key term - Alkylborane

Definition

An alkylborane is an organic compound consisting of a boron atom bonded to one or more alkyl groups. These compounds are important intermediates in organic synthesis, particularly in the hydration of alkynes and the reactions of carboxylic acids.

5 Must Know Facts For Your Next Test

1. Alkylboranes are formed as intermediates in the hydroboration-oxidation reaction, which is a useful method for the anti-Markovnikov hydration of alkynes.
2. The alkylborane intermediate in the hydroboration-oxidation reaction can be oxidized to form an alcohol, allowing for the conversion of an alkyne to an alcohol.
3. Alkylboranes can be used as reducing agents in the conversion of carboxylic acids to alcohols, a process known as carboxylic acid reduction.
4. Organoborane reagents, including alkylboranes, are versatile synthetic tools that can participate in a variety of carbon-carbon bond-forming reactions.
5. The reactivity of alkylboranes is influenced by the nature of the alkyl group, with more substituted alkyl groups generally being more reactive.

Review Questions

• Explain the role of alkylboranes in the hydration of alkynes using the hydroboration-oxidation reaction.
  • In the hydroboration-oxidation reaction, an alkyne is first reacted with a borane (BH3) to form an alkylborane intermediate. This alkylborane then undergoes oxidation, typically using hydrogen peroxide and sodium hydroxide, to convert the boron-carbon bond into a carbon-oxygen bond, resulting in the formation of an alcohol. This process allows for the anti-Markovnikov hydration of alkynes, where the hydrogen and hydroxyl group are added to the less substituted carbon of the alkyne.
• Describe how alkylboranes are used in the reduction of carboxylic acids to alcohols.
  • Alkylboranes, such as borane (BH3) or diborane (B2H6), can be used as reducing agents to convert carboxylic acids to alcohols. In this reaction, the carboxylic acid is first converted to an acylborane intermediate through a nucleophilic addition reaction. The acylborane is then reduced, typically using sodium borohydride, to form the corresponding alcohol. This carboxylic acid reduction process allows for the selective conversion of a carboxylic acid functional group to an alcohol, while maintaining the integrity of other functional groups present in the molecule.
• Discuss the versatility of organoborane reagents, including alkylboranes, in organic synthesis.
  • Organoborane reagents, such as alkylboranes, are widely used in organic synthesis due to their versatility. In addition to their roles in the hydroboration-oxidation of alkynes and the reduction of carboxylic acids, alkylboranes can participate in a variety of carbon-carbon bond-forming reactions. These include Suzuki cross-coupling reactions, allylboration of carbonyl compounds, and the use of alkylboranes as nucleophiles in substitution reactions. The reactivity of alkylboranes can be tuned by modifying the nature of the alkyl group, allowing for the selective and controlled formation of new carbon-carbon bonds in complex organic molecules.

Related terms

Hydroboration-Oxidation:

A two-step reaction process where an alkene is first reacted with a borane (BH3) to form an alkylborane intermediate, which is then oxidized to form an alcohol.

Carboxylic Acid Reduction: The reduction of a carboxylic acid to an alcohol using an alkylborane reagent, such as borane (BH3) or diborane (B2H6).

Organoborane Reagents: A class of boron-containing compounds that can be used as nucleophiles, reducing agents, and in various carbon-carbon bond-forming reactions in organic synthesis.