A Unique Intramolecular Redox Reaction by Unidirectional Migration of Three Hydrogen Atoms: Theoretical Elucidation of the 3H‐Transfer Sequence

The intramolecular migration of three hydrogen atoms from one moiety of a gaseous radical cation to the other prior to fragmentation is an extremely rare type of redox reaction. Within the scope of this investigation, this scenario requires an ionized but electron-rich arene acceptor bearing a para-(3-hydroxyalkyl) residue. The precise mechanism of such unidirectional 3H transfer processes, including the order of the individual H transfer steps, has remained unclear in spite of previous isotope labelling and recent infrared ion spectroscopy (IRIS) studies. Herein, the details of this peculiar process have been investigated for ionized 4-(N,N-dimethylaminophenyl)-2-butanol, 2, by state-of-the-art density functional theory (DFT) calculations. The energetically most favorable pathway consists of a sequence of successive 1,4-, 1,6- and 1,5-H steps. During these steps the secondary alcohol functionality is oxidized to a carbonyl group and the radical-cationic aniline ring is reduced to an ionized 2,3-dihydroaniline unit. Several alternative sequences, such as three successive 1,5-H shifts, could be excluded. A concomitant unidirectional 2H migration reaction of ion 2 was also investigated and the intermediacy of ion-neutral complexes (INCs) enabling sequential hydride and proton transfer was confirmed by the calculations.