Necessity to consider a three-water chain in modelling the hydration of ketene imines and carbodiimides
A theoretical study of the hydration of a model ketene imine (R2CCNH) and carbodiimide (RNCNR) has been undertaken. The detailed hydration mechanism of the simplest cumulenes by water and water clusters (HXCNH + n H2O→H2XCONH2 + (n – 1) H2O, n = 1, 2, 3 and X = CH, N) was modelled using high-level ab initio MO methods. Geometric and energetic parameters were determined for two possible reaction channels involving water attack across both CC and CN bonds of ketene imine. Using one and two actively participating water molecules to model the hydration, calculated results consistently show that the CN addition, giving first an amide enol, is favoured over the CC yielding immediately the amide product. A reverse situation occurs when a chain of three water molecules is used. Since attack in two different planes is possible in the latter case, reducing the unfavourable distortion of the methylene group, the CC addition becomes easier to perform than the CN, with an energy barrier of 48 kJ mol–1 found at the CCSD(T)/6-31G(d,p) level, the lowest barrier of all the calculated water chain models. These findings are consistent with experimental evidence for direct formation of CC products in non-hindered ketene imines. Thus, water oligomers higher than the dimer seem to make a primordial contribution to the rate of the hydration and are really needed to perform a concerted reaction. These gas-phase results are confirmed when the effect of the solvent bulk is taken into account in PCM calculations. Hydration of the analogous carbodiimide, in which addition can only occur across the CN bond, was also studied. The CN addition with the aid of a three-water cluster is rate-determining followed by a tautomerization of the primary adduct leading to urea. Carbodiimide hydration turns out to be easier to achieve than ketenimine hydration.