Dendrimeric organochalcogen catalysts for the activation of hydrogen peroxide: origins of the "dendrimer effect" with catalysts terminating in phenylseleno groups

Several scenarios were evaluated to explain the large "dendrimer effect" observed in the bromination of cyclohexene with H(2)O(2) and NaBr catalyzed by the addition of Frechét-type dendrimers terminating in -O(CH(2))(3)SePh groups. Although phenylseleninic acid was an efficient catalyst for the oxidation of NaBr with H(2)O(2), first-order rate constants for the selenoxide elimination were too small to produce PhSeO(2)H at a rate sufficient to explain the rates of catalysis and no dendrimer effect was observed in the rates of selenoxide elimination. An induction period was observed using 1-SePh as a catalyst for the oxidation of Br(-) with H(2)O(2). The addition of preformed selenoxide 1-Se(=O)Ph gave immediate catalysis with no induction period. However, rates of oxidation of the selenides with H(2)O(2) under homogeneous or biphasic conditions or with t-BuOOH under homogeneous conditions were too slow to account for the rates of catalysis, and no dendrimer effect was observed in the rates of oxidation. The primary oxidant for converting selenides to selenoxides was "Br(+)" produced initially by the uncatalyzed background reaction of H(2)O(2) with NaBr and then produced catalytically following formation of selenoxide groups. Autocatalysis is observed, and the rate of oxidation increases with the number of SePh groups. Autocatalysis is the source of the large dendrimer effect observed with the SePh series of catalysts.     

l-Isofucoselenofagomine and derivatives: dual activities as antioxidants and as glycosidase inhibitors

The synthesis of a series of l-fuco-configured selenosugars, isosters of the potent glycosidase inhibitor isofucofagomine has been accomplished by a double nucleophilic displacement of a dimesylated derivative with selenide anion generated in situ. Se-Alkylation and oxidation of the corresponding selenane afforded a selenonium and a selenoxide, respectively. The biological activities of such compounds have been evaluated, finding a dual activity caused by the presence of the selenium atom: the selenane exerted a good glutathione peroxidase mimicry by efficiently scavenging H₂O₂ in the presence of thiols, whereas the stable selenoxide derivative was found to be the first example of a selenosugar acting as a good α-l-fucosidase inhibitor.     

Stabilizing effect of intramolecular lewis base toward racemization of optically active selenoxides

2‐(Methylchalcogenomethyl)diphenyl selenoxides 1 and 2‐{2′‐(N,N‐dimethylamino)ethyl}phenyl alkyl (or aryl) selenoxides 2 , which were expected to be stabilized toward racemization by intramolecular coordination, were synthesized and optically resolved into their enantiomers on an optically active column using high‐performance liquid chromatography. Relationship between the absolute configurations and the chiroptical properties of the enantiomers was clarified by comparing with those of sulfur analogues. Stabilities toward racemization of optically active selenoxides 1a and 1b were nearly equal to that of 2‐{(N,N‐dimethylamino)methyl}diphenyl selenoxide and mesityl phenyl selenoxide. The rates of racemization for optically active selenoxides 2 were found to be faster than that of 2‐{(N,N‐dimethylamino)methyl}phenyl alkyl (or aryl) selenoxides. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:301–311, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20299     

Mechanistic studies of the tellurium(II)/tellurium(IV) redox cycle in thiol peroxidase-like reactions of diorganotellurides in methanol

Di-n-hexyl telluride (2), di-p-methoxyphenyl telluride (3), and (S)-2-(1-N,N-dimethylaminoethyl)phenyl phenyl telluride (4) catalyzed the oxidation of PhSH to PhSSPh with H(2)O(2) in MeOH. Telluride 2 displayed greater rate acceleration than the diaryltellurides 3 and 4 as determined by the initial velocities, v(0), for the rate of appearance of PhSSPh determined at 305 nm by stopped-flow spectroscopy. Rate constants for the oxidation of tellurides 2-4 (k(ox)), rate constants for the introduction of PhSH as a ligand to the Te(IV) center (k(PhSH)) of oxidized tellurides 5-7, and thiol-independent (k(1)) and thiol-dependent (k(2)) rate constants for reductive elimination at Te(IV) in oxidized tellurides 5-7 were determined using stopped-flow spectroscopy. Oxidation of the Te atom of the electron-rich dialkyl telluride 2 was more rapid than oxidation of diaryl tellurides 3 and 4. The dimethylaminoethyl substituent of 4, which acts as a chelating ligand to Te(IV), did not affect k(ox). Values of k(PhSH) for the introduction of PhSH to oxidized dialkyl tellurane 5 and oxidized diaryl tellurane 6 were comparable in magnitude, while the chelating dimethylaminoethyl ligand of oxidized telluride 7 diminished k(PhSH) by a fator of 10(3). Reductive elimination by both first-order, thiol-independent (k(1)) and second-order, thiol-dependent (k(2)) pathways was slower from dialkyl Te(IV) species derived from 2 than from diaryl Te(IV) species derived from 3. The chelating dimethylaminoethyl ligand of Te(IV) species derived from 4 diminished k(1) by a factor of 50 and k(2) by a factor of 3 (relative to the 3-derived species).     

Mechanistic investigations on the efficient catalytic decomposition of peroxynitrite by ebselen analogues

In this study, ebselen and its analogues are shown to be catalysts for the decomposition of peroxynitrite (PN). This study suggests that the PN-scavenging ability of selenenyl amides can be enhanced by a suitable substitution at the phenyl ring in ebselen. Detailed mechanistic studies on the reactivity of ebselen and its analogues towards PN reveal that these compounds react directly with PN to generate highly unstable selenoxides that undergo a rapid hydrolysis to produce the corresponding seleninic acids. The selenoxides interact with nitrite more effectively than the corresponding seleninic acids to produce nitrate with the regeneration of the selenenyl amides. Therefore, the amount of nitrate formed in the reactions mainly depends on the stability of the selenoxides. Interestingly, substitution of an oxazoline moiety on the phenyl ring stabilizes the selenoxide, and therefore, enhances the isomerization of PN to nitrate.     

Experimental and theoretical evidence for cyclic selenurane formation during selenomethionine oxidation

The oxidation products of selenomethionine (SeMet) have been studied via experimental (77)Se NMR and theoretical (77)Se chemical shifts. Four signals are observed: a diastereomeric pair of selenoxides at 840 ppm and two unidentified resonances at 703 and 716 ppm. Theoretical DeltaG and chemical shifts suggest the 703 and 716 ppm resonances correspond to hypervalent selenium heterocycles, called selenuranes, formed by reaction with the amine or acid group of the amino acid and the selenoxide. To identify which of these selenuranes is formed, the amine and acid groups were individually protected. The N-formyl SeMet formed only the selenoxide pair at 840 ppm. The oxidized SeMet methyl ester produced signals at 703 and 716 ppm which are assigned as the Se-N selenurane.     

Evidence of an equilibrium between selenides and osmium(VIII) reagents and selenoxides and osmium(VI) reagents

Driving the equilibrium between selenides and osmium(VIII) reagents with selenoxides and osmium(VI) by a subsequent reaction (rearrangement of allyl selenoxides to allyl alcohols or addition of osmium(VIII) species on C=C double bonds) to one side, allows the transformation of methyl geranyl selenides to linalool and of methyl citronellyl selenoxide to 6,7-dihydroxy citronellyl selenide.     

Selective deoxygenation of aryl selenoxides by triaryl phosphites. Evidence for a concerted transformation

Triaryl phosphites selectively reduce aryl selenoxides to selenides. The Hammett plot of the reactions of para-phenyl substituted triaryl phosphites with diphenyl selenoxide gave ρ=+2.3, whereas with bis(p-methoxyphenyl) selenoxide, ρ=−2.1. The results are consistent with a concerted mechanism for the oxygen transfer from Se to P.     

Efficient trapping of the intermediates in the photooxygenation of sulfides by aryl selenides and selenoxides

Aryl selenides and selenoxides trap efficiently the intermediates in the reaction of singlet oxygen with sulfides. In the co-photooxygenation of 1 equiv of an aryl selenoxide with 1.3 equiv of dimethyl sulfide, the aryl selenone is formed quantitatively. Aryl selenides require 4-5 equiv of sulfide for their complete co-oxidation to selenones.     

Evidence of desulfurization in the oxidative cyclization of thiosemicarbazones. Conversion to 1,3,4-oxadiazole derivatives

The addition of pyridine-2-carbaldehyde 4N-methylthiosemicarbazone (C8H10N4S) to an aqueous solution of copper(II) nitrate yields [[Cu(C8H9N4S)(NO3)]2] (1). This complex consists of centrosymmetric dinuclear entities containing square-pyramidal copper(II) ions bridged through the sulfur thioamide atoms. The oxidation of 1 with KBrO3 or KIO3 gives rise to a compound with formula [[Cu(C8H8N4O)(H2O)2(SO4)]2]*2H2O (2) (C8H8N4O = 2-methylamino-5-pyridin-2-yl-1,3,4-oxadiazole). The structure of 2 is made up of centrosymmetric dimers where the copper(II) ions exhibit a distorted octahedral coordination and are connected by the oxadiazole moiety. The metal ions in 2 can be removed by addition of K4[Fe(CN)6], and then the oxadiazole ligand can be isolated and recrystallized as (C8H8N4O)*3H2O (3).     

Deoxygenation and other photochemical reactions of aromatic selenoxides

Atomic oxygen O(3P) is a potent oxidant that has been well-studied in the gas phase. However, exploration of its reactivity in the condensed organic phase has been hampered by the lack of an appropriate source. Dibenzothiophene-S-oxide (DBTO) and related derivatives have been promoted as photochemical O(3P) sources but suffer from low quantum yields. Photolysis of dibenzoselenophene-Se-oxide (DBSeO) results in the formation of dibenzoselenophene and oxidized solvent in significantly higher quantum yields, ca. 0.1. The oxidation product ratios from toluene obtained from the photolysis of dibenzothiophene-S-oxide and the corresponding selenoxide are the same, strongly suggesting a common oxidizing intermediate, which is taken to be O(3P). An additional product, proposed to be the corresponding selenenic ester, is also observed under deoxygenated conditions. The photochemistry of diphenyl selenoxide includes a minor portion of oxidant-forming deoxygenation, in contrast to previous conclusions (Yamazaki, Y.; Tsuchiya, T.; Hasegawa, T. Bull. Chem. Soc. Jpn. 2003, 201-202).     

Antiradical activity of dimethyl selenoxide and sodium selenite

Determination of the oxygen radical absorption capacity (ORAC) served to discover antiperoxyradical activity of dimethyl selenoxide (DMSeO). The antiperoxyradical capacity of DMSeO is higher than that of the water-soluble analog of α-tocopherol and trolox and close to the value determined for the butylated hydroxy toluene (BHT). The redox parameters of DMSeO were determined using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The anodic oxidation peaks of DMSeO on the CV and DPV voltammograms in the potential range from ?1200 to 1500 mV relative to Ag/Ag⁺ in CH₂Cl₂ confirm antioxidant activity of DMSeO towards oxidants and peroxyl radicals. It was established that Na₂SeO₃ in the acidic medium at pH = 3 shows the antiradical activity towards the stable radical of 2,2′-diphenyl-1-picryl hydrazyl (DPPH) and acts as a two-electron oxidant by trapping two DPPH radicals.     

Synthesis and Crystal Structure of Tris(N-p- methylphenyl-3-hydroxy- 2-ethyl-4-pyridinonato)iron(III)

1 INTRODUCTION A number of hydroxypyrones and hydroxypyridinones are being assessed or considered as orally effective chelators for treatment iron or aluminum overload[1,2]. Almost all present and potential applications involve the tris-ligand complexes of metal(III) cations, as for example in administration of iron(III) complexes for the treatment of anaemia[3], and the appropriate isotopes (e.g. 67Ga, 111In, 90Y) for radiotherapy or the isotopes of gadolinium for magnetic resonance …     

Synthesis and Crystal Structure of Tris(N-p- methylphenyl-3-hydroxy- 2-ethyl-4-pyridinonato)iron(III)①

The complex [Fe(C14H14NO2)3](2H2O has been prepared by reaction of N-p-methylphenyl-3-hydroxy-2-ethyl-4-pyridinone with FeCl3(6H2O. A single-crystal X-ray study shows that the iron atoms lie in a trigonally distorted octahedral environment coordinated to the hydroxy and ketone oxygen atoms of three ligands in the mer configuration Mr=773.57(C42H46N3O8Fe). The crystal is hexagonal with space group P1c; a=15.943(2), c=17.612(4)?, V=3877.0(12)?3, Z=4, Dc=1.325g/cm3, (=0.445mm－1, F(000)=1634, R=0.0446, wR= 0.1154 for 3085 reflections with I >2((I). The bond lengths from iron to oxygens are 1.980(1)? for the ketone oxygens and 2.071(1)? for the hydroxy oxygens. The molecule exhibits the expected propeller shape, and the angle of the trigonal twist is 48.37(. The dihedral angles are 0.5(2)° between chelate ring plane and pyridine ring plane and 71.31(7)° between pyridine ring plane and benzene ring plane. The solvent H2O(O(3) and O(4)) molecules are linked with O(2) and O(1) by hydrogen bonds with bond lengths 2.900(1) and 2.999(1), respectively.     

Selective oxidation of allylic sulfides by hydrogen peroxide with the trirutile-type solid oxide catalyst LiNbMoO(6).

Chemoselective sulfur oxidation of allylic sulfides containing double bonds of high electron density due to multiple alkyl substituents or extended conjugation was developed using the composite metal oxide catalyst, LiNbMoO(6), without any epoxidation of the electron-rich double bond(s). Selective oxidation to either the corresponding sulfoxides or the sulfones was realized by controlling the stoichiometry of the quantitative oxidant, H(2)O(2). This new oxidant system had general applicability for chemoselective oxidation of various allylic, benzylic, or propargylic sulfides containing unsaturated carbon-carbon bonds with different electron properties. Various functional groups including hydroxy, formyl, and ethers of THP or TBDMS are compatible under this mild oxidation reaction condition.     

Photocatalytic Oxidation of Aromatic Hydrocarbons

In acidic aqueous solutions UO(2)(2+) serves as a photocatalyst (lambda(irr) >/= 425 nm) for the oxidation of benzene by H(2)O(2). Under conditions where 50% of the excited state UO(2)(2+) is quenched by H(2)O(2) (k = 5.4 x 10(6) M(-)(1) s(-)(1)) and 50% by benzene (k = 2.9 x 10(8) M(-)(1) s(-)(1)), the quantum yield for the formation of phenol is 0.70. The yield does not change when benzene is replaced by benzene-d(6), but decreases by a factor of approximately 4 upon the change of solvent from H(2)O to D(2)O. Photocatalytic oxidation of toluene by UO(2)(2+)/H(2)O(2) produces PhCHO, PhCH(2)OH, and a mixture of cresols with a total quantum yield of 0.28 under conditions where 50% of UO(2)(2+) is quenched by H(2)O(2). The quenching of UO(2)(2+) by benzene and substituted benzenes takes place with k > 10(8) M(-)(1) s(-)(1). The system UO(2)(2+)/t-BuOOH/C(6)H(6)/hnu does not result in the oxidation of benzene, but instead yields methane and ethane.     

Theoretical study of the mechanism of alkane hydroxylation and ethylene epoxidation reactions catalyzed by diiron bis-oxo complexes. The effect of substrate molecules

The hybrid density functional method B3LYP was used to study the mechanism of the hydrocarbon (methane, ethane, methyl fluoride, and ethylene) oxidation reaction catalyzed by the complexes cis-(H(2)O)(NH(2))Fe(mu-O)(2)(eta(2)-HCOO)(2)Fe(NH(2))(H(2)O), I, and cis-(HCOO)(Imd)Fe(mu-O)(2)(eta(2)-HCOO)(2)Fe(Imd)(HCOO) (Imd = Imidazole), I_m, the "small" and "medium" model of compound Q of the methane monooxygenase (MMO). The improvement of the model from "small" to "medium" did not change the qualitative conclusions but significantly changed the calculated energetics. As in the case of methane oxidation reported by the authors previously, the reaction of all the substrates studied here is shown to start by coordination of the substrate molecule to the bridging oxygen atom, O(1) of I, an Fe(IV)-Fe(IV) complex, followed by the H-atom abstraction at the transition state III leading to the bound hydroxy alkyl intermediate IV of Fe(III)-Fe(IV) core. IV undergoes a very exothermic coupling of alkyl and hydroxy groups to give the alcohol complex VI of Fe(III)-Fe(III) core, from which alcohol dissociates. The H(b)-atom abstraction (or C-H bond activation) barrier at transition state III is found to be a few kcal/mol lower for C(2)H(6) and CH(3)F than for CH(4). The calculated trend in the H(b)-abstraction barrier, CH(4) (21.8 kcal/mol) > CH(3)F (18.8 kcal/mol) > or = C(2)H(6) (18.5 kcal/mol), is consistent with the C-H(b) bond strength in these substrates. Thus, the weaker the C-H(b) bond, the lower is the H(b)-abstraction barrier. It was shown that the replacement of a H-atom in a methane molecule with a more electronegative group tends to make the H(b)-abstraction transition state less "reactant-like". In contrast, the replacement of the H-atom in CH(4) with a less electronegative group makes the H(b)-abstraction transition state more "reactant-like". The epoxidation of ethylene by complex I is found to proceed without barrier and is a highly exothermic process. Thus, in the reaction of ethylene with complex I the only product is expected to be ethylene oxide, which is consistent with the experiment.     

Determination of carboxymethyl-D-glucoses in the product of hydrolytic depolymerization of carboxymethylcellulose by isotachophoresis

Optimal conditions for isotachophoretic separation of carboxymethyl-D-glucoses formed by acidic depolymerization of carboxymethylcellulose were found. 6-O-carboxymethyl-D-glucose, 2-O-carboxymethyl-D-glucose and 3-O-carboxymethyl-D-glucose were identified and determined in the reaction mixture after carboxymethylcellulose hydrolysis. Relative reactivity of hydroxy groups in the glucopyranose unit of cellulose decreased in the following order: O(6)H greater than O(2)H much greater than O(3)H. This was found to be in agreement with the data published by other authors.     

Synthesis, characterization, and structures of Mn(DMHP)3 x 12H2O and Mn(DMHP)2Cl x 0.5H2O

This report describes the synthesis, characterization, and X-ray crystal structures of two Mn(III) complexes, Mn(DMHP)3 x 12H2O and Mn(DMHP)2Cl x 0.5H2O (DMHP = 1,2-dimethyl-3-hydroxy-4-pyridinone). Mn(DMHP)2Cl was prepared from the reaction of Mn(II) chloride with 2 equiv of DMHP under reflux in the presence of triethylamine. Mn(DMHP)3 was obtained by reacting Mn(II) acetate with 3 equiv of DMHP in the presence of sodium acetate. Mn(DMHP)3 could also be prepared by reacting Mn(OAc)3 x 2H2O with 3 equiv of DMHP in the presence of triethylamine. Both Mn(III) complexes have been characterized by elemental analysis, infrared spectroscopy, electronic paramagnetic resonance, electrospray ionization spectroscopy, electrochemical method, and X-ray crystallography. The X-ray crystal structure of Mn(DMHP)2Cl x 0.5H2O revealed a rare example of five-coordinated Mn(III) complexes with two bidentate ligands and a square pyramidal coordination geometry. Surprisingly, the average Mn-O (hydroxy) bond distance in Mn(DMHP)2Cl x 0.5H2O is approximately 0.025 A longer than that of the average Mn-O (carbonyl) bond, suggesting an extensive delocalization of electrons in the two pyridinone rings. The structure of Mn(DMHP)3 x 12H2O, a rare example of six-coordinate high-spin Mn(III) complexes without Jahn-Teller distortion, is isostructural to M(DMHP)3 x 12H2O (M = Al, Ga, Fe, and In). The electrochemical data for Mn(DMHP)3 suggests that the Mn(III) oxidation state is highly stabilized by three DMHP ligands. DMHP has the potential as a chelator for the removal of excess intracellular Mn and the treatment of chronic Mn toxicity.     

Synthesis and Crystal Structure of cis-Bis(3-hydroxy2-ethyl-4-pyranonato)dioxomolybdenum(Ⅵ)

1 INTRODUCTION 3-Hydroxy-2-methyl-4-pyranone (maltol) and 3-hydroxy-2-ethyl-4-pyranone (ethylmaltol) are nontoxic compounds that have been applied to bio- inorganic chemistry over several decades[1, 2]. Their iron(III) complexes are relevant to the control of iron levels in the human body. Such complexes have been assessed for the amelioration of anaemia[3] and their respective ligands have been tested for the removal of excess burdens of iron in diseases such as siderosis, haemochroma…