Theoretical study of the gas‐phase thermal isomerization of isoxazoles. Part I. Isoxazole

An MNDO study was made of the gas‐phase thermal isomerization of isoxazole to explore theoretically the proposed reaction mechanism. The results showed that isoxazole isomerizes through an azirine, as an intermediate, to oxazole via a nitrile ylide or to ketenimine, with similar activation energies, and that the first step is rate limiting, in agreement with the experimental results. These results also show that isomerization to a nitrile is possible, but in this case the energy barrier is grater than the corresponding isomerization to oxazole. The MNDO study also supports a concerted process for the rate‐limiting step, as reported earlier on the basis of experimental studies.