Having established the scope of the isomerization procedure we turned our attention toward the investigation of the reaction mechanism. Over the last decades, different mechanisms for olefin isomerization by transition metals have been proposed. Among those, the alkyl and radical mechanism are the most prevalent. The alkyl mechanism involves the insertion of an olefin into a metal hydride followed by β-hydride elimination to furnish the product. Following a similar sequence, the radical mechanism is based on the transfer of hydrogen radicals. In contrast to the previously mentioned pathways, the π-allyl mechanism proceeds exclusively in an intramolecular fashion through a 1,3hydrogen shift. A well-established method to distinguish the possible pathways described above is the crossover labeling experiment. We performed a crossover isomerization of allylbenzene-d2 (2-d2) and non-deuterated 4allylanisole (4) (Scheme 3). An intramolecular hydrogen shift involved in the allyl mechanism would confine deuterium to the previously labeled substrate 2-d2. However, deuterium scrambling was found to extend to 4-allylanisole yielding partially deuterated (E)-anethole-d (4a-d). Furthermore, a distribution of deuterium throughout the product propene chains was observed. Due to these findings, the involvement of an π-allyl mechanism in the isomerization catalyzed by Mn4 was excluded.