title: Structurally diagnostic ion-molecule reactions: acylium ions with alpha-, beta- and gamma-hydroxy ketones.
abstract: Gas-phase reactions of four acylium ions and a thioacylium ion with three isomeric alpha-, beta- and gamma-hydroxy ketones are performed by pentaquadrupole mass spectrometric experiments. Novel structurally diagnostic reactions are observed, and found to correlate directly with interfunctional group separation. All five ions tested (CH(3)CO(+), CH(2)(double bond)CHCO(+), PhCO(+), (CH(3))(2)NCO(+) and (CH(3))(2)NCS(+)) react with the gamma-hydroxy ketone (5-hydroxy-2-pentanone) to form nearly exclusively a cyclic oxonium ion of m/z 85 that formally arises from hydroxy anion abstraction. With the beta-hydroxy ketone (4-hydroxy-2-pentanone), CH(2)(double bond)CHCO(+), PhCO(+) and (CH(3))(2)NCO(+) form adducts that undergo fast cyclization via intramolecular water displacement, yielding resonance-stabilized cyclic dioxinylium ions. With the alpha-hydroxy ketone (3-hydroxy-3-methyl-2-butanone), PhCO(+), (CH(3))(2)NCO(+) and (CH(3))(2)NCS(+) form stable adducts. Evidence that these adducts display cyclic structures is provided by the triple-stage mass spectra of the (CH(3))(2)NCS(+) adduct; it dissociates to (CH(3))(2)NCO(+) via a characteristic reaction-dissociation pathway that promotes sulfur-by-oxygen replacement. If cyclizations are assumed to occur with intramolecular anchimeric assistance, relationships between structure and reactivity are easily recognized.
authors.affiliation: Institute of Chemistry, State University of Campinas-UNICAMP, CP 6154, 13083-970 Campinas, SP, Brazil.
authors.forename: L A B
authors.initials: LA
authors.lastname: Moraes
authors.forename: Marcos N
authors.initials: MN
authors.lastname: Eberlin
mesh_terms: D000080:Acetals / Q000032:analysis / Q000737:chemistry*; D007659:Ketones / Q000737:chemistry*; D013058:Mass Spectrometry; D015394:Molecular Structure; D014835:Volatilization
chemical_list: D000080:Acetals; D007659:Ketones