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

Abstract

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 R1E(CH2)nCHCR2H, the olefin and/or the heteroatom can be oxidized. It is known that molybdenum oxodiperoxo [MoO(O2)2L] complexes can selectively oxidize sulfide groups in the presence of alkenes. Thus, the effects of the substituents (R1 and R2) 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@CC) and at the heteroatom (TS@E) oxidation pathways, ΔΔ‡G=Δ‡G(TS@CC)−Δ‡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 R1 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 R1, namely, differences between sp2 and sp3 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, ΔΔ‡G is still large enough (4.3kcal/mol) to provide high chemoselectivity. This agrees with the experimental observations related to the oxidations by [MoO(O2)2L] 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.