Tomohisa Hirano

Palladium(II) Containing γ-Keggin Silicodecatungstate That Efficiently Catalyzes Hydration of Nitriles

A mixture of Pd(OAc)(2) and TBA(4)[γ-SiW(10)O(34)(H(2)O)(2)] (TBA-SiW10, TBA = [(n-C(4)H(9))(4)N](+)) showed high catalytic activities for hydration of various kinds of structurally diverse nitriles including aromatic, aliphatic, heteroaromatic, and double bond-containing ones. For hydration of 3-cyanopyridine, the turnover frequency was 860 h(-1), and the turnover number reached up to 670. A dipalladium-substituted γ-Keggin silicodecatungstate, [γ-H(2)SiW(10)O(36)Pd(2)(OAc)(2)](4-) (I), was successfully synthesized by the reaction of [γ-SiW(10)O(34)(H(2)O)(2)](4-) with Pd(OAc)(2) in a mixed solvent of acetone and water. The crystal structure of I was a monomeric, dipalladium-substituted, γ-Keggin silicodecatungstate with bidentate acetate ligands. Compound I showed similar activities and selectivities to those of a simple mixture of Pd(OAc)(2) and TBA-SiW10. The kinetic, mechanistic, and density functional theory calculation studies show that the dipalladium site plays an important role in the present hydration, and the nucleophilic attack of a hydroxide or water to the nitrile carbon atom is included in the rate-determining step.

Hydrogen-bond-assisted epoxidation of homoallylic and allylic alcohols with hydrogen peroxide catalyzed by selenium-containing dinuclear peroxotungstate.

The reaction of peroxotungstates (H(2)WO(4) + H(2)O(2)) with H(2)SeO(4) gave the novel selenium-containing dinuclear tungsten species, (TBA)(2)[SeO(4){WO(O(2))(2)}(2)] (I, TBA = [(n-C(4)H(9))(4)N](+)), which was characterized by elemental analysis, IR, Raman, UV-vis, (77)Se NMR, (183)W NMR, and CSI-MS. Various kinds of homoallylic and allylic alcohols were efficiently epoxidized to the corresponding epoxy alcohols in high yields with 1 equiv. H(2)O(2) with respect to the substrates. Compound I showed the highest catalytic activity for H(2)O(2)-based epoxidation of homoallylic and allylic alcohols among selenium and tungsten complexes. The turnover frequency reached up to 150 h(-1) in a 10 mmol-scale epoxidation of cis-3-hexen-1-ol and this value was the highest among those reported for the transition-metal catalyzed epoxidation of homoallylic alcohols with H(2)O(2). The kinetic, mechanistic, computational studies showed that the stabilization of the transition-state by the hydrogen bonding between I and the substrates results in the high reactivity for the I-catalyzed epoxidation of homoallylic and allylic alcohols. The nature of the hetero atoms in the di- and tetranuclear peroxotungstates with XO(4)(n-) ligands (X = As(V), P(V), S(VI), Si(IV), etc.) was crucial in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen transfer reactivity. All the data of the structural, kinetic, spectroscopic, and computational comparison show that the dimeric peroxotungstate unit, {WO(O(2))(2)}(2), in I is activated by the SeO(4)(2-) ligand.

Cyanosilylation of Carbonyl Compounds with Trimethylsilyl Cyanide Catalyzed by an Yttrium‐Pillared Silicotungstate Dimer

Cyanohydrins are a very important class of compounds in chemistry as well as biology, and have been widely utilized as important synthetic intermediates for organic compounds, such as a-hydroxy acids, a-hydroxy aldehydes, and b-amino alcohols. For the synthesis of cyanohydrins, various cyanating reagents have been employed, and trimethylsilyl cyanide (TMSCN) is one of the most useful and safe cyanating reagents for nucleophilic addition to carbonyl compounds to give cyanohydrin trimethylsilyl ether. Hence, the development of efficient catalysts for cyanosilylation of carbonyl compounds with TMSCN is a very important subject in current research, and several efficient catalysts have been developed so far. Lewis acid catalysts can act as electrophilic catalysts to activate carbonyl compounds and have been extensively investigated for cyanosilylation (see Table S1 in the Supporting Information). 3] Several nucleophilic catalysts, such as amines, phosphines, phosphazanes, and alkalineearth metal oxides, can activate TMSCN and promote cyanosilylation (Table S1). 3] Asymmetric cyanosilylations have also been successfully developed by employing customly designed chiral ligands. Polyoxometalates (POMs) are a large family of anionic metal–oxygen clusters that consist of Group V and VI metals in their highest oxidation states, and are thermally and oxidatively stable in comparison with commonly utilized organometallic catalysts and organocatalysts. The chemical properties of POMs, for example, redox potentials, (multi)electron-transfer properties, acidities, and solubilities, and negative charges, can be finely tuned by choosing the constituent anion and countercations, and diverse structures can be synthesized. In addition to the above-mentioned properties, the important feature of POMs is the presence of bare nucleophilic surfaces as a result of external oxygen atoms (M O M and M=O species, M = W or Mo), which might act as nucleophilic sites as well as stabilizers of cationic intermediates. 6] In particular, it is expected that POMs with large negative charges can nucleophilically activate TMSCN, thus resulting in promotion of cyanosilylations, as observed for Lewis base catalyzed cyanosilylations. 3h] Initially, the cyanosilylation of 2-adamantanone (1a) with TMSCN was carried out with the 8-charged POM TBA4H4[g-SiW10O36] (SiW10, TBA = tetra-n-butylammonium) in 1,2-dichloroethane at 30 8C. SiW10 promoted the cyanosilylation, giving the corresponding cyanohydrin trimethylsilyl ether 2a in 15% yield after 20 minutes (Scheme 1). In the case of TBA4H4[a-SiW11O39] (SiW11), 2a was also obtained in 16% yield (Table S2). In the absence of the catalysts, cyanosilylation was not observed under the present conditions.

Highly efficient oxidation of sulfides with hydrogen peroxide catalyzed by [SeO4{WO(O2)2}2]2―

By using the selenium-containing dinuclear peroxotungstate at 0.005-0.1 mol%, various kinds of sulfides could be converted into the corresponding sulfoxides or sulfones in excellent yields with one or two equivalents of H(2)O(2) with respect to the sulfide, respectively.

Light-induced Retinal Damage In Mice: Hydrogen Peroxide Production and Superoxide Dismutase Activity in Retina

The oxidative mechanism in retinal damage due to exposure to intense light was investigated histochemically and biochemically. SMA mice (albino mice) of 2 to 3 months of age were exposed to intense light (1000-1400 lux). In the cyclic light-reared group (without dark adaptation), the outer and inner segments of the photoreceptor cells were damaged after 3 days of exposure, and severe outer nuclear layer damage was observed after 5 to 7 days of exposure. Hydrogen peroxide (H2O2) production in the outer nuclear layer increased with the progress of retinal damage. In the dark-reared group (dark adaptation of 16-18 hours), outer and inner segment damage was noted after 4 hours of light exposure, and severe outer nuclear layer damage was noted after 12 hours of light exposure. H2O2 production increased in the outer nuclear layer with retinal damage. Superoxide dismutase (SOD) activity did not change before the occurrence of retinal damage, and decreased by 25% after 3 days of exposure to light in the cyclic light-reared group. The decrease in total SOD activity corresponded to that of manganese-SOD (Mn-SOD). In the dark-reared group, SOD activity did not change, even after 1 day of exposure. There appears to be some relationship between retinal light damage and H2O2 production in the outer nuclear layer. Superoxide dismutase activity failed to provide protection against retinal oxidative damage due to intense light exposure.

Epoxidation of alkenes with hydrogen peroxide catalyzed by selenium-containing dinuclear peroxotungstate and kinetic, spectroscopic, and theoretical investigation of the mechanism.

The dinuclear peroxotungstate with a SeO(4)(2-) ligand, (TBA)(2)[SeO(4){WO(O(2))(2)}(2)] (I; TBA = [(n-C(4)H(9))(4)N](+)), could act as an efficient homogeneous catalyst for the selective oxidation of various kinds of organic substances such as olefins, alcohols, and amines with H(2)O(2) as the sole oxidant. The turnover frequency (TOF) was as high as 210 h(-1) for the epoxidation of cyclohexene catalyzed by I with H(2)O(2). The catalyst was easily recovered and reused with maintenance of the catalytic performance. The SeO(4)(2-) ligand in I played an important role in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen-transfer reactivity. Several kinetic and spectroscopic results showed that the present catalytic epoxidation included the following two steps: (i) formation of the subsequent peroxo species [SeW(m)O(n)](o-) (II; m = 1 and 2) by the reaction of I with an olefin and (ii) regeneration of I by the reaction of II with H(2)O(2). Compound I was the dominant species under steady-state turnover conditions. The reaction rate for the catalytic epoxidation showed a first-order dependence on the concentrations of olefin and I and a zero-order dependence on the concentration of H(2)O(2). The rate of the stoichiometric epoxidation with I agreed well with that of the catalytic epoxidation with H(2)O(2) by I. All of these kinetic and spectroscopic results indicate that oxygen transfer from I to the C=C double bond is the rate-determining step. The computational studies support that the oxygen-transfer step is the rate-determining step.

Reversible deprotonation and protonation behaviors of a tetra-protonated γ-Keggin silicodecatungstate.

The potentiometric titration of a γ-Keggin tetra-protonated silicodecatungstate, [γ-SiW(10)O(34)(H(2)O)(2)](4-) (H(4)·I), with TBAOH (TBA = [(n-C(4)H(9))(4)N](+)) showed inflection points at 2 and 3 equiv of TBAOH. The (1)H, (29)Si, and (183)W NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [γ-SiW(10)O(34)(OH)(OH(2))](5-) (H(3)·I), [γ-SiW(10)O(34)(OH)(2)](6-) (H(2)·I), and [γ-SiW(10)O(35)(OH)](7-) (H·I), with C(1), C(2v), and C(2) symmetries, respectively. Single crystals of TBA(6)·H(2)·I suitable for the X-ray structure analysis were successfully obtained and the anion part was a monomeric γ-Keggin divacant silicodecatungstate with two protonated bridging oxygen atoms. Compounds H(3)·I, H(2)·I, and H·I were reversibly monoprotonated to form H(4)·I, H(3)·I, and H(2)·I, respectively.

Synthesis and structural characterization of inorganic-organic-inorganic hybrids of dipalladium-substituted γ-Keggin silicodecatungstates.

Three inorganic-organic-inorganic hybrids of dipalladium-substituted γ-Keggin silicodecatungstates with organic linkers of different lengths, TBA8[{(γ-H2SiW10O36Pd2)(O2C(CH2)nCO2)}2] (n = 1 (II), 3 (III), and 5 (IV), TBA = [(n-C4H9)4N](+)), were synthesized by exchange of the acetate ligands in TBA4[γ-H2SiW10O36Pd2(OAc)2] (ITBA) with malonic, glutaric, and pimelic acids, respectively. The X-ray crystallographic analysis of II, IIIA (IIIA: III with DCE, DCE = 1,2-dichloroethane), and IVA (IVA: IV with 10DCE) revealed that the anion parts of II, IIIA, and IVA were inorganic-organic-inorganic hybrids composed of two dipalladium-substituted γ-Keggin silicodecatungstates connected by two dicarboxylate ligands. In the crystal structure of IVA, 10 DCE molecules per polyanion were present in the vicinity of polyanions. Compound IVB (IVB: IV with 0.2DCE) was obtained by the evacuation of IVA. The DCE sorption-desorption isotherms of IVB showed that the amount of DCE sorbed was saturated at 10.5 mol mol(-1), of which the amount was close to that (10 mol mol(-1)) of crystallographically assigned DCE molecules. In the DCE sorption-desorption isotherms, a low-pressure hysteresis was observed probably because of hydrogen-bonding interaction between DCE molecules and polyanions. The powder X-ray diffraction (XRD) pattern of IVA changed with decrease in the relative DCE vapor pressure to form IVC (IVC: IV with 0.7DCE) at P/P0 = 0.0. The in situ powder XRD study showed reversible structure transformation between IVA and IVC driven by the sorption-desorption of DCE.

Novel Iron Complexes Behave Like Superoxide Dismutase In Vivo

Novel iron and copper complexes having tris[N-(5-methyl-2-pyridylmethyl)-2-aminoethyl]amine (5MeT-PAA), tris[N-(3-methyl-2-pyridylmethyl)-2-aminoethyl]amine (3MeTPAA), tris[N-(5-methoxycarbonyl-2-pyridylmethyl)-2-aminoethyl]amine (TNAA), tris[(2-thienylmethyl)-2-aminoethyl]amine (TTAA), tris[(2-furylmethyl)-2-aminoethyl]amine (TFAA) or tris[(2-imidazoyl)-2-aminoethyl]amine (TIAA) as ligand, were synthesized to examine the superoxide dismutase (SOD) activity. The concentrations of Fe-3MeTPAA and Fe-TIAA equivalent to 1 unit of SOD (IC50) were 0.5 microM and 1.0 microM, respectively. Fe-3MeTPAA and Fe-TIAA had higher SOD activity than other Fe and Cu complexes and protected Escherichia coli cells from paraquat toxicity. In case of using tris[N-(6-methyl-2-pyridylmethyl)-2-aminoethyl]amine (6MeTPAA) as ligand, the Fe complex could not be obtained, which may be due to the steric hindrance of 6-methyl substituent. Generally, Cu complexes had low SOD activity, compared with Fe complexes, and could not suppress paraquat toxicity.