Protonation study of some enamine systems

Amino, Ammonio and Aminioethenes: A Theoretical Study of their Structure and Energetics

We have performed high level ab initio quantum mechanical calculations for aminoethene and the three isomeric 1,1- (Z)- or (E)-1,2-diaminoethenes as well as their singly and doubly charged cations derived by loss of electrons and/or upon protonation. Gas phase molecular structures were computed at the MP2/6-311+G(3df,2p) level. Standard molar enthalpies of formation in the gas phase, at T = 298.15 K were estimated using the G3 composite method and atomization, isodesmic and homodesmotic reactions. Other energetic parameters were also calculated at the G3 level: proton affinities, basicities and adiabatic ionization enthalpies. Theoretical and experimental data are compared. The reported experimental data refer only to aminoethene wherein the standard molar enthalpy of formation has a considerable uncertainty, although the molecular structure is well-established. There are no such data, neither structural nor thermochemical, for any of the three isomeric diaminoethenes. Isoelectronic comparisons are made. For example, the diprotonated diaminoethenes are isoelectronic to isobutene and (Z)- and (E)-butene, while the doubly ionized diaminoethenes are likewise related to trimethylenemethane and 1,3-butadiene.

Fragmentation of plumeran indole alkaloids from Aspidosperma spruceanum by electrospray ionization tandem mass spectrometry

The fragmentation of six plumeran indole alkaloids (PIAs) previously isolated from Aspidosperma spruceanum has been investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS) in the positive ion mode. The fragmentation pathways have been established on the basis of MS/MS experiments using fragment ions generated in-source and deuterium-labeled alkaloids as precursor ions and on the basis of accurate mass measurements. Our results demonstrated that the fragmentation routes observed for the protonated PIAs are essentially derived from a pericyclic reaction and from the opening of rings D and E, followed by 1,4-hydrogen rearrangements. Product ions resulting from radical eliminations were also observed, contrary to the 'even-electron rule'. Our data reveals that some product ions from protonated PIAs provide crucial information for the characterization of the acyl substituent at N-1, the methoxyl and hydroxyl groups at the aromatic moiety, and give evidence of an ether bridge between C-18 and C-21. The data reported here were used for the dereplication of these compounds in a stem bark methanolic extract of Aspidosperma spruceanum.