Oxydation des alcènes : Syn-dihydroxylation avec le tétraoxyde d'osmium

Syn dihydroxylation adds two hydroxyl groups across an alkene double bond with both groups attaching to the same face of the molecule, forming a cis-diol. Anti dihydroxylation adds hydroxyl groups to opposite faces, forming a trans-diol. Osmium tetroxide catalyzes syn dihydroxylation through a concerted mechanism, while anti dihydroxylation uses peroxycarboxylic acids. The stereochemical outcome depends on the reaction pathway and reagent used.

Osmium tetroxide adds across the alkene double bond in a concerted, single-step process. The electrophilic osmium accepts an electron pair from the alkene π bond, forming a five-membered cyclic osmate ester intermediate. During this addition, osmium's oxidation state reduces from +8 to +6. Both oxygen atoms add simultaneously to the same face of the double bond, establishing syn stereochemistry before the intermediate is reduced.

Sodium bisulfite acts as a reducing agent in the second step of syn dihydroxylation. It cleaves the osmium-oxygen bonds in the cyclic osmate ester without altering the stereochemistry of the newly formed carbon-oxygen bonds. This hydrolysis releases the cis-diol product while preserving the syn stereochemistry established in the first step, yielding the final 1,2-diol or glycol.

Osmium tetroxide is toxic and expensive, limiting its practical use. Co-oxidants such as N-methylmorpholine N-oxide or tert-butyl hydroperoxide regenerate osmium tetroxide by reoxidizing the reduced osmium +6 species back to +8. This catalytic recycling allows a small amount of osmium tetroxide to oxidize multiple alkene molecules, improving efficiency and reducing the quantity of expensive reagent required.

Since syn dihydroxylation is stereospecific, the E and Z geometry of the starting alkene determines the product structure. Dihydroxylation of (E)-hex-3-ene produces a pair of enantiomers because the two faces of the double bond are different. Dihydroxylation of (Z)-hex-3-ene yields a single meso compound, which is achiral despite containing stereogenic centers.

Sharpless asymmetric dihydroxylation is an enantioselective method developed by Karl Barry Sharpless for syn dihydroxylation of alkenes. It uses osmium tetroxide with chiral amine ligands and co-oxidants to produce optically pure diols from prochiral or racemic alkenes. This breakthrough earned Sharpless the Nobel Prize and provides a friendlier, more practical alternative to stoichiometric osmium tetroxide reactions.

The first step is a concerted addition where osmium tetroxide, an electrophilic strong oxidizing agent, accepts electrons from the alkene π bond. The two oxygen atoms add simultaneously across the same face of the double bond, reducing osmium from +8 to +6 oxidation state. This forms a five-membered cyclic osmate ester intermediate that can be isolated and characterized before proceeding to hydrolysis.