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Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene01:13

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Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

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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.
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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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Electrophilic Addition to Alkynes: Hydrohalogenation02:35

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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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Preparation of Alkynes: Alkylation Reaction02:27

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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.
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Introduction
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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes
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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes

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Alkynylation with Hypervalent Iodine Reagents.

Jerome Waser1

  • 1Laboratory of Catalysis and Organic Synthesis, EPFL SB ISIC LCSO, BCH 4306, 1015, Lausanne, Switzerland. jerome.waser@epfl.ch.

Topics in Current Chemistry
|August 31, 2015
PubMed
Summary
This summary is machine-generated.

Hypervalent iodine reagents, particularly ethynylbenziodoxol(on)e (EBX) reagents, provide stable electrophilic alkyne synthons for organic synthesis. These reagents enable efficient alkynylation of various nucleophiles and C-H bonds under mild conditions.

Keywords:
AlkynesAlkynyliodonium saltsEthynylbenziodoxol(on)e (EBX) reagents

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Area of Science:

  • Organic Chemistry
  • Synthetic Methodology
  • Hypervalent Iodine Chemistry

Background:

  • Alkynes are versatile building blocks in organic synthesis, chemical biology, and materials science.
  • Traditional alkyne chemistry involves nucleophilic addition, limiting electrophilic alkyne synthon development.
  • Hypervalent iodine reagents offer a unique approach to electrophilic alkynylation.

Purpose of the Study:

  • To review the development and applications of hypervalent iodine reagents as electrophilic alkyne synthons.
  • To highlight the advantages of ethynylbenziodoxol(on)e (EBX) reagents over earlier alkynyliodonium salts.
  • To showcase the broad utility of these reagents in modern organic synthesis.

Main Methods:

  • Development of alkynyliodonium salts and ethynylbenziodoxol(on)e (EBX) reagents.
  • Application of EBX reagents in alkynylating nucleophiles (soft carbon, heteroatoms, thiols).
  • Utilizing EBX reagents in transition metal-catalyzed reactions (C-H bond functionalization, domino processes).

Main Results:

  • Alkynyliodonium salts enabled ynamide synthesis but suffered from instability.
  • EBX reagents offer superior stability and reactivity as electrophilic alkyne synthons.
  • EBX reagents facilitate alkynylation of acidic C-H bonds, aromatic C-H bonds, radicals, and heteroatom nucleophiles under mild conditions.

Conclusions:

  • Hypervalent iodine reagents, especially EBX, are powerful tools for electrophilic alkynylation.
  • These reagents enable efficient synthesis of alkynes via non-classical disconnections.
  • The demonstrated utility expands synthetic possibilities in organic chemistry, chemical biology, and materials science.