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3-Pyridinecarboxaldehyde (7) has been studied as a model system for superelectrophilic activation. When compound 7 is compared with benzaldehyde (3) in acid-catalyzed condensation reactions with arenes, 7 is more reactive than 3. Compound 7 reacts with chlorobenzene, o-dichlorobenzene, or nitrobenzene in CF3SO3H (triflic acid, TfOH) to give diaryl-3-pyridylmethanes, while 3 does not react with these deactivated arenes in TfOH. Moreover, 7 reacts with benzene in solutions as weakly acidic as H o = −9, while 3 requires acidity in the range of H o = −11.5 to −14 to reach a comparable level of electrophilic reactivity. Compound 7 was studied in acidic solution by 13C NMR, and the diprotonated, dicationic species was observed at −60 °C in a solution of FSO3H−SbF5.

Ninhydrin (1) reacts with aromatic compounds in acid solution to give condensation products. In H2SO4, 1 reacts with arenes to give 2,2-diaryl-1,3-indanediones (2a−f). In superacidic triflic acid (CF3SO3H, TfOH), 1 reacts with arenes to give 3-(diarylmethylene)isobenzofuranones (3a−e). Products 3a−e are proposed to have formed by a condensation and rearrangement involving dicationic intermediates. Benzo[f]ninhydrin also reacts with C6H6 in H2SO4 to give a similar condensation product.

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