C(sp 3 )-H bond activation by the carboxylate-adduct of osmium tetroxide (OsO 4 ).

The reaction of osmium tetroxide (OsO 4 ) and carboxylate anions (acetate: X - = AcO - and benzoate: X - = BzO - ) gave 1 : 1 adducts, [OsO 4 (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 OsO 4 itself. Despite the negative shift of E 1/2 , the reactivity of these adduct complexes 1X was enhanced compared to that of OsO 4 in benzylic C(sp 3 )-H bond oxidation. The reaction obeyed the first-order kinetics on both 1X and the substrates, giving the second-order rate constant ( k 2 ), 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 ( k 2 (xanthene- h 2 )/ k 2 (xanthene- d 2 )). 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.