A chemical ionization study of deuteron transfer initiated propene loss from propoxypyridines

Abstract

The mechanism of propene loss from the metastable [M + D] + ions of isomeric 2-, 3-, and 4- n-propoxypyridines and the related isopropoxypyridines has been examined by chemical ionization (CI) and tandem mass spectrometry in combination with deuterium labeling. The [M + D] + ions were generated with CD 3OD, CD 3CN, (CD 3) 2CO, or pyrrole-D 5 (listed in order of increasing proton affinity) as the CI reagent. The results reveal that the deuteron added in the CI process is not interchanged with the hydrogen atoms of the propyl group prior to propene loss from the metastable [M + D] + ions of the propoxypyridines. The site selective labeling of the α-, β-, or γ-position of the propyl group indicates that the [M + D] + ions of 2- n-propoxypyridine expel propene with formation of an ion-neutral complex composed of a propyl carbenium ion and 2-pyridone. By contrast, the [M + D] + ions of 3- n-propoxypyridine expel propene by: (1) Formation of ion-neutral complexes, and (2) a conventional 1,5-hydride shift from the β-position of the n-propyl group to the ring and/or a 1,2-elimination type process. For the 4-isomer, the results suggest the occurrence of propene loss by a 1,2-elimination in addition to the intermediate formation of ion-neutral complexes. Loss of propene with one deuterium atom is the only reaction of the [M + D] + ions of the isopropoxypyridines labeled at the α-position of the isopropyl group. The results for the isopropoxypyridines labeled with three deuterium atoms at the β-position are consistent with: (1) The loss of propene by ion-neutral complex formation and the occurrence of a substantial isotope effect in the subsequent proton/deuteron transfer within the complex, and/or (2) the loss of propene by a 1,2-elimination type reaction.