Metal complexes, catalysis

Olefin Hydroformylation (The Oxo Process). One of the most important iadustrial applications of transition-metal complex catalysis is the hydroformylation of olefins (23), ihusttated for propjdene ... 

A. D. Pomogailo, Immobilized Polymeric-Metal Complex Catalysis (in Russian), Khimiya, Moscow (1986). 

Such a broad range of classical elementary reactions of homogeneous catalysis with metal complexes, that can be facilitated by photons, make illumination of reaction solution a very useful instrument for substantial increase of the possibilities of homogeneous metal complex catalysis in organic synthesis. Particular examples of light-assisted syntheses will be given in section 3. 

In 1981, it was demonstrated (70) that anions of nitro compounds can be involved in C,C-coupling with allyl acetates at the allylic carbon atom with the use of metal complex catalysis. For many years, this observation did not come to the attention of chemists interested in the synthesis of cyclic nitronates. However, Trost demonstrated (71) that this process can be used in the synthesis of five-membered cyclic nitronates from olefins (18) containing two acyl groups in the different allylic positions (Scheme 3.21). 

In the very recent past, metal complex catalysis has been used with advantage for the stereo- and enantio selective syntheses based on the Henry and Michael reactions with SENAs (454-458). The characteristic features of these transformations can be exemplified by catalysis of the reactions of SENAs (327) with functionalized imides (328) by ligated trivalent scandium complexes or mono-and divalent copper complexes (454) (Scheme 3.192). Apparently, the catalyst initially forms a complex with imide (328), which reacts with nitronate (327) to give the key intermediate A. Evidently, diastereo- and enantioselectivity of the process are associated with preferable transformations of this intermediate. 

It should be noted that the demarcation between metal complex catalysis of the Henry (454-458) and Mannich reactions is arbitrary, and that the catalyzed process is sometimes called the Mannich reaction (see, e.g., Ref. 456). 

Molecular sieves, chiral metal complex catalysis, 395... 

Conversely, other processes are totally original. This is especially encountered when the electrochemical act is associated with a transition metal complex catalysis. These methods have the advantage of affording the organozinc compound synthesis under simple and mild conditions that are compatible with the presence of reactive functional groups on the substrate. Importantly, these procedures are reproducible and can be run by any chemist. Besides, the preparation from a few millimoles to tens of millimoles of the organometallic compound is easy at the laboratory scale. 

Simandi, L. I., ed. Advances in Catalytic Activation of Dioxygen by Metal Complexes . Catalysis by Metal Complexes, Series Eds. van Leeuven, P. James, B. Vol. 26. Kluwer Dordrecht, 2003. 

Supramolecular Construction of Chelating Bidentate Ligand Libraries through Hydrogen Bonding Concept and Applications in Homogeneous Metal Complex Catalysis... 

Catalysis in liquid-liquid biphasic systems has developed recently into a subject of great practical interest because it provides an attractive solution to the problems of separation of catalysts from products and of catalyst recycle in homogeneous transition metal complex catalysis. Two-phase systems consist of two immiscible solvents, e.g., an aqueous phase or another polar phase containing the catalyst and an organic phase containing the products. The reaction is homogeneous, and the recovery of the catalyst is facilitated by simple phase separation. 

Hydrogenation was reviewed recently by Chaloner etal. (85). The history of aqueous metal complex catalysis started with the hydrogenation reaction. Recent work has focused on the search for mechanistic clues to understand the changes in reaction rates and selectivities of aqueous hydrogenation reactions compared with the nonaqueous analogues. 

The interest and activity in metal complex catalysis will continue to be marked by further advances in asymmetric catalysis and catalysis by solid metal complexes as well as in the discovery of new complexes capable of catalytically activating molecules such as hydrocarbons, nitrogen, oxygen, and carbon dioxide in a manner that permits new uses for these abundant materials. 

Before any catalysis can occur, at least one of the substrates must coordinate to the catalyst. This means that the catalyst must have a vacant active site. In homogeneous metal complex catalysis and biocatalysis, this will be a vacant coordination site at the metal atom. In heterogeneous catalysis, the vacant site could be a metal crystallite or an ion on the surface. For the latter, we speak of desorption and adsorption instead of dissociation and coordination. Remember that our reactions are not in vacuum, so there is no vacant site . Thus, before any chemical species can coordinate to the metal complex (or to the active site in heterogeneous catalysis or biocatalysis) the species already occupying this space must first vacate it. This happens constantly, as the system is dynamic (Figure 3.3) [15]. At any given moment... 

---