A Reusable Palladium Catalyst for Allylic Arylation of Baylis‐Hillman Adducts via Heck Arylation‐Isomerization Cascade Under Solvent‐Free Conditions

The direct arylation of several common heterocycles, using homogeneous and heterogeneous palladium (pre)catalysts, has been examined by initial rate analysis. The study reveals that apparently distinct palladium catalysts can display similar activities in such transformations, implying formation of a comparable active palladium catalyst phase. A substrate dependence was noted for the palladium catalysts examined.

Arylation of indole at C2 catalyzed by palygorskite grafted covalent organic frameworks supported palladium catalyst

There are a number of protocols for Ullmann coupling-type S-arylation reactions, many of them suffer from the use of homogenous and often corrosive catalyst, cumbersome workup procedures, and long reaction times. Besides, many of these reagents are expensive and non-recoverable, leading to the generation of a large amount of toxic waste particularly when large-scale applications are considered. The aim of this study was to prepare a new Pd catalyst bonded on the surface of zeolite as a heterogeneous catalyst. A heterogeneous palladium catalyst has been prepared by immobilizing Pd ions on Clinoptilolite. This novel developed heterogeneous catalyst was thoroughly examined for Ullmann coupling-type S-arylation reaction using different bases, solvents and 0.003 mg of the catalyst. The structural and morphological characterizations of the catalyst were carried out using XRD, TGA, BET and TEM techniques. Highly efficient heterogeneous palladium catalyst has been developed by immobilizing Pd ions on Clinoptilolite, as one of the most abundant naturally occurring zeolites for Ullmann Sarylation. By using this method, we provide an efficient way to a wide variety of substituted thiolic compounds. Moreover, the catalyst is easily recovered using simple filtration and reused for 5 consecutive runs. In this effort, we developed a new Pd catalyst bonded on the surface of zeolite as a substrate to prepare the heterogeneous catalyst. We demonstrate that this novel catalyst offers reliable and convincing data that may offer a valuable application in further developing the science and technology of Ullmann reaction protocols and allied industries. Additionally, the catalyst was reusable and kept its high activities over a number of cycles.

ConspectusPalladium-catalyzed oxidations involving cascade processes providea versatile platform for streamlined conversion of simple feedstocksinto functional molecules with high atom and step economy. However,the achievement of high palladium efficiency and selectivity in Pd-catalyzedoxidative cascade reactions is still challenging in many cases, asa result of the aggregation of active palladium species to Pd blackand the possible side reactions during each bond-forming step. Thetwo current solutions for addressing these issues are either to utilizeoxidant-stable ligands or to use electron transfer mediators (ETMs).The former solution, which includes the use of amines, pyridines,sulfoxides, and carbene derivatives, inhibits aggregation of Pd0 during the catalytic cycle, while the latter solution facilitatesreoxidation of Pd0 to PdII to improve the activityand selectivity. Following our long-standing interest in Pd-catalyzedoxidations, very recently we developed heterogeneous catalysts toresolve the issues mentioned above in oxidative cascade reactions.The heterogeneous palladium catalysts (Pd-AmP-MCF or Pd-AmP-CNC) comprisepalladium nanoclusters (1–2 nm) immobilized on amino-functionalizedsiliceous mesocellular foam (MCF) or on crystalline nanocellulose(CNC), exhibiting high activity, selectivity as well as excellentrecycling ability.In this Account, we will discuss the synthesisand characterizationsof the heterogeneous palladium catalysts, as well as their catalyticbehaviors, and the mechanisms involved in their reactions. An importantaspect of these catalysts in oxidation reactions is the generationof active Pd(II) species within the heterogeneous phase. Typical oxidativecascade reactions of our recent research on this topic include oxidativecarbocyclization-carbonylation, oxidative carbocyclization-borylation,oxidative alkynylation-cyclization, oxidative carbonylation-cyclization,and oxidative carbocyclization-alkynylation. These reactions provideaccess to important compounds attractive in medicinal chemistry andfunctional materials, such as γ-lactone/γ-lactam-basedpoly rings, cyclobutenols, highly substituted furans, and oxaboroles.During these processes, the heterogeneous catalysts exhibited muchhigher turnover numbers (TONs) than their homogeneous counterparts(e.g., Pd(OAc)2) as well as unique selectivity that cannotbe achieved by homogeneous palladium catalysts. The origin of thehigh efficiency and unique selectivity of the heterogeneous catalystswas also investigated. Asymmetric syntheses for the construction ofoptically pure compounds were realized based on the excellent selectivityin these heterogeneous processes. Kinetic studies revealed that therate and yield of the reactions were essentially maintained duringrecycling, which demonstrates that Pd-AmP-MCF and Pd-AmP-CNC are robustand highly active in these oxidative cascade reactions. In addition,inductively coupled plasma optical emisson spectroscopy (ICP-OES)analysis and hot filtration test suggest that these processes mostlikely proceed via a heterogeneous pathway.Recent progressin our group has shown that the activity of Pd-AmP-MCFand Pd-AmP-CNC could be improved even further by the addition of Ag+ to generate cationic Pd(II). Furthermore, intriguing solventeffects were observed in a Pd-AmP-MCF-catalyzed oxidative cascadeprocess, and solvent-controlled chemoselective transformations weredeveloped based on this property of the catalyst. The heterogeneousstrategy of this Account provides solutions to palladium deactivationand selectivity issues in Pd(II)-catalyzed oxidative cascade reactionsand enables efficient catalyst recycling, which will open up new opportunitiesin oxidative cascade reactions.

An Efficient and Recyclable Mesostructured Polymer-Supported N-Heterocyclic Carbene-Palladium Catalyst for Sonogashira Reactions

A new method named "the polymer incarcerated (PI) method" for preparing a heterogeneous palladium catalyst has been developed. The method is operationally simple, and the Pd catalyst prepared (PI Pd) is highly active for hydrogenation, carbon-carbon, and carbon-oxygen bond-forming reactions. Remarkable points are that the activity of PI Pd is higher than that of homogeneous Pd catalysts and that PI Pd is recovered by simple filtration and reused several times without loss of activity. The catalyst is expected to replace many heterogeneous palladium catalysts, especially Pd/C, which is often used in academia and industry, but recovery of which is difficult.

For Abstract see ChemInform Abstract in Full Text.

Telomerization of butadiene with water catalyzed by heterogeneous palladium catalysts

Designing reusable high‐performance heterogeneous palladium (Pd) catalysts via convenient, economic synthesis is of great importance to the industrial applications of various carbon‐carbon cross‐coupling reactions. We demonstrate herein a convenient one‐pot self‐encapsulation synthesis of a heterogeneous Pd catalyst [Pd@PDEB, PDEB=poly(1,3‐diethynylbenzene)] directly from commercially available, economic precursors. In the synthesis, the formation of the cross‐linked polymer networks and Pd encapsulation are accomplished simultaneously, turning a homogeneous Pd polymerization catalyst into the heterogeneous cross‐coupling catalyst. As a unique, practical heterogeneous catalyst, Pd@PDEB shows remarkably high activity, high reusability, and high versatility towards at least four types of cross‐coupling reactions (Suzuki–Miyaura, Stille, allylic arylation, and Mizoroki–Heck reactions) with even difficult reactants (aryl chlorides and heteroaryl halides) under aerobic conditions with Pd loadings down to ppm or even ppb levels. Evidences from hot filtration and 3‐phase tests demonstrate the heterogeneous nature of the catalyst with very low Pd leaching and negligible contributions of leached homogeneous Pd species towards the coupling reactions.magnified image

Highly active and recyclable heterogeneous palladium catalyst derived from guar gum for fabrication of biaryl compounds.

Synthesis of polystyrene-supported Pd(II)-NHC complex derived from theophylline as an efficient and reusable heterogeneous catalyst for the Heck-Matsuda cross-coupling reaction

Highly porous chitosan microspheres supported palladium catalyst for coupling reactions in organic and aqueous solutions

Heterogeneous palladium catalysts anchored on functionalized silica were prepared by sol–gel methods and their catalytic properties for the oxidative carbonylation of phenol to diphenyl carbonate (DPC) were investigated. The catalysts were characterized by means of IR, XPS, EA and BET. The Pd loading in the heterogeneous catalysts and leaching in solution were detected by atomic absorption. The effects of different reaction parameters such as temperature, solvent and inorganic cocatalyst on the yield of DPC and Pd leaching were also studied. It was found that Cu2O and tetrahydrofuran (THF) were the best partners with these heterogeneous catalysts. In the presence of 3 Å molecular sieves as dehydrating agent, the heterogeneous palladium catalyst prepared from 2‐acylpyridine revealed excellent catalytic performance and stability at 110 °C for 5 h, giving 13.7% yield of DPC based on phenol and 4.0% Pd loss in solution. The heterogeneous catalyst was more active and stable compared with traditional supported PdC catalyst under the same reaction conditions. Copyright © 2005 John Wiley & Sons, Ltd.

A covalently anchored Pd(II)-Schiff base complex over a modified surface of mesoporous silica SBA-16: An efficient and reusable catalyst for the Heck-Mizoroki coupling reaction in water

Preparation, structural characterization, and catalytic performance of Pd(II) and Pt(II) complexes derived from cellulose Schiff base