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Preparation of Alkynes: Alkylation Reaction

Preparation of Alkynes: Alkylation Reaction

Introduction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.

Preparation of Alkynes: Dehydrohalogenation

Preparation of Alkynes: Dehydrohalogenation

Introduction
Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ammonia. The reaction proceeds with the loss of two equivalents of hydrogen halide (HX) via two successive E2 elimination reactions.

Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction

Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction

The radical dimerization of ketones or aldehydes gives vicinal diols through a pinacol coupling reaction. However, the behavior of titanium metals used for the reaction as a source of electrons is unusual. When the reaction is carried out in the presence of titanium, diols can be isolated at low temperatures. Else titanium further reacts with diols, forming alkenes through the McMurry reaction.

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

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.

Acidity of 1-Alkynes

Acidity of 1-Alkynes

The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.

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Access to Terminal Alkynes via Palladium-Catalyzed Coupling of ArCl with a Low Catalyst Loading of 0.08 mol.

Lei Xu¹, Gen-Qiang Chen², Xumu Zhang^1,2,3

¹Shenzhen Key Laboratory of Small Molecule Drug Discovery and Synthesis, Department of Chemistry and Medi-X Pingshan, Southern University of Science and Technology, Shenzhen 518055, People's Republic of China.

The Journal of Organic Chemistry

|May 29, 2025

Summary

This summary is machine-generated.

This study presents a new method for synthesizing terminal alkynes from aryl chlorides using a palladium-catalyzed Heck-Cassar reaction. This efficient process allows for the preparation of valuable aryl-substituted alkynes, useful in drug discovery.

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Area of Science:

Organic Synthesis
Catalysis
Medicinal Chemistry

Background:

Synthesizing terminal alkynes from aryl chlorides is a significant challenge in organic chemistry.
Existing methods often require expensive starting materials or harsh reaction conditions.

Purpose of the Study:

To develop an efficient and robust method for the synthesis of aryl-substituted terminal alkynes.
To utilize inexpensive aryl chlorides as starting materials.
To enable late-stage functionalization in drug discovery.

Main Methods:

A Heck-Cassar reaction employing aryl chlorides and (triisopropylsilyl)acetylene.
Palladium(0) catalysis at low catalyst loading (0.08 mol %).
Subsequent desilylation to yield terminal alkynes.

Main Results:

Successful synthesis of terminal alkynes from aryl chlorides.
The reaction demonstrates robustness and good functional group tolerance.
The method is suitable for late-stage modification of complex molecules.

Conclusions:

A novel and efficient palladium-catalyzed route to aryl-substituted terminal alkynes has been established.
This method offers a practical approach for accessing valuable synthetic intermediates.
The reaction's compatibility makes it applicable to pharmaceutical research and development.