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Eur. J. Mass Spectrom. DOI: 10.1255/ejms.550

Reactions of the ionized enol tautomer of acetanilide: elimination of HNCO via a novel rearrangement

Lisa N. Heydorn,a Lynn M. Carter,b Richard D. Bowenb and Johan K. Terlouwa*
aDepartment of Chemistry, McMaster University, 1280 Main St W., Hamilton, ON, L8S 4M1, Canada. E-mail:
bDepartment of Chemical and Forensic Sciences, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK

ABSTRACT:
The reactions of ionised acetanilide, C6H5NH(=O)CH3·+, and its enol, C6H5NH(OH)=CH2·+, have been studied by a combination of tandem mass spectrometric and computational methods. These two isomeric radical cations have distinct chemistries at low internal energies. The keto tautomer eliminates exclusively CH2=C=O to give ionised aniline. In contrast, the enol tautomer loses HN=C=O, via an unusual skeletal rearrangement, to form predominantly ionised methylene cyclohexadiene. Hydrogen atom loss also occurs from the enol tautomer, with the formation of protonated oxindole. The mechanisms for HN=C=O and hydrogen atom loss both involve cyclisation; the former proceeds via a spiro transition state formed by attachment of the methylene group to the ipso position, whereas the latter entails the formation of a five-membered ring by attachment to the ortho position. The behaviour of labelled analogues reveals that these two processes have different site selectivities. Hydrogen atom loss involves a reverse critical energy and is subject to an isotope effect. Surprisingly, attempts to promote the enolisation of ionised acetanilide by proton-transport catalysis were unsuccessful. In a reversal of the usual situation for ionised carbonyl compounds, ionised acetanilide is actually more stable than its enol tautomer. The enol tautomer was resistant to proton-transport catalysed ketonisation to ionised acetanilide, possibly because the favoured geometry of the encounter complex with the base molecule is inappropriate for facilitating tautomerisation.

Keywords: ketoenol tautomerism, proton-transport catalysis, ab initio calculations, tandem mass spectrometry

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