Palladium pre-catalysts

The virtually unlimited availability of CO renders it extremely appealing as a monomer in copolymerization reactions. Herrmann and co-workers [21] reported the copolymerization of CO and ethene using dicationic chelating carbene complexes of palladium(II) (Fig. 2). Given the large molecular weight of the obtained copolymer and the relatively modest TONs they observed, the authors postulated that only a small percentage of palladium pre-catalyst actually participates in the production of copolymer. 

Dicationic NHG chelate complexes of Pd have been employed in the catalytic co-polymerization of ethylene and GO with modest activity under mild conditions of temperature and pressure. It was concluded that only a very small percentage of the palladium pre-catalyst is active in the co-polymerization reaction, although no suggestion of what the actual active species might be was provided. This is an interesting report chain-growth reactions (oligomerization... 

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). 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

The enantioselective hydrosilylation catalyst system based on the chiral pre-catalyst dichloro l- (/ )-l- [(S)-2 (dipheaylphosphinoKP) ferrocenyl] ethyl -3- trimethyl- phenyl-5-1 H- pyrazole-tc/V) palladium, 1 (shown in Figure 2) exhibits a interesting inversion of selectivity with a series of para-... 

Commonly used metal salts and palladium precursors include Pd(OAc)2, Pd (acac)2 (acac = acetylacetonato) and Pd2(dba)3 or Pd(dba)2 (dba = dibenzylide-neacetone). If a Pd(II) salt is used as pre-catalyst, reduction by base or by excess phosphine ligand is required. The exact nature of the reducing agent is somewhat contended, but it is often assumed that the phosphine takes up this role, for which evidence has been reported [26]. A typical catalytic system consists of a palladium source and an aryl- or alkylphosphine (typically PPh3), in at least 2 eq., as ligand. The addition of bases, typically amines and alkoxides, is often found to be beneficial for activity, which is also reflected in the patent literature [27-29]. The bases are thought to facilitate in the attack of the nucleophile in the rate-determining step and, in the case of the amines, in the reduction of Pd(II) to Pd(0). 

Astruc D (2007) Palladium nanoparticles as efficient green homogeneous and heterogeneous carbon-carbon coupling pre-catalysts aunifying view. Inorg Chem 46 1884-1894... 

The effect of nitrile-functionalised ionic liquids on the recycling potential was investigated in the coupling between iodobenzene and tributylphenyltin.[15] Several, closely related palladium-precursors, based on the ionic liquid cation [C3CNpy]+ and PdCP (Figure 6.5), were tested in the reaction, but the nature of the pre-catalyst did not have a significant influence on the rate. Independent from the ionic liquid used, rather low catalytic activity was observed and with 5 mol% catalyst, yields ranged between 43-65% after 12 hours at 80°C. 

While carefully isolating all products from a reaction of 2-bromopyridine 19 with palladium catalyst, bimetallic complex 148 was isolated and determined to be a pre-catalyst for the Suzuki reaction [52]. Upon examining the utility of this complex, it was found to be able to carry out Suzuki reactions, such as the conversion of 149 and 150 into 151. 

The palladium-catalysed arylation of olefins with aryl halides, the Heck reaction, is usually performed in polar solvents such as acetonitrile or dimethyl sulfoxide, in combination with a base and a Pd(ii) pre-catalyst that may or may not be associated with a phosphorus ligand. Given that quaternary ammonium or phosphonium salts are known to increase reaction rates, ILs emerged as promising solvents for this reaction.In the case of imidazolium-based ionic liquids, the solvent, beside providing an unusual coulombic environment. 

Scheme 9 Palladium-catalyzed hydroxylation of (hetero)aryl bromides and chlorides employing an L13-ligated palladacyclic pre-catalyst...

Scheme 14 Palladium-catalyzed cross-coupling of (hetero)aryl triflates with fluoride employing the (L5) Pd (COD) pre-catalyst. Ad = I-adamantyl COD= 1,5-cychoctadiene...

The first step of the Catellani reaction involves the oxidative addition of a palladium(O) species into an aryl halide bond to generate an arylpalladium(II) species [5, 6], Catellani s initial reports employed the pre-formed phenyl-norbomylpalladium(II) dimer [7] (PNP dimer) (10, Fig. 1) as a pre-catalyst which could directly enter the catalytic cycle and generate a palladium(O) species after one catalyst turnover [4],... 

On the other hand, a Pd(II) pre-catalyst, such as palladium acetate, together with a ligand (or as a preformed complex) can be used and has the benefit of better stability for storage. 

Chlorinated diarylamines have also been successfully cyclized using (V-hetero-cyclic carbene (NHC) palladium catalysts. It has been demonstrated that the turnover number of the catalytic system is substantially improved by using imida-zohum salt additives along with a mono-NHC-paUadium(ll) pre-catalyst [201]. 

The development of Pd colloids as catalyst for C—C coupling reactions is rather recent [5]. The first example was reported by BeUer et al. in 1996 they used preformed Pd coUoids stabihsed by tetra-octylammonium bromide prepared following the Bonneman procedure in the Heck arylation [6]. The colloidal system was effective for the Heck arylation of styrene or butyl acrylate by activated aryl bromides, but showed only moderate to little activity for deactivated aryl bromides and aryl chlorides. To obtain these results, the authors found that the colloidal pre-catalysts must be added slowly to the reaction mixture to avoid the formation of inactive palladium black at the beginning of the reaction. 

Scheme 8.13 MIzorokl-Heck reaction catalysed by a palladium pincer pre-catalyst.

Apart from the original Kharasch reaction-promoters, the following palladium or nickel (pre)catalysts appeared to be efficient Pd(PPh3)4 [12], Pd(PPh3)2Cl2 [20,21],... 

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