In hydroboration–oxidation, borane adds to the double bond with the boron atom attaching to the sp² carbon carrying more hydrogen atoms, contrary to Markovnikov’s rule. The reaction yields excellent alcohol formation.
What distinguishes the addition of borane to alkenes in hydroboration–oxidation, and why is the alcohol yield excellent in this reaction?
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The addition of borane to alkenes in hydroboration–oxidation is distinctive because it occurs with anti-Markovnikov selectivity, contrary to typical electrophilic addition reactions. Boron adds to the carbon with fewer hydrogen substituents, leading to the formation of boron intermediates. In the subsequent oxidation step, the boron is replaced by a hydroxyl group. The excellent yield of alcohols in this reaction results from the syn-addition of boron and hydrogen across the alkene double bond, producing a boron intermediate that undergoes facile hydroxyl substitution. This unique reactivity provides a valuable method for the synthesis of anti-Markovnikov alcohols.