Catalysis involving metal -complex

Nonlinear mechanisms are very common in heterogeneous catalytic reactions. They are also characteristic of chain reactions and, perhaps, of homogeneous catalysis involving metal complexes. Because of this, the classification of these mechanisms is of considerable interest. 

Catalysis by Metal Complexes deals with all aspects of catalysis which involve metal complexes and seeks to pnblish authoritative, state-of-the-art volumes which serve to document the progress being made in this interdisciplinary area of science. 

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. 

One of the most remarkable recent advances in metal carbonyl substitution chemistry has been the discovery by Coville and co-workers of the homogeneous and heterogeneous catalytic labilization of the metal-carbon bond in metal-carbonyl complexes (26-31). Considering that restrictions to catalysis involving metal carbonyl species can, in some instances, be related to the strength of the metal-carbon bond, these discoveries could have far-reaching implications. To exemplify these catalytic substitution processes, comparisons in the systems M(CO)6(M = Cr, Mo, W), CpMoI(CO)3, CpFeI(CO)2, Fe(CO)5, Fe(CO)4(olefin), and Ir4(CO)12 will be made. 

Phase-transfer catalysis (PTC) is the most widely used method for solving the problem of the mutual insolubility of nonpolar and ionic compounds. Basic principles, synthetic uses, industrial applications of PTC, and its advantages over conventional methods are well documented [1-3]. PTC has become a powerful and widely accepted tool for organic chemists due to its efficiency, simplicity, and cost effectiveness. The main merit of the method is its universality. It may be applied to many types of reactions involving diverse classes of compounds. An important feature of PTC is its computability with other methods for the intensification of biphasic reactions (sonolysis, photolysis, microwaving, etc.) as well as with other types of catalysis, in particular, with transition-metal-complex catalysis. Homogeneous metal-complex catalysis under PTC conditions involves the simul-... 

The mechanism of metal catalysis is multifaceted and it always involves metal complexes with reacting species, but true nature of the transition states is open to debate... 

The coverage in this chapter is not comprehensive and no tabulation of rate data has been attempted. A more extensive account of the material will appear in Volume 9 of the series the presentation of the material also differs slightly from previous volumes. There have been many important developments in this general area. A comprehensive review of one-electron reduction potentials for nonmetallic substrates has appeared. Detailed kinetic studies have been reported for several important reactions where metal ions serve as catalysts in the transformation of organic substrates.Catalytic oxidation reactions involving metal complexes and macrocyclic metal complexes have been reviewed. Continued interest centers on catalysis by metalloporphyrins, and the role of metal complexes in electrocatalytic reductions has been reviewed. ... 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

The same reasoning applies to the synthesis of pure enantiomers as to organic synthesis in general processes should be atom efficient and have low E factors, i.e. involve catalytic methodologies. This is reflected in the increasing attention being focused on enantioselective catalysis, using either enzymes or chiral metal complexes. 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

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