Vicinal Syn Dihydroxylation with Osmium Tetroxide

Osmium tetroxide reacts with alkenes in a two-step process to give the corresponding vicinal diols in a stereospecihcally syn manner. The process therefore complements the epoxidation-hydrolysis seqnence described in the previons section, which proceeds with anti selectivity. 

The process leads initially to an isolable cyclic ester, which is rednctively hydrolyzed with H2S or bisulfite, NaHSOs. For example, 

What is the mechanism of this transformation The initial reaction of the ir bond with osmium tetroxide constitutes a concerted (Section 6-4) addition in which three electron pairs move simultaneously to give a cyclic ester containing Os(VI). This process can be viewed as an electrophilic attack on the alkene Two electrons flow from the alkene onto the metal, which is reduced [Os(VIII) Os(VI)]. For steric reasons, the product can form only in a way that introduces the two oxygen atoms on the same face of the double bond—syn. This intermediate is usually not isolated but converted upon reductive work-up into the free diol. 

Because OSO4 is expensive and highly toxic, a commonly nsed modification calls for the nse of only catalytic quantities of the osmium reagent and stoichiometric amounts of another oxidizing agent such as H2O2, which serves to reoxidize reduced osmium. 

Potassium Permanganate Test for Alkene Donble Bonds 

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