Editing Osmium tetroxide (section)

===Oxidation of alkenes===
Alkenes add to OsO<sub>4</sub> to give [[alkoxide|diolate]] species that hydrolyze to ''cis''-diols. The net process is called dihydroxylation. This proceeds via a [3 + 2] [[cycloaddition]] reaction between the OsO<sub>4</sub> and alkene to form an intermediate osmate ester that rapidly hydrolyses to yield the [[Diol#Vicinal diols|vicinal diol]]. As the oxygen atoms are added in a concerted step, the resulting stereochemistry is ''[[Cis-trans isomerism|cis]]''.

:[[File:Dihydroxylation with OsO4.png|thumb|center|600px|Idealized depiction of the cis-dihydroxylation of alkenes.]]

OsO<sub>4</sub> is expensive and highly toxic, making it an unappealing reagent to use in [[stoichiometric]] amounts. However, its reactions are made  [[Catalysis|catalytic]] by adding [[reoxidant]]s to reoxidise the Os(VI) by-product back to Os(VIII). Typical reagents include [[hydrogen peroxide|H<sub>2</sub>O<sub>2</sub>]] ([[Milas hydroxylation]]), [[N-Methylmorpholine N-oxide|N-methylmorpholine N-oxide]] ([[Upjohn dihydroxylation]]) and [[potassium ferricyanide|K<sub>3</sub>Fe(CN)<sub>6</sub>]]/water. These reoxidants do not react with the alkenes on their own. Other osmium compounds can be used as catalysts, including osmate(VI) salts ([OsO<sub>2</sub>(OH)<sub>4</sub>)]<sup>2−</sup>, and osmium trichloride hydrate (OsCl<sub>3</sub>·''x''H<sub>2</sub>O). These species oxidise to osmium(VIII) in the presence of such oxidants.<ref>{{ cite journal | title = On the timing of hydrolysis / reoxidation in the osmium-catalyzed asymmetric dihydroxylation of olefins using potassium ferricyanide as the reoxidant |author1=Ogino, Y. |author2=Chen, H. |author3=Kwong, H.-L. |author4=Sharpless, K. B. | journal = [[Tetrahedron Letters]] | year = 1991 | volume = 32 | issue = 32 | pages = 3965–3968 | doi = 10.1016/0040-4039(91)80601-2 }}</ref>

Lewis bases such as tertiary [[amine]]s and [[pyridine]]s increase the rate of dihydroxylation. This "ligand-acceleration" arises via the formation of [[adduct]] OsO<sub>4</sub>L, which adds more rapidly to the alkene. If the amine is chiral, then the dihydroxylation can proceed with enantioselectivity (see [[Sharpless asymmetric dihydroxylation]]).<ref name=catalysis>{{ cite journal |author1=Berrisford, D. J. |author2=Bolm, C. |author3=Sharpless, K. B. | title = Ligand-Accelerated Catalysis | year = 1995 | journal = [[Angewandte Chemie International Edition]] | volume = 34 | issue = 10 | pages = 1059–1070 | doi = 10.1002/anie.199510591 }}</ref> OsO<sub>4</sub> does not react with most carbohydrates.<ref name=stain/>

The process can be extended to give two [[aldehyde]]s in the [[Lemieux–Johnson oxidation]], which uses [[periodate]] to achieve [[diol cleavage]] and to regenerate the catalytic loading of OsO<sub>4</sub>. This process is equivalent to that of [[ozonolysis]].

:[[File:Lemieux–Johnson oxidation.svg|600px]]