Hydration of Alkynes
Hydration of Alkynes - Hydroboration
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.
Because the formed alkenylborane still contains a π bond, a second hydroboration can occur. This second hydroboration can be prevented if a sterically-hindered borane is applied. Suitable for this purpose are diisopentyl borane and dicyclohexyl borane, for example.
With asymmetrically substituted, non-terminal alkynes, the reaction sequence of hydroboration, oxidation and hydrolysis yields both possible ketones. If a terminal alkyne is applied, the aldehyde and not the ketone is formed. This is an important difference to the mercury(II)-catalyzed hydration (oxymercuration) in which terminal alkynes are transformed into methyl ketones.
The hydration of alkynes by oxymercuration yields the Markovnikov enol while the hydroboration-oxidation-hydrolysis sequence results in the anti-Markovnivkov enol. If a terminal alkyne is applied, the product of the subsequent tautomerization is a methyl ketone (oxymercuration method) or an aldehyde (hydroboration method).