Unidirectional triple and double hydrogen rearrangement reactions in the radical cations of γ-arylalkanols

A novel fragmentation reaction accompanied by the unidirectional migration of three hydrogen atoms has been found in the radical cations of γ‐arylpropanols with electron‐donating substituents in the para position. This triple hydrogen (3H) rearrangement reaction is the dominant fragmentation channel of the long‐lived molecular ions of trans‐2‐(4′‐dimethylaminobenzyl)‐l‐indanol, 2, but it occurs also in simpler γ‐arylpropanol ions. Deuterium labelling of 2 reveals that the three hydrogen atoms originate with extraordinarily high specificity from the C(l), C(2) and O positions of the alcohol moiety. Cis‐ and 3′‐substituted isomers do not undergo this reaction. Along with the 3H rearrangement reaction a unidirectional double hydrogen (2H) rearrangement reaction takes place independently and with less specificity in the trans‐2‐(4′‐X‐benzyl)‐l‐indanol ions 1+˙ and 2+˙. No hydrogen exchange occurs during the 3H and 2H rearrangement reactions. Mechanistic alternatives of these unusual fragmentation reactions are discussed; the experimental evidence strongly favours pathways via several intermediate ion–neutral complexes.