Dihydroxylation

Dihydroxylation is the chemical process by which two hydroxyl groups are added across an unsaturated bond, typically converting alkenes into vicinal diols (glycols). The reaction is usually stereospecific, delivering the two hydroxyls on the same face of the former double bond (syn addition), and thus reflects the geometry of the starting alkene.

Two principal methods are used. Osmium tetroxide (OsO4)–based dihydroxylation, often called Upjohn dihydroxylation, uses catalytic OsO4 with a co-oxidant such as N-methylmorpholine N-oxide (NMO) or tert-butyl hydroperoxide to regenerate the catalyst. This approach is highly versatile, compatible with a broad range of substrates, and yields cis-1,2-diols with high efficiency. A notable variant is Sharpless asymmetric dihydroxylation, which employs chiral ligands (AD-mix) to produce enantioenriched vicinal diols from achiral alkenes.

Cold, dilute potassium permanganate (KMnO4) can also effect dihydroxylation under aqueous conditions, typically giving syn diols as well but with less control over stereochemistry and a greater risk of over-oxidation or cleavage for certain substrates.

Applications and limitations. Dihydroxylation is a fundamental tool in organic synthesis for installing adjacent hydroxyl groups, often used in natural product and carbohydrate chemistry. Practical considerations include the expense and toxicity of OsO4, the need for re-oxidants, and the substrate’s sensitivity to oxidative conditions. Protective group strategy is frequently employed after dihydroxylation.

See also: vicinal diol; Upjohn dihydroxylation; Sharpless asymmetric dihydroxylation.