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Halogenation of Alkenes02:46

Halogenation of Alkenes

15.2K
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.
15.2K
Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

3.6K
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...
3.6K
Formation of Halohydrin from Alkenes02:41

Formation of Halohydrin from Alkenes

12.7K
An alkene, such as propene, reacts with bromine in the presence of water to yield a halohydrin. Halohydrins contain a halogen and a hydroxyl group attached to adjacent carbons. When the halogen is bromine, it is called a bromohydrin, while a chlorohydrin has chlorine as the halogen.
12.7K
Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

9.8K
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.
9.8K
Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene01:14

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

2.3K
Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.
2.3K
Base-Promoted α-Halogenation of Aldehydes and Ketones00:51

Base-Promoted α-Halogenation of Aldehydes and Ketones

3.4K
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...
3.4K

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Photoexcited Transition-Metal Catalyzed Carbon-Halogen Bond Formation.

Ramon Arora1, Philip Samokhin1, Mark Lautens1

  • 1Davenport Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George St, Toronto, Ontario, M5S 3H6, Canada.

Angewandte Chemie (International Ed. in English)
|February 21, 2025
PubMed
Summary

Transition-metal photocatalysis enables new carbon-halogen (C-X) bond formation pathways, overcoming limitations of traditional methods. This approach utilizes photoexcited species, offering alternatives to high temperatures and additives for broader synthetic access.

Keywords:
CarbohalogenationHalogenationPhotocatalysisTransition-Metal

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Area of Science:

  • Organic Chemistry
  • Catalysis
  • Photochemistry

Background:

  • Transition-metal catalysis is crucial for carbon-halogen (C-X) bond formation.
  • Existing methods face limitations, necessitating innovative strategies.
  • Metallaphotoredox catalysis has emerged as a powerful alternative.

Purpose of the Study:

  • To review recent advances in transition-metal photocatalyzed C-X bond formation.
  • To highlight novel mechanistic pathways enabled by photoredox catalysis.
  • To provide a resource for chemists seeking efficient synthetic methods.

Main Methods:

  • Utilizing photoexcited transition metal complexes.
  • Employing visible light to drive catalytic cycles.
  • Exploring new reaction mechanisms for C-X bond formation.

Main Results:

  • Metallaphotoredox catalysis offers new routes to C-X bonds.
  • These methods bypass limitations of traditional catalysis, such as high temperatures.
  • Access to previously unattainable molecular space is achieved.

Conclusions:

  • Transition-metal photocatalysis represents a significant advancement in C-X bond formation.
  • This strategy provides milder reaction conditions and broader substrate scope.
  • The review aims to facilitate synthetic endeavors in both academic and industrial settings.