Dihydroxylation and Hydroxylamination

1 Syn-Dihydroxylation. When the reaction was first discovered, the syn-dihydroxylation of alkenes was carried out by using a stoichiometric amount of osmium tetroxide in dry organic solvent. Hoffman made the observation that alkenes could react with chlorate salts as the primary oxidants together with a catalytic quantity of osmium tetroxide, yielding syn-vicinal diols (Eq. 3.11). This catalytic reaction is usually carried out in an aqueous and tetrahydrofuran solvent mixture, and silver or barium chlorate generally give better yields.  

Syn-hydroxylation of alkenes is also effected by a catalytic amount of osmium tetroxide in the presence of hydrogen peroxide. Originally developed by Milas, the reaction can be performed with aqueous hydrogen peroxide in solvents such as acetone or diethyl ether. Allyl alcohol is quantitatively hydroxylated in water (Eq. 3.12).  

Catalytic dihydroxylations using the osmium tetroxide-tert-butyl hydroperoxide system are largely due to Sharpless and co-workers. The aqueous 70%-t-butyl hydroperoxide is commercially available and ideal for direct use in this dihydroxylation process.  

A very effective way of carrying out syn-dihydroxylation of alkenes is by using an osmium tetroxide-tertiary amine N-oxide system. This dihydroxylation is usually carried out in aqueous acetone in either one-or two-phase systems, but other solvents may be required to overcome problems of substrate solubility.  

Potassium permanganate, usually in alkaline conditions, using aqueous or aqueous-organic solvents, is a widely used oxidant for effecting syn-vicinal hydroxylation of alkenes (Eq. 3.13). However, overoxidation or alternative oxidation pathways may pose a problem, and the conditions must be carefully controlled. 

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