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.
9.9:
Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
Mechanism
The hydroboration-oxidation reaction is a two-step process. It begins with the hydroboration step, which involves a concerted syn addition of BH3 across the carbon–carbon triple bond to form an alkenylborane. The concerted nature of the reaction also accounts for the anti-Markovnikov regiochemistry, where the BH2 group adds to the less substituted carbon and H to the more substituted carbon of the triple bond.
Three successive hydroboration reactions convert an alkene into a trialkenylborane intermediate. The second part of the sequence is oxidation, where the trialkenylborane is treated with alkaline hydrogen peroxide to form an enol. The enol eventually converts into a stable carbonyl product via keto-enol tautomerism.
Hydroboration of Alkynes with Disubstituted Boranes
Unlike alkenes, hydroboration of alkynes does not stop at the first addition of BH3. This is because alkynes have two π bonds, each capable of reacting with BH3. The first addition forms an organoborane, which is an alkene derivative that can react further with another equivalent of BH3.
Terminal alkynes being less hindered than internal alkynes are more susceptible to a second BH3 addition. With internal alkynes, the addition of BH3 stops after the first stage and proceeds in a direction to give the trialkenylborane.
Nevertheless, hydroboration of terminal alkynes can be stopped at the first step by using bulky disubstituted boranes (R2BH) such as disiamylborane and 9-BBN instead of BH3.
The first addition of the bulky reagent forms a sterically hindered alkenylborane that resists any further additions and helps in the efficient conversion of alkynes to stable carbonyl compounds.
