To differentiate between these mechanistically distinct options, we tried to intercept the proposed intermediates and submit each of them to our reaction conditions. We first studied the formation of imine and enamine when treating propiophenone with aniline in the absence of azide. Under our reaction conditions, imine was observed while enamine was not detected (Figure .h.). We then probed the imineenamine tautomerization step by synthesizing and purifying the imines of acetophenone and propiophenone, followed by treatment with azide under our standard reaction conditions (Figure .j.). Product formation was observed with acetophenone, whereas the propiophenone imine remained unreacted. We then sought to test whether the reaction can proceed upon the addition of azide to enamines. The enamines that were postulated to form under our reaction conditions are not stable, thus we synthesized and isolated stable enamines from secondary amines with acetophenone (6a) and propiophenone (6b) and submitted them to our reaction conditions (Figure .j.). In both cases, amidine products were obtained, implying that the cyclization/de-cyclization steps are not limiting to propiophenone reactivity. Overall, this set of experiments strongly suggested that whereas propiophenone can form an imine, it could not proceed to form enamine under our reaction conditions. Given these results, we postulated that the sensitivity of the reaction to the nature of the alkyl substituent on the ketone could provide us with a powerful methodology to control regioselective amidine transformations of aliphatic substrates. Upon the addition of aliphatic ketones, theoretically, the reaction may proceed through two possible pathways (see figure .a.): (I) the imine-enamine tautomerization would occur on the mesial position with the elimination of the corresponding diazoalkane by-product; (II) the imine-enamine tautomerization would occur on the distal methyl position with the elimination of diazomethane. As a proof-of-concept we submitted butanone to our reaction conditions to probe its regioselectivity. Indeed, the tautomerization occurs only on the distal methyl position, leaving the mesial position intact and providing a highly regioselective transformation (2y Figure3.b.). Similar behavior was observed with phenyl acetone as substrate affording product 2z exclusively (Figure3.b.).