Acetic acid, hydrogen bonding cooperativity

As outlined in the previous paragraph, chiral Brpnsted base organocatalysts appear to be most effective when equipped with an additional Brpnsted acidic moiety, for example, hydrogen bond donors like (thio)urea. Apparently, both functionalities catalyze the asymmetric Mannich reaction in a cooperative fashion, that is, simultaneous activation of both the nucleophile and the electrophile. However, activation of the electrophile can also be accomplished with a single, enantiomerically pure Brpnsted acid. In this respect, readily available chiral phosphoric acids are most commonly applied [88-90]. In 2004, the groups of Akiyama and Terada independently from each other reported the first asymmetric Mannich reaction of silyl ketene acetals or acetyl acetone with imines utilizing chiral phosphoric acid catalysts, which... 

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