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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.

Hydrogen Bonds

Hydrogen Bonds

Hydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, and has a partial negative charge. The other is a hydrogen atom that has bonded with another electronegative atom and has a partial positive charge.
Hydrogen Bonds Control the World!
Because hydrogen has very weak electronegativity when it binds with a strongly electronegative atom, such as oxygen or nitrogen, electrons in the bond are unequally shared....

Hydrogen Bonds

Hydrogen Bonds

A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another polar molecule, such as water (H2O), hydrogen fluoride (HF), or ammonia (NH3). The huge electronegativity difference between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for an N atom), combined with the very small size of an H atom...

Nomenclature of Alkynes

Nomenclature of Alkynes

Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of alkynes follows a set of rules similar to alkanes and alkenes; however, alkynes bear the suffix "-yne" instead of "-ane" or "-ene." There are two approaches to naming alkynes:

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.

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.

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Quantitating Iron Transport Across the Mouse Placenta In Vivo Using Nonradioactive Iron Isotopes

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Stereoselective Alkyne Hydrogenation by using a Simple Iron Catalyst.

Bernhard J Gregori¹, Felix Schwarzhuber², Simon Pöllath²

¹Dept. of Chemistry, University of Hamburg, Martin Luther King Pl 6, 20146, Hamburg, Germany.

|July 3, 2019

Summary

This study introduces a simple iron catalyst for Z-selective alkyne semihydrogenation. The method avoids toxic noble metals and operates under mild conditions, offering a greener alternative for alkene synthesis.

Keywords:

alkynes hydrogenation iron nanoparticles stereoselectivity

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Area of Science:

Organic Chemistry
Catalysis
Materials Science

Background:

Stereoselective hydrogenation of alkynes is crucial for creating stereodefined alkenes.
Conventional methods often rely on expensive and toxic noble metal catalysts.
Developing sustainable and efficient catalytic systems is a key challenge in organic synthesis.

Purpose of the Study:

To develop a simple, cost-effective, and environmentally friendly catalyst for Z-selective alkyne semihydrogenation.
To investigate the catalytic activity of an iron-based system for alkyne reduction.
To understand the underlying catalytic mechanism.

Main Methods:

Utilized a catalyst system composed of iron(II) acetylacetonate and diisobutylaluminum hydride.
Performed semihydrogenation of alkynes under near ambient conditions (1-3 bar H2, 30°C).
Conducted mechanistic studies including kinetic poisoning, X-ray absorption spectroscopy, and transmission electron microscopy (TEM).

Main Results:

Achieved Z-selective semihydrogenation of alkynes with high efficiency.
The iron catalyst operates effectively without requiring elaborate preparation or additional ligands.
Mechanistic investigations suggest the involvement of small iron clusters and particle catalysts.

Conclusions:

The developed iron catalyst offers a practical and sustainable alternative for Z-alkene synthesis.
The catalyst system is simple to prepare and utilizes readily available commercial chemicals.
The findings provide insights into the catalytic behavior of iron nanoparticles in hydrogenation reactions.