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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.

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.

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.

Electrophilic Addition to Alkynes: Halogenation

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.

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Updated: May 24, 2025

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

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Published on: April 19, 2019

Harnessing transient CAAC-stabilized mesitylborylenes for chalcogen activation.

Maximilian Michel^1,2, Lukas Endres^1,2, Felipe Fantuzzi³

¹Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg Am Hubland 97074 Würzburg Germany h.braunschweig@uni-wuerzburg.de.

Chemical Science

|March 5, 2025

Summary

New boron compounds, cyclic(alkyl)(amino)carbene-bound mesitylborylenes, efficiently generate dicoordinate borylenes. These activate chalcogens like sulfur, selenium, and tellurium, forming novel boron chalcogenides with unique E-E bonds.

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Published on: May 26, 2019

Area of Science:

Organoboron Chemistry
Main Group Chemistry
Inorganic Synthesis

Background:

Dicoordinate borylenes are reactive intermediates.
Cyclic(alkyl)(amino)carbenes (CAACs) stabilize low-coordinate boron species.
Efficient precursors for borylene generation are needed.

Purpose of the Study:

To synthesize novel precursors for dicoordinate borylenes.
To investigate the reactivity of these borylenes with elemental chalcogens.
To characterize the resulting boron chalcogenides and understand their electronic structures.

Main Methods:

Synthesis of CAAC-bound mesitylborylene adducts with CO and PMe3.
Thermal and photolytic activation of precursors.
Reaction with elemental sulfur, selenium, and tellurium.
Spectroscopic characterization (NMR, X-ray crystallography).
Quantum chemical calculations (DFT).

Main Results:

Novel adducts serve as efficient precursors for the in situ generation of [(CAAC)BMes].
Reaction with sulfur and selenium yields boron chalcogenides with terminal B=E double bonds.
Reaction with tellurium produces an unusual diradical ditelluride with a Te-Te bond and an open-shell singlet ground state.
Redox studies reveal E-E bond formation and cleavage.

Conclusions:

CAAC-stabilized borylene precursors offer a versatile route to novel boron chalcogenides.
The reactivity with chalcogens is dependent on the element, leading to diverse structures.
The diradical ditelluride exhibits unique electronic properties.
Boron chalcogenides show interesting redox behavior with potential for E-E bond manipulation.