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Facile Smiles-type rearrangement in radical cations of N-acyl arylsulfonamides and analogs

Published

Author(s)

Karl Irikura, Nino G. Todua

Abstract

RATIONALE: N-Alkylation of sulfonylbenzamides was reported recently to cause a dramatic and surprising change in electron ionization mass spectrometry (EIMS), leading to a closed-shell base peak. Only an incomplete, speculative mechanism was available at that time. The fragmentation mechanism is determined in the present work and set in the context of related compounds. METHODS: Candidate reaction mechanisms were evaluated theoretically using modest density-functional calculations. The fragmentation mechanism with the lowest barriers was identified and one of its implications tested successfully by experimental 18O-isotopic substitution. RESULTS: The amide oxygen atom attacks the arylsulfonyl group at the ipso position (Smiles-type rearrangement), displacing a molecule of SO2. The resulting carboximidate radical cation has a weak C-O bond that breaks easily. The incipient aryloxyl radical abstracts a proton from the amide nitrogen to form the dominant product ion, but if the molecule is N-alkylated this cannot occur. Instead, the neutral aryloxyl radical is lost and a closed-shell, N-alkyl nitrilium ion is the major product. CONCLUSIONS: The Smiles-type ion fragmentation mechanism is facile for the title compounds, despite the necessity for carbonyl oxygen to serve as a nucleophile. This rearrangement probably occurs in many of the mass spectra reported for structurally similar compounds, in which the nucleophile may be a thione, arylthio, imine, methylene, or methine moiety.
Citation
Rapid Communications in Mass Spectrometry
Volume
28

Keywords

density functional theory, electron ionization mass spectrometry, sulfabenzamide, carboximidate radical cation, quantum chemistry, 18O-labeling experiments, oxygen rearrangement, reaction mechanism

Citation

Irikura, K. and Todua, N. (2014), Facile Smiles-type rearrangement in radical cations of N-acyl arylsulfonamides and analogs, Rapid Communications in Mass Spectrometry, [online], https://doi.org/10.1002/rcm.6850, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=912656 (Accessed April 20, 2024)
Created January 20, 2014, Updated October 14, 2021