title: Double transacetalization of diacylium ions.
abstract: A novel gas-phase reaction of diacylium ions of the O=C=X(+)=C=O type (X = N, CH) is reported: double transacetalization with cyclic acetals or ketals. The reaction is exothermic and highly efficient, and forms members of a new class of highly charged-delocalized ions: cyclic ionic diketals. Pentaquadrupole double- and triple-stage mass spectrometric (MS(2) and MS(3)) experiments reveal the high double transacetalization reactivity of O=C=N(+)=C=O and O=C=CH(+)=C=O, whereas the synthesis of differently substituted cyclic ionic diketals is performed in MS(3) experiments via sequential mono- and double transacetalization of O=C=N(+)=C=O and O=C=CH(+)=C=O with different acetals. With cyclic acetals, the acylium-thioacylium ion O=C=N(+)=C=S reacts promptly and selectively by mono-transacetalization at its acylium site, but the free thiacylium site of its cyclic ionic ketal is nearly unreactive by double transacetalization. Therefore, only the acylium site of O=C=N(+)=C=S can be efficiently protected by transacetalization. Low-energy MS(3) collision-induced dissociation of the cyclic ionic diketals of O=C=N(+)=C=O and O=C=CH(+)=C=O sequentially frees each of the protected acylium site to form the mono-derivatized ion, and then the fully deprotected diacylium ion.
authors.affiliation: State University of Campinas-UNICAMP, Institute of Chemistry, CP 6154, 13083-970 Campinas, SP, Brazil.
authors.forename: R
authors.initials: R
authors.lastname: Sparrapan
authors.forename: M A
authors.initials: MA
authors.lastname: Mendes
authors.forename: M N
authors.initials: MN
authors.lastname: Eberlin
mesh_terms: D000080:Acetals / Q000737:chemistry*; D017953:Isocyanates / Q000737:chemistry*; D013058:Mass Spectrometry
chemical_list: D000080:Acetals; D017953:Isocyanates