Bifunctional organocatalysis catalysts

Another important class of covalent catalysis is carried ont by chiral iV-heterocyclic carbenes [15] that react with carbonyl componnds forming chiral acyl anion eqniva-lents whose reaction with the corresponding electrophile complete the catalytic cycle (F, Fig. 2.2). Finally, it is worthy to mention the ability of certain bifnnctional organo-catalysts to perform simultaneous activation of the nncleophile and the electrophile. The nse of bifunctional organocatalysis has been shown to be very snccessfnl in conjngate additions. 

Finally in Chapters 11-13, some of the more recent discoveries that have led to a renaissance in the field of organocatalysis are described. Included in this section are the development of chiral Brdnsted acids and Lewis acidic metals bearing the conjugate base of the Bronsted acids as the ligands and the chiral bifunctional acid-base catalysts. 

This reaction encompasses a number of interesting features (general Brpnsted acid/ Brpnsted base catalysis, bifunctional catalysis, enantioselective organocatalysis, very short hydrogen bonds, similarity to serine protease mechanism, oxyanion hole), and we were able to obtain a complete set of DFT based data for the entire reaction path, from the starting catalyst-substrate complex to the product complex. 

Keywords Asymmetric organocatalysis Bifunctional catalyst Brpnsted base Chiral scaffold Cinchona akaloid Cyclohexane-diamine Guanidine... 

In 1981, Hiemstra and Wynberg reported a thorough investigation of the cinchona alkaloid-catalyzed addition of thiols to a,p-unsaturated enones [24] (Scheme 6.30). Mechanistic studies revealed that the free OH group is ctu-cial in this reaction and thus cinchona alkaloids most likely act as bifunctional catalysts herein. This report may now be considered as one of the major breakthroughs in asymmetric organocatalysis and has inspired a lot of further research toward the development of asymmetric Lewis or Brpnsted base catalysts. 

This gives chapter an overview of natural cinchona alkaloids and synthetic derivatives together with examples of their use in asymmetric organocatalysis. In recent years, the emphasis has been on the development of cinchona-based bifunctional catalysts, in particular species with a thiourea moiety. The search for new cinchona-based organocatalysts continues and new derivatives are relentlessly being prepared and applied for specific enantioselective reactions. The design of these new... 

Although boron is a metalloid, one can nonetheless include boron-derived compounds in a book on organocatalysis as not being transition metal containing catalysts Planar chiral boronic acids have been employed as amide couphng catalysts, actually as bifunctional Lewis acid/base activators. Whiting s [51] group reported the preparation [52] of two amino-boronic ferrocenes (see Scheme 8.17). Their use... 

Bifunctional catalysts bearing both an acidic and a basic/nucleophilic functionality are interesting for asymmetric organocatalysis. The catalysts 39 and 40 consist... 

Organocatalysis Continuing efforts to harness the cooperative activity of bifunctional organocatalysts have led to the examinations of chiral calix[4]arenes containing amino phenol stractures and chiral per-6-amino-P-cyclodextrin in the asymmetric sulfa-Michael reactions to cyclic and acylic enones.The preliminary results indicated that further structural modification would be required to allow more efficient asymmetric catalyst systems. 

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