We then sought to obtain experimental information on the reaction mechanism (Figure ). The use of deuterated amide d 1 -1b led to selective deuterium incorporation at the carbon -to the ketone carbonyl (Fig. ). This is suggestive of a hydride transfer event. On the other hand, the use of bisdeuterated -methylstyrene d2-3a led to deuterium incorporation in the α-position of ketone 3 (Fig. ). Finally, quenching the reaction with H2 18 O (20 equiv.) generated a ketone product whereby the incorporation of O is > 90% (Fig. ). These observations allow us to put forth a mechanistic proposal as depicted in Fig. . Thus, activation of the amide starting material is likely to generate an N-allyl nitrilium species 6, for which we also have obtained in situ NMR evidence (see Supporting Information for details, pages S23). This electrophile is captured by the alkene partner, setting up the stage for intramolecular 1,5-hydride delivery. This step simultaneously achieves reduction of the transient carbocation 7 and formation of an azoniaallene intermediate 8,