Mechanism of the condensation reaction between 1‐aryl‐ and heteroaryl‐1,4‐dihydro‐3(2 H )‐isoquinolinones and aldehydes

4‐Benzylidene‐1‐phenyl‐1,4‐dihydro‐3(2 H )‐isoquinolinone, the intermediary product of the carbonyl condensation reaction between 1‐phenyl‐1,4‐dihydro‐3(2 H )‐isoquinolinone and benzaldehyde, rearranges in the presence of an equivalent quantity of sodium hydride into 4‐benzyl‐1‐phenyl‐3(2 H )‐isoquinolinone. As the possibility of the migration of the hydrogen at C‐1 in the form of a proton or a hydrogen atom (radical reaction) was excluded, the mechanism of the rearrangement could be depicted as an intermolecular hydride anion migration. In case of the 1‐(4‐pyridyl)‐ and 1‐(3‐pyridyl)‐1,4‐dihydro‐3(2 H )‐isoquinolinones, however, the rearrangement can be carried out also in polyphosphoric acid and in this case a proton loss‐proton gain mechanism was proved.