Concave reagents catalysts

Luning U. Concave Reagents and Catalysts from Lamps to Selectivity in Mol. Recognit. Inclusion, Proc. Int. Symp., 9th. 1998 203, Ed. Coleman A. W., Pb. Kluwer, Dordrecht... 

In the case of catalytic systems, the tedious and expensive synthesis of a concave catalyst is compensated by its (theoretically) unlimited recyclability. Reagents, in contrast, are used up in a reaction. Therefore, concave reagents will only be attractive when, after the reaction, the used functional groups can be returned into the active original functionality. They must be rechargeable . This is trivial for acids and bases but in principle should also be realizable for redox reagents. 

When the concave reagents are compared to other reactions in Supramole-cular Chemistry a distinct difference must be noted Most other approaches try to bind the substrate in a host first. Then this complex reacts with a reagent which either is present in solution or attached to the host. For concave reagents and concave catalysts, however, there is no need for binding of the educt. In contrast, the protonation reactions can be interpreted as a reagent (H ) host complex. 

For a reagent or a catalyst, a reactive functional group is needed. Inspired by the structure of enzymes, the functional group must be located in the concave region. The challenge is an enc(o-functionalization. In 1987, we presented the concept of concave reagents... 

If the two-step syntheses for bimacrocyclic concave reagents are compared with one another, the use of a pyridine-free macrocycle as the starting material seems to be attractive because numerous macrocycles are easily available, some of which can even be purchased. Consequently, for instance calixarenes have been spanned with pyridine (and other) bridges. Although a very powerful ligand for a copper(I) catalyst has been found... 

As presented in the introduction, concave reagents can be used as reagents or as catalysts. As reagents they will be used up with the need of recycling, as catalysts they can carry out numerous catalytic cycles and can be recovered from the mixture. 

Besides the use as a reagent, concave reagents can be used in a catalytic fashion. From the vast possibility of potential reactions, so far nucleophilic catalyses and metal catalyses have been investigated with several types of concave catalysts. 

So far, most approaches to enzyme-like activity have used just one of the functional groups which are present in enzymes. However, many enzymes only operate by a cooperation of functional groups (see for instance the catalytic triade in peptidases). There, the enzyme s functional groups perform a multifunctional catalysis. Therefore in (organo) catalysis, bifunctional catalysis has been developed, too. In the field of concave reagents, first bifunctional catalysts have been constructed (Figure 7.28), and future will tell how capable they are to catalyse reactions with their acidic and basic functionalities. 

S. Petersen, U. Liining, Concave reagents, 28 comparison of bimacrocyclic, monomacrocyclic and non-macrocychc bis(amidomethyl)pyridines as catalysts in the base catalysed addition of alcohols to ketenes, Em. J. Org. Chem., 1999, 847-854. 

To justify the synthetic efforts for the synthesis of concave reagents, the gain in selectivity must be combined with an easy recovery. Usually, polymer fixation allows easy recycling, and therefore, the concave pyridine la has been attached to a Merrifield resin (27, see Fig. 8a).Indeed, the polymeric material 27 was able to catalyze the alcohol addition to diphenylketene comparably to the soluble concave pyridine la. but complications may exist due to swelling of the resin and substrate depletion deep within the polymer. Therefore, soluble polymers loaded with the concave pyridine I a have also been synthesized. However, this material behaved unreproducibly, especially because side reactions led to cross-polymerized insoluble material. Furthermore, due to reptation (leakage through membrane), an easy separation of product and catalyst would not be possible by ultrafiltration. 

A variety of concave reagents and catalysts is now accessible In protonations, base catalyses and metal- on-assisted catalyses, they have been successfully employed... 

Luning. U. Miiller. M. Gelbert. M. Peters. K. von Schnering, H.G. Keller. M. Concave reagents. 15 New concave 1.10-phenanthrolines Catalysts for the alcohol addition to ketenes. and ligands in transition metal complexes. Chem. Ber. 1994. 127. 2297-2306. 

Gelbert, M. Liining, U. Concave reagents. 25 Transition metal complexes of concave 1.10-phenanthroiines as catalysts for [4+2]-cycloadditions. Ligand effects on the exo/endo-selecuvity. Supramolecular Chem. 2001. 72, 435-444. 

As expected, axial OH groups were easier to differentiate from equatorial ones than equatorial OH groups from one another. In the case of methyl cholate 66a, a standard reagent (pyridine, 50) does a good job. But in the glucose derivative 67a, the two equatorial OH groups are much more similar to one another. Therefore it is not surprising that they react with almost the same rate in the uncatalyzed reaction. When pyridine (50) was used as catalyst, the acylation of the 2-position (67c) was preferred by a factor of 4 but also a bis-acylated product 67d was formed. Concave pyridine 3r showed the best results. With a selectivity of 9 1, the 2-acylated product 67c was formed and no diacylated product 67d could be determined. 

Liming successfully attached a concave arrangement of pyridine units to Fre-chet-type dendrimers in homogeneous phase. A remarkable selectivity was thus achieved in base-catalysed addition of ketenes to alcohols and polyols (e.g. monosaccharides). The functionalised dendrimer catalysts exhibit a greater molar mass than conventional non-dendritic catalysts, thus permitting subsequent recycling of the catalyst by nanofiltration. These dendrimers are thus suitable as reagents for selective acylation of polyols [4]. 

But most related to enzymes are those concave entities in which reactive functional groups are pointing inwards. Several subclasses can be identihed there are reagents and catalysts, and these can be divided into metal-containing and metal-free compounds. 

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