Alkyne Hydroboration: Videos & Practice Problems

Alkyne hydroboration involves the addition of boron to the less substituted carbon of the alkyne, followed by oxidation. The mechanism starts with the addition of a boron source like BH₃ to the alkyne, forming a trialkylborane intermediate. This step is anti-Markovnikov, meaning the boron attaches to the less substituted carbon. The intermediate is then oxidized, typically using hydrogen peroxide (H₂O₂) in a basic medium (NaOH), converting the boron-carbon bond into a carbon-oxygen bond. This results in the formation of an enol, which tautomerizes to form an aldehyde or ketone, depending on the alkyne's substitution pattern.

Hydroboration is considered an anti-Markovnikov addition because the boron atom attaches to the less substituted carbon of the alkyne. In typical Markovnikov additions, the more substituted carbon receives the electrophile, but in hydroboration, the boron (acting as the electrophile) adds to the less substituted carbon. This results in the formation of an enol, which upon oxidation, yields an aldehyde or ketone. The anti-Markovnikov nature of this reaction is crucial for synthesizing specific products that are not accessible through Markovnikov addition.

Common boron sources used in alkyne hydroboration include borane (BH₃), borane-tetrahydrofuran (BH₃-THF), and diborane (B₂H₆). These reagents are effective in facilitating the addition of boron to the less substituted carbon of the alkyne. The choice of boron source can depend on the specific conditions and preferences of the reaction setup, but all serve the same purpose of initiating the hydroboration process.

The products of Markovnikov and anti-Markovnikov hydration of alkynes differ in their functional groups and positions. Markovnikov hydration of alkynes typically yields ketones, where the more substituted carbon of the alkyne becomes the carbonyl carbon. In contrast, anti-Markovnikov hydration, achieved through hydroboration-oxidation, results in aldehydes, where the less substituted carbon becomes the carbonyl carbon. This distinction is crucial for synthesizing specific compounds in organic chemistry.

In hydroboration-oxidation, the enol formed initially undergoes tautomerization to form an aldehyde. Tautomerization is a chemical process where the enol (a compound with a hydroxyl group attached to a carbon-carbon double bond) rearranges to form a carbonyl compound (aldehyde or ketone). This involves the migration of a hydrogen atom and the shift of the double bond. The enol's hydroxyl group (OH) and the adjacent hydrogen atom switch places, resulting in the formation of a carbonyl group (C=O) and converting the enol into an aldehyde.