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 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 al so 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 inter conversion of both the ions and the neutrals into their 2-methyl substituted counterparts, thus accounting for their stability in the dilute gas phase. (C) 2000 Elsevier Science B.V.