THE ROLE OF ION-NEUTRAL COMPLEXES IN THE REACTIONS OF ONIUM IONS AND RELATED SPECIES

An account is given of the development of the proposal that ion-neutral complexes are involved in the unimolecular reactions of onium ions (R1R2C-Z+R3 ;Z = 0, S, NR4; R1, R2, R3, R4 = H, CnH2n+1), with particular emphasis on the informative C4H9O+ oxonium ion system (Z = 0; R1, R2 = H; R3 = C3H7). Current ideas on the role of ion-neutral complexes in cation rearrangements, hydrogen transfer processes and more complex isomerizations are illustrated by considering the behaviour of isomeric CH3CH2CH2X+ and (CH3)2CHX+ species [X = CH2O, CH3CHO, H2O, CH3OH, NH3, NH2CH3, NH(CH3)2, CH2-NH, CH2-NCH3, CO, CH3., Br. and I.]. Attention is focused on the importance of four energetic factors (the stabilization energy of the ion-neutral complex, the energy released by rearrangement of the cationic component, the enthalpy change for proton transfer between the partners of the ion neutral complex and the ergicity of recombination of the components) which influence the reactivity of the complexes. The nature and extent of the chemistry involving ion-neutral complexes depend on the relative magnitudes of these parameters. Thus, when the magnitude of the stabilization energy exceeds the energy released by cation rearrangement, the ergicity of proton transfer is small, and recombination of the components in a new way is energetically favourable, extensive complex-mediated isomerizations tend to occur. Loss of H2O from metastable CH2=O+C3H7 ions is an example of such a reaction. Conversely, if the stabilization energy is small compared with the magnitude of the energy released by cation rearrangement, the opportunities for complex-mediated processes to become manifest are decreased, especially if proton transfer is endoergic. Thus, CH3CH2CH2CO+ expels CO, with an increased kinetic energy release, after rate-limiting isomeriZation of CH3CH2CH2+---CO to (CH3)2CH+---CO has taken place. When proton transfer between the components of the complex is strongly exoergic, fragmentation corresponding to single hydrogen transfer occurs readily. The proton-transfer step is often preceded by cation rearrangement for CH3CH2CH2X+ Species. In such circumstances, the involvement of ion-neutral complexes can be detected by the observation of unusual site selectivity in the hydrogen-transfer step. Thus, C3H6 loss from CH2=N+(R1)CH2CH2CH3 (R1 = H, CH3, C3H7) immonium ions is found by H-2-labelling experiments to proceed via preferential alpha- and gamma-hydrogen transfer; this finding is explained if the incipient +CH2CH2CH3 ion isomerizes to CH3CH+CH3 prior to proton abstraction. In contrast, the isomeric CH2=N+(R1)CH(CH3)2 Species undergo specific beta-hydrogen transfer because the developing CH3CH+CH3 Cation is stable with respect to rearrangements involving a 1,2-H shift.