title: 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, [RC(6)H(4)CH=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 C(α)-CONHPh bond results in another ion/neutral complex 2, [PhNHCO(+)/RC(6)H(4)CH=CH(2)]. 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 C(α)-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 RC(6)H(5), 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.
authors.affiliation: Department of Chemistry, Zhejiang University, Hangzhou 310027, China.
authors.forename: Cheng
authors.initials: C
authors.lastname: Guo
authors.forename: Kezhi
authors.initials: K
authors.lastname: Jiang
authors.forename: Lei
authors.initials: L
authors.lastname: Yue
authors.forename: Ziming
authors.initials: Z
authors.lastname: Xia
authors.forename: Xiaoxia
authors.initials: X
authors.lastname: Wang
authors.forename: Yuanjiang
authors.initials: Y
authors.lastname: Pan
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry; D000970:Antineoplastic Agents / Q000737:chemistry*; D005030:Ethylenes / Q000737:chemistry; D017953:Isocyanates / Q000737:chemistry; D007659:Ketones / Q000737:chemistry; D008958:Models, Molecular; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization; D053719:Tandem Mass Spectrometry; D013816:Thermodynamics; D020032:Tyrphostins / Q000737:chemistry*
chemical_list: D000814:Aniline Compounds; D000970:Antineoplastic Agents; D005030:Ethylenes; D017953:Isocyanates; D007659:Ketones; D011522:Protons; D020032:Tyrphostins; C025319:phenyl isocyanate; C008223:ketene; C023650:aniline