Chemoselective oxidation of unsaturated organosulfur, selenium and phosphorus compounds by molybdenum oxodiperoxo complexes: A computational investigation

Oxidation is fundamental for many chemical processes and the search for chemoselective oxidants is relevant because most substrates have different functional groups. For unsaturated organo-heteroatom (E) substrates (RE)-E-1(CH2)(n)CH=(CRH)-H-2, the olefin and/or the heteroatom can be oxidized. It is known that molybdenum oxodiperoxo [MoO(O-2)(2)L] complexes can selectively oxidize sulfide groups in the presence of alkenes. Thus, the effects of the substituents (R-1 and R-2) and of the separation n between the functional groups on the chemoselectivity were investigated by methods based on the density functional theory (DFT) for sulfides (E = S). The chemoselectivity was quantified by the difference between the Gibbs energy of activation at the double bond (TS@C=C) and at the heteroatom (TS@E) oxidation pathways, Delta Delta(double dagger)G = Delta(double dagger)G(TS@C=C) - Delta(double dagger)G(TS@E). Consistent with experimental observations, this oxidation is chemoselective towards the heteroatom (E = S, Se, and PCH3) for any unsaturated substrate or ancillary ligand L in the complex. For unsaturated organosulfur compounds, it is shown that an increase in the electronegativity of R-1 leads to a decrease of the chemoselectivity, which can be correlated with the atomic charge at the sulfur center. The separation n affects the chemoselectivity similarly to the electronegativity of R-1, namely, differences between sp(2) and sp(3) carbon centers. The ancillary ligand L = OPH3, pyrazole, pyridine N-oxide, and Si(OH)(4), affects the reactivity of the complex and its chemoselectivity, where L = Si(OH)(4) shows the highest reactivity and the least selectivity; however, Delta Delta(double dagger)G is still large enough (4.3 kcal/mol) to provide high chemoselectivity. This agrees with the experimental observations related to the oxidations by [MoO(O-2)(2)L] complexes supported on silica. These quantitative results and the qualitative trends and correlations can be helpful in the design of more efficient and greener Mo-based oxidants. (C) 2017 Elsevier B.V. All rights reserved.