Catalysis/catalysts metal complexes

It is important to realize that there is a great deal of overlap in the topics covered in this chapter. For example, the chemistry of metal carbonyls is intimately related to metal alkene complexes, because both types of ligands are soft bases and many complexes contain both carbonyl and alkene ligands. Also, both areas are closely associated with catalysis by complexes discussed in Chapter 22, because some of the best-known catalysts are metal carbonyls and they involve reactions of alkenes. Therefore, the separation of topics applied is certainly not a clear one. Catalysis by metal complexes embodies much of the chemistry of both metal carbonyls and metal alkene complexes. 

Brunner s concept of attaching dendritic wedges to a catalytically active metal complex represented the first example of asymmetric catalysis with metal complex fragments located at the core of a dendritic structure [5,6]. Important early examples of catalysts in core positions were Seebach s TAD-DOL systems (TADDOL = 2,2-dimethyl-a,a,a/,a/-tetraphenyl-l,3-dioxolane-4,5-dimethanol) [38,39]. In general, the catalytic performance of such systems was either unchanged with respect to the simple mononuclear reference system or significantly lower. In no case has the potential analogy of this core fixation and the existence of efficient reactive pockets in enzymes been vindicated. This may be due to the absence of defined secondary structures in the dendrimers that have been employed to date. 

At the synthetic level we may expect increased emphasis on enantioselective catalysis usin metal complex catalysts as a key component of the manufacturing process (84). For biocatalysts there will unquestionably continue to be increasing interest in the "custom synthesis" of enzymes engineered for specific functions and conditions. The first example of the "ultimate" enzyme has been reported with the synthesis of the all-D form of HIV-1 protease (85-87). This enzyme exhibits a chiral specificity opposite to that of the naturally occurring L form and it may be generally predicted that enantiomeric proteins will exhibit reciprocal chiral specificity in all aspects of their interactions. These reciprocal chiral... 

Solid-Supported Surface Catalysis by Metal Complexes. Hong et al. (1987a, b, in press) have prepared a variety of hybrid catalysts between Co(II) phthalocyanine complexes and the surfaces of silica gel, polystyrene-divinylben-zene, and Ti02 and tested these hybrids for catalytic activity with respect to the autoxidation of hydrogen sulfide, sulfur dioxide, 2-mercaptoethanol, cysteine, and hydrazine ... 

This is an important area with many available methods. We shall look first at organic catalysis and then change to catalysis by metal complexes. The same type of intermediate 117 used for conjugate addition is clearly also suitable for Diels-Alder reactions with the same proviso it must be more reactive then the a,(5-unsaturated carbonyl compound as that too can do Diels-Alder reactions. And of course the first formed product 118 must hydrolyse rapidly to release the catalyst 102. 

The synthesis of the chiral copper catalyst is very easy to reproduce. The complex catalyses the asymmetric alkylation of enolates of a range of amino acids, thus allowing the synthesis of enantiomeric ally enriched a,a disubstituted amino acids with up to 92% ee. The procedure combines the synthetic simplicity of the Phase Transfer Catalyst (PTC) approach, with the advantages of catalysis by metal complexes. The chemistry is compatible with the use of methyl ester substrates, thus avoiding the use of iso-propyl or ferf-butyl esters which are needed for cinchona-alkaloid catalyzed reactions[4], where the steric bulk of the ester is important for efficient asymmetric induction. Another advantage compared with cinchona-alkaloid systems is that copper(II)(chsalen) catalyses the alkylation of substrates derived from a range of amino acids, not just glycine and alanine (Table 2.4). 

One of the most important uses for metal complexes is in the homogeneous catalysis of reactions. Studies of metal enzymes (physiological catalysts) show that the site of reaction in the biological system is frequently a complexed metal ion. Many industrial processes depend directly on catalysis hy metal complexes. The reaction of an alkene with carbon monoxide and hydrogen takes place in the presence of a cobalt complex, reaction (1). 

D. J. Cole-HamUton and R. P. Tooze, Catalyst Separation, Recovery and Recycling. Chemistry and Process Design. (Catalysis by Metal Complexes, Vol. 30), Springer, Dordrecht, the Netherlands, 2006. 

Photo- and Peroxide-Initiated Catalysis by Metal Complexes. Photogenerated catalysts obtained by the light-induced generation of a ground-state catalyst obtained from a catalytically inactive precursor have become a topic of current interest in catalysis by transition-metal complexes. Such methods have also been applied in hydrosilylation. 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

Hirai, H. and Toshima, N. In Catalysis by Metal Complexes, Tailored Metal Catalysts, Iwasawa, Y., Ed., D. Reidel Publishing Conq)any Dordrecht, 1986. Bradley, J. S. In Clusters and Colloids. From Theory to Applications, Schmid, G., Ed., VCH Weinheim, 1994. 

Xuereb, D., Dzierzak, J., and Raja, R. (2010) in Heterogenized Homogeneous Catalysts for Fine Chemicals Production, Catalysis by Metal Complexes, vol. 33, Chapter 2 (eds P. Barbaro and F. Ligiari), Springer Science and Business Media B.V, 37 -63. 

In recent years, the studies in the field of homogeneous catalytic oxidation of hydrocarbons with molecular oxygen were developed in two directions, namely, the free-radical chain oxidation catalyzed by transition metal complexes and the catalysis by metal complexes that mimic enzymes. Low yields of oxidation products in relation to the consumed hydrocarbon (RH) caused by the fast catalyst deactivation are the main obstacle to the use of the majority of biomimetic systems on the industrial scale. 

Catalysis by metal complexes in the liquid phase is presently a very important area in chemistry. Intense development of this field is due to several evident advantages of such catalysts. They are characterized by high catalytic activity, capability of reacting only with specific substrates (specificity) and in a specific position (selectivity). 

Homogeneous Catalysts Jbr Fine Chemicals Production Materials and Processes (Catalysis by Metal Complexes), Springer. (h) Gmttadauria, M. and Giacalone, F. (eds) (2011) Catalytic Methods in Asymmetric Synthesis Advanced Materials, Techniques, and Applications, John Wiley Sons, Inc., Hoboken. 

I n this chapter we discuss some of the basic chemical concepts that are of special relevance for homogeneous catalysis and metal complexes. Most homogeneous catalysts that we will discuss in this book are coordination or organometallic complexes of d elements. There are also a few complexes of f elements, i.e., lanthanides and actinides, that have shown promising catalytic activities, but their industrial use as homogeneous catalysts is insignificant. 

Reactions 33 and 35 constitute the two principal reactions of alkyl hydroperoxides with metal complexes and are the most common pathway for catalysis of LPOs (2). Both manganese and cobalt are especially effective in these reactions. There is extensive evidence that the oxidation of intermediate ketones is enhanced by a manganese catalyst, probably through an enol mechanism (34,96,183—185). 

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

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