Substituent effects on tautomerization of oxepine to benzene oxide: a Hammett study via ab initio

Abstract

In order to find the susceptibility of the oxepine to benzene oxide tautomerization to the substituent effects, equilibrium constants are found for conversion of 3-(X)-oxepines to their corresponding 2-(X)-benzene oxides (Series 1), as well as 4-(X)-oxepines to analogous 3-(X)- benzene oxides (Series 2) where X=H, NH 2, OCH 3, CH 3, CF 3, F, Cl, Br, NO 2 and CN. For the equilibria related to Series 1, the K eq are: 31.29 ( H), 0.01 (NH 2), 498.58 (OCH 3), 2.71 (CH 3), 4.19 (CF 3), 0.31 (F), 1.03 (Cl), 1.72 (Br), 0.31 (NO 2) and 1.89 (CN). For Series 2, the K eq are: 31.29 ( H), 47.73 (NH 2), 7.97 (OCH 3), 16.48 (CH 3), 44.61 (CF 3), 52.81 (F), 52.81(Cl), 64.67 (Br), 374.21 (NO 2) and 107.31(CN). Experimental K eq has only been reported for X=H, which appears to consistent with that calculated in this report. Using B3LYP/6-31G* Hammett ρ value of 0.8 and 0.6 are obtained for Series 1 and 2, respectively; suggesting that electron withdrawing groups slightly shift the equilibrium in favor of benzene oxide. All rate constants ( k r), activation electronic energies (Δ E #), enthalpies (Δ H #) and Gibbs free energies (Δ G #) for 1 ⇌ 2 tautomerizations, are calculated at HF, DFT (B3LYP) and MP2 levels of theory, using 6-31G* standard basis set.