Anion Structure Determination in the Gas Phase: Chemical Reactivity as a Probe

摘要

In the gas phase, the discrimination between two isomeric anion structures is a challenge that requires different solutions for different applications. The anionic oxy-Cope rearrangement involves the rearrangement of an alkoxide to an isomeric enolate; the mechanistic study of such a process in the gas phase requires a simple and selective probe process. Using a flowing afterglow mass spectrometer, we have examined the utility and limitations of using chemical reactivity to discriminate between alkoxides and enolates in the gas phase. A series of alkoxides and enolates were allowed to react with three chemical probe reagents: methanol-O-d, methyl nitrite, and dimethyl disulfide. Quantitative and qualitative characterization of each probe reagent reveals the especially broad and flexible utility of dimethyl disulfide as a chemical probe. Dimethyl disulfide is a selective reagent with ambident behavior that reacts efficiently with all anions studied and fully capitalizes on the structure/reactivity differences between alkoxides and enolates. Alkoxides behave as classical "hard bases" when allowed to react with dimethyl disulfide, effecting elimination across the C—S bond, whereas enolates, "soft bases", attack at sulfur. Methyl nitrite is also a selective ambident probe reagent but, due to its particularly slow reaction with enolates, is useful only in conjunction with a more reliable probe such as dimethyl disulfide. Methanol-O-d, for a variety of reasons detailed in the paper, is unsuitable as a chemical probe reagent for the unequivocal discernment between alkoxides and enolates.