Catalytic cycles transition-metal complexes

Useful multicomponent catalyst systems as well as multifunctional catalysts both offer new possibilities for the performance of catalytic processes this potential, however, can hardly be used as yet. One of the reasons for this difficulty stems from the fact that the preparation of such catalytic systems requires highly selective as well as sufficiently active catalytic components which, in addition, all reach their optimal catalytic properties for the same reaction conditions. This demand can be fulfilled by the use of tailor-made, catalytically active, transition metal complexes. The problem, however, is that these catalysts normally work via a relatively complex catalytic cycle. In a one-pot reaction system, therefore, a large number of different chemical species must be expected. Such a complex structured system can lead to several problems since it cannot be assumed that in a homogeneously catalyzed reaction system all components do not negatively interact. Even if a sufficiently stable catalyst system can be found by applying one or more of the different heterogenization techniques, this type of problem is hard to solve be-... 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

Various other transition metal complexes are also useful, including rhodium,195 palladium,196 and molybdenum197 compounds. The catalytic cycle can generally be represented as shown below.198... 

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. 

Koga, N., and K. Morokuma, Ab initio molecular orbital studies of catalytic elementary reactions and catalytic cycles of transition-metal complexes. Chem. Rev., 91, 823-842 (1991). 

The primary product of the oxidation of organic compounds is hydroperoxide, which is known as an effective electron acceptor. Hydroperoxides are decomposed catalytically by transition metal salts and complexes with the generation of free radicals via the following cycle of reactions [1-6] ... 

However, for Group Ylb transition metal complexes (M = Cr, Mo, and W) in basic solution, their studies led them to propose a different type of catalytic cycle involving the decomposition of formate intermediate.25,33,54 Below (Scheme 18a) is an example for Mo, but one can readily replace Mo with Cr or W. 

The efficient catalytic cycle is ascribed to the characteristic feature that Pd(0) is more stable than Pd(II). Reactions of 7t-allylpalladium complexes with carbon nucleophiles are called Tsuji Trost reactions. In addition to Pd, other transition metal complexes, such as those of Mo [26], Rh [27] and other metals, are used for catalytic allylation. 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

A second basic interaction pathway between transition metal complexes and organic substrates is SET (Path B). The overall processes can involve one individual or several sequential SET steps. For the latter, timing and direction of SET steps determine the reaction outcome significantly. The catalyzed reaction can proceed either as redox-neutral processes, in which oxidative and reductive SET steps are involved in the catalytic cycle, or as overall oxidative or reductive catalytic reactions, where two oxidative or reductive SET steps occur consecutively in the catalytic cycle. The third pathway (Path C) consists of a direct atom or group abstraction by the metal complex, which is possible for a weak R-X bond. 

A transition metal complex catalyzes the trimerization of ethylene to give 1-hexene. Draw a catalytic cycle like the one shown in Fig. 6.3 for this reaction. 

Nature accomplishes many syntheses-even those of complex molecules-by sequences of elementary steps. In the last few decades, the blueprint of catalyzed cascade reactions has found fertile soil through the advent of transition metal catalysis in laboratories. Scrutinizing catalytic cycles and mechanistic insight has paved the way for designing new sequential transformations catalyzed by transition metal complexes in a consecutive or domino fashion. In particular, transition metal-catalyzed sequences considerably enhance structural complexity by multiple iterations of organometalhc elementary steps. All this has fundamentally revolutionized synthetic strategies and conceptual thinking. 

Strictly speaking, a catalyst is some species directly involved in the catalytic cycle and, in the reactions discussed here, these species are usually low-valent, coordinatively unsaturated transition metal complexes. Metal halides, e-.g., chloroplatinic acid, PdCl, etc., although often claimed as catalysts are more properly catalyst precursors, since in the presence of silyl hydrides the metal halides are reduced. If no stabilizing ligands, e.g., olefins, phosphines, etc. are present, the reduction normally proceeds to a finely divided form of the metal or to insoluble metal silyl/hydride clusters which may act as heterogeneous catalysts. 

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