Br0nsted base catalysts

Planar Chiral Ferrocenes as Lewis- or Br0nsted-Base Catalysts.163... 

Scheme 5.25 Cinchona alkaloids as chiral Br0nsted-base catalysts...

TABLE 11.4. Direct a-Amination of a-Snbstitnted a-Cyanoacetates with Different Br0nsted Base Catalysts... 

More recently, Tan and co-workers reported the reaction of p,7-unsaturated thioesters with di-tert-butyl azodicarboxylate promoted by the chiral guanidine 32 as the Br0nsted base catalyst (Scheme 11.16) [57]. By judicious choice of the double bond geometry of the (3,y-unsaturated thioester compound, the reaction can deliver either enantiomeric product with excellent enantioselectivity. 

In general, most chiral Br0nsted base catalysts are equipped with an additional hydrogen bond donor (Brpnsted acid), which activates the electrophile. Moreover, coordination of both the nucleophile and electrophile to the rigid chiral backbone of the bifunctional catalyst via hydrogen bonding and a basic tertiary amine anchors the electrophile and nucleophile in an optimal transition state, which seems essential for the highly stereoselective and predictable formation of a... 

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