手性催化剂负载的一种新方法: 自负载策略

较系统地介绍了手性催化剂负载的一种新方法(即“自负载”策略)及其在非均相不对称催化反应中应用的最新进展. 与传统的负载模式不同, “自负载”策略中利用含双或多官能团的配体与金属通过自组装形成的有机-无机聚合物做为催化剂, 因此不需使用任何载体. “自负载”手性催化剂在若干非均相不对称催化反应中显示了优秀的催化活性和对映选择性并且能够简单回收再利用, 为手性催化剂的负载化提供了一个新的策略.     

Transformation of Sugars into Chiral Polyols over a Heterogeneous Catalyst

Transformation of sugars, while maintaining the intrinsic stereochemical structure, is desirable. However, such a transformation requires multistep synthesis with protection and deprotection of the OH groups. Herein, a new method for selective transformation of sugar derivatives into chiral building blocks and a diol synthon, with retention of the intrinsic configuration (stereo‐ and regioselectively), is demonstrated. The method is based on the selective recognition of cis‐vicinal OH groups in sugars and leads to the one‐pot removal of the cis‐vicinal OH groups, without protection of OH groups (except the OH group of the hemiacetal group), over a heterogeneous CeO₂‐supported ReO_x and Pd (ReO_x‐Pd/CeO₂) catalyst by using H₂ as a reducing agent.     

Using a Nitrophenol Cocktail Screen to Improve Catalyst Down-selection

The catalytic reduction of 4-nitrophenol ( 4NP ) with excess NaBH₄ is the benchmark model for quantifying catalytic activity of nanoparticles. Although broadly useful, the reaction can be very selective. This can lead to false positives and negatives when utilized for catalyst down-selection from a broader materials candidate pool. We report a multi-nitrophenol cocktail screening methodology incorporating 4NP and other amino-nitrophenols, utilizing Ag, Au, Pt, and Pd nanoparticles on carbon support. The reduction of the cocktail proceeds with no deleterious side reactions on the time-scale tested. The resulting kinetic rates provide an improved correlation of relative catalyst activity when compared to performance with other reducible moieties (e. g. azo bonds), or when compared to solely 4NP screening.     

Cascade Synthesis of Pyrroles from Nitroarenes with Benign Reductants Using a Heterogeneous Cobalt Catalyst

A bifunctional 3d-metal catalyst for the cascade synthesis of diverse pyrroles from nitroarenes is presented. The optimal catalytic system Co/NGr-C@SiO₂-L is obtained by pyrolysis of a cobalt-impregnated composite followed by subsequent selective leaching. In the presence of this material, (transfer) hydrogenation of easily available nitroarenes and subsequent Paal–Knorr/Clauson-Kass condensation provides >40 pyrroles in good to high yields using dihydrogen, formic acid, or a CO/H₂O mixture (WGSR conditions) as reductant. In addition to the favorable step economy, this straightforward domino process does not require any solvents or external co-catalysts. The general synthetic utility of this methodology was demonstrated on a variety of functionalized substrates including the preparation of biologically active and pharmaceutically relevant compounds, for example, (+)-Isamoltane.     

Preparation of a Series of Supported Nonsymmetrical PNP-Pincer Ligands and the Application in Ester Hydrogenation

In contrast to their symmetrical analogues, nonsymmetrical PNP-type ligand motifs have been less investigated despite the modular pincer structure. However, the introduction of mixed phosphorus donor moieties provides access to a larger variety of PNP ligands. Herein, a facile solid-phase synthesis approach towards a diverse PNP-pincer ligand library of 14 members is reported. Contrary to often challenging workup procedures in solution-phase, only simple workup steps are required. The corresponding supported ruthenium-PNP catalysts are screened in ester hydrogenation. Usually, industrially applied heterogeneous catalysts require harsh conditions in this reaction (250–350 °C at 100–200 bar) often leading to reduced selectivities. Heterogenized reusable Ru-PNP catalysts are capable of reducing esters and lactones selectively under mild conditions.     

A State-of-the-Art Heterogeneous Catalyst for Efficient and General Nitrile Hydrogenation

Cobalt-doped hybrid materials consisting of metal oxides and carbon derived from chitin were prepared, characterized and tested for industrially relevant nitrile hydrogenations. The optimal catalyst supported onto MgO showed, after pyrolysis at 700 °C, magnesium oxide nanocubes decorated with carbon-enveloped Co nanoparticles. This special structure allows for the selective hydrogenation of diverse and demanding nitriles to the corresponding primary amines under mild conditions (e.g. 70 °C, 20 bar H₂). The advantage of this novel catalytic material is showcased for industrially important substrates, including adipodinitrile, picolinonitrile, and fatty acid nitriles. Notably, the developed system outperformed all other tested commercial catalysts, for example, Raney Nickel and even noble-metal-based systems in these transformations.     

Mechanochemical Immobilisation of Metathesis Catalysts in a Metal–Organic Framework

A simple, one‐step mechanochemical procedure for immobilisation of homogeneous metathesis catalysts in metal–organic frameworks was developed. Grinding MIL‐101‐NH₂(Al) with a Hoveyda–Grubbs second‐generation catalyst resulted in a heterogeneous catalyst that is active for metathesis and one of the most stable immobilised metathesis catalysts. During the mechanochemical immobilisation the MIL‐101‐NH₂(Al) structure was partially converted to MIL‐53‐NH₂(Al). The Hoveyda–Grubbs catalyst entrapped in MIL‐101‐NH₂(Al) is responsible for the observed catalytic activity. The developed synthetic procedure was also successful for the immobilisation of a Zhan catalyst.     

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.     

Sustainable Heterogeneous Platinum Catalyst for Direct Methylation of Secondary Amines by Carbon Dioxide and Hydrogen

Pt and MoO_x co‐loaded TiO₂ is found to be highly effective for direct methylation of aliphatic and aromatic secondary amines by CO₂ and H₂ under solvent‐free conditions. This is the first additive‐free and reusable heterogeneous catalytic system with acceptable turnover number.     

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.     

Solid-Phase Immobilization of a New Epoxidation Catalyst

The heterogenization of a titanium(IV ) silsesquioxane complex in an MCM-41 molecular sieve , by tailoring the polarity of MCM-41, results in self-assembled materials (see the space-filling model on the right) that are active, truly heterogeneous, and recyclable catalysts for liquid-phase alkene epoxidation.     

Chemical Immobilization of Heteropolyacid Catalyst on Inorganic Mesoporous Material for use as an Oxidation Catalyst

Recent progress on the chemical immobilization of heteropolyacid (HPA) catalyst on inorganic mesoporous material is reported in this review. Mesostructured cellular foam silica, mesoporous carbon, and nitrogen-containing mesoporous carbon were used as supporting materials. The mesoporous materials were modified to have a positive charge, and thus, to provide sites for the immobilization of HPA catalyst. By taking advantage of the overall negative charge of heteropolyanion, the HPA catalyst was chemically immobilized on the surface-modified mesoporous material as a charge-compensating component. Characterization results showed that the HPA catalyst was finely and molecularly dispersed on the surface of mesoporous material via strong chemical immobilization, and that the pore structure of mesoporous material was still maintained even after the immobilization of HPA catalyst. The supported HPA catalysts were applied to the model vapor-phase ethanol conversion, 2-propanol conversion, and methacrolein oxidation reactions. The supported HPA catalyst showed a better oxidation catalytic activity than the unsupported HPA catalyst in the model reactions. The enhanced oxidation catalytic performance of the supported HPA catalyst was attributed to the finely dispersed HPA catalyst, which was chemically immobilized on the positive site of mesoporous material by sacrificing its proton (Brönsted acid site). The HPA catalyst chemically immobilized on mesoporous material served as an excellent oxidation catalyst.     

Recycling asymmetric hydrogenation catalysts by their immobilization onto ion-exchange resins

Ion-exchange resins can be used as supports for the preparation of single-site, heterogenised asymmetric hydrogenation catalysts. The immobilised catalysts obtained can be efficiently and conveniently recovered and recycled. This article reviews the significant contributions in the field including the main concepts behind the design and the applications of this type of catalyst.     

Functionalized Periodic Mesoporous Organosilica: A Highly Enantioselective Catalyst for the Michael Addition of 1,3‐Dicarbonyl Compounds to Nitroalkenes

A functionalized periodic mesoporous organosilica with incorporated chiral bis(cyclohexyldiamine)‐based Ni^II complexes within the silica framework was developed by the co‐condensation of (1R,2R)‐cyclohexyldiamine‐derived silane and ethylene‐bridge silane, followed by the complexation of NiBr₂ in the presence of (1R,2R)‐N,N′‐dibenzylcyclohexyldiamine. Structural characterization by XRD, nitrogen sorption, and TEM disclosed its orderly mesostructure, and FTIR and solid‐state NMR spectroscopy demonstrated the incorporation of well‐defined single‐site bis(cyclohexyldiamine)‐based Ni^II active centers within periodic mesoporous organosilica. As a chiral heterogeneous catalyst, this functionalized periodic mesoporous organosilica showed high catalytic activity and excellent enantioselectivity in the asymmetric Michael addition of 1,3‐dicarbonyl compounds to nitroalkenes, comparable to those with homogeneous catalysts. In particular, this heterogeneous catalyst could be recovered easily and reused repeatedly up to nine times without obviously affecting its enantioselectivity, thus showing good potential for industrial applications.     

手性Ru复合物催化剂的固载化及其不对称氢转移反应催化应用

Chiral Ru-BsDPEN, (1R,2R)-N-p-benzenesulfonyl-1,2-diphenylethylenediamine, catalyst has been immobilized on a mesoporous molecular sieve of MCM-41 type successfully. A hybrid mesoporous molecular sieve was synthesized using a precursor bearing benzene group, which in organosilica were sulfonylated and reacted with (1R,2R)-l,2-diphenylethylenediamine and [RuC1E(p-cymene)]2 successively to afford immobilized catalyst. The Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore size decreased after immobilization of catalyst onto the mesoporous material. Enantioselective transfer hydrogenation of ketones catalyzed by immobilized catalyst showed the highest yield of 22.36% and e.e. value of 31.47% by using acetophenone as substrate when reaction time was 48 and 16 h respectively.     

Ni2P Nanoalloy as an Air-Stable and Versatile Hydrogenation Catalyst in Water: P-Alloying Strategy for Designing Smart Catalysts

Non-noble metal-based hydrogenation catalysts have limited practical applications because they exhibit low activity, require harsh reaction conditions, and are unstable in air. To overcome these limitations, herein we propose the alloying of non-noble metal nanoparticles with phosphorus as a promising strategy for developing smart catalysts that exhibit both excellent activity and air stability. We synthesized a novel nickel phosphide nanoalloy (nano-Ni₂P) with coordinatively unsaturated Ni active sites. Unlike conventional air-unstable non-noble metal catalysts, nano-Ni₂P retained its metallic nature in air, and exhibited a high activity for the hydrogenation of various substrates with polar functional groups, such as aldehydes, ketones, nitriles, and nitroarenes to the desired products in excellent yields in water. Furthermore, the used nano-Ni₂P catalyst was easy to handle in air and could be reused without pretreatment, providing a simple and clean catalyst system for general hydrogenation reactions.