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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.

Fig.1
Hydroboration of alkynes

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.

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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.

Fig.6
Comparison of oxymercuration and hydroboration

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).

Exercise: Hydration of alkynes - hydroboration

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