Kinetic and theoretical considerations in the hydrolysis of iminocarbonates

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 gem-dimethyl 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.