ION-NEUTRAL COMPLEX-FORMATION AND HYDROGEN-SHIFTS IN THE ALKENE LOSS FROM IONIZED ALKYL PHENYL THIOETHERS IN THE GAS-PHASE

The mechanistic aspects of alkene loss from ionized ethyl- and n-propyl phenyl thioethers have been studied by use of denterium labelling and tandem mass spectrometry. Loss of ethene from the molecular ion of ethyl phenyl thioether proceeds predominantly by a specific H-shift from the beta-position of the ethyl group to the remaining part of the ion. In contrast, deuterium labelling reveals that elimination of propene from the molecular ions of the n-propyl phenyl and n-propyl 2,6-dimethylphenyl thioethers involves three competing reactions: (i) a 1,2-hydride shift assisted cleavage of the bond between the propyl group and the sulfur atom yielding an ion-neutral complex of a thiophenyoxy radical and a secondary propyl carbenium ion, which reacts further by proton transfer prior to dissociation; (ii) a partially reversible 1,5-H shift from the beta-position to the 2- or 6-position of the ring; and (iii) a 1,3-H shift from the beta-position to the sulfur atom. The preference for transfer of a hydrogen atom from the beta-position is particularly pronounced for the reactions of the metastable molecular ions of the n-propyl phenyl and n-propyl 2,6-dimethylphenyl thioethers. This indicates that the critical energies of the processes involving H-shifts are lower than the critical energy for propene loss with intermediate formation of an ion-neutral complex. In addition to ethene elimination, ionized ethyl phenyl thioether expels a HS+. radical. This reaction is preceded by an interchange between the hydrogen atoms at the alpha-methylene group and;he hydrogen atoms at the 2- and 6-positions of the ring as observed also for ionized methyl phenyl thioether.