Ken Motokura

Copper-Catalyzed Formic Acid Synthesis from CO2 with Hydrosilanes and H2O

A copper-catalyzed formic acid synthesis from CO2 with hydrosilanes has been accomplished. The Cu(OAc)2·H2O-1,2-bis(diphenylphosphino)benzene system is highly effective for the formic acid synthesis under 1 atm of CO2. The TON value approached 8100 in 6 h. The reaction pathway was revealed by in situ NMR analysis and isotopic experiments.

Silica-supported aminopyridinium halides for catalytic transformations of epoxides to cyclic carbonates under atmospheric pressure of carbon dioxide

Silica-supported 4-pyrrolidinopyridinium iodide was prepared by quaternization of 4-pyrrolidinopyridine with silica-supported alkyl iodide. The pyrrolidinopyridinium structure on the silica surface was confirmed by solid-state 13C CP MAS NMR. The silica-supported 4-pyrrolidinopyridinium iodide showed excellent catalytic performances for transformations of various epoxides to cyclic carbonates under atmospheric pressure of carbon dioxide (CO2). The reactions took place without any solvents or additives other than the catalyst. The catalyst was reusable with retention of activity and selectivity. 1-n-Hexyl-4-pyrrolidinopyridinium as a homogeneous catalyst showed a lower catalytic performance than the supported catalyst. Bifunctional catalysis involving acidic surface silanol and the basic 4-pyrrolidinopyridinium iodide was proposed.

Layered materials with coexisting acidic and basic sites for catalytic one-pot reaction sequences.

Acidic montmorillonite-immobilized primary amines (H-mont-NH(2)) were found to be excellent acid-base bifunctional catalysts for one-pot reaction sequences, which are the first materials with coexisting acid and base sites active for acid-base tamdem reactions. For example, tandem deacetalization-Knoevenagel condensation proceeded successfully with the H-mont-NH(2), affording the corresponding condensation product in a quantitative yield. The acidity of the H-mont-NH(2) was strongly influenced by the preparation solvent, and the base-catalyzed reactions were enhanced by interlayer acid sites.

Acid–Base Bifunctional Catalysis of Silica–Alumina‐Supported Organic Amines for Carbon–Carbon Bond‐Forming Reactions

Acid-base bifunctional heterogeneous catalysts were prepared by the reaction of an acidic silica-alumina (SA) surface with silane-coupling reagents possessing amino functional groups. The obtained SA-supported amines (SA-NR2) were characterized by solid-state 13C and 29Si NMR spectroscopy, FT-IR spectroscopy, and elemental analysis. The solid-state NMR spectra revealed that the amines were immobilized by acid-base interactions at the SA surface. The interactions between the surface acidic sites and the immobilized basic amines were weaker than the interactions between the SA and free amines. The catalytic performances of the SA-NR2 catalysts for various carbon-carbon bond-forming reactions, such as cyano-ethoxycarbonylation, the Michael reaction, and the nitro-aldol reaction, were investigated and compared with those of homogeneous and other heterogeneous catalysts. The SA-NR2 catalysts showed much higher catalytic activities for the carbon-carbon bond-forming reactions than heterogeneous amine catalysts using other supports, such as SiO2 and Al2O3. On the other hand, homogeneous amines hardly promoted these reactions under similar reaction conditions, and the catalytic behavior of SA-NR2 was also different from that of MgO, which was employed as a typical heterogeneous base. An acid-base dual-activation mechanism for the carbon-carbon bond-forming reactions is proposed.

Highly Active and Selective Catalysis of Copper Diphosphine Complexes for the Transformation of Carbon Dioxide into Silyl Formate

Copper diphosphine complexes have been found to be highly active and selective homogeneous catalysts for the hydrosilylation of CO2. The structure of the phosphine ligands strongly affects their catalytic activity. Turnover number (TON) reaches 70,000 after 24 hours with 1,2-bis(diisopropylphosphino)benzene as a ligand under 1 atmosphere of CO2. (1)H and (13)C NMR spectra, carried out under the reaction conditions, showed the reaction mechanism through insertion of CO2 into Cu-H to afford Cu/formate species.

Influence of Si distribution in framework of SAPO-34 and its particle size on propylene selectivity and production rate for conversion of ethylene to propylene.

To investigate the effect of SAPO-34 particle size (with a fixed Si mole fraction in its framework) and that of the Si mole fraction (in a SAPO-34 framework with fixed particle size) on propylene selectivity and production rate for the conversion of ethylene to propylene, SAPO-34 was prepared by hydrothermal synthesis using tetraethyl ammonium hydroxide or morpholine as a structural agent. The conversion of ethylene was carried out at 473 K using SAPO-34. The selectivity for propylene, the rate of propylene production, and the lifetime of the catalyst were strongly influenced by the catalyst crystal size. The SAPO-34 with a approximately 2.5 microm particle size had the highest selectivity for propylene (approximately 80%) up to a high conversion of ethylene (approximately 70%), while SAPO-34 with smaller particles had a longer catalyst lifetime, implying that catalyst deactivation was suppressed. The mole fraction of Si in the SAPO-34 framework with fixed particle size had little influence on the selectivity for propylene, indicating that the acid strength of SAPO-34 is independent of the Si mole fraction and all protons in SAPO-34 behave equivalently. Furthermore, the acid strength of protons determined by the measurements of NH(3)-TPD (temperature-programmed desorption) spectra did not depend on either the Si mole fraction or the SAPO-34 particle size. This result was also evident in the cracking rate of n-butane, which increased proportionally with increasing number of protons in SAPO-34.The number of protons generated by the incorporation of Si4+ into the SAPO-34 lattice increased proportionally, up to one Si atom introduced into every cage of SAPO-34, but did not continue to increase with further introduction of Si4+ into the lattice.

Copper-diphosphine complex catalysts for N-formylation of amines under 1 atm of carbon dioxide with polymethylhydrosiloxane

N-formylation of a wide range of amines proceeded using copper-diphosphine complexes as homogeneous catalysts with polymethylhydrosiloxane (PMHS) under 1 atm of CO2. In the reaction of piperidine, for example, the turnover number (TON) reached 11 700 in 23 h with 90% yield of the formylated product. This TON value is much higher than those of the reported catalysts for the formylation of amines under 1 atm of CO2 with hydrosilanes. The Cu complexes with phosphines having ortho-phenylene structures acted as good ligands for the formylation, as compared to a bidentate ligand connected with a propyl chain and a monodentate ligand. Among these diphosphines, ligands with alkyl functionalities, such as isopropyl and cyclohexyl groups, produced better results than the phenyl group. Not only cyclic secondary amines, but also linear secondary amines and aromatic and aliphatic primary amines were found to be reactive substrates. In the case of 2,2,6,6-tetramethylpiperidin-4-amine, the formylation proceeded regioselectively. A catalytic reaction pathway was proposed from a separate experiment using [Me2NCO2][Me2NH2].

Acid–Base Bifunctional Catalytic Surfaces for Nucleophilic Addition Reactions

This article illustrates the modification of oxide surfaces with organic amine functional groups to create acid-base bifunctional catalysts, summarizing our previous reports and also presenting new data. Immobilization of organic amines as bases on inorganic solid-acid surfaces afforded highly active acid-base bifunctional catalysts, which enabled various organic transformations including C--C coupling reactions, though these reactions did not proceed with either the homogeneous amine precursors or the acidic supports alone. Spectroscopic characterization, such as by solid-state MAS NMR and FTIR, revealed not only the interactions between acidic and basic sites but also bifunctional catalytic reaction mechanisms.

Bifunctional Heterogeneous Catalysis of Silica–Alumina‐Supported Tertiary Amines with Controlled Acid–Base Interactions for Efficient 1,4‐Addition Reactions

We report the first tunable bifunctional surface of silica-alumina-supported tertiary amines (SA-NEt(2)) active for catalytic 1,4-addition reactions of nitroalkanes and thiols to electron-deficient alkenes. The 1,4-addition reaction of nitroalkanes to electron-deficient alkenes is one of the most useful carbon-carbon bond-forming reactions and applicable toward a wide range of organic syntheses. The reaction between nitroethane and methyl vinyl ketone scarcely proceeded with either SA or homogeneous amines, and a mixture of SA and amines showed very low catalytic activity. In addition, undesirable side reactions occurred in the case of a strong base like sodium ethoxide employed as a catalytic reagent. Only the present SA-supported amine (SA-NEt(2)) catalyst enabled selective formation of a double-alkylated product without promotions of side reactions such as an intramolecular cyclization reaction. The heterogeneous SA-NEt(2) catalyst was easily recovered from the reaction mixture by simple filtration and reusable with retention of its catalytic activity and selectivity. Furthermore, the SA-NEt(2) catalyst system was applicable to the addition reaction of other nitroalkanes and thiols to various electron-deficient alkenes. The solid-state magic-angle spinning (MAS) NMR spectroscopic analyses, including variable-contact-time (13)C cross-polarization (CP)/MAS NMR spectroscopy, revealed that acid-base interactions between surface acid sites and immobilized amines can be controlled by pretreatment of SA at different temperatures. The catalytic activities for these addition reactions were strongly affected by the surface acid-base interactions.

Photoinduced Reversible Structural Transformation and Selective Oxidation Catalysis of Unsaturated Ruthenium Complexes Supported on SiO2

The photoirradiation of a SiO2-supported Ru complex was found to promote the selective formation of two different, novel unsaturated Ru structures on the surface, dependent on an O2 or N2 atmosphere, differing in the orientation of an Ru H moiety. One of these structures, owing to an appropriate Ru H conformation, catalyzed the selective photooxidation of cycloalkanes with O2. The two surface-bound unsaturated Ru complexes undergo reversible structural interconversion by photoexcitation at different wavelengths under different atmospheres. On heterogeneous catalyst surfaces, owing to the limited accessibility of reactants, rate-enhancement and new catalytic strategies can often be developed using novel, coordinatively unsaturated metal structures, which are hard to isolate in homogeneous solutions. Attachment of metal complexes onto a surface results in their stabilization and prevents aggregation and decomposition. Recently, we produced a novel three-coordinate unsaturated ruthenium complex on a SiO2 surface by coupling with SiO2-bound p-styryltrimethoxysilane. The unsaturated Ru complex was highly active for selective alkene epoxidation using a mixture of isobutyraldehyde and O2. However, the Ru complex was inactive for selective oxidation of saturated hydrocarbons with O2 as a sole oxidant, which may be more important from the viewpoint of practical use as a catalyst. A SiO2-supported Ru complex (B) was prepared using a N-sulfonyl-1,2-ethylenediamine–Ru complex (A) and pstyryltrimethoxysilane-functionalized SiO2 (Scheme 1, Supporting Information 1). The local coordination structure of B was similar to that of A. Ultraviolet irradiation (l> 275 nm) of B under N2 was found to cause the stoichiometric elimination of a coordinated p-cymene ligand from the supported Ru complex. 87% of free p-cymene was detected in a solution after the photoirradiation of B under N2 for 2 h (Table 1) affording the coordinatively unsaturated Ru complex C2. The elimination of p-cymene was also evidenced by C solid-state magic-angle spinning (MAS) NMR spectroscopy (Figure 1). X-ray photoelectron spectroscopy (XPS) revealed a shift in binding energy of Ru 3d5/2, on elimination of p-cymene, from 282.0 eV for B, to 282.2 eV for C2, (Table 1 and Supporting Information 2). The shift in the binding energy of Ru 3d5/2 indicates that the surface Ru complex is positively charged by the photoirradiation. However, the similar ratio of the XPS signal intensities for Cl 2p to Ru 3p3/2 in B and C2 indicates that the supported Ru complex C2 retains a Cl ligand. A change was also evident in the Ru K-edge X-ray absorption near-edge structure (XANES) spectroscopic signal (see Supporting Information 3). Ru K-edge extended X-ray absorption fine structure (EXAFS) spectroscopic analysis revealed two coordinations, Ru O(N) and Ru Cl, with bond orders of 3.2 and 1.0 , and bond distances of (2.10 0.01) and (2.38 0.01) , respectively (see Supporting Information 4), which confirms the retention of Cl, suggested by XPS, and also indicates surface coordination to Ru by oxygen, alongside the immobilization by silane coupling. C solid-state NMR spectroscopy indicated that the organic diamine ligand was retained during photoinduced p-cymene elimination (Figure 1). Photoirradiation (l> 275 nm) of B under an O2 atmosphere also resulted in dissociation of a p-cymene ligand (Table 1, Figure 1, Scheme 1) but afforded a different structure, C1, as evidenced by a very different UV/Vis spectrum to that of C2 (Figure 2). The spectrum for C2, produced under N2, shows two signals in the visible-light region, at 468 nm and 696 nm (Figure 2d), whereas that for C1, produced under O2, has one signal, at around 517 nm (Figure 2e). However, the XPS Ru 3d5/2 signal (at 282.2 eV), solid-state NMR spectrum, and Ru K-edge EXAFS spectrum of C1 were almost the same as those of C2. Notably, C1 and C2 are interconverted reversibly: C1 was converted into C2 by photoirradiation (l> 275 nm) under N2, and C2 was converted into C1 by photoirradiation (l> 370 nm) under O2 (Figure 2e–h). Neither O2 [*] Dr. M. Tada, Y. Akatsuka, Dr. Y. Yang, M. Kinoshita, Dr. K. Motokura, Prof. Dr. Y. Iwasawa Department of Chemistry, Graduate School of Science The University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan). Fax: (+ 81)3-5800-6892 E-mail: iwasawa@chem.s.u-tokyo.ac.jp