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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.

Conjugate Addition to α,β-Unsaturated Carbonyl Compounds

Conjugate Addition to α,β-Unsaturated Carbonyl Compounds

α,β-Unsaturated carbonyl compounds are molecules bearing a carbonyl and alkene functionality in conjugation with each other. The conjugation in the molecule leads to three resonance structures. The hybrid form exhibits two probable electrophilic sites: the carbonyl carbon and the β carbon.

Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene

Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene

The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.

Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.

Conjugate Addition (1,4-Addition) vs Direct Addition (1,2-Addition)

Conjugate Addition (1,4-Addition) vs Direct Addition (1,2-Addition)

α,β-Unsaturated carbonyl compounds with two electrophilic sites, the carbonyl carbon, and the β carbon, are susceptible to nucleophilic attack via two modes: conjugate or 1,4-addition and direct or 1,2-addition.
Conjugate addition results in a thermodynamically stable product. The reaction retains the stronger C=O bond at the expense of the weaker C=C π bond. The process is slow as the β carbon is less electrophilic than the carbonyl carbon.
Direct addition products are formed faster owing to...

Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene

Electrophilic 1,2- and 1,4-Addition of X₂ to 1,3-Butadiene

Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.

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Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

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Ruthenium-catalyzed two-component addition to form 1,3-dienes: optimization, scope, applications, and mechanism.

B M Trost¹, A B Pinkerton, M Seidel

¹Department of Chemistry, Stanford University, Stanford, California 94305, USA.

Journal of the American Chemical Society

|December 14, 2001

Summary

A new method efficiently synthesizes 1,3-dienes from allenes and olefins using ruthenium catalysts. This atom-economical process tolerates diverse functional groups and is useful for creating cyclic structures via Diels-Alder reactions.

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Area of Science:

Organic Chemistry
Catalysis
Synthetic Methodology

Background:

1,3-dienes are crucial building blocks in organic synthesis.
Efficient and selective methods for 1,3-diene synthesis are highly sought after.
Ruthenium-catalyzed reactions offer unique pathways for C-C bond formation.

Purpose of the Study:

To develop a novel two-component coupling reaction for synthesizing 1,3-dienes.
To explore various synthetic routes for preparing diverse allene precursors.
To optimize reaction conditions for efficient and selective 1,3-diene formation.

Main Methods:

Synthesis of mono- to tetrasubstituted allenes from terminal alkynes, propargylic alcohols, or ketenes.
Ruthenium(II)-catalyzed coupling of allenes with activated olefins.
Optimization of reaction parameters including catalyst, cocatalyst, solvent, and temperature.
Investigation of reaction mechanism and selectivity factors.

Main Results:

Developed a chemoselective coupling reaction yielding 1,3-dienes with high atom economy.
Tolerated a wide range of functional groups (esters, alcohols, nitriles, amides).
Achieved good selectivity with substituted allenes, influenced by steric factors and beta-hydrogen elimination.
Demonstrated the utility of synthesized 1,3-dienes in Diels-Alder reactions for cyclic compound synthesis.

Conclusions:

A robust and versatile method for synthesizing 1,3-dienes has been established.
The reaction offers a convergent and atom-economical approach to complex cyclic structures.
Ruthenium catalysis provides a powerful tool for controlled C-C bond formation and functionalization.