Fundamentals of Asymmetric Catalysis

Walsh PJ, Kozlowski MC (2009) Fundamentals of asymmetric catalysis. University Science, Sausalito... 

Fundamentals of Asymmetric Catalysis, University Science Books,... 

P.J. Walsh and M.C. Kozlowski, Fundamentals of Asymmetric Catalysis, University Science Book, Sausalito, California, 2008. ISBN 9781891389542. 

The concepts and reactions outlined in this chapter are intended to convey the essential features of catalysis involving the organoinetallic reactions outlined in this book. However, these fundamentals apply to catalysis in general. Further reading on the fundamentals of asymmetric catalysis - and more comprehensive treatises are listed in the references and notes. 

Walsh, P. J. Kozlowski, M. C. Fundamentals of Asymmetric Catalysis University Science Books Sausalito, CA, 2009. 

Implementation of the concept of combined acids in the field of asymmetric catalysis has been known for over 20 years. Several excellent reviews containing historical background and theoretical perspectives on this subject have appeared [93]. Fundamentally, a combined acid system involves the association of an acceptor atom A with a donor atom D that is chemically bonded to another... 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

The asymmetric processes discussed so far in this chapter have focused on reactions that create non-racemic, chiral products from achiral reagents by selective reaction at one prochiral face or position over the other. However, these principles can also be applied to reactions that separate enantiomers of an existing racemic mixture, channel both enantiomers of such a mixture to a single enantiomeric product, or that select between reaction at one of two diastereotopic functional groups in an achiral substrate. These reactions are also synthetically valuable and are called kinetic resolutions, dynamic kinetic resolutions, and desymmetrizations. An understanding of these reactions draws from the principles established so far in this chapter, but they also require some additional principles to be established that apply in a specific way to these classes of asymmetric transformations. Thus, the remainder of Chapter 14 introduces the fundamentals of these classes of asymmetric catalysis. 

I - FUNDAMENTALS AND RECENT ADVANCES n - SOME ASPECTS OF ASYMMETRIC CATALYSIS WITH TRANSITION - METAL COMPLEXES... 

Enantioselective -Functionalization of Aldehydes and Ketones The direct and enantiosective functionalization of enolates or enolate equivalents with carbon-, nitrogen-, oxygen-, sulfur- or halogen-centered electrophiles represents a powerful transformation of chemical synthesis and of fundamental importance to modem practitioners of asymmetric molecule constmction. Independent studies from List, J0rgensen, Cordova, Hayashi, and MacMiUan have demonstrated the power of enamine catalysis, developing catalytic enantioselective reactions such as... 

The chapter by Reetz includes many examples and explanations of methods illustrating how directed evolution has been applied to prepare new enzyme catalysts. Directed evolution of enantioselective enzymes has emerged as a fundamentally new approach to asymmetric catalysis. It involves the combination... 

The quest for new chiral ligands is of fundamental importance for progress in asymmetric catalysis [1], The ultimate goal is the development of a chiral ligand that is readily available in both enantiomeric forms, which imparts high turnover numbers and enantioselectivity to the catalyst. Although such ligands are known... 

Chiral asymmetric epoxidations have been intensively investigated due to the fundamental importance of epoxides in organic chemistry [69, 70], Nevertheless, catalytic asymmetric Lewis acid epoxidation of a,/i-unsaturated aldehydes remains a challenge to chemists. Recently, Jorgensen and co-workers developed the first asymmetric approach to epoxides of enals, in which chiral pyrrolidine 11 was used as catalyst and H2O2 as oxidant, thus following the concept of iminium catalysis (Scheme 3.9) [71-73]. Importantly, reaction conditions are tolerant to a variety of functionalities and this chemical transformation proceeds in different solvents, with no loss of enantioselectivity. (For experimental details see Chapter 14.13.1). 

As has been emphasized at the beginning of this overview of asymmetric den-drimer catalysis, the kinetically controlled stereoselection depends on very small increments of free activation enthalpy. It is therefore an excellent sensitive probe for dendrimer effects and will continue to be studied in this fundamental context. As mono dispersed macromolecules, chiral dendrimer catalysts provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers. 

The hydrocyanation reaction is important not only because it is practiced industrially on a large scale, but also because it clearly illustrates some of the fundamental postulates of homogeneous catalysis. The potential of the hydrocyanation reaction in asymmetric catalysis has also been explored and appears to be promising (see Chapter 9). 

The roots for the activity in the field of preparation of enantiopure amino acids in the Leibniz-Institut fur Organische Katalyse an der Universitat Rostock e.V. (formerly known as Bereich Komplexkatalyse which was a part of the Zentral-institut fur Organische Chemie der Akademie der Wissenschaften der DDR ) were planted at the end of the 1960s by Horst Pracejus, who was its director at that time (Fig. 1). In the 1950s Pracejus had previously worked on asymmetric catalysis and published outstanding results on the reaction of nucleophiles with ke-tenes catalyzed by chiral bases and developed a fundamental understanding of the mechanism of such enantioselective processes controlled by opposed entropic and enthalpic parts of the free activation enthalpy [1],... 

The chapters on asymmetric catalysis will describe catalytic versions of the reactions we are about to describe. Catalysis is a fundamentally more efficient way of asymmetric induction and we shall restrict our discussion here to cases where catalysis proved ineffective. 

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