Hydroboration–oxidation reaction
In organic chemistry, the hydroboration–oxidation reaction is a two-step organic reaction that converts an alkene into a neutral alcohol by the net addition of water across the double bond. The hydrogen and hydroxyl group are added in a syn addition leading to cis stereochemistry. Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction was first reported by Herbert C. Brown in the late 1950s and it was recognized in his receiving the Nobel Prize in Chemistry in 1979.
The general form of the reaction is as follows:
Tetrahydrofuran (THF) is the archetypal solvent used for hydroborations.
In the first step, borane (BH3) adds to the double bond, transferring one of the hydrogen atoms to the carbon adjacent to the one that becomes bonded to the boron. This hydroboration is repeated two additional times, successively reacting each B–H bond so that three alkenes add to each BH3. The resulting trialkylborane is treated with hydrogen peroxide in the second step. This process replaces the B-C bonds with HO-C bonds. The boron reagent is converted to boric acid. The reaction was originally described by H.C. Brown in 1957 for the conversion of 1-hexene into 1-hexanol.
Knowing that the group containing the boron will be replaced by a hydroxyl group, it can be seen that the initial hydroboration step determines the regioselectivity. Hydroboration proceeds in an antimarkovnikov manner. The reaction sequence is also stereoselective, giving syn addition (on the same face of the alkene): the hydroboration is syn-selective and the oxidation replaces the boron with hydroxyl having the same geometric position. Thus 1-methylcyclopentene reacts with diborane predominantly to give trans-1-hydroxy-2-methylcyclpentane—the newly added H and OH are cis to each other.
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