title: Mannich-type reactions in the gas-phase: the addition of enol silanes to cyclic N-acyliminium ions.
abstract: The intrinsic gas-phase reactivity of cyclic N-acyliminium ions in Mannich-type reactions with the parent enol silane, vinyloxytrimethylsilane, has been investigated by double- and triple-stage pentaquadrupole mass spectrometric experiments. Remarkably distinct reactivities are observed for cyclic N-acyliminium ions bearing either endocyclic or exocyclic carbonyl groups. NH-Acyliminium ions with endocyclic carbonyl groups locked in s-trans forms participate in a novel tandem N-acyliminium ion reaction: the nascent adduct formed by simple addition is unstable and rearranges by intramolecular trimethylsilyl cation shift to the ring nitrogen, and an acetaldehyde enol molecule is eliminated. An NSi(CH(3))(3)-acyliminium ion is formed, and this intermediate ion reacts with a second molecule of vinyloxytrimethylsilane by simple addition to form a stable acyclic adduct. N-Acyl and N,N-diacyliminium ions with endocyclic carbonyl groups, for which the s-cis conformation is favored, react distinctively by mono polar [4(+) + 2] cycloaddition yielding stable, ressonance-stabilized cycloadducts. Product ions were isolated via mass-selection and structurally characterized by triple-stage mass spectrometric experiments. B3LYP/6-311G(d,p) calculations corroborate the proposed reaction mechanisms.
authors.affiliation: State University of Campinas-UNICAMP Intitute of Chemistry, SP, Brazil.
authors.forename: Daniela Maria
authors.initials: DM
authors.lastname: Tomazela
authors.forename: Luiz Alberto B
authors.initials: LA
authors.lastname: Moraes
authors.forename: Ronaldo A
authors.initials: RA
authors.lastname: Pilli
authors.forename: Marcos N
authors.initials: MN
authors.lastname: Eberlin
authors.forename: Marcelo G M
authors.initials: MG
authors.lastname: D'Oca