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-cSeCH(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-OMeC6H4 < C6H5 < p-BrC6H4 < p-CF3C6H4 < mNO(2)C(6)H(4), giving a Hammett plot with R-2 = 0.98 and p = +1.66. The two main pathways include direct transfer of the selenium moiety to the incoming alkene (Nligand 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 electronwithdrawing 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 N-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 N-ligand exchange as the leaving group was changed from ethene to acrylic acid.