Transacetalization of 1,3-dioxane with acylium and sulfinyl cations in the gas phase

Abstract

Transacetalization occurs extensively in gas phase ion–molecule reactions of 1,3-dioxane with a variety of acylium ions [R–C⁺O; R = CH₃, C₂H₅, Ph, CH₃O, Cl, CH₂CH, (CH₃)₂N] and a sulfur analogue, the thioacetyl ion CH₃–C⁺S. Six-membered 1,3-dioxanylium ions and analogues, i.e. cyclic `ionic (thio)ketals', are formed, as evidenced by pentaquadrupole triple-stage collision-dissociation mass spectra and MP2/6–311G(d,p)//6–311G(d,p) + ZPE ab initio calculations, as well as by <sup>18</sup>O labelling experiments. Transacetalization with 1,3-dioxane is not a general reaction for sulfinyl cations (R–S<sup>+</sup>O). They react either moderately (CH<sub>3</sub>–S<sup>+</sup>O) or extensively (CH<sub>2</sub>CH–S<sup>+</sup>O) by transacetalization, form abundant intact adducts (Ph–S<sup>+</sup>O) or undergo mainly proton transfer and/or hydride abstraction reactions (Cl–S<sup>+</sup>O, CH<sub>3</sub>O–S<sup>+</sup>O and C<sub>2</sub>H<sub>5</sub>O–S<sup>+</sup>O). Competitive MS<sup>2</sup> experiments are employed to compare the transacetalization reactivity of different acylium ions, and that of two cyclic neutral acetals, that is 1,3-dioxane and 1,3-dioxolane. All the cyclic `ionic ketals' dissociate exclusively under low-energy collision conditions to regenerate the original reactant ion species, a simple dissociation chemistry that is amply demonstrated to be a very general characteristic of the transacetalization products. The cyclic `ionic thioketal' formed in transacetalization with CH₃–C⁺S is found, however, to dissociate exclusively to the oxygen analogue ion CH₃–C⁺O, a triple-stage mass spectrometric (MS³) experiment that constitutes a novel gas-phase strategy for conversion of thioacylium ions into acylium ions.