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.