Effects of Oxygenation on the Reactions of the Iodiranium Ion <i>c</i>-C₂H₄I⁺ with Unsaturated Compounds in the Gas Phase

The potential for ion-molecule reactions to identify alkene functional groups in substrates with increasing oxygenation is explored by using linear ion-trap mass spectrometry. Electrophilic attack by the iodiranium ion, <i>c</i>-C₂H₄I⁺ (<i>m</i>/<i>z</i> 155), proceeds at the collision rate with terminal alkenes RCH═CH₂ (R = <i>n</i>-Pr), allyl ethers (R = MeOCH₂), and nonconjugated methyl esters (R = MeO₂CCH₂), favoring transfer of the iodenium cation to the point of unsaturation by π-ligand exchange. Density functional theory (DFT) calculations at M06-2X-D3/def2-TZVP revealed a single energy well for π-ligand exchange, allowing for a barrierless and hence kinetically favorable reaction pathway. In contrast, ring-opening of the iodiranium ion with subsequent elimination of HI proceeds via a relatively high barrier and is only a minor product channel despite being thermodynamically preferable. Conjugation of the carbon-carbon double bond to an ester group in methyl acrylate had a significant impact on the reaction partitioning with exclusive attack by the carbonyl oxygen to give an allylic stabilized oxonium ion. However, substitution at the point of unsaturation with an inductively donating methyl group enabled the formation of a π-ligand exchange product, which underwent secondary reactions with the neutral. Comparison of the ion-molecule reactions of methyl crotonate, methyl methacrylate, and methyl 3-butenoate with <i>c</i>-C₂H₄I⁺ show this shift in reaction partitioning between ring-opening by the ester carbonyl oxygen and π-ligand exchange is related to the nucleophilicity of the carbon-carbon double bond. The branching ratios of these product channels reveal significant differences between these three structural isomers encompassing conjugated and nonconjugated unsaturated esters.