Enzyme catalysis Diels—Alder reaction

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

Keywords enzyme mimics for catalysis of Diels-Alder reaction... 

In subsequent studies it has been found that a combination of Lewis-acid and micellar catalysis can lead to huge (in fact, enzyme like) rate acceleration in water. In the absence of Lewis-acid catalysts, micelles tend to inhibit Diels-Alder reactions, largely because of the particular nature of the substrate binding sites at the micelle. This problem can be solved by adding Lewis-acid catalysts that bind effectively at the micellar surface. 

Micellar catalysis, conducted in the absence of Lewis acid tends to inhibit the Diels-Alder reaction, relative to the reaction in water. The reason is that the local reaction medium in the Stern region is less favorable than bulk water. However, by combining Lewis-acid and micellar catalysis, enzyme-hke rate accelerations can be obtained (Table 7.5) in case the Lewis acid acts as the counterion for the miceUe. " ... 

Lewis-acid catalysis of Diels-Alder reactions involving bidentate dienophiles in water is possible also if the beneficial effect of water on the catalyzed reaction is reduced relative to pure water. There are no additional effects on endo-exo selectivity. As expected, catalysis by Cu ions is much more efficient than specific-acid catalysis.Using a-amino acids as chiral ligands, Lewis-acid enan-tioselectivity is enhanced in water compared to organic solvents. Micelles, in the absence of Lewis acids, are poor catalysts, but combining Lewis-acid catalysis and micellar catalysis leads to a rate accelaration that is enzyme-like. 

Enzymic catalysis of the Diels-Alder reaction in the biosynthesis of natural... 

Sangwan and Schneider have studied the effect of cyclodextrins on a number of aqueous Diels-Alder reactions between acrylate, fumarate and maleate derivatives of varying hydrophobicities and (mainly) cyclopenta-diene [26]. No simple correlation between substrate hydrophobicity and cyclodextrin-catalyzed rate enhancement was found. However, those systems that did respond to p-cyclodextrin catalysis exhibited enzyme-like saturation kinetics. This led these workers to conclude that the hydrophobic effect can, in fact, be counterproductive to the Diels-Alder reaction if it leads to unproductive orientation of the reactants. The same can be said about the effect of amphiphiles (detergents capable of micellization) on aqueous Diels-Alder reactions since sodium dodecylsulfate (SDS) decelerated the reaction between cyclopentadiene and methyl acrylate. Those cases in the literature claiming micellar catalysis of the aqueous Diels-Alder reaction may simply be benefiting from the solubilizing effect of the amphiphilic additives rather than any bona fide preorganization of the reactants within a micelle [27,28]. 

Abstract In the first part of this mini review a variety of efficient asymmetric catalysis using heterobime-tallic complexes is discussed. Since these complexes function at the same time as both a Lewis acid and a Bronsted base, similar to enzymes, they make possible many catalytic asymmetric reactions such as nitroal-dol, aldol, Michael, Michael-aldol, hydrophosphonyla-tion, hydrophosphination, protonation, epoxide opening, Diels-Alder and epoxi-dation reaction of a, 3-unsaturated ketones. In the second part catalytic asymmetric reactions such as cya-nosilylations of aldehydes... 

Thus, room-temperature ionic liquids have the potential to provide environmentally friendly solvents for the chemical and pharmaceutical industries. The ionic liquid environment is very different from normal polar and nonpolar organic solvents both the thermodynamics and the kinetics of chemical reactions are different, and so the outcome of a reaction may also be different. Organic reactions that have been successfully studied in ionic liquids include Friedel-Crafts, Diels-Alder,Heck catalysis, chlorination, enzyme catalysis,polymeriz-... 

Another important research direction is the mimieking of enzymes and the construction of selective catalysts. For these purposes, the polymer is imprinted with the desired reaetion-product or better, a molecule which resembles the transition state of the reaction adducts. If the educts bind specifically to the recognition site, they become confined into these micro-reactors and are supposed to react faster and more defined than outside the cavities [445]. Examples for reactions in the presence of such synthetic enzymes can be found in [452,453,454,455,456,457] (cf Figure 40c). First positive results have been reported, e.g. an synthetic esterase , increasing the rate of alkaline hydrolysis of substituted phenyl-(2-(4-carboxy-phenyl)-acetic esters for 80 times [488] and Diels-Alder catalysis fiuic-tional holes containing titanium lewis-acids [489]... 

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