Transacetalization with gaseous carboxonium and carbosulfonium ions

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.

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.

Gas-phase reactivity of protonated 2-oxazoline derivatives: mass spectrometry and computational studies

RATIONALE

Oxazolines have attracted the attention of researchers worldwide due to their versatility as carboxylic acid protecting groups, chiral auxiliaries, and ligands for asymmetric catalysis. Electrospray ionization tandem mass spectrometric (ESI-MS/MS) analysis of five 2-oxazoline derivatives has been conducted, in order to understand the influence of the side chain on the gas-phase dissociation of these protonated compounds under collision-induced dissociation (CID) conditions.

METHODS

Mass spectrometric analyses were conducted in a quadrupole time-of-flight (Q-TOF) spectrometer fitted with electrospray ionization source. Protonation sites have been proposed on the basis of the gas-phase basicity, proton affinity, atomic charges, and a molecular electrostatic potential map obtained on the basis of the quantum chemistry calculations at the B3LYP/6-31 + G(d,p) and G2(MP2) levels.

RESULTS

Analysis of the atomic charges, gas-phase basicity and proton affinities values indicates that the nitrogen atom is a possible proton acceptor site. On the basis of these results, two main fragmentation processes have been suggested: one taking place via neutral elimination of the oxazoline moiety (99 u) and another occurring by sequential elimination of neutral fragments with 72 u and 27 u. These processes should lead to formation of R(+).

CONCLUSIONS

The ESI-MS/MS experiments have shown that the side chain could affect the dissociation mechanism of protonated 2-oxazoline derivatives. For the compound that exhibits a hydroxyl at the lateral chain, water loss has been suggested to happen through an E2-type elimination, in an exothermic step.

Absolute assignment of constitutional isomers via structurally diagnostic fragment ions: the challenging case of α- and β-acyl naphthalenes

A general mass spectrometric method is described for the absolute assignment of α- or β-acyl naphthalenes, via which the gaseous α- and β-naphthoyl cations of m/z 155 are used as structurally diagnostic fragment ions (SDFI). These stable acylium ions are common and normally abundant fragment ions of acylnaphthalenes in general. Using a pentaquadrupole mass spectrometer, CID experiments with argon and ion/molecule reactions with 2-methyl-1,3-dioxolane, isoprene, acetonitrile and propionitrile were performed but failed to distinguish the two SDFI. Reactions with ethyl vinyl ether and several homologues as well as ethyl vinyl thioether were, however, successful. In reactions with ethyl vinyl ether, the α-SDFI form a pair of diagnostic product ions of m/z 165 and m/z 181, which are absent in the corresponding spectrum of the β-SDFI. Methyl 4-(1-naphthyl)-2,4-dioxobutanoate was used as a test molecule for this class of constitutional isomers and absolute structural assignment as an α-acyl naphthalene was correctly performed via the characterization of its α-SDFI.

Ion/molecule reactions in a miniature RIT mass spectrometer

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH3OP(+)(O)CH3 (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry.

Meerwein reaction of phosphonium ions with epoxides and thioepoxides in the gas phase

Phosphonium ions are shown to undergo a gas-phase Meerwein reaction in which epoxides (or thioepoxides) undergo three-to-five-membered ring expansion to yield dioxaphospholanium (or oxathiophospholanium) ion products. When the association reaction is followed by collision-induced dissociation (CID), the oxirane (or thiirane) is eliminated, making this ion molecule reaction/CID sequence a good method of net oxygen-by-sulfur replacement in the phosphonium ions. This replacement results in a characteristic mass shift of 16 units and provides evidence for the cyclic nature of the gas-phase Meerwein product ions, while improving selectivity for phosphonium ion detection. This reaction sequence also constitutes a gas-phase route to convert phosphonium ions into their sulfur analogs. Phosphonium and related ions are important targets since they are commonly and readily formed in mass spectrometric analysis upon dissociative electron ionization of organophosphorous esters. The Meerwein reaction should provide a new and very useful method of recognizing compounds that yield these ions, which includes a number of chemical warfare agents. The Meerwein reaction proceeds by phosphonium ion addition to the sulfur or oxygen center, followed by intramolecular nucleophilic attack with ring expansion to yield the 1,3,2-dioxaphospholanium or 1,3,2-oxathiophospholanium ion. Product ion structures were investigated by CID tandem mass spectrometry (MS(2)) experiments and corroborated by DFT/HF calculations.

Ionic Transacetalization with Acylium Ions: A Class-Selective and Structurally Diagnostic Reaction for Cyclic Acetals Performed under Unique Electrospray and Atmospheric Pressure Chemical Ionization In-Source Ion−Molecule Reaction Conditions

Ionic transacetalization of cyclic acetals with the gaseous (CH3)2NCO+ acylium ion has been performed under unique in-source ion-molecule reaction (in-source IMR) conditions of electrospray (ESI) and atmospheric pressure chemical ionization (APCI). In-source IMR under ESI and APCI greatly expands the range of neutral molecules that can be brought to the gas phase to react by ionic transacetalization, a general, class-selective and structurally diagnostic reaction for cyclic acetals (Moraes, L. A. B.; Gozzo, F. C.; Vainiotalo, P.; Eberlin, M. N. J. Org. Chem. 1997, 62, 5096). Heavier, more polar, and less volatile cyclic acetals than those previously employed in quadrupole collision cells are shown to react efficiently by ionic transacetalization under the ESI and APCI in-source IMR conditions. Tetramethylurea (TMU) acts as an efficient dopant, being co-injected with the acetal in either benzene, toluene, methanol, or water/methanol solutions. Under APCI or ESI, the basic TMU dopant is protonated preferentially, and the labile protonated TMU then undergoes dissociation to (CH3)2NCO+, the least acidic and the most transacetalization-reactive acylium ion so far tested. Under the relatively high-pressure, low-energy collision conditions set to favor associative reactions, (CH3)2NCO+ reacts competitively both with TMU to form acylated TMU and with the acetal via ionic transacetalization to form the respective cyclic ionic acetals. Spectrum subtraction removes the ionic products of the dopant (TMU) self-reactions, thus providing clean ion-molecule reaction product ion mass spectra, which are used for the selective, structurally diagnostic detection of cyclic acetals. Information on ring substituents comes from characteristic mass shifts resulting from aldehyde/ketone by acylium ion replacement. Enhanced selectivity in structural characterization or chemical recognition for cyclic acetal monitoring is gained by performing on-line collision-induced dissociation via tandem mass spectrometric experiments. Most cyclic ionic acetals dissociate exclusively or nearly exclusively to re-form the reactant (CH3)2NCO+ acylium ion whereas the presence of additional functional groups with increased structural complexity tends to favor other specific but likewise selective dissociation channels.

Ketalization of phosphonium ions by 1,4-dioxane: selective detection of the chemical warfare agent simulant DMMP in mixtures using ion/molecule reactions

Phosphonium ions CH(3)P(O)OCH(3)(+) (93 Th) and CH(3)OP(O)OCH(3)(+) (109 Th) react with 1,4-dioxane to form unique cyclic ketalization products, 1,3,2-dioxaphospholanium ions. By contrast, a variety of other types of ions having multiple bonds, including the acylium ions CH(3)CO(+) (43 Th), CH(3)OCO(+) (59 Th), (CH(3))(2)NCO(+) (72 Th), and PhCO(+) (105 Th), the iminium ion H(2)C[double bond]NHC(2)H(5)(+) (58 Th) and the carbosulfonium ion H(2)C[double bond]SC(2)H(5)(+) (75 Th) do not react with 1,4-dioxane under the same conditions. The characteristic ketalization reaction can also be observed when CH(3)P(OH)(OCH(3))(2)(+), viz. protonated dimethyl methylphosphonate (DMMP), collides with 1,4-dioxane, as a result of fragmentation to yield the reactive phosphonium ion CH(3)P(O)OCH(3)(+) (93 Th). This novel ion/molecule reaction is highly selective to phosphonium ions and can be applied to identify DMMP selectively in the presence of ketone, ester, and amide compounds using a neutral gain MS/MS scan. This method of DMMP analysis can be applied to aqueous solutions using electrospray ionization; it shows a detection limit in the low ppb range and a linear response over the range 10 to 500 ppb.

Mono and double polar [4 + 2(+)] Diels-Alder cycloaddition of acylium ions with O-heterodienes

Gas-phase reactions of acylium ions with alpha,beta-unsaturated carbonyl compounds were investigated using pentaquadrupole multiple-stage mass spectrometry. With acrolein and metacrolein, CH(3)-C(+)(double bond)O, CH(2)(double bond)CH-C(+)(double bond)O, C(6)H(5)-C(+)(double bond)O, and (CH(3))(2)N-C(+)(double bond)O react to variable extents by mono and double polar [4 + 2(+)] Diels-Alder cycloaddition. With ethyl vinyl ketone, CH(3)-C(+)(double bond)O reacts exclusively by proton transfer and C(6)H(5)-C(+)(double bond)O forms only the mono cycloadduct whereas CH(2)(double bond)CH-C(+)(double bond)O and (CH(3))(2)N-C(+)(double bond)O reacts to great extents by mono and double cycloaddition. The positively charged acylium ions are activated O-heterodienophiles, and mono cycloaddition occurs readily across their C(+)(double bond)O bonds to form resonance-stabilized 1,3-dioxinylium ions which, upon collisional activation, dissociate predominantly by retro-addition. The mono cycloadducts are also dienophiles activated by resonance-stabilized and chemically inert 1,3-dioxonium ion groups, hence they undergo a second cycloaddition across their polarized C(double bond)C ring double bonds. (18)O labeling and characteristic dissociations displayed by the double cycloadducts indicate the site and regioselectivity of double cycloaddition, which are corroborated by Becke3LYP/6-311++G(d,p) calculations. Most double cycloadducts dissociate by the loss of a RCO(2)COR(1) molecule and by a pathway that reforms the acylium ion directly. The double cycloadduct of the thioacylium ion (CH(3))(2)N-C(+)(double bond)S with acrolein dissociates to (CH(3))(2)N-C(+)(double bond)O in a sulfur-by-oxygen replacement process intermediated by the cyclic monoadduct. The double cycloaddition can be viewed as a charge-remote type of polar [4 + 2(+)] Diels-Alder cycloaddition reaction.