Assessing the rates of ring-opening of aziridinium and azetidinium ions: a dramatic ring size effect.

Rates for the ring-opening of aziridinium and azetidinium ions by DMAP were measured. The four-membered ring appears to be ca. 17,000 times less reactive compared to the three-membered ring but is still highly relevant from a synthetic viewpoint. The electrophilicity of these strained ammonium ions is measured for the first time.