Palladacycles as catalysts

Many palladacycles have also been described as useful catalysts for cross-coupling and the related reactions [184-192]. However, strong evidence has been accumulated indicating that the palladacycles merely act as a reservoir of Pd(ll), which requires reduction to Pd(0) to enter into the catalytic cycle [189,190,193]. Thus, in a 

Palladium nanoparticles and other heterogeneous catalysts are often invoked as catalysts in cross-coupling processes [194, 195). Direct evidence in support of an oxidative-addition-promoted leaching mechanism has been recently obtained in the Suzuki-Miyaura reactions with nanoparticle catalysts, suggesting that true surface catalysis remains largely unknown with these heterogeneous catalysts [196]. 

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Gruber AS, Zim D, Ebeling G, Monteiro AL Dupont (2000) Sulfur-containing palladacycles as catalyst precursors for the Heck reaction. J Org Lett 2 1287-1290... 

In 2007, Wu et al. showed that arylboronic acids and the corresponding boronates reacted with terminal alkynes in the presence of a palladacycle as catalyst and silver oxide (Scheme 10.80).132 As in the Mori reaction using basic silver salts, stoichiometric amounts of silver ion were required for high efficiency. 

Scheme 2 Heck reaction with palladacycles as catalysts...

The increase of the catalyst turnover numbers is indeed one other major area where further improvements could be expected. Such improvements have recently been achieved for the standard Heck reaction by the use of high pressure conditions [86], the use of preformed palladacycles as catalysts [87], or by using a macrocyclic tetraphole as hgand [88].Dendritic diphosphine-palladium complexes as catalysts for Heck reactions have also been reported to possess superior stabihty compared to the monomeric parent compounds [89]. Transferring such iimovations to the AHR remains an important goal. 

An entirely different mode of interaction of electron-rich cyclopalladated Pd(II) complexes with water has been described, involving the oxidation of Pd(IV) species through oxidative addition of water to Pd(ll) with subsequent elimination of dihydrogen.It is not clear whether this process, proved for Pd complex with a very specific coordination sphere, may have a relation to a hypothetical involvement of Pd(ll)/Pd(IV) catalytic cycles in the reactions generally considered as Pd(0)/Pd(n) processes (Heck reaction, cross-coupling, etc.), particularly in those involving palladacycles as catalysts. ... 

We have shown that the direct arylation of acrolein toward the synthesis of cinnamaldehyde derivatives was an efficient procedure. Using the palladacycle 1 as catalyst, substituted aldehydes 3 were prepared with up to 87% isolated yield from condensed aiyl bromides (Scheme 21.1, Route 1) that was extended successfully to heteroaiyl bromides, like bromoquinolines (6). Alternatively, the acrolein diethyl acetal was used as olefin and a selective formation of the saturated ester 4 was attained under the same reaction conditions (Scheme 21.1, Route 2). The expected aldehydes 3 were, however, obtained from most of the aiyl halides used under modified conditions. It was shown that the addition of n-Bu4NOAc in the medium... 

The first palladium-catalyzed coupling between a secondary amine and an aryl chloride was described by Beller, Hermann and co-workers in 1997 [70]. Use of palladacycle 12 as catalyst resulted in C-N bond formation in good yield for several secondary amines and electron-deficient aryl chlorides, Eq. (40). In these transformations, varying amounts of the regioisomeric product was observed, indicative that to some degree the reaction proceeds via a benzyne intermediate. 

Many types of palladacyclic complexes have been used as precursors to catalysts for a variety of coupling processes146,147. Mixtures of dimeric palladacycles containing bridging halide, acetate or trifluoroacetate ions and a phosphine or carbene ligand have been studied as catalysts for the animation of aryl halides. The isolated phosphine adducts can also be applied in catalysis. 

In a preparation of ArCN from ArX and K4Fe(CN)g, the employment of a palladacycle as a catalyst under rather drastic conditions (NMP, 140°) is perhaps of questionable value. 

Palladacycles as simple, and efficient catalyst precursors for homogeneous catalysis 01EJI1917. 

More recently, Najera et al. performed a number of Heck reactions of deactivated aryl halides and styrenes under phosphane-free conditions using oxime-derived palladacycles or palladium acetate as catalyst [113], Coupling can be performed either vith chcyclohexylmethylamine as base and TBAB as PTC catalyst or in neat vater vith triethylamine in N,N-dimethylacetamide (DMA) solutions under the action of MW irradiation. 

Recently, Suzuki-type reactions in air and water have also been studied, first by Li and co-workers. They found that the Suzuki reaction proceeded smoothly in water under an atmosphere of air with either Pd(OAc)2 or Pd/C as catalyst (Eq. 6.36). Interestingly, the presence of phosphine ligands prevented the reaction. Subsequently, Suzuki-type reactions in air and water have been investigated under a variety of systems. These include the use of oxime-derived palladacycles and tuned catalysts (TunaCat). A preformed oxime-carbapalladacycle complex covalently anchored onto mercaptopropyl-modified silica is highly active (>99%) for the Suzuki reaction of p-chloroacetophenone and phenylboronic acid in water no leaching occurs and the same catalyst sample can be reused eight times without decreased activity. ... 

The intermediacy of palladacycles in the reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as catalyst and KOAc as a base in anisole as solvent was initially suggested by the isolation, among other products, of two compounds (Eq. 9), the former resulting from norbornene insertion into an alkylpalladium bond, the latter clearly deriving from palladium migration from the alkyl to the aryl site and double norbornene insertion. In both cases the termination step involved p,y-C-C bond cleavage followed by P-H elimination. The stereochemistry of the norbornane unit invariably was exo [19]. 

In Sect. 3 we have seen that 0,0 -dialkylation via palladacycle is accompanied by norbornene deinsertion. Since this process is essentially due to the steric effects of the two ortho substituents, we could expect that norbornene deinsertion should be less favorable when steric hindrance is smaller and other reactants are absent. This is what happens with o,o -methyl groups. At 105 °C in DMF o,o -dimethyliodobenzene reacted with norbornene in the presence of Pd(OAc)2 as catalyst and KOAc as a base to give a 60% yield of hexahy-dromethanofluorene. The reaction implies palladation of the ortho methyl group followed by reductive elimination (Eq. 35) [39]. 

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