Remote fragmentations of protonated aromatic carbonyl compounds via internal reactions in intermediary ion-neutral complexes

• Published in: Journal of the American Society for Mass Spectrometry 1992-05, Vol.3 (4), p.417-426
• Main Authors: Thielking, G., Filges, U., Grützmacher, H.-Fr
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Mass spectrometry; Organic chemistry; Reactivity and mechanisms
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1016/1044-0305(92)87069-B

Protonated aromatic aldehydes and methyl ketones 1a–10a, carrying initially the proton at the carbonyl group, are prepared by electron impact-induced loss of a methyl radical from 1-arylethanols and 2-aryl-2-propanols, respectively. The aryl moiety of the ions corresponds to a benzene group, a naphthalene group, a phenanthrene group, a biphenyl group, and a terphenyl group, respectively, each substituted by a CH 3OCH 2 side-chain as remote from the acyl substituent as possible. The characteristic reactions of the metastable ions, studied by mass-analyzed ion kinetic energy spectrometry, are the elimination of methanol, the formation of CH 3OCH + 2 ions, and the elimination of an ester RCOOCH 3 (R = H and CH 3). The mechanisms of these fragmentations were studied by using D-labeled derivatives. Confirming earlier results, it is shown that the ester elimination, at least from the protonated aryl methyl ketones, has to proceed by an intermediate [acyl cation/arylmethyl ether]-complex. The relative abundances of the elimination of methanol and of the ester decrease and increase, respectively, with the size of the aromatic system. Clearly, the fragmentation via intermediate ion-neutral complexes is favored for the larger ions. Furthermore, the acyl cation of these complexes can move unresitricted over quite large molecular distances to react with the remote CH 3OCH 2-side-chain, contrasting the restricted migration of a proton by 1,2-shifts (“ring walk”) in these systems.