Hydrolysis of oxaziridines. Part 3. Evidence for both O- and N-conjugate acid formation with 2,3,3-triethyloxaziridine

Abstract

Both the kinetics and the products for the hydrolysis of 2,3,3-triethyloxaziridine in aqueous acid at 25 °C are reported. The reaction is subject to general acid or base catalysis in acetic acid buffers, but in aqueous perchloric acid, the acid catalysis is slight with the rate reaching an unusually sharp maximum in ca. 0.9M HClO₄ and then decreasing at higher acidities. Comparison with 2-[αα′-²H₂]ethyl-3,3-diethyloxaziridine reveals a substantial primary deuterium isotope effect at low acidities (k₀^H/k₀^Dca. 2.7) which steadily decreases to become negligible in 6.68M HClO₄. The variation in kinetic parameters relates to a change in the reaction products. In ca. 1M HClO₄ the major products are diethyl ketone and acetaldehyde, but N-ethylhydroxylamine is formed at the expense of acetaldehyde as the acidity increases.

The results are analysed and shown to be consistent with a mechanism requiring concurrent protonation of both O and N atoms of the oxaziridine ring. The O-conjugate acid intermediate undergoes relatively rapid decomposition to diethyl ketone and acetaldehyde by an E-2 process where cleavage of the O–N bond is synchronous with H⁺-abstraction from the N-ethyl group. The N-conjugate acid is much more stable. N-Ethylhydroxylamine is formed either by thermal decomposition or by a slow breakdown of the N-conjugate acid. Calculations suggest that O and N basicities are very similar and this unusual result is attributed to steric hindrance to solvation of the N-conjugate acid.