Electronic and Steric Effects on the Reactivity of Seleniranium Ions with Alkenes in the Gas Phase

Gas phase ion-molecule reactions between seleniranium ions, R-<i>c</i>-SeCH₂CH₂⁺, and <i>cis</i>-cyclooctene were used to probe electronic and steric effects of substituents on kinetics and branching ratios. The second-order rate coefficients increased in the order <i>p</i>-OMeC₆H₄ < C₆H₅ < <i>p</i>-BrC₆H₄ < <i>p</i>-CF₃C₆H₄ < <i>m</i>-NO₂C₆H₄, giving a Hammett plot with <i>R</i>² = 0.98 and ρ = +1.66. The two main pathways include direct transfer of the selenium moiety to the incoming alkene (π-ligand exchange) and the less favored ring-opening by attack at an iranium carbon to give a <i>cis</i>-bicyclic selenonium ion as supported by density functional theory (DFT) calculations. Branching ratios of each pathway indicated that electron-withdrawing groups directed more attack at carbon than selenium in agreement with previous solution-phase results. Increased steric bulk on selenium was investigated by changing the R group from a methyl to <i>t</i>-butyl, which not only shut down π-ligand exchange but also significantly reduced the overall reactivity. Finally, the reactivity of the iranium ion derived from <i>Se</i>-methylselenocysteine was investigated and shown to react faster and favor π-ligand exchange as the leaving group was changed from ethene to acrylic acid.