Issue 3, 2022

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

Abstract

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 (E1/2) and a red shift of the νOs[double bond, length as m-dash]O stretches as compared to those of tetrahedral OsO4 itself. Despite the negative shift of E1/2, 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 (BDEC–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.

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

Supplementary files

Article information

Article type
Paper
Submitted
11 Қар. 2021
Accepted
13 Жел. 2021
First published
14 Жел. 2021

Dalton Trans., 2022,51, 1123-1130

Author version available

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

T. Fujimoto, Y. Hirata, H. Sugimoto, M. Miyanishi, Y. Shiota, K. Yoshizawa and S. Itoh, Dalton Trans., 2022, 51, 1123 DOI: 10.1039/D1DT03819B

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