substitution of anhydrides by amphoteric vanadyl triflate

[reaction--see text] Among four vanadyl species examined, vanadyl triflate was the most efficient catalyst to facilitate nucleophilic acyl substitution of anhydrides with a myriad array of alcohols, amines, and thiols in high yields and high chemoselectivity. By using mixed-anhydride technique, one can achieve oleate and peptide syntheses. In marked contrast to common metal triflates, the amphoteric character of the V=O unit in vanadyl species was proven to be responsible for the catalytic profile in this process.

Syntheses, structures, stability, and insulin-like activities of peroxovanadium(V) complexes with a heteroligand

Several peroxovanadium(V) complexes were prepared with a tripodal or a quasi-tripodal tetradentate ligand. The structures of K(2)[VO(O(2))(nta)].2H(2)O and K[VO(O(2))(DL-cmhist)].H(2)O have been determined by X-ray crystallography (nta, nitrilotriacetate; cmhist, N-carboxymethylhistidinate). The structure of Cs[VO(O(2))(pda)].2H(2)O (pda, N-pyridylmethyliminodiacetate) has been estimated to be similar to that of K[VO(O(2))(DL-cmhist)].H(2)O. Each complex anion in these compounds adopts a distorted pentagonal bipyramidal structure, which is typical for heptacoordinate oxoperoxovanadium(V) complexes. The peroxide ion binds in a side-on fashion to the vanadium(V) center in the pentagonal plane. The peroxide anion in the cmhist complex dissociates rather easily in an acidic solution (pH approximately 3), while that in the other complexes stays intact under similar conditions. The in vitro insulin mimetic effect of the peroxovanadium(V) complexes has been evaluated by the inhibitory effect on free fatty acid (FFA) release in isolated rat adipocytes treated with epinephrine. The cmhist complex is effective, while the others are almost totally ineffective.

Vanadocene-mediated in vivo male germ cell apoptosis

Vanadocenes are potent apoptosis-inducing cytotoxic agents against human testicular cancer cells in vitro. The present study investigated the ability of four vanadocenes-vanadocene diazide (VDA), vanadocene dicyanate (VDCN), vanadocene dioxycyanate (VDOCN), and vanadocene monochloro oxycyanate (VDCO)-to induce male germ cell apoptosis in vivo in mouse testes by repetitive intratesticular injection of vanadocenes (7.5 mg/kg/testis) for 28 days. Germ cell loss in vivo was measured by epididymal sperm count, testes weights, and histologic evaluation of the testes. Repetitive intratesticular injection of vanadocenes led to decreased sperm counts and reduced testicular weights. Histopathological examination revealed seminiferous tubular atrophy, inhibition of spermatogenesis, and the preferential loss of maturing and elongated spermatids. In situ evaluation by the terminal deoxynucleotidyl transferase-mediated FITC-deoxyuridine triphosphate nick-end labeling (TUNEL) of seminiferous tubule cross sections and laser confocal microscopy showed characteristic apoptotic cells identified primarily as pachytene spermatocytes delineating the periphery of the seminiferous tubules. The ability of vanadocenes to induce germ cell apoptosis in vivo may have potential utility in the treatment of testicular seminomas in humans.

Redox reactions via vanadium-induced electron transfer

One-electron reduction and oxidation induced by vanadium complexes are demonstrated to be useful in oxidative and reductive transformations of carbonyl compounds. The redox interaction between vanadium complexes and redox-active ligands is achieved with coenzyme PQQ and polyanilines that afford the corresponding redox systems.

Synthesis and aqueous solution properties of multinuclear oxo-bridged vanadium(IV/V) complexes

Reaction of the multifunctional phenolic ligands 2,5-bis[N,N-bis(carboxymethyl)aminomethyl]hydroquinone (H6cahq), 2,2'-bis[N,N-bis(carboxymethyl)aminomethyl]-4,4'-isopropylidenediphen ol(H6capd),2,2',2''-tris[N,N-bis(carboxymethyl)aminomethyl]-1,1 ,1-tris(4-hydroxyphenyl)ethane (H9catp) and the monofunctional 2-[N,N-bis(carboxymethyl)aminomethyl]-4-carboxyphenol (H3cacp), with VOSO4 and NaVO3 affords the oxo-bridged mixed-valence vanadium(IV/V) Na6[(VO)4(mu-O)2(mu-cahq)2] x Na2SO4 x 20H2O (1), HnNa(3-n)[(VO)2(mu-O)(mu-cacp)2] (2), HnNa(3-n)[(VO)4(mu-O)2(mu-capd)2] (3), HnNa(9-n)[(VO)6(mu-O)3(mu3-catp)2] (4). In addition to the synthesis, we report the infrared, magnetic, optical and electrochemical properties of these complexes. The hydrolytic stability at different pH values was also investigated using visible spectroscopy.

Chemistry and insulin-mimetic properties of bis(acetylacetonate)oxovanadium(IV) and derivatives

The syntheses and the solid state structural and spectroscopic solution characterizations of VO(Me-acac)2 and VO(Et-acac)2 (where Me-acac is 3-methyl-2,4-pentanedionato and Et-acac is 3-ethyl-2,4-pentanedionato) have been conducted since both VO(acac)2 and VO(Et-acac)2 have long-term in vivo insulin-mimetic effects in streptozotocin-induced diabetic Wistar rats. X-ray structural characterizations of VO(Me-acac)2 and VO(Et-acac)2 show that both contain five-coordinate vanadium similar to the parent VO(acac)2. The unit cells for VO(Et-acac)2 and VO(Me-acac)2 are both triclinic, P1, with a = 9.29970(10) A, b = 13.6117(2) A, c = 13.6642(2) A, alpha = 94.1770(10) degrees, beta = 106.4770(10) degrees, gamma = 106.6350(10) degrees for VO(Et-acac)2 and a = 7.72969(4) A, b = 8.1856(5) A, c = 11.9029(6) A, alpha = 79.927(2) degrees, beta = 73.988(2)degrees, gamma = 65.1790(10)degrees for VO(Me-acac)2. The total concentration of EPR-observable vanadium(IV) species for VO(acac)2 and derivatives in water solution at 20 degreesC was determined by double integration of the EPR spectra and apportioned between individual species on the basis of computer simulations of the spectra. Three species were observed, and the concentrations were found to be time, pH, temperature, and salt dependent. The three complexes are assigned as the trans-VO(acac)2.H2O adduct, cis-VO(acac)2.H2O adduct, and a hydrolysis product containing one vanadium atom and one R-acac- group. The reaction rate for conversion of species was slower for VO(acac)2 than for VO(malto)2, VO(Et-acac)2, and VO(Me-acac)2; however, in aqueous solution the rates for all of these species are slow compared to those of other vanadium species. The concentration of vanadium(V) species was determined by 51V NMR. The visible spectra were time dependent, consistent with the changes in species concentrations that were observed in the EPR and NMR spectra. EPR and visible spectroscopic studies of solutions prepared as for administration to diabetic rats documented both a salt effect on speciation and formation of a new halogen-containing complex. Compound efficacy with respect to long-term lowering of plasma glucose levels in diabetic rats traces the concentration of the hydrolysis product in the administration solution.

Spermicidal activity of oxovanadium(IV) complexes of 1, 10-phenanthroline, 2,2'-bipyridyl, 5'-bromo-2'-hydroxyacetophenone and derivatives in humans

We have recently reported that tetrahedral metallocene complexes containing vanadium(IV) (vanadocene) have potent spermicidal activity against human sperm. The spermicidal activity was dependent on vanadium(IV) as the central metal ion within the bis-cyclopentadienyl (Cp2)-metal complex, but the variation of diacido groups and/or replacement with bidentate ligands coordinated to the Cp2-vanadium(IV) moiety also significantly modulated the spermicidal potency. To assess the structure-activity relationship between vanadocenes and other coordination complexes of vanadium(IV), a set of 11 oxovanadium(IV) complexes with different geometrical configurations were synthesized and evaluated for spermicidal activity by computer-assisted sperm analysis. These complexes included mono and bis ancillary ligands, 1,10-phenanthroline (phen): [VO(phen), VO(phen)2, VO(Me2-phen), VO(Me2-phen)2, VO(Cl-phen), and VO(Cl-phen)2]; 2,2'-bipyridyl (bipy): [VO(bipy), VO(bipy)2, VO(Me2-bipy), and VO(Me2-bipy)2], linked via nitrogen atoms; and 5'-bromo-2'-hydroxyacetophenone (acph): [VO(Br,OH-acph)2], linked via oxygen donor atoms. All 11 oxovanadium(IV) complexes elicited concentration-dependent spermicidal activity at micromolar concentrations (EC50 values: 5.5-118 microM). The bis-phenanthroline complex of oxovanadium(IV), VO(Cl-phen)2, was the most active, and the mono bipyridyl complex, VO(bipy), was the least active; the order of efficacy was VO(Cl-phen)2 > VO(phen)2 > VO(Br,OH-acph)2 > VO(Me2-phen) > VO(bipy)2 > VO(phen) > VO(Cl-phen) > VO(Me2-phen)2 > VO(Me2-bipy)2 > VO(Me2-bipy) > VO(bipy). The neutral complex, VO(Br, OH-acph)2, induced rapid sperm immobilization (T1/2 = 38 sec). The sperm-immobilizing activity of mono- and bis-ligated oxovanadium(IV) complexes was irreversible, since the treated sperm underwent apoptosis, as determined by the flow cytometric quantitation of mitochondrial membrane potential, surface Annexin V binding assay, and in situ DNA nick-end labeling of sperm nuclei. The percentages of apoptotic sperm quantitated by the flow cytometric assay correlated well with the spermicidal potency of oxovanadium(IV) complexes. These results provide unprecedented evidence that the spermicidal and apoptosis-inducing activities of vanadium(IV) complexes are determined by the oxidation state of vanadium as well as their geometry. Because of its rapid and potent sperm-immobilizing activity, the bromo-hydroxyacetophenone complex, [VO(Br,OH-acph)2], may be useful as a contraceptive agent.