The reaction mechanism for this reaction and the related Skraup synthesis is a matter of debate. A 2006 study proposes a fragmentation-recombination mechanism based on carbon isotope scrambling experiments. In this study 4-isopropylaniline 1 is reacted with a mixture (50:50)of ordinary pulegone and the 13C-enriched isomer 2 and the reaction mechanism is outlined in scheme 2 with the labeled carbon identified with a red dot. The first step is a nucleophilic conjugate addition of the amine with the enol to the amine ketone 3 in a reversible reaction. This intermediate then fragments to the imine 4a and the saturated cyclohexanone 4b in a non-reversible reaction and both fragments recombine in a condensation reaction to the conjugated imine 5. In the next step 5 reacts with a second aniline molecule in a nucleophilic conjugate addition to imine 6 and subsequent electrophilic addition and proton transfer to leads to 7. elimination of one aniline molecule through 8 and rearomatization leads to final product 9. Because α-amino protons are not available in this model compound the reaction is not taken to the fully fledged quinoline.
