Showing papers by "Keigo Kamata published in 2010"

Efficient stereo- and regioselective hydroxylation of alkanes catalysed by a bulky polyoxometalate

Abstract: Direct functionalization of alkanes by oxidation of C-H bonds to form alcohols under mild conditions is a challenge for synthetic chemistry. Most alkanes contain a large number of C-H bonds that present difficulties for selectivity, and the oxidants employed often result in overoxidation. Here we describe a divanadium-substituted phosphotungstate that catalyses the stereo- and regioselective hydroxylation of alkanes with hydrogen peroxide as the sole oxidant. Both cyclic and acyclic alkanes were oxidized to form alcohols with greater than 96% selectivity. The bulky polyoxometalate framework of the catalyst results in an unusual selectivity that can lead to the oxidation of secondary rather than the weaker tertiary C-H bonds. The catalyst also avoids wasteful decomposition of the stoichiometric oxidant, which can result in the production of hydroxyl radicals and lead to non-selective oxidation and overoxidation of the desired products.

Green Oxidation Reactions by Polyoxometalate-Based Catalysts: From Molecular to Solid Catalysts

Abstract: This article summarizes our recent developments for H2O2- and O2-based green oxidation reactions by polyoxometalates (POMs) and related compounds. POM-based structurally controlled molecular catalysts exhibit high catalytic performance for epoxidation and sulfoxidation with H2O2. These molecular catalysts can be immobilized on the organic–inorganic hybrid support with covalently anchoring N-octyldihydroimidazolium cation fragments via the anion exchange. In addition, 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.

Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by Selenium-Containing Dinuclear Peroxotungstate and Kinetic, Spectroscopic, and Theoretical Investigation of the Mechanism

Abstract: The dinuclear peroxotungstate with a SeO42− ligand, (TBA)2[SeO4{WO(O2)2}2] (I; TBA = [(n-C4H9)4N]+), could act as an efficient homogeneous catalyst for the selective oxidation of various kinds of organic substances such as olefins, alcohols, and amines with H2O2 as the sole oxidant. The turnover frequency (TOF) was as high as 210 h−1 for the epoxidation of cyclohexene catalyzed by I with H2O2. The catalyst was easily recovered and reused with maintenance of the catalytic performance. The SeO42− 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 [SeWmOn]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 H2O2. Compound I was the dominant species under steady-state ...

Sulfoxidation with hydrogen peroxide catalyzed by [SeO4{WO(O2)2}2]2−

Abstract: The selenium-containing dinuclear peroxotungstate, [(n-C4H9)4N]2[SeO4{WO(O2)2}2] (I), acts as a homogeneous catalyst for the selective oxidation of various kinds of cyclic mono- and disulfides with 30% aqueous H2O2. The cyclic disulfides were selectively oxidized to the corresponding monosulfoxides with one equivalent of H2O2 with respect to the sulfides. In the presence of two equivalents of H2O2, the oxidation of dibenzothiophene gave the corresponding sulfone in 98% yield under the mild conditions. The negative Hammett ρ value (−0.62) for the competitive oxidation of p-substituted thioanisoles and the low XSO (XSO = (nucleophilic oxidation)/(total oxidation)) value of 0.14 for the I-catalyzed oxidation of thianthrene 5-oxide (SSO) revealed that I is a strong electrophilic oxidant. The reactivities of the di- and tetranuclear peroxotungstates with XO4n− ligands (X = Se(VI), As(V), P(V), S(VI), and Si(IV)) were strongly dependent on the kinds of hetero atoms. The reaction rates for the sulfoxidation decreased with an increase in the XSO values and a peroxotungstate with a stronger electrophilicity was more active for the sulfoxidation. The kinetic and mechanistic investigations showed that the electrophilic attack of the peroxo oxygen at the sulfur atom is a key step in the sulfoxidation. The computational investigation supported the high chemoselectivitiy for the sulfoxidation of diallyl sulfide.

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

Abstract: 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)

Green Oxidation Reactions by Polyoxometalate-Based Catalysts: From Molecular to Solid Catalysts

Abstract: This article summarizes our recent developments for H2O2- and O2-based green oxidation reactions by polyoxometalates (POMs) and related compounds. POM-based structurally controlled molecular catalysts exhibit high catalytic performance for epoxidation and sulfoxidation with H2O2. These molecular catalysts can be immobilized on the organic–inorganic hybrid support with covalently anchoring N-octyldihydroimidazolium cation fragments via the anion exchange. In addition, 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.

Cyclopropanation of Olefins with Diazo Compounds Catalyzed by a Dicopper-substituted Silicotungstate [γ-H2SiW10O36Cu2(μ-1,1-N3)2]4−

Abstract: The dicopper-substituted γ-Keggin silicotungstate (TBA) 4 -[γ-H 2 SiW 10 O 36 Cu II 2 (μ-1,1-N 3 ) 2 ] (I, TBA = tetra-n-butylammonium) could act as an efficient precatalyst for the chemoselective cylopropanation of olefins with diazo compounds. Various kinds of olefins were efficiently converted to the corresponding cyclopropane derivatives in good yields.

Cyclopropanation of Olefins with Diazo Compounds Catalyzed by a Dicopper-Substituted Silicotungstate [γ-H2SiW10O36Cu2 (μ-1,1-N3)2]4-.

Abstract: The dicopper-substituted γ-Keggin silicotungstate (TBA) 4 -[γ-H 2 SiW 10 O 36 Cu II 2 (μ-1,1-N 3 ) 2 ] (I, TBA = tetra-n-butylammonium) could act as an efficient precatalyst for the chemoselective cylopropanation of olefins with diazo compounds. Various kinds of olefins were efficiently converted to the corresponding cyclopropane derivatives in good yields.