Hydroboration oxidation: Mechanism and application

Table of Contents

The hydroboration oxidation of alkene, alkynes, mechanism, and its application has been discussed here.

Hydroboration oxidation

When diborane reacts with alkenes, the addition of diborane to the carbon-carbon double bond takes place to form alkyl borane. This reaction is called hydroboration. On oxidation of alkyl borane with H2O2 in presence of NaOH gives alcohol as a product. This step is called oxidation. The whole process is called a hydroboration oxidation reaction.

Let’s see an example of a hydroboration oxidation reaction.

hydroboration oxidation reaction

Hydroboration oxidation mechanism

The mechanism of hydroboration oxidation reaction can be illustrated in the following steps:

Step 1: Addition of diborane to double bond

The first step is hydroboration in which the boron and hydrogen atom from the diborane is added to the carbon-carbon double bond in a concerted manner from the same direction i.e syn addition. The addition follows the anti-markovnikov’s rule i.e the hydrogen is added to the carbon having a fewer number of the hydrogen atom and boron is added to the carbon having more number of hydrogen.

hydroboration oxidation mechanism

Step 2: Deprotonation of hydrogen peroxide

The base NaOH abstracts a hydrogen atom from the hydrogen peroxide to form a hydroperoxide ion.

hydroboration reaction

Step 3: The hydroperoxide anion attacks the boron atom of the organoboron compound(trialkyl boron).

hydroboration oxidation mechanism

Step 4: The alkyl group migrates from Boron atom to oxygen atom as shown below:

hydroboration oxidation mechanism

Step 5: Hydrolysis

Therefore, the final product is alcohol and we can say that the net result is the same as the addition of water to the alkene.

hydroboration oxidation mechanism

Hydroboration oxidation stereochemistry

The stereochemistry of hydroboration oxidation reaction can be explained by using the following example.

hydroboration oxidation stereochemistry

Do you get any idea? First of all, we can see in the figure, the hydrogen atom and BH2 are added to the carbon-carbon double bond from the same side of the alkene. It means the hydrogen atom is present on the wedged bond and the BH2 group is also present on the wedged bond. Similarly, the hydrogen atom is present on the dashed bond, and the BH2 group is also present on the dashed bond. It indicates that the addition is syn addition.

The regiochemistry of the reaction can be explained on the basis of anti-markovnikov’s rule. This means the product is formed by the addition of a hydrogen atom (of BH3) to the carbon atom( of the double bond) having fewer hydrogen atoms in the alkene.

Hydroboration oxidation of alkenes

The hydroboration oxidation reaction of alkenes produces alcohol as the main product. Some examples of such reaction can be illustrated as shown below:

hydroboration oxidation of alkenes

Hydroboration oxidation of propene

Hydroboration oxidation of propene gives propanol as a product. The whole process can be explained as:

hydroboration oxidation of propene

Hydroboration oxidation of alkynes

The terminal alkynes also undergo hydroboration oxidation reaction in accordance with anti-markovnikov’s rule. The terminal alkyne reacts with diborane followed by oxidation with hydrogen peroxide in presence of Sodium hydroxide to give vinyl alcohol. The vinyl alcohol undergoes tautomerism to give carbonyl compound. Therefore, the mechanism for the hydroboration oxidation for alkynes takes place in the following steps:

hydroboration oxidation of alkynes

Hydroboration and oxidation of 1 hexene to make 1 hexanol

When 1-hexene is treated with diborane followed by hydrogen peroxide in presence of a base such as sodium hydroxide, 1-hexanol is obtained as a product. Let’s see the reaction.

hydroboration and oxidation of 1 hexene to make 1 hexanol

Application of Hydroboration oxidation

The main application of hydroboration oxidation is the preparation of alcohol. The alkenes can be converted into alcohol by hydroboration oxidation. We have already seen some examples of this reaction above.

Hydroboration oxidation reaction video:

FAQs/MCQs:

what is meant by hydroboration oxidation reaction?

When diborane reacts with alkenes, the addition of diborane to the carbon-carbon double bond takes place to form alkyl borane. This reaction is called hydroboration. On oxidation of alkyl borane with H2O2 in presence of NaOH gives alcohol as a product. This step is called oxidation. The whole process is called a hydroboration oxidation reaction.

hydroboration oxidation reagents

The reagents of hydroboration oxidation reaction are diborane(B2H6), hydrogen peroxide(H2O2), and base like NaOH

advantages of hydroboration oxidation reaction

The main advantage of hydroboration oxidation reaction is that it can be used to prepare alcohol from alkenes

References:

  • Skyes, P., A Guide Book to Mechanism in Organic Chemistry, Second edition, Orient Longman Ltd., 1988
  • Morrison, R. T., & Boyd, R. N., Organic chemistry, Allyn and Bacon, Inc. 1987
  • March, J., Advanced Organic Chemistry, Wiley Eastern Limited, 1986.

Hydroboration oxidation

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