Template effect catalysis

Templating effects are important in the context of supramolecular catalysis where, by analogy with enzymatic processes, the reactants are organized by the template so that the formation of a particular linking of atoms is accelerated. An early example of this approach reported by Kelly is shown in Scheme 41 [104]. 

One of the most spectacular and useful template reactions is the Curtis reaction , in which a new chelate ring is formed as the result of an aldol condensation between a methylene ketone or inline and an imine salt. The initial example of this reaction was the formation of a macrocyclic nickel(II) complex from tris(l,2-diaminoethane)nickel(II) perchlorate and acetone (equation 53).182 The reaction has been developed by Curtis and numerous other workers and has been reviewed.183 In mechanistic terms there is some circumstantial evidence to suggest that the nucleophile is an uncoordinated aoetonyl carbanion which adds to a coordinated imine to yield a coordinated amino ketone (equation 54). If such a mechanism operates then the template effect is largely, if not wholly, thermodynamic in nature, as described for imine formation. Such a view is supported by the fact that the free macrocycle salts can be produced by acid catalysis alone. However, this fact does not... 

Inoue Y, Kojima 1, Moriki S, Yasumori 1 (1976) Template effects in paUadiiun catalysis. In Proceedings of the sixth international congress on catalysis, vol 1, p 139 Abdelrehim IM, Caldwell TE, Land DP (1996) Coverage effects on the kinetics of benzene formation from acetylene on Pd(lll) A laser-induced thermal desorption Fourier transform mass spectrometry investigation. J Phys Chem 100 10265... 

We proceeded, through overlapping control experiments [29, 30a, 30b], to exclude each individual function of the product molecule as a source of simple chemical catalysis. The control additives are shown in Figure 15 and the results are summarized in Table 1. In CHCI3 at 2.2 mM concentrations of (4) and (5), a 50% increase in initial rate was observed when the reaction was seeded with half an equivalent of product (6) (Table 1, entry 2). Our control experiments, detailed below, show that this 50% increase is the result of replication, i.e. a template effect as shown in Figure 3, complex (7). 

Polymerised preformed [(N,N -dimethyl-l,2-diphenylethane diamine)2Rh] complex allows us to obtain enantioselective material. We have then shown that it is possible to imprint an optically pure template into the rhodium-organic matrix and to use the heterogeneous catalyst in asymmetric catalysis with an obvious template effect. The study of yield versus conversion graphs has shown that the mechanism occurs via two parallel reactions on the same site without any inter-conversion of the final products. Adjusting the cross-linker ratio at 50/50 allows us to find a compromise between activity and selectivity. Phenyl ethyl ketone (propiophenone) was reduced quantitatively in 2 days to (R)-l-phenyl propanol with 7tf% enantiomeric excess We have then shown that the imprinting effect is obvious for molecules related in structure to the template (propiophenone, 4 -trifluoromethyl acetophenone). It is not efficient if the structure of the substrate is too different to that of the template. 

Figure 2 Common roles for metal Ions In catalysis, (a) Lewis acid, (b) electrostatic catalyst, and (c) template effect.

Remote epoxidation. Breslow and Maresca have found that Sharpless epoxidation of olefins bearing hydroxyl groups (5, 76 77) is amenable to direction by a template (for an example of this method of control see 5, 352-353). Thus the steroid ester 1 can be epoxidized in high yield by t-butyl hydroperoxide with catalysis by Mo(CO)e. In contrast the ester 2 under the same conditions is recovered unchanged. Another striking example of the template effect is that only the 17,20-double bond of the ester 3 can be epoxidized by the Sharplcss method. Ordinarily A - stenols are epoxidized readily. In these examples both the template and the steroid are rigid molecules. 

Molecular Clefts and Tweezers, p. 887 Organometallic Anion Receptors, p. 1006 Phase-Transfer Catalysis in Environmentally Benign Reaction Media, p. 1042 Siderophores, p. 1278 Swfactants, Part 1 Fundamentals, p. 1458 Surfactants, Part 11 Applications, p. 1470 The Template Effect, p. 1493... 

The role of the transition metal in these reactions can be rationaUzed by the classical concepts of coordination catalysis template effects, activation or, in some cases, stabilization of labile intermediates. Electron transfer processes and the participation of polynuclear complexes are also involved in some particular reactions. 

The benefit of the LbL technique is that the properties of the assemblies, such as thickness, composition, and function, can be tuned by varying the layer number, the species deposited, and the assembly conditions. Further, this technique can be readily transferred from planar substrates (e.g., silicon and quartz slides) [53,54] to three-dimensional substrates with various morphologies and structures, such as colloids [55] and biological cells [56]. Application of the LbL technique to colloids provides a simple and effective method to prepare core-shell particles, and hollow capsules, after removal of the sacrificial core template particles. The properties of the capsules prepared by the LbL procedure, such as diameter, shell thickness and permeability, can be readily adjusted through selection of the core size, the layer number, and the nature of the species deposited [57]. Such capsules are ideal candidates for applications in the areas of drug delivery, sensing, and catalysis [48-51,57]. 

Self-assembly processes in nature are sometimes catalyzed by enzymes. Zeolites are, in many ways, the inorganic counterparts of enzymes, with their ability to selectively bind other substances and perform catalysis. Can templates or catalysts be effective in increasing rates and reducing defects in a wide range of nanostructured materials ... 

Comparable effects were observed in the catalysis of the dehydrofluorination of 4-fluoro-4-(p-nitrophenyl)-2-butanone using Mi-polymers or even Ml-pro-teins (see later) imprinted with suitable TSA templates. Focusing on the two sp3 hybridized carbons (C2 and C3) of the butanone which are converted to sp2 carbons as a result of dehydrofluorination, the search for the right TSA led to N-benzyl-AT-isopropylamine (Fig. 13) either containing a nitro-group [62] at the aromatic ring or not [63,64]. 

B. Auten, H. Lang, and B. D. Chandler, Support effects on dendrimer templated Pt—Au catalysts II CO oxidation catalysis, manuscript in preparation (2004). 

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