An experimental and computational investigation on the fragmentation behavior of enaminones in electrospray ionization mass spectrometry

Abstract

The dissociation pathways of protonated enaminones with different substituents were investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS) in positive ion mode. In mass spectrometry of the enaminones, ArCOCHCHN(CH(3) )(2) , the proton transfers from the thermodynamically favored site at the carbonyl oxygen to the dissociative protonation site at ipso-position of the phenyl ring or the double bond carbon atom adjacent to the carbonyl leading to the loss of a benzene or elimination of C(4) H(9) N, respectively. And the hydrogendeuterium (H/D) exchange between the added proton and the proton of the phenyl ring via a 1,4-H shift followed by hydrogen ring-walk was witnessed by the D-labeling experiments. The elemental compositions of all the ions were confirmed by ultrahigh resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR-MS/MS). The enaminones studied here were para-monosubstituted on the phenyl ring and the electron-donating groups were in favor of losing the benzene, whereas the electron-attracting groups strongly favored the competing proton transfer reaction leading to the loss of C(4) H(9) N to form a benzoyl cation, Ar-CO(+) . The abundance ratios of the two competitive product ions were relatively well-correlated with the σ(p) (+) substituent constants. The mechanisms of these reactions were further investigated by density functional theory (DFT) calculations.

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

“…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. The proposed mechanism is shown in Scheme 3.…”

Negative charge induced dissociation: fragmentation of deprotonated N‐benzylidene‐2‐hydroxylanilines in electrospray ionization mass spectrometry

“…Electrospray ionization (ESI) mass spectrometry combined with collision-induced dissociation (CID) technique is an indispensable method for the analysis of numerous molecules and the structural determination for organic chemists. [1][2][3] The ions generated in ESI mass spectrometry can be [M + H] + , [4][5][6][7][8][9] [M À H] À , [10][11][12][13][14] complex of M and alkali metal cations [15][16][17][18][19][20][21] and other metal cations. [22][23][24][25][26][27][28][29][30] The widespread ionization routes enrich the capacity of ESI-MS and increase the variability of the fragmentation pattern of gaseous ions, which have been widely studied since the early days of organic mass spectrometry.…”

Gas phase retro‐Michael reaction resulting from dissociative protonation: fragmentation of protonated warfarin in mass spectrometry

“…The positive charge brought in by protonation is usually the driving force for fragmentation reactions of the protonated molecules, which is termed the dissociative proton attachement . However, in some cases, no fragmentation occurs when protonation takes place at the thermodynamically preferred site, but a major reaction is found when the added proton is transferred to a different position, which is described as the dissociative protonation site …”

“…[10][11][12] However, in some cases, no fragmentation occurs when protonation takes place at the thermodynamically preferred site, but a major reaction is found when the added proton is transferred to a different position, which is described as the dissociative protonation site. [13][14][15][16] Dissociative protonation sometimes does not directly form the final products but result in the non-conventional ion-neutral complex (INC). [13,17] INCs are frequently formed as reactive intermediates in the course of unimolecular dissociation reactions in a mass spectrometer.…”