4.6.9 Dihydroxylation of Alkenes
Addition of a hydroxyl group to the double bond is called hydroxylation. For example, the hydroxylation of propene gives propane-1,2-diol in the presence of osmium tetroxide (Scheme 27).
Scheme 27
Osmium tetroxide reacts with alkene in a concerted step to form a cyclic osmate ester which is hydrolyzed into cis-1,2-diol. The two carbon-oxygen bonds are formed simultaneously and the oxygen atoms add to the same face of the double bond ( syn- addition) (Scheme 28).
Scheme 28
A cold, dilute solution of potassium permanganate (KMnO4) can also hydroxylate alkenes with syn stereochemistry (Scheme 29).
Scheme 29
4.6.10 Cleavage with Ozone
Ozone can cleave double bond to give ketones or aldehydes (Scheme 30).
Scheme 30
Regarding the mechanism, ozone reacts with an alkene to form a cyclic compound called a primary ozonide which has peroxy (-O-O-) linkages, so it is quite unstable. It rearranges rapidly to form an ozonide that could be reduced by reducing agents such as dimethyl sulfide (Scheme 31).
Scheme 31
Treatment of an alkene with hot basic potassium permanganate oxidatively cleaves the double bond. Monosubstituted double bond is oxidatively cleaved to salts of carboxylic acids (Scheme 32). Disubstituted alkenes are oxidatively cleaved to ketones while unsubstituted alkenes are oxidized to carbon dioxide.
Scheme 32