Abstract

The mechanism of the uncatalyzed hydrolysis of various classes of N-sulfinyl compounds (N=S=O) has been studied with the B3LYP/6-31+G(2d,2p) model chemistry, which was validated to provide reliable results in terms of the reproduction of experimental geometries and determination of energies of the reaction. N-sulfinylamines (R–N=S=O), -hydrazines (R–NH–N=S=O), -hydrazides (R–CO–NH–N=S=O) and amides (R–CO–N=S=O), known to possess a widely varying hydrolytic behaviour, were studied in comparison, and the electronic structures of many were determined for the first time. The effect of the substituent was investigated through R = H, CH3 and Ph for each class of N-sulfinyl species; the series was expanded by eight more substituents for N-sulfinylamines. The hydrolysis of selected N-sulfinylamines was compared to that of the related isocyanates (R–N=C=O).
Hydrolysis of all N-sulfinyl species involves the electrostatic interaction of water towards sulfur, with either close-to-perpendicular or in-plane alignment of a water molecule, with respect to the NSO plane, in a pre-reaction complex. A two-water-molecule model in gas phase calculations was determined to be sufficient for an adequate description of the hydrolysis. The rate-determining step of the reaction consists of the concerted hydroxylation of sulfur with protonation of either nitrogen (attack across the N=S bond) or oxygen (attack across the S=O bond) atoms of the NSO group. Electron-withdrawing substituents were found to decrease, electron-donating substituents to increase the reaction barrier. A fundamental difference in the resonance description of the various classes of N-sulfinyl species was found to be responsible for their distinctively varying sensitivity towards moisture.
Activation barriers for hydrolysis correlate well with the strength of the S…O interaction, governed by the charge on the electrophilic sulfur atom, which can be accurately determined by both the Quantum Theory of Atoms in Molecules and the Natural Bond Orbital Analysis. The computationally determined order of reactivity of the various classes of NSO compounds, quantitatively defined through the charge on sulfur, agrees well with the available qualitative experimental data. Thus, N-sulfinylamides and aliphatic N-sulfinylamines are most reactive, N-sulfinylhydrazides are much less reactive, and N-sulfinylhydrazines are completely inert to water.
The clear dependence of the hydrolytic reactivity of N-sulfinyl species on the charge of the reactive center suggests the possibility of using such dependences in the prediction of the general reactivity of N-sulfinyl compounds, as most of their reactions involve the sulfur atom or the N=S bond.