Tineke A. Molenaar-Langeveld

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

Abstract 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 5 H 4 NO · /(CH 3 ) 2 CH + ] 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 5 H 4 NO · /(CH 3 ) 2 CH + ] complex and the occurrence of proton transfer within this species. The [C 5 H 5 NO +· /C 3 H 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.

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

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 CD3OD, CD3CN, (CD3)2CO, or pyrrole-D5 (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.

Isomerization of the molecular ion of cyclohexyl cyanide to an ionized bicycloheptane imine species competing with direct ring-opening

Abstract The mechanisms of the dissociation reactions of the molecular ion of cyclohexyl cyanide have been studied with nitrogen-15, carbon-13 and deuterium labelling in combination with tandem mass spectrometric experiments. The most important unimolecular reactions of ionized cyclohexyl cyanide involve the losses of CH • 3 , NH 3 , HCN, C 3 H 3 N and C 4 H • 7 . These reactions are all significant for the molecular ions dissociating in the ion source, whereas the metastable molecular ions react only by the competing losses of CH • 3 , NH 3 and HCN. The results of the labelling experiments reveal that the carbon atoms at the 2- and 6-positions of the ring become structurally indistinguishable from the carbon atoms at the 3- and 5-positions prior to the losses of CH • 3 and HCN. This finding is suggested to be a result of a rearrangement of the molecular ion of cyclohexyl cyanide with formation of a symmetrical and ionized [2,2,1]-bicycloheptane-7-imine as the key intermediate. The elimination of C 3 H 3 N is indicated to involve a direct ring-opening of the molecular ion of cyclohexyl cyanide followed by dissociation to yield a C 4 H +· 8 ion. The loss of a C 4 H • ; radical may involve the same initial step followed by an H-shift in the ring-opened species prior to dissociation.