Cyclization of acylium ions with nitriles: gas-phase synthesis and characterization of 1,3,5-oxadiazinium ions

Gas phase reactions of mass-selected acylium ions [CH3–C ¢O (1), CH2¢CH–C ¢O (2), C6H5–C ¢O (3), and (CH3)2N–C ¢O (4)] with nitriles (CH3CN, C2H5CN, CH2¢CHCN, and C6H5CN) were investigated using pentaquadrupole multiple-stage mass spectrometry. In analogy with the solution behavior, the ions were found to react readily with benzonitrile by cyclization via double nitrile addition to form aromatic 1,3,5-oxadiazinium ions. Cyclization with acetonitrile, propionitrile, and acrylonitrile is less general and occurs readily only for 4, by far the most reactive acylium ion tested. In “one-pot” reactions of 4 with two-component nitrile mixtures, cyclization via double nitrile addition occurs readily and forms both equally and differently 4,6-disubstituted isomeric 1,3,5-oxadiazinium ions. Using MS experiments, the 1,3,5-oxadiazinium ions were mass-selected and then either reacted with nitrogen nucleophiles or dissociated by low-energy collisions with argon. The nucleophiles add readily to the ions, whereas the symmetry of the 1,3,5-oxadiazinium ring allows two competitive dissociation pathways: double retro-addition that re-forms the reactant acylium ion, or an analogous dissociation that, formally and combined with cyclization, promotes group exchange between one nitrile and the acylium ion: RCO 2 RCN 3 cyclic 1,3,5-oxadiazinium ion 3 RCO RCN RCN. Isomeric 4,6-disubstituted 1,3,5-oxadiazinium ions are easily distinguished because the nitrile added second is lost first via the double retro-addition dissociation. (Int J Mass Spectrom 212 (2001) 445–454) © 2001 Elsevier Science B.V.