Transacetalization with gaseous carboxonium and carbosulfonium ions

Primary carboxonium (H2C=O+-R) and carbosulfonium (H2C=S+-R) ions (R=CH3, C 2H5, Ph) and the prototype five-membered cyclic carboxonium ion are found t o react in the gas phase with cyclic acetals and ketals by transacetalizati on to form the respective O-alkyl-1,3-dioxolanium and S-alkyl-1,3-oxathiola nium ions. The reaction, which competes mainly with proton transfer and hyd ride abstraction, initiates by O-alkylation and proceeds by ring opening an d recyclization via intramolecular displacement of the carbonyl compound pr eviously protected in its ketal form. As indicated by product ion mass spec tra, and confirmed by competitive reactions, carbosulfonium ions are, by tr ansacetalization, much more reactive than carboxonium ions. For acyclic sec ondary and tertiary carboxonium ions bearing acidic alpha -hydrogens, littl e or no transacetalization occurs and proton transfer dominates. This struc turally related reactivity distinguishes primary from both secondary and te rtiary ions, as exemplified for the two structural isomers H2C=O+-C2H5 and CH3C(H)=O+-CH3. The prototype five- and six-membered cyclic carboxonium ion s react mainly by proton transfer and adduct formation, but the five-member ed ring ion also reacts by transacetalization to a medium extent. Upon CID, the transacetalization products of the primary ions often dissociate by lo ss of formaldehyde, and a +44 u neutral gain/-30 u neutral loss MS3 scan is shown to efficiently detect reactive carboxonium and carbosulfonium ions. Transacetalization with either carboxonium or carbosulfonium ions provides a route to 1,3-oxathiolanes and analogs alkylated selectively either at the sulfur or oxygen atom. (C) 2001 American Society for Mass Spectrometry.