Calixarenes, catalysts based

Calixarene 17 is one of such water-soluble calixarene hgands. The main advantages of catalysts based on such calixarenes include the substantial increase in activity compared with other catalysts and high selectivity of the processes (Scheme 4.11) [41,62]. 

Calixarene Catalysts Containing Bronsted Acid/Base Moieties... 

It has already been mentioned that metal complexes with confined binding pockets often display unusual chemical reactivities (see Section II). Thus, complexes of substituted hydrotris (pyrazolyl)borates, in which the substituents serve to from a hydrophobic binding pocket, have already been shown to exhibit enhanced chemical reactivity when compared with their unmodified analogs (282,283). Likewise, cyclodextrin and calixarene-based metallocavitands have been used as catalysts for selective organic transformations, and even as catalysts for reactions that... 

The parent / -f-butylcalix[n] arenes (n = 4, 6, 8) are almost completely insoluble in water. However, their resemblance to crown ethers and spherands makes them interesting from the point of view of applications as phase transfer catalysts (Section 3.8.2). Table 3.20 shows the selectivity of calixarene 3.118 and its hexameric and octameric homologues for the extraction of various metal hydroxides into an organic receiving phase such as chloroform. Fortunately, in aqueous base the calixarenes are sufficiently soluble to act as phase transfer catalysts as a consequence of deprotonation of one of their phenolic hydroxyl groups. This solubility contrasts to [18] crown-6, which is much more effective in neutral solution. 

What could be further developments Enantioselective catalysts may be named as one important field. The construction of calixarene-based, eventually biomimetic, catalysts is still in its infancy.291 The simultaneous attachment of catatytically active groups in combination with chiral functions or with the inherent chirality of the calixarene skeleton itself opens numerous possibilities that are far from being exhausted. Even the combination of achiral calixarenes with chiral catalysts may lead to useful results.292... 

The calix[4]arene-based diphosphine (119) (R = Me, Pb, Ph R = Pb) on reaction with [Rh(/r-Cl)(COD)]2 and then TlPFg forms the dinuclear species (120). These were tested as catalysts for hex-l-ene hydroformylation (see Carbonylation Processes by Homogeneous Catalysis) and that with R = Ph was best, but the performance was not better than extant systems and no special role for the calixarene moieties could be discerned. Several papers have used DFT calculations as part of QM/MM studies (see Molecular Orbital Theory) to probe the course of the hydroformylation process (see Carbonylation Processes... 

In addition to calixarenes which have been made water-soluble as new inverse phase-transfer catalysts [85], novel ligands have been designed, based on a cydo-propene moiety bearing pyridyl substituents and called [hexakis(2- or 3-pyridyl) 3 -radialene] 37. A metallosupramolecular chemistry is envisioned by the authors and the solubility in water and their stability offer innovative perspectives [86]. 

The goal of designing calixarene-based catalysts has its roots at least as far back as the early 1970s when this book s author entered the field. In the 1980s a few examples of calixarene-catalyzed processes appeared, and a few more have been added in recent years. However, this remains the least well developed facet of calixarene chemistry and continues to pose an interesting intellectual and synthetic challenge to organic chemists. 

Recently, Sirit and co-workers [45] developed calixarene-based chiral phase-transfer catalysts derived from cinchona alkaloids successfully used for alkylation of glycine-imine esters. In 2010, Itsuno et al. [46] published quartemary ammonium sulfonate polymers used for a-alkylation reaction of a glycine imine ester with high yields and enantioselectivity. 

Abstract. Several facets of calixarene chemistry have been investigated including the mechanism of their formation by the base-induced condensation of phenols and formaldehyde, procedures for introducing functional groups onto the upper rim and lower rim of calixarenes, the conformational behavior of calixarene oxyanions, the formation of complexes in aqueous and nonaqueous systems, and the application of calixarenes as catalysts. 

Systems Based on Calixarenes as the Basis for the Creation of Catalysts and Nanocontainers... 

Construction of supramolecular associates is an important and promising trend in the field of catalysis. Catalytic systems based on calixarenes exhibit high selectivity toward most organic reactions. The activity of these catalysts depends on the conformity of the size of the host molecule cavity and that of the guest molecule so that the substrate selectivity is observed. The size of these cavities and consequently the selective conversion of certain substrates can be adjusted by the receptor composition. 

Karakhanov EA, Buchneva TS, Maksimov AL, Runova EA. Calixarene-based catalyst systems in biphasic Wacker oxidation of oleflns. Neftekhimiya 2003 43(l) 42-9. 

In the same context, the catalytic activity of novel calixarene-based chiral phase-transfer catalysts derived from cinchona alkaloids was evaluated by Sirit et al. by carrying out the phase-transfer benzylation of A -diphenylmethylene glycine ethyl ester with benzyl bromide. Although a good yield (89%) was... 

Reinhoudt et al. [2] developed one of the first examples of efficient calixarene-based artificial nuclease and introduced the concept of dynamic preorganization to describe the capability of calixarenes to secure a good compromise between a suitable preorganization of a set of catalytic and/or recognition moieties in reciprocal close proximity and a residual flexibility in the molecular scaffold that greatly helps in the dynamic fit of the catalyst to the bound substrate through the reaction path. 

Up to now, a great deal of attention on catalytic applications of calixarenes has been paid to achieve a promising catalyst which efficiently catalyzes the specific reactions in a faster and an easy way. It has been reported that some of calixarene-based organocatalysts exhibited efficient catalytic properties for some organic reactions [5-7]. 

Another calixarene-based phase-transfer catalyst was designed by Srivastava and Srivastava [20]. In that study, two different quaternary ammonium salts of calix [4]arene (Calix[4]arene 2a and 2b) were employed as a catalyst in the Darzens condensation to synthesize a,p-epoxy carbonyl compounds (see Fig. 27.5). It was found that these phase-transfer catalysts exhibited significant catalytic activity, especially when the calix[4]arene with long aUcyl chain ( -butyl) was chosen (see Table 27.3). 

Table 27.6 Biginelli reaction in the presence of calixarene-based organo catalyst [1]...

Investigations on calixarene-based pseudo-biomimetic catalyst has received considerable attention because calixarenes can be functionalized on this purpose, and particularly they are nonpeptidic molecules that mimic structural and substitution aspect of the biomolecules [35, 36]. In principle, calixarene-based pseudo-biocatalysts... 

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