Template catalytic systems

Chapter 3 presents data about enantioselective hydrogenation reactions on metal catalysts supported on chiral carriers. Discussed are palladium supported on modified silica gels, effective chiral colloidal catalysts, template catalytic systems, palladium-silk, palladium-wool, and palladium polypeptide catalysts. 

Native enzymes, which can spatially and chemically recognize substrate molecules, are powerful catalytic systems in many biochemical processes under mild reaction conditions. The preparation of artificial enzymatic catalysts with the capability of molecular recognition capability, by a molecular-imprinting method, which creates cavities with a similar shape and size to the template molecule in polymer matrices has been developed [1-14]. The technique has been mainly established in the field of analytical chemistry - molecular receptors [15-23], chromatographic separations [24-28], fine chemical sensing [29-33]. All of the methods rely on the selective adsorption of target molecules on imprinted adsorption sites. The number of papers reported per year on molecular imprinting is summarized in Fig. 22.1. 

The catalytic macrocyclization of thietanes [65] appears to be described particularly conclusively, since (1) it results from stoichiometric preliminary studies of reactivity on clusters (2) the activation of the first building block occurs through the coordination of the sulfur atom to two metal centers (3) the reaction is slow enough to intercept most of the intermediates and to characterize them, including X-ray crystal structure determination (4) the selectivity of the reaction originates from the cluster template, whose nuclearity seems to be maintained along the catalytic cycle and (5) sulfur is particularly well adapted to the catalytic system. 

Esters can be cleaved by template catalysts that use a metal ion as both a binding group and part of the catalytic system [79-81]. However, metal ion catalysis has also been extended to cases in which the principal substrate binding involves cyclodextrin inclusion indeed, the first catalyst described as an artificial enzyme was. such an example [82]. A cyclodextrin dimer 47 with a bound metal ion between the two cyclodextrins is a particularly effective hydrolytic catalyst for esters that can bind into both cyclodextrin units (Scheme 6-20) [83, 84]. 

This chapter will concentrate on examples of templated synthesis where the template takes the form of a temporary, covalent tether. The intermediate molecule containing both reacting species is in all cases isolable (or potentially so). This distinguishes this intra molecularization approach from substrate-directed stereoselective synthesis. In this case, the template, most frequently a metal, is used in a much more transient sense allowing the potential for developing catalytic systems, which is obviously not possible using a covalently bound template. 

The first example of DNA-templated metal catalysis was reported by the gronp of Kramer [136]. hi this approach, metal-catalyzed hydrolysis of an ester attached to a DNA template was performed. The catalytic system consisted of three parts a DNA template strand, a ligand for Ctf attached to a PNA strand complementary to half of the template, and an ester substrate connected to a PNA strand complementary to the other half of the DNA template (Fig. 10). Complexation of both PNA strands to the template brings the catalytic center and the substrate in close proximity, and hydrolysis of the ester group is accelerated approximately 150-fold relative to the background hydrolysis rate, which makes it suitable as a detection method for DNA sequences [137]. 

Functionalisation of these templates with phosphine ligands, followed by complexation with Co or Pd precursors, led to the demonstration of remarkable dendritic effects on the activity and selectivity of the catalytic systems in the Pauson-Khand [63] and Heck reactions (Scheme 5) [64]. The advantage of the weakly coordinating polyether dendritic backbone, as compared with the coordinating dendritic backbones (e.g. polyamide), was demonstrated for the Heck reaction [65]. 

More recently imidazoiium ILs have been used as a template for the synthesis of a plethora of stable transition-metal NPs with small size and narrow size distributions [23, 24]. These particles immobilized in ILs constitute highly active multiphase catalytic systems for various reactions. The main goal of this chapter is to disclose the mechanistic and structural aspects of the formation and stabilization of transition-metal NPs in imidazoiium ILs, especially those that have been used in catalytic reactions. 

Inorganic systems are much less sophisticated than the immunoresponse system, but similar in that, in principle, information on the transition state through the correct choice of template can be incorporated into the catalytic system such as in the case of zeolite synthesis. The mechanism of the zeofite synthesis reaction, discussed in the next, section has combinatorial evolutionary characteristics. 

More interestingly, anil 17 was found to cross-catalyze the condensation of amine 15 and aldehyde 13 to form 16 with a first-order relationship hence, addition of preformed template of 17 led to a proportional increase in the formation of 16. Crucial for such behavior are different stabilities of the ternary and the product duplex. In case of a stable duplex, strong association leads to a square root dependence of rate with respect to the total template concentration. However, if the product duplex proves less stable than the ternary complex of building blocks and template, the system can reach its full catalytic potential. 

Another example of diastereoisomeric heredity in a replicating system using simple organic molecules was demonstrated within a multicyclic set up for a cross-catalytic system in which two complementary templates were found to stereospecifically catalyze the formation of each other (Figure 23). Combining the chemistry based on the Diels-Alder reaction between furan and... 

An asymmetric a-alkylation of 2-oxindoles has been developed (Scheme 40). The chemistry utilizes a catalytic system with a Brpnsted acid and bis-cinchona alkaloid template (i.e., 191). Thus, the 2-oxindole (188) is alkylated by Michler s hydrol (189) to give the 2-oxindole product (190) in good yield and modest enantioselectivity. 

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