Carboxylate-Directed Addition of Aromatic C–H Bond to Aromatic Aldehydes under Ruthenium Catalysis

We report that ruthenium complexes effectively catalyzed the carboxylate-directed addition of aromatic C–H bonds to aldehydes. The reactions of aromatic acids with a variety of aromatic aldehydes including unactivated ones proceeded efficiently to give the corresponding isobenzofuranone derivatives in high yields. The combination of ruthenium­(II) complexes with tricyclohexylphosphine led to highly nucleophilic aryl–metal species, which enabled versatile [3 + 2] cycloaddition in the absence of a Lewis acid. This paper also demonstrates the application of supported ruthenium catalysts to the title reaction

In Situ Time-Resolved XAFS Study of the Reaction Mechanism of Bromobenzene Homocoupling Mediated by [Ni(cod)(bpy)]

A homocoupling reaction mechanism of bromobenzene mediated by the [Ni­(cod)­(bpy)] (cod = 1,5-cyclooctadiene; bpy = 2,2′-bipyridine) complex was investigated by means of in situ time-resolved X-ray absorption fine structure (XAFS) and factor analysis. A dimer intermediate [Ni­(bpy)­(Ph)­Br]₂ proposed in the previous studies by other groups is too dilute to observe with the XAFS technique; however, the structures and concentrations on the time course of a reactant [Ni­(cod)­(bpy)], an intermediate [Ni­(bpy)­(Ph)­Br­(dmf)₂], and a byproduct [Ni­(bpy)­Br₂(dmf)] during reaction are revealed by this combination

Borylation of Stable C(sp³)–O Bonds of Alkyl Esters over Supported Au Catalysts

We report herein that supported gold catalysts efficiently promote the borylation of stable C(sp3)–O bonds of alkyl esters. The use of a disilane as an electron source and gold nanoparticles as a single-electron transfer catalyst is the key to generating alkyl radicals via the homolysis of stable C(sp3)–O bonds, thereby enabling cross-coupling between bis(pinacolato)diboron and linear and cyclic alkyl esters to afford the diverse alkyl boronates

Reaction Mechanism of Selective Photooxidation of Hydrocarbons over Nb₂O₅

Selective aerobic photooxidation of hydrocarbons to oxidized products, mainly ketones, was conducted over Nb2O5 in the absence of solvent. For the oxidation of cyclohexane and ethylbenzene, Nb2O5 shows significantly higher selectivity to partial oxidation products than does TiO2. In the oxidation of cyclohexane, Nb2O5 gave a much higher ketone/alcohol ratio than TiO2. On the basis of spectroscopic and kinetic studies, the following mechanism is proposed. Both hydrocarbon and dioxygen adsorb on Nb2O5. The C–H bond of the hydrocarbon is activated to produce an alkyl radical by a photogenerated positive hole, and adsorbed oxygen is reduced by a photogenerated electron. The alkyl radical reacts with superoxide to produce a hydroperoxide. Then, the hydroperoxide is converted to a ketone, which subsequently desorbs from Nb2O5. This proposed mechanism is a typical Langmuir–Hinshelwood mechanism, including the formation of the hydroperoxide species as an intermediate. A linear correlation between logarithms of the reaction rates and bond dissociation enthalpies of several hydrocarbons indicates that the rate-determining step is C–H bond activation. An electron spin resonance (ESR) study suggests that the higher selectivity of Nb2O5 for partial oxidation products than TiO2 is due to the absence of O3–, which is highly active for the complete oxidation of hydrocarbons

Quantitative Evaluation of the Effect of the Hydrophobicity of the Environment Surrounding Brønsted Acid Sites on Their Catalytic Activity for the Hydrolysis of Organic Molecules

Sulfo-functionalized siloxane gels with a variety of surface hydrophobicities were fabricated to elucidate the effect of the environment surrounding the Brønsted acid site on their catalytic activity for the hydrolysis of organic molecules. A detailed structural analysis of these siloxane gels by elemental analysis, X-ray photoelectron spectroscopy, Fourier-transformed infrared (FT-IR), and 29Si MAS NMR revealed the formation of gel catalysts with a highly condensed siloxane network, which enabled us to quantitatively evaluate the hydrophobicity of the environment surrounding the catalytically active sulfo-functionality. A sulfo group in a highly hydrophobic environment exhibited excellent catalytic turnover frequency for the hydrolysis of acetate esters with a long alkyl chain, whereas not only conventional solid acid catalysts but also liquid acids showed quite low catalytic activity. Detailed kinetic studies corroborated that the adsorption of oleophilic esters at the Brønsted acid site was facilitated by the surrounding hydrophobic environment, thus significantly promoting hydrolysis under aqueous conditions. Furthermore, sulfo-functionalized siloxane gels with a highly hydrophobic surface showed excellent catalytic activity for the hydrolytic deprotection of silyl ethers

Ruthenium-Catalyzed Addition of Aromatic Amides to Internal Alkynes and Subsequent Intramolecular Cyclization for the Atom-Economical Synthesis of Isoindolinones

A selective and atom-economical synthesis of isoindolinones is described. This novel synthetic strategy involves two catalytic reactions: the ruthenium-catalyzed regioselective alkenylation of aromatic C–H bond of aromatic amides with internal alkynes, and subsequent intramolecular cyclization of the resulting alkene with amide functionalities. The addition of only a catalytic amount of bases is required for efficient construction of the desired isoindolinones, and no byproducts are formed in the tandem catalytic reactions. Various kinds of aromatic amides and internal alkynes can be used in the present reactions, and the corresponding isoindolinones bearing a quaternary carbon at the C3 position are obtained in good to high yields

π Back-Bonding of Iron(II) Complexes Supported by Tris(pyrid-2-ylmethyl)amine and Its Nitro-Substituted Derivatives

The electronic and geometric structures of a series of iron(II) complexes supported by tetradentate tris(pyrid-2-ylmethyl)amine-type ligands with different numbers of 4-nitropyridine groups, [(PyCH2)3–n(4-NO2PyCH2)nN] (n = 0–3), were examined by X-ray absorption fine-structure and variable-temperature 1H NMR spectroscopies and theoretical calculations to reveal how the low-spin state is stabilized through π back-bonding interactions between iron(II) and 4-nitropyridine donor group(s)

Local Structure and La L₁ and L₃‑Edge XANES Spectra of Lanthanum Complex Oxides

La L₁ and L₃-edge X-ray absorption near-edge structure (XANES) of various La oxides were classified according to the local configuration of La. We found a correlation between both of the areas of the pre-edge peaks of the La L₁-edge XANES spectra and the full width at half-maximum of white line of La L₃-edge XANES spectra and the local configuration of La. Theoretical calculation of the XANES spectra and local density of states reveals the difference of La L₁ and L₃-edge XANES spectra of various La compounds is related to the p–d hybridization of the unoccupied band and broadening of the d band of La induced by the difference of local configuration. In addition, simplified bond angle analysis parameters defined by the angles of the La atom and the two adjacent oxygen atoms are correlated to the pre-edge peak intensity of the La L₁-edge XANES spectra. These results indicate that quantitative analysis of La L₁ and L₃-edge XANES spectra could be an indicator of the local structure of La materials

Photocatalytic Oxidation of Alcohols over TiO₂ Covered with Nb₂O₅

A selectivity enhancement in alcohol photooxidation using TiO₂ covered with Nb₂O₅ is demonstrated. A series of TiO₂ covered with Nb₂O₅ catalysts (Nb₂O₅/TiO₂, loading of Nb₂O₅; 0–5 mol %) were prepared and characterized. XPS studies suggest that the TiO₂ surface was completely covered with Nb₂O₅ at a 3.5 mol % loading. UV–vis spectra of TiO₂ and the Nb₂O₅/TiO₂ series revealed that the band gap energy of the catalyst was not changed upon addition of Nb₂O₅. The amounts of photogenerated oxygen anion radical species (O₂^– and O₃^–) over the catalyst, as estimated by ESR, drastically decreased with increased loadings of Nb₂O₅. The O₃^– anion, in particular, which can be formed on anatase TiO₂, completely disappeared at Nb₂O₅ loadings over 4 mol %. In the oxidations of several alcohols (1-pentanol, 2-pentanol, 3-pentanol, and cyclohexanol), the Nb₂O₅/TiO₂ catalysts exhibited higher selectivities than TiO₂ with comparable conversion levels. Furthermore, the Nb₂O₅/TiO₂ system gave a higher photocatalytic activity compared with Nb₂O₅ without lowering the selectivity

Supported Palladium–Gold Alloy Catalysts for Efficient and Selective Hydrosilylation under Mild Conditions with Isolated Single Palladium Atoms in Alloy Nanoparticles as the Main Active Site

Supported Pd–Au alloy catalysts were developed for the highly efficient and selective hydrosilylation of α,β-unsaturated ketones and alkynes. The Pd/Au atomic ratio of the Pd–Au alloy and the supporting material affected the catalytic activity, and supported Pd–Au alloy nanoparticles with a low Pd/Au atomic ratio functioned as highly active heterogeneous catalysts under mild reaction conditions. Structural characterization of supported Pd–Au alloy catalysts by X-ray diffraction, X-ray absorption spectroscopy (XAS), and transmission electron microscopy revealed the formation of random Pd–Au alloy nanoparticles with a uniform size of around 3 nm on the support. Furthermore, XAS and X-ray photoelectron spectroscopy elucidated the charge transfer from Pd to Au and the formation of isolated single Pd atoms in random Pd–Au alloys with a low Pd/Au ratio, which enabled efficient hydrosilylation of a variety of substrates under mild reaction conditions