Dehydrobenzoyl cations: Distonic ions with dual free radical and acylium ion reactivity

In the gas phase, m- and p-dehydrobenzoyl cations display strong duality of chemical behavior. The ions react selectively as either free radicals or acylium ions, depending on the choice of the neutral reaction partner. Transacetalization with 2-methyl-1,3-dioxolane, ketalization with 2-methoxyethanol, and epoxide ring expansion with epichlorohydrin demonstrate their acylium ion reactivity, whereas (SCH3)-S-. abstraction with dimethyl disulfide demonstrates their free radical reactivity. In one-pot reactions with gaseous mixtures of epichlorohydrin and dimethyl disulfide, the m- and p-dehydrobenzoyl cations react selectively at either site to form the two monoderivatized ions in variable but controlled yields; further reaction at either the remaining radical or the acylium charge site forms a single biderivatized ion as the final product. The o-dehydrobenzoyl cation also displays the expected radical and acylium ion reactivities. But for the ortho isomer, binding of the nucleophilic neutral to the free or derivatized C+=O group facilitates reactions at the radical site. Hence, the ortho isomer displays a unique behavior; its acylium ion reactions either occur simultaneously with, or are followed by, H-abstraction radical reactions. As shown by ab initio calculations, the three isomers display sigma-localized odd-spin and pi-delocalized charge densities, which characterize distonic structures with molecular orbital-separated radical and charge sites. The dehydrobenzoyl cations are also, according to the calculations, the most stable among 19 of the most feasible C7H4O+. isomers.12043111361114