Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

Acid-catalyzed hydration of terminal alkynes yields methyl ketones following Markovnikov's regioselectivity, where the hydroxyl group adds to the more substituted carbon. In contrast, hydroboration-oxidation produces aldehydes with anti-Markovnikov regioselectivity, where boron adds to the less substituted carbon. Both methods convert alkynes to carbonyl compounds but differ in regiochemistry and product type.

Hydroboration begins with concerted syn addition of borane across the triple bond, forming an alkenylborane intermediate. Three successive hydroboration steps convert borane into a trialkenylborane. Oxidation with alkaline hydrogen peroxide then produces an enol, which undergoes keto-enol tautomerism to form a stable aldehyde product.

Unlike alkenes, alkynes possess two π bonds equally capable of reacting with borane. Terminal alkynes are less hindered than internal alkynes, making them susceptible to a second BH3 addition. Bulky disubstituted boranes like disiamylborane form sterically hindered alkenylboranes that resist further additions, ensuring selective monohydroboration and efficient aldehyde formation.

Internal alkynes yield ketones as final products following hydroboration-oxidation. Symmetrical internal alkynes produce a single ketone product, while unsymmetrical internal alkynes generate a mixture of ketones due to the lack of regioselectivity when both carbons of the triple bond are similarly substituted.

Alkaline hydrogen peroxide oxidizes the trialkenylborane intermediate to form an enol intermediate. This oxidation step is essential for converting the carbon-boron bond into a carbon-oxygen bond, enabling the subsequent keto-enol tautomerism that produces the stable aldehyde or ketone final product.

The concerted syn addition of borane across the triple bond accounts for the anti-Markovnikov regioselectivity observed in hydroboration-oxidation. This mechanism ensures that boron adds to the less substituted carbon while hydrogen adds to the more substituted carbon, directing the reaction toward aldehyde formation in terminal alkynes.

The key difference is that alkynes contain two π bonds capable of reacting with borane, whereas alkenes have only one. This allows alkynes to undergo multiple hydroboration reactions, necessitating the use of bulky disubstituted boranes to prevent unwanted secondary additions and achieve selective conversion to aldehydes or ketones.