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Benzoate substituted with a cationic quaternary ammonium group at the para‐position, [4‐(CH3)3N(C6H4)CO2], forms mononuclear alkali metal ion complexes, [4‐(CH3)3N(C6H4)CO2M]+ (M=Li, Na, K, Rb and Cs), which are observed by electrospray‐ionisation mass spectrometry. When mass selected and subjected to collision‐induced dissociation each of these complexes undergoes decarboxylation to produce the mononuclear arylalkali complex cations, [4‐(CH3)3N(C6H4)M]+, which subsequently react with water via hydrolysis to yield the quaternary ammonium cation, (CH3)3NC6H5+. The unexpected product ions, [M((CH3)2NC6H5)]+, [M(CH3OH)]+, and M+, which are associated with the hydrolysis pathway, suggest an unusual reaction occurring within the exit channel ion‐molecule complex, [(CH3)3NC6H5+MOH]+. DFT calculations were used to examine the: decarboxylation reaction of [4‐(CH3)3N(C6H4)CO2M]+, which readily proceeds via a four‐centred transition state; the hydrolysis and SN2 reactions accounting for formation of products in the exit channel, which are connected via a roaming mechanism in which the metal hydroxide moiety migrates from the para‐hydrogen of (CH3)3NC6H5+ to one of the methyl groups. [ABSTRACT FROM AUTHOR]

Rationale: There are a growing number of examples of protomers formed via electrospray ionization (ESI) that do not fragment under mobile proton conditions, giving rise to distinct tandem mass spectra. To model the N‐protomer of 4‐aminobenzoic acid, here we study the gas‐phase unimolecular and bimolecular chemistry of the 4‐(carboxyphenyl)trimethylammonium ion. Methods: 4‐(Carboxyphenyl)trimethylammonium iodide was synthesized, purified via recrystallization and transferred to the gas phase via ESI. 4‐(Carboxyphenyl)trimethylammonium ion, 7, was mass selected and subjected to collision‐induced dissociation and ion–molecule reactions in a linear ion trap mass spectrometer. Results: The major fragmentation channel for the fixed‐charge cation 7 is methyl radical loss, whereas loss of trimethylamine and CO2 represents minor pathways. The free carboxylic acid functional group of 7 is unreactive toward a number of neutral reagents (methanol, acetone, acetonitrile, and N,N′‐diisopropylcarbodiimide). 7 reacts very slowly with trimethylborate via addition‐elimination, consistent with density functional theory (DFT) calculations that show this reaction is slightly endothermic. The deuterated cation 7(D) undergoes slow D/H exchange with ethanol, and DFT calculations reveal that a flip‐flop mechanism operates. Conclusions: The free carboxylic group of 7 is not very reactive toward neutral reagents in the gas phase. [ABSTRACT FROM AUTHOR]

Multistage mass spectrometry (MSn) experiments were used to explore extrusion-insertion (ExIn) reactions of the palladium complex [(phen)Pd(O2CPh)]+ (phen, 1,10-phenanthroline). Under collision-induced dissociation (CID) conditions, the organopalladium cation [(phen)Pd (Ph)]+ was formed via decarboxylation and was found to react with phenylmethylketene to yield the enolate [(phen)Pd(CPhMeC(O)Ph)]+ via an insertion reaction. A further stage of CID revealed that the enolate fragments via loss of styrene to form the acyl complex [(phen)Pd (C(O)Ph)]+. Formation of both the coordinated enolate and acyl anions is supported by density functional theory (DFT) calculations. Attempts to develop a palladium-mediated one-pot synthesis of ketones from 2,6-dimethoxybenzoic acid as the key substrate and the ketene substrates R¹R²"C"O (R¹ = Ph, R² = Me; R¹ = R² = Ph) proved challenging owing to low yields and side product formation. [ABSTRACT FROM AUTHOR]

Multistage mass spectrometry experiments, isotope labelling and DFT calculations were used to explore whether selective decarbonylation of formic acid could be mediated by molybdate anions [(MoO3)x(OH)]− (x = 1 and 2) via a formal catalytic cycle involving two steps. In step 1, both molybdate anions undergo gas-phase ion-molecule reactions (IMR) with formic acid to produce the coordinated formates [(MoO3)x(O2CH)]− and H2O. In step 2, both coordinated formates [(MoO3)x(O2CH)]− undergo decarbonylation under collision-induced dissociation (CID) conditions to reform the molybdate anions [(MoO3)x(OH)]− (x = 1 and 2), thus closing a formal catalytic cycle. In the case of [MoO3(O2CH)]− an additional decarboxylation channel also occurs to yield [MoO3(H)]−, which is unreactive towards formic acid. The reaction between [Mo18O3(18OH)]− and formic acid gives rise to [Mo18O3(O2CH)]− highlighting that ligand substitution occurs without 18O/16O exchange between the coordinated 18OH ligand and HC16O2H. The reaction between [(MoO3)x(OD)]− (x = 1 and 2) and DCO2H initially produces [(MoO3)x(OH)]− (x = 1 and 2), indicating that D/H exchange occurs. DFT calculations were carried out to investigate the reaction mechanisms and energetics associated with both steps of the formal catalytic cycle and to better understand the competition between decarbonylation and decarboxylation, which is crucial in developing a selective catalyst. The CO and CO2 loss channels from the monomolybdate anion [MoO3(O2CH)]− have similar barrier heights which is in agreement with experimental results where both fragmentation channels are observed. In contrast, the dimolybdate anion is more selective, since the decarbonylation pathway of [(MoO3)2(O2CH)]− is both kinetically and thermodynamically favoured, which agrees with experimental observations where the CO loss channel is solely observed. [ABSTRACT FROM AUTHOR]

Nitroimidazoles are important compounds in medicine, biology, and the food industry. The growing need for their structural assignment, as well as the need for the development of the detection and screening methods, provides the motivation to understand their fundamental properties and reactivity. Here, we investigated the decomposition of protonated ronidazole [Roni+H]+ in low-energy and high-energy collision-induced dissociation (CID) experiments. Quantum chemical calculations showed that the main fragmentation channels involve intramolecular proton transfer from nitroimidazole to its side chain followed by a release of NH2CO2H, which can proceed via two pathways involving transfer of H+ from (1) the N3 position via a barrier of TS2 of 0.97 eV, followed by the rupture of the C–O bond with a thermodynamic threshold of 2.40 eV; and (2) the –CH3 group via a higher barrier of 2.77 eV, but with a slightly lower thermodynamic threshold of 2.24 eV. Electrospray ionization of ronidazole using deuterated solvents showed that in low-energy CID, only pathway (1) proceeds, and in high-energy CID, both channels proceed with contributions of 81% and 19%. While both of the pathways are associated with small kinetic energy release of 10–23 meV, further release of the NO• radical has a KER value of 339 meV. [ABSTRACT FROM AUTHOR]

The gas-phase extrusion–insertion (ExIn) reactions of the palladium complexes [(phen) n Pd(O2SC6H5)]+ (phen = 1,10-phenanthroline, n = 1 or 2), were investigated in the gas phase by multistage mass spectrometry (MS n ) experiments consisting of electrospray ionisation and a linear ion trap combined with density functional theory (DFT) calculations. Desulfination of palladium sulfinate cations under collision-induced dissociation (CID) generates the organopalladium intermediates [(phen) n Pd(C6H5)]+. Of these two organometallic cations, only [(phen)Pd(C6H5)]+ reacts with phenyl isocyanate via insertion to yield [(phen)Pd(NPhC(O)C6H5)]+. The formation of a coordinated amidate anion is supported by DFT calculations. In exploring this reactivity in the solution phase, we found that heating a mixture of benzenesulfinic acid, phenylisocyanate and palladium trifluoroacetate under a range of different conditions (ligand free versus with ligand, different solvents, addition of acid or base) failed to lead to the formation N -phenyl-benzamide in all cases. Instead, biphenyl was formed and could be isolated in a yield of 46%. DFT calculations using a solvent continuum reveal that the barrier associated with the insertion reaction lies above the competing sequential reactions of desulfination of a second phenyl sulfinate followed by reductive elimination of biphenyl. While gas-phase studies suggest that both palladium-mediated extrusion–insertion (ExIn) and condensation reactions operate for the generation of a N -phenyl-benzamide anion from the phenylsulfinate anion, a double desulfinative biaryl homocoupling reaction is favoured in the solution phase. [ABSTRACT FROM AUTHOR]