1.5: Hydration of Alkynes

Objectives

After completing this section, you should be able to

1. write the equation for the reaction of water with an alkyne in the presence of sulfuric acid and mercury(II) sulfate.
2. describe keto-enol tautomerism.
3. predict the structure of the ketone formed when a given alkyne reacts with sulfuric acid in the presence of mercury(II) sulfate.
4. identify the reagents needed to convert a given alkyne to a given ketone.
5. identify the alkyne needed to prepare a given ketone by hydration of the triple bond.
6. write an equation for the reaction of an alkyne with borane.
7. write the equation for the reaction of a vinylic borane with basic hydrogen peroxide.
8. identify the reagents, the alkyne, or both, needed to prepare a given ketone or a given cis alkene through a vinylic borane intermediate.
9. identify the ketone produced when a given alkyne is reacted with borane followed by basic hydrogen peroxide.
10. explain why it is necessary to use a bulky, sterically hindered borane when preparing vinylic boranes from terminal alkynes.
11. predict the product formed when the vinylic borane produced from a terminal alkyne is treated with basic hydrogen peroxide.
12. identify the alkyne needed to prepare a given aldehyde by a vinylic borane.

Key Terms

Make certain that you can define, and use in context, the key terms below.

• enol
• keto-enol tautomeric equilibrium
• tautomerism
• tautomers

Study Notes

Rapid interconversion between tautomers is called tautomerism; however, as the two tautomers are in equilibrium, the term tautomeric equilibrium may be used. This section demonstrates the equilibrium between a ketone and an enol; hence, the term keto-enol tautomeric equilibrium is appropriate. The term “enol” indicates the presence of a carbon-carbon double bond and a hydroxyl (i.e., alcohol) group. 

Notice how hydroboration complements hydration in the chemistry of both alkenes and alkynes.

Mercury(II)-Catalyzed Hydration of Alkynes

As with alkenes, hydration (addition of water) of alkynes requires a strong acid, usually sulfuric acid, and is facilitated by the mercuric ion (Hg²⁺). However, the hydration of alkynes gives ketone products while the hydration of alkenes gives alcohol products. Notice that the addition of oxygen in both reactions follows Markovnikov rule.

During the hydration of an alkyne, the initial product is an enol intermediate (a compound having a hydroxyl substituent attached to a double-bond), which immediately rearranges to the more stable ketone through a process called enol-keto tautomerization.

Tautomers are defined as rapidly inter-converted constitutional isomers, usually distinguished by a different bonding location for a labile hydrogen atom and a differently located double bond. The keto and enol tautomers are in equilibrium with each other and with few exceptions the keto tautomer is more thermodynamically stable and therefor favored by the equilibrium. This mechanism for tautomerization will be discussed in greater detail in Section 6-2.

General Reaction

For terminal alkynes, the addition of water follows the Markovnikov rule, and the final product is a methyl ketone. For internal alkynes the addition of water is not regioselective. Hydration of symmetrical internal alkynes produces a single ketone product. However, hydration of asymmetrical alkynes, (i.e. if R & R' are not the same ) produces two isomeric ketone products.

Mechanism

TMechanis

The mechanism starts with the eletrophilic addition of the mercuric ion (Hg²⁺) to the alkyne producing a mercury-containing vinylic carbocation intermediate. Nucleophilic attack of water on the vinylic carbocation forms a C-O bond to produce a protonated enol. Deprotonation of the enol by water then produces a organomurcury enol. The murcury is substituted with H⁺ to produced a neutral enol and regenerate the Hg²⁺ catalyst. The enol is converted to the ketone product through keto-enol tautomerization the mechanism of which is provided in the Carbonyl alpha-Substitution Chapter.

1) Electrophilic addition of Hg²⁺

2) Nucleophilic attack by water

3) Deprotonation

4) Substitution

5) Tautomerization

Hydroboration–Oxidation of Alkynes

The hydroboration-oxidation of alkynes is analagous to the reaction with alkenes. However, where alkenes for alcohol products, alkynes for aldehyde or ketone products. In both cases the addition is anti-Markovnikov and an oxygen is placed on the less alkyl substituted carbon. With the hydroboration of an alkyne the presences of a second pi bond allows the initial product to under tautomerization to become the final aldehyde product.

Alkynes have two pi bonds both of which are capable of reacting with borane (BH₃). To limit the reactivity to only one alkyne pi bond, a dialkyl borane reagent (R₂BH) is used. Replacing two of the hydrogens on the borane with alkyl groups also creates steric hindrance which enhances the anti-Markovnikov regioselective of the reaction. Disiamylborane (Sia₂BH) and 9-borabicyclo[3.3.1]nonane (9-BBN) are two common reagents for this hydroboration reaction. The oxidation reagents (a basic hydrogen peroxide solution) are the same for both alkenes and alkynes.

General Reaction

The hydroboration of terminal alkynes produces aldehyde products while internal alkynes produce ketone products. The hydroboration of symmetrical alkynes produces one ketone product and asymmetrical alkynes produce a mixture of product ketones.

Mechanism

The mechanism starts with the electrophilic addition of the B-H bond of the borane. The hydrogen atom and the borane all on the same side of the alkyne creating a syn addition configuration in the alkene product. Also, the addition is anti-Markovnikov regioselective which means the borane adds to the less substituted carbon of the alkyne and the hydrogen atom adds to the more substituted. The oxidative work-up replaces the borane with a hydroxy group (-OH) creating an enol intermediate. The enol immediately tautomerizes to the product aldehyde for terminal alkynes and the product ketone for internal alkynes.

Comparison of Mercury(II)-Catalyzed Hydration and Hydroboration–Oxidation of Alkynes

These two reactions are complementary for the reaction of a terminal alkyne because the produce distinctly different products. The mercury(II) catalyzed hydration of a terminal alkyne produces a methyl ketone, while the hydroboration-oxidation produces an aldehyde.

For internal alkynes, the regioslectivity of these reactions are rendered ineffective. The reactions are redundant in that they both produce the same ketone products.

 

Exercise \(\PageIndex{1}\)

Draw the structure of the keto form of the enol compound below. Which form would you expect to be the most stable? Draw the mechanism for the enol to keto transformation under both acidic and basic conditions.

Answer

Our prediction is that the keto form will be the most stable. (This will be true unless the enol form is part of a highly conjugate system, like a benzene ring.) Check out Section 6-2 for the enol to keto tautomerization mechanism.

Exercise \(\PageIndex{2}\)

Draw the product(s) of the following reactions.

Answer

Terminal alkynes will yield ketones upon acidic hydration and aldehydes upon hydroboration-oxidation. Internal alkynes will yield the same ketone products regardless of the reaction conditions.

Exercise \(\PageIndex{3}\)

What alkyne starting material would you use to synthesize the following targets?

Answer

Remember, using a terminal alkyne to make a ketone avoids generating a mixture of ketones products.