Identifying ylide ions and methyl migrations in the gas phase: the decarbonylation reactions of simple ionized N-heterocycles

The decarbonylation reactions of ionized 2-acetylpyridine, 2-acetyIpyrazine , and 2-acetylthiazole have been investigated using the multiple collision technique of neutralization-reionization/collision-induced dissociation mas s spectrometry and related techniques. The resultant heterocyclic C6H7N.+, C5H6N2.+, and C4H5NS.+ ions were identified as 2-methylene-1,2-dihydropyrid ine, 6(.+);, 2-methylene-1,2-dihydropyrazine, 9(.+), and 2-methylene-2,3-di hydrothiazole, 12(.+), respectively. This result refutes proposals in the o lder literature that the decarbonylation would involve a methyl transfer yi elding the 2-methyl species 2-methylpyridine (2(.+)), 2-methylpyrazine (8(. +)), and 2-methylthiazole (11(.+)). Literature proposals for a methyl trans fer in the dissociation of ionized dimethyl-2,3-pyridinedicarboxylate and m ethyl-4-pyridinecarboxylate were al so examined, but could not be substanti ated either. However, the proposed gas phase synthesis of the N-methylpyrid inium ylide, 1(.+), from ionized methyl-2-pyridinethiocarboxylate did provi de 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 B3 LYP/6-31G* level of theory) indicate that 6(.+) and 12(.+) are among the mo st stable isomers in the C6H7N.+ and C4H5NS.+ systems, lying 24 and 19 kcal /mol lower in energy than 2(.+) and 11(.+), respectively. Their neutral cou nterparts, however, are considerably less stable: 6 is calculated to be 27 kcal/mol higher in energy than 2. Nevertheless, from neutralization-reioniz ation experiments it follows that the neutral counterparts of the ionized d ecarbonylation products 6, 9, and 12 are stable molecules on the microsecon d time scale. Significant 1,3-hydrogen shift barriers hinder the inter conv ersion of both the ions and the neutrals into their 2-methyl substituted co unterparts, thus accounting for their stability in the dilute gas phase. (C ) 2000 Elsevier Science B.V.