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Base-Promoted α-Halogenation of Aldehydes and Ketones00:51

Base-Promoted α-Halogenation of Aldehydes and Ketones

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α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base.  The reaction begins with the abstraction of  α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction...
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Halogenation of Alkenes02:46

Halogenation of Alkenes

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Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
15.5K
Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

9.9K
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.
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ortho–para-Directing Deactivators: Halogens01:24

ortho–para-Directing Deactivators: Halogens

5.5K
Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate...
5.5K
Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

3.7K
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...
3.7K
Radical Substitution: Allylic Chlorination01:31

Radical Substitution: Allylic Chlorination

2.2K
Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic...
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Late-Stage Halogenation of Peptides, Drugs and (Hetero)aromatic Compounds with a Nucleophilic Hydrazide Catalyst.

Angewandte Chemie (International ed. in English)·2023
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Halogen and Chalcogen Activation by Nucleophilic Catalysis.

Haripriyo Mondal1

  • 1Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|July 23, 2024
PubMed
Summary

This review explores nucleophilic activators for halogen activation in organic synthesis. It highlights advances in catalytic strategies for creating halogenated compounds, crucial for drug discovery.

Keywords:
CatalysisHalogen activationHalogenationNucleophileNucleophilic catalyst

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Area of Science:

  • Organic Chemistry
  • Medicinal Chemistry

Background:

  • Halogenated organic compounds are vital in drug discovery due to their unique properties.
  • Traditional synthesis methods are often inefficient and environmentally unfavorable.
  • Solid halogen carriers have enabled new catalytic approaches to halofunctionalization.

Purpose of the Study:

  • To review the role of nucleophilic activators in halogen activation.
  • To discuss advancements in catalytic halofunctionalization reactions.
  • To cover both racemic and asymmetric synthesis strategies.

Main Methods:

  • Review of recent literature on catalytic halofunctionalization.
  • Analysis of nucleophilic activators and their mechanisms.
  • Discussion of catalyst development and reaction kinetics.

Main Results:

  • Emergence of efficient catalytic strategies for installing carbon-halogen bonds.
  • Development of novel catalysts and activation modes for halofunctionalization.
  • Progress in both racemic and asymmetric halogenation reactions.

Conclusions:

  • Nucleophilic activators are key to modern catalytic halofunctionalization.
  • Catalytic methods offer greener and more efficient routes to halogenated compounds.
  • Continued research promises further innovation in synthesizing functionalized molecules.