TRANSACETALIZATION WITH ACYLIUM IONS - A STRUCTURALLY DIAGNOSTIC ION MOLECULE REACTION FOR CYCLIC ACETALS AND KETALS IN THE GAS-PHASE/

Transacetalization takes place in high yields in gas phase ion/molecul e reactions of acylium ions (RC+=O) with a variety of cyclic acetals a nd ketals, that is, five-, six-, and seven-membered 1,3-O,O-heterocycl es and their mono-sulfur and nitrogen analogues. A general, structural ly diagnostic method for the gas phase characterization of cyclic acet als and ketals is therefore available. Transacetalization occurs via i nitial O(or S)-acylation, followed by a ring-opening/ring-re-forming p rocess 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 e liminated in the course of the reaction, is found to affect considerab ly the extent of transacetalization. Substituents not at the 2-positio n remain in the ionic products; hence positional isomers produce diffe rent cyclic ''ionic ketals'' and are easily differentiated. The triple -stage (MS3) mass spectra of the cyclic ''ionic ketals'' show in all c ases 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 O-18-labe led transacetalization product of 1,3-dioxolane shows dissociation to both CH3C+=O-18 and CH3C+=O to the same extent, which confirms its cyc lic ''ionic ketal'' structure and the ''oxygen-scrambling'' mechanism of transacetalization. Ab initio MP2/6-31G(d,p)//6-31G-(d,p) + ZPE ene rgy surface diagrams show that transacetalization is the most exotherm ic, 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.