Unidirectional Double- and Triple-Hydrogen Rearrangement Reactions Probed by Infrared Ion Spectroscopy

Dennis Zeh; Marcel Bast; Jonathan Martens; Giel Berden; Jos Oomens; Sandra Brünken; Stephan Schlemmer; Mathias Schäfer; Dietmar Kuck

doi:10.1021/jasms.2c00039

Unidirectional Double- and Triple-Hydrogen Rearrangement Reactions Probed by Infrared Ion Spectroscopy

J Am Soc Mass Spectrom. 2022 Aug 3;33(8):1377-1392. doi: 10.1021/jasms.2c00039. Epub 2022 Apr 26.

Authors

Dennis Zeh¹, Marcel Bast², Jonathan Martens³, Giel Berden³, Jos Oomens³, Sandra Brünken³, Stephan Schlemmer², Mathias Schäfer¹, Dietmar Kuck⁴

Affiliations

¹ Department of Chemistry, Institute of Organic Chemistry, University of Cologne, Greinstraße 4, 50939 Köln, Germany.
² I. Physikalisches Institut, University of Cologne, Zülpicher Straße 77, 50937 Köln, Germany.
³ Institute for Molecules and Materials, FELIX Laboratory, Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands.
⁴ Department of Chemistry, Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany.

PMID: 35471836
DOI: 10.1021/jasms.2c00039

Abstract

Unidirectional double-hydrogen (2H) and triple-hydrogen (3H) rearrangement reactions occur upon electron-ionization-induced fragmentation of trans-2-(4-N,N-dimethylaminobenzyl)-1-indanol (1), trans-2-(4-methoxybenzyl)-1-indanol (2), 4-(4-N,N-dimethylaminophenyl)-2-butanol (3), and related compounds, as reported some 35 years ago (Kuck, D.; Filges, U. Org. Mass Spectrom. 1988, 23, 643-653). These unusual intramolecular redox processes were found to dominate the mass spectra of long-lived, metastable ions. The present report provides independent evidence for the structures of the product ions formed by the 2H and 3H rearrangement in an ion trap instrument. The radical cations 1^•+ and 3^•+ as well as ionized 1-(4-N,N-dimethylaminophenyl)-5-(4-methoxyphenyl)-3-pentanol, 5^•+, were generated by electrospray ionization from anhydrous acetonitrile solutions. The 2H and 3H fragment ions were obtained by collision-induced dissociation and characterized by IR ion spectroscopy and density functional theory calculations. Comparison of the experimental and calculated infrared ion spectra enabled the identification of the 2H rearrangement product ion, C₉H₁₄N⁺ (m/z 136), as an N,N-dimethyl-para-toluidinium ion bearing the extra proton ortho to the amino group, a tautomer which was calculated to be 31 kJ/mol less stable than the corresponding N-protonated form. The 3H rearrangement product ion, C₈H₁₃N^•+ (m/z 123), formerly assumed to be a distonic ammonium ion bearing a cyclohexadienyl radical, was now identified as a conventional radical cation, ionized N,N-dimethyl-2,3-dihydro-para-toluidine. Thus, the 3H rearrangement represents an intramolecular transfer hydrogenation between a secondary alcohol and an ionized aromatic ring. Based on these structural assignments, more detailed mechanisms for the unidirectional 2H and 3H rearrangement reactions are proposed.

Keywords: IR ion spectroscopy; anilines; density functional theory; intramolecular transfer hydrogenation; multiple hydrogen rearrangements; radical cations; unidirectional hydrogen rearrangements.