Oxidations of Alkenes Syn 1,2-Dihydroxylation

Alkenes undergo a number of reactions in which the carbon-carbon double bond is oxidized. 

Osmium tetroxide is widely used to synthesize 1,2-diols (the products of 1,2-dihydroxy-lation, sometimes also called glycols). Potassium permanganate can also be used, although because it is a stronger oxidizing agent it is prone to cleave the diol through further oxidation (Section 8.17). 

After formation of the cyclic intermediate with osmium, cleavage at the oxygen-metal bonds takes place without altering the stereochemistry of the two new C—O bonds. 

The syn stereochemistry of this dihydroxylation can readily be observed by the reaction of cyclopentene with osmium tetroxide. The product is cw-l,2-cyclopentanediol. 

Osmium tetroxide is highly toxic, volatile, and very expensive. For these reasons, methods have been developed that permit OSO4 to be used catalytically in conjunction with a co-oxi-dant. A very small molar percentage of OSO4 is placed in the reaction mixture to do the dihydroxylation step, while a stoichiometric amount of co-oxidant reoxidizes the OSO4 as it 

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Oxidation of Alkenes Syn 1,2-Dihydroxylation 368 THE CHEMISTRY OF... Catalytic Asymmetric Dihydroxylation 370... 

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