Search

The decarbonylation reactions of ionized 2-acetylpyridine, 2-acetylpyrazine, and 2-acetylthiazole have been investigated using the multiple collision technique of neutralization–reionization/collision-induced dissociation mass spectrometry and related techniques. The resultant heterocyclic C6H7N·+, C5H6N2·+, and C4H5NS·+ ions were identified as 2-methylene-1,2-dihydropyridine, 6·+, 2-methylene-1,2-dihydropyrazine, 9·+, and 2-methylene-2,3-dihydrothiazole, 12·+, respectively. This result refutes proposals in the older literature that the decarbonylation would involve a methyl transfer yielding the 2-methyl species 2-methylpyridine (2·+), 2-methylpyrazine (8·+), and 2-methylthiazole (11·+). Literature proposals for a methyl transfer in the dissociation of ionized dimethyl-2,3-pyridinedicarboxylate and methyl-4-pyridinecarboxylate were also examined, but could not be substantiated either. However, the proposed gas phase synthesis of the N-methylpyridinium ylide, 1·+, from ionized methyl-2-pyridinethiocarboxylate did provide evidence for a genuine methyl migration. To reinforce these conclusions, the dissociation characteristics of isomers structurally related to 6·+ (3·+– 5·+ and 7·+), to 9·+ (10·+) and to 12·+ (13·+– 15·+) were also considered. Exploratory quantum chemical calculations (at the B3LYP/6-31G∗ level of theory) indicate that 6·+ and 12·+ are among the most stable isomers in the C6H7N·+ and C4H5NS·+ systems, lying 24 and 19 kcal/mol lower in energy than 2·+ and 11·+, respectively. Their neutral counterparts, however, are considerably less stable: 6 is calculated to be 27 kcal/mol higher in energy than 2. Nevertheless, from neutralization–reionization experiments it follows that the neutral counterparts of the ionized decarbonylation products 6, 9, and 12 are stable molecules on the microsecond time scale. Significant 1,3-hydrogen shift barriers hinder the interconversion of both the ions and the neutrals into their 2-methyl substituted counterparts, thus accounting for their stability in the dilute gas phase.

The intriguing gas-phase ion chemistry of β-hydroxypyruvic acid (HPA), HOCH2C(==O)COOH, has been investigated using tandem mass spectrometry (metastable ion (MI) and (multiple) collision-induced dissociation (CID) experiments, neutralization-reionization mass spectrometry (NRMS),18O and D isotopic labelling on both the acid and its methyl ester) in conjunction with computational chemistry (ab initio MO and density functional theories). HPA does not enolize upon evaporation, but it retains its keto structure. When ionized, decarbonylation occurs and, depending on the internal-energy content, this dissociation reaction proceeds via two distinct routes. The source-generated, high-energy ions lose the keto C==O, not via a least-motion extrusion into ionized glycolic acid, HOCH2COOH.+, but via a rearrangement that yields the title H-bridged radical cation CH2==O...H...O==C-OH.+for which Δ Hf0= 99 ± 3 kcal/mol. The long-lived low-energy ions enolize prior to decarbonylation and lose the carboxyl C==O. Again, this is not a least-motion extrusion (which would produce the most stable isomer, HOC(H)==C(OH)2.+Δ Hf0= 73 kcal/mol) but a rearrangement yielding the ion-dipole complex HOC(H)C==C==O.+/H2O. The methyl ester of HPA behaves analogously, yielding CH2==O...H...O==C-OCH3.+and HOC(H)C==C==O.+/ CH3OH upon decarbonylation of the high- and low-energy ions, respectively. Decarboxylation into the ylidion CH2OH2.+characterizes the dissociation chemistry of both the title H-bridged ion and its glycolic acid isomer HOCH2COOH.+. A computational analysis of this reaction (which satisfies the experimental observations) leads to the proposal that the decarboxylation of the acid occurs via CH2-O(H)...H...==C==O.+as the key intermediate, whereas the title H-bridged ion follows a higher energy route that involves ion-dipole rotations leading to the ionized carbene HO(H2)CO-C-OH.+and the distonic ion H2O-C(H2)-O-C==O.+as key intermediates.Key words: tandem mass spectrometry, hydrogen-bridged radical cation, hydroxypyruvic acid, ab initio calculations, keto-enol tautomerization,18O labelling.

Tandem mass spectrometry based experiments on commercial oxalacetic acid (OAA) samples confirm the indirect evidence from the elegant study of Flint et al. (J. Org. Chem. 57 (1992) 7270) that solid OAA exists solely in the (Z)-enol form of HOOCC(H)C(OH)COOH. It is further shown that: (1) the samples contain a minor impurity in the low percent range, assigned as a dehydration product of 4-hydroxy-4-methyl-2-ketoglutaric acid; (2) careful evaporation of solid OAA samples yields mass spectra representative of the (Z)-enol form. The (E)-enol is not present in the solid, nor is its ion generated in the gas phase via isomerization of the ionized (Z)-enol form. However, even under gentle sample introduction conditions, a partial ketonization of the neutral (Z)-enol takes place. The resulting keto OAA molecules are either ionized intact or decarboxylate to yield a mixture of α-hydroxyacrylic acid, CH2C(OH)COOH and its keto isomer pyruvic acid, CH3C(O)COOH. From computational quantum chemistry (CBS-4) and experimental data, ionic enthalpies of formation of 95 and 109 kcal/mol were derived for CH2C(OH)COOH·+ and CH3C(O)COOH·+, respectively; and (3) under less controlled sample introduction conditions, thermal dehydration may take place to yield C4H2O4 molecules whose mass spectral characteristics are indistinguishable from those of ionized hydroxymaleic anhydride.

The fascinating unimolecular chemistry of ionized 1,2-propanediol, CH 3 C(H)OHCH 2 OH ·+ , 1 , has been re-examined using computational chemistry (ab initio MO and density functional theories) in conjunction with modern tandem mass spectrometric and 13 C labelling experiments. The calculations allow a considerable simplification of a previously proposed complex mechanism (Org. Mass Spectrom., 23 (1988) 355). Again, the central intermediates are proposed to be stable hydrogen bridged ion—dipole complexes, but our present calculations indicate that the key transformation now is the rearrangement CH 3 C(H)OH + ···O(H)-CH 2 . → CH 3 C(H)OH + ··· . OCH 3 , which can best be viewed as the cation-catalyzed 1,2-hydrogen shift . CH 2 OH → CH 3 O . , a rearrangement which does not occur so easily in the unassisted system. Another important process is the electron transfer CH 3 C(H)O···CH 3 OH ·+ → OCH(CH 3 ) ·+ ···O(H)CH 3 which allows proton transfer to generate CH 3 OH 2 + + CH 3 CO . . Other dissociation processes (loss of CH 3 . , H 2 O, H 2 O + CH 3 . , H 2 O + CH 4 ) are interpreted in terms of Bohme's ‘methyl cation shuttle’ (J. Am. Chem. Soc., 118 (1996) 4500) taking place in ion-dipole complexes. The most stable intermediate is the hydrogen bridged ion-dipole complex CH 2 CHOH .+ ···O(H)CH 3 , which is the reacting configuration for loss of methanol.