The arene oxede-oxepine valence isomerization; a dynamic nmr investigation using a prochiral substituent

The geminal -methytene protons in 3-ethylnaphthalene-l,2-oxide show chemical shift nonequivalence in ¹H NMR spectra recorded at ambient temperature consistent with a high barrier to enantiomerization via the orepine tautomer. The methylene protons in -benzoyloxytoluene-2,3-oxide which are a singlet at ambient temperature split into an AB system at -135 °C and there is evidence of ca. 5% of the oxepine isomer. Lineshape analysis of the coalescing arene oxide AB Signals provides the activation parameters for the valence isomerization/degenerate racemization process: ΔGXXX = 7.6 kcal mol^-1, ΔHXXX = 8.6 kcal mol^-l, ΔSXXX= 5.8 cal K^-l mol^-1.