Unidirectional Triple Hydrogen Rearrangement in the Radical Cations of Electron-Rich 3-Aryl-1-Propanols: Further Evidence and Limitation

The unidirectional triple-hydrogen (3H) rearrangement of the radical cations of 3-aryl-1-propanols bearing an electron-rich substitutent in the para-position was investigated for the diastereomeric 2-(4-dimethylamino)benzylcyclohexanols and 2-(4-dimethylamino)-benzylcyclopentanols and confirmed to be a highly stereospecific feature. Whereas the standard electron ionization (EI) (70 eV) mass spectra of the trans-isomers exhibit very minor (∼ 2%–3%) albeit stereospecific peaks for the relevant C8H13N•+ ions ( m/z 123), the metastable ion [mass-analyzed ion kinetic energy (MIKE)] spectra show these peaks with significant relative intensity (8%–17%). The respective cis-isomers do not undergo the 3H rearrangement, be it under standard or under metastable-ion conditions. The stereospecific 3H rearrangement is suppressed in the radical cations of cis- and trans-3-(4-dimethylamino)phenylcyclohexanol, the mass and MIKE spectra of which are governed by cleavage processes of the cyclohexane ring, which impedes the stereochemical assignment of the isomers by mass spectrometry. A multistep mechanism for the unidirectional 3H rearrangement is discussed in view of the present and previous experimental data.