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 -SeCH 2 CH 2 + , 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 -OMeC 6 H 4 < C 6 H 5 < p -BrC 6 H 4 < p -CF 3 C 6 H 4 < m -NO 2 C 6 H 4 , giving a Hammett plot with R 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 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.