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

Article abstract of DOI:10.1002/oms.1210251005

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.