Pierluigi Giacomello

Acid-catalysed glucose dehydration in the gas phase: a mass spectrometric approach.

Understanding on a molecular level the acid-catalysed decomposition of the sugar monomers from hemicellulose and cellulose (e.g. glucose, xylose), the main constituent of lignocellulosic biomass is very important to increase selectivity and reaction yields in solution, key steps for the development of a sustainable renewable industry. In this work we reported a gas-phase study performed by electrospray triple quadrupole mass spectrometry on the dehydration mechanism of D-glucose. In the gas phase, reactant ions corresponding to protonated D-glucose were obtained in the ESI source and were allowed to undergo collisionally activated decomposition (CAD) into the quadrupole collision cell. The CAD mass spectrum of protonated D-glucose is characterized by the presence of ionic dehydrated daughter ion (ionic intermediates and products), which were structurally characterized by their fragmentation patterns. In the gas phase D-glucose dehydration does not lead to the formation of protonated 5-hydroxymethyl-2-furaldehyde, but to a mixed population of m/z 127 isomeric ions. To elucidate the D-glucose dehydration mechanism, 3-O-methyl-D-glucose was also submitted to the mass spectrometric study; the results suggest that the C3 hydroxyl group plays a key role in the reaction mechanism. Furthermore, protonated levulinic acid was found to be formed from the monodehydrated D-glucose ionic intermediate, an alternative pathway other than the known route consisting of 5-hydroxymethyl-2-furaldehyde double hydration.

The Mechanism of 2-Furaldehyde Formation from d -Xylose Dehydration in the Gas Phase. A Tandem Mass Spectrometric Study

AbstractThe mechanism of reactions occurring in solution can be investigated also in the gas phase by suited mass spectrometric techniques, which allow to highlight fundamental mechanistic features independent of the influence of the medium and to clarifying controversial hypotheses proposed in solution studies. In this work, we report a gas-phase study performed by electrospray triple stage quadrupole mass spectrometry (ESI-TSQ/MS) on the dehydration of d-xylose, leading mainly to the formation of 2-furaldehyde (2-FA). It is generally known in carbohydrate chemistry that the thermal acid catalyzed dehydration of pentoses leads to the formation of 2-FA, but several aspects on the solution-phase mechanism are controversial. Here, gaseous reactant ions corresponding to protonated xylose molecules obtained from ESI of a solution containing d-xylose and ammonium acetate as protonating reagent were allowed to undergo collisionally activated decomposition (CAD) into the triple stage quadrupole analyzer. The product ion mass spectra of protonated xylose are characterized by the presence of ionic intermediates arising from xylose dehydration, which were structurally characterized by their fragmentation patterns. As expected, the xylose triple dehydration leads to the formation of the ion at m/z 97, corresponding to protonated 2-FA. On the basis of mass spectrometric evidences, we demonstrated that in the gas phase, the formation of 2-FA involves protonation at the OH group bound to the C1 atom of the sugar, the first ionic intermediate being characterized by a cyclic structure. Finally, energy resolved product ion mass spectra allowed to obtain information on the energetic features of the d-xylose→2-FA conversion. Figureᅟ

A mass spectrometric study of the acid-catalysed d-fructose dehydration in the gas phase

5-hydroxymethylfuraldehyde (5-HMF) and simpler compounds, such as levulinic acid (LA) and glyceraldehyde, are platform molecules produced by the thermal acid-catalyzed dehydration of carbohydrates coming from biomass. Understanding sugar degradation pathways on a molecular level is necessary to increase selectivity, reduce degradation by-products yields and optimize catalytic strategies, fundamental knowledge for the development of a sustainable renewable industry. In this work gaseous protonated d-fructose ions, generated in the ESI source of a triple quadrupole mass spectrometer, were allowed to undergo Collisionally Activated Decomposition (CAD) into the quadrupole collision cell. The ionic intermediates and products derived from protonated d-fructose dehydration were structurally characterized by their fragmentation patterns and the relative water-loss dehydration energies measured by energy-resolved CAD mass spectra. The data were compared with those obtained from protonated d-glucose decomposition in the same experimental conditions. In the gas phase, d-fructose dehydration leads to the formation of a mixed population of isomeric [C6H6O3]H(+) ions, whose structures do not correspond exclusively to 5-hydroxymethyl-2-furaldehyde protonated at the more basic aldehydic group.

Gas-phase reaction of free isopropyl ions with phenol and anisole

Unsolvated isopropyl ions, obtained in the dilute gas state from the radiolysis of propane, react with PhOH yielding isomeric isopropylphenols and isopropyl phenyl ether, in the ratio of ca. 3 : 1 at 320 Torr. At lower pressures, the ratio is further shifted in favour of ring alkylation, reaching a value in excess of 10 : 1 at 22 Torr. The isomeric composition of isopropylphenols passes from 83%ortho, 3%meta, 14%para at 720 Torr to 61%ortho, 3%meta, 36%para at 22 Torr. A similar pressure dependence characterizes the isomeric composition of products from the alkylation of PhOMe, which passes from 85%ortho, 5%meta, 10%para in C3H8 at 720 Torr to 64%ortho, 21%meta, and 15%para at 20 Torr. The substrate selectivity of isopropyl ion, referred exclusively to the alkylation, is measured by the apparent ratios kPhOH:kPhH= 1.0–1.6 and kPhOMe:kPhMe= 0.6–0.9 at 720 Torr. The results are consistent with a model involving kinetically predominant attack to the oxygen atom, i.e. to the n-type nucleophilic centre of the ambident substrate, and competition between proton transfer and alkylation channels, reflecting respectively the Bronsted acid and the Lewis acid reactivity of the alkyl cation. The role of isomerization processes in determining the orientation has been independently evaluated by protonating isopropyl phenyl ether with gaseous Bronsted acids, such as H3+ and CnH5+(n= 1 or 2), thus obtaining via an independent route the same oxonium ion arising from the attack of isopropyl ion on PhOH. The results demonstrate dealkylation to PhOH and intramolecular isomerization to o- and p-isopropylphenols, whose ratio depends on the nature of the Bronsted acid, and the exothermicity of the proton transfer to the ether.

Gas-phase basicity of 2-furaldehyde

2-Furaldehyde (2-FA), also known as furfural or 2-furancarboxaldehyde, is an heterocyclic aldehyde that can be obtained from the thermal dehydration of pentose monosaccharides. This molecule can be considered as an important sustainable intermediate for the preparation of a great variety of chemicals, pharmaceuticals and furan-based polymers. Despite the great importance of this molecule, its gas-phase basicity (GB) has never been measured. In this work, the GB of 2-FA was determined by the extended Cookss kinetic method from electrospray ionization triple quadrupole tandem mass spectrometric experiments along with theoretical calculations. As expected, computational results identify the aldehydic oxygen atom of 2-FA as the preferred protonation site. The geometries of O-O-cis and O-O-trans 2-FA and of their six different protomers were calculated at the B3LYP/aug-TZV(d,p) level of theory; proton affinity (PA) values were also calculated at the G3(MP2, CCSD(T)) level of theory. The experimental PA was estimated to be 847.9 ± 3.8 kJ mol(-1), the protonation entropy 115.1 ± 5.03 J mol(-1) K(-1) and the GB 813.6 ± 4.08 kJ mol(-1) at 298 K. From the PA value, a ΔH°(f) of 533.0 ± 12.4 kJ mol(-1) for protonated 2-FA was derived.

Gaseous isomeric Ph-C3H+6 ions: a radiolytic and mass spectrometric study

Abstract Collisional activation mass spectrometry and radiolytic techniques have been used to study the structure, the stability and the interconversion of gaseous C9H+11 ions from different reactions, i.e. (i) protonation of 3-phenyl-1-propene, 2-phenylpropene, cis- and trans-1-phenyl-1-propene, cyclopropylbenzene and indane; (ii) protodehydration and protodehydrohalogenation of Ph-C3H6X (X = OH, Br) substrates; (iii) addition of C3H+5 ions to PhH and of Ph+ ions to propene and cylcopropane. Several model structures have been identified, namely, Ph C + (Me)2 (a), Ph C + HEt (b), PhCH2 C + HMe (c) and C9H+11 arenium ions from indane. Protonation of cyclopropylbenzene in the presence of MeOH yields exclusively Ph-CH(OMe)Et, which is likely to arise from the cleavage of the protonated, intact C3 ring induced by the attack of the nucleophile. No rearrangement of the linear-chain ions b and c nor of protonated phenylcyclopropane into the branched-chain ion a has been detected under chemical ionization conditions, nor in the radiolytic systems, while significant c → b isomerization occurs in the period of time, ca. 10−8 s, required for the capture of the ionic intermediates by the nucleophile.

Aromatic substitution in the gas phase. Alkylation of arenes by C4H9+ ions from the protonation of C4 alkenes and cycloalkanes with gaseous Brønsted acids

Use of a reactive aromatic substrate (o-xylene) to sample the isomeric population of the butyl ions from the gas-phase protonation of the C4H8 hydrocarbons shows that the primary product from linear olefins and methylcyclopropane is the s-butyl ion, or at least an s-butylating charged species displaying a positional selectivity very similar to that measured for the thermal s-butyl cations obtained from n-butane. A fraction of the primary C4H9+ ions isomerizes to the most stable tertiary structure, to an extent which depends, inter alia, on the exothermicity of the proton transfer process. Protonation of cyclobutane gives an alkylating reagent whose s-butylation/t-butylation ratio and positional selectivity set it apart from the C4H9+ reagents obtained from the other C4H8 hydrocarbons, suggesting the intervention of a different electrophile, conceivably protonated cyclobutane. Finally, protonation of isobutene yields exclusively a t-butyl ion that does not isomerize despite the large exothermicity of its formation.

Gas-Phase Reactions of Free Methyl Cations from the Decay of [3H4]-Methane. A Study of their Attack on the Halobenzenes

The gas-phase reaction of labelled methyl cations, generated from the decay of multitritiated methane, with the halobenzenes (PhX, X = F, CI, Br,) leads to formation of the corresponding halotoluenes, of the tritiated substrate itself and of minor amounts of methyl halides and toluene (except for X = F). The electrophilic aromatic substitution by this free charged reagent is characterized in the gaseous phase by low substrate selectivity (fcphX : ^PhCH = 0.8; 0.6; 0.5 for X = F, CI, Br) and by high relative yields of the meta isomer. The general mechanistic picture is dominated by the high exothermicity of the initial condensation step, leading to formation of excited haloarenium ions which can either undergo isomerization towards a thermodynamic distribution of products and subsequent deprotonation to the halotoluenes, or hydrogen scrambling between the side chain and the ring to yield eventually the labelled substrate. Evidence is presented for the direct attack of CTJ to the η-do nor substituent to give methylphenyl halonium ions, following the trend Br > Cl > F. Intervention of this species as selective methylating reagent for toluene is suggested to account for the observed decrease of bromotoluene yields, relative to the xylenes, by increasing the mole fraction of PhBr with respect to PhCH3 in competition experiments.

Ab-initio and experimental study of pentose sugar dehydration mechanism in the gas phase

In this work pentose sugar (D-xylose, D-ribose and D-arabinose) gas phase dehydration reaction was investigated by means of mass spectrometric techniques and theoretical calculations. The ionic species derived from the dehydration reaction of protonated D-ribose and D-arabinose were structurally characterized by their fragmentation patterns and the relative dehydration energies measured by energy resolved CAD mass spectra. The results were compared with those recently obtained for D-xylose in the same mass spectrometric experimental conditions. Dehydration of C1-OH protonated sugars was theoretically investigated at the CCSD(T)/cc-pVTZ//M11/6-311++G(2d,2p) level of theory. Protonated pentoses are not stable and promptly lose a water molecule giving rise to the dehydrated ions at m/z 133. D-xylose, D-ribose and D-arabinose dehydration follows a common reaction pathway with ionic intermediates and transition states characterized by similar structures. Slightly different dehydration energies were experimentally measured and the relative trend was theoretically confirmed. The overall dehydration activation energy follows the order arabinose < ribose < xylose. Gas-phase pentose sugar dehydration leads to the formation of protonated 2-furaldehyde as final product. Based on the experimental and theoretical evidence a new mechanistic hypothesis starting from C1-OH protonation was proposed.

Acid-catalyzed aldehyde-ketone rearrangements in the gas phase. Cyclopentane- and cyclohexanecarboxaldehyde.

Abstract Protonation by gaseous H + 3 , C n H + 5 (n=1,2), s-C 3 H + 7 and t-C 4 H + 9 cations promotes rearrangement of cyclopentane- and cyclohexanecarboxaldehyde to cyclohexanone and cyclopentylmethyl ketone, respectively. The reaction was investigated by radiolytic and mass spectrometric methods.