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Relevant academic research and scientific papers

Unimolecular Reactions of the Isolated Immonium Ions CH3CH=NH+C4H9, CH3CH2CH=NH+C4H9 and (CH3)2C=NH+C4H9

The reactions of ten metastable immonium ions of general structure R1R2C=NH+C4H9 (R1 = H, R2 = CH3, C2H5; R1 = R2 = CH3) are reported and discussed.Elimination of C4H8 is usually the dominant fragmentation pathway.This process gives rise to a Gaussian metastable peak; it is interpreted in terms of a mechanism involving ion-neutral complexes containing incipient butyl cations.Metastable immonium ions containing an isobutyl group are unique in undergoing a minor amount of imine (R1R2C=NH) loss.This decomposition route, which also produces a Gaussianmetastable peak, decreases in importance as the basicity of the imine increases.The correlation between imine loss and the presence of an isobutyl group is rationalized by the rearrangement of the appropriate ion-neutral complexes in which there are isobutyl cations to the isomeric complexes containing the thermodynamically more stable tert-butyl cations.A sizeable amount of a third reaction, expulsion of C3H6, is observed for metastable n-C4H9+NH=CR1R2 ions; in contrast to C4H8 and R1R2C=NH loss, C3H6 elimination occurs with a large kinetic energy release (40-48 kJ mol-1) and is evidenced by a dish-topped metastable peak.This process is explained using a two-step mechanism involving a 1,5-hydride shift, followed by cleavage of the resultant secondary open-chain cations, CH3CH+CH2CH2NHCHR1R2.

Trends in alkyl substituent effects on nucleophilic reactions of carbonyl compounds: Gas phase reactions between ammonia and R1R2COCH3+ oxonium ions

The reactivity of carbonyl substituted methyl oxonium ions (R1R2COCH3-) towards ammonia has been investigated using an FT-ICR mass spectrometer and ab initio calculations. The monosubstituted ions (R1=H: R2 = H, CH3, C2H5 and i-C3H7) show different reaction patterns with variable degree of: (1) nucleophilic substitution, (2) addition elimination and (3) proton transfer, when reacted with ammonia. In all cases addition-elimination dominates over nucleophilic substitution, and the observed reactions are slow. The trends in reactivity are consistent with the alkyl group's electronic properties, as expressed by a single parameter linear or slightly non-linear model.

Site of Gas-phase Methylation of 1-Phenyl-2-aminopropane

The regioselectivity of methyl cation transfer from (CH3)2F(1+), (CH3)2Cl(1+) and (CH3)3O(1+) to 1-phenyl-2-aminopropane was studied by Fourier transform ion cyclotron resonance in combination with collision-induced dissociation and neutralization-reionization mass spectrometry of the stable (1+) ions formed in a chemical ionization source.The (CH3)2F(1+) ion transfers a methyl cation to the NH2 group and the phenyl ring with almost equal probability.Predominant CH3(1+) transfer to the NH2 group is observed for the (CH3)2Cl(1+) ion whereas the (CH3)3O(1+) ion reacts almost exclusively at the amino group.The preference for m ethylation at NH2 is discussed in terms of a lower methyl cation affinity of the phenyl ring than of the amino group and the existence of an energy barrier for methylation of the phenyl moiety.

Proton Affinities and the Site of Protonation of Enamines in the Gas Phase

The gas-phase proton affinities of a number of methyl-substituted enamines and imines have been measured using ion cyclotron resonance spectroscopy.Comparison of the effect of substituents on the proton affinities of the enamines with those of corresponding amines is used to show that protonation in the gas phase occurs at carbon leading to the formation of an iminium ion.The observation of a large substituent effect for substitution of an α-methyl group also suggests that there is a significant amount of delocalization of positive charge in the iminium ion.A comparison with solution-phase basicities of enamines is also presented.

Unimolecular Reactions of Isolated Organic Ions. The Importance of Ion-Dipole Interaction

Possible mechanisms are discussed for the decomposition of "onium" ions of general formula R1CH=X+CR2R3CHR4R5 (R = H or alkyl; X = O, S, NH, or NCH3).For olefin elimination, to form R1CH=X+H and R2R3C=CR4R5, a simple concerted, "four-center" process can account for a considerable proportion of 2H-labeling results.However, such a mechanism would be symmetry forbidden and may be excluded on orbital symmetry and energetic grounds.A highly nonsynchronous mechanism is proposed, involving the formation of a loose complex of the carbonyl component, R1CH=X, and the carbonium ion, R2R3C+CHR4R5.Extensive stabilization of this intermediate is possible, by ion-dipole attraction; subsequent rearrangement can lead to a second complex, in which the carbonyl component and the incipient olefin are coordinated to a common proton.Isomerization of the nascent olefin fragment may take place, in the second complex, by protonation followed by deprotonation at a different site.Finally, the second complex breakes down, with elimination of an olefin or carbonyl component, the incipient fragment with the greater proton affinity remaining bound to the common proton.