Palladium complexes reaction patterns

The current high level of interest in binuclear metal complexes arises from the expectation that the metal centers in these complexes will exhibit reactivity patterns that differ from the well-established modes of reactivity of mononuclear metal complexes. The diphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexibility in the distance between the two metal ions involved (1). This chapter presents an overview of the reaction chemistry and structural parameters of some palladium complexes of dpm that display the unique properties found in some binuclear complexes. Palladium complexes of dpm are known for three different oxidation states. Palladium(O) is present in Pd2(dpm)3 (2). Although the structure of this molecule is unknown, it exhibits a single P-31 NMR reso-... 

Cocyclization of acetylene with isocyanides gives interesting new cyclic compounds 103, 116). The reaction patterns are generally similar to the cocyclization wdth carbon monoxide which is already known 103, 117). Low-valent nickel, palladium, or cobalt complexes are active in the following reactions 102, 103) for which intervention of acetylene complexes has been suggested ... 

The basic pattern of the Heck reaction in its classical form is depicted in Scheme 1. It involves, for instance, the reaction of an aryl halide (1) with an olefin in the presence of a base and a catalytic amount of a palladium complex to give a styrene derivative (2) under elimination of HX. 

The chemistry of palladium is dominated by two stable oxidation states the zero-valent state [Pd(0), d ] and the +2 state [(Pd(II), d ]. Each oxidation state has its own characteristic reaction pattern. Thus, Pd(0) complexes are electron-rich nucleophilic species, and are prone to oxidation, ligand dissociation, insertion, and oxidative-coupling reactions. Pd(II) complexes are electrophilic and undergo ligand association and reductive-coupling reactions. A large amount of literature deals with these reactions. However, a few fundamental principles, such as oxidative addition, transmetalation, and reductive elimination, provide a basis for applying the chemistry of palladium in research. 

Conversion of monocyclic and polycyclic vinylcyclopropanes with low-valent transition-metal complexes, (e.g. iron, rhodium, zirconium, cobalt, nickel, palladium) mainly leads to ring opening and rearrangement products. A typical reaction pathway of vinylcyclopropanes with transition-metal complexes leads to f/Calkyl-j/ -allyl complexes, which as homodiene complexes exhibit interesting reaction patterns (e.g. carbonylation) leading to organic products. ... 

Organoplatinum complexes have a strong resemblance to the palladium analogs in their synthetic methodology and reaction patterns, but because of their high stability. 

However, the substitution pattern on the arene is crucial for the success of the Heck reaction. When a second Heck coupling takes place in an ortho position of another alkenyl unit, cyclization of the intermediately formed o--(/3-arylalkyl)palladium complex may occur, as formation of alkylideneindanes and alkylindenes, especially under classical Heck conditions with phosphines in the catalyst cocktail, was observed (Scheme 2, Table 1). 

The protocols for the utilization of ketone-derived silyl enol ethers in Tsuji-Trost reactions were preceded by a report of Morimoto and coworkers on the enantioselective allylation of sUyl ketene acetals 88. Without external activation, they reacted with the allylic substrate 19d in the presence of the palladium complex derived from the amidine ligand 89 to give y,5-unsaturated esters 90 in moderate chemical yield but high enantiomeric excess (Scheme 5.29) [46]. Presumably, the pivalate anion hberated during the oxidative addition functions as an activator of the silyl ketene acetal. The protocol is remarkable in view of the fact that asymmetric allylic alkylations of carboxylic esters are rare. Interestingly, the asymmetric induction originates from a ligand with an uncomplicated structure. The protocol seems however rather restricted with respect to the substitution pattern of allylic component and sUyl ketene acetal. 

The palladium complex 3.16 mimics the type of catalytic intermediates that are present when certain single-site catalysts are used in polymerization reactions (see Section 6.5.2). It is important to note that an agostic interaction is postulated in its structure. Here again the chemical shifts and the coupling patterns provide direct evidence of structural assignment. The agostically bound hydrogen appears as a broad triplet at -86. 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

Negishi E (2002) Fundamental Properties of Palladium and Patterns of the Reactions of Palladium and Its Complexes. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 17... 

The triethoxysilyl endgroup is a popular functional group to bind the catalyst to a polymeric support [238]. Polymeric supports include silica gel, MCM-41 (mesoporous silica gel) and ITQ-2 (delaminated zeolite) [247]. Corma et al. used this approach to synthesise gold(I) and palladium(II) NHC complexes for Suzuki cross-coupling reactions between iodobenzene and various arylboronic acids (see Figure 4.78) [247]. The results were very modest at 35-80% dependent upon the substitution pattern of the arylboronic acid. Yields with gold(I) catalysts were marginally better than those for palladium(II) complexes. 

Fundamental Properties of Palladium and Patterns of the Reactions of Palladium and Its Complexes... 

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