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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.

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.

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.

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.

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...

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Pd-Catalyzed Alkene-Relayed Intermolecular C-H Alkylation Using Aryl Halide Substrates.

Shuai Yang¹, Yanghui Zhang¹

¹School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, 1239 Siping Road, Shanghai 200092, China.

Organic Letters

|December 1, 2022

Summary

This study introduces a new palladium-catalyzed reaction for C-H bond functionalization using alkenes as relays. The method efficiently synthesizes complex fused polycyclic aromatic hydrocarbons from simple precursors.

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Area of Science:

Organic Chemistry
Catalysis
Synthetic Methodology

Background:

C-H bond functionalization is crucial for organic synthesis.
Developing selective and efficient C-H activation strategies remains a challenge.
Alkene-relayed C-H activation offers a novel approach to complex molecule synthesis.

Purpose of the Study:

To develop a palladium-catalyzed annulation reaction for C-H alkylation using alkenes as relays.
To synthesize succinimide-fused 9,10-dihydrophenanthrenes and dibenzo[j,l]fluoranthenes.
To establish a versatile and efficient method for constructing polycyclic aromatic hydrocarbons.

Main Methods:

Palladium-catalyzed annulation reaction.
Utilizing 2-iodobiphenyls and maleimides as substrates.
Employing acenaphthylene as an alternative relay molecule.

Main Results:

Successful synthesis of succinimide-fused 9,10-dihydrophenanthrenes via alkene-relayed C-H alkylation.
Demonstrated broad substrate scope and high reaction efficiency.
Developed a complementary method for dibenzo[j,l]fluoranthene synthesis using acenaphthylene.

Conclusions:

Alkene-relayed C-H activation is a powerful strategy for selective C-H bond functionalization.
The developed methods provide straightforward access to valuable fused polycyclic aromatic compounds.
This work expands the toolkit for modern organic synthesis.