Palladium well-defined catalysts

Monocarbene-palladium(O) complexes bearing a dvds group [224] are the best well-defined catalysts to date for the telomerization of butadiene with... 

The pivotal mechanistic role of zerovalent palladium results from the known activity seen for well-defined catalysts of the type Pd[P(aryl)3] (n = 2-4). The dissociation equilibria regarding n determine the catalytic performance, with excess phosphine, as expected, suppressing the reaction rate according to r [P(aryl)3] [50]. A sixfold excess of phosphine over palladium practically kills catalysis [50 a]. 

J Schwarz, VPW Bdhm, MG Gardiner, M Grosche, WA Herrmann, W Hieringer, G Raudaschl-Sieher. Polymer-supported Carbene Cotrplexes of Palladium Well-defined, Air-stable, Recyclable Catalysts for the Heck Reaction. Chem Eur J 6 1773-1780,2000. 

A step forward in the design of catalysts enabling the N-arylation of amines is to eliminate the activation step (reduction of palladium(II) to palladium(O) prior to oxidative addition). One approach is to use well-defined palladium(O) complexes of (NHC)2Pd or mixed phosphine/NHC,(R3P)Pd(NHC) type [80]. These complexes are efficient catalysts for this transformation at mild temperatures. A second approach is to sidestep two required activation stages in the catalytic cycle and eliminate the need for the preactivation and the oxidative addition processes by using well-defined catalysts that are oxidative addition adducts such as NHC-stabilized palladacycles (Scheme 25) [81]. 

At the beginning, palladacyles were considered as a long-awaited class of palladium catalysts with a stable and very robust coordination sphere and were, thus, expected to open almost infinite possibilities for the development of so-called well-defined catalysts. In order to reconcile the coordination requirements of the classic Mizoroki-Heck reaction... 

However, the Buchwald-Hartwig reaction with NHCs as hgands is not limited to palladium. Nickel has also been successfully employed in this catalytic amination. In situ procedures have been described for the coupling of aryl chlorides [163] and tosylates [164] and, more interestingly, anisoles [165]. The use of well-defined Ni(0) catalysts has also been studied [166] (Scheme 6.49). 

Very recently, well-defined complexes with general formula [PdCl(T -Cp) (NHC)] were synthesised and tested for the homocoupling of non-electrodeficient arylboronic acids at room temperature with good results (Scheme 7.7) [51]- This new class of catalysts were synthesised from commercially available NHC palladium(II) chloride dimers and are air-stable. 

In the present chapter, we focus on the catalyst nature in solution using well-defined metal NPs as catal 4 ic precursors it means, soluble (or dispersible) heterogeneous pre-catalysts, as stated by Finke [6]. Some experiments described in the literature concerning the distinction between homogeneous and heterogeneous catalysts are discussed (see Section 3), followed by a particular case studied by us with regard to the catalyst nature in the allylic alkylation reaction, using preformed palladium NPs as catalytic precursors (see Section 4). 

Using a monodentate ligand does not necessarily mean that only one ligand coordinates to the metal. Since these monoligated metal species are very important for catalytic activity, their synthesis is highly desirable. More details on the development of well-defined and highly active monoligated palladium NHC catalysts will be provided in later parts of this volume [103-109]. 

Less attention has been paid to the use of amines as nucleophiles in the telomerization reaction. A single report from Nolan and co-workers [233] has shown that well-defined cationic palladium complexes are efficient catalysts in the telomerization of butadiene with amines under mild conditions (Table 10). In the case of primary amines, the concentration of the reactants and their steric hinderance dictates the formation of a mono- or double-alkylated product. 

Apart from palladium(O) and palladium(II) salts, such as Pd2(dba)3, PdCl2 and Pd(OAc)2, several well-defined Pd-carbene complexes have been employed in ionic liquids, as have some palladacycle complexes. However, ligands used are in most cases neutral and have not been included to facilitate catalyst retention in the ionic liquid. 

A further step toward technological catalysts is the use of medium-to-low surface area supports on which relatively large metal particles ( 5nm) of well-defined shape are grown. Figure 3h displays a TEM micrograph of palladium nanoparticles supported on MgO (105,108). In favorable cases, the powder oxide support also allows the use of scanning probe microscopy (AFM) or electron spectroscopy. 

The discovery that group IV metallocenes can be activated by methylaluminox-ane (MAO) for olefin polymerization has stimulated a renaissance in Ziegler-Natta catalysis [63]. The subsequent synthesis of well-defined metallocene catalysts has provided the opportunity to study the mechanism of the initiation, propagation, and termination steps of Ziegler-Natta polymerization reactions. Along with the advent of cationic palladium catalysts for the copolymerization of olefins and carbon monoxide [64, 65], these well-defined systems have provided extraordinary opportunities in the field of enantioselective polymerization. 

Arylation of ketones, aryl amination and suzuki-miyaura cross coupling using a well-defined palladium catalyst bearing an w-heterocyclic carbene ligand... 

ARYLATION OF KETONES, ARYL AMINATION AND SUZUKI-MIYAURA CROSS COUPLING USING A WELL-DEFINED PALLADIUM CATALYST BEARING AN /V-HETEROCYCLIC CARBENE LIGAND... 

Easily synthesized, highly active, yet air- and shelf-stable palladium complexes have added to the existing arsenal of cross-coupling catalysts. Well-defined [(NHC)PdCl2]2 complexes have been used in aryl amination of more economical and more difficult to activate aryl chlorides and amines/anilines in DME as solvent and potassium fcrf-amylate (KO Am) as base [77]. When... 

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