Concave reagents protonations

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. 

While stoichiometric amounts of base were used in the nitronate anion protonations, catalytic amounts of concave macrocycles 156,157 and 159 were found to influence the addition of alcohols to diphenylketene. The chiral concave macrocycle 161 catalyzed the addition of l -l-phenylethanol 20% faster than the addition of the S-enantiomer to the ketene thus demonstrating the enantioselectivity of the concave reagent [8]. 

All concave reagents carrying a proton within the cavity can be used as concave acids. This includes protonated concave bases. If a proton is delivered directly from the proton source to the substrate (general protonation ) and the protonation is irreversible, the concave shielding will... 

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

Liining. U. Baumgartner, H. Manthey, C. Meynhardt. B. Concave reagents. 20 Sterically shielded m-terphenyls as selective agents in general protonations. J. Org. Cheni. 1996. 67, 7922-7926-... 

The unsubstituted pyrrolo[l,2-f]oxazol-3-one 295 underwent a dihydroxylation with catalytic osmium tetroxide to exclusively give diol 296. This diol resulted from attack of the oxidizing agent from the concave face of the molecule due to the pseudoaxial protons that sterically hinder the /3-face to be attacked by the osmium reagent (Equation 51) <2004AJC669>. 

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. 

For weak acids, the proton is directly transferred from the acid to the substrate in a reagent-controlled manner and, in order to increase the selectivity, extremely shielded 2 -substituted m-terphenyls have been developed as concave protonating reagents inspired by the geometry of enzymes. Thus, the diastereoselective protonation by a series of substituted phenols of endocyclic keto enolates, obtained by the stereocontrolled 1,4-addition of lithiocuprates onto substituted cyclohexenones, was reported by Krause and coworkers354 355 and applied to the synthesis of racemic methyl dihydroepijasmonate356. 

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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