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Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

9.8K
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.
9.8K
Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

4.6K
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...
4.6K
Base-Promoted α-Halogenation of Aldehydes and Ketones00:51

Base-Promoted α-Halogenation of Aldehydes and Ketones

4.0K
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...
4.0K
Preparation of Alkynes: Dehydrohalogenation02:34

Preparation of Alkynes: Dehydrohalogenation

17.7K
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.
17.7K
Halogenation of Alkenes02:46

Halogenation of Alkenes

18.2K
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.
18.2K
Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

11.1K
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.
11.1K

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Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
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Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

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Visible-Light-Induced Dehydrohalogenative Coupling for Intramolecular α-Alkenylation: A Way to Build Seven- and

Dawen Xu1,2, Han Li2, Guangxing Pan3

  • 1Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438, P.R. China.

Organic Letters
|May 21, 2020
PubMed
Summary

A novel metal-free reaction uses visible light to create medium-sized rings through an intramolecular alkenylation. This process involves a unique [2 + 2]-photocycloaddition mechanism, offering a new pathway for cyclic diene synthesis.

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Area of Science:

  • Organic Chemistry
  • Photochemistry
  • Synthetic Methodology

Background:

  • Visible-light photoredox catalysis has emerged as a powerful tool in organic synthesis.
  • Developing metal-free catalytic systems remains a significant challenge for sustainable chemistry.

Purpose of the Study:

  • To develop a metal-free, visible-light-induced intramolecular α-alkenylation reaction.
  • To synthesize seven- and eight-membered rings via dehydrohalogenative C(sp2)-C(sp2) coupling.
  • To elucidate the reaction mechanism involving [2+2]-photocycloaddition pathways.

Main Methods:

  • Visible-light irradiation of substrates under metal-free conditions.
  • Dehydrohalogenative C(sp2)-C(sp2) coupling reaction.
  • Mechanistic investigations including computational studies and intermediate trapping.

Main Results:

  • Successful synthesis of seven- and eight-membered rings with high stereoselectivity.
  • Identification of cyclobutane and cyclobutene intermediates.
  • Exclusive formation of cyclic (1Z,3Z)-1,3-diene products via photochemically allowed disrotation.

Conclusions:

  • The developed protocol offers a new metal-free approach for medium-sized ring synthesis.
  • The reaction proceeds via a novel [2+2]-photocycloaddition, elimination, and retro-[2+2]-photocycloaddition mechanism.
  • This work expands the utility of [2+2]-photocycloadditions in organic synthesis.