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Syn-dihydroxylation of olefins occur via the reaction with osmium tetroxide. Cycloaddition of osmium tetroxide with ethene proceeds via a cyclic transition state to form an osmate ester. Addition occurs on the same side of the ethene molecule to give a syn-addition. Work-up with water gives the syn-diol and reduced Osmium(VI).
Osmium(VIII) is reduced to Os(VI) over the reaction which is an oxidation.