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We developed a novel system for the synthesis of phenols through aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones by Mg–Al-layered double hydroxide-supported Au–Pd alloy nanoparticles., Phenol, an important industrial chemical, is widely produced using the well-developed cumene process. However, demand for the development of a novel alternative method for synthesizing phenol from benzene has been increasing. Herein, we report a novel system for the synthesis of phenols through aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones, including ketone–alcohol (KA) oil, catalyzed by Mg–Al-layered double hydroxide (LDH)-supported Au–Pd alloy nanoparticles (Au–Pd/LDH). Alloying of Au and Pd and basicity of LDH are key factors in achieving the present transformation. Although monometallic Au/LDH, Pd/LDH, and their physical mixture showed almost no catalytic activity, Au–Pd/LDH exhibited markedly high catalytic activity for the dehydrogenative phenol production. Mechanistic studies showed that β-H elimination from Pd-enolate species is accelerated by Au species, likely via electronic ligand effects. Moreover, the effect of supports was critical; despite the high catalytic performance of Au–Pd/LDH, Au–Pd bimetallic nanoparticles supported on Al2O3, TiO2, MgO, and CeO2 were ineffective. Thus, the basicity of LDH plays a deterministic role in the present dehydrogenation possibly through its assistance in the deprotonation steps. The synthetic scope of the Au–Pd/LDH-catalyzed system was very broad; various substituted cyclohexanols and cyclohexanones were efficiently converted into the corresponding phenols, and N-substituted anilines were synthesized from cyclohexanones and amines. In addition, the observed catalysis was truly heterogeneous, and Au–Pd/LDH could be reused without substantial loss of its high performance. The present transformation is scalable, utilizes O2 in air as the terminal oxidant, and generates water as the only by-product, highlighting the potential practical utility and environmentally benign nature of the present transformation. Dehydrogenative aromatization of cyclohexanols proceeds through (1) oxidation of cyclohexanols to cyclohexanones; (2) dehydrogenation of cyclohexanones to cyclohexenones; and (3) disproportionation of cyclohexenones to afford the desired phenols. In the present Au–Pd/LDH-catalyzed transformation, the oxidation of the Pd–H species is included in the rate-determining step.

In this paper, we report the synthesis of a Ti-substituted phosphotungstate, TBA6[(γ-PW10O36)2Ti4(μ-O)2(μ-OH)4] (I, TBA = tetra-n-butylammonium), and its application to H2O2-based oxidation. Firstly, an organic solvent-soluble dilacunary phosphotungstate precursor, TBA3[γ-PW10O34(H2O)2] (PW10), has been synthesized. By the reaction of PW10 and TiO(acac)2 (acac = acetylacetonate) in an organic medium (acetonitrile), I can be obtained. Compound I possesses a tetranuclear Ti core which can effectively activate H2O2 and shows high catalytic performance for several oxidation reactions, such as epoxidation of alkenes, oxygenation of sulfides, oxidative bromination of unsaturated compounds, and hydroxylation of anisole, giving the corresponding oxidation products with high efficiencies and selectivities. The catalytic performance of I is much superior to those of previously reported Ti-substituted polyoxometalates. In addition, I is highly durable during catalysis and can be reused several times while keeping its high catalytic performance. Furthermore, we have successfully isolated the truly catalytically active species for the present I-catalyzed oxidation, TBA6[(γ-PW10O36)2Ti4(μ-η2:η2-O2)4] (II), and its anion structure has been determined by X-ray crystallographic analysis. All of the four Ti2-μ-η2:η2-peroxo species in II are active for stoichiometric oxidation (without H2O2), and II is included in the catalytic cycle for I-catalyzed oxidation.

Chiral ligand-modified polyoxometalates (POMs), (Bu4N)4[ﾎｳ-SiTi2W10O36(ﾎｼ-OH)2(ﾎｼ-BINOLate)], were successfully synthesized by the introduction of (R)- or (S)-BINOL into the dititanium-substituted tungstosilicate POM in an org. medium and characterized by x-ray crystallog., spectroscopy, and elemental analyses. These BINOL-modified POMs were stable in the soln. state and showed catalytic activity for asym. oxidn. of thioanisole. [on SciFinder(R)]

The supported phosphorus-contg. tetranuclear peroxotungstate ([PO4{WO(O2)2}4]3-, denoted by PW4) catalysts by using zinc-modified SnO2 supports with different zinc contents [PW4-Zn(x)/SnO2, in which x denotes the zinc content (wt%)] were prepd. The supported catalysts, in particular PW4-Zn(0.8)/SnO2, could act as efficient and reusable heterogeneous catalysts for selective oxidn. with aq. H2O2 as the terminal oxidant. The catalytic performance of PW4-Zn(0.8)/SnO2 was much superior to those of the corresponding homogeneous analog THA3PW4 (THA = tetra-n-hexylammonium) and the previously reported tungstate-based heterogeneous catalysts such as our W-Zn/SnO2. In the presence of PW4-Zn(0.8)/SnO2, various types of org. substrates such as alkenes, amines, silanes, and sulfides could be converted into the corresponding oxygenated products in high to excellent yields by using near-stoichiometric amts. of H2O2 with respect to the substrates (typically 1.2 equiv.). The PW4 species interacting with highly dispersed Zn2+ species on SnO2 likely plays an important role in the present oxidn. [on SciFinder(R)]

The title catalyst is effective for dehydration of primary amides to nitriles. Various kinds of primary amides could be converted into nitriles in good yields. [on SciFinder(R)]

A divanadium-substituted phosphotungstate [ﾎｳ-PW10O38V2(ﾎｼ-OH)2]3- (I) could act as an efficient homogeneous catalyst for selective oxidn. of sulfides with 30% aq. H2O2. By using I at 0.01-0.1 mol%, various kinds of sulfides including aryl, alkyl, and vinyl ones could be converted into the corresponding sulfoxides in excellent yields with one equiv. of H2O2 with respect to sulfides. The neg. Hammett ﾏ�value (-0.83) for competitive oxidn. of p-substituted thioanisoles and the low XSO (XSO = (nucleophilic oxidn.)/(total oxidn.)) value (

A divanadium-substituted phosphotungstate TBA4[γ-PW10O38V2(μ-OH)(μ-O)] (I, TBA = tetra-n-butylammonium) reacts with one equivalent H+ to form a bis-μ-hydroxo species [γ-PW10O38V2(μ-OH)2]3− (I′) in organic media. The strong electrophilic oxidants such as [γ-PW10O38V2(μ-OH)(μ-OOH)]3− (II) and [γ-PW10O38V2(μ-η2:η2-O2)]3− (III) are formed by the reaction of the bis-μ-hydroxo species with H2O2. In the presence of I and H+, H2O2-based oxidations such as (i) epoxidation of alkenes (17 examples including electron-deficient ones), (ii) hydroxylation of alkanes (11 examples), and (iii) oxidative bromination of alkenes, alkynes, and aromatics with Br− as a bromo source (12 examples including chlorination) chemo-, diastereo-, and regioselectively proceed to give the corresponding oxidized products in moderate to high yields with high efficiencies of H2O2 utilization.

Efficient polyoxometalate (POM)-based Lewis acid-base catalysts of the rare-earth-metal-containing POMs (TBA(6)RE-POM, RE = Y(3+), Nd(3+), Eu(3+), Gd(3+), Tb(3+), or Dy(3+)) were designed and synthesized by reactions of TBA(4)H(4)[γ-SiW(10)O(36)] (TBA = tetra-n-butylammonium) with RE(acac)(3) (acac = acetylacetonato). TBA(6)RE-POM consisted of two silicotungstate units pillared by two rare-earth-metal cations. Nucleophilic oxygen-enriched surfaces of negatively charged POMs and the incorporated rare-earth-metal cations could work as Lewis bases and Lewis acids, respectively. Consequently, cyanosilylation of carbonyl compounds with trimethylsilyl cyanide ((TMS)CN) was efficiently promoted in the presence of the rare-earth-metal-containing POMs via the simultaneous activation of coupling partners on the same POM molecules. POMs with larger metal cations showed higher catalytic activities for cyanosilylation because of the higher activation ability of C═O bonds (higher Lewis acidities) and sterically less hindered Lewis acid sites. Among the POM catalysts examined, the neodymium-containing POM showed remarkable catalytic performance for cyanosilylation of various kinds of structurally diverse ketones and aldehydes, giving the corresponding cyanohydrin trimethylsilyl ethers in high yields (13 substrates, 94-99%). In particular, the turnover frequency (714,000 h(-1)) and the turnover number (23,800) for the cyanosilylation of n-hexanal were of the highest level among those of previously reported catalysts.

Authors have synthesized a novel yttrium-pillared silicotungstate dimer by the reaction of TBA4H4[ﾎｳ-SiW10O36] (SiW10) with Y(acac)3 in an org. medium (acetone). In the presence of TBA8H2[(ﾎｳ-SiYW10O36)2]ﾂｷ7H2O (Y2), cyanosilylation of several structurally diverse ketones and aldehydes with TMSCN selectively proceeded to afford the corresponding cyanohydrin trimethylsilyl ethers. In particular, the catalytic performance for aldehydes was very significant; for example, TON = 18000 and TOF = 540000 h-1 were obsd. for n-hexanal. [on SciFinder(R)]

We have recently reported that oral gavage of a potato extract (Potein®) suppressed the food intake in rats. The satiating effect of the potato extract was compared in the present study to other protein sources, and the involvement of endogenous cholecystokinin (CCK) secretion was examined. Food consumption was measured in 18-h fasted rats after oral gavage of the potato extract or other protein sources. The CCK-releasing activity of the potato extract was then examined in anesthetized rats with a portal cannula. Oral gavage of the potato extract reduced the food intake in the rats, the effect being greater than with casein and a soybean β-conglycinin hydrolysate. The suppressive effect on appetite of the potato extract was attenuated by treating with a CCK-receptor antagonist (devazepide). The portal CCK concentration was increased after a duodenal administration of the potato extract to anesthetized rats. These results indicate that the potato extract suppressed the food intake in rats through CCK secretion.

The di-copper-substituted γ-Keggin-type silicotungstate TBA4[γ-H2SiW10O36Cu2(μ-1,1-N3)2] (I, TBA = tetra-n-butylammonium) could act as an efficient reusable homogeneous catalyst for the aerobic oxidative alkyne homocoupling. Various kinds of structurally diverse terminal alkynes including aromatic, heteroaromatic, aliphatic, double bond-containing, silylacetylene, propargylic alcohol, and propargylic amine derivatives could selectively be converted into the corresponding diynes in the presence of I. The catalytic activity of I was much higher than those of the mono-copper-substituted silicotungstate, monomeric copper complexes, and simple copper salts, showing that the di-copper core in I plays an important role in the present alkyne homocoupling. The reaction mechanism involving the formation of the di-copper(II)-alkynyl intermediate, reductive elimination of a diyne, and re-oxidation of reduced copper species by O2 has been proposed.

A review. This article summarizes our recent developments for H2O2- and O2-based green oxidn. reactions by polyoxometalates (POMs) and related compds. POM-based structurally controlled mol. catalysts exhibit high catalytic performance for epoxidn. and sulfoxidn. with H2O2. These mol. catalysts can be immobilized on the org.-inorg. hybrid support with covalently anchoring N-octyldihydroimidazolium cation fragments via the anion exchange. In addn., we have developed supported metal hydroxide catalysts such as Ru(OH)x/Al2O3 and Cu(OH)x/TiO2 on the basis of the information of the catalytically active sites with POMs. By using these supported metal hydroxide catalysts, novel aerobic oxidative synthesis of nitriles and amides, and efficient aerobic oxidative homocoupling of alkynes can be realized. [on SciFinder(R)]

The supported copper hydroxide Cu(OH)x/Al2O3 acts as an efficient heterogeneous precatalyst for the 1,3-dipolar cycloaddn. of org. azides to terminal alkynes. The recovered Cu(OH)x/Al2O3 retains its high activity and can be reused. [on SciFinder(R)]

A review. [on SciFinder(R)]

The di-copper-substituted ﾎｳ-Keggin silicotungstate with bis-ﾎｼ-1,1-azido ligands TBA4[ﾎｳ-H2SiW10O36Cu2(ﾎｼ-1,1-N3)2] (I, TBA = tetra -n-butylammonium) was synthesized in an aq. medium. The crystal structure of I showed that the anion part of I is a monomer of the basal-basal end-on diazido-bridged di-copper-substituted ﾎｳ-Keggin silicotungstate. The NMR and CSI-MS spectra of I in org. solvents, such as acetonitrile, benzonitrile, and 1,2-dichloroethane, showed that complex I was present as a monomer of the di-copper-substituted ﾎｳ-Keggin silicotungstate. Complex I could act as an effective homogeneous catalyst for the oxidative homocoupling of various types of alkynes, including arom., aliph., and heteroatom-contg. ones. The reaction possibly proceeds as follows: first, the ligand exchange proceeds between the azido groups in I and alkynyl groups to form the corresponding diyne with the reduced copper(I) species via the di-copper(II)-alkynyl intermediate, then the reduced species is reoxidized by mol. oxygen, and the oxidized species reacts with an alkyne to regenerate the alkynyl intermediate. [on SciFinder(R)]

Efficient H2O2-based oxidation systems with three kinds of polyoxometalates, [γ-SiW10O34(H2O)2]4− (1), [γ-1,2-H2SiV2W10O40]4− (2), and [W2O3(O2)4(H2O)2]2− (3) are reported. The compound 1 can catalyze epoxidation of various olefins including non-activated terminal olefins such as propylene and 1-octene with ≥99% selectivity to epoxide and ≥99% efficiency of H2O2 utilization. It is notable that 2 shows unique stereospecificity, regioselectivity, and diastereoselectivity for the epoxidation of cis/trans olefins, non-conjugated dienes, and 3-substituted cyclohexenes, respectively. The epoxidation of various allylic alcohols with only 1 equiv. H2O2 in water can be efficiently catalyzed by 3 to give the corresponding epoxy alcohols in high yields. Further, the catalyst 3 can be recycled with the maintenance of the catalytic performance.

Epoxides are an important class of industrial chemicals that have been used as chemical intermediates. Catalytic epoxidation of olefins affords an interesting technology for their production. This review summarizes recent developments for the epoxidation of olefins with hydrogen peroxide catalyzed by polyoxometalates. The versatility, accessibility, and tunability of polyoxometalates open up the development of effective epoxidation systems. Polyoxometalates for H2O2-based epoxidation can be classified into two groups according to the structural and mechanistic aspects: (i) catalyst precursors of peroxotungstate or peroxomolybdate species and (ii) oxidatively and hydrolytically stable transition-metal-substituted polyoxometalates. In addition, we describe our recent studies on the silicotungstate compound, [γ-SiW10O34(H2O)2]4− (I*), synthesized by protonation of divacant lacunary Keggin-type [γ-SiW10O36]8−. The tetra-n-butylammonium salt derivative of I* (TBA-I*) exhibits high catalytic activity in the epoxidation of various olefins including non-reactive terminal olefins such as propylene with hydrogen peroxide under mild conditions.

The oxidative coupling reaction can efficiently be promoted by supported ruthenium catalyst Ru(OH)x/Al2O3. A variety of 2-naphthols and substituted phenols can be converted to the corresponding biaryl compounds in moderate to excellent yields using molecular oxygen as a sole oxidant in water without any additives. The catalysis is truly heterogeneous in nature, and Ru(OH)x/Al2O3 can easily be recovered after the reaction. The catalyst can be recycled seven times with the maintenance of the catalytic performance, and the total turnover number reaches up to 160. The results of competitive coupling reactions suggest that the present oxidative biaryl coupling reaction proceeds via the homolytic coupling of two radical species and the Ru(OH)x/Al2O3 catalyst acts as an one-electron oxidant. Two radical species are coupled to give the corresponding biaryl product, and the one-electron reduced catalyst is reoxidized by molecular oxygen. The amounts of O(2) uptake and H(2)O formation were almost one-quarter and one-half the amount of substrate consumed, respectively, supporting the reaction mechanism. The kinetic data and kinetic isotope effect show that the reoxidation of the reduced catalyst is the rate-limiting step for the coupling reaction.

In the presence of a simple combined catalyst of a tungstate monomer (TBA2WO4, TBA: tetra-n-butylammonium) and Rh2(OAc)4 hydrosilylation of various types of substances including ketone, aldehyde, carbon dioxide, alkene, nitrile, and furan derivs. efficiently proceeded, affording the corresponding hydrosilylation products in moderate to high yields (竕･63% yields). In addn., the present system was also applicable to the one-pot redn. of benzamide to benzylamine (95% yield). [on SciFinder(R)]

A divacant, lacunary, Keggin-type silicotungstate, [ γ -SiW 10 O 34 (H 2 O) 2 ] 4− , exhibits high catalytic performance for the epoxidation of various nonconjugated dienes using hydrogen peroxide under mild conditions, high regioselectivity to the more accessible double bonds, and high efficiency of hydrogen peroxide utilization. The high regioselectivity for the [ γ -SiW 10 O 34 (H 2 O) 2 ] 4− -catalyzed epoxidation would be caused by the steric hindrance of the active site.

In the presence of Rh2(OAc)4 (OAc = acetate) and TBA2WO4 (TBA = tetra-n-butylammonium), the N-silylation of indole derivs. with hydrosilanes efficiently proceeded to give the corresponding N-silylated indoles in high yields. Pyrrole and carbazole were also N-silylated with the combined catalysts. [on SciFinder(R)]

Composites of [γ-H2PV2W10O40]3− and [α-SiW12O40]4− supported on Fe2O3 (PV2-SiW12/Fe2O3, in particular, the molar ratio of PV2/SiW12 = 1/1) could act as effective and reusable heterogeneous catalysts for selective oxidation with aqueous hydrogen peroxide. In the presence of PV2-SiW12/Fe2O3, various kinds of organic substrates such as alkenes, sulfides, arenes, and alkanes could selectively be converted into the corresponding oxygenated products in moderate to high yields. The observed catalyses for the present oxidations were intrinsically heterogeneous, and PV2-SiW12/Fe2O3 could be reused at least three times for each oxidation (epoxidation, sulfoxidation, and arene hydroxylation) without appreciable losses of the high catalytic performance.

Epoxides are an important class of industrial chemicals that have been used as chemical intermediates. Catalytic epoxidation of olefins affords an interesting production technology. We found a widely usable green route to the production of epoxides: A silicotungstate compound, [γ-SiW 10 O 34 (H 2 O) 2 ] 4– , is synthesized by protonation of a divacant, lacunary, Keggin-type polyoxometalate of [γ-SiW 10 O 36 ] 8– and exhibits high catalytic performance for the epoxidation of various olefins, including propylene, with a hydrogen peroxide (H 2 O 2 ) oxidant at 305 kelvin. The effectiveness of this catalyst is evidenced by ≥99% selectivity to epoxide, ≥99% efficiency of H 2 O 2 utilization, high stereospecificity, and easy recovery of the catalyst from the homogeneous reaction mixture.

A divanadium-substituted phosphotungstate TBA4[ﾎｳ-HPV2W10O40] (TBA = tetra-n-butylammonium) could act as an effective homogeneous catalyst for the H2O2-based oxidative bromination. [on SciFinder(R)]

It goes on the dicopper core! A monomeric ﾎｳ-Keggin silicotungstate with a dicopper core that is bridged by two ﾎｼ-1,1-azido ligands catalyzes oxidative alkyne homocoupling reactions whereby various kinds of arom. and aliph. alkynes are selectively converted into the corresponding diynes. [on SciFinder(R)]

The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-SiW(10)O(34)(H(2)O)(2)] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide. The negative Hammett rho(+) (-0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), X(SO)=0.04, for I-catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans-epoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I-catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. (29)Si and (183)W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA](4)[gamma-SiW(10)O(32)(O(2))(2)] (II), with retention of a gamma-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III.

A novel titanium-substituted silicotungstate cluster of [{gamma-SiTi2W10O36(OH)2}2(mu-O)2]8- (1) is synthesized by the introduction of titanium(IV) ions into a divacant lacunary gamma-Keggin-type silicotungstate of [gamma-SiW10O36]8-. This titanium-substituted polyoxometalate, 1, exhibits a dimeric structure. One half of the gamma-Keggin fragment of 1 contains a dinuclear titanium center bridged by two hydroxo groups, and the resulting Ti2(mu-OH)2 core connects to the other Ti2(mu-OH)2 core of the paired gamma-Keggin subunit through Ti-O-Ti linkages. The Ti2(mu-OH)2 core of 1 reacts with MeOH to form the corresponding alkoxo derivative, [{gamma-SiTi2W10O36(OH)(OMe)}2(mu-O)2]8- (2). Two of four hydroxo groups of the Ti2(mu-OH)2 cores in 1 are replaced by methoxo groups to give the Ti2(mu-OH)(mu-OMe) core, and the Ti-O-Ti linkages connecting two gamma-Keggin subunits are maintained in 2. The gamma-Keggin dititanium-substituted silicotungstate 1 catalyzes mono-oxygenation reactions, such as the epoxidation of olefins and sulfoxidation of sulfides with hydrogen peroxide under mild conditions, while the monotitanium-substituted silicotungstate, [alpha-SiTiW11O39]4- (3), and the fully occupied silicododecatungstate, [gamma-SiW12O40]4-, are inactive. The epoxidation with 1 is stereospecific; the configurations around the C=C double bonds of the cis- and trans-olefins are completely retained in the corresponding epoxides. For the competitive epoxidation of cis- and trans-2-octenes, the ratio of the formation rate of cis-2,3-epoxyoctane to that of the trans isomer (R(cis)/R(trans)) is relatively high (21.3) in comparison with those observed for the tungstate catalysts, including [gamma-SiW10O34(H2O)2]4-. The epoxidation of 3-methyl-1-cyclohexene is highly diastereoselective and gives the corresponding epoxide with an anti configuration. The molecular structure of 1 is preserved during the catalysis because the 29Si and 183W NMR spectra of the catalyst recovered after completion of the oxidation are consistent with those of as-prepared compound 1. All these facts suggest the contribution of rigid nonradical oxidants generated on the multinuclear titanium center of 1.

The bis(ﾎｼ-hydroxo)-bridged dioxovanadium site in [ﾎｳ-1,2-H2SiV2W10O40]4- catalyzes the epoxidn. of alkenes in the presence of only one equiv. of H2O2 with a high yield of epoxide, high efficiency of H2O2 utilization, unusual regioselectivity, and unprecedented diastereoselectivity. [on SciFinder(R)]

[reaction: see text] Ru(OH)x/Al2O3 efficiently catalyzes the heterogeneous aerobic oxygenation or oxidative dehydrogenation of alkylarenes to give the corresponding oxygenated or dehydrogenated products. Catalyst/product separation is very easy, and the recovered catalyst is reusable with retention of the high catalytic performance.

A novel Ru-contg. silicotungstate of [Me2NH2]8K2H2[(ﾎｳ-SiW10O36)2Ru4Cl4O2(OH)4]窶｢23H2O (1) was obtained as a single crystal suitable for x-ray crystallog. anal. by the reaction of a divacant lacunary ﾎｳ-Keggin silicotungstate K8[ﾎｳ-SiW10O36] with K2[RuCl5(OH2)]. The unprecedented dimeric silicotungstate consisted of two [ﾎｳ-SiW10O36]8- units sandwiching an adamantanoid Ru-O cluster core [Ru4Cl4O2(OH)4]4+. [on SciFinder(R)]