Benzaldehyde-derived iminium ions

Mayr has extended his electrophilicity scale to benzaldehyde-derived iminium ions through measurement of rate constants for their reactions with C-nucleophiles such as enamines, silylated ketene acetals and enol ethers." With an E value of -9.27 for Ph-CH=NMe2" (in a range from -8.34 to -10.69 forpara-C j, andpura-OMe, respectively), these iminium ions are 10 orders more reactive than the parent aldehydes. However, the values are restricted to C-nucleophiles the iminium ions react 10 -10 times faster with water and amines than these E values would predict. Such reactions benefit from the anomeric stabilization of 0,Af-acetals and Af,Af-aminals. 

Triphenylhydrazine reacts in a similar way with enol ethers to cinnoline derivatives (Table 12, number 5). The electrooxidation of A -alkylhydrazines leads to an iminium ion, which can react with olefins to five-membered rings [Eq. (28)] [254]. The anodic oxidation of phenylhydrazones of benzaldehyde affords diphenylnitrilimines, which add to dipolarophilic compounds [255]. 

In the Vilsmeier-Haack reaction, a reagent such as A(A-dimethylformamide (DMF) is the source of the formyl group. Reaction of DMF with phosphorus oxychloride (POCI3), creates an electrophilic iminium ion in situ that reacts with activated aromatic compounds. Hydrolysis of the resulting imininm ion results in a benzaldehyde derivative. 

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