Polar Acetalization and Transacetalization in the Gas Phase: The Eberlin Reaction

A general, most basic rule for ion dissociation: Protonated molecules

Contrary to the common but potentially misleading belief that when a protonated molecule is excited, it is its most stable protomer that will mandatorily dissociate, we demonstrate herein that, when rationalizing or predicting the chemistry of such ions, we should always search for the most labile protomer. This "most labile protomer" rule, based on the mobile proton model, states therefore that when a protonated molecule is heated, during ionization or by collisions for instance, the loosely bonded proton (H+ ) can acquire enough energy to detach itself from the most basic site of the molecule and then freely "walk through" the molecular framework to eventually find, if available, another protonation site, forming other less stable but more labile protomers, that is, protomers that may display lower dissociation thresholds. To demonstrate the validity of the "most labile protomer" rule as well as the misleading nature of the "most stable protomer" rule, we have selected several illustrative molecules and have collected their ESI(+)-MS/MS. To compare energies of precursors and products, we have also performed PM7 calculations and elaborated potential energy surface diagrams for their possible protomers and dissociation thresholds. We have also applied the "most labile protomer" rule to reinterpret-exclusively via classical charge-induced dissociation cleavages-several dissociation processes proposed for protonated molecules. In an accompanying letter, we have also applied a similar "most labile electromer" rule to ionized molecules.

Kinetic Study of the Maillard Reaction in Thin Film Generated by Microdroplets Deposition

The Maillard reaction kinetics in the confined volume of the thin film produced by ESI microdroplet deposition was studied by mass spectrometry. The almost exclusive formation of the Amadori product from the reaction of D-xylose and D-glucose toward L-glycine and L-lysine was demonstrated. The thin film Maillard reaction occurred at a mild synthetic condition under which the same process in solution was not observed. The comparison of the thin film kinetics with that of the reaction performed in solution showed strong thin film rate acceleration factors.

Accelerated d-Fructose Acid-Catalyzed Reactions in Thin Films Formed by Charged Microdroplets Deposition

Thin films derived by the deposition of charged microdroplets generated in the ESI source of a mass spectrometer act as highly concentrated reaction vessels in which the final products of an ion-molecule reaction can be isolated by their precipitation onto a solid surface under ambient conditions. In this study, the ESI Z-spray source supplied to a Q-TOF Ultima mass spectrometer was used to investigate the d-fructose acid-catalyzed reactions by microdroplets deposition onto a stainless-steel target surface. High conversion ratios of d-fructose into 5-hydroxymethylfuraldehyde (5-HMF), 5-methoxymethylfuraldehyde (5-MMF), and difructrose anhydrides (DFAs) were obtained with HCl and KHSO₄ as metal-free catalysts by using synthetic conditions under which the same products in bulk are not formed. Furthermore, the reaction outcome was found to be highly sensitive to the catalyst and the solvent employed as well as to the ESI source parameters influencing the thin film formation from microdroplets deposition onto the solid surface.

Reaction Acceleration at Solid/Solution Interfaces: Katritzky Reaction Catalyzed by Glass Particles

The Katritzky reaction in bulk solution at room temperature is accelerated significantly by the surface of a glass container compared to a plastic container. Remarkably, the reaction rate is increased by more than two orders of magnitude upon the addition of glass particles with the rate increasing linearly with increasing amounts of glass. A similar phenomenon is observed when glass particles are added to levitated droplets, where large acceleration factors are seen. Evidence shows that glass acts as a "green" heterogeneous catalyst: it participates as a base in the deprotonation step and is recovered unchanged from the reaction mixture. Reaction acceleration at two separate interfaces is recognized in this study: i) air/solution phase acceleration, as is well known in microdroplets; ii) solid/solution phase, where such acceleration appears to be a new phenomenon.

Mass Spectrometry-Based Discovery of New Chemical Scaffold Rearrangement Ions: Aza-biphenylene as a Novel Potent Biradical Agent in Cancer Chemotherapy

Discovery of a new drug is time-consuming, laborious, and expensive. Herein, a novel integrative strategy for discovering potential new lead compounds has been developed, which was based on the characteristics of mass spectrometry (MS). MS was used to predict the potential forced degradation products (DPs) and metabolites of drugs by electrospray ionization and collision-induced dissociation (CID). Special rearrangement ions representing unique predicted DPs and metabolites were identified. The consistency between the predicted and the measured results was proven by in vitro metabolism and forced degradation of a commercial drug, respectively. From this, new chemical scaffold rearrangement ions named (aza)-biphenylenes, as potent anticancer agents, were discovered. As a representative aza-biphenylene analogue, 2-azabiphenylene was proven in vitro to induce apoptosis and inhibit the growth of various human cancer cells in a dose-dependent manner. Surprisingly, 2-azabiphenylene exhibited the best comparable bioactivity with the positive control sorafenib, but showed significantly lower in vitro cytotoxicity than sorafenib (at least a 5-fold decrease in cytotoxicity) because it could be targeted to the tumor microenvironment at low pH. A biradical mechanism accompanied by a mitochondrion-dependent oxidative stress mechanism was proposed to explore its anticancer mechanism. The highly reactive intermediate aza-biphenylenediyl worked as an active pharmaceutical ingredient and induced apoptosis of cancer cells. This provided the basis for the potential applications of CID-induced special rearrangement ions in developing new lead compounds.

Accelerated Reaction Kinetics in Microdroplets: Overview and Recent Developments

Various organic reactions, including important synthetic reactions involving C-C, C-N, and C-O bond formation as well as reactions of biomolecules, are accelerated when the reagents are present in sprayed or levitated microdroplets or in thin films. The reaction rates increase by orders of magnitude with decreasing droplet size or film thickness. The effect is associated with reactions at the solution-air interface. A key factor is partial solvation of the reagents at the interface, which reduces the critical energy for reaction. This phenomenon is of intrinsic interest and potentially of practical value as a simple, rapid method of performing small-scale synthesis.

High-yield gram-scale organic synthesis using accelerated microdroplet/thin film reactions with solvent recycling

A closed system has been designed to perform microdroplet/thin film reactions with solvent recycling capabilities for gram-scale chemical synthesis. Claisen-Schmidt, Schiff base, Katritzky and Suzuki coupling reactions show acceleration factors relative to bulk of 15 to 7700 times in this droplet spray system. These values are much larger than those reported previously for the same reactions in microdroplet/thin film reaction systems. The solvent recycling mode of the new system significantly improves the reaction yield, especially for reactions with smaller reaction acceleration factors. The microdroplet/thin film reaction yield improved on recycling from 33% to 86% and from 32% to 72% for the Katritzky and Suzuki coupling reactions, respectively. The Claisen-Schmidt reaction was chosen to test the capability of this system in gram scale syntheses and rates of 3.18 g per h and an isolated yield of 87% were achieved.

Heterocumulenic carbene nitric oxide radical OCCNO˙

The elusive heterocumulenic radical OCCNO˙ and its isotopologues OC13CNO˙ and OCC15NO˙ have been prepared by reacting photolytically generated unsaturated carbene OCC/OC13C with ˙NO/15˙NO in cryogenic N2-, Ar-, and Ne-matrices. Upon UV-light (365 nm) irradiation, the C-C bond in OCCNO˙ breaks and yields a long-sought ground-state radical CNO˙ (X2Π), which has also been identified with matrix-isolation infrared spectroscopy.

A Simple and Versatile Method for the Formation of Acetals/Ketals Using Trace Conventional Acids

An important and surprising finding that the acetalization and ketalization of aldehydes and ketones with alcohols, respectively, proceed smoothly in the presence of 0.1 mol % acid, without removing water, has been presented. This process has many merits, such as commercial available catalysts with low cost and low loadings (as low as 0.03 mol %), quite a broad substrate scope (including various aldehydes, ketones, acid-sensitive substrates, and diols), a wide range of reaction temperature (-60 to 50 °C), high yields, large-scale preparation, environmental friendliness, and simple work-up procedure. This new protocol has also been successfully applied to protect the important organic compounds, such as 1,3-diols, 1,2-diols, acid-sensitive substrates, glucose, and 1,3-dicarbonyl compounds.

Diastereoselective and regiodivergent oxa-[3 + 2] cycloaddition of Achmatowicz products and cyclic 1,3-dicarbonyl compounds

Development of two novel oxa-[3 + 2] cycloaddition reactions of Achmatowicz products with 1,3-dicarbonyl compounds for rapid and highly efficient assembly of polycyclic furopyranones is described. Plausible mechanisms were proposed.

Evaluation of the α-phenylvinyl cation as a chemical ionization reagent for the differentiation of isomeric substituted phenols in an ITMS

Ion-molecule reactions between the α-phenylvinyl cation and isomeric naturally occurring phenols were investigated using a quadruple ion trap mass spectrometer. The α-phenylvinyl cation m/z 103, generated by chemical ionization from phenylacetylene, reacts with neutral aromatic compounds to form the characteristic species: [M + 103](+) adduct ions and the trans-vinylating product ions [M + 25](+) , which correspond to [M + 103](+) adduct after the loss of benzene. Isomeric differentiation of several ring-substituted phenols was achieved by using collision-induced dissociation of the [M + 103](+) adduct ions. This method also showed to be effective in the differentiation of 4-ethylguaiacol from one of its structural isomers that displays identical EI and EI/MS/MS spectra. The effects of gas-phase alkylation with phenylvinyl cation on the dissociation behavior were examined using mass spectrometry(n) and labeled derivatives. Copyright © 2015 John Wiley & Sons, Ltd.

Mass spectrometric study of the gas-phase difluorocarbene expulsion of polyfluorophenyl cations via F-atom migration

An increasing number of fluorinated drugs, pesticides, and fine chemicals are now produced and applied, especially those containing polyfluorinated aromatic moieties. However, at present, the extent of literature covering the special mass spectrometric behaviors of these compounds remains limited. Herein, we report an unexpected but also general gas-phase dissociation mode of polyfluorinated aromatics in mass spectrometry: expulsion of difluorocarbene (50-Da neutral loss). Results from accurate mass measurements, tandem mass spectrometric experiments, and density functional theory (DFT) calculations support an intramolecular F-atom “ring-walk” migration mechanism for gas-phase CF2 loss. Based on an assessment of the electron ionization-mass spectrometry (EI-MS) data of more than 40 polyfluorinated aromatic compounds from the National Institute of Standards and Technology data bank, we generalized on the substitution group effects on the difluorocarbene dissociation process of polyfluorinated aromatic compounds in EI-MS. These studies have enriched our knowledge of the special gas-phase reactivity of polyfluorinated aromatics and will provide valuable information in further analytical research of these compounds by mass spectrometry.

Ion-molecule reactions for the differentiation of primary, secondary and tertiary hydroxyl functionalities in protonated analytes in a tandem mass spectrometer

A mass spectrometric method utilizing gas-phase ion-molecule reactions of 1-butanethiol and di-tert-butyl peroxide has been developed for the differentiation of primary, secondary and tertiary hydroxyl functionalities in protonated analytes in a FT-ICR mass spectrometer.

Study of the gas-phase intramolecular aryltrifluoromethylation of phenyl(trifluoromethyl)iodonium by ESI-MS/MS

The gas-phase reactions of the reactive λ(3)-phenyl(trifluoromethyl)iodonium (PhI(+)(III)CF3, 1 at m/z 273) to the radical cation of iodobenzene (PhI(•+), 2 at m/z 204) via the loss of ·CF3 and the radical cation of trifluoromethylbenzene (PhCF3(•+), 3 at m/z 146) via the loss of ·I, were studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Interestingly, the gas-phase intramolecular coupling reaction of CF3 with phenyl via the CF3 migration process of 1 at m/z 273 from iodine to the phenyl to give 3 at m/z 146 could only occur according to an intramolecular aromatic substitution mechanism. Density functional theory (DFT) calculations showed that the gas-phase intramolecular aryltrifluoromethylation of 1 at m/z 273 to 3 at m/z 146 occurred via a Meisenheimer complex intermediate (MC), where the triplevalent I center of 1 was reduced to monovalent I. Most importantly, the structure of 3 at m/z 146 derived from 1 at m/z 273 in ESI-MS/MS process was confirmed by comparison of its MS/MS with that of an authentic PhCF3(•+) at m/z 146 acquired from the electron ionization (EI)-MS/MS analysis of PhCF3. Thus, our studies revealed the intrinsic reactivity tendencies of λ(3)-phenyl(trifluoromethyl)iodonium under solvent-free conditions.

Ambient ion/molecule reactions in low-temperature plasmas (LTP): reactive LTP mass spectrometry

RATIONALE

Ion/molecule reactions are commonly used to characterize structures due to their high specificity. Herein, we present ambient ion/molecule reactions performed in the course of low-temperature plasma (LTP) ionization of condensed-phase analytes in order to increase the specificity of LTP-based ambient analysis.

METHODS

The ion population of the cold plasma is modified by addition of a reagent to the plasma before it is directed at a surface bearing the analyte. Desorbed ions were analyzed using linear ion trap-Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS).

RESULTS

Acylium ions generated from tetramethylurea react with 1,3-dioxane analyte to afford distinctive Eberlin product ions. Reactions of alkylamines, such as n-hexylamine and n-octylamine, with benzaldehyde produce the corresponding imines. Reaction of ruthenocene with trifluoroacetic anhydride forms the unusual trifluoroacetate ruthenocene.

CONCLUSIONS

A LTP source can be used to generate reagent ions that can undergo ion/molecule reactions in the ambient environment with an analyte at condensed phase on a surface. The experiment is a 'reactive' version of the standard low-temperature plasma (LTP) ambient ionization experiment. This approach provides additional information by combining ion/molecule chemistry with conventional MS and MS/MS data to characterize particular analytes.

Probing the reactivity and radical nature of oxidized transition metal-thiolate complexes by mass spectrometry

Transition metal thiolate complexes such as [PPN](+)[RuL3](-) (PPN = bis(triphenylphosphoranylidene) ammonium and L = diphenylphosphinobenzenethiolate) are known to undergo addition reactions with unsaturated hydrocarbons via the formation of new C-S bonds in solution upon oxidation. The reaction mechanism is proposed to involve metal-stabilized thiyl radical intermediates, a new type of distonic ions such as [RuL3](+) ion in the case of [PPN](+)[RuL3](-). This study presents the reactivity and structure investigation of [RuL3](+) by mass spectrometry (MS) in conjunction with ion/molecule reactions. The addition reactions of [RuL3](+) with alkenes or methyl ketones in the gas phase are indeed observed, in agreement with the proposed mechanism. Such reactivity is also maintained by several fragment ions of [RuL3](+), indicating the preserved thiyl diradical core structure is responsible for the addition reaction. The thiyl radical nature of [RuL3](+) was further verified by the ion/molecule reaction of [RuL3](+) with dimethyl disulfide, in which the characteristic CH3S• transfer occurs, both at atmospheric pressure and also at low pressure (~mTorr). These results provide, for the first time, clear mass spectrometric evidence of the radical nature of [RuL3](+) (i.e., the distonic ion structure of [RuL3](+)), arising from the oxidation of non-innocent thiolate ligands of the complex [PPN](+)[RuL3](-). Similar thiolate complexes, including ReL3 and NiL2, were also examined. Although reactions of oxidized ReL3 or NiL2 with CH3SSCH3 take place at atmospheric pressure, the corresponding reaction did not occur in vacuum. Consistent with these data, the addition of ethylene was not observed either, indicating lower reactivities of [ReL3](+) and [NiL2](+) in comparison to [RuL3](+).

Exploring the intrinsic polar [4+2(+)] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions

Gas-phase reactions of model carbosulfonium ions (CH(3)-S(+)=CH(2;) CH(3)CH(2)-S(+)=CH(2) and Ph-S(+)=CH(2)) and an O-analogue carboxonium ion (CH(3)-O(+)=CH(2)) with acyclic (isoprene, 1,3-butadiene, methyl vinyl ketone) and cyclic (1,3-cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6-311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4+2(+)] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH(+) transfer) products. In great contrast to its S-analogues, CH(3)-O(+)=CH(2) (as well as C(2)H(5)-O(+)=CH(2) and Ph-O(+)=CH(2) in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH(2)=CH-O(+)=CH(2) ion forms an abundant [4+2(+)] cycloadduct with isoprene, but similar to the behavior of such α,β-unsaturated carboxonium ions in solution, seems to occur across the C=C bond.

Comparison of functional group selective ion-molecule reactions of trimethyl borate in different ion trap mass spectrometers

We report here a comparison of the use of diagnostic ion-molecule reactions for the identification of oxygen-containing functional groups in Fourier-transform ion cyclotron resonance (FTICR) and linear quadrupole ion trap (LQIT) mass spectrometers. The ultimate goal of this research is to be able to identify functionalities in previously unknown analytes by using many different types of mass spectrometers. Previous work has focused on the reactions of various boron reagents with protonated oxygen-containing analytes in FTICR mass spectrometers. By using a LQIT modified to allow the introduction of neutral reagents into the helium buffer gas, this methodology has been successfully implemented to this type of an ion trap instrument. The products obtained from the reactions of trimethyl borate (TMB) with various protonated analytes are compared for the two instruments. Finally, the ability to integrate these reactions into LC-MS experiments on the LQIT is demonstrated.

Investigation of coordination of Mg(II) cations to 2-pyrimidinyloxy-N-arylbenzylamines by electrospray mass spectrometry: insights for Mg(II) catalyzed Smiles rearrangement reactions

The CH(3)OH solutions of pyrimidinyloxy-N-arylbenzylamines (1-5) in the presence of Mg(II)X(2) salts (X = Cl or ClO(4)) were investigated by electrospray ionization mass spectrometry and tandem mass spectrometry (MS/MS) subsequently, showing that the cationic Mg(II) complexes 1-5·MgX(+) were important active complexes or intermediates for initiating interesting Smiles rearrangement reactions in both the gas and solution phases. By using different MgX(2) salts and selecting a set of reactants with different substitutes, the role of the counter-ion (X(-)) and the structure effect of the reactants on the Mg(II) catalyzed Smiles rearrangement reactions were studied. Moreover, the solvent effect on Mg(II) catalyzed Smiles rearrangement reactions was revealed by studying the CH(3)OH adduct complexes of 1-5·MgCl(+), which showed that the coordination of CH(3)OH to the Mg(II) center in the complexes decreased the reaction tendency. The mechanisms involved in the gas-phase Mg(II) catalyzed Smiles rearrangement reactions were proposed on the basis of MS/MS experiments and theoretical computations, showing some unique chemistries initiated by introducing Mg(II) into the template molecules.

Experimental cross-sections energy dependence and an ab initio electronic structure survey of the ground singlet potential surface for reactive Li(+) + n-C(3)H(7)Cl collisions at low energies

Reactive collisions between n-C(3)H(7)Cl molecules and lithium ions both in their ground electronic state have been studied in the 0.05-7.00 eV center of mass energy range using an octopole radio frequency guided-ion beam apparatus developed in our laboratory and recently modified. At low collision energies, dehydrohalogenation reactions leading to Li(C(3)H(6))(+) and Li(HCl)(+) are the main reaction channels, while on increasing energies C(3)H(7)(+) and C(2)H(3)(+) formation become dominant. Cross section energy dependences in arbitrary units for all these reactions have been measured. Also, ab initio electronic structure calculations at the MP2 level have been performed to obtain information about the potential energy surface on which the reactive processes take place. The reactants' entrance channel leads to the formation of a stable [Li-n-C(3)H(7)Cl](+) ion-molecule adduct that, following an intrinsic-reaction-coordinate pathway and surmounting a transition state, isomerizes to [Li-i-C(3)H(7)Cl](+). From this second minimum, dehydrohalogenation reactions for both n-C(3)H(7)Cl and i-C(3)H(7)Cl share a common reaction pathway leading to the same products. All potential barriers explored by reactions always lie below the reactants' energy. The entrance reaction channel [Li-n-C(3)H(7)Cl](+) adduct also leads adiabatically to C(3)H(7)(+) formation which, on increasing collision energy generates C(2)H(3)(+)via a unimolecular decomposition. A qualitative interpretation of the experimental results based on our ab initio calculations is also given.

Gas-phase meerwein reaction of epoxides with protonated acetonitrile generated by atmospheric pressure ionizations

Ethylnitrilium ion can be generated by protonation of acetonitrile (when used as the LC-MS mobile phase) under the conditions of atmospheric pressure ionizations, including electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) as well as atmospheric pressure photoionization (APPI). Ethylnitrilium ion (CH(3)-C≡N+H and its canonical form CH(3)-C+=NH) is shown to efficiently undergo the gas-phase Meerwein reaction with epoxides. This reaction proceeds by the initial formation of an oxonium ion followed by three-to-five-membered ring expansion via an intramolecular nucleophilic attack to yield the Meerwein reaction products. The density functional theory (DFT) calculations at the B3LYP/6-311+G(d,p) level show that the gas-phase Meerwein reaction is thermodynamically favorable. Collision-induced dissociation (CID) of the Meerwein reaction products yields the net oxygen-by-nitrogen replacement of epoxides with a characteristic mass shift of 1 Da, providing evidence for the cyclic nature of the gas-phase Meerwein reaction products. The gas-phase Meerwein reaction offers a novel and fast LC-MS approach for the direct analysis of epoxides that might be of genotoxic concern during drug development. Understanding and utilizing this unique gas-phase ion/molecule reaction, the sensitivity and selectivity for quantitation of epoxides can be enhanced.

Dimerization of ionized 4-(methyl mercapto)-phenol during ESI, APCI and APPI mass spectrometry

A novel ion/molecule reaction was observed to occur under electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo ionization (APPI) conditions, leading to dimerization of ionized 4-(methyl mercapto)-phenol followed by fast H(*) loss. The reaction is particularly favored during ESI, which suggests that this ion/molecule reaction can occur both in the solution inside the ESI-charged droplets and in the gas-phase environment of most other atmospheric pressure ionization techniques. The dimerization reaction is inherent to the electrolytic process during ESI, whereas it is more by ion/molecule chemistry in nature during APCI and APPI. From the tandem mass spectrometry (MS/MS) data, accurate mass measurements, hydrogen/deuterium (H/D) exchange experiments and density functional theory (DFT) calculations, two methyl sulfonium ions appear to be the most likely products of this electrophilic aromatic substitution reaction. The possible occurrence of this unexpected reaction complicates mass spectral data interpretation and can be misleading in terms of structural assignment as reported herein for 4-(methyl mercapto)-phenol.

An ion/molecule reaction for the identification of analytes with two basic functional groups

A mass spectrometric method is presented for the identification of analytes with two basic functionalities and PA between 222 and 245 kcal/mol, including diamines. This method utilizes gas-phase ion-molecule reactions of protonated analytes with neutral 1,1-diethoxyethene (DEE) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR). A variety of protonated mono-, bi-, and trifunctional analytes containing different functional groups, namely, amido, amino, N-oxide, hydroxy, carboxylic acid, keto, thio, thioether, alkene, phosphite, and phosphonate, were tested in the FT-ICR. The results demonstrate that basic protonated bifunctional compounds (PA between 222 and 245 kcal/mol) react selectively with DEE by forming a specific addition/elimination product ion (adduct - EtOH) (this product was also observed for lysine with three functionalities). The diagnostic reaction sequence involves proton transfer from the protonated analyte to the basic vinyl group in DEE, followed by addition of one of the functional groups of the analyte to the electrophilic alpha-carbon in protonated DEE. The next step involves proton transfer from this functionality to the other analyte functionality, followed by proton transfer to DEE and elimination of ethanol. Since the mechanism involves proton transfer between two functional groups of the analyte, the reaction does not occur for analytes where the two functionalities cannot be in close proximity (i.e., meta-phenylenediamine), and where no proton is available (i.e., dimethylaminoketone).

Electron impact ion fragmentation pathways of peracetylated C-glycoside ketones derived from cyclic 1,3-diketones

Monosaccharide C-glycoside ketones have been synthesized by aqueous-based Knoevenagel condensation of isotopically labeled and unlabeled aldoses with cyclic diketones, 5,5-dimethyl-1,3-cyclohexanedione (dimedone) and 1,3-cyclohexanedione (1,3-CHD). The reaction products and their corresponding acetylated analogs produce characteristic molecular adduct ions by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS). Analysis of the peracetylated C-glycosides by electron ionization (EI) gas chromatography/mass spectrometry (GC/MS) revealed diagnostic fragment ions that have been used to deduce the EI fragmentation pathways and the structure of each C-glycoside ketone. Characteristic gluco- and ribo-specific ions were observed at m/z 350 and 278, respectively. Ions common to both carbohydrate fragmentation pathways were observed at m/z 193 and 169 for the dimedone-C-glycosides, and m/z 165 and 141 for the 1,3-CHD-C-glycosides. Ions with m/z 169 and 141 retain the anomeric carbon (carbon-1) of the original sugar, while m/z 193 and 165 are shown to retain carbons-1, 2, and 3.

Recognition and resolution of isomeric alkyl anilines by mass spectrometry

Two MS techniques have been used to recognize and resolve a representative isomeric pair of N-alkyl and ring-alkyl substituted anilines. The first technique (1) uses MS/MS to perform ion/molecule reactions of structurally-diagnostic fragment ions (SDFI) whereas the second (2) uses traveling wave ion mobility spectrometry (TWIMS) of the pair of protonated molecules followed by on-line collision-induced dissociation (CID), that is, MS/TWIMS-CID/MS. Isomeric C(7)H(7)N(+) ions of m/z 106 (1' from 4-butylaniline and 2 from N-butylaniline) are formed as abundant fragments by 70 eV EI of the anilines, and found to function as suitable SDFI. Ions 1' and 2 display nearly identical unimolecular dissociation chemistry, but contrasting bimolecular reactivity with ethyl vinyl ether, isoprene, acrolein, and 2-methyl-1,3-dioxolane. Ion 2 forms adducts to a large extent whereas 1' is nearly inert towards all reactants tested. The intact protonated anilines are readily resolved and recognized by MS/TWIMS-CID/MS in a SYNAPT mass spectrometer (Waters Corporation, Manchester, UK). The protonated N-butyl aniline (the more compact isomer) displays shorter drift time and higher lability towards CID than its 4-butyl isomer. The general application of SDFI 1' and 2 and other homologous and analogous ions and MS/TWIMS-CID/MS for absolute recognition and resolution of isomeric families of N-alkyl and ring-alkyl mono-substituted anilines and analogues is discussed.

Electrophilic aromatic substitution and single-electron transfer (SET) by the phenylium ion in the gas phase: characterization of a long-lived SET intermediate

Gas-phase mass spectrometric studies and calculations were performed for the reaction of naked phenylium ion with several benzene halides. From these reactions, the molecular ion for biphenyl as the predominant product was obtained only from the reaction of phenylium ions with iodobenzene and bromobenzene. Furthermore, through the collision-induced dissociation (CID) of the ion at m/z 281, the only dissociation observed is the loss of a phenyl radical, which indicates that a single-electron transfer (SET) mechanism might have occurred within the reaction. Additionally, according to the comparison between the CID experiments of those isomeric compounds of the sigma-complexes and the CID experiment of the ion at m/z 281 captured in the ion trap, we have also defined the captured ion at m/z 281 as an SET-intimate ion pair rather than those of sigma-complexes or the diphenyliodonium.

Alternative reagents for chemical noise reduction in liquid chromatography-mass spectrometry using selective ion-molecule reactions

Reduction of ionic chemical background noise based on selective gas-phase reactions with chosen neutral reagents has been proven to be a very promising approach in liquid chromatography-mass spectrometry (LC-MS). In this study further investigations on alternative reagents including the disulfides (dimethyl disulfide, diethyl disulfide, methyl propyl disulfide), dimethyl trisulfide, ethylene oxide, and butadiene monoxide, for example, have been carried out. Tandem mass spectrometric studies of ion/molecule reactions indicate that-besides dimethyl disulfide-ethylene oxide and butadiene monoxide also exhibit very efficient reactions with background ions. Furthermore, it is confirmed that the reactions are very selective according to the test with some analyte ions. In contrast to its rapid reactions with background ions, ethylene oxide does not react, or reacts much less, with these analytes. Therefore, it can be used as an alternative reagent for noise reduction. Although reactions of the other tested neutral reagents with background ions are evaluated, they are generally not suitable as reagents for this purpose because of lack of reactivity or dramatic ion losses during reactions.

Recognizing alpha-, beta- or gamma-substitution in pyridines by mass spectrometry

A general mass spectrometric method able to recognize the site of substitution of monosubstituted pyridines is described. The method requires that the molecule under investigation forms, upon ionization and dissociation, the respective alpha-, beta- or gamma- pyridinium ion of m/z 78. Pyridinium ions are stable and common fragments of ionized and protonated pyridines and are found to function as appropriate structurally diagnostic fragment ions. They can be identified by their characteristic and nearly identical collision-induced dissociation behavior and distinguished by the combined use of two structurally diagnostic ion/molecule reactions with acetonitrile and 2-methyl-1,3-dioxolane. alpha-, beta- or gamma-substitution in pyridines can, therefore, be securely recognized via an MS-only method based on structurally diagnostic ions and by the inspection of a single molecule (no need for intracomparisons within the whole set of isomers).

Tandem mass spectrometry and hydrogen/deuterium exchange studies of protonated species of 1,1'-bis(diphenylphosphino)-ferrocene oxidative impurity generated during a Heck reaction

Oxidation of 1,1'-bis(diphenylphosphino)-ferrocene (DPPF) was found to occur when it served as the ligand for Pd(II)(CH3COO)2 in a Heck reaction. This oxidative impurity of DPPF, referred to as DPPF(O), was identified by high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) and exact mass measurements. Protonated DPPF(O) exhibited unique fragmentation pathways in the gas phase. Hydrogen/deuterium (H/D) exchange experiments provided important insights into the dissociation mechanisms of protonated DPPF(O), suggesting the existence of isomeric structures of the product ions by retaining or losing a proton (or deuteron) upon collision-induced dissociation (CID). The specific fate of the proton (or deuteron) upon CID is postulated to be dependent on the distance between the exchangeable proton (or deuteron) and the sites of bond cleavage. Density functional theory (DFT) calculations at the B3LYP/LANL2DZ level of theory showed that oxygen in DPPF(O) plays a pivotal role in invoking pi-cation interactions between the p-type lone pair electrons (n pi) in oxygen and the anti-bonding orbital of Fe(II), accounting for the major fragmentation pathways of protonated DPPF(O). Facile formation of organometallic distonic ions in dissociation of protonated DPPF(O), and especially of protonated DPPF, could be useful for further exploration of their chemical properties by gas-phase ion/molecule reactions.