Single electron redox via an ion-neutral complex in the fragmentation of protonated benzoylferrocenes

Proton-bound complex mediating retro-Michael-type fragmentation of protonated 3-substituted oxindoles in the Orbitrap high-energy collisional dissociation cell

RATIONALE

Oxindole derivatives are valuable building blocks for indole chemistry. Systematically exploring the fragmentation behavior of the protonated 3-pyrazole-substituted oxindoles by kinetic methods combined with density functional theory (DFT) calculations is useful for further understanding their basic properties, and might provide some insights into their reactivity trends in synthesis and metabolism.

METHODS

All high-resolution high-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) experiments were carried out using electrospray ionization hybrid Quadrupole-Orbitrap mass spectrometry in positive ion mode. Theoretical calculations were carried out by the DFT method at the B3LYP level with the 6-311G (d, p) basis set in the Gaussian 03 package of programs.

RESULTS

In the fragmentation of protonated 3-pyrazole-substituted oxindoles, the characterized protonated 3-(3-methyl-5-oxo-1H-pyrazol-4(5H)-ylidene)indolin-2-one derivatives and the protonated 5-methylpyrazolone were observed, which were proposed from the cleavage of the C(β)-C(γ) bond in a retro-Michael reaction. With the kinetic plot, a linear correlation was established between the intensities of this two competitive product ions and the difference in proton affinities of the corresponding neutral molecules, which demonstrated that the retro-Michael reaction was mediated by a proton-bound complex.

CONCLUSIONS

Using the kinetic method combined with theoretical calculations, a proton-bound complex mediating retro-Michael reaction was proposed for the fragmentation of protonated 3-pyrazole-substituted oxindoles in the high-energy collisional dissociation tandem mass spectrometry for the first time, which provided potential evidence to further understand their intrinsic bioactivities.

Fragmentation reactions of N-benzyltetrahydroquinolines in electrospray ionization mass spectrometry: the roles of ion/neutral complex intermediates

RATIONALE

Electrospray ionization mass spectrometry (ESI-MS) combined with the collision-induced dissociation (CID) technique has assumed increasing importance as an invaluable tool for the structural analysis of organic and biological molecules. However, general rules for elucidating the fragmentation behaviors of charged molecules in the gas phase are still lacking. Therefore, explorations on the mechanistic information are desirable at all times.

METHODS

CID experiments of protonated N-benzyltetrahydroquinolines were carried out on ESI ion trap mass spectrometer and accurate mass measurements were performed on a high-resolution ESI quadrupole time-of-flight (Q-TOF) mass spectrometer in positive ion mode.

RESULTS

An ion/neutral complex, [RC6H4CH2(+)/tetrahydroquinoline], resulting from cleavage of the C-N bond induced by the positive charge brought in by protonation, was proposed to be the intermediate to elucidate the fragmentation reactions. For all the compounds investigated, benzyl cation transfer, electron transfer and hydride transfer reactions mediated by the complex were observed. Moreover, for the compound substituted by a methyl group at the para-position of the benzylic phenyl ring, proton transfer reaction via the complex also occurs.

CONCLUSIONS

This study is a case for better understanding the intriguing roles of ion/neutral complexes in gas-phase fragmentation reactions and enriching the knowledge about the gas-phase chemistry of the benzyl cation. In addition, it provides useful information for researchers working on analysis or structural elucidation of complicated compounds which contain the N-benzyltetrahydroquinoline substructure.

Identifying the proton transfer reaction mechanism via a proton-bound dimeric intermediate for esomeprazoles by a kinetic method combined with density functional theory calculations

RATIONALE

Esomeprazole analogs are a class of important proton pump inhibitors for the treatment of gastro-esophageal reflux diseases. Understanding the fragmentation reaction mechanism of the protonated esomeprazole analogs will facilitate the characterization of their complex metabolic fate in humans. In this paper, the kinetic method and theoretical calculations were applied to evaluate the fragmentation of protonated esomeprazole analogs.

METHODS

All collision-induced dissociation (CID) mass spectrometry experiments were carried out using electrospray ionization (ESI) ion trap mass spectrometry in positive ion mode. Also the accurate masses of fragments were measured on by ESI quadrupole time-of-flight (QTOF) MS in positive ion mode. Theoretical calculations were carried out by the density functional theory (DFT) method with the 6-31G(d) basis set in the Gaussian 03 program.

RESULTS

In the fragmentation of the protonated esomeprazole analogs, C-S bond breakage is observed, which gives rise to protonated 2-(sulfinylmethylene)pyridines and protonated benzimidazoles. DFT calculations demonstrate that the nitrogen atom of the pyridine part is the thermodynamically most favorable protonation site, and the C-S bond cleavage is triggered by the transfer of this ionizing proton from the nitrogen atom of the pyridine part to the carbon atom of the benzimidazole part to which the sulfinyl is attached. Moreover, with the kinetic plot, the intensity ratios of two protonated product ions yield a linear relationship with the differences in proton affinities of the corresponding neutral molecules, which provides strong experimental evidence that the reaction proceeds via proton-bound 2-(sulfinylmethylene)pyridine/benzimidazole complex intermediates.

CONCLUSIONS

The kinetic method combined with theoretical calculations was successfully applied to probe the proton transfer reaction by proton-bound 2-(sulfinylmethylene)pyridine/benzimidazole complexes in the fragmentation of protonated esomeprazole analogs by ESI CID MS, which is a strong evidence that the kinetic method can be applied in identifying a proton-bound dimeric intermediate in the fragmentation of protonated ions.