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

Abstract

Protonated aromatic aldehydes and methyl ketones la-lOa, 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-Bpropanols, 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 CH30CH2 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,OCH$ ions, and the elimination of an ester RCOOCH, (R = H and CH,). The mechanisms of these fragmentations were studied by using D-labeled derivatives. Confrrming 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 methyl etherl-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 unrestricted over quite large molecular distances to react with the remote CH30CH,-side-chain, contrasting the restricted migration of a proton by 1,2-shifts ("ring walk") in these systems. (J Am Sot Mass Spectrom 1992, 3,[417][418][419][420][421][422][423][424][425][426]

Ion-neutral complexes resulting from dissociative protonation: Fragmentation of α-furanylmethyl benzyl ethers and 4-N,N-dimethylbenzyl benzyl ethers

“…Thus, proton transfer before fragmentation is proposed. Although the proton transfer over the phenyl ring seems difficult, it could take place by ring walk [28]. As with the series of ␣-furanylmethyl benzyl ethers, the oxygen and the phenyl ␣-carbons are the probable dissociative protonation sites.…”

Gas Phase Chemistry of Li+with Amides: the Observation of LiOH Loss in Mass Spectrometry

“…The existence of ion at m/z 351, which results from the loss of LiOH, implies that the deuteron migrates to the phenyl ring and exchanges with the proton on the phenyl ring. In fact, the proton transfer and H/D exchange processes of protonated aromatics have been studied in the past decades [51][52][53][54][55][56][57][58][59][60][61][62]. After the deuteron transfers from the nitrogen to one of the orthopositions of the phenyl ring, the proton may migrate back to the nitrogen while the deuteron retains on the phenyl ring [63,64], which allows the interconversion of f-1 and f-2 via g. The subsequent loss of LiOD from f-1 or LiOH from f-2 results in the formation of the ion at m/z 350 or 351, respectively.…”

[(7-chloroquinolin-4-yl)amino]acetophenones and their copper(II) derivatives: synthesis, characterization, computational studies and antimalarial activity

“…This pathway was possible if one considers the migration of a proton from the initial protonation site (quinolinic-N) to the site of dissociative protonation. So, proton transfer is proposed before fragmentation occurs (Thielking et al, 1992[43]; Pengyuan et al, 2010[32]; Demarque et al, 2016[8]).…”

“…Route 1 was given by cleavage of the methyl-carbonyl α-bond, releasing methane (16 Da) and producing the ion at m/z 281, in turn, this fragment generated the ion at m/z 253-254 by loss of a mass of 28 Da (carbon monoxide). Route 2 implies a process of fragmentation that is explained through the formation of a neutral/ion complex, but where apparently two mechanisms competed, one of dissociation (Thielking et al, 1992[43]; Filges and Grützmacher, 1987[12]) and another of hydride abstraction by an intermolecular electrophilic reaction (Thielking et al, 1992[43]; Wenthold and Liu, 2001[46]). The first mechanism generated the observed acyl ion at m/z 43, while the second mechanism provided the main product ion at m/z 253-254.…”