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

Article abstract of DOI:10.1021/ja00528a038

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.