Cycloadditions of alkenes with osmium tetroxide

Two very important synthetic reactions cycloaddition of alkenes with osmium tetroxide and with ozone... 

From the mechanism shown in Scheme 7.23, we would expect the dihydroxylation with syn-selectivity. The cyclic intermediate may be isolated in the osmium reaction, which is formed by the cycloaddition of OSO4 to the alkene. Since osmium tetroxide is highly toxic and very expensive, the reaction is performed using a catalytic amount of osmium tetroxide and an oxidizing agent such as TBHP, sodium chlorate, potassium ferricyanide or NMO, which regenerates osmium tetroxide. For example, Upjohn dihydroxylation allows the syn-selective preparation of 1,2-diols from alkenes by the use of catalytic amount of OSO4 and a stoichiometric amount of an oxidant such as NMO. 

In the absence of tertiary amines, osmium tetroxide reacts with alkenes via 1,3-dipolar addition to generate a monomeric Os(VI) ester such as 252,352 where L is a ligand that can be a solvent molecule or an added substrate such as pyridine. Sharpless et al. proposed that hydroxylation proceeds by an allowed [2-1-2]- cycloaddition reaction, producing an Os(VII) intermediate, followed by reductive insertion of the Os—C bond into an Os=0 bond.353 This complex can be decomposed in aqueous or alcoholic solution, but the hydrolysis is... 

Several oxidizing reagents react with alkenes under mild conditions to give, as the overall result, addition of hydrogen peroxide as HO—OH. Of particular importance are alkaline permanganate (MnO40) and osmium tetroxide (0s04), both of which react in an initial step by a suprafacial cycloaddition mechanism like that postulated for ozone. 

Computations using density functional theory predict that the concerted permanganate hydroxylation of alkenes follows the 3 - - 2-cycloaddition mechanism established earlier for osmium tetroxide rather than a stepwise process involving a 2 -I- 2-cycloaddition. Calculated carbon and deuterium KIEs for a substituted case (frans-pentane-2,4-dienoic acid) are in good agreement with experimental data for frans-cinnamic acid." ... 

---