Pincer palladacycles

Palladacycles, Pincers and Other Palladium Complexes as Precursors of Palladium Nanoclusters... 

The pincer-type palladacycle (120) (R = 1Pr), which is actually a derivative of a dialkylphos-phinous acid (themselves excellent ligands see Section 9.6.3.4.6) was shown to allow the crosscoupling of aryl chlorides with terminal acetylenes ((120), ZnCl2, Cs2C03, dioxane, 160 °C). However the high reaction temperature may be prohibitive for the actual application of this catalytic system, as acetylenes are known to be thermally sensitive.433 The same palladacycle (R = Ph) is effective in the Suzuki-Miyaura reaction with aryl bromides and activated aryl chlorides (K2C03, toluene, 130 °C). 

Palladacycles [17] have been established as important class of catalysts, an unusual phosphine-free sulfur containing catalyst was introduced by Zim [18] et al. A phosphinite based palladacycle [19] proved to be very efficient in Suzuki coupling reactions. An N,P-Ligand type was synthesized by Kocovsky [20] et al. Tridentate pincer ligands [21] have been proved use-fill in the Heck reaction. Recent developments regarding Heck (22] and Suzuki [23] reactions have been reviewed by Fu and Littke. A new catalyst especially suitable for Heck couplings has been introduced by Beller [24] et al. 

Ligands with a carbon anion directly bonded to a metal ion are more common than perhaps supposed. The family of ligands (82) (Figure 14) invariably act as a tridentate N2C donor ligand to Pt, via deprotonation of the carbon marked in the figure. The C-deprotonated N2C pincer ligand (83) forms a totally flat palladacycle with sp hybridization of C and both N donors. A diimine tridentate (84) with the carbanion as a terminal rather than central donor of the N2C ligand has... 

Also worth mentioning here are studies based around the preparation and use of silica-supported palladacyclic complexes. It was the use of these that gave valuable evidence for the decomposition of half-pincer and SCS pincer " complexes during Heck reactions, generating soluble Pd(0) species that are the true catalysts. 

Herrmann-Beller palladacycle Fig. 10.12 Palladacycles and pincers used as pre-catalysts in the Heck and Suzuki reactions... 

Several excellent reviews have appeared recently in which published results regarding C—C bond forming reactions catalysed by several sources of palladium, including palladacycles and pincers, are critically evaluated with emphasis on mechanistic aspects [58, 105, 134—136]. 

It seems highly likely that all palladium-catalysed reactions that commence with an oxidative addition as the first step of the catalytic cycle proceed though a Pd(0) / Pd(ll) mechanism. Thus one needs to conclude that all palladacycles and pincers are converted to some form of Pd(0) in these reactions. In many cases this was shown to be in the form of palladium nanoparticles. However, with the more reactive iodoarenes it is possible that most of the catalyst is in the form of an anionic or neutral monomeric or dimeric palladium species. 

In conclusion, for C-C bond forming reactions on aryl bromides and iodides there is no need for the use of complexes, palladacycles or pincers as simple ligand-free Pd(OAc)2 will perform as well. For C—C bond forming reactions on aryl chlorides some palladacycles may be the catalyst of choice. For problematic cases, the palladium complexes based on bulky electron-rich phosphines or catalysts based on carbene Hgands are likely to give the best results. 

Two specific reactions were studied the arylation of n-butyl vinyl ether (25) with two different trifiates, 1-naphthyl triflate (26), and 4-cyanophenyl trifiate (27). The four catalytic systems studied were based on palladacycles 30 and 31, pincer complex 32, Pd(OAc)2 and polyurea encapsulated Pd EnCat30. The authors used dppp (l,3-bis(diphenylphosphino)propane) (33) and (2S,4S)-(-)-2,4-bis(diphenylpho-... 

Novel Palladium Chloride-Based Catalysts for Carbon-Carbon / Carbon-Heteroatom Bond Formations. The past decade has witnessed the development of novel palladacycles as a new class of catalysts for carbon-carbon/carbon-heteroatom bond-forming reactions. Several types of palladacycles (derived from PdCl2) have appeared in the literature. These include PC type, PCP pincer type, phosphite palladacycles, NC type, NCN pincer type, and sulfur containing palladacycles. Heterogeneous palladacycles have also been reported in the literature. These palladacycles are obtained via direct metallation from appropriate ligands with either PdCl2 or Na2PdCl4. Typical examples are shown in Scheme 1. 

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