Unimolecular reactions of isolated organic ions: olefin elimination from immonium ions R1R2NCH2

Abstract

The slow unimolecular reactions of numerous tertiary immonium ions of general formula R¹R²NCH₂ are reported and discussed. Two distinct processes involving olefin loss are observed; the first reaction results in elimination of an olefin having the same number of carbon atoms as R¹(or R²), whilst the second proceeds with expulsion of a smaller olefin, having one less carbon atom than R¹(or R²). Evidence is presented to show that the latter reaction proceeds via transfer of a γ-hydrogen atom from R¹ or R² to the isolated CH₂ group, followed by σ-cleavage in the resulting open-chain carbonium ion. Whenever this process involves a secondary or tertiary cation, it dominates over the alternative route, for energetic reasons. However, when γ-hydrogen transfer produces a primary cation, both classes of olefin elimination occur in comparable abundance. Loss of olefins from R¹ and/or R², in competition, is likewise controlled by the nature of the intermediate cation.