Concave reagents reactions

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. 

Section 2 discusses the syntheses of different classes of concave acids and bases. Convergent synthetic strategies were chosen for an easy structural variation of the reagents (modular assembly). Section 3 characterizes the concave acids and concave bases and checks whether the acid/base properties of the parent compounds benzoic acid, pyridine and 1,10-phenanthroline are conserved in the bimacrocyclic structures. In Section 4, the influence of the concave shielding on the reactivity and selectivity of the concave reagents is measured in model reactions. In principle, the concave shielding should be able to influence inter- and intramolecular competitions as well as chemoselectivity and (dia)stereoselectivity. If the reagent is chiral, enantioselectivity should also be observable. 

Therefore, the attempt to direct the protonation of allyl anions systematically by using concave acids is doomed because every change in reagent, reaction conditions and solvent has a strong influence on the aggregates of the anions with the lithium counter ions and these changes cannot be separated from one another. 

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. 

Concave reagents have been employed in many reactions but for the reasons mentioned above, most of the reactions were catalyses. Besides acid or base catalyses, especially transition metal ion catalysed reactions are of interest and have been investigated. The following chapters will hrst present major classes of concave reagents and will then discnss some reactions and catalyses and the influence of the concave shielding on rate and especially selectivity. 

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. 

Probably the largest catalytic potential lies in the metal complexes of concave reagents. Two sub-structures are well established as metal ligands 1,10-phenanthroline and NHC. Numerous transition metal complexes have been synthesized and some of them have been investigated in metal catalysed reactions such as Lewis-acid catalysed Diels-Alder reactions, palladium-catalysed aUylations, and copper(l)-catalysed cyclopropa-nations. " In the latter reaction, the full potential of concave reagents and the importance of the tine structure of the concave shielding has become obvious. With two different types of concave ligands, two complementary stereoselectivities have been found. 

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. 

Three different classes of reactions have been investigated using concave reagents ... 

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. 

Luning. U. Abbass. M. Fahrenkrug. F. Concave reagents, 37 A facile route to aryl-substituted 1.10-phenanthrolines by means of Suzuki coupling reactions between substituted areneboronic acids and halogeno-1.10-phenanthrolines. Fur. J. Org. Chem. 2002. 3294-3303. 

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. 

If the transition metal could exist in two different oxidation states in the complex 87, one would have a concave redox reagent which could be useful for instance in epoxidation reactions [56]. The concave shielding of the metal ion should influence the regio-, stereo- and, if the concave 1,10-phenanthroline 21 is chiral, enantioselectivity of an epoxidation. 

In many cyclic or bicyclic molecules a stereo structure is present in which one can identify a convex and a concave side. Because reactions usually take place in such a way that the reacting reagent is exposed to the least possible steric hindrance, convex/concave substrates are generally react on their convex side. 

In the case of a reagent, a functional group is altered in the course of the reaction. An acid for instance can carry out a protonation which resnlts in the corresponding base at the end of the reaction. Or a redox-reagent oxidizes or rednces a substrate and alters its redox state. This is a problem for expensive reagents like concave ones, and therefore recycling is essential in these cases (but easy to carry out in case of protonations/depro-tonations or redox-reactions). 

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