Many important compounds—the carbohydrates, for example—have two hydroxyl groups on adjacent carbon atoms. They are called 1,2-diols. A good way of making a 1,2-diol is to add two hydroxyl groups across a double bond. This can be done in two ways, each of which can give a different diastereoisomer of the product. The fi rst way uses chemistry you have already met. When a nucleophile opens an epoxide, it generates an alcohol. If the nucleophile is water, the product is the diol. The epoxide open ing in an SN2 reaction goes with stereochemical inversion, so in this example the two hydroxyl groups end up on opposite sides of the six-membered ring: the product is an anti-diol. The epoxide opening reaction can be done in acid or in base.

To get the syn diol, a completely different method is used, involving the reagent osmium tetroxide, OsO4. OsO4 reacts with alkenes to deliver two hydroxyl groups—one to each end of the double bond—in a single step. Because both groups are delivered at the same time, they are always syn to one another: OsO4 carries out a syn dihydroxylation of the double bond. The mechanism of the reaction is different from ones you have met before and goes like this: the Os starts as tetrahedral osmium (VIII) and ends up as osmium (VI). The immediate product of the reaction is an osmate ester, but these reactions are carried out in the presence of water, and hydrolysis always follows on fast, giving the diol.

Because Os (VI) is produced in the reaction, and a simple oxidation will restore it to Os (VIII), the most effective version of this reaction makes use of just a catalytic amount of Os (VIII) and a stoichiometric amount of a reoxidant, often the compound NMO, or N-methylmorpholine N-oxide. In the example below there is only one new chiral centre, so no possibility of diastereoisomers.

Because OsO₄ adds two hydroxyl groups to an alkene in a syn fashion, the overall product depends on the geometry of the alkene starting material: it is stereospecific. It is similar to bromination in that respect, although of course bromination is an anti-addition. You can see how two different diastereoisomers are produced from different alkenes in these two examples: both dihydroxylations are mechanistically syn, but redrawing the product from the Z alkene in its more extended form reveals anti-stereochemistry.

مواضيع ذات صلة

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific

The regioselectivity of epoxide opening can depend on the conditions