Study on the Gas-Phase Reactivity of Charged Pyridynes.

The reactivities of three isomeric, charged ortho-pyridynes, the 1,2-, 2,3-, and 3,4-didehydropyridinium cations, were examined in the gas phase using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. The structures of selected product ions were probed using collision-activated dissociation (CAD) experiments in a linear quadrupole ion trap (LQIT) mass spectrometer. Mechanisms based on quantum chemical calculations are proposed for the formation of all major products. The products of the reactions of the charged ortho-pyridynes in the gas phase were found to closely resemble those formed upon reactions of neutral ortho-arynes in solution, but the mechanisms of these reactions exhibit striking differences. Additionally, no radical reactions were observed for any of the charged ortho-pyridynes examined, in contrast to previous proposals that ortho-benzyne can occasionally react via radical mechanisms. Finally, the relative reactivities of those charged gaseous ortho-pyridynes that yielded similar product distributions were found to be affected mainly by the (calculated) vertical electron affinities of the dehydrocarbon sites, which suggests that the reactivity of these species is controlled by polar effects.

Measurement of the Proton Affinities of a Series of Mono- and Biradicals of Pyridine.

The proton affinity (PA) of a neutral molecule is defined as the negative of the enthalpy change for the gas-phase reaction between a proton and the neutral molecule to produce the (charged) conjugate acid of the molecule. PA is a fundamental property that is related to the structure of a molecule and affects its reactivity. Very few PA values are available for basic organic monoradicals and none for biradicals. Here, the PA values for several σ-type carbon-centered pyridine-based monoradicals and biradicals have been experimentally determined by monitoring proton transfer from the protonated mono- and biradicals to reference bases with known proton affinities as a function of time in Fourier-transform ion cyclotron resonance (FT-ICR) and linear quadrupole ion trap (LQIT) mass spectrometers. A procedure was developed for both instruments that permits differentiation between exo- and endothermic proton transfer reactions. The PA values of all the (bi)radicals studied were found to be lower than that of pyridine. This is rationalized based on the electron-withdrawing nature of the radical site(s). Thus, the PA values decrease in the order: pyridine > monoradicals > biradicals. The PA values of the monoradicals were also found to increase (making the protonated radicals less acidic) as the distance between the basic nitrogen atom and the radical site increases. Similar behavior was found for the biradicals, with one exception: 3,5-didehydropyridine has a larger PA (215.3 ± 3.3 kcal mol-1) than 3,4-didehydropyridine (PA = 213.4 ± 3.3 kcal mol-1) even though the latter biradical has one radical site farther away from the basic nitrogen atom. Quantum chemical calculations of the PAs of the (bi)radicals are in reasonably good agreement with the experimentally determined values. At the DFT (B3LYP), CCSD(T), and CASPT2 levels of theory, the mean unsigned errors are 2.3, 1.7, and 2.1 kcal mol-1.

Effects of hydrogen bonding on the gas-phase reactivity of didehydroisoquinolinium cation isomers.

Two previously unreported isomeric biradicals with a 1,4-radical topology, the 1,5-didehydroisoquinolinium cation and the 4,8-didehydroisoquinolinium cation, and an additional, previously reported isomer, the 4,5-didehydroisoquinolinium cation, were studied to examine the importance of the exact location of the radical sites on their reactivities in the gas phase. The experimental results suggest that hydrogen bonding in the transition state enhances the reactivity of the 1,5-didehydroisoquinolinium cation towards tetrahydrofuran but not towards allyl iodide, dimethyl disulfide or tert-butyl isocyanide. The observation of no such enhancement of reactivity towards tetrahydrofuran for the 4,8-didehydroisoquinolinium and 4,5-didehydroisoquinolinium cations supports this hypothesis as these two biradicals are not able to engage in hydrogen bonding in their transition states for hydrogen atom abstraction from tetrahydrofuran. Quantum chemical transition state calculations indicate that abstraction of a hydrogen atom from tetrahydrofuran by the 1,5-didehydroisoquinolinium cation occurs at the C-1 radical site and that the transition state is stabilized by hydrogen bonding.

Reactivity Controlling Factors for an Aromatic Carbon-Centered σ,σ,σ-Triradical: The 4,5,8-Tridehydroisoquinolinium Ion.

The chemical properties of the 4,5,8-tridehydroisoquinolinium ion (doublet ground state) and related mono- and biradicals were examined in the gas phase in a dual-cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The triradical abstracted three hydrogen atoms in a consecutive manner from tetrahydrofuran (THF) and cyclohexane molecules; this demonstrates the presence of three reactive radical sites in this molecule. The high (calculated) electron affinity (EA=6.06 eV) at the radical sites makes the triradical more reactive than two related monoradicals, the 5- and 8-dehydroisoquinolinium ions (EA=4.87 and 5.06 eV, respectively), the reactivity of which is controlled predominantly by polar effects. Calculated triradical stabilization energies predict that the most reactive radical site in the triradical is not position C4, as expected based on the high EA of this radical site, but instead position C5. The latter radical site actually destabilizes the 4,8-biradical moiety, which is singlet coupled. Indeed, experimental reactivity studies show that the radical site at C5 reacts first. This explains why the triradical is not more reactive than the 4-dehydroisoquinolinium ion because the C5 site is the intrinsically least reactive of the three radical sites due to its low EA. Although both EA and spin-spin coupling play major roles in controlling the overall reactivity of the triradical, spin-spin coupling determines the relative reactivity of the three radical sites.

Experimental and computational studies on the formation of three para-benzyne analogues in the gas phase.

Experimental and computational studies on the formation of three gaseous, positively-charged para-benzyne analogues in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer are reported. The structures of the cations were examined by isolating them and allowing them to react with various neutral reagents whose reactions with aromatic carbon-centered σ-type mono- and biradicals are well understood. Cleavage of two iodine-carbon bonds in N-deuterated 1,4-diiodoisoquinolinium cation by collision-activated dissociation (CAD) produced a long-lived cation that showed nonradical reactivity, which was unexpected for a para-benzyne. However, the reactivity closely resembles that of an isomeric enediyne, N-deuterated 2-ethynylbenzonitrilium cation. A theoretical study on possible rearrangement reactions occurring during CAD revealed that the cation formed upon the first iodine atom loss undergoes ring-opening before the second iodine atom loss to form an enediyne instead of a para-benzyne. Similar results were obtained for the 5,8-didehydroisoquinolinium cation and the 2,5-didehydropyridinium cation. The findings for the 5,8-didehydroisoquinolinium cation are in contradiction with an earlier report on this cation. The cation described in the literature was regenerated by using the literature method and demonstrated to be the isomeric 5,7-didehydro-isoquinolinium cation and not the expected 5,8-isomer.

Reactivity of a σ,σ,σ,σ-tetraradical: the 2,4,6-tridehydropyridine radical cation.

The 2,4,6-tridehydropyridine radical cation, an analogue of the elusive 1,2,3,5-tetradehydrobenzene, was generated in the gas phase and its reactivity examined. Surprisingly, the tetraradical was found not to undergo radical reactions. This behavior is rationalized by resonance structures hindering fast radical reactions. This makes the cation highly electrophilic, and it rapidly reacts with many nucleophiles by quenching the N-C ortho-benzyne moiety, thereby generating a relatively unreactive meta-benzyne analogue.

Reactivity of the 4,5-didehydroisoquinolinium cation.

The chemical properties of a 1,8-didehydronaphthalene derivative, the 4,5-didehydroisoquinolinium cation, were examined in the gas phase in a dual-cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. This is an interesting biradical because it has two radical sites in close proximity, yet their coupling is very weak. In fact, the biradical is calculated to have approximately degenerate singlet and triplet states. This biradical was found to exclusively undergo radical reactions, as opposed to other related biradicals with nearby radical sites. The first bond formation occurs at the radical site in the 4-position, followed by that in the 5-position. The proximity of the radical sites leads to reactions that have not been observed for related mono- or biradicals. Interestingly, some ortho-benzynes have been found to yield similar products. Since ortho-benzynes do not react via radical mechanisms, these products must be especially favorable thermodynamically.

Analysis of xyloglucans by ambient chloride attachment ionization tandem mass spectrometry.

Xyloglucan oligomers obtained upon enzyme digestion from Hymenaea courbaril, Arabidopsis Columbia-0 and mur3 were ionized and analyzed by using chloride anion attachment electrospray ionization (ESI) and tandem mass spectrometry. MW determination and structural elucidation of several xyloglucan oligomers was performed directly from the mixture solutions without sample pretreatment or derivatization. Sodium cation attachment was used to determine the number of xyloglucans present in the mixtures and their MWs. However, tandem mass spectrometry results showed that structure elucidation based on the sodium adducts is ambiguous. Chloride anion also forms stable adducts with these xyloglucans upon ESI. These adducts can be readily identified due to the chlorine isotope pattern. The mass spectral profile of xyloglucans obtained for the mixtures matches the HPAEC results, thus validating this methodology for the determination of the xyloglucan composition and the MW of each xyloglucan. Upon collisional activation in MS(2) experiments, the chloride anion adducts readily lose HCl, which helps verify the molecular weight of each xyloglucan. Isolating the resulting anion (deprotonated oligomer) and subjecting it to further collision-activated dissociation experiments (MS(n); n=3-4) yields useful structural information that allows the differentiation between isomeric anions and hence determination of the sequence of the xyloglucan oligomers. The deprotonated oligomers fragment by a stepwise loss of sugar units from the reducing end.

Does the 2,6-didehydropyridinium cation exist?

Reactive intermediates are key species involved in many chemical and biochemical processes. For example, carbon-centered aromatic σ,σ-biradicals formed in biological systems from naturally occurring enediyne antitumor antibiotics are responsible for the irreversible cleavage of double-stranded DNA caused by these prodrugs. However, because of their high reactivity, it is very difficult or impossible to isolate and investigate these biradicals. The aromatic σ,σ-biradical, 2,6-didehydropyridine, has been speculated for many years to be formed in certain organic reactions; however, no definitive proof of its generation has been obtained. We report here the successful generation of protonated 2,6-didehydropyridine and the examination of its chemical properties in the gas phase by using a Fourier transform ion cyclotron resonance mass spectrometer. The results suggest that a mixture of singlet (ground) state and triplet (excited) state 2,6-didehydropyridinium cations was generated. The two different states show qualitatively different reactivity, with the triplet state showing greater Brønsted acidity than that of the singlet state. The triplet state also shows much greater radical reactivity than that of the singlet state, as expected because of the coupling of the nonbonding electrons in the singlet state.

Differentiation of regioisomeric aromatic ketocarboxylic acids by positive mode atmospheric pressure chemical ionization collision-activated dissociation tandem mass spectrometry in a linear quadrupole ion trap mass spectrometer.

Positive-mode atmospheric pressure chemical ionization tandem mass spectrometry (APCI-MS(n)) was tested for the differentiation of regioisomeric aromatic ketocarboxylic acids. Each analyte forms exclusively an abundant protonated molecule upon ionization via positive-mode APCI in a commercial linear quadrupole ion trap (LQIT) mass spectrometer. Energy-resolved collision-activated dissociation (CAD) experiments carried out on the protonated analytes revealed fragmentation patterns that varied based on the location of the functional groups. Unambiguous differentiation between the regioisomers was achieved in each case by observing different fragmentation patterns, different relative abundances of ion-molecule reaction products, or different relative abundances of fragment ions formed at different collision energies. The mechanisms of some of the reactions were examined by H/D exchange reactions and molecular orbital calculations.