9.9 : Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

The addition of water to alkynes can proceed via two complementary pathways, namely acid-catalyzed hydration, and hydroboration-oxidation.

Recall that acid-catalyzed hydration of terminal alkynes yields methyl ketones following Markovnikov's regioselectivity.

In contrast, the hydroboration-oxidation of terminal alkynes gives aldehydes with anti-Markovnikov's regioselectivity.

For example, the addition of borane to 1-propyne followed by oxidation with hydrogen peroxide in the presence of sodium hydroxide yields an enol that rearranges into a stable carbonyl compound to form propanal as the final product.

The hydroboration mechanism begins with a concerted syn addition of borane across the carbon–carbon triple bond forming an alkenyl borane. In terminal alkynes, the addition is regioselective with boron adding to the less substituted carbon of the triple bond.

Three successive hydroboration steps eventually convert borane into a trialkenylborane.

Next, the trialkenylborane undergoes oxidation in the presence of alkaline hydrogen peroxide to form an enol.

The final step involves the tautomerization of the enol into a stable aldehyde.

Although the mechanism is similar to that of alkenes, there is a vital difference in the hydroboration step.

Unlike alkenes, alkynes have two π bonds that are equally suited to react with BH₃. Therefore, the alkenylborane formed from the first addition of BH₃ can undergo a second hydroboration reaction.

Terminal alkynes are less hindered compared to internal alkynes, and therefore, more susceptible to a second addition of BH₃.

However, this can be prevented by using a bulky disubstituted borane such as di-sec-isoamylborane, where the branched alkyl substituents replace the two hydrogen atoms of BH₃.

Under these conditions, the first hydroboration step forms a sterically hindered alkenyl borane that resists further additions, thereby facilitating the conversion of terminal alkynes to aldehydes.

Analogous to acid-catalyzed hydration, hydroboration-oxidation of internal alkynes gives ketones as the final product. Symmetrical internal alkynes give a single ketone product, whereas unsymmetrical alkynes yield a mixture of ketones.