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

A mixture of Pd­(OAc)₂ and TBA₄[γ-SiW₁₀O₃₄(H₂O)₂] (TBA-SiW10, TBA = [(<i>n</i>-C₄H₉)₄N]⁺) 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-H₂SiW₁₀O₃₆Pd₂(OAc)₂]^4– (<b>I</b>), was successfully synthesized by the reaction of [γ-SiW₁₀O₃₄(H₂O)₂]^4– with Pd­(OAc)₂ in a mixed solvent of acetone and water. The crystal structure of <b>I</b> was a monomeric, dipalladium-substituted, γ-Keggin silicodecatungstate with bidentate acetate ligands. Compound <b>I</b> 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

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

A mixture of Pd­(OAc)₂ and TBA₄[γ-SiW₁₀O₃₄(H₂O)₂] (TBA-SiW10, TBA = [(<i>n</i>-C₄H₉)₄N]⁺) 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-H₂SiW₁₀O₃₆Pd₂(OAc)₂]^4– (<b>I</b>), was successfully synthesized by the reaction of [γ-SiW₁₀O₃₄(H₂O)₂]^4– with Pd­(OAc)₂ in a mixed solvent of acetone and water. The crystal structure of <b>I</b> was a monomeric, dipalladium-substituted, γ-Keggin silicodecatungstate with bidentate acetate ligands. Compound <b>I</b> 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

Efficient [WO₄]^2–-Catalyzed Chemical Fixation of Carbon Dioxide with 2‑Aminobenzonitriles to Quinazoline-2,4(1<i>H</i>,3<i>H</i>)‑diones

A simple monomeric tungstate, TBA₂[WO₄] (<b>I</b>, TBA = tetra-<i>n</i>-butylammonium), could act as an efficient homogeneous catalyst for chemical fixation of CO₂ with 2-aminobenzonitriles to quinazoline-2,4­(1<i>H</i>,3<i>H</i>)-diones. Various kinds of structurally diverse 2-aminobenzonitriles could be converted into the corresponding quinazoline-2,4­(1<i>H</i>,3<i>H</i>)-diones in high yields at atmospheric pressure of CO₂. Reactions of inactive 2-amino-4-chlorobenzonitrile and 2-amino-5-nitrobenzonitrile at 2 MPa of CO₂ also selectively proceeded. The present system was applicable to a g-scale reaction of 2-amino-5-fluorobenzonitrile (10 mmol scale) with CO₂ and 1.69 g of analytically pure quinazoline-2,4­(1<i>H</i>,3<i>H</i>)-dione could be isolated. In this case, the turnover number reached up to 938 and the value was the highest among those reported for base-mediated systems so far. NMR spectroscopies showed formation of the corresponding carbamic acid through the simultaneous activation of both 2-aminobenzonitirile and CO₂ by <b>I</b>. Kinetic and computational studies revealed that <b>I</b> plays an important role in conversion of the carbamic acid into the product

Reversible Deprotonation and Protonation Behaviors of a Tetra-Protonated γ-Keggin Silicodecatungstate

The potentiometric titration of a γ-Keggin tetra-protonated silicodecatungstate, [γ-SiW₁₀O₃₄(H₂O)₂]^4– (H₄·<b>I</b>), with TBAOH (TBA = [(<i>n</i>-C₄H₉)₄N]⁺) showed inflection points at 2 and 3 equiv of TBAOH. The ¹H, ²⁹Si, and ¹⁸³W NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [γ-SiW₁₀O₃₄(OH)­(OH₂)]^5– (H₃·<b>I</b>), [γ-SiW₁₀O₃₄(OH)₂]^6– (H₂·<b>I</b>), and [γ-SiW₁₀O₃₅(OH)]^7– (H·<b>I</b>), with <i>C</i>₁, <i>C</i>_2<i>v</i>, and <i>C</i>₂ symmetries, respectively. Single crystals of TBA₆·H₂·<b>I</b> 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 H₃·<b>I</b>, H₂·<b>I</b>, and H·<b>I</b> were reversibly monoprotonated to form H₄·<b>I</b>, H₃·<b>I</b>, and H₂·<b>I</b>, respectively

Reversible Deprotonation and Protonation Behaviors of a Tetra-Protonated γ-Keggin Silicodecatungstate

The potentiometric titration of a γ-Keggin tetra-protonated silicodecatungstate, [γ-SiW₁₀O₃₄(H₂O)₂]^4– (H₄·<b>I</b>), with TBAOH (TBA = [(<i>n</i>-C₄H₉)₄N]⁺) showed inflection points at 2 and 3 equiv of TBAOH. The ¹H, ²⁹Si, and ¹⁸³W NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [γ-SiW₁₀O₃₄(OH)­(OH₂)]^5– (H₃·<b>I</b>), [γ-SiW₁₀O₃₄(OH)₂]^6– (H₂·<b>I</b>), and [γ-SiW₁₀O₃₅(OH)]^7– (H·<b>I</b>), with <i>C</i>₁, <i>C</i>_2<i>v</i>, and <i>C</i>₂ symmetries, respectively. Single crystals of TBA₆·H₂·<b>I</b> 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 H₃·<b>I</b>, H₂·<b>I</b>, and H·<b>I</b> were reversibly monoprotonated to form H₄·<b>I</b>, H₃·<b>I</b>, and H₂·<b>I</b>, respectively

Electronic Effect of Ruthenium Nanoparticles on Efficient Reductive Amination of Carbonyl Compounds

Highly selective synthesis of primary amines over heterogeneous catalysts is still a challenge for the chemical industry. Ruthenium nanoparticles supported on Nb₂O₅ act as a highly selective and reusable heterogeneous catalyst for the low-temperature reductive amination of various carbonyl compounds that contain reduction-sensitive functional groups such as heterocycles and halogens with NH₃ and H₂ and prevent the formation of secondary amines and undesired hydrogenated byproducts. The selective catalysis of these materials is likely attributable to the weak electron-donating capability of Ru particles on the Nb₂O₅ surface. The combination of this catalyst and homogeneous Ru systems was used to synthesize 2,5-bis­(aminomethyl)­furan, a monomer for aramid production, from 5-(hydroxymethyl)­furfural without a complex mixture of imine byproducts

Strategic Design and Refinement of Lewis Acid–Base Catalysis by Rare-Earth-Metal-Containing Polyoxometalates

Efficient polyoxometalate (POM)-based Lewis acid–base catalysts of the rare-earth-metal-containing POMs (TBA₆RE-POM, RE = Y³⁺, Nd³⁺, Eu³⁺, Gd³⁺, Tb³⁺, or Dy³⁺) were designed and synthesized by reactions of TBA₄H₄[γ-SiW₁₀O₃₆] (TBA = tetra-<i>n</i>-butylammonium) with RE­(acac)₃ (acac = acetylacetonato). TBA₆RE-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 <i>n</i>-hexanal were of the highest level among those of previously reported catalysts

Formation of 5‑(Hydroxymethyl)furfural by Stepwise Dehydration over TiO₂ with Water-Tolerant Lewis Acid Sites

The reaction mechanism for the formation of 5-(hydroxymethyl)­furfural (HMF) from glucose in water over TiO₂ and phosphate-immobilized TiO₂ (phosphate/TiO₂) with water-tolerant Lewis acid sites was studied using isotopically labeled molecules and ¹³C nuclear magnetic resonance measurements for glucose adsorbed on TiO₂. Scandium trifluoromethanesulfonate (Sc­(OTf)₃), a highly active homogeneous Lewis acid catalyst workable in water, converts glucose into HMF through aldose–ketose isomerization between glucose and fructose involving a hydrogen transfer step and subsequent dehydration of fructose. In contrast to Sc­(OTf)₃, Lewis acid sites on bare TiO₂ and phosphate/TiO₂ do not form HMF through the isomerization–dehydration route but through the stepwise dehydration of glucose via 3-deoxyglucosone as an intermediate. Continuous extraction of the evolved HMF with 2-<i>sec</i>-butylphenol results in the increase in the HMF selectivity for phosphate/TiO₂, even in highly concentrated glucose solution. These results suggest that limiting the reactions between HMF and the surface intermediates improves the efficiency of HMF production

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)_<i>n</i>CO₂)}₂] (<i>n</i> = 1 (<b>II</b>), 3 (<b>III</b>), and 5 (<b>IV</b>), TBA = [(<i>n</i>-C₄H₉)₄N]⁺), were synthesized by exchange of the acetate ligands in TBA₄[γ-H₂SiW₁₀O₃₆Pd₂(OAc)₂] (<b>I</b>_<b>TBA</b>) with malonic, glutaric, and pimelic acids, respectively. The X-ray crystallographic analysis of <b>II</b>, <b>III_A</b> (<b>III</b>_<b>A</b>: <b>III</b> with DCE, DCE = 1,2-dichloroethane), and <b>IV</b>_<b>A</b> (<b>IV</b>_<b>A</b>: <b>IV</b> with 10DCE) revealed that the anion parts of <b>II</b>, <b>III</b>_<b>A</b>, and <b>IV</b>_<b>A</b> were inorganic-organic-inorganic hybrids composed of two dipalladium-substituted γ-Keggin silicodecatungstates connected by two dicarboxylate ligands. In the crystal structure of <b>IV</b>_<b>A</b>, 10 DCE molecules per polyanion were present in the vicinity of polyanions. Compound <b>IV</b>_<b>B</b> (<b>IV</b>_<b>B</b>: <b>IV</b> with 0.2DCE) was obtained by the evacuation of <b>IV</b>_<b>A</b>. The DCE sorption–desorption isotherms of <b>IV</b>_<b>B</b> 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 <b>IV</b>_<b>A</b> changed with decrease in the relative DCE vapor pressure to form <b>IV</b>_<b>C</b> (<b>IV</b>_<b>C</b>: <b>IV</b> with 0.7DCE) at <i>P</i>/<i>P</i>₀ = 0.0. The in situ powder XRD study showed reversible structure transformation between <b>IV</b>_<b>A</b> and <b>IV</b>_<b>C</b> driven by the sorption–desorption of DCE