The First Step In The Hydration of Alkynes Is Formation Of An “Enol” The “Enol” Is Converted To A Ketone Through A Process Called “Tautomerization” Alkynes Can Also Be “Hydrated” via Oxymercuration (HgSO4/H2O) Hydroboration Of Alkynes (R2BH) Occurs With Anti-Markovnikov Selectivity, Giving Aldehydes From Terminal Alkynes The mechanism of the hydroboration of alkynes is similar to that of alkene hydroboration. The addition of BH3 (from B2H6) to an alkyne is a stereospecific cis addition and the boron atom is regiospecifically added to the lower-substituted carbon atom. Hydration of Alkynes - Hydroboration. With asymmetrically substituted, non-terminal alkynes, the reaction sequence of hydroboration, oxidation and hydrolysis yields both possible ketones. Suitable for this purpose are diisopentyl borane and dicyclohexyl borane, for example. However, a bulky borane reagent needs to be used to stop at the alkenyl-borane stage. This second hydroboration can be prevented if a sterically-hindered borane is applied. If a terminal alkyne is applied, the product of the subsequent tautomerization is a methyl ketone (oxymercuration method) or an aldehyde (hydroboration method). The reaction proceeds in an Anti-Markovnikov manner, where the hydrogen (from BH 3 or BHR 2) attaches to the more substituted carbon and the boron attaches to the least substituted carbon in the alkene bouble bond. (If the alkyne is not terminal, it will just yield a ketone). If a terminal alkyne is applied, the aldehyde and not the ketone is formed. Borane (BH 3) and alkyl boranes (R 2 BH) add to alkynes in the same way as they do with alkenes. Hydroboration-Oxidation of Alkenes. Concept #1: Anti-Markovnikov … The mechanism of the hydroboration of alkynes is similar to that of alkene hydroboration. The addition of BH 3 (from B 2 H 6) to an alkyne is a stereospecific cis addition and the boron atom is regiospecifically added to the lower-substituted carbon atom. In this reaction, an alkyne reacts with a borane in the first step and then you oxidize the resulting intermediate with hydrogen peroxide in basic media. Step 1: Hydroboration of terminal alkynes reacts in an anti-Markovnikov fashion in which the Boron attacks the less substituted carbon which is the least hindered. Because the formed alkenylborane still contains a π bond, a second hydroboration can occur. Hydration of alkynes MECHANISM FOR HYDROBORATION OF ALKYNES Mechanistically analgous to the hydroboration of alkenes to form the enol. Hydroboration-Oxidation of Alkynes with Practice Problems. Copyright © 1999-2016 Wiley Information Services GmbH. It is a stereospecific reaction where syn addition is observed as the hydroboration occurs on the same side of the alkyne and results in cis stereochemistry. Hydroboration-Oxidation of Alkynes This is another reaction similar to what you already know for alkenes. The hydration of alkynes by oxymercuration yields the Markovnikov enol while the hydroboration-oxidation-hydrolysis sequence results in the anti-Markovnivkov enol. This is an important difference to the mercury(II)-catalyzed hydration (oxymercuration) in which terminal alkynes are transformed into methyl ketones. All rights reserved. Learn Additional Practice Summary Hydroboration-oxidation of terminal alkynes leads to formation of aldehydes. Hydroboration-Oxidation is a two step pathway used to produce alcohols. The resulting enol tautomerises to the aldehyde or ketone. Exercise: Hydration of alkynes - hydroboration.