Transition states catalysis

The mechanistic significance of the terms in Eq. (27) for pseudobase decomposition must, of course, be the microscopic reverse of the interpretations given for the pseudobase formation reactions. Thus, k,[H+] is the microscopic reverse of the kHl0 term, and may be formally interpreted as either the spontaneous loss of a molecule of water from the O-protonated pseudobase (i.e., specific-acid catalysis transition state C) or alternatively as elimination of hydroxide ion from the neutral pseudobase molecule with the aid of H30+ as a general-acid catalyst (transition state D). The k2 term is the microscopic reverse of fc0H[OH ], and so formally represents either the spontaneous decomposition of the pseudobase to heterocyclic cation and hydroxide ion (transition state A) or the kinetically equivalent general-acid catalysis of this reaction by a water molecule (transition state B). 

In conclusion, enzymes use a variety of strategies to achieve high rates of catalysis. Transition state stabilization seems to be the dominant factor in catalysis, but in some enzymes, the more sophisticated strategies such as substrate destabilization and protein dynamics seem to play an important role. 

Turning to non-transition-metal catalysis, transition-state structures for the reduction of 2-methyl- and 2-isopropyl-cyclohexanone by LAH have been identified by DFT, and LUMO maps and NBO analysis have been used to examine the uneven distribution of the molecular orbital about the carbonyl r-plane, in order to explain the product ratio " substituent effects, the conformational ratio in the reactant, and... 

After some preliminary discussion of history and nomenclature, the review will briefly cover issues of rearrangement temperature, substituent effects and catalysis. Transition state stracture as determined by isotope effects combined with theoretical studies will then be described. Stereochemical aspects of the reaction will be examined in detail with accompanying examples. The remainder of the chapter will describe the various methods of ketene acetal formation, structural variety in the allyUc ester substrates and apphcations to natural product synthesis. The presentation of the material is somewhat arbitrary, and indeed most examples could well have been placed under more than one sub-heading. 

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

Section 16 13 Under conditions of acid catalysis nucleophiles attack the carbon that can better support a positive charge Carbocation character is developed m the transition state... 

B. S. Green, Y. Ashani, and D. Chipman, eds.. Chemical Approaches to Understanding En me Catalysis Biomimetic Chemistry and Transition State Analogs, Elsevier, Amsterdam, 1982. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

In this chapter we shall illustrate some fundamental aspects of enzyme catalysis using as an example the serine proteinases, a group of enzymes that hydrolyze peptide bonds in proteins. We also examine how the transition state is stabilized in this particular case. 

A structural anomaly in subtilisin has functional consequences Transition-state stabilization in subtilisin is dissected by protein engineering Catalysis occurs without a catalytic triad Substrate molecules provide catalytic groups in substrate-assisted catalysis Conclusion Selected readings... 

There is an intermediate mechanism between these extremes. This is a general acid catalysis in which the proton transfer and the C—O bond rupture occur as a concerted process. The concerted process need not be perfectly synchronous that is, proton transfer might be more complete at the transition state than C—O rupture, or vice versa. These ideas are represented in a three-dimensional energy diagram in Fig. 8.1. 

The transition state for the rapid hydrolysis of the monoanion has been 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... 

Each of the following molecules has been considered to be capable of some form of intramolecular catalysis of ester hydrolysis. For each reactant, indicate one or more mechanisms by which intramolecular catalysis might occur. Depict a transition-state arrangement that shows this catalysis. 

By using imidazole catalysis, it is possible to get a better understanding of the active forms that water takes in enzymatic processes Thus, at low concentrations m the presence of an enzyme, the water may not be fully hydrogen bonded and therefore more reactive [61] The rate of hydrolysis of p-nitrotrifluoroacetanilide in acetonitrile shows a strong dependence on water concentration at low levels in the presence of imidazole The imidazolium complex is the approximate transition state (equation 60)... 

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