Dihydroxylation of alkenes using a Tp–osmium complex

The reaction of TpOsVI(N)(OH)2 (1) [Tp = hydrotris(1-pyrazolyl)borate], m-chloroperbenzoic acid (m-CPBA), and trans-stilbene in C6H6 at room temperature gives the diolate complex TpOsVI(N)(trans-O2C2H2Ph2) (2) and m-chlorobenzoic acid (m-CBA). The trans configuration of the stilbene is retained in the diolate complex as shown in the crystal structure of 2. Complex 2 is hydrolyzed by 2 equivalents of aqueous HCl in CD2Cl2 to give rac-hydrobenzoin, the product of cis-dihydroxylation, and TpOsVI(N)Cl2. cis-Stilbene, styrene, cyclohexene, trans-dimethyl fumarate, trans-methyl cinnamate, and trans-4-dimethylamino-4′-nitrostilbene are also converted to their corresponding free diol products by reaction with 1 and m-CPBA in CD2Cl2, and then aqueous HCl. In aqueous HBF4, oxidation of the water-soluble olefin, 4-styrenesulfonic acid, by 1 is modestly catalytic (15 turnovers in 3 h) with excess PbO2 as the oxidant. Under non-acidic conditions, the diolate complexes are inert and catalysis is precluded. Mechanistic experiments implicate an oxidizing osmium complex as an intermediate in these reactions.