Isomeric dependence of the formation of ion/neutral complexes in dissociation reactions of ionized propoxypyridines

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The mechanistic aspects of the dissociation reactions of the molecular ions of isomeric propoxypyridines have been examined with the use of deuterium labelling and tandem mass spectrometry. The ionized 2- and 3-propoxypyridines dissociate predominantly by the loss of propene, whereas the molecular ions of 4-propoxy- and 4-isopropoxypyridine react by the competing losses of a propene molecule and an allyl radical. The loss of propene from the metastable molecular ions of the 2-isomer involves a 1,5-H shift from the 2-position of the propyl group to the ring as evidenced by the labelling experiments. For the metastable molecular ions of 3-propoxypyridine, the results are in agreement with propene loss by a pathway involving formation of a [C 5H 4NO ·/(CH 3) 2CH +] complex which reacts further by proton transfer prior to dissociation. In contrast to these findings, interchange between the hydrogen atoms at the 2-position of the propyl group, and the 3- or 5-position of the pyridine ring, occurs in ionized 4-propoxypyridine. This interchange can precede the formation of a [C 5H 4NO ·/(CH 3) 2CH +] complex and the occurrence of proton transfer within this species. The [C 5H 5NO +·/C 3H 6] complex formed in the proton transfer step may either expel propene or react by hydrogen atom transfer prior to the loss of an allyl radical. For the metastable ions of 4-isopropoxypyridine, interchange between a hydrogen atom from one of the methyl groups of the alkyl chain and the aromatic ring appears to be of minor importance.