Isotope effects and the mechanism of fragmentation of epoxy imino-1,3,4-oxadiazolines

The fragmentation of an epoxy imino-1,3,4-oxadiazoline is studied by a combination of isotope effects and theoretical calculations. Significant primary 13C isotope effects are observed at the two oxadiazoline carbons but negligible isotope effects are observed at the remaining carbons. This is consistent with a rate-limiting fragmentation of the oxadiazoline without fragmentation of the adjacent epoxide ring. Theoretical calculations support this interpretation. This formal coarctate reaction avoids two complicated coarctate fragmentations in favor of a multi-step mechanism with a simple coarctate final step.