Uncommon aqueous media for nitrilimine cycloadditions. II. Computational study of the effect of water on reaction rate

The effect of water on the 1,3-dipolar cycloaddition reaction of nitrilimines to alkenes has been studied by HF and DFT ab initio calculations. As solvation models we used the polarizable continuum model and a model comprising small clusters consisting of reactants hydrogen-bonded to water molecules. In addition, a combination of these two models was considered. Reactivity has been qualitatively estimated by the familiar frontier molecular orbital approach and quantitatively computed by a recent extension to the hard-soft acid-base principle involving molecular reactivity indices. The main conclusions drawn from the extensive computational results are that (i) the effect of water is in general rather small and (ii) therefore, water is not directly responsible for the large acceleration of the 1,3-dipolar cycloaddition reaction of nitrilimines to alkenes. Its role is surely minor with respect to that played by the ionic catalysts present in the reaction mixture, in agreement with the conclusions drawn in the experimental companion paper.