Chemistry of small‐ring compounds. Part 42. The rearrangement of methyleneaziridinium ions to cyclopropaniminium ions

Abstract

Abstract A kinetic study in FSO 3 H/SbF 5 reveals that the 1‐methyl‐2‐methyleneaziridinium ion 4 rearranges in a first order reaction to the ring‐opened enammonium ion 6. At temperatures below 80° this process takes place via isomerisation of the methyleneaziridinium ion to an intermediary cyclopropaniminium ion 5. At higher temperatures the direct conversion 4 → 6 ‐ which is negligible in the lower temperature region ‐ must also be taken into account (cf. Scheme 4). 1,1‐Dimethyl‐2‐methyleneaziridinium fluorosulfate undergoes a similar heterocyclic ‐ homocyclic ring rearrangement in FSO 3 H, but higher temperatures (115°) are required. The rearranged product, N,N ‐dimethylcyclopropaniminium fluorosulfate is much more stable than the mono‐ N ‐methyl derivative i.e. it can be heated up to 150° without decomposition. By analogy the methyleneaziridinium → cyclopropaniminium isomerisation is thought to take place via a ring‐opened biradical cationic species rather than via a diamagnetic 2‐(mono‐ or dialkylamino)allyl cation.