Highly Efficient,Catalytic Meerwein–Ponndorf–Verley Reduction with a Novel Bidentate Aluminum Catalyst

Double electrophilic activation of carbonyl groups allows a modern variant of the Meerwein–Ponndorf–Verley reduction to be carried out under mild conditions with bidentate catalyst 1 (see reaction). Various carbonyl substrates can be reduced efficiently at room temperature in CH₂Cl₂ with 2-propanol or sec-phenethyl alcohol in the presence of a catalytic amount of 1 . This system is also applicable to the Oppenauer oxidation of secondary alcohols to the corresponding ketones.     

Porous Zirconium–Phytic Acid Hybrid: a Highly Efficient Catalyst for Meerwein–Ponndorf–Verley Reductions

The utilization of compounds from natural sources to prepare functional materials is of great importance. Herein, we describe for the first time the preparation of organic–inorganic hybrid catalysts by using natural phytic acid as building block. Zirconium phosphonate (Zr‐PhyA) was synthesized by reaction of phytic acid and ZrCl₄ and was obtained as a mesoporous material with pore sizes centered around 8.5 nm. Zr‐PhyA was used to catalyze the mild and selective Meerwein–Ponndorf–Verley (MPV) reduction of various carbonyl compounds, e.g., of levulinic acid and its esters into γ‐valerolactone. Further studies indicated that both Zr and phosphate groups contribute significantly to the excellent performance of Zr‐PhyA.     

Zr-TUD-1: A novel heterogeneous catalyst for the Meerwein–Ponndorf–Verley reaction

A series of Schiff-base complexes has been synthesized by the condensation of 1,2-diaminocyclohexane with salicylaldehyde, 2-pyridinecarboxaldehyde, and 2-hydroxy-1-naphthaldehyde, followed by the metallation with manganese (1, 2, 3a), cobalt (3b), copper (3c) and iron (3d) salts. These Schiff-base ligands L₁–L₃ and complexes 1, 2, 3a–d were then characterized by IR, ¹H NMR, ¹³C NMR, UV–vis spectra, and DSC measurement. Schiff-base Mn complex (3a) resulting from N,N′-bis(2-hydroxy-1-naphthalidene)cyclohexanediamine (L₃) ligand was considerably active for the catalytic epoxidation of styrene under mild conditions, in which the highest yield of styrene oxide reached 91.2 mol%, notably higher than those achieved from simple salt catalysts Mn(Ac)₂·4H₂O and MnSO₄·H₂O. However, another two salen–Mn complexes 1 and 2 derived from ligands N,N′-bis(salicylidene)cyclohexanediamine (L₁) and N,N′-bis(2-pyridine carboxalidene)cyclohexanediamine (L₂) exhibited relatively poor activity under identical experimental conditions.     

A Well‐Defined Aluminum‐Based Lewis Acid as an Effective Catalyst for Diels–Alder Transformations

A catalytically active aluminum‐based system for Diels–Alder transformations is reported. The system was generated by mixing a β‐diketiminate‐stabilized aluminum bistriflate compound with Na[BAr^Cl₄] (Ar^Cl=3,5‐Cl₂C₆H₃). Solid‐state analysis of the catalytic system reveals a unique structure incorporating a two‐dimensional coordination polymer. According to the experimental results obtained from several Diels–Alder transformations, the aluminum‐based system appears to be a more practical and more robust alternative to the recently reported compounds based on carbon and silicon cations.     

Nitrogen‐Doped Carbon Nanosheets with Size‐Defined Mesopores as Highly Efficient Metal‐Free Catalyst for the Oxygen Reduction Reaction

Nitrogen‐doped carbon nanosheets (NDCN) with size‐defined mesopores are reported as highly efficient metal‐free catalyst for the oxygen reduction reaction (ORR). A uniform and tunable mesoporous structure of NDCN is prepared using a templating approach. Such controlled mesoporous structure in the NDCN exerts an essential influence on the electrocatalytic performance in both alkaline and acidic media for the ORR. The NDCN catalyst with a pore diameter of 22 nm exhibits a more positive ORR onset potential than that of Pt/C (?0.01 V vs. ?0.02 V) and a high diffusion‐limited current approaching that of Pt/C (5.45 vs. 5.78 mA cm^?2) in alkaline medium. Moreover, the catalyst shows pronounced electrocatalytic activity and long‐term stability towards the ORR under acidic conditions. The unique planar mesoporous shells of the NDCN provide exposed highly electroactive and stable catalytic sites, which boost the electrocatalytic activity of metal‐free NDCN catalyst.     

Designed Synthesis of Well‐Defined Pd@Pt Core–Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt‐based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well‐defined Pd@Pt core–shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4–1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as‐synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well‐defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single‐cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low‐cost and high‐efficient applications of PEMFCs.     

Metal‐Dependent Reaction Tuning with Cyclopentylmetal Reagents: Application to the Asymmetric Synthesis of (+)‐α‐Conhydrine and (S)‐2‐Cyclopentyl‐2‐phenylglycolic Acid

Tuning in : The reaction of halocyclopentane organometallic reagents can be tuned by the choice of metal (see scheme). Cyclopentylmagnesium bromide reduces aldehydes and ketones to the corresponding alcohols. However, in the presence of ZnCl₂, normal Grignard addition to the ketones gives tertiary alcohols with complete diastereoselectivity. These protocols were used in the asymmetric synthesis of two medicinally important compounds.

     

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.     

A Water-Soluble Sodium Pectate Complex with Copper as an Electrochemical Catalyst for Carbon Dioxide Reduction

A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO₂ reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO₂ to CH₄ in water. Stability and selectivity of conversion of CO₂ to CH₄ as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO₂ reduction reaction (CO₂RR) proceeds at −1.5 V vs. Ag/AgCl at ~10 mA/cm² current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO₂). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.     

Preparation and Characterization of Chiral Oxazaborolidine Complex Immobilized SBA‐15 and Its Application in the Asymmetric Reduction of Prochiral Ketones

The immobilization of chiral oxazaborolidine complex in the well‐ordered mesochannels of SBA‐15 is demonstrated by a postsynthetic approach using 3‐aminopropyltriethoxysilane as a reactive surface modifier. The immobilized catalysts are characterized by various techniques, such as XRD, nitrogen adsorption, HRSEM, UV/Vis diffuse reflectance spectroscopy, and FTIR spectroscopy. The catalysts are used for the enantioselective reduction of aromatic prochiral ketones. The activity of the chiral oxazaborolidine complex immobilized SBA‐15 catalysts is also compared with that of the pure chiral oxazaborolidine complex, which is a homogeneous catalyst. It is found that the activity of the chiral complex immobilized SBA‐15 heterogeneous catalyst is comparable with that of the homogeneous catalyst.     

A Nitrogen,Sulfur co-Doped Porphyrin-based Covalent Organic Framework as an Efficient Catalyst for Oxygen Reduction

Oxygen reduction reaction(ORR) is a significant reaction for energy conversion systems(such as fuel cells, metal-air batteries, etc.). It is an urgent need to develop cheap, durable and highly-active catalysts for efficient ORR. Hence, we report a metal-free nitrogen and sulfur co-doped porphyrin-based covalent organic framework(COF) as a high-efficiency ORR catalyst[the onset potential(E_o) is 0.79 V and the half-wave potential(E_1/2) is 0.70 V]. The double doping of N and S atoms causes uneven charge distribution around carbon atoms, which can act as catalytic active centers, improving ORR activity. Compared with single-atom doping, double atoms doping exhibits a higher activity due to the synergistic effect between different elements. These results demonstrate that reasonable design of stable metal-free COFs with a high electrochemical activity can promote their wide applications.     

KF‐Melamine Formaldehyde Resin (KF‐MFR) as a Versatile and Efficient Heterogeneous Reagent for Aldol Condensation of Aldehydes and Ketones under Microwave Irradiation

KF‐Melamine formaldehyde resin (KF‐MFR) was demonstrated to be a highly efficient heterogenious catalyst for cross‐aldol condensation under microwave irradiation. In this synthesis, various aldehydes and ketones were condensed together in the presence of supported KF on melamine‐formaldehyde resin to afford different chalcone derivatives in good to excellent yields. KF‐MFR proved to have unique termal and chemical resistance and can be reused for many consecutive runs without remarkable loss in catalytic activity.     

Metal Amidoboranes: Superior Double‐Hydrogen‐Transfer Agents in the Reduction of Ketones and Imines

Metal amidoboranes (MABs), such as lithium amidoborane (LiAB), show superior ability in reducing ketones and imines directly into their corresponding secondary alcohols and amines, respectively, at room temperature with high conversion and yields. A mechanistic study indicates that the reduction proceeds through a double‐hydrogen‐transfer process. Both protic H(N) and hydridic H(B) protons in the amidoborane participate in the reaction. Theoretical investigations show that the first (and rate‐determining) step of the reduction reaction is the elimination of LiH from LiAB, followed by the transfer of H(Li) to the C site of the unsaturated bond.