Q7(b). OsO₄/pyridine hydroxylation of maleic and fumaric acids

Introduction. OsO₄ adds to a C=C in one concerted step (a [3+2] cycloaddition) to form a cyclic osmate ester; both C–O bonds form on the same face. Hydrolysis (aqueous NaHSO₃ or H₂S) cleaves Os–O and leaves the two OH groups syn. Syn addition to a cis alkene gives the meso (erythro) diol; to a trans alkene, the racemic (threo) diol.

Scheme: Syn addition: cis alkene → meso diol; trans alkene → racemic diol.

1. Maleic acid (cis-butenedioic acid): both OH enter on one face of the cis alkene; the product has a mirror plane: meso-tartaric acid, (2R,3S), optically inactive (internally compensated).
2. Fumaric acid (trans): attack on either face gives the two enantiomers equally: ()-tartaric acid, (2R,3R) + (2S,3S), racemic (externally compensated).
3. The reaction is stereospecific: the geometry of the alkene fixes the relative configuration of the diol.

Role of pyridine

1. Pyridine is a ligand for osmium: OsO₄.py and the osmate ester OsO₂(O₂C₂R₂)(py)₂ form. Coordination makes Os(VIII) more reactive and accelerates the addition (ligand-accelerated reaction, important for the electron-poor C=C of maleic and fumaric acids).
2. It gives a stable, crystalline osmate(VI) ester complex, which can be isolated and then hydrolysed cleanly, so no over-oxidation (C–C cleavage) occurs.
3. As a base it takes up the acid and keeps the medium from hydrolysing the ester prematurely.

Conclusion. Maleic acid gives meso-tartaric acid; fumaric acid gives ()-tartaric acid, both by syn addition through a cyclic osmate ester. Pyridine binds osmium, speeds up the addition and stabilises the osmate ester.

Application. Replacing pyridine by chiral cinchona-alkaloid amines turns this reaction into the Sharpless asymmetric dihydroxylation (AD-mix), used in drug synthesis; catalytic OsO₄ with NMO avoids using stoichiometric toxic osmium.