Osmium tetroxide

Other names: Tetraoxoosmium, osmium (VIII) oxide

OsO₄

22/50 common

Osmium tetroxide is a volatile crystalline solid that serves as a powerful oxidizing agent in organic synthesis, particularly for the dihydroxylation of alkenes. This highly toxic compound appears as pale yellow crystals that sublime readily at room temperature, releasing vapors that can cause severe eye damage and respiratory distress. Despite these hazards, its remarkable ability to perform syn-dihydroxylation with exceptional stereospecificity has made it indispensable in synthetic chemistry. OsO₄ achieves this transformation through a [3+2] cycloaddition with alkenes, forming an osmate ester that can be hydrolyzed to yield vicinal diols. This reactivity forms the foundation of the Sharpless asymmetric dihydroxylation using AD-mix α and β, where catalytic OsO₄ is paired with chiral ligands to achieve enantioselective oxidations. Modern syntheses often employ safer catalytic conditions with potassium osmate as the osmium source, reducing exposure to free OsO₄ while maintaining its valuable synthetic utility.