Gas-phase Base-induced Elimination-reactions In Onium Intermediates .2. Stereochemistry and Orientation In Alkene Formation From Gaseous Halonium Ions

The stereochem. and orientation in gas-phase NMe3-induced elimination reactions of halonium ions were investigated under different exptl. conditions. The halonium ion intermediates were generated in the gas phase at 760 Torr by attack of radiolytically formed CnH5+ (n = 1,2) acids on selected 2,3-dihalobutanes. Isolation and identification of the neutral haloalkenes formed demonstrated that, under the used exptl. conditions, 3-halo-1-butene is preferentially formed over isomeric 2-halo-2-butenes. In addn., gas-phase base-induced elimination reactions occur predominantly via a transition state with an anti configuration. The elimination pathway takes place in competition with an intramol. nucleophilic displacement within protonated 2,3-dihalobutene yielding the inverted 2,3-butanediylhalonium ion, which further rearranges slowly to the 2-halo-2-Bu cation. No stereoelectronic control is exerted in the NMe3-induced ring opening of 2,3-butanediylhalonium ions, whereas 2-halo-2-Bu cation generates preferentially trans-2-halo-2-butene (ca. 70%). Kinetic treatment of the exptl. results provided a quant. est. of the relative extent of bimol. elimination reactions and unimol. displacement and rearrangement processes within protonated 2,3-dihalobutanes. Furthermore, the influence of the nature and the configuration of the substrate upon the relative extent of these processes could be evaluated. A mechanistic model is proposed for gas-phase base-induced eliminations in onium ions at relatively high pressures, which is contrasted with those from related low-pressure ion cyclotron resonance mass spectrometry and soln.-chem. studies.