Supramolecular catalyst complexes

In this work we present hyperbranched polymers as platforms for catalysts that fall into three major classes, according to their topology and binding mode to the polymeric support (Fig. 2) (i) defined multiple site catalysts (ii) dendritic core-shell catalysts (iii) supramolecular catalyst complexes. 

The FePcY-PDMS supramolecular catalyst resembles the architecture of natural enzymes. In this system the PDMS membrane takes over the role of the phospholipid double layer likewise, the zeolite imitates the protein and the FePc complex the Fe-protoporphyrin. Zeolite-encaged Cu-histidine complexes were also studied as mimics of natural Cu-enzyme complexes.173... 

Figure 1.14 Generation of supramolecular catalysts for asymmetric hydrogenation (a) Assembly of heterodimeric chelating ligands, (b) Structure of the optimal rhodium-diphosphonite complex for asymmetric hydrogenation (other ligands from the metal center omitted for clarity), (c) Enantioselective hydrogenation of functionalized alkenes.

This chapter describes the various ways that the rates of acyl transfer reactions can be enhanced in supramolecular complexes or by supramolecular catalysts in which the crucial ingredients are such simple and relatively featureless chemical species as alkaline-earth metal ions, mainly Sr and Bi ions, and occasionally Ci . ... 

Earlier work in this field has been thoroughly reviewed [1,2]. However, to illustrate in a sensible and logical way the evolution from simple metal ion promotion of acyl transfer in supramolecular complexes to supramolecular catalysts capable of turnover catalysis, an account of earlier work is appropriate. The following sections present a brief overview of our earlier observations related to the influence of alkaline-earth metal ions and their complexes with crown ethers on the alcoholysis of esters and of activated amides under basic conditions. 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

The systems described in this chapter possess properties that define supramolecular reactivity and catalysis substrate recognition, reaction within the supermolecule, rate acceleration, inhibition by competitively bound species, structural and chiral selectivity, and catalytic turnover. Many other types of processes may be imagined. In particular, the transacylation reactions mentioned above operate on activated esters as substrates, but the hydrolysis of unactivated esters and especially of amides under biological conditions, presents a challenge [5.77] that chemistry has met in enzymes but not yet in abiotic supramolecular catalysts. However, metal complexes have been found to activate markedly amide hydrolysis [5.48, 5.58a]. Of great interest is the development of supramolecular catalysts performing synthetic... 

FIGURE 22 Preparation of supramolecular catalysts for hydrocyanation reactions (82) (A) assembly of heterodimeric chelating ligands (B) structure of the optimal nickel-diphosphine complex for hydrocyanation (other ligands of the metal center are omitted for clarity) and (C) hydrocyanation of functionalized styrenes. (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

FIGURE 24 Generation of supramolecular catalysts for allylic amination (89) (A) assembly of pseudoenantiomeric bisoxazoline derivatives. CAChe-minimized model of the palladium-containing supramolecular complex and (B) palladium-mediated enan-tioselective allylic amination. (See Color Insert in the back of this book.)... 

Roelfes, Feringa, and Kraemer (104,105) developed a similar hybrid supramolecular catalyst, which they named DNAzyme, relating to DNA as the biomacromolecular chiral host. 9-Aminoacridine-modified Cu(II) complexes were employed as metal fragments with high DNA affinities (Figure 29). The acridine unit is likely to intercalate into the double helix of the DNA. The Cu(II) catalytic center is thus brought into close proximity to the DNA, which creates a chiral environment around the Cu(II) site. Stereoinduction can be expected in any transformation mediated by this Cu(II) center. 

Ballester, Vidal-Ferran, and van Leeuwen evaluate concepts and strategies in the field of supramolecular catalysis. The authors describe what characterizes supramolecular catalysts, formulating a definition on the basis of the nature of interactions between catalyst and substrate or between building blocks of the catalyst. Examples are cited that demonstrate how supramolecular catalysts are superior to simple molecular catalysts in a broad range of reactions. Ballester et al. consider supramolecular catalysts as enzyme models, guided in their comparisons by the various mechanisms by which enzymes accelerate chemical transformations such as the binding of a reactant next to the catalytic site, the simultaneous complexation of two reactants, or desolvation. Addressing the synthesis of supramolecular catalysts, the authors describe how... 

While multivalency, self-assembly, and template effects provide strategies aiming at generating more and more complex architectures, supramolecular chemistry can also be utilized for controlling reactivity and even catalyzing reactions. Closely related to organocatalysis, supramolecular catalysts [39] accelerate reactions by... 

Fig. 18 Example of a supramolecular catalyst by Reek and PM3 optimized molecular structure of an active Pd-aUyl complex of the catalyst. Reprinted from [45], Copyright 2005, with permission from Elsevier...

Despite their limited set of functional groups, ribozymes can accelerate complex organic transformations like Diels-Alder reactions between small molecules in a way similar to protein enzymes or supramolecular catalysts featuring multiple turnover, substrate specificity and stereoselectivity. The three-dimensional sbucture shows striking similarities with proteins evolved for similar reactions, and the catalytic strategies used appear to be similar as well. 

The photocatalytic reduction of CO2 to CO using supramolecular organometallic complexes has been described. In particular, irradiation of [(dmb)2Ru"(bpyC3bpy)Re (CO)3Cl] (dmb =4,4 -dimethyl-2,2 -bipyridine) in DMF/ TEOA solution (DMF = N,N-dimethylformamide TEOA = triethanol amine) containing a sacrificial electron source gave CO. The mechanism (Scheme 16) was proposed to involve initial excitation to an MLCT excited state of the Re moiety. Electron transfer then occurs from the reduced bpy ligand bonded to the Re center to the bpy ligand bonded to the Re center. The latter species is the catalyst for the CO2 reduction reaction. 

These. supramolecular catalysts showed high substrate selectivity in competition hydrogenation experiments and exceptional activity in the hydroformylation reactions. In contrast to the simple methylated P-cyclodextrin previously mentioned, even internal and cyclic olefins were converted into aldehydes. Such improvements were explained with the formation of an inclusion complex at the phase boundary, with the cylodextrin host fixing the substrate in the proximity of the catalytically active metal center (Fig. 

The structured binding sites of metalloclefts mean that they are often highly selective hosts. An example is the salophen-uranyl-based metallocleft 3, which complexes neutral molecules, provided that the two phenyl groups of the host are parallel to one another. The salophen-uranyl unit may be employed as a catalyst for Michael additions, and the high steric requirements of the metallocleft of 3 result in it acting as a highly selective supramolecular catalyst. Metalloclefts of Type 4 selectively bind SO2 molecules and may be incorporated into dendritic materials that show catalytic activity. ... 

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