Reactivity of the Onium Carbanion

Cation exchange from the metal cation to the onium carbanion improves the intrinsic reactivity of the latter due to formation of the naked anion . At the same time, the 

An asymmetric SN2 reaction and SNAr reaction are included as other examples of this category. 

Unlike the nucleophilic substitution reactions which generate stable onium halide after the reaction, nucleophilic additions to electrophilic C=X double bonds (X=C, N, O) provide rather basic onium anion species as an initial product. If the anion is sufficiently stable under the reaction conditions, onium anion will then exchange the counter ion for the other metal carbanion at the interface to regenerate the reactive onium carbanion Q+R. In another scenario, the basic onium anion may abstract the acidic hydrogen atom of the other substrate to provide Q 1 R directly. Such a reaction system ideally requires only a catalytic amount of the base although, in general, a substoichiometric or excess amount of the base is used to lead the reaction to completion. An additional feature of this system is the substantial possibility of a retro-process at the crucial asymmetric induction step, which might be problematic in some cases. 

Direct Asymmetric Aldol Reaction of Glycine Schiff Base 

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