Dioxaborirane: a highly reactive peroxide that is the likely intermediate in borate catalysed electrophilic reactions of hydrogen peroxide in alkaline aqueous solution

Abstract

This paper reports on a kinetic and theoretical study into the borate mediated reaction of dimethyl sulfide with hydrogen peroxide in both acid and alkaline conditions. At high pH, whilst the kinetic data is consistent with the catalytic species being monoperoxoborate, formed from the rapid equilibrium between hydrogen peroxide and boric acid, DFT calculations show that this species is in fact less reactive than hydrogen peroxide, requiring us to seek an alternative catalytic mechanism. DFT provides an important insight for this, showing that although boric acid and peroxoboric acid are primarily Lewis acids, they can exhibit a small degree of Brønsted acidity, allowing, respectively, the B(O)(OH)₂⁻ and HOOB(OH)(O)⁻ anions to exist in small concentrations. Whilst the peroxoborate anion, HOOB(OH)(O)⁻, is predicted to have only marginal catalytic activity, its tautomer, dioxaborirane, (HO)₂BO₂⁻, a three membered cyclic peroxide, has a very low activation barrier of 2.8 kcal/mol. Hence, even though dioxaborirane is likely to be present in very low concentrations, it is still sufficiently reactive for overall rate enhancements to be observed for this system. This is the first literature report of this species. The observed low selectivity observed for borate catalysed reactions of hydrogen peroxide with a range of substituted phenyl methyl sulfides in our previous study (D. M. Davies, M. E. Deary, K. Quill and R. A. Smith, Chem.–Eur. J. 2005, 11, 3552–3558) is further evidence in favour of a highly reactive catalytic species. At low pH, kinetic data shows that borate catalyses the reaction between hydrogen peroxide and dimethyl sulfide; this is supported by DFT calculations that predict peroxoboric acid to be an effective catalytic intermediate, with an energy barrier of 7.4kcal mol⁻¹ compared to 10.1kcal mol⁻¹ for the uncatalysed system. Nevertheless, the overall contribution of this pathway is small because of the unfavourable equilibrium between hydrogen peroxide and boric acid to form peroxoboric acid.