An Osmium(III)/Osmium(V) Redox Couple Generating OsV(O)(OH) Center for cis-1,2-Dihydroxylation of Alkenes with H2O2: Os Complex with a Nitrogen-Based Tetradentate Ligand

For the synthesis of the 1,2-diols, cis-1,2-dihydroxylation of alkenes catalyzed by osmium­(VIII) tetroxide (OsO4) is a powerful method. However, OsO4 is quite toxic due to its highly volatile and sublimable nature. Thus, the development of alternative catalysts for cis-1,2-dihydroxylation of alkenes is highly challenging. Our approach involves the use of a nitrogen-based tetradentate ligand, tris­(2-pyridylmethyl)­amine (tpa), for an osmium center to develop a new osmium catalyst and hydrogen peroxide (H2O2) as a cheap and environmentally benign oxidant. The new Os–tpa complex acts as a very efficient turnover catalyst for syn-selective dihydroxylation of various alkenes (turnover number ∼1000) in aqueous media, and H2O2 oxidant is formally incorporated into the products quantitatively (100% atom efficiency). The reaction intermediates involved in the catalytic cycle have been isolated and characterized crystallographically as [OsIII(OH)­(H2O)­(tpa)]2+ and [OsV(O)­(OH)­(tpa)]2+ complexes. The observed syn-selectivity, structural characteristics of the intermediates, and kinetic studies have suggested a concerted [3 + 2]-cycloaddition mechanism between [OsV(O)­(OH)­(tpa)]2+ and alkenes, which is strongly supported by DFT calculations.