Cooperative Catalysis of Primary and Tertiary Amines Immobilized on Oxide Surfaces for One‐Pot CC Bond Forming Reactions

Concerted Hydrosilylation Catalysis by Silica-Immobilized Cyclic Carbonates and Surface Silanols

Developing a method for creating a novel catalysis of organic molecules is essential because of the growing interest in organocatalysis. In this study, we found that cyclic carbonates immobilized on a nonporous or mesoporous silica support showed catalytic activity for hydrosilylation, which was not observed for the free cyclic carbonates, silica supports, or their physical mixture. Analysis of the effects of linker lengths and pore sizes on the catalytic activity and carbonate C=O stretching frequency revealed that the proximity of carbonates and surface silanols was crucial for synergistic hydrosilylation catalysis. A carbonate and silanol concertedly activated the silane and aldehyde for efficient hydride transfer. Density functional theory calculations on a model reaction system demonstrated that both the carbonate and silanol contributed to the stabilization of the transition state of hydride transfer, which resulted in a reasonable barrier height of 16.8 kcal mol-1. Furthermore, SiO2/carbonate(C4) enabled the hydrosilylation of an aldehyde with an amino group without catalyst poisoning, owing to the weak acidity of surface silanols, in sharp contrast to previously developed acid catalysts. This study demonstrates that immobilization on a solid support can convert inactive organic molecules into active and heterogeneous organocatalysts.

Multistep Cascade Catalytic Reactions Employing Bifunctional Framework Compounds

Multistep cascade reactions are important to achieve atom as well as step economy over conventional synthesis. This approach, however, is limited due to the incompatibility of the available reactive centers in a catalyst. In the present study, new MOF compounds, [Zn₂(SDBA)(3-ATZ)₂]·solvent, I and II, with tetrahedral Zn centers as good Lewis acidic sites and the free amino group of the 3-amino triazole ligand as a strong Lewis base center were shown to perform 4-step cascade/tandem reaction in a facile manner. Effective conversion of benzaldehyde dimethyl acetal in the presence of excess nitromethane at 100 °C in water to 1-(1,3-dinitropropan-2-yl) benzene was achieved in 10 h with yields of ∼95% (I) and ∼94% (II). This 4-step cascade reaction proceeds via deacetalization (Lewis acid), Henry (Lewis base), and Michael (Lewis base) reactions. The present work highlights the importance of spatially separated functional groups in multistep tandem catalysis─the examples of which are not common.

Plasmonic Heating-Promoted Photothermal Synthesis of α-Cyanoacrylonitriles Over Au/h-BN Catalysts

Plasmonic nanoparticle-involved materials play an essential role in the field of photothermal conversion. Herein, we report the application of photothermal heterogeneous catalysts consisting of gold nanoparticles decorated on defect-rich h-BN sheets (Au/h-BN) for the photocatalytic synthesis of α-cyanoacrylonitriles under mild conditions. It has been demonstrated the–NH₂ groups present in the defect-rich h-BN act as the catalytically active sites, while plasmonic heating from the gold nanoparticles can drive the reaction by providing local heat. Au/h-BN catalyst can work for a broad substrate scope in the synthesis of α-cyanoacrylonitriles, and a plausible –NH₂ group-involved reaction mechanism has been proposed. This work may open up new avenues in photothermal catalysis by combining plasmonic materials and catalytic sites in one system.

Deep eutectic solvent as solvent and catalyst: one-pot synthesis of 1,3-dinitropropanes via tandem Henry reaction/Michael addition

The Henry reaction was performed using microwave heating within the deep eutectic solvent (DES) choline chloride/urea (ChCl/urea) which acted as both the catalyst and solvent for the reaction. The optimisation of the conditions (temperature, heating mode, time, DES) allowed 1,3-dinitropropane derivatives to be obtained via tandem Henry reaction/Michael addition, in one step from a range of different aromatic aldehydes in high yields and under mild reaction conditions.

Multifunctional Catalytic Surface Design for Concerted Acceleration of One-Pot Hydrosilylation-CO2 Cycloaddition

Silica-supported Rh-ammonium iodide catalyst showed high performance for hydrosilylation-CO₂ cycloaddition reaction sequences. The catalyst was prepared by surface grafting of Rh and the silane-coupling reaction of the ammonium iodide moiety. The acceleration of each catalytic reaction was realized due to the concerted catalysis between Rh species, immobilized organic functions, and surface Si-OH groups. As a result, good to excellent yields of silyl carbonates were obtained from epoxyolefins, hydrosilanes, and CO₂ under mild reaction conditions.

Effects of ball-milling treatment on physicochemical properties and solid base activity of hexagonal boron nitrides

Hexagonal boron nitride solid base catalysts were prepared by simple ball-milling at various rotation speeds of a commercial low-surface area boron nitride.

Phosphor-doped hexagonal boron nitride nanosheets as effective acid–base bifunctional catalysts for one-pot deacetalization–Knoevenagel cascade reactions

Phosphor doping creates well-defined Brønsted acid and base sites on h-BN nanosheets, ensuring the smooth proceeding of deacetalization–Knoevenagel cascade reaction.

Electric-Field-Assisted Anion-π Catalysis

This report focuses on the remote control of anion-π catalysis by electric fields. We have synthesized and immobilized anion-π catalysts to explore the addition reaction of malonic acid half thioesters to enolate acceptors on conductive indium tin oxide surfaces. Exposed to increasing electric fields, anion-π catalysts show an increase in activity and an inversion of selectivity. These changes originate from a more than 100-fold rate enhancement of the disfavored enolate addition reaction that coincides with an increase in selectivity of transition-state recognition by up to -14.8 kJ mol^-1. The addition of nitrate with strong π affinity nullified (IC₅₀ = 2.2 mM) the responsiveness of anion-π catalysts to electric fields. These results support that the polarization of the π-acidic naphthalenediimide surface in anion-π catalysts with electric fields increases the recognition of anionic intermediates and transition states on this polarized π surface, that is, the existence and relevance of electric-field-assisted anion-π catalysis.

A dual-functional heterogeneous ruthenium catalyst for the green one-pot synthesis of biphenols

A green one-pot synthesis of biphenols using O₂ and H₂ was achieved using a magadiite-supported Ru nanoparticle catalyst.

A Novel Strategy for Biomass Upgrade: Cascade Approach to the Synthesis of Useful Compounds via C-C Bond Formation Using Biomass-Derived Sugars as Carbon Nucleophiles

Due to the depletion of fossil fuels, biomass-derived sugars have attracted increasing attention in recent years as an alternative carbon source. Although significant advances have been reported in the development of catalysts for the conversion of carbohydrates into key chemicals (e.g., degradation approaches based on the dehydration of hydroxyl groups or cleavage of C-C bonds via retro-aldol reactions), only a limited range of products can be obtained through such processes. Thus, the development of a novel and efficient strategy targeted towards the preparation of a range of compounds from biomass-derived sugars is required. We herein describe the highly-selective cascade syntheses of a range of useful compounds using biomass-derived sugars as carbon nucleophiles. We focus on the upgrade of C2 and C3 oxygenates generated from glucose to yield useful compounds via C-C bond formation. The establishment of this novel synthetic methodology to generate valuable chemical products from monosaccharides and their decomposed oxygenated materials renders carbohydrates a potential alternative carbon resource to fossil fuels.

Direct Estimation of the Surface Location of Immobilized Functional Groups for Concerted Catalysis Using a Probe Molecule

The location of active sites during concerted catalysis by a metal complex and tertiary amine on a SiO2 surface is discussed based on the interaction between the functionalized SiO2 surface and a probe molecule, p-formyl phenylboronic acid. The interactions of the probe molecule with the surface functionalities, diamine ligand, and tertiary amine, were analyzed by FT-IR and solid-state (13)C and (11)B MAS NMR. For the catalyst exhibiting high 1,4-addition activity, the diamine ligand and tertiary amine base exist in closer proximity than in the catalyst with low activity.

One “Click” to controlled bifunctional supported catalysts for the Cu/TEMPO-catalyzed aerobic oxidation of alcohols

A modular strategy is described for the preparation and molecular engineering of multifunctional surfaces using CuAAC chemistry and is applied to the model Cu/TEMPO-catalyzed aerobic oxidation of benzyl alcohol.

Yolk–shell-structured mesoporous silica: a bifunctional catalyst for nitroaldol–Michael one-pot cascade reaction

A site-isolated yolk–shell-structured mesoporous silica for the nitroaldol–Michael one-pot enantio-relay reaction to convert aldehydes, nitromethane and acetylacetone to chiral diones is developed.

Post-grafting amination of alkyl halide-functionalized silica for applications in catalysis, adsorption, and 15N NMR spectroscopy

An anhydrous synthesis of aminosilica materials from alkyl halide-functionalized mesoporous SBA-15 silica by post-grafting amination is introduced for applications in CO2 adsorption, cooperative catalysis, and (15)N solid-state NMR spectroscopy. The synthesis is demonstrated to convert terminal alkyl halide-functionalized silica materials containing Cl, Br, and I to primary alkylamines using anhydrous ammonia in a high-pressure reactor. The benefits of the post-grafting amination procedure include (i) use of anhydrous isotopically labeled ammonia, (15)NH3, to create aminosilica materials that can be investigated using (15)N solid-state NMR to elucidate potential intermediates and surface species in CO2 adsorption processes and catalysis, (ii) similar CO2 uptake in experiments extracting CO2 from dry simulated air experiments, and (iii) improved activity in acid-base bifunctional catalysis compared to traditional amine-grafted materials. The effects of the type of halide, the initial halide loading, and the total reaction time on the conversion of the halides to primary amines are explored. Physical and chemical characterizations of the materials show that the textural properties of the silica are unaffected by the reaction conditions and that quantitative conversion to primary amines is achieved even at short reaction times and high initial alkyl halide loadings. Additionally, preliminary (15)N solid-state NMR experiments indicate formation of nitrogen-containing species and demonstrate that the synthesis can be used to create materials useful for investigating surface species by NMR spectroscopy. The differences between the materials prepared via post-grafting amination vs traditional aminosilane grafting are attributed to the slightly increased spacing of the amines synthesized by amination because the alkylhalosilanes are initially better spaced on the silica surface after grafting, whereas the aminosilanes likely cluster to a greater extent when grafted on the silica surface. A slight increase in amine spacing allows for more effective amine-silanol interactions in cooperative catalysis without reducing the amine efficiency in CO2 uptake under the conditions used here.

Organoamine-functionalized graphene oxide as a bifunctional carbocatalyst with remarkable acceleration in a one-pot multistep reaction

In this work, we reported the synthesis of bifunctional carbocatalyst with acid-base dual-activation mechanism by introducing organoamines on the basal planes of graphene oxide (GO). Interestingly, GO-supported primary amine (AP-GO) exclusively promoted one-pot Henry-Michael reactions with excellent activity to give synthetically valuable multifunctionalized nitroalkanes. Notably, it also exhibited significantly higher activity than those using propylamine, activated carbon-supported primary amine, and mesoporous silica-supported primary amine as the catalysts. This superior catalytic performance originated from the unique properties of AP-GO, which provided the acid-base cooperative effect by the appropriate positioning of primary amines on their basal planes and carboxyl acids along their edges and the decreased diffusion resistance of the reactants and the intermediates during the multistep catalytic cycles because of its open two-dimensional sheet-like structure. Moreover, it could be readily recycled by simple filtration and subsequently reused without significant loss of its catalytic activity in a six times run test.

Heterogeneous double-activation catalysis: Rh complex and tertiary amine on the same solid surface for the 1,4-addition reaction of aryl- and alkylboronic acids

Double-activation catalysis by a rhodium complex/tertiary amine catalyst for the 1,4-addition of organoboronic acids was investigated.

Nitroaldol (Henry) reaction of 2-oxoaldehydes with nitroalkanes as a strategic step for a useful, one-pot synthesis of 1,2-diketones

The nitroaldol (Henry) reaction of 2-oxoaldehydes with a variety of nitroalkanes, under basic heterogeneous conditions and microwave irradiation, affords 1,2-diketones in a one-pot way.

Ethylenediamine-functionalized magnetic Fe3O4@SiO2 nanoparticles: cooperative trifunctional catalysis for selective synthesis of nitroalkenes

The designed nanocatalyst Fe₃O₄@SiO₂–NH₂ was highly active for selective synthesis of nitroalkenes through cooperative trifunctional catalysis of primary amine, secondary amine and Si–OH groups.

Aminosilanes grafted to basic alumina as CO2 adsorbents--role of grafting conditions on CO2 adsorption properties

Solid oxide-supported amine sorbents for CO2 capture are amongst the most rapidly developing classes of sorbent materials for CO2 capture. Herein, basic γ supports are used as hosts for amine sites through the grafting of 3-aminopropyltrimethoxysilane to the alumina surface under a variety of conditions, yielding the expected surface-grafted alkylamine groups, as demonstrated by FTIR spectroscopy and (29)Si and (13)C cross-polarization magic-angle spinning (CPMAS NMR) spectroscopy. Grafting amine sites on the surface in the presence of water leads to a high density of amine sites on the surface whereas simultaneously creating a unique type of aluminum species on the surface, as demonstrated by both 1D and 2D (27)Al MAS NMR spectroscopy. The thus prepared sorbents result in higher CO2 adsorption capacities and amine efficiencies compared to sorbents prepared in the absence of water or similar amine loading sorbents prepared using silica supports. In situ FTIR spectra of the sorbents exposed to CO2 at various pressures show no distinct difference in the nature of the adsorbed CO2 species on alumina- versus silica-supported amines, whereas water adsorption isotherms show that the improved performance of the amine-grafted alumina support is not a consequence of retained water on the more hydrophobic aminoalumina materials. The findings demonstrate that amine-grafted, basic alumina materials can be tuned to be more efficient than the corresponding silica-supported materials at comparable amine loadings, further demonstrating that the properties of amine sites can be tuned by controlling or adjusting the support surface properties.

An erbium-based bifuctional heterogeneous catalyst: a cooperative route towards C-C bond formation

Heterogeneous bifuctional catalysts are multifunctional synthetic catalysts enabling efficient organic transformations by exploiting two opposite functionalities without mutual destruction. In this paper we report the first Er(III)-based metallorganic heterogeneous catalyst, synthesized by post-calcination MW-assisted grafting and modification of the natural aminoacid L-cysteine. The natural acid–base distance between sites was maintained to assure the cooperation. The applicability of this new bifunctional heterogeneous catalyst to C-C bond formation and the supposed mechanisms of action are discussed as well.