Catalyst acid/base

In an intramolecular aldol condensation of a diketone many products are conceivable, since four different ends can be made. Five- and six-membered rings, however, wUl be formed preferentially. Kinetic or thermodynamic control or different acid-base catalysts may also induce selectivity. In the Lewis acid-catalyzed aldol condensation given below, the more substituted enol is formed preferentially (E.J. Corey, 1963 B, 1965B). 

The oxidation of methacrolein to methacrylic acid is most often performed over a phosphomolybdic acid-based catalyst, usually with copper, vanadium, and a heavy alkaU metal added. Arsenic and antimony are other common dopants. Conversions of methacrolein range from 85—95%, with selectivities to methacrylic acid of 85—95%. Although numerous catalyst improvements have been reported since the 1980s (120—123), the highest claimed yield of methacryhc acid (86%) is still that described in a 1981 patent to Air Products (124). 

With phosphoric acid-based catalysts, in which the active component is Hquid acid absorbed in the pores of the support, the reaction probably follows the path (119) for the hydration of olefins in aqueous solution ... 

K. Tanabe and WF. Holderich, Industrial Applications of Solid Acid-Base Catalysts , Applied Catalysis A, General, 1999, 181, 399. 

A non-acidic isomerization catalyst system has unexpectedly emerged from recent studies by French workers [4] in the area of Mo-oxycarbides. Although at an early stage of development, these new materials exhibit high selectivities for the isomerization of paraffins such as n-heptane. An alternative non-carbenium ion mechanistic route to achieve isomerization of higher alkanes could potentially overcome some of the limitations of conventional solid acid based catalyst systems. 

Homocitrate acting as acid/base catalyst for protonating bound substrates or metal centers to produce hydrides... 

Glycine acts as an acid-base catalyst in this reaction. C8 and Cl 1 are very acidic, and once deprotonated they are very nucleophilic, so they can attack C2 and C3 in an aldol reaction. Dehydration gives a key cyclopentadienone intermediate. (The mechanism of these steps is not written out below.) Cyclopentadienones are antiaromatic, so they are very prone to undergo Diels-Alder reactions. Such a reaction occurs here with norbomadiene. A retro-Diels-Alder reaction followed by a [4 + 1] retrocycloaddition affords the product. 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

A new dimension in the development of nucleic acid based catalysts was introduced by Breaker and Joyce in 1994 when they isolated the first deoxyribozyme [111]. It is not unexpected that DNA is also able to catalyze chemical reactions because it was shown previously that ssDNA aptamers which bind to a variety of ligands can be isolated by in vitro selection [141]. In the meantime, several deoxyribozymes have been described which expand the range of chemical transformations accelerated by nucleic acid catalysts even further and raising question whether even catalytic DNA might have played some role in the pre-biotic evolution of hfe on earth [69-71]. 

As part of a search for environmentally friendly solid acid-base catalysts, a modified Mg-Al hydrotalcite has been used as a base catalyst for aldol and Knoevenagel condensations. Yields are often quantitative, reaction times are about Ih, the catalyst can be recovered by filtration, and only moderate temperatures are required (60 °C for the aldol, ambient for the Knoevenagel). 

Abstract The last few years have seen a considerable increase in our understanding of catalysis by naturally occurring RNA molecules, called ribozymes. The biological functions of RNA molecules depend upon their adoption of appropriate three-dimensional structures. The structure of RNA has a very important electrostatic component, which results from the presence of charged phosphodiester bonds. Metal ions are usually required to stabilize the folded structures and/or catalysis. Some ribozymes utilize metal ions as catalysts while others use the metal ions to maintain appropriate three-dimensional structures. In the latter case, the correct folding of the RNA structures can perturb the pKa values of the nucleo-tide(s) within a catalytic pocket such that they act as general acid/base catalysts. The various types of ribozyme exploit different cleavage mechanisms, which depend upon the architecture of the individual ribozyme. 

Fig. 4A The mechanism of cleavage by ribonuclease A. Two imidazole residues function as general acid-base catalysts. B The single-metal-ion mechanism proposed for cleavage by the hammerhead ribozyme. One metal ion binds directly to the pro-Rp oxygen and functions as a general base catalyst. C The double-metal-ion mechanism proposed for cleavage by the hammerhead ribozyme. Two metal ions bind directly to the 2 -oxygen and the 5 -oxygen...

A novel finding related to the mechanism of catalysis by the genomic HDV ribozyme is that the pKa of C75 is perturbed to neutrality in the ri-bozyme-substrate complex and, more importantly, that C75 acts as a general acid catalyst in combination with a metal hydroxide which acts as a general base catalyst (Fig. 9A) [105]. The discovery of this phenomenon provided the first direct proof that a nucleobase can act as an acid/base catalyst in RNA. As a result, as shown by the solid curve in Fig. 9B, the curve that represents the dependence on the pH of the self-cleavage of the precursor genomic HDV ribozyme has a slope of unity at pH values that are below 7 (the activity increases linearly as the pH increases, with a slope of +1). Then, at higher pH values, the observed rate constant is not affected by the pH. 

The most effective acid-base catalyst is one whose pATa is 7.0, since at pH 7.0 the concentrations of acid and conjugate base are equal (see Section 4.9). With just a slight decrease in pH it would become... 

In addition to imprinted acid-base catalysts [49-55], attempts to imprint metal complexes have been reported and constitute the current state of the art [46, 47]. In most cases of metal-complex imprinting, ligands of the complexes are used as template molecules, which aims to create a cavity near the metal site. Molecular imprinting of metal complexes exhibits several notable features (i) attachment of metal complex on robust supports (ii) surrounding of the metal complex by polymer matrix and (iii) production of a shape selective cavity on the metal site. Metal complexes thus imprinted have been appHed to molecular recognition [56, 57], reactive complex stabilization [58, 59], Hgand exchange reaction [60] and catalysis [61-70]. 

There has been great interest in the area of chiral acid catalysts in organic synthesis over the past few decades. This topic has been the subject of several previous reviews. For example, the book Lewis Acids in Organic Synthesis (edited by Hisashi Yamamoto) was published by Wiley-VCH in 2000. In this chapter, successful and significant chiral Brpnsted acid catalysts, chiral Lewis acid catalysts [typical Lewis acidic elements main group elements, B(III) and Al(III), and early transition metal, Ti(IV)], and Lewis acid-assisted chiral Brpnsted acid catalysts developed after 2000 are discussed. Chiral acid/base catalysts wdl be discussed in Chapter 13 by Shibasaki and Kanai. 

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