Catalysts, general acid centers

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

The most commonly used type of catalyst is a relatively small, bifunctional molecule that contains both a Lewis base and a Bronsted acid center, the catalytic properties being based on the activation of both the donor and the acceptor of the substrates. The majority of organocatalysts are chiral amines, e.g. amino acids or peptides. The acceleration of the reaction is either based on a charge-activated reaction (formation of an imminium ion 4), or involves the generalized enamine catalytic cycle (formation of an enamine 5). In an imminium ion, the electrophilicity compared to a keton or an oxo-Michael system is increased. If the imminium ion is deprotonated to form an enamine species, the nucleophilicity of the a-carbon is increased by the electron-donating properties of the nitrogen. ... 

The question as to whether the acid properties of cracking catalysts are due to protons (Bronsted acids) or to electron-deficient atoms (Lewis acids) is somewhat more difficult to answer. Before considering this question, the origin of acid centers in silica-alumina compositions should be discussed. It is generally believed that acid centers, either the... 

It is now generally agreed, with some exceptions, that the cracking of hydrocarbons over silica-alumina and related catalysts involves a cationic mechanism in which the acidity of the catalyst plays a decisive role. There is some disagreement among workers in this field concerning the actual part played by the acid centers of the catalyst in the initiating step of the mechanism. 

The TS for the rapid hydrolysis of the monoanion is depicted as involving an intramolecular general acid catalysis by the carboxylic acid group, with participation by the anionic carboxylate group, which becomes bound at the developing electrophilic center. The un-ionized carboxylic acid group acts as a general acid catalyst and the... 

This example of side-chain alkylation of toluene with methanol serves not only to demonstrate the interaction between basic and acidic centers in a zeolite but also to illustrate another type of zeolite catalysis or of catalysis in general. The key phrases "computer graphics" or "computer-aided catalyst design" are used to describe this novel catalyst research. With the aid of computer graphics, it is possible to simulate zeolite structures and produce images of these structures. Computer graphics can also be used to produce pictures of molecules such as toluene in the zeolite pores or cages. 

The stereogenic centers may be integral parts of the reactants, but chiral auxiliaries can also be used to impart facial diastereoselectivity and permit eventual isolation of enantiomerically enriched product. Alternatively, use of chiral Lewis acids as catalysts can also achieve facial selectivity. Although the general principles of control of the stereochemistry of aldol addition reactions have been well developed for simple molecules, the application of the principles to more complex molecules and the... 

These examples serve to illustrate several general points about use of chiral catalysts for D-A reactions. A cationic metal center is present in nearly all of the catalysts developed to date and has several functions. It is the anchor for the chiral ligands and also serves as a Lewis acid with respect to the dienophile. The chiral ligands establish the facial selectivity of the complexed dienophile. There are several indications of the importance of the anions to catalytic activity. Anions, in general,... 

Arthur L. Weber (1998), now working at the Seti Institute of the Ames Research Center at Moffett Field, reports the successful synthesis of amino acid thioesters from formose substrates (formaldehyde and glycolaldehyde) and ammonia synthesis of alanine and homoserine was possible when thiol catalysts were added to the reaction mixture. On the basis of his experimental results, Weber (1998) suggests the process shown in Fig. 7.10 to be a general prebiotic route to amino acid thioesters. 

Pd(dba)2 [palladium(O)] generally affords the best results and thus an oxidation to the metal center must occur. The most likely mechanism for this to occur is by net oxidative addition of the acidic phosphonium P-H moiety (Scheme 3). This hypothesis is supported by the observation that the pKa of the phosphonium-hydro-gen bond directly affects the activity of catalysts generated in situ with more basic ligands being inactive. 

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