Hydroboration–oxidation involves the reaction of alkenes with diborane to form trialkyl boranes, which are then oxidized to alcohols by hydrogen peroxide in the presence of aqueous sodium hydroxide. This process contradicts Markovnikov’s rule as the boron atom attaches to the sp² carbon with more hydrogen atoms.
Explain the process of hydroboration–oxidation in alkene reactions and how it contradicts Markovnikov’s rule.
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Hydroboration–oxidation is an alkene reaction where boron adds to the less substituted carbon, contrary to Markovnikov’s rule. In the hydroboration step, the boron compound (usually boron trifluoride diethyl etherate, BF₃•Et₂O) reacts with the alkene, forming a boron intermediate. The boron atom adds to the carbon with fewer hydrogen substituents, following anti-Markovnikov addition. Subsequently, in the oxidation step, the boron is replaced by a hydroxyl group, resulting in alcohol formation. This process contradicts Markovnikov’s rule, showcasing a unique pathway for the addition of boron and providing access to anti-Markovnikov alcohols.