Question

What is meant by hydroboration-oxidation reaction? Illustrate it with an example.

Short answer

Hydroboration-oxidation is a two-step reaction that converts alkenes into alcohols using borane and hydrogen peroxide. It proceeds with anti-Markovnikov regioselectivity.

Step-by-step solution

Understanding Hydroboration

Hydroboration is the first part of the hydroboration-oxidation reaction. It involves the addition of borane (BH3) or one of its derivatives to a double bond in an alkene, resulting in the formation of an organoborane intermediate. This step is regioselective and anti-Markovnikov, meaning the boron atom attaches to the less substituted carbon of the double bond.

Detailing the Oxidation

Oxidation is the second part of the reaction, where the organoborane intermediate is treated with hydrogen peroxide (H2O2) in the presence of a base like sodium hydroxide (NaOH). This step converts the C-B bond to a C-OH bond, resulting in the formation of an alcohol. The overall reaction converts an alkene into an alcohol.

Choosing an Example

For a clear illustration, consider the hydroboration-oxidation of propene. In the hydroboration step, borane adds across the double bond, giving an organoborane intermediate. Subsequent oxidation with hydrogen peroxide and base yields 1-propanol, as the boron initially attaches to the less substituted terminal carbon of the alkene. This exemplifies the anti-Markovnikov addition.

Key concepts

Organoborane Intermediate

The organoborane intermediate plays a crucial role in the hydroboration-oxidation reaction. During the hydroboration process, borane (BH₃) reacts with an alkene, adding across the double bond to form this organoborane intermediate. This step is vital because it initiates the transformation of the alkene into an alcohol.

Let's take a closer look:

• Borane attaches to one end of the double bond.
• This results in the formation of a stable organoborane compound.
• Because of borane's empty p-orbital, it can form a new bond easily.

Once formed, the organoborane intermediate then undergoes oxidation, where it is converted into an alcohol. This step is essential for the completion of the hydroboration-oxidation reaction cycle.

Anti-Markovnikov Addition

The concept of anti-Markovnikov addition is central to understanding the regioselectivity of the hydroboration step. In most addition reactions, new groups preferentially add to the more substituted carbon. However, in anti-Markovnikov addition, as seen in hydroboration, the opposite occurs.

Here’s what happens:

• Boron prefers to attach to the less substituted carbon atom.
• This choice is driven by factors such as sterics and electronic effects, which favor the positioning of the larger boron atom.
• This results in the boron being placed where there is less crowding from other carbon atoms nearby.

This selectivity is useful in organic synthesis because it allows chemists to predict the structure of the final alcohol product formed after subsequent oxidation.

Conversion of Alkene to Alcohol

The conversion of an alkene to an alcohol is the ultimate goal of the hydroboration-oxidation reaction. The process involves two main steps: forming the organoborane intermediate and then oxidizing it to yield an alcohol.

Here’s how the conversion works:

• First, the alkene reacts with borane (BH₃) to form an organoborane intermediate, as previously discussed.
• The next step involves oxidizing this intermediate using hydrogen peroxide (H₂O₂) and a base, such as sodium hydroxide (NaOH).
• The oxidation converts the C-B bond into a C-OH bond, thereby forming the desired alcohol.

This reaction is especially valuable because it provides a straightforward method to synthesize alcohols from alkenes with specific regioselectivity and without rearrangements.