Does the Reaction of Cyclopropyl Acid Chlorides and Imines To Form 1,3-Oxazin-4-enone Heterocycles Proceed via a Ketene or an N‑Acyl-iminium Mechanism?

The mechanism of the reaction between cyclopropyl acid chlorides and imines to form 1,3-oxazin-4-enones was probed through physical and computational experiments. The data gathered strongly support the reaction proceeding through an N-acyl iminium intermediate mechanism rather than a ketene intermediate mechanism.