On the mechanism of disproportionation reactions of 1,2-dihydroquinolines

The commonly accepted mechanism for the acid-catalyzed disproportionation of 1,2-dihydroquinolines involves a hydride transfer from C-2 of one dihydroquinoline molecule to C-4 of another molecule which has already been protonated at C-3. This mechanism is shown to be incorrect. The proposed intermediate is shown to react by solvent addition and not by reduction under the conditions of the reaction. Evidence has been obtained which shows that the reaction proceeds by way of a 3,4-dihydroquinoline intermediate. A possible mechanism for the formation of the intermediate is discussed.