Differentiation between the concerted and stepwise mechanisms for aromatization (NIH-shift) of arene epoxides

Abstract

In order to differentiate between the kinetically equivalent mechanisms of either concerted proton migration and epoxide ring opening or stepwise formation of carbonium ion followed by proton migration to form in either case the keto form of the phenol, the primary kinetic isotope effects for aromatization of perdeuteriodihydroepoxybenzene and [1-²H]dihydro-1,2-epoxynaphthalene (k^H/k^D=ca. 1) as well as the substituent effects on the rate for benzene epoxide (ρ–7·3 and –7·6 for spontaneous and acid-catalysed paths, respectively) have been determined; the stepwise mechanism is strongly favoured.