Kinetic and theoretical considerations in the hydrolysis of iminocarbonates

Abstract

Analysis of the pH–rate profiles for the hydrolysis of 4-methyl-2-(N-phenylimino)-1,3-dioxolane (1) and 2-[N-(p-fluorophenyl) imino]-4-phenyl-1,3-dioxolane (2) together with product distribution of compounds (1)–(5) and pK values estimated for the various ionic forms T⁰, T⁺, T^±, and T^– of the tetrahedral intermediates, enables the determination of the reactivity of the tetrahedral intermediates along their reaction pathways, At pH <6, the only products formed are the amine and the carbonate (C–N fission) and at pH < 6 C–O bond fission increases to form carbamates. The transition point corresponds to pK values of 9.2 and 8.5 for compounds (1) and (2), respectively. It is proposed that in acidic solution C–N bond fission proceeds via T⁺⇌ T^±→ P while at a moderate pH this bond fission proceeds via a proton-switch mechanism T⁰→ T^±. At pH > 11 the only product produced is the hydroxycarbamate (C–O fission)via its anionic intermediate T^–. Mono- and gemdimethyl substituted iminocarbonates T^– undergo two modes of ring-opening. Employing MNDO/H calculations the relative stability (ΔH_F) of the intermediates and their formed products was predicted and correlated with actual experimental product distribution.