Asymmetric catalysis Subject

Chiral oxazolines developed by Albert I. Meyers and coworkers have been employed as activating groups and/or chiral auxiliaries in nucleophilic addition and substitution reactions that lead to the asymmetric construction of carbon-carbon bonds. For example, metalation of chiral oxazoline 1 followed by alkylation and hydrolysis affords enantioenriched carboxylic acid 2. Enantioenriched dihydronaphthalenes are produced via addition of alkyllithium reagents to 1-naphthyloxazoline 3 followed by alkylation of the resulting anion with an alkyl halide to give 4, which is subjected to reductive cleavage of the oxazoline moiety to yield aldehyde 5. Chiral oxazolines have also found numerous applications as ligands in asymmetric catalysis these applications have been recently reviewed, and are not discussed in this chapter. ... 

Perhaps the most compelling research objective in this area will involve the development of a chiral metathesis catalyst that effects C-C bond formation efficiently and with excellent levels of enantioselectivity [41 ]. In such a case, all the reactions discussed herein, in addition to those expertly developed in other laboratories [40], will become subject to asymmetric catalysis. Such a development should prove to have an enormous impact on the field of inorganic, organome-tallic and synthetic organic chemistry. 

The chiral auxiliaries anchored to the substrate, which is subjected to diastereoselective catalysis, is another factor that can control these reactions. These chiral auxiliaries should be easily removed after reduction without damaging the hydrogenated substrate. A representative example in this sense is given by Gallezot and coworkers [268], They used (-)mentoxyacetic acid and various (S)-proline derivates as chiral auxiliaries for the reduction of o-cresol and o-toluic acid on Rh/C. A successful use of proline derivates in asymmetric catalysis has also been reported by Harada and coworkers [269,270], The nature of the solvent only has a slight influence on the d.e. [271],... 

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... 

Quinazolines take part in the same types of reactions as pyrimidines, but because of their additional benzene ring, the products of these reactions may have the added feature of hindered rotation. An example of this is the synthesis of 2-phenyl-Quinazolinap by Guiry and co-workers <99TA2797>. Suzuki coupling of 4-chloro-2-phenylquinazoline (115) with boronic acids 116 led to 117 (R = OMe). These intermediates were parlayed into phosphinamines 117 (R = PPh2) and then subjected to chiral resolution to produce new chiral phosphinamine ligands for asymmetric catalysis. 

This book deals with the basic principles of asymmetric catalysis and places particular emphasis on its synthetic significance. The mechanisms of most of the chemical reactions that I will discuss are obscure and are therefore treated only briefly. My talks at Cornell relied heavily on chemistry developed in our laboratories at Nagoya University, and the materials in Chapters 2, 3, 5, and 6 are highly subjective. Because asymmetric synthesis with molecular catalysts is a very attractive and rich subject, many academic and industrial laboratories all over the world have contributed to its development. In an attempt to balance my coverage of the entire field, I have tried to include most of the major achievements recorded by the fall of 1992 within Chapter 4. 

We wished to develop a macroscopic model of the interactions between molecular ligands and receptors. Molecular recognition is a broad subject that describes selective assembly in chemistry and biology, with examples from DNA-protein complex formation to asymmetric catalysis. The principle behind molecular recognition dictates that the molecules that mate have complementary shapes and interfacial characteristics. Our extension of this principle to the mesoscale involved the self-assembly of objects that matched both... 

Diels-Alder reactions have been very successfully subjected to asymmetric catalysis by binaphthyl complexes. Accordingly, the synthesis of tetrahydropyranes 41 and 42 can be realized by reaction of glyoxylic esters 39 with methoxy-dienes 38 (Scheme 8) [20]. Some of these reactions take place with excellent endo-conxro and... 

The interest in all these themes really is not decreasing, indeed some fascinating areas of research are emerging or are the subject of many investigations the medicinal chemistry of bisphosphonates, the role of phosphorus in biology, phosphorus ligands in radiopharmaceutical chemistry, phosphorus in material science, new polymers and dendrimers incorporating phosphorus, and asymmetric catalysis to name but a few. 

A number of important review articles have appeared in the area of pentaco-ordinate and hexaco-ordinate phosphorus chemistry. Robert Holmes has provided an extremely informative comparison of the hypervalency, stereochemistry and reactivity of silicon and phosphorus including the application of the latter to enzyme systems. The coordination chemistry of hydrophosphoranes including the formation of complexes from bicyclic-, tricyclic- and tetracyclic hydrophosphoranes has also been the subject of a comprehensive review with literature coverage to 1995. Numerous metal complexes are mentioned including Rh, Ru, Pd, Co, Fe, Mo, and W and the relevance to asymmetric catalysis is discussed. Neutral six-coordinate compounds of phosphorus, including mono-, di-, tri-, and tetracyclic examples, have also been reviewed. 

A classical method for the preparation of enantiopure compounds is the resolution of racemate. However, it is much more effective to use the selective synthesis of the desired enantiopure substance via enantioselective approach. Stereoselective methods of synthesis have been widely developed in organic chemistry. The method of asymmetric synthesis has been known since the nineteenth century and asymmetric catalysis has witnessed an enormous amount of development in recent decades as shown in Chapter 3. In contrast, the asymmetric synthesis of coordination compounds has only recently become a subject of systematic investigation. This is no doubt related to the fact that the chirality of coordination compounds is a much more complex phenomenon than that of organic compounds, because of higher coordination and the multitude of possible central atoms. Furthermore, while in organic chemistry the chiral tetrahedral carbon centres can be prepared without racemization, in contrast T-4 metal centres are very often labile. In fact it is even difficult to prepare compounds with a metal centre coordinated to four different monodentate ligands, and thus the possibility of obtaining one enantiomer is excluded in most cases. 

The same group of workers has proceeded to develop an intramolecular version of the reaction. The aldehyde acids (35, n = 1 or 2) on treatment with Mukaiyama s reagent, 2-chloro-l-methylpyridinium iodide, and triethylamine afforded the cis substituted bicyclic lactones (36, n = 1 or 2). The authors have adduced evidence in support of a nucleophile-catalysed aldol lactonisation (NCAL) reaction mechanism rather than the alternative thermal [2+2] cycloaddition. They have also found that the intramolecular reaction, like the intermolecular process, is subject to asymmetric catalysis. When an optically active base such as 0-acetylquinidine was present in the reaction mixture, the bicyclic lactones were produced with high ee <01 JA7945>. 

Whilst there are many cycle addition reactions which could be subjected to asymmetric catalysis, the maj ority of work has been involved with the Diels-Alder and related reactions. Nevertheless, 1,3-dipolar cycloadditions have provided fairly good... 

I am grateful to my former mentors. Professor S. G. Davies (Oxford) and Professor D. A. Evans (Harvard), who introduced me to asymmetric synthesis and asymmetric catalysis. I am also indebted to my current coworkers and students who help to keep organic chemistry alive for me with their enthusiasm for the subject. 

A classification of the different synthetic routes for chiral induction was presented in Table 9.3. Included in the table is asymmetric catalysis which forms the subject matter of the rest of the chapter. 

For two recent books on this subject, see (a) Noyori, R., Asymmetric Catalysis in Organic Synthesis, Wiley Interscience New York 1994 (b) Ojima, I. Catalytic Asymmetric Synthesis, VCH New York, 1993. Other reviews include (c) Ojima,... 

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