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 (R2C--C=NH) an d carbodiimide (RN=C=NR) has been undertaken. The detailed hydration mechan ism of the simplest cumulenes by water and water dusters (HX=C=NH + 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 determ ined for two possible reaction channels involving water attack across both C=C and C=N bonds of ketene imine. Using one and two actively participating water molecules to model the hydration, calculated results consistently sh ow that the C=N addition, giving first an amide enol, is favoured over the C-C yielding immediately the amide product. A reverse situation occurs when a chain of three water molecules is used. Since attack in two different pl anes is possible in the latter case, reducing the unfavourable distortion o f the methylene group, the C-C addition becomes easier to perform than the C=N, 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 C=C products in non-hindered ketene imines. Thus, water oligomers higher t han the dimer seem to make a primordial contribution to the rate of the hyd ration and are really needed to perform a concerted reaction. These gas-pha se results are confirmed when the effect of the solvent bulk is:taken into account in PCM calculations. Hydration pf the analogous carbodiimide, in wh ich addition can only occur across the C=N:bond, was also studied. The C=N addition with the aid of a three-water cluster is rate-determining followed by a tautomerization of the primary adduct leading to urea. Carbodiimide h ydration turns out to be easier to achieve than ketenimine hydration.