Silver(I)‐ or Copper(II)‐Mediated Dearomatization of Aromatic Ynones: Direct Access to Spirocyclic Scaffolds

A high‐yielding silver(I)‐ or copper(II)‐catalyzed dearomatizing spirocyclization strategy allows the conversion of simple aromatic compounds that contain ynone substituents, including indole, anisole, pyrrole, and benzofuran derivatives, into functionalized spirocyclic scaffolds. A high‐yielding asymmetric variant furnishes spirocyclic indolenines in up to 89:11 e.r.     

A Simple and Fast Aqueous‐Phase Synthesis of Ultra‐Highly Concentrated Silver Nanoparticles and Their Catalytic Properties

A simple and fast synthetic route to ultra‐highly concentrated silver nanoparticles with long‐term stability by reducing AgNO₃ with ascorbic acid in the presence of polyethyleneimine (PEI) as a stabilizer in an aqueous phase is reported. The concentration of silver precursor was as high as 2000 mm (200 g of Ag nanoparticle per liter of water) and the reaction time was less than 10 min. The resulting silver nanoparticles show long‐term stability after two months of storage at room temperature without any signs of particle aggregation or precipitation in an aqueous phase. The successful ligand exchange of PEI‐stabilized silver nanoparticles to polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) without particle aggregation is also demonstrated. In addition, the catalytic activities of silver nanoparticles stabilized by various stabilizers prepared by the ligand exchange method was investigated. The PEI‐stabilized silver nanoparticles exhibited a higher stability than those of PEG‐ and PVP‐stabilized silver nanoparticles in the diffusion‐controlled catalytic reduction of 4‐nitrophenol to 4‐aminophenol by NaBH₄.     

Fabrication of Ultrafine Palladium Phosphide Nanoparticles as Highly Active Catalyst for Chemoselective Hydrogenation of Alkynes

Monodisperse palladium phosphide nanoparticles (Pd–P NPs) with a smallest size ever reported of 3.9 nm were fabricated using cheap and stable triphenylphosphine as phosphorous source. After the deposition and calcination at 300 °C and 400 °C, the resulting Pd–P NPs increased in size to 4.0 nm and 4.8 nm, respectively. Notably, the latter NPs probably crystallized with a single phase of Pd₃P_0.95, which acted as a highly active catalyst in semi‐ and stereoselective hydrogenation of alkynes. X‐ray photoelectron spectroscopy analysis determined a positive shift of binding energy for Pd(3d) in Pd–P NPs compared to that in Pd on carbon. It indicated the electron flow from metal to phosphorus and the larger electron deficiency of Pd in Pd–P NPs, which suppressed palladium hydride formation and subsequently increased the selectivity. Thus, this result may also indicate the applications of Pd–P and other metal–P NPs in various selective hydrogenation reactions.     

Well‐Defined Silica‐Supported Calcium Reagents: Control of Schlenk Equilibrium by Grafting

Bound to stay heteroleptic : Grafting of homoleptic benzyl and amide calcium reagents onto silica affords well‐defined supported heteroleptic species, catalytically active in hydrosilylation, hydroamination, and styrene polymerization. The fact that attempts to generate a molecular model were thwarted by the Schlenk equilibrium affording the homoleptic compound (see scheme) emphasizes the importance of immobilization as an approach for the syntheses of heteroleptic calcium complexes.

     

Graphene‐Supported Ultrafine Metal Nanoparticles Encapsulated by Mesoporous Silica: Robust Catalysts for Oxidation and Reduction Reactions

Graphene nanosheet‐supported ultrafine metal nanoparticles encapsulated by thin mesoporous SiO₂ layers were prepared and used as robust catalysts with high catalytic activity and excellent high‐temperature stability. The catalysts can be recycled and reused in many gas‐ and solution‐phase reactions, and their high catalytic activity can be fully recovered by high‐temperature regeneration, should they be deactivated by feedstock poisoning. In addition to the large surface area provided by the graphene support, the enhanced catalytic performance is also attributed to the mesoporous SiO₂ layers, which not only stabilize the ultrafine metal nanoparticles, but also prevent the aggregation of the graphene nanosheets. The synthetic strategy can be extended to other metals, such as Pd and Ru, for preparing robust catalysts for various reactions.     

Nanoparticles as Semi‐Heterogeneous Catalyst Supports

Nanoparticles can serve as semi‐heterogeneous supports since they readily disperse in common solvents and combine high surface area with excellent accessibility. Reversible agglomeration through solvent changes and magnetic separation provide technically attractive alternatives to classical catalyst filtration. This account places emphasis on recent developments in this emerging area.     

Encapsulation of Silver Nanoparticles in an Amine‐Functionalized Porphyrin Metal–Organic Framework and Its Use as a Heterogeneous Catalyst for CO2 Fixation under Atmospheric Pressure

A new porphyrin‐based compound, [Zn₃(C₄₀H₂₄N₈)(C₂₀H₈N₂O₄)₂(DEF)₂](DEF)₃ ( 1 ; DEF=N,N‐diethylformamide), has been synthesized by employing 5,10,15,20‐tetrakis(4‐pyridyl)porphyrin, 1,2‐diamino‐3,6‐bis(4‐carboxyphenyl)benzene, and Zn²⁺ salt at 100 °C under solvothermal conditions. The structure, as determined by single‐crystal XRD studies, is three‐dimensional with threefold interpenetration. The usefulness of free −NH₂ groups in the ligand was exploited for anchoring silver nanoparticles through a simple solution‐based route. The silver‐loaded sample, Ag@ 1 , was characterized by powder XRD, energy‐dispersive X‐ray spectroscopy, high‐resolution TEM, SEM, X‐ray photoelectron spectroscopy, and inductively coupled plasma MS analysis, which clearly indicated that silver nanoparticles with a size of 3.83 nm were uniformly distributed within the metal–organic framework (MOF). The Ag@ 1 sample was evaluated for possible catalytic activity for the carboxylation of a terminal alkyne by employing CO₂ under atmospheric pressure; this gave excellent results. The Ag@ 1 catalyst was found to be robust, active, and recyclable. The present studies suggest that porphyrin MOFs not only exhibit interesting structures, but also show good heterogeneous catalytic activity towards the fixation of CO₂.     

Calix[4]arene‐Functionalised Silver Nanoparticles as Hosts for Pyridinium‐Loaded Gold Nanoparticles as Guests

A series of lipophilic gold nanoparticles (AuNPs) circa 5 nm in diameter and having a mixed organic layer consisting of 1‐dodecanethiol and 1‐(11‐mercaptoundecyl) pyridinium bromide was synthesised by reacting tetraoctylammonium bromide stabilised AuNPs in toluene with different mixtures of the two thiolate ligands. A bidentate ω‐alkylthiolate calix[4]arene derivative was instead used as a functional protecting layer on AgNPs of approximately 3 nm. The functionalised nanoparticles were characterised by transmission electron microscopy (TEM), and by UV/Vis and X‐ray photoelectron spectroscopy (XPS). Recognition of the pyridinium moieties loaded on the AuNPs by the calix[4]arene units immobilised on the AgNPs was demonstrated in solution of weakly polar solvents by UV/Vis titrations and DLS measurements. The extent of Au‐AgNPs aggregation, shown through the low‐energy shift of their surface plasmon bands (SPB), was strongly dependent on the loading of the pyridinium moieties present in the organic layer of the AuNPs. Extensive aggregation between dodecanethiol‐capped AuNPs and the Ag calix[4]arene‐functionalised NPs was also promoted by the action of a simple N‐octyl pyridinium difunctional supramolecular linker. This linker can interdigitate through its long fatty tail in the organic layer of the dodecanethiol‐capped AuNPs, and simultaneously interact through its pyridinium moiety with the calix[4]arene units at the surface of the modified AgNPs.     

Graphite‐Supported Gold Nanoparticles as Efficient Catalyst for Aerobic Oxidation of Benzylic Amines to Imines and N‐Substituted 1,2,3,4‐Tetrahydroisoquinolines to Amides: Synthetic Applications and Mechanistic Study

Selective oxidation of amines using oxygen as terminal oxidant is an important area in green chemistry. In this work, we describe the use of graphite‐supported gold nanoparticles (AuNPs/C) to catalyze aerobic oxidation of cyclic and acyclic benzylic amines to the corresponding imines with moderate‐to‐excellent substrate conversions (43–100 %) and product yields (66–99 %) (19 examples). Oxidation of N‐substituted 1,2,3,4‐tetrahydroisoquinolines in the presence of aqueous NaHCO₃ solution gave the corresponding amides in good yields (83–93 %) with high selectivity (up to amide/enamide=93:4) (6 examples). The same protocol can be applied to the synthesis of benzimidazoles from the reaction of o‐phenylenediamines with benzaldehydes under aerobic conditions (8 examples). By simple centrifugation, AuNPs/C can be recovered and reused for ten consecutive runs for the oxidation of dibenzylamine to N‐benzylidene(phenyl)methanamine without significant loss of catalytic activity and selectivity. This protocol “AuNPs/C+O₂” can be scaled to the gram scale, and 8.9 g (84 % isolated yield) of 3,4‐dihydroisoquinoline can be obtained from the oxidation of 10 g 1,2,3,4‐tetrahydroisoquinoline in a one‐pot reaction. Based on the results of kinetic studies, radical traps experiment, and Hammett plot, a mechanism involving the hydrogen‐transfer reaction from amine to metal and oxidation of M‐H is proposed.     

Polymeric Carbon Nitride/Mesoporous Silica Composites as Catalyst Support for Au and Pt Nanoparticles

Small and homogeneously dispersed Au and Pt nanoparticles (NPs) were prepared on polymeric carbon nitride (CN_x)/mesoporous silica (SBA‐15) composites, which were synthesized by thermal polycondensation of dicyandiamide‐impregnated preformed SBA‐15. By changing the condensation temperature, the degree of condensation and the loading of CN_x can be controlled to give adjustable particle sizes of the Pt and Au NPs subsequently formed on the composites. In contrast to the pure SBA‐15 support, coating of SBA‐15 with polymeric CN_x resulted in much smaller and better‐dispersed metal NPs. Furthermore, under catalytic conditions the CN_x coating helps to stabilize the metal NPs. However, metal NPs on CN_x/SBA‐15 can show very different catalytic behaviors in, for example, the CO oxidation reaction. Whereas the Pt NPs already show full CO conversion at 160 °C, the catalytic activity of Au NPs seems to be inhibited by the CN_x support.     

Nucleation of Tiny Silver Nanoparticles by Using a Tetrafacial Organic Molecular Barrel: Potential Use in Visible-Light-Triggered Photocatalysis

Coordination-driven self-assembly of discrete molecular architectures of diverse shapes and sizes has been well studied in the last three decades. Use of dynamic imine bonds for designing analogous metal-free architectures has become a growing challenge recently. This article reports an organic molecular barrel ( OB4^R ) as a potential template for nucleation and stabilization of very tiny (<1.5 nm) Ag nanoparticles (AgNPs). Imine bond condensation of a rigid tetra-aldehyde with a flexible diamine followed by imine-bond reduction yielded the discrete tetragonal organic barrel ( OB4^R ). The presence of a molecular pocket ornamented with eight diamine moieties gives the potential for encapsulation of silver(I). The organic barrel was finally used as a molecular vessel for the controlled nucleation of silver nanoparticles (AgNPs) with fine size tuning through binding of Ag^I ions in the confined space of the barrel followed by reduction. Transmission electron microscopy (TEM) analysis of the Ag⁰@OB4^R composite revealed that the mean particle size is 1.44±0.16 nm. The composite material has approximately 52 wt % silver loading. The barrel-supported ultrafine AgNPs [ Ag⁰@OB4^R ] are found to be an efficient photocatalyst for facile Ullmann-type aryl-amination coupling of haloarenes at ambient temperature without using any additives. The catalyst was stable for several cycles of reuse without any agglomeration. The new composite Ag⁰@OB4^R represents the first example of discrete organic barrel-supported AgNPs employed as a photocatalyst in Ullmann-type coupling reactions at room temperature.     

Silver Nanoparticles Supported on CeO2‐SBA‐15 by Microwave Irradiation Possess Metal–Support Interactions and Enhanced Catalytic Activity

Metal–support interactions (MSIs) and particle size play important roles in catalytic reactions. For the first time, silver nanoparticles supported on CeO₂‐SBA‐15 supports are reported that possess tunable particle size and MSIs, as prepared by microwave (MW) irradiation, owing to strong charge polarization of CeO₂ clusters (i.e., MW absorption). Characterizations, including TEM, X‐ray photoelectron spectroscopy, and extended X‐ray absorption fine structure, were carried out to disclose the influence of CeO₂ contents on the Ag particle size, MSI effect between Ag nanoparticles and CeO₂‐SBA‐15 supports, and the strong MW absorption of CeO₂ clusters that contribute to the MSIs during Ag deposition. The Ag particle sizes were controllably tuned from 1.9 to 3.9 nm by changing the loading amounts of CeO₂ from 0.5 to 2.0 wt %. The Ag nanoparticle size was predominantly responsible for the high turnover frequency (TOF) of 0.41 min^?1 in ammonia borane dehydrogenation, whereas both particle size and MSIs contributed to the high TOF of 555 min^?1 in 4‐nitrophenol reduction for Ag/0.5CeO₂‐SBA‐15, which were twice as large as those of Ag/SBA‐15 without CeO₂ and Ag/CeO₂‐SBA‐15 prepared by conventional oil‐bath heating.     

CuxCo1−xO Nanoparticles on Graphene Oxide as A Synergistic Catalyst for High‐Efficiency Hydrolysis of Ammonia–Borane

Ammonia–borane (AB) is an excellent material for chemical storage of hydrogen. However, the practical utilization of AB for production of hydrogen is hindered by the need of expensive noble metal‐based catalysts. Here, we report Cu_xCo_1?xO nanoparticles (NPs) facilely deposited on graphene oxide (GO) as a low‐cost and high‐performance catalyst for the hydrolysis of AB. This hybrid catalyst exhibits an initial total turnover frequency (TOF) value of 70.0 (H₂) mol/(Cat‐metal) mol?min, which is the highest TOF ever reported for noble metal‐free catalysts, and a good stability keeping 94 % activity after 5 cycles. Synchrotron radiation‐based X‐ray absorption spectroscopy (XAS) investigations suggested that the high catalytic performance could be attributed to the interfacial interaction between Cu_xCo_1?xO NPs and GO. Moreover, the catalytic hydrolysis mechanism was studied by in situ XAS experiments for the first time, which reveal a significant water adsorption on the catalyst and clearly confirm the interaction between AB and the catalyst during hydrolysis.     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]?n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was ?67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.     

A Brush‐Gel/Metal‐Nanoparticle Hybrid Film as an Efficient Supported Catalyst in Glass Microreactors

A polymer‐brush‐based material was applied for the formation and in situ immobilization of silver and palladium nanoparticles, as a catalytic coating on the inner wall of glass microreactors. The brush film was grown directly on the microchannel interior by means of atom‐transfer radical polymerization (ATRP), which allows control over the polymer film thickness and therefore permits the tuning of the number of nanoparticles formed on the channel walls. The wide applicability of the catalytic devices is demonstrated for the reduction of 4‐nitrophenol and for the Heck reaction.     

In Situ Generated Gold Nanoparticles on Active Carbon as Reusable Highly Efficient Catalysts for a C −C Stille Coupling

Gold nanoparticle catalysts are important in many industrial production processes. Nevertheless, for traditional C ?C cross‐coupling reactions they have been rarely used and Pd catalysts usually give a superior performance. Herein we report that in situ formed gold metal nanoparticles are highly active catalysts for the cross coupling of allylstannanes and activated alkylbromides to form C ?C bonds. Turnover numbers up to 29 000 could be achieved in the presence of active carbon as solid support, which allowed for convenient catalyst recovery and reuse. The present study is a rare case where a gold metal catalyst is superior to Pd catalysts in a cross‐coupling reaction of an organic halide and an organometallic reagent.