3-pyridinecarboxaldehyde: A model system for superelectrophilic activationand the observation of a diprotonated electrophile

3-Pyridinecarboxaldehyde (7) has been studied as a model system for superel ectrophilic activation. When compound 7 is compared with benzaldehyde (3) i n acid-catalyzed condensation reactions with arenes, 7 is more reactive tha n 3. Compound 7 reacts with chlorobenzene, o-dichlorobenzene, or nitrobenze ne 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 ac idity in the range of H-o = -11.5 to -14 to reach a comparable level of ele ctrophilic reactivity. Compound 7 was studied in acidic solution by C-13 NM R, and the diprotonated, dicationic species was observed at -60 degrees C i n a solution of FSO3H-SbF5.