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-c-SeCH₂CH₂⁺, and cis-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 p-OMeC₆H₄ < C₆H₅ < p-BrC₆H₄ < p-CF₃C₆H₄ < m-NO₂C₆H₄, giving a Hammett plot with R² = 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 cis-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 t-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 Se-methylselenocysteine was investigated and shown to react faster and favor π-ligand exchange as the leaving group was changed from ethene to acrylic acid.