Catalytic concept

Using microkinetic expressions, we have discussed the most important catalytic concepts that describe heterogeneous catalytic reactions. We have related these concepts with the energies, entropies, and transition-state features that are accessible through current state-of-the-art DFT techniques. 

The Catalysis Concept of Iminium Activation In 2000, the MacMillan laboratory disclosed a new strategy for asymmetric synthesis based on the capacity of chiral amines to function as enantioselective catalysts for a range of transformations that traditionally use Lewis acids. This catalytic concept was founded on the mechanistic postulate that the reversible formation of iminium ions from a,p-unsaturated aldehydes and amines [Eq. (11.10)] might emulate the equilibrium dynamics and 7i-orbital electronics that are inherent to Lewis acid catalysis [i.e., lowest unoccupied molecular orbital (LUMO)-lowering activation] [Eq. (11.9)] ... 

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. 

Aldehydes and ketones can also be directly a-brominated using the catalytic concepts presented in Scheme 2.34 and Eq. (7) [30]. The easily synthesized and air-stable 4,4-dibromo-2,6-di-ter -butyl-cyclohexa-2,5-dienone was found to be the best reagent for the functionalization of aldehydes (enantioselectivities in the range of 68-96% ee) and for the preparation of the optically active a-... 

Such studies, when well designed, provide for a more fundamental understanding of catalysis. Catalytic concepts and models are thereby developed and "catalysts by design" come within the realm of reality. This area of catalyst characterization is exciting, complex and requires researchers of many talents. This is where the issue of true fundamental interest, the structure and associated energetics of the transition state should be identified as a major challenge. 

Catalytica Associates, Inc. is conducting a study to produce a systematic assessment of the role of catalysis in thermochemical conversion via gasification and liquefaction. This study is also examining the potential impact of catalytic concepts under development in other areas, such as coal conversion, and new reactor technology on biomass conversion. 

About five basic catalytic concepts have been used in the history of catalytic emission control (Fig. 20(a)). 

In addition, the use of a whole-cell biocatalyst consisting of a racemase, hydantoi-nase, and carbamoylase allows a dynamic biocatalytic resolution. Besides resolution processes, asymmetric (bio-)catalytic concepts have been applied successfully on an industrial scale. The different types of asymmetric (bio-)catalytic syntheses of L-amino acids, based on the use of prochiral starting materials, are shown in Fig. 3. 

The Catalytic Concept Catalyst Design and Development of an Efficient Catalysis in Model Reactions... 

In this chapter we summarize the catalytic principles highlighted in this book. This provides the reader with an umbrella of important theoretical catalytic concepts and founding laws. Each concept is listed along with the chapter(s) in which it was introduced. 

Since its rediscovery by MacMillan in 2000, iminium activation catalysis has become a key catalytic concept in organocatalysis. After initial work centered on cycloadditions, Michael additions became the main area of interest and it is now estabhshed as a general strategy for the asymmetric conjugate addition of nucleophiles to a,P-unsaturated compounds. 

Iminium catalysis is another key catalytic concept in aminocatalysis. Initialworkwas disclosed by MacMillan for the Diels-Alder reaction of cyclopentadiene and a,P-unsaturated aldehydes [12], but it was rapidly extended to Michael additions (including Friedel-Crafts reactions). Now iminium catalysis has been established as a general mode for nucleophilic addition to a,]3-unsaturated carbonyl compounds. 

Grdger H, Hammer B. Catalytic concepts for the enantiose-lective synthesis of a-amino and a-hydroxy phosphonates. Chem. Eur. J. 2000 6 943-948. 

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