Chloride ion pairs as catalysts for the alkylation of aldehydes and ketones with C–H acidic compounds

Chloride anions associated with various soft cations (like tetraalkyl ammoniums, alkyl imidazoliums or pyridiniums) were shown to be able to promote the alkylation of carbonyl derivatives with acidic compounds, as exemplified on Knoevenagel and aldol condensations under relatively mild conditions. This activity was attributed to an enhanced nucleophilicity of the chlorine anion, originating from a softness/hardness mismatch between the anion and the cation.