Introduction
Dihydroxylation is a chemical transformation that converts an alkene into a vicinal diol. This oxidation process is typically achieved using transition metals in a high oxidation state, such as osmium or manganese, which often act as catalysts in the presence of another stoichiometric oxidant.
The mechanism of dihydroxylation begins with the coordination of a ligand to the metal catalyst, for instance, osmium. Following this, the alkene coordinates to the metal via a (3+2) cycloaddition, leading to the dissociation of the ligand from the metal catalyst. The subsequent hydrolysis of the olefin produces the vicinal diol, and the catalyst is regenerated through oxidation by a stoichiometric oxidant, allowing the cycle to continue.
Osmium tetroxide (OsO4) is a widely used oxidant in the dihydroxylation of alkenes due to its reliability and efficiency in producing syn-diols. However, due to its high cost and toxicity, only catalytic quantities of OsO4 are used, supplemented by a stoichiometric oxidizing agent. Manganese is another metal used in dihydroxylation. Its high oxidation potential can, however, lead to the over-oxidation of substrates.
Reaction
Reaction using osmium tetroxide:
Reaction using potassium permanganate:
Mechanism
Please note that these are simplified mechanisms which don't go into the details of the metal catalytic cycle which involves steps such as coordination, migratory insertion, reductive elimination, etc.
Mechanism using osmium tetroxide:
Mechanism using potassium permanganate:
