Syn Dihydroxylation of Alkene

Osmium tetroxide oxidizes alkenes to give glycols through syn addition. A glycol, also known as a vicinal diol, is a compound with two -OH groups on adjacent carbons.

The reaction with OsO4 is a concerted process that has a cyclic intermediate and no rearrangements. Vicinal syn dihydroxylation complements the epoxide-hydrolysis sequence which constitutes an anti dihydroxylation of an alkene. When an alkene reacts with osmium tetroxide, stereocenters can form in the glycol product. Cis alkenes give meso products and trans alkenes give racemic mixtures.

Mechanism

• Electrophilic attack on the alkene
  • Pi bond of the alkene acts as the nucleophile and reacts with osmium (VIII) tetroxide (OsO₄)
  • 2 electrons from the double bond flows toward the osmium metal
    • In the process, 3 electron pairs move simultaneously
  • Cyclic ester with Os (VI) is produced
• Reduction
  • H₂S reduces the cyclic ester
    • NaHSO₄ with H₂O may be used
  • Forms the syn-1,2-diol (glycol)

Example: Dihydroxylation of 1-ethyl-1-cycloheptene