Hydroboration followed by oxidation is a versatile and widely used laboratory method for the synthesis of alcohols
, two of the commonest classes of organic compounds. Such a synthesis involves adding water
across the double bond in the alkene, and although it is sometimes possible to simply react the alkene with water itself (an acid-catalysed process), it is more often impossible, so other methods are required.
Hydroboration-oxidation is a general method for the synthesis of what may be called the "anti-Markovnikov" alcohol, in which the negative species OH- adds to the less substituted carbon in an unsymmetrical alkene. This is, formally, a violation of Markovnikov's rule, which states that the negative part of the addendum, in this case OH, should add to the more substituted carbon. However hydroboration-oxidation involves the use of boron, which is less electronegative than hydrogen, so the "rule" must in this case be treated with caution.
In this step, the compound borane, BH3, is added across the double bond on the alkene. Borane normally exists as diborane, B2H6, its dimer. However if borane is dissolved in an ether, such as diethyl ether or THF, it coordinates to the oxygen and is effectively cleaved into the monomer BH3.
When borane in ether is reacted with the alkene, the borane adds across the double bond so that the boron itself is bonded to the less substituted carbon. This happens partly because the boron is less sterically hindered this way, and partly because BH3 actually adds as BH2+ and H-, so it is in accordance with expectation that the negative addendum, H-, should add to the more substituted carbon. At first this produces a molecule in which H has been added to one carbon and BH2 to the other. However in most circumstances hydroboration will continue to the point where each boron atom bonds to a carbon chain not once but three times, giving the species BR3 where R is the carbon chain.
The boron is then removed by reaction with hydrogen peroxide, H2O2, in alkaline conditions. The contribution of the hydrogen peroxide is to insert oxygen between the boron and the carbon group. The contribution of the alkali is to provide a hydroxy group, OH, to replace the boron in BR3 and give an alcohol, R-OH. Because the boron was in the less substituted position on the carbon chain, so is the hydroxy group, giving the "anti-Markovnikov" alcohol.
For example, imagine treating 1-butene, H2C=CHCH2CH3, in this way. The boron adds to the terminal carbon, which is less substituted, and when it is replaced by OH this gives a terminal hydroxy group and thus a primary alcohol, in this case 1-butanol, CH3CH2CH2CH2OH.
Hydroboration-oxidation is among the most useful laboratory-scale syntheses of alcohols from alkenes. In giving the "anti-Markovnikov" alcohol, it is complementary to the method known as oxymercuration-demercuration, which gives the "Markovnikov" alcohol in which the hydroxy group instead bonds to the more substituted carbon atom.