Mechanism
The reaction involves the formation of cyclic osmate ester, which undergoes oxidative cleavage with NaIO4 to give the dicarbonyl compounds (Scheme 9).
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Scheme 9 |
1.1.3 Sharpless Asymmetric Dihydroxylation
Although osmylation of alkenes is an attractive process for the conversion of alkenes to 1,2-diols, the reaction produces racemic products. Sharpless group attempted to solve this problem by adding chiral substrate to the osmylation reagents, with the goal of producing a chiral osmate intermediate (Scheme 10). The most effective chiral additives were found to be the cinchona alkaloids, especially esters of dihydorquinidines such as DHQ and DHQD. The % ee of the diol product is good to excellent with a wide range of alkenes.
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Scheme 10 |
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Scheme 11 |