Discuss the regioselectivity in hydroboration of isobutylene and give the mechanism involved.

Answer 1

Hydroboration is a chemical reaction that involves the addition of borane (\(BH_3\)) to alkenes, leading to the formation of organoboranes. This reaction is especially noted for its regioselectivity and stereoselectivity.

Regioselectivity in Hydroboration of Isobutylene

Isobutylene (or 2-methylpropene) is a branched alkene with the structure:

       CH3
        |
   H2C=C–CH3

When isobutylene undergoes hydroboration, borane adds across the double bond. The regioselectivity of this reaction favors the less substituted carbon atom due to the mechanism of hydroboration, which involves a concerted, sterically hindered approach.

1. Formation of the Boron-Centered Intermediate: The addition of borane to the double bond occurs in a syn-addition manner. This means that both the boron and the hydrogen end up on the same side of the planar alkene. Given that isobutylene is a substituted alkene where one end (the double bond) is more branched than the other, borane will preferentially add to the less hindered (less substituted) terminal carbon atom.

2. End Result: For isobutylene, the product formed will typically be:

   • Butylborane: where the boron is attached to the less substituted carbon (the terminal carbon).

Mechanism of Hydroboration

The mechanism of hydroboration can be summarized in several key steps:

1. Approach of Borane: Borane (\(BH_3\)) acts as an electrophile and approaches the alkene. The double bond of the alkene acts as a nucleophile and attacks the boron atom.

2. Formation of the Boron-Carbon Bond: The π bond of the alkene is broken as the boron atom forms a bond with one of the carbons (the less substituted carbon in the case of isobutylene).

3. Hydride Shift: After the initial boron addition, simultaneously, a hydrogen atom from borane adds to the more substituted carbon of the double bond in a concerted fashion, leading to the formation of the organoborane product.

   Here’s a simplified schematic of the hydroboration mechanism:

   • Step 1: \[ \text{R-CH=CH2} + \text{BH}_3 \rightarrow \text{R-CH(BH2)-CH3} \]

   • Step 2: \[ \text{R-CH(BH2)-CH3} + \text{H}_2\text{O}_2/\text{OH}^- \rightarrow \text{R-CH(OH)-CH3} + \text{B(OH)}_3 \text{(oxidation step)} \]

4. Oxidation: The organoborane can subsequently undergo oxidation using hydrogen peroxide (\(H_2O_2\)) in basic conditions, converting the boron atom into an alcohol, yielding the final product.

Conclusion

The regioselectivity of hydroboration allows for the synthesis of alcohols with a specific orientation regarding the placement of the hydroxyl group in the oxidation step that follows. For isobutylene, the hydroboration reaction delivers 2-butanol as the primary alcohol product because hydroboration adds across the double bond in a manner that places the hydroxyl group on the more substituted carbon after oxidation, following Markovnikov's rule in reverse due to the initial syn-addition of boron.