Transacetalization with Acylium Ions. A Structurally Diagnostic Ion/Molecule Reaction for Cyclic Acetals and Ketals in the Gas Phase

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Abstract

Transacetalization takes place in high yields in gas phase ion/molecule reactions of acylium ions (RC+O) with a variety of cyclic acetals and ketals, that is, five-, six-, and seven-membered 1,3-O,O-heterocycles and their mono-sulfur and nitrogen analogues. A general, structurally diagnostic method for the gas phase characterization of cyclic acetals and ketals is therefore available. Transacetalization occurs via initial O(or S)-acylation, followed by a ring-opening/ring-re-forming process in which a neutral carbonyl compound is eliminated and cyclic “ionic ketals” (that is, cyclic 1,3-dioxonium ions and analogues) are formed. The nature of the substituents at the 2-position, which are eliminated in the course of the reaction, is found to affect considerably the extent of transacetalization. Substituents not at the 2-position remain in the ionic products; hence positional isomers produce different cyclic “ionic ketals” and are easily differentiated. The triple-stage (MS3) mass spectra of the cyclic “ionic ketals” show in all cases major dissociation to re-form the reactant acylium ion, a unique dissociation chemistry that is equivalent to the hydrolysis of neutral acetals and ketals and which is then determined to be a very general characteristic of cyclic “ionic ketals”. Additionally, the 18O-labeled transacetalization product of 1,3-dioxolane shows dissociation to both CH3C+18O and CH3C+O to the same extent, which confirms its cyclic “ionic ketal” structure and the “oxygen-scrambling” mechanism of transacetalization. Ab initio MP2/6-31G(d,p)//6-31G(d,p) + ZPE energy surface diagrams show that transacetalization is the most exothermic, thermodynamically favorable process in reactions of CH3C+O with 1,3-dioxolane and 1,3-oxathiolane, whereas 1,3-dithiolane is unreactive due to the endothermicity of the initial acylation step.

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