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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.

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.

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.

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.

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.

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...

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A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

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Aryl-Allene Cyclization via a Hg(OTf)2-Catalytic Pathway.

Hirofumi Yamamoto¹, Maho Ueda¹, Naoto Yamasaki¹

¹Faculty of Pharmaceutical Sciences, Tokushima Bunri University , Yamashiro-cho, Tokushima 770-8514, Japan.

Organic Letters

|May 28, 2016

Summary

Mercury(II) triflate catalyzed aryl-allene cyclization creates a quaternary carbon center. A novel pathway involves direct H-transfer and mercury migration, confirmed by deuterium labeling and computational studies.

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Published on: September 18, 2016

Area of Science:

Organic Chemistry
Catalysis
Synthetic Methodology

Background:

Quaternary carbon centers are crucial structural motifs in pharmaceuticals and natural products.
Efficient methods for constructing quaternary carbon centers are highly sought after in organic synthesis.

Purpose of the Study:

To develop a novel mercury-catalyzed reaction for the synthesis of molecules containing a quaternary carbon center.
To elucidate the catalytic mechanism of the aryl-allene cyclization.

Main Methods:

Mercury(II) triflate [Hg(OTf)2] catalyzed cyclization of aryl-allenes.
Deuterium labeling experiments to trace reaction pathways.
Computational modeling (e.g., DFT calculations) to investigate the reaction mechanism.

Main Results:

Successful realization of Hg(OTf)2-catalyzed aryl-allene cyclization.
Formation of a sterically hindered quaternary carbon center.
Proposed a novel catalytic pathway involving direct aromatic C-H activation and subsequent mercury migration.

Conclusions:

The developed method provides an efficient route to complex molecules with quaternary carbon centers.
The proposed catalytic cycle offers new mechanistic insights into mercury-catalyzed transformations.
This work expands the scope of catalytic C-H functionalization reactions.