Mechanism, catalysis

If this intermediate, in turn, is more n idly attacked by water or hydroxide ion than the original ester, the overall reaction will be faster in the presence of the nucleophile than in its absence. These are the requisite conditions for nucleophilic catalysis. Esters of relatively acidic alcohols (in particular, phenols) are hydrolyzed by the nucleophilic catalysis mechanism in the presence of imidazole ... 

The nucleophilic catalysis mechanism only operates when the alkoxy group being hydrolyzed is not much more basic than the nucleophilic catalyst. This relationship can be imderstood by considering the tetrahedral intermediate generated by attack of the potential catalyst on the ester ... 

The relative importance of the potential catalytic mechanisms depends on pH, which also determines the concentration of the other participating species such as water, hydronium ion, and hydroxide ion. At low pH, the general acid catalysis mechanism dominates, and comparison with analogous systems in which the intramolecular proton transfer is not available suggests that the intramolecular catalysis results in a 25- to 100-fold rate enhancement At neutral pH, the intramolecular general base catalysis mechanism begins to operate. It is estimated that the catalytic effect for this mechanism is a factor of about 10. Although the nucleophilic catalysis mechanism was not observed in the parent compound, it occurred in certain substituted derivatives. 

Table 6-3. Kinetically Equivalent Intramolecular Catalysis Mechanisms...

However, it may be that the acid catalysis mechanism is more complex than that described by Scheme 6-5. Broxton and McLeish (1983) investigated the (ii)-diazo ether of a benzene derivative with a carboxy group in the 2-position. They also observed acid catalysis by added carboxylic acids, but, interestingly enough, the reactivity decreased as the strength of the added acid increased. 

FIG. 13 Alkylation of benzene with olefin under HF catalysis (mechanism). 

The commonly accepted mechanism for the catalytic reaction of lysozyme is the so-called general acid catalysis mechanism. 

FIGURE 7.2. Two alternative mechanisms for the catalytic reaction of serine proteases. Route a corresponds to the electrostatic catalysis mechanism while route b corresponds to the double proton transfer (or the charge relay mechanism), gs ts and ti denote ground state, transition state and tetrahedral intermediate, respectively. 

Another way to esterify a carboxylic acid is to treat it with an alcohol in the presence of a dehydrating agent. One of these is DCC, which is converted in the process to dicyclohexylurea (DHU). The mechanism has much in common with the nucleophilic catalysis mechanism the acid is converted to a compound with a better leaving group. However, the conversion is not by a tetrahedral mechanism (as it is in nucleophilic catalysis), since the C—O bond remains intact during this step ... 

In the pH range 2—3.5 the phosphonate (78) hydrolyses with loss of ROH at approximately 10 times the rate of comparable esters lacking the vicinal oxime function or in which this function is methylated on oxygen. An intramolecular general-acid catalysis mechanism was proposed, but it was not possible to exclude entirely an intramolecular nucleophilic attack at phosphorus. Intramolecular attack by the vicinal dimethylamino-group takes place preferentially at carbon rather than phosphorus in the phos-phonofluoridate (79). ... 

Generally, HNLs utilize an acid-base catalysis mechanism. The amino acid residues at active sites of these enzymes differ significantly, but share the common motif for cyanogenesis. 

Let us consider the basic enzyme catalysis mechanism described by the Michaelis-Menten equation (Eq. 2). It includes three elementary steps, namely, the reversible formation and breakdown of the ES complex (which does not mean that it is at equilibrium) and the decomposition of the ES complex into the product and the regenerated enzyme ... 

Bhaduri, S., and Mukesh, D. (2000). Homogeneous Catalysis Mechanisms and Industrial Applications. Wiley-Interscience, New York. 

The catalysis mechanism of horseradish PX and, in particular, the C isoenzyme, has been investigated extensively. However, although there is a wide range of PX isoenzymes, the following equation describes most of the reactions catalyzed by PX ... 

M. R Halpem (ed.), Phase-Transfer Catalysis Mechanism and Syntheses (ACS Symposium Series 659), American Chemical Society, Washington, DC, 1997. 

Enzymatic catalysis mechanisms may well be considerably more complex than the basic ping-pong mechanism, involving inhibition and hysteresis phenomena. Horseradish peroxidase offers a remarkable example... 

Conformational effects, on reactivity of cycloamyloses, 23 242, 245-249 Constant Ci, values for, 33 273, 274 Contact catalysis, mechanism of, 2 251 Contact catalysts, surface area measurements for studying, 1 65... 

The third and last part of the book (Chapters 12-16) deals with zeolite catalysis. Chapter 12 gives an overview of the various reactions which have been catalyzed by zeolites, serving to set the reader up for in-depth discussions on individual topics in Chapters 13-16. The main focus is on reactions of hydrocarbons catalyzed by zeolites, with some sections on oxidation catalysis. The literature review is drawn from both the patent and open literature and is presented primarily in table format. Brief notes about commonly used zeolites are provided prior to each table for each reaction type. Zeolite catalysis mechanisms are postulated in Chapter 13. The discussion includes the governing principles of performance parameters like adsorption, diffusion, acidity and how these parameters fundamentally influence zeolite catalysis. Brief descriptions of the elementary steps of hydrocarbon conversion over zeolites are also given. The intent is not to have an extensive review of the field of zeolite catalysis, but to select a sufficiently large subset of published literature through which key points can be made about reaction mechanisms and zeolitic requirements. 

So far 18 different members of HDACs have been discovered in humans and classified into four classes based on their homology to yeast histone deacetylases [33]. Class I includes four different subtypes (HDACl, 2, 3, 8), class II contains six subtypes tvhich are divided into two subclasses class Ila with subtypes HDAC4, 5, 7, 9 and class Ilb with HDAC6, 10. Class I and class II HDAC share significant structural homology, especially within the highly conserved catalytic domains. HDACs 6 and 10 are unique as they have two catalytic domains. HDACll is referred to as class IV. While the activity of class I, II and IV HDACs depends on a zinc based catalysis mechanism, the class III enzymes, also called sirtuins, require nicotinamide adenine dinucleotide as a cofactor for their catalysis. 

Phase-Transfer Catalysis Mechanisms and Syntheses, Ed. M. Halpem, American Chemical Society,... 

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