Li + -Coordinated Polyglycol Radicals: Ion—Molecule Reactions with Alkenes and Ethers

Abstract

The Li⁺ complexes of the radicals HOCH₂CH₂O^• (“oxy” radical), HOCH₂CH₂OCH₂^• (“methyl” radical) and HOCH₂CH₂OCH₂CH₂^• (“ethyl” radical) are produced in the gas phase by fast atom bombardment of poly(ethylene glycol) 200 and the ion–molecule reactions of these novel distonic ions, [R^•+Li]⁺, with 1-butene, dimethyl ether and methyl vinyl ether are examined in the hexapole collision cell of a sector/quadrupole tandem mass spectrometer. The products arising from these reactions depend on the reagent used and the type of radical site present in [R^•+Li]⁺. With 1-butene, predominantly radical-site addition–eliminations take place, initiated by addition of the unpaired electron of [R^•+Li]⁺ to the olefinic double bond; fragmentations, induced by the unpaired electron of the adduct, follow to create stable closed-shell products. Dimethyl ether attaches to the charge site of [R^•+Li]⁺ (i.e. at the metal ion) and the adduct reacts further by elimination of (mainly) small radicals from either the original [R^•+Li]⁺ segment or the added reagent. Methyl vinyl ether, which blends the structural features of alkenes and ethers, gives rise to a combination of such radical-and charge-site addition–fragmentation processes. The dissociations promoted by the unpaired electron involve β-scissions and ^•H rearrangements, which are the typical reactions of neutral radicals. All lithiated radicals undergo addition to double bonds. A significant fraction of the oxy radical also reacts via hydrogen atom abstraction from the added reagent. The charge site (metal ion) does not directly participate in the radical-site reactions observed, but facilitates specific channels by transiently coordinating to dissociating ligands, so that inter-ligand hydrogen atom rearrangements between these ligands can occur. This type of metal ion involvement in radical reactivity could be particularly important in biological milieus where metal ions abound.

Keywords

adicals distonic ions ion–molecule reactions addition–fragmentation pathways polyglycol tandem mass spectrometry

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