Question

Draw the product(s) of the following hydroborati

Draw the product(s) of the following hydroboration-oxidation reactions. Disregard stereochemistry.

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Answer 1

Concepts and reason

Hydroboration:

The addition of a hydrogen-boron bond to carbon-carbon, carbon-nitrogen and carbon-oxygen double bonds is known as a hydroboration reaction. Hydroboration also involves the addition of hydrogen-boron bond to carbon-carbon triple bonds.

Fundamentals

Hydroboration is also known as the oxidation of alkenes. Hydroboration is regioselective as well as stereoselective.

Regioselectivity:

Boron has an empty p-orbital and is electrophilic in nature. Since boron is electrophilic, the addition to an alkene follows Anti-Markovnikov’s rule.

Anti-Markovnikov’s addition:

It states that when there is an addition of two different atoms to unsymmetrical alkenes, the electrophile adds to the more hindered carbon atom and the nucleophile adds to the less hindered carbon atom of the double bond.

Stereoselectivity:

The addition of hydrogen-boron to the alkene proceeds through four membered intermediates; the product formed is only syn addition. That is, the hydrogen and boron add to the same side.

Mechanism of hydroboration reaction involves two steps:

1.The addition of a hydrogen-boron bond to a carbon-carbon double bond:

An addition of ${{\rm{B}}_{\rm{2}}}{{\rm{H}}_{\rm{6}}}$ over an alkene forms a four membered ring transition state. The cleavage of the transition state yields boron to less substituted carbon and hydrogen to more substituted carbon according to Anti-Markonikav’s addition.

2.The treatment of peroxide to a hydroboration product:

The treatment of peroxide results in the replacement of a carbon-boron bond with carbon-hydroxyl bond.

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