Catalysts supramolecular

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... 

Fig. 10. Structure of the supramolecular catalyst generated in situ from 24 and TBHP.

The advancements in supramolecular catalysis are not limited to transitions-metal catalyzed reactions. Clarke and coworkers recently reported the preparation of a library of organocatalysts and their application in the asymmetric Michael addition of ketones to nitroalkenes [37]. They proposed use of a supramolecular catalyst formed... 

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.

Figure 1.15 Supramolecular catalysts for organocatalysis (a) Route to catalyst libraries (b) asymmetric nitro-Michael reaction.

Supramolecular control of reactivity and catalysis is among the most important functions in supramolecular chemistry. Since catalysis arises from a differential binding between transition and reactant states, a supramolecular catalyst is, in essence, chemical machinery in which a fraction of the available binding energy arising from noncovalent interactions is utilized for specific stabilization of the transition state or, in other words, is transformed into catalysis. 

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. 

Figure 6.2 (a) Structure of rhodium host 3. (b) Catalytic isomerization of allylbenzene derivatives carried out by supramolecular catalyst 3 (solid lines) or by reference catalyst HRh (CO)[P(OPh3)]3 (dashed lines), depending on the R-group ofthe substrate (< and 4-allylcatechol and allylbenzene). 

New Supramolecular Approaches in Transition Metal Catalysis Template-Ligand Assisted Catalyst Encapsulation, Self-Assembled Ligands and Supramolecular Catalyst Immobilization... 

In the last decade several new approaches have been introduced which avoid the use of elaborate syntheses. Both cavities and ligands are prepared via assembly processes which speed up the process enormously and assembly also leads to a large number of catalyst systems where only a limited number of building blocks have to be synthesized. In just a few years time this has led to an outburst of supramolecular catalysts, of astounding beauty, with unprecedented selectivities, or with high practicality. Of the latter group a few hold even promise for industrial application. 

A number of studies have made use of functionalized cyclophanes for developing supramolecular catalysts and enzyme models [4.31-4.34, 5.37, 5.38]. Their catalytic behaviour is based on the implementation of electrostatic, hydrophobic and metal coordination features for effecting various reactions in aqueous media. 

The design of supramolecular catalysts may make use of biological materials and processes for tailoring appropriate recognition sites and achieving high rates and selectivities of reactions. Modified enzymes obtained by chemical mutation [5.70] or by protein engineering [5.71] represent biochemical approaches to artificial catalysts. 

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... 

Supramolecular catalysts are by nature abiotic chemical reagents that may perform the same overall processes as enzymes, without following the detailed pathway by which the enzymes actually effect them or under conditions in which enzymes do not operate. Furthermore and most significantly, this chemistry may develop systems realizing processes that enzymes do not perform while displaying comparable high efficiencies and selectivities. 

In addition to the mimicking of natural enzymes, a fascinating goal in supramolecular chemistry is the development of entirely artificial catalysts. While both heterogeneous and homogeneous catalysts based in particular on coordination of ligands to transition metals are common, supramolecular catalysts use either exclusively, or in addition, some aspect of molecular recognition and non-covalent... 

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. 

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