Ryo Ishimoto

Highly Selective Oxidation of Organosilanes to Silanols with Hydrogen Peroxide Catalyzed by a Lacunary Polyoxotungstate

Organosilicon compounds have attracted much attention, and silanols are widely utilized as building blocks for the production of silicon-based polymer materials and organic donors in metal-catalyzed cross-coupling reactions. Silanols are conventionally synthesized by hydrolysis of chlorosilanes and treatment of siloxanes with alkaline reagents. Strictly controlled reaction conditions are needed and it is difficult to synthesize sterically exposed silanols that readily condense to form disiloxanes. Oxidation with stoichiometric oxidants such as silver salts, peracids, permanganate, dioxiranes, osmium tetroxide, oxaziridines, and ozone has also been reported. However, most of them lead to undesired production of the corresponding siloxanes instead of silanols. In contrast to these approaches, transitionmetal-catalyzed oxidation of silanes to silanols is a promising synthetic method. Hydrolytic oxidation systems based on Ir, Ru, Ag, Cu, Cr, Re, Pt, and Au catalysts with water as oxygen donor have been reported, and some of them show broad substrate scope and high catalytic activity. Another approach is oxidation of silanes by Re and Tizeolite catalysts with H2O2 as oxygen donor. However, these systems have disadvantages: 1) applicability to a limited number of silanes, 2) use of an excess of highly concentrated H2O2 or urea/H2O2 adduct (UHP), 3) low turnover frequencies (TOF = 1–7), and 4) significant formation of undesirable siloxanes. Therefore, catalytic systems for efficient and selective H2O2-based oxidation of various silanes to silanols are scarcely known. Polyoxometalates have attracted considerable attention in the fields of structural chemistry, biological chemistry, catalysis, and materials science. To date, numerous catalytic H2O2-based oxidations by polyoxometalates such as peroxometalates, lacunary polyoxometalates, and transition metal substituted polyoxometalates have been developed. However, application of polyoxometalate catalysts to the oxidation of silanes with H2O2 has never been reported. Here we report highly selective oxidation of organosilanes to silanols with 30–60% aqueous H2O2 catalyzed by divacant lacunary polyoxotungstate (TBA)4[g-SiW10O34(H2O)2] [I, TBA = (n-C4H9)4N , Figure 1, Eq. (1)]. The present system

A Highly Active Protonated Tetranuclear Peroxotungstate for Oxidation with Hydrogen Peroxide

Selective oxygen transfer to organic substrates with H2O2 as a terminal oxidant is important in industrial and synthetic chemistry because of the high content of active oxygen species in H2O2 and coproduction of only water. Therefore, a large number of complexes of titanium, vanadium, iron, manganese, tungsten, rhenium, and platinum have been developed for catalytic oxidation of various kinds of organic substrates with H2O2. [1–4] Among these systems, selective oxidation by tungsten-based catalysts with H2O2 has been widely investigated because of their high reactivity for oxidation as well as an inherent poor activity for the decomposition of H2O2. While peroxotungstates, [5–8] transition-metal-substituted polyoxotungstates, and lacunary polyoxotungstates efficiently catalyze H2O2-based selective oxidation of alkenes, alcohols, amines, sulfides, and silanes, these systems have some drawbacks: 1) use of an excess of H2O2 with respect to the substrates, 2) low turnover numbers (TONs), 3) limited substrate scope, and/or 4) requirement of microwave irradiation or additives. Therefore, developments of efficient tungsten-based catalysts for green oxidation with H2O2 applicable to a wide range of substrates are still in great demand. Various d-transition-metal peroxo complexes with hO2, [5, 6, 11] m-h:h-O2, [7] and m-h:h-O2 [8] groups have been proposed as active species for the epoxidation of alkenes with H2O2. For titanium and molybdenum complexes, spectroscopic and computational studies show that alkyl hydroperoxides, H2O2 , and protons play an important role in formation of active species such as metal-bound alkylperoxides and hydroperoxides. 13] While it has also been reported that epoxidation rates for tungsten complexes increase as the acidity of the reaction media increases, the reaction mechanism, including the role of the acidity, is still unclear. During the course of our investigation into the effects of protons on the epoxidation of alkenes catalyzed by a dinuclear peroxotungstate [{WO(O2)2}2(m-O)] 2 , we successfully synthesized a novel protonated tetranuclear peroxotungstate TPA3[H{W2O2(O2)4(m-O)}2] (1, TPA = [(nC3H7)4N] ). Herein, we report the 1-catalyzed selective oxidation of various kinds of alkenes, sulfides, amines, and a silane under almost stoichiometric conditions (1.0– 1.5 equivalents). Compound 1 shows the highest catalytic activity reported to date for epoxidation of cyclooctene with H2O2 among various peroxotungstates including diand tetranuclear peroxotungstates with XO4 n ligands (X = Se, S, As, P, and Si). The effects of protons on epoxidation of cyclooctene catalyzed by TPA2[{WO(O2)2}2(m-O)] were investigated (Figure 1). While epoxidation barely occurred in the absence

Efficient epoxidation of electron-deficient alkenes with hydrogen peroxide catalyzed by [γ-PW10O38V2(μ-OH)2]3-.

A divanadium-substituted phosphotungstate, [γ-PW(10)O(38)V(2)(μ-OH)(2)](3-) (I), showed the highest catalytic activity for the H(2)O(2)-based epoxidation of allyl acetate among vanadium and tungsten complexes with a turnover number of 210. In the presence of I, various kinds of electron-deficient alkenes with acetate, ether, carbonyl, and chloro groups at the allylic positions could chemoselectively be oxidized to the corresponding epoxides in high yields with only an equimolar amount of H(2)O(2) with respect to the substrates. Even acrylonitrile and methacrylonitrile could be epoxidized without formation of the corresponding amides. In addition, I could rapidly (≤10 min) catalyze epoxidation of various kinds of terminal, internal, and cyclic alkenes with H(2)O(2) under the stoichiometric conditions. The mechanistic, spectroscopic, and kinetic studies showed that the I-catalyzed epoxidation consists of the following three steps: 1) The reaction of I with H(2)O(2) leads to reversible formation of a hydroperoxo species [γ-PW(10)O(38)V(2)(μ-OH)(μ-OOH)](3-) (II), 2) the successive dehydration of II forms an active oxygen species with a peroxo group [γ-PW(10)O(38)V(2)(μ-η(2):η(2)-O(2))](3-) (III), and 3) III reacts with alkene to form the corresponding epoxide. The kinetic studies showed that the present epoxidation proceeds via III. Catalytic activities of divanadium-substituted polyoxotungstates for epoxidation with H(2)O(2) were dependent on the different kinds of the heteroatoms (i.e., Si or P) in the catalyst and I was more active than [γ-SiW(10)O(38)V(2)(μ-OH)(2)](4-). On the basis of the kinetic, spectroscopic, and computational results, including those of [γ-SiW(10)O(38)V(2)(μ-OH)(2)](4-), the acidity of the hydroperoxo species in II would play an important role in the dehydration reactivity (i.e., k(3)). The largest k(3) value of I leads to a significant increase in the catalytic activity of I under the more concentrated conditions.

A Flexible Nonporous Heterogeneous Catalyst for Size‐Selective Oxidation through a Bottom‐Up Approach

The bottom-up approach has the potential to create novel devices with a wide range of applications such as in electronics, medicine, and energy, as the arrangement of molecular building blocks into nanostructures can be controlled. 2] It is still a great challenge to fabricate not only devices but also heterogeneous catalysts with intended structures and functions by a bottom-up approach, while biominerals such as shells and bones have been already formed by the bottom-up approach through the self-assembly of inorganic building blocks with organic molecules in water. The control of the self-organization of nanobuilding blocks with well-defined sizes, shapes, and physical and chemical properties would lead to progress in science and technology. Various catalytically active sites, such as metal nodes, framework nodes, and molecular species, can be introduced into metal–organic frameworks (MOFs) through self-assembly. Efficient sizeand enantioselective catalysis by crystalline and porous MOFs has been reported for reduction, C C bond formation, and acid–base reactions, and hydrolytic and oxidative stabilities are critical for the development of MOF-based oxidation systems that are efficient, chemoand size-selective, and recyclable, and use the green oxidant H2O2. [5–7] Therefore, the development of efficient, easily recoverable, and recyclable heterogeneous oxidation catalysts with H2O2 by a bottom-up approach has received particular research interest. Polyoxometalates (POMs) are discrete early transitionmetal oxide cluster anions with applications in broad fields, such as catalysis, materials, and medicine, because their structures and chemical properties can be finely tuned by choose of the constituent elements. Various POMs such as peroxometalates, lacunary POMs, and transition-metal-substituted POMs have been developed for H2O2or O2-based green oxidations. Therefore, POMs are suitable nanobuilding blocks to construct heterogeneous oxidation catalysts. Recently, the development of heterogeneous oxidation catalysts based on POMs and the related compounds has been attempted according to the following strategies: “solidification” of POMs (formation of insoluble solid ionic materials with appropriate countercations) and “immobilization” of POMs through adsorption, covalent linkage, and ion exchange. In most cases, however, the catalytic activities and selectivities of the parent homogeneous POMs are somewhat or much decreased by the heterogenization, and there are only a few successful examples. We are interested in a bottom-up approach to the design and synthesis of artificial heterogeneous catalysts with POMs and herein report that the nonporous tetra-n-butylammonium salt of [g-SiW10O34(H2O)2] 4 ([(n-C4H9)4N]4[g-SiW10O34(H2O)2]·H2O, 1·H2O) synthesized through a bottom-up approach sorbs ethyl acetate (EtOAc), which is highly mobile in the solid bulk of the compound, probably contributing to the easy co-sorption of the olefins and H2O2. The compound heterogeneously catalyzes size-selective oxidation of various organic substances including olefins, sulfides, and silanes with aqueous H2O2 in EtOAc. The compound can easily be separated by filtration and reused several times with retention of its high catalytic activity. The catalysis is truly heterogeneous in nature because the filtrate after removal of the solid catalyst is completely inactive. Notably, sizeselective oxidation catalysis is observed: small olefins are much more preferentially epoxidized than large olefins. To the best of our knowledge, this study provides the first example for the heterogeneously catalyzed size-selective liquid-phase oxidation with H2O2 by a POM-based catalyst. Compound 1·H2O was synthesized by a bottom-up approach as described below. The silicodecatungstate [gSiW10O34(H2O)2] 4 was synthesized in situ by the addition of concentrated HNO3 to an aqueous solution of [g-SiW10O36] 8 . Then, tetra-n-butylammonium bromide [(n-C4H9)4N]·Br was added to the solution, and white powder of 1·H2O was formed. The use of other cations, such as tetramethylammonium [(CH3)4N] , formed single crystals. The powder Xray diffraction (XRD) pattern, crystal structure, and spacefilling model of 1·H2O are shown in Figure 1a–c. The [*] Prof. Dr. N. Mizuno, Dr. S. Uchida, Dr. K. Kamata, R. Ishimoto, S. Nojima Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)

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.

Composites of [γ-H2PV2W10O40]3− and [α-SiW12O40]4− supported on Fe2O3 as heterogeneous catalysts for selective oxidation with aqueous hydrogen peroxide

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.

Highly Selective Epoxidation of Cycloaliphatic Alkenes with Aqueous Hydrogen Peroxide Catalyzed by [PO4{WO(O2)2}4]3−/Imidazole

In the presence of imidazole as an additive, a phosphorus‐containing tetranuclear peroxotungstate, THA3[PO4{WO(O2)2}4] (I, THA=tetra‐n‐hexylammonium), could act as an efficient catalyst for epoxidation of cycloaliphatic alkenes with 30 % aqueous hydrogen peroxide (H2O2). Compound I showed higher catalytic activity and selectivity to epoxide than other tungstates. By using the I/imidazole system, various kinds of cycloaliphatic alkenes could be highly selectively converted into the acid‐sensitive epoxides including industrially important diepoxides in high to excellent yields under the almost stoichiometric conditions. The 1H NMR spectroscopy showed that imidazole would work not only as a proton acceptor but also as a Lewis base to remarkably suppress the acid‐catalyzed ring opening of epoxides.