Vanadate Esters Research Articles

Convergent Protein Phosphatase Inhibitor Design for PTP1B and TCPTP: Exchangeable Vanadium Coordination Complexes on Graphene Quantum Dots

AbstractDevelopment of potent and specific inhibitors of protein tyrosine phosphatase 1B (PTP1B) with desired drug‐like properties is still a challenge. Based on the crystal structures of PTP1B transition state analog consisting of a vanadate peptide, a novel approach is proposed to design PTP1B inhibitors, in which the tyrosyl vanadate ester of a PTP1B peptide mimic (PL1) is stably integrated on the membrane permeable graphene quantum dots (GQDs). The vanadate complexes (GQD‐PL1‐VV) prepared exhibit high potency (Ki = 6 ± 1 × 10−9 m) and selectivity (selectivity index SI >200 for PTP1B versus the T‐cell protein tyrosine phosphatase, TCPTP) in solution and in HepG2 cells. Oral administration of GQD‐PL1‐VV in db/db model mice shows selective PTP1B inhibition in liver and fat tissues and exhibits improved anti‐diabetic activity compared to bis(maltolato)oxovanadium(IV). Moreover, exchange of PL1 to a TCPTP‐specific ligand (PL2) results in potent TCPTP inhibition (Ki = 59 ± 12 × 10−9 m) as expected with SI ≈23 versus PTP1B. Overall, the present results provide a paradigm shift and a general design of phosphatase inhibitors consisting of GQDs, a complexing targeting ligand and vanadium (V) for selective regulation of PTP1B both in vitro and vivo.

ChemInform Abstract: Antidiabetic, Chemical, and Physical Properties of Organic Vanadates as Presumed Transition‐State Inhibitors for Phosphatases

AbstractReview: 183 refs.

Antidiabetic, Chemical, and Physical Properties of Organic Vanadates as Presumed Transition-State Inhibitors for Phosphatases.

Studies of antidiabetic vanadium compounds, specifically the organic vanadate esters, are reviewed with regard to their chemistry and biological properties. The compounds are described from the perspective of how the fundamental chemistry and properties of organic vanadate esters impact their effects as inhibitors for phosphatases based on the structural information obtained from vanadium-phosphatase complexes. Vanadium compounds have been reported to have antidiabetic properties for more than a century. The structures and properties of organic vanadate complexes are reviewed, and the potency of such vanadium coordination complexes as antidiabetic agents is described. Because such compounds form spontaneously in aqueous environments, the reactions with most components in any assay or cellular environment has potential to be important and should be considered. Generally, the active form of vanadium remains elusive, although studies have been reported of a number of promising vanadium compounds. The description of the antidiabetic properties of vanadium compounds is described here in the context of recent characterization of vanadate-phosphatase protein structures by data mining. Organic vanadate ester compounds are generally four coordinate or five coordinate with the former being substrate analogues and the latter being transition-state analogue inhibitors. These studies demonstrated a framework for characterization of five-coordinate trigonal bipyramidal vanadium inhibitors by comparison with the reported vanadium-protein phosphatase complexes. The binding of the vanadium to the phosphatases is either as a five-coordinate exploded transition-state analogue or as a high energy intermediate, respectively. Even if potency as an inhibitor requires trigonal bipyramidal geometry of the vanadium when bound to the protein, such geometry can be achieved upon binding from compounds with other geometries. Desirable properties of ligands are identified and analyzed. Ligand interactions, as reported in one peptidic substrate, are favorable so that complementarity between phosphatase and coordinating ligand to the vanadium can be established resulting in a dramatic enhancement of the inhibitory potency. These considerations point to a frameshift in ligand design for vanadium complexes as phosphatase inhibitors and are consistent with other small molecule having much lower affinities. Combined, these studies do suggest that if effective delivery of potentially active antidiabetic compound such a the organic vanadate peptidic substrate was possible the toxicity problems currently reported for the salts and some of the complexes may be alleviated and dramatic enhancement of antidiabetic vanadium compounds may result.

Synthesis, Structure, and Reactivity of Dinuclear Nickel Amino-Thiophenolate Complexes Bearing Bridging VO2(OH)2–and VO2(OR)2– Coligands

A series of novel mixed ligand dinickel complexes of the type [Ni(II)(2)L(μ-L')](+), where L' is a tetrahedral oxo-alkoxo vanadate (L' = [O(2)V(V)(OR)(2)](-), R = H or alkyl) and L a macrocyclic N(6)S(2) supporting ligand, have been prepared, and their esterification reactivity has been studied. The orthovanadate complex [Ni(2)L(μ-O(2)V(OH)(2))](+) (2), prepared by reaction between [Ni(2)L(μ-Cl)]ClO(4) with Na(3)VO(4) and a phase transfer reagent in CH(3)CN, reacts smoothly with MeOH and EtOH forming the vanadate diesters [Ni(2)L(μ-O(2)V(OMe)(2))](+) (3) and [Ni(2)L(μ-O(2)V(OEt)(2))](+) (4). The dialkyl orthovanadate esters in 3 and 4 are readily transesterified with mono- and difunctional alcohols. Complex 3 can also be generated from 4 by transesterification with MeOH. Complexes 3 and 4 react with diols (ethylene glycol, propylene glycol and diethylene glycol) as well to afford the complexes [Ni(2)L(μ-O(2)V(OH)(OCH(2)CH(2)OH))](+) (5), [Ni(2)L(μ-O(2)V(OCH(2))(2)CH(2))](+) (6), and [Ni(2)L(μ-O(2)V(OCH(2)CH(2))(2)O)] (7). The crystal structures of the tetraphenylborate salts of complexes 3-7 reveal in each case four-coordinate O(2)V(V)(OR)(2)(-) groups bonded in a μ(1,3)-bridging mode to generate trinuclear complexes with a central N(3)Ni(μ-S)(2)(μ(1,3)-O(2)V(OR)(2))NiN(3) core. The stabilization of the four-coordinate V(V)O(2)(OR)(2)(-) moieties is a consequence of both the two-point coordinative fixation to and the steric protection of the bowl-shape binding pocket of the [Ni(2)L](2+) fragment. Cyclic voltammetry experiments reveal that the encapsulated vanadate esters are not reduced in a potential window of -2.0 to +2.5 V vs SCE. The spins of the nickel(II) (S(i) = 1 ions) in 3 are weakly ferromagnetically coupled (J = +23 cm(-1), (H = -2JS(1)S(2))) to produce an S = 2 ground state.

The chemistry of vanadate chelate esters of monoionized aromatic diols and methoxyphenolate

Two ONO donor tridentate Schiff base ligands, two aromatic 1,2-diols and a monomethyl ether derivative of aromatic 1,2-diol are used in this work. The ONO donor ligands are deprotonated forms of N-(1-hydroxyphenyl)salicylaldimine (H 2L 1) and N-(1-hydroxyphenyl)napthaldimine(H 2L 2) (general abbreviation H 2L). The concerned diols are catechol (H 2cat), its 3 , 5 - Bu 2 t derivative (H 2dbcat) and the monomethyl ether derivative of aromatic 1,2-diol is 2-methoxyphenol (HMecat), commonly known as guaiacol. The syntheses of hexa-/penta-coordinated vanadate esters of monoionized aromatic 1,2-diols and 2-methoxyphenolate, with diionized N-salicylidene/ N-napthalidine-α-aminophenol as coligands are described and the reactivities of the compounds are reported. The concerned vanadate esters are [VO(L)(Hcat)], [VO(L)(Hdbcat)] and [VO(L)(Mecat)], where Hcat, Hdbcat and Mecat are monoionised forms of H 2cat, H 2dbcat and HMecat, respectively. The complexes are electroactive in MeCN solution. Molecular oxygen reacts smoothly with the oxovanadium(V) complexes quantitatively, furnishing the corresponding o-quinone. In the presence of the aromatic diol or guaiacol the reaction becomes catalytic. The X-ray structure of [VO(L 2)(Mecat)] has also been reported.

Transition State Analogues for Nucleotidyl Transfer Reactions: Structure and Stability of Pentavalent Vanadate and Phosphate Ester Dianions

The structures and energy of phosphate dimethyl ester and vanadate dimethyl ester have been calculated using B3LYP/TZVP density functional quantum chemical methods and polarized continuum (PCM) and Langevin dipoles solvation models. These calculations were carried out to obtain fundamental information on the ability of vanadate esters to function as transition state analogues for the nucleotidyl transfer reaction catalyzed by DNA polymerases. Base-catalyzed methanolysis of the phosphate and vanadate dimethyl esters were the model reactions examined in this study. The structures of the phosphate and vanadate dimethyl esters and pentavalent intermediates in aqueous solution were optimized and evaluated at the PCM/B3LYP/TZVP level. The three-dimensional free energy surfaces for the studied reactions were determined at the PCM/B3LYP/TZVP//B3LYP/TZVP level. Comparison with experimental structural data obtained from the Cambridge Structural Database and with the observed kinetics of phosphate diester hydrolysis demonstrated that the level of theory chosen for these studies was appropriate. The results showed that structurally and electrostatically the vanadate dimethylester and a five-coordinate nearly trigonal bipyramidal intermediate were reasonable analogues for the parent phosphorus systems. Despite these similarities in structure, the energetics of the two systems were different, and the transition states of the two model reactions were found on different areas of the potential energy surface. When the binding energy of a transition state-DNA polymerase complex was extrapolated to a transition state analogue-DNA polymerase complex, the formation of a simple dianionic pentavalent vanadate ester adduct in the enzyme active site was not found to be sufficiently favorable. This finding suggests that additional stabilization of this adduct is needed before this type of transition state analogue will be likely to yield stable adducts with this class of enzymes. New possible candidates for such complexes are suggested.

Complexation of vanadium(V) oxyanions with hexopyranose- and mannopyranoseuronic acid-containing polysaccharides: stereochemical considerations

Carbohydrates containing galactopyranosyl and mannopyranosyl units with vicinal cis-diols were treated with NaVO 3 in D 2O, and complexation was determined by 51V NMR spectroscopy. Me α-Gal p, Me β-Gal p (3,4- cis-diols), and Me α-Man p (2,3- cis-diol) complexed, but Me β-Man p barely did so. This low degree of complexation also occurred with a β-mannan containing alternate (1 → 3)- and (1 → 4)-linkages and an alginate having β-Man pA blocks. In contrast, branched α-mannans complexed readily, although the 51V resonances for one with side chains terminated with α-Man p-(1 → 3)-α-Man p-(1 → differed from another with only α-Man p-(1 → 2)-α-Man p-(1 → groups. The anomeric configuration of Me α-Gal p and Me β-Gal p, each with 3,4- cis-diols remote from C-1, gave rise to three 51V signals of complexes with similar shifts and proportions. The shifts of a galactomannan with terminal α-Gal p-(1 → 2)-α-Man p- were the same as those with α-Gal p-(1 → 6)-β-Man p- groups, but fewer complexes were formed with the former structure, probably due to greater steric crowding of the vanadate esters. Most of the complexes gave rise to a signal in the δ −515 region, consistent with the dimeric trigonal–bipyramidal structure.

Synthesis and structure of vanadate esters of glycerol and propane-1,3-diol

Complexes of the type [VO(L)(H2gl)] and [VO(L)(Hpd)] have been synthesized in excellent yields by reacting bis(acetylacetonato)oxovanadium(IV) with H2L in the presence of excess glycerol (H3gl) and propane-1,3-diol (H2pd) in methanol–acetone (hydroxyphenylmethylenehydrazones of 4-hydroxy-4-phenylbut-3-en-2-one, salicylaldehyde and 2-hydroxynaphthaldehyde are respectively abbreviated as H2L1, H2L2 and H2L3 and generally as H2L). The crystal structures of [VO(L2)(H2gl)] and [VO(L2)(Hpd)] have revealed tridentate ONO co-ordination by [L2]2– while [H2gl]– and [Hpd]– form five- and six-membered V(O,O) chelate rings respectively. In the distorted octahedral VO5N co-ordination sphere the V–O (alkoxide) bond is 0.5–0.6 A shorter than the V–O (alcoholic) bond which lies trans to the oxo oxygen atom. In [VO(L2)(Hpd)] the lattice consists of dimers held together by O· · ·N hydrogen bonding between the OH group of [Hpd]– in one molecule and the unco-ordinated [L2]2– nitrogen in an adjacent molecule. In the [VO(L2)(H2gl)] lattice such dimers self-assemble into an infinite pattern via additional O· · ·O hydrogen bonding between the unco-ordinated OH group of [H2gl]– of one molecule with the co-ordinated alkoxidic oxygen of the adjacent molecule. In [VO(L)(H2gl)] both the metal site and the [H2gl]– ligand are chiral and the two equally abundant diastereoisomers are present in solution (1H NMR). The 51V chemical shifts are diagnostic of the alkoxidic chelate ring size: the shifts of [VO(L)(H2gl)] being ≈30 ppm downfield from that of [VO(L)(Hpd)]. The complexes have low VO3+–VO2+ reduction potentials.

Comparison of vibrational frequencies of critical bonds in ground-state complexes and in a vanadate-based transition-state analog complex of muscle phosphoglucomutase. Mechanistic implications.

The symmetric stretching frequency of the P-O bonds of the enzymic phosphate group in muscle phosphoglucomutase was measured via 16O/18O Raman difference spectroscopy. This frequency, and its shift on isotopic substitution, is characteristic of a dianionic phosphate ester. The P-O stretching frequency is not detectably altered by the binding of the metal ion activators Mg2+, Zn2+, or Cd2+ nor by the subsequent binding of glucose phosphate. Hence, a binding-induced distortion/polarization of the enzymic phosphate group in the ground state, or enzyme-substrate complex, cannot serve as a rationale for the large value of kcat in the phosphoglucomutase reaction. By contrast, the stretching frequency of the V-O bonds within a vanadate group bound at the same site in the transition-state analog complex involving glucose 1-phosphate 6-vanadate is much lower than for a normal dianionic vanadate. This low V-O stretching frequency is best rationalized in terms of the extensive polarization of all three nonbridging oxygens of the vanadate ester dianion plus the formation of a weak, fifth bond to the vanadium atom. This distortion/polarization of the VO3(2-) group depends on the metal ion activator, since it is largely abolished, and the involvement of the fifth ligand eliminated, by substitution of Li+ for Mg2+ at the metal activation site.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure of vanadate esters of vicinal diols

Structure of vanadate esters of vicinal diols

Substituent effects in organic vanadate esters in imidazole-buffered aqueous solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSubstituent effects in organic vanadate esters in imidazole-buffered aqueous solutionsDebbie C. Crans, Susan M. Schelble, and Lisa A. TheisenCite this: J. Org. Chem. 1991, 56, 3, 1266–1274Publication Date (Print):February 1, 1991Publication History Published online1 May 2002Published inissue 1 February 1991https://doi.org/10.1021/jo00003a060RIGHTS & PERMISSIONSArticle Views118Altmetric-Citations34LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Study of the oxyvanadium constellation in transition-state-analog complexes of phosphoglucomutase and ribonuclease. Structural deductions from electron-transfer spectra

The absorbance peak in the near ultraviolet electron-transfer spectrum of the oxyvanadium constellation in the "transition-state-analogue complexes" obtained by treating the dephospho form of phosphoglucomutase with inorganic vanadate in the presence of either glucose 1-phosphate or glucose 6-phosphate, as described in an accompanying paper [Ray, W. J., Jr., Burgner, J. W., II, & Post, C. B. (1990) Biochemistry (second of four papers in this issue)], is centered at a wavelength of 312 nm. The position of this peak amounts to a change in oscillator frequency of about -5000 cm-1 relative to that of tetrahedral VO4(3-). To provide a rationale for this spectral change, the near ultraviolet spectra of the di- and monoanions of inorganic vanadate and a number of derivatives of these anions are compared with that of vanadium (V) in the enzymic complexes, in terms of both what is observed experimentally and what is expected from crystal field theory. Comparisons in water and in largely anhydrous solvents show that water is not an essential element in the coordination sphere of inorganic vanadate or its mono- or diesters and hence that the coordination number of V(V) in such compounds likely is four. These comparisons also show that loss of solvating water from a 4-coordinate vanadate on binding cannot provide a rationale for the spectra of the enzymic complexes. Other comparisons show that neither the binding of metal ions nor protonation nor the binding of vanadate at a site with an unusually high or an unusually low dielectric constant can provide such a rationale. Further comparisons with vanadates known to be pentacoordinate strongly suggest that the coordination number of V(V) in the transition-state-analogue complexes of phosphoglucomutase does not exceed four. In fact, from the standpoint of crystal field theory the marked red shift observed in the electron-transfer absorbance spectrum of the oxyvanadium constellation in these complexes is more reasonably interpreted in terms of a decreased coordination at vanadium (V), viz., in terms of a weakened bonding between vanadium and one or more of its coordinating oxygens. This decreased coordination could be produced by a physical stretching of the vanadate ester linkage. By contrast, the near ultraviolet spectrum of the transition-state-analogue complex that ribonuclease forms with an adduct of uridine and vanadate [Lindquist, R. N., Lynn, J. L., & Lienhard, G. E. (1973) J. Am. Chem. Soc. 95, 8762] is similar to spectra of pentacoordinate model compounds of vanadium(V).(ABSTRACT TRUNCATED AT 400 WORDS)

Multinuclear NMR study of the interaction of vanadate with mononucleotides, ADP, and ATP.

The interaction of vanadate with 5'-mononucleotides, ADP, ATP, and various molecules containing some of their chemical moieties was studied in aqueous solution in the pH region of 5-9 using proton, 13C, 31P, and 51V nuclear magnetic resonance (NMR) spectroscopy. All the compounds studied formed noncyclic vanadate esters through interaction of monovanadate or divanadate with the hydroxyl groups of the ribose ring. Noncyclic anhydrides were also formed with the phosphate groups of ribose 5-phosphate, the mononucleotides, ADP, ATP, phosphate, pyrophosphate, and tripolyphosphate. In particular, ADP and ATP analogs resulted from AMP (AMPV and AMPV2) and from ADP (ADPV). Cyclic esters of trigonal bipyramidal geometry resulted from the interaction of vanadate with two ribose ring cis hydroxyl groups. AMP, CMP, and UMP formed two such complexes of 1:1 and 1:2 stoichiometries, similar to what has been observed for uridine and other nucleosides. However, 2'-deoxy-AMP does not yield this type of complexes. ADP and ATP also form similar cyclic ester complexes with vanadate, which does not chelate their pyrophosphate and tripolyphosphate moieties. Nevertheless, the separate pyrophosphate (PP) and tripolyphosphate (PPP) ligands form cyclic anhydrides of octahedral geometry with vanadate. However, their binding to vanadate is weaker than that of the ribose ring of nucleotides. Competition experiments between ethylene glycol and phosphate (P), pyrophosphate (PP), or tripolyphosphate (PPP) show that the relative strength of the interaction of these ligands with vanadate is PP greater than ethylene glycol greater than PPP greater than P.

The dependence of vanadate phenyl ester formation on the acidity of the parent phenols

The condensation of vanadate (VO4H21−) with a variety of substituted phenols in aqueous acetone solution has been studied by 51V nuclear magnetic resonance spectroscopy. The formation constants for the various product esters were determined, as were their respective pKa values. Inductive and resonance contributions to vanadate ester formation were investigated using the dual substituent parameter approach. This procedure did not succeed particularly well, although trends were clearly observed. Correlations with Hammett constants were compared with those of the dual parameter approach. The observation of some linear free-energy relationships was interpreted to mean that the inductive and resonance parameters generally ascribed to the various phenols are not directly applicable to vanadate ester formation where d orbitals are probably involved. The results did indicate that resonance effects have a significantly greater influence on ester formation than do inductive effects.

Vanadium(V) oxyanions: interactions of vanadate with 1,1,1-tris(hydroxymethyl)ethane and with the buffer tris(hydroxymethyl)aminomethane

/sup 51/V NMR spectroscopy has been used to study the interactions of vanadate with 1,1,1-tris(hydroxymethyl)ethane (tris-eth) and with the buffer tris(hydroxymethyl)aminomethane. The reaction of vanadate with tris-eth was found to proceed readily at pH 7.5 to give a variety of products. From variation of vanadate and ligand concentration it was possible to obtain the ligand stoichiometry and formation constants for the various products. The products identified included the monoester of vanadate, the mono- and diesters of divanadate, and a binuclear product for which a bipyramidal coordination geometry about each vanadium nucleus was proposed. The reaction of Tris buffer with vanadate was similar to that of tris-eth. Studies at pH 7.6 and 9.0 showed the formation of vanadate esters of tetrahedral coordination, a bis(ligand) product, assigned an octahedral coordination, and, as well, two binuclear products, one with a single Tris ligand and the second with two ligands. The results were consistent with a bipyramidal/tetrahedral mixed anhydride in the first case and a symmetrical bipyramidal anhydride analogous to that formed with tris-eth in the second case. A rather surprising observation was that these two products exhibited a large chemical shift dependence on pH, presumably reflecting the protonation/deprotonation of the amino group ofmore » Tris. Investigation of vanadate in the presence of cis- or trans-1,2-cyclohexanedial and with or without Tris buffer revealed a strong synergistic reaction of tris with the products of the reaction of vanadate with either of the diols. This result suggests that Tris buffer should be used with extreme caution when vanadate/ligand interactions are studied. 14 refs., 6 figs.« less

Interaction of inorganic vanadate with glucose-6-phosphate dehydrogenase. Nonenzymic formation of glucose 6-vanadate.

Inorganic vanadate (Vi) activates catalysis by glucose-6-phosphate dehydrogenase of the oxidation of glucose by NADP+. As the concentration of Glu-6-P dehydrogenase is increased, the rate of the vanadate-activated glucose oxidation becomes less sensitive to increases in enzyme concentration. The rate of glucose oxidation in the absence of Vi increases linearly with Glu-6-P dehydrogenase concentration. These results are interpreted in terms of nonenzymic formation of glucose 6-vanadate. At high enzyme concentration, vanadate ester formation becomes partially rate-limiting, and extrapolation to infinite Glu-6-P dehydrogenase concentration allows determination of the second order rate constant for formation of the ester. In separate experiments designed to test the proposed mechanism, it was found that Vi, at concentrations at which it strongly activates catalysis by Glu-6-P dehydrogenase of glucose oxidation, has no effect on the rates of oxidation of glucose 6-phosphate or 6-deoxyglucose catalyzed by Glu-6-P dehydrogenase. Sulfate, which is known to activate glucose oxidation and to inhibit glucose 6-phosphate oxidation, strongly activates 6-deoxyglucose oxidation. These experiments show that the 6-hydroxyl group of glucose is essential for the observed activation by Vi and are also consistent with the formation of glucose 6-vanadate. Also, the rate of the sulfate-activated glucose oxidation increases linearly with Glu-6-P dehydrogenase concentration. These results are consistent with the proposed mechanism for sulfate activation which involves sulfate binding to the enzyme (Anderson, W. B., Horne, R. N., and Nordlie, R. C. (1968) Biochemistry 7, 3997-4004). The second order rate constant calculated for formation of glucose 6-vanadate at pH 7.0 is 2.4 M-1 s-1. The corresponding values for glucose 6-phosphate and glucose 6-arsenate formation are approximately 9 X 10(-11) M-1 s-1 and 6.3 X 10(-6) M-1 s-1 (Lagunas, R. (1980) Arch. Biochem. Biophys. 205, 67-75).

Metal catalysis in oxidation by peroxides. Part 10. On the nature of the peroxovanadium(V) species in non-aqueous solvents

The equilibrium formation of mono- and diperoxovanadium(V) compounds from vanadate esters and hydrogen peroxide in ethanol and dioxane—ethanol mixtures has been studied by electron spectroscopy. The acid—base behaviour of the peroxo species identified by the spectroscopic investigation was studied potentiometrically. The results show that in ethanol a monoperoxo species, which behaves as a weak acid, is formed at low hydrogen peroxide concentrations and that such a species is transformed into a diperoxo compound, which behaves as a strong acid, at higher [H 2O 2] 0. In dioxane—ethanol, on the other hand, only the monoperoxo species can be detected, even at high hydrogen peroxide concentrations. The consistency of these findings with the outcome of previous kinetic investigations is discussed.

Metal catalysis in oxidation by peroxides. Part 3. Significance of ligand exchange and acid–base equilibria of vanadium(V) catalyst in oxidation by t-butyl hydroperoxide in alcohols

Kinetic and spectroscopic evidence suggests that, in alcoholic solvents (ROH) and in the presence of t-butyl hydroperoxide, bis(acetylacetonato)oxovanadium(IV)[VO(acac)2] is rapidly converted to vanadate esters VO(OR)3. These vanadium(V) triesters undergo acid–base equilibria leading to anionic or cationic species depending on the acidity of the medium. Kinetic studies on the vanadium(V) catalysed oxidation of di-n-butyl sulphide by t-butyl hydroperoxide in methanol, ethanol, and propan-2-ol, respectively, at 25° indicate that the most active catalyst is the neutral species VO(OR)3. The significant depression of oxidation rates observed when vanadium anion species are present, even at low concentration, suggests that vanadium(V) species might be involved in aggregation phenomena which become more significant as the acidity decreases. Thus, the seemingly large differences observed in the rates of catalysed oxidation in the three alcohols mentioned stem from changes in the position of acid–base equilibria involving the catalyst species in the three solvents. Indeed, the dependence of oxidation rates upon the medium acidity shows that rather small differences in rates are found in the range where the neutral catalyst ester VO(OR)3 is the dominant species.

ChemInform Abstract: USE OF OXO‐METALLIC DERIVATIVES IN ISOMERIZATION. REACTIONS OF UNSATURATED ALCOHOLS

AbstractDie Isomerisierung von α‐acetylenischen und α‐äthylenischen Alkoholen zu Carbonylderivaten und Allylalkoholen gelingt in einer Stufe mit Sauerstoff‐Derivaten von Metallen.

Use of oxo-metallic derivatives in isomerisation: Reactions of unsaturated alcohols

The isomerisation of α-acetylenic alcohols to ethylenic carbonyl derivatives and the isomerisation of α-ethylenic alcohols to the corresponding allyl alcohols may be effected in good yields in a single step using oxometallic derivatives. The vanadate esters are particularly efficient. Mechanistic explanations are given.