Resonance benzoin formation

The apparent fickleness of the acyl-pyrroles and -indoles in their reaction with carbanions to form new C—C bonds arises from the contribution made by the zwitterionic structure, e.g. (410b), to the resonance hybrid and the choice of the reaction conditions is critical for a successful nucleophilic reaction. Thus, formyl-pyrroles and -indoles do not normally undergo the Cannizzaro reaction nor do they form stable cyanohydrins or undergo benzoin-type reactions. However, surprisingly, 2-formylpyrrole reacts with arylaldehydes in the presence of potassium cyanide to yield (428), which is easily oxidized to (429) (B-77MI30505). It is noteworthy that the presence of an ester substituent adjacent to the formyl group modifies the mesomeric interaction to such an extent to allow the formation of (430) in low yield, as a result of an initial benzoin-type self-condensation (Scheme 76) (68BSF637). 

Aromatic aldehydes generally do not produce cyanohydrins on reaction with hydrogen cyanide, but undergo the benzoin condensation (Scheme 6.12). The initial product from nucleophilic attack by cyanide ion is depro-tonated to form a resonance-stabilized carbanion, which attacks a second molecule of the aldehyde. Elimination of HCN leads to an a-hydroxy ketone, benzoin (2-hydroxy-1,2-diphenylethanone). The benzoin condensation is catalysed specifically by cyanide ion, which assists in both the formation and stabilization of the carbanion. The reaction is limited to aromatic aldehydes, since the aryl ring also stabilizes the anion. 

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