The key role of zwitterionic species in the conversion of ureas into isocyanates in aqueous solution. Hydrolysis of 1-phenylcarbamoylimidazole

Abstract

N-Phenylcarbamoylimidazole (2) is converted into aniline and imidazole near neutral pH but is relatively unreactive in acidic and basic solution. The rate of hydrolysis of (2) varies in a complex way as the pH of the medium [H₂O; 30°; µ= 1·0 (KCl)] is changed, reaching two maxima at pH ca. 4·5 and 10·0. It is proposed that the zwitterionic tautomer (4) of (2) is the reactive species over the entire pH range [except in highly basic solution where the conjugate base of (2) reacts]. The rate of decomposition of the zwitterion (to imidazole and phenyl isocyanate initially) is so rapid (t_½ca. 10^–4 s) that the diffusion-controlled proton transfers involved in the formation of the zwitterion become rate limiting in the pH region 4–10. Addition of sufficient concentration of general acids or bases (except imidazole) can cause a change in the rate-limiting step to zwitterion decomposition in this pH region. Data are also presented for 1-phenylcarbamoylbenzimidazole (14), 1-(p-bromophenylcarbamoyl)imidazole (11), and 3,5-dimethyl-1-phenylcarbamoylpyrazole (12) and are interpreted similarly. The catalysis of the diffusion-controlled proton transfer steps by primary, secondary, and tertiary amine catalysts is investigated in detail for the hydrolysis of (14). Addition of low concentrations (< 5 × 10^–5M) of imidazole in neutral solution initially increases the rate of hydrolysis of (2)(imidazole acting as a general acid–base) but high concentrations of imidazole actually retard the overall rate of hydrolysis of (2). This is interpreted in terms of imidazole acting as an efficient trap for the phenyl isocyanate intermediate, thus regenerating the substrate.