Acyclic distonic acylium ions: dual free radical and acylium ion reactivity in a single molecule

Abstract

Three gaseous acyclic distonic acylium ions: •CH 2–CH 2–C +O, •CH 2–CH 2–CH 2–C +O, and •CH 2C(CH 2)–C +O, are found to display dual free radical and acylium ion reactivity; with appropriate neutrals, they react selectively either as free radicals with inert charge sites, or (and more pronouncedly) as acylium ions with inert radical sites. The free radical reactivity of the ions is demonstrated via the Kenttämaa reaction: CH 3S • abstraction with the spin trap dimethyl disulfide; their ion reactivity by two reactions most characteristic of acylium ions: transacetalization with 2-methyl-1,3-dioxolane and the gas-phase Meerwein reaction, that is, expansion of the three-membered epoxide ring of epichlorohydrin to the five-membered 1,3-dioxolanylium ion ring. In “one-pot” reactions with gaseous mixtures of epichlorohydrin and dimethyl disulfide, the ions react selectively at either site, but more readily at the acylium charge site, to form the two mono-derivatized ions. Further reaction at either the remaining free radical or acylium charge site forms a single bi-derivatized ion as the final product. Becke3LYP/6-31G( d) calculations predict the reactions at the acylium charge sites of the three distonic ions to be highly exothermic, and both the “hot” transacetalization and epoxide ring expansion products of •CH 2–CH 2–CH 2–C +O to dissociate rapidly by H 2CCH 2 loss in overall exothermic processes. The calculations also predict highly spatially separate odd spin and charge sites for the novel cyclic distonic ketal ions formed by the reactions at the acylium charge sites.