Cyclodextrin catalytic properties

Numerous examples of modiflcations to the fundamental cyclodextrin structure have appeared in the literature.The aim of much of this work has been to improve the catalytic properties of the cyclodextrins, and thus to develop so-called artificial enzymes. Cyclodextrins themselves have long been known to be capable of catalyzing such reactions as ester hydrolysis by interaction of the guest with the secondary hydroxyl groups around the rim of the cyclodextrin cavity. The replacement, by synthetic methods, of the hydroxyl groups with other functional groups has been shown, however, to improve remarkably the number of reactions capable of catalysis by the cyclodextrins. For example, Breslow and CO workersreported the attachment of the pyridoxamine-pyridoxal coenzyme group to beta cyclodextrin, and thus found a two hundred-fold acceleration of the conversion of indolepyruvic acid into tryptophan. 

Kaifer et al. were particularly interested in the catalytic properties of Pd nanoparticles derivatized with surface-attached perthiolated cyclodextrins and their use in various catalytic reactions such as Suzuki reactions [57] or the hydrogenation of alkenes [58] or allylamine [59]. The modified cyclodextrins play the role of a ligand, leading to a steric stabilization. To our knowledge, only one report describes the catalytic hydrogenation of olefins using colloidal Rh dispersions embedded by nahve cyclodextrins [60], generating steric stabilization via hydrophobic interactions. 

The catalytic properties of the sulfonated diphosphine-stabilized RuNPs and sulfonated diphosphine/cyclodextrin-stabilized RuNPs were compared in the hydrogenation of unsaturated model substrates (styrene, acetophenone, and w-methylanisole) in biphasic liquid-hquid conditions (i.e., ruthenium aqueous colloidal solution and organic substrate no added solvent). Whilst all of these RuNPs displayed suitable performances in catalysis, different activities and selec-tivities were observed. This highhghted that supramolecular interactions on the metallic surface in the presence of a cyclodextrin control the catalytic reactivity of the nanocatalysts. Interestingly the CD acts as a phase-transfer promotor, which... 

Asghari and collaborators demonstrated the catalytic properties of sulfonated /J-cyclodextrin (CAT-34, Table 6). The catalyst works better under solvent-free conditions, and the Biginelli adducts were obtained in good yields (>71%) under these conditions [45]. Zhou and collaborators showed that /3-cyclodextrin, in combination with FlCl (CAT-35), provided the Biginelli adduct in 48% yield after 12 h (Table 6) [46]. Aliphatic aldehydes were good substrates, and excellent yields (>84%) were observed when aromatic aldehydes were employed [46]. 

An important feature of the cyclodextrins is that they can also accelerate chemical reactions, and therefore serve as models for the catalytic as well as the binding properties of enzymes. The rapid reaction is not catalysis, since the dextrin enters reaction but is not regenerated presumably it arises from approximation, where complex formation forces the substrate and the cyclodextrin into intimate contact. In particular, cyclodextrins can increase the rate of cleavage of phenyl pyrophosphate by factors of as much as 100 (Cramer, 1961). More recent work has improved upon this early example. 

C02Me) in the presence of 2,4,6-triphenylpyrylium tetrafluoroborate as sensitizer gives cyclopropenes together with 2H-pyrroles which arise by solvent addition to the 1,3-radical cation intermediate. Phenylmercaptotetrazole has been photo-catalytically oxidised on aerated Ti02 dispersions (Xjn- >330 nm) and a mechanism suggested which involves the formation of CO2, S04, NOa, and NH4. Photooxidation of phenothiazine in either benzene or cyclohexane in the presence of molecular oxygen has been shown by EPR experiments to give the phenothiazine nitroxyl radical rather than the radical cation. These observations are supported by the results of AMI calculations. The photooxidation of Azure A (81) as well as its fluorescence properties have been studied in the presence of P-cyclodextrin, and this has enabled an induced fluorimetric method to be developed for the determination of (81). ... 

The cyclodextrins were the first compounds that have been studied with regard to their complexing and catalytic — in other words enzyme mimicking — properties. The enzymatic catabolism of starch mainly yields a-, p-, and y-cyclodextrin (CyD, 1-3). Those are macrocyclic oligosaccharides, in which six (a), seven (P) and eight (y) a-D-glucopyranose units, respectively, are connected by 1,4-glycosidic bonds. 

Solubilization of water-insoluble substrate in the aqueous phase containing the organometalhc catalyst can be achieved by using catalytic amounts of water-soluble receptors such as cyclodextrins [1] or calixarenes [2], The beneficial effect of these water-soluble host compounds on the mass transfer is ascribed to their complexing properties and it is postulated that these compounds operate like inverse phase-transfer catalysts according to Figure 1. 

Cyclodextrins are cyclic glucose oligomers consisting of six or more monomer units. The cyclodextrin structure is such that the hydrogens of C-H bonds are directed towards the inside of the cavity of the molecule and hydroxyl groups are directed towards the outside (Fig. 11-7). Therefore, the molecule has a hydrophobic cavity, which owing to its hydrophobicity can bind nonpolar molecules into host-guest complexes and transfer them into the polar phase. This property of cyclodextrins allows us to use them as components of catalytic systems in two-phase catalysis by metal complexes. [20-23, 182-196] this essentially increased the activity of these catalytic systems. 

Cyclodextrins have had valuable industrial uses for a considerable time, particularly as agents to bind or release volatile molecules. Accurate predictions concerning the selectivity and stability of cyclodextrin-guest complexes are therefore of considerable interest both academically and practically." MD was used to simulate cyclodextrin hydrates" as a test of the applicability of the GROMOS program package to systems beyond proteins and nucleic acids. Other early MD simulations focused on interactions with guests such as enantiomers of methyl-2-chloropropionate. Comparisons between calculated thermodynamic properties for complexes formed by O -cyclodextrin with para-substituted phenols and the results of MM simulations led to improvements in force fields that described the interactions. MM2 simulations were used to support NMR data for the -cyclodextrin inclusion complex with benzoic acid. " The well-known catalytic effect of cyclodextrins has been modeled. For example, the relative rate increase of hydrolysis of S over R phenyl ester stereoisomers in the presence of -cyclodextrin... 

The cyclodextrin cage holds the ester while the metal ion positions other groups for attack. If cyclohexanol is added in the solution, it competes with the substrate and the efficiency of the system falls to 60%. Most of the catalytic power of the system is attributed to the binding of the substrate by the cyclodextrin moiety of the complex. In the absence of cyclodextrin, the rate enhancement is 350-fold. Thus, by combining the properties of cyclodextrin with those of a metal ion, a much more efficient catalytic system can be obtained that mimics enzyme features. 

---