C(sp³)-H bond activation by the carboxylate-adduct of osmium tetroxide (OsO₄)

The reaction of osmium tetroxide (OsO₄) and carboxylate anions (acetate: X^- = AcO^- and benzoate: X^- = BzO^-) gave 1 : 1 adducts, [OsO₄(X)]^- (1X), the structures of which were determined by X-ray crystallographic analysis. In both cases, the carboxylate anion X coordinates to the osmium centre to generate a distorted trigonal bipyramidal osmium(VIII) complex. The carboxylate adducts show a negative shift of the redox potentials (E_1/2) and a red shift of the ν_OsO stretches as compared to those of tetrahedral OsO₄ itself. Despite the negative shift of E_1/2, the reactivity of these adduct complexes 1X was enhanced compared to that of OsO₄ in benzylic C(sp³)-H bond oxidation. The reaction obeyed the first-order kinetics on both 1X and the substrates, giving the second-order rate constant (k₂), which exhibits a linear correlation with the C-H bond dissociation energy (BDE_C-H) of the substrates (xanthene, 9,10-dihydroanthracene, fluorene and 1,2,3,4-tetrahydronaphthalene) and a kinetic deuterium isotope effect (KIE) of 9.7 (k₂(xanthene-h₂)/k₂(xanthene-d₂)). On the basis of these kinetic data together with the DFT calculation results, we propose a stepwise reaction mechanism involving rate-limiting benzylic hydrogen atom abstraction and subsequent rebound of the generated organic radical intermediate to a remaining oxido group on the osmium centre.