Bifunctional organocatalysis Lewis bases

FIGURE 2.43. Representative bifunctional hydrogen-bond donor/Lewis base organocatalysis. 

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. 

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. 

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... 

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