Calixarenes catalytic systems

C. David Gutsche is retired from the position of Robert A. Welch Professor at Texas Christian University. He is a pioneer in the use of calixarenes in systems designed to mimic enzymes. He has prepared numerous calixarene-type molecules, evaluated their host-guest binding abilities, and studied their catalytic potentials [48,49],... 

Although promiscuous in their choice of guest-compounds, the only well-defined catalytic system based upon the calixarenes, which has been documented, is the one described by Shinkai [61]. He has studied the acid-catalyzed addition of water (Scheme 6) to lV-benzyl-l,4-dihydronicotinamide in the presence of a para-sulfonatocalix[6]arene and its methoxycarbonyl derivative. The polyanionic nature of the upper rim, containing the sulfonate groups, provides a site for the stabilization of the cationic intermediate. The lower rim, which carries the phenolic... 

Scheme 6. In spite of being good host molecules, there is only one well-documented [61] catalytic system based upon the calixarenes. The acid-catalyzed addition of water to yV-benzyl-l,4-dihydronicotinamide 22 to give 24 via 23 is accelerated by the calix[6]arene derivative 25...

R. Cacciapaglia, L. Mandolini, Calixarene based catalytic systems, in Calixarenes in Action, L. Mandolini, R. Ungaro (Eds.), Imperial College Press, London, 2000, pp. 241-264. 

A phosphite-modified calixarene with unsubstituted hydroxyl groups was used as a ligand in 1-hexene hydroformylation catalyzed by rhodium complexes [224], The reaction was carried out at a synthesis gas pressure of 6.0 MPa and 160 °C. Rh(acac)(CO)2 was a catalyst precursor. In 3 h, the conversion of the initial alkene virtually reached its theoretically predicted value the yield of aldehydes was 80-85%, and the normal-to-isomeric aldehyde ratio was approximately 1 1. Some similar phosphites 83 were also studied as components of catalytic systems for 1-octene hydroformylation [225]. It was shown that the nature and steric volume of substituent R have no essential effect on the main laws of the process. For example, the conversion was 80-90% at a selectivity with respect to nonanal of about 60% in all cases. The regioselectivity with respect to nonanal was considerably increased to 90-92% by using the chelate biphosphite 84 [220]. 

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. 

In [51], Wacker oxidation of olefins was studied in the presence of catalytic systems comprising water-soluble calixarenes (sulfonated and glycydylated derivatives), palladium salt, and copper salt. The presence of nonpolar cavities in these molecules enables binding nonpolar substrates and their transfer into the aqueous phase where the reaction takes place. The activity of these catalysts depends on the complementarity between the cavity size of the host molecule and the size of the guest molecule. Therefore, substrate selectivity was exhibited. For example, the addition of calixarene increased the reaction rate for linear 1-alkenes which size corresponded to the size of the calixarene cavity (1-hexene for calix[4]arene and 1-octene for calix[6]arene). The activity of catalytic system applied for the oxidation of substituted styrenes also depended on the ratio of the size of the substrate molecule and that of the calixarene cavity. 

Water-soluble calix[n]arenes 20-24 with sulfonated fragments 4-6 aromatic rings) and aminomethylated calix[4]arenes were used as components in the catalytic system for the cross-coupling reactions of l-iodo-4-phenylbenzene and phenylboronic acid (Suzuki reaction) catalyzed by palladium complexes with sulfonated triphenylphosphine (Scheme 4.13) [65], Addition of water-soluble calixarenes increased the yield of reaction products more than twofold. The maximum activity was exhibited by calixarenes containing amino groups 22-24. The appropriate constants of the formation of respective inclusion complexes were significantly higher. 

Calixarene phosphite 30 with unsubstituted hydroxyl group was used as a ligand in the rhodium complex-catalyzed hydroformylation of 1-hexene (Scheme 4.19) [78]. The almost quantitative conversion of initial alkene was observed in 3 hours with aldehyde yields of 80-85%. Unfortunately, the selectivity was low because the aldehydes with normal and isomerized chains were formed in similar amounts. A number of structurally similar phosphites 31 as components of catalytic systems for the hydroformylation of 1-octene are described (Scheme 4.19) [79], It was shown that the nature and steric volume of R did not significantly affect main processes. Thus, the depth of conversion was 80-90% with about 60% selectivity in all cases. 

Thus, the data discussed in this chapter indicate broad possibilities for the use of macrocycles in catalysis and as nanoreactors. The use of macrocyclic receptors on calixarene platform as components of catalytic systems results in a significant increase of catalytic activity, as well as the attainment of substrate regioselectivity. 

Maksimov AL, Buchneva TS, Karakhanov EA. Supramolecular calixarene-based catalytic systems in the Wacker-oxidation of higher alkenes. J Mol Cat A Chem 2004 217(l-2) 59-67. 

Recently, a new bidentate hemispherical chelating bisphosphite ligand based on a calixarene backbone has been designed for linear selective hydroformylation of alkenes (Scheme 9) [54], Excellent levels of regioselectivity have been observed, and even the intrinsic branched-selective hydroformylation of styrene could be overruled by this system. However, the system suffers from low catalytic activity. 

At first, the catalytic effect of the single micellar solutions based on surfactants of different structure was studied. The data covering this period are reviewed in refs. The regularities when generalized made it possible to proceed with the more complicated systems, in particular, the binary surfactant solutions, the polymer-surfactant and polymer-cyclophane (calixarene, pyrimidinophane, and etc.) systems. For these systems, the terms supramolecular systems (ensembles, compositions, complexes) are further used. 

Calixarenes similarly to natural detergents easily embed the micelles and bilayers formed by other amphiphiles [54] and can exhibit highly selective catalytic activity as enzyme-mimicking systems. Thus, water-soluble calix[6]arenes 8-11 catalyzed the hydration of 1,4-dihydronicotinamide derivatives, important model compound used to study the properties of the reduced form of the coenzyme nicotinamide adenine dinucleotide (NADH) (Scheme 4.8) [55], The reaction proceeded according to the Michaelis-Menten kinetics. 

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