Osmium(III) and osmium(V) complexes bearing a macrocyclic ligand: a simple and efficient catalytic system for cis-dihydroxylation of alkenes with hydrogen peroxide.

A simple protocol that uses Os(III)(OH)(H2O)(L-N4Me2)2 (1; L-N4Me2 = N,N'-dimethyl-2,11-diaza3.3pyridinophane) as a catalyst and H2O2 as a terminal oxidant for efficient cis-1,2-dihydroxylation of alkenes is presented. Unfunctionalized (or aliphatic) alkenes and alkenes/styrenes containing electron-withdrawing groups are selectively oxidized to the corresponding vicinal diols in good to excellent yields (46-99 %). In the catalytic reactions, the stoichiometry of alkene:H2O2 is 1:1, and thus the oxidant efficiency is very high. For the dihydroxylation of cyclohexene, the catalytic amount of 1 can be reduced to 0.01 mol % to achieve a very high turnover number of 5500. The active oxidant is identified as the Os(V)(O)(OH) species (2), which is formed via the hydroperoxide adduct, an Os(III)(OOH) species. The active oxidant 2 is successfully isolated and crystallographically characterized.