Posted by Tutorial covering the alkene hydroboration/oxidation reaction, which converts an alkene into an alcohol with anti-Markovnikov regioselectivity and syn stereoselectivity.
Introduction
In the alkene hydration reaction, covered previously, an alkene is converted to an alcohol. This electrophilic addition reaction follows Markovnikov’s rule, which means that the nucleophile (a water molecule) is added to the alkene carbon that has more alkyl substituents. This regioselectivity is explained by the reaction mechanism, and depends on the stability of a carbocation intermediate (see the alkene hydration reaction page for more information).
The alkene hydroboration/oxidation reaction is another reaction that converts an alkene into an alcohol. Unlike the alkene hydration reaction, the hydroboration/oxidation reaction gives the anti-Markovnikov product, in which the alcohol hydroxyl group ends up on the less substituted alkene carbon.
Reaction
The hydroboration/oxidation reaction is a two-step process, converting an alkene into an alcohol, and favouring the anti-Markovnikov product.
In the first step, the hydroboration step, the alkene reacts with borane (BH3), forming a monoalkyl borane. This makes a boron-carbon bond (on the less substituted alkene carbon) and a hydrogen-carbon bond (on the more substituted alkene carbon), and breaks a boron-hydrogen bond:
If there’s enough of the alkene present, the monoalkyl borane can react again with another molecule of the alkene, forming a dialkyl borane, and then with another alkene to form a trialkyl borane.
In the second step, the oxidation step, the alkyl borane reacts with hydrogen peroxide and sodium hydroxide, breaking the boron-carbon bond, and placing a hydroxyl group on the carbon which was bonded to boron:
The net result of these two steps (hydroboration and oxidation) is that the hydroxyl group ends up on the less substituted alkene carbon, which is known as the anti-Markovnikov product. This regioselectivity (see below) is a result of the hydroboration step, where the boron forms a bond to the less substituted alkene carbon. The reason for this selectivity is described in the next tutorial.
Regioselectivity
As described above, the alkene hydroboration/oxidation reaction favours the anti-Markovnikov product, where the hydroxyl group ends up on the less substituted alkene carbon. This preferential formation of one product is an example of regioselectivity.
Usually, Markovnikov and anti-Markovnikov products are regioisomers: structural isomers that differ based on the position of a functional group. As the hydroboration/oxidation reaction favours the formation of one of these regioisomers, it is regioselective.
However, the regioselectivity of the alkene hydroboration/oxidation reaction is not perfect, and some of the Markovnikov product can be formed as a side product. This can be overcome by using a sterically bulky alkyl borane in the place of borane (BH3). An example is 9-borabicyclo[3.3.1]nonane (or 9-BBN for short); this is a cyclononane (nine-carbon cycloalkane) with a bridging boron atom. The reason for this enhanced regioselectivity is explained in the next tutorial.
Stereoselectivity
In the mechanism of the hydroboration step, the boron atom and the hydrogen atom are added to the same face of the alkene (more on this in the next tutorial). As an example, consider the reaction of borane with a cyclohexene:
Here, the boron and hydrogen atoms (in red) have approached from the same face of the cyclohexene ring. This addition of two substituents to the same face of a double bond is known as a syn addition. This contrasts anti addition, where two substituents are added to opposite faces of a double bond. As the hydroboration reaction preferentially forms a specific stereoisomer, it is stereoselective.
Summary
Alkene hydroboration/oxidation is a two-step reaction that converts an alkene into an alcohol, forming the anti-Markovnikov product regioisomer.
In the hydroboration step, the alkene reacts with a borane, forming an alkyl borane. The hydroboration step is stereoselective, forming the syn addition product (where boron and hydrogen are added to the same face of the alkene). The regioselectivity of the hydroboration step can be improved with bulky boranes (such as 9-BBN). In the oxidation step, the alkyl borane is converted into an alcohol through the use of hydrogen peroxide and sodium hydroxide.
Related
Alkene Hydration Reaction – another reaction that converts an alkene to an alcohol, which preferentially forms the Markovnikov product.
Periodic Chemistry 