Electron ionization mass spectra and tautomerism of substituted 2‐phenacylquinolines

Abstract

Tautomerism has been studied conventionally in solutions or in the solid state. However, the importance of mass spectrometry in the gas phase was realized relatively late. 2-Phenacylquinolines are known to undergo ketimine-enaminone tautomerism. The ratio of tautomers is dependent on the nature of the phenyl ring substituent and the Hammett substituent constants sigma. Theoretical calculations indicate the presence of ketimine and enaminone tautomers in the gas phase. The electron ionization mass spectra of eight 2-phenacylquinolines (ketimine form) were recorded at 70 eV in order to determine the fragmentation routes and to screen for the presence of their enaminone tautomers, (Z)-2-benzoylmethylene-1,2-dihydroquinolines, in the gas phase. The relative abundances or total ion currents of some ions correlated with the Hammett substituent constants and Hammett-Brown constants. The product ions [M-CO](+.) and [M-HCO](+) were observed. A reaction mechanism is suggested for the formation of these ions, requiring skeletal rearrangements. The results furnish information relating to tautomerism in the gas phase.

Tautomeric equilibrium, proton affinity and mass spectrometry fragmentation of flexible hydrogen-bonded precursors and rigid $$\hbox {N}\longrightarrow \hbox {BF}_2$$ fluorescent dyes

“…The mechanism of formation of [( )-CH-OH] and [( )-CH=O] ions would involve the complex intramolecular rearrangements, the detachment of hydrogen atoms from the benzene rings, cleavage of several bonds and is unlikely, whereas the mechanism leading to the formation of products requires the transfer of only one proton and simple intramolecular rearrangement (in case of protonated keto tautomeric form of singly charged ions of 1 this mechanism is even simpler). Furthermore, also the analysis of previous results of gas-phase fragmentation behavior of similar structure compounds leads to the conclusion that formation of [( )-C=O] is most likely 69 .…”

Gas Phase Decarbonylation and Cyclization Reactions of Protonated N-Methyl-N-Phenylmethacrylamide and Its Derivatives Via an Amide Claisen Rearrangement

“…Gas phase Claisen rearrangements of protonated allyl phenyl ethers [26], protonated Nallylaniline [27], and protonated benzyloxy indoles have been investigated by mass spectrometry [28]. Similar gas phase Claisen rearrangement reactions have been reported in the dissociation process of radical cations formed by electron ionization mass spectrometry (EI-MS) [29][30][31][32][33]. Most of these gas phase Claisen rearrangement reactions of carbonyl compounds involve a CO-loss fragmentation pathway.…”

Tautomerism, Hammett σ, and QSAR

“…In spite of this, because tautomerism involves the competition of two sites on the molecule for a proton, it can be expressed as the competition between the pK a 's of the two sites [4]. Thus it is not surprising that many investigations show that log K t is a function of some type of Hammett r (for example [4][5][6][7][8][9]). …”