Issue 35, 2012

Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

Abstract

In mass spectrometry of protonated N-phenylcinnamides, the carbonyl oxygen is the thermodynamically most favorable protonation site and the added proton is initially localized on it. Upon collisional activation, the proton transfers from the carbonyl oxygen to the dissociative protonation site at the amide nitrogen atom or the α-carbon atom, leading to the formation of important reactive intermediates. When the amide nitrogen atom is protonated, the amide bond is facile to rupture to form ion/neutral complex 1, [RC6H4CH[double bond, length as m-dash]CHCO+/aniline]. Besides the dissociation of the complex, proton transfer reaction from the α-carbon atom to the nitrogen atom within the complex takes place, leading to the formation of protonated aniline. The presence of electron-withdrawing groups favored the proton transfer reaction, whereas electron-donating groups strongly favored the dissociation (aniline loss). When the proton transfers from the carbonyl oxygen to the α-carbon atom, the cleavage of the Cα–CONHPh bond results in another ion/neutral complex 2, [PhNHCO+/RC6H4CH[double bond, length as m-dash]CH2]. However, in this case, electron-donating groups expedited the proton transfer reaction from the charged to the neutral partner to eliminate phenyl isocyanate. Besides the cleavage of the Cα–CONHPh bond, intramolecular nucleophilic substitution (a nucleophilic attack of the nitrogen atom at the β-carbon) and stepwise proton transfer reactions (two 1,2-H shifts) also take place when the α-carbon atom is protonated, resulting in the loss of ketene and RC6H5, respectively. In addition, the H/D exchanges between the external deuterium and the amide hydrogen, vinyl hydrogens and the hydrogens of the phenyl rings were discovered by D-labeling experiments. Density functional theory-based (DFT) calculations were performed to shed light on the mechanisms for these reactions.

Graphical abstract: Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

Supplementary files

Article information

Article type
Paper
Submitted
24 May 2012
Accepted
11 Jul 2012
First published
13 Jul 2012

Org. Biomol. Chem., 2012,10, 7070-7077

Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

C. Guo, K. Jiang, L. Yue, Z. Xia, X. Wang and Y. Pan, Org. Biomol. Chem., 2012, 10, 7070 DOI: 10.1039/C2OB26011E

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