Catalytic activity Future directions

As a catalytic concept, asymmetric enamine catalysis has been the subject of several recent reviews [8-23], In this concept review, we will focus on some of the key aspects of this mode of activation, and probe the current limitations and possible future directions of enamine catalysis. 

There will undoubtedly be many new catalysts described in the near future. In particular, we anticipate that later transition-metal complexes will play a much larger role in this type of chemistry. Although bis-Cp complexes of Groups 3 and 4, the lanthanides, and the actinides have shown exceptional activity and thus far have dominated reported investigations on catalytic homodehydrocoupling, it is most unlikely that they are unique in this property. It is to be expected that other classes of complexes, such as mono-Cp and -rj -alkyl complexes of these elements, will also be active. Other directions for future evolution are the development of catalysts that are air stable, that are tolerant of more functionalities on substrates, and that are more easily manipulated. 

A novel transition metal-catalyzed hydrosilylation process is described. The use of an ionic liquid in this process allows for the immobilization, heterogenization, and recovery of the expensive precious metal catalyst as well as its direct reuse in a subsequent hydrosilylation reaction. From an economic and ecological point of view, this process perfectly fits in the concept of "Sustainable Chemistry". Future research activities will aim at the prolongation of the catalyst life-time. For this, it is necessary to gain a deeper understanding of the catalytically active species in the catalyst/ionic liquid solution. 

In this chapter, we will present some contemporary results from our laboratory aimed toward blending the desirable properties of homogeneous and heterogeneous catalysts to create highly active, selective, and recoverable immobilized catalysts. The chapter covers organometaUic catalytic systems for polymerization reactions as well as small molecule reactions. Formation of single-site catalysts, recyclability, and stability/leaching issues will be addressed, as will studies on the effects of the immobilization procedure and the nature of the support structure on catalytic performance. Finally, a brief discussion on projected future directions for immobilized catalysts is presented. 

Further defluorination to octafluoronapthalene, which is often the product of similar reactions, does not occur. Coupled with terminal reductants such as aqueous dithionite this reaction can be performed under catalytic conditions with respect to (C6H6)2Cr [74]. Together these reactions provide a glimpse of possible future directions for research into the application of C-F bond activation chemistry to synthetic problems. 

In addition, to demonstrate the reaction principle for a wider range of applications including in particular asymmetric catalysis, future efforts will be directed toward the utilization of the chemical characteristics of the system in synthetic procedures. The carbonic acid equilibria (Scheme 1) provide many opportunities to interact directly with substrates, products, or catalytically active intermediates as, for example, demonstrated in Scheme 2. Up to now, the main focus has been to avoid detrimental interference of such processes in the reactions under scrutiny. As our imderstanding of the mechanisms and catalytic cycles in this medium increases, it should, however, become more and more possible to take advantage of the CO2/H2O reactivity similarly to the interaction of SCCO2 with secondary amines [47, 48],... 

As illustrated by other chapters of this book, a diverse array of pincer and pincer-type ligands are known in the literature and can be incorporated to make new nickel complexes. Their catalytic activity in various cross-coupling reactions is likely to be the focus of future research. Another research direction that deserves more attention is carbon-heteroatom bond-forming reactions very few nickel pincer complexes have been studied for these catalytic applications. 

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