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Nucleophilic Aromatic Substitution: Elimination–Addition01:11

Nucleophilic Aromatic Substitution: Elimination–Addition

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Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is...
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Aryldiazonium Salts to Azo Dyes: Diazo Coupling01:11

Aryldiazonium Salts to Azo Dyes: Diazo Coupling

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The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the para...
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Preparation of Alkynes: Dehydrohalogenation02:34

Preparation of Alkynes: Dehydrohalogenation

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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.
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Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)01:30

Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)

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Nucleophilic substitution in aromatic compounds is feasible in substrates bearing strong electron-withdrawing substituents positioned ortho or para to the leaving group. The reaction proceeds via two steps: the addition of the nucleophile and the elimination of the leaving group.
The reaction begins with an attack of the nucleophile on the carbon that holds the leaving group. This results in the delocalization of the π electrons over the ring carbons. The resonance interaction between...
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Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN101:14

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Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo,...
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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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Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-phosphinetriyltripiperidine]}palladium Under Mild Reaction Conditions
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Reversible Dehalogenation in On-Surface Aryl-Aryl Coupling.

Samuel Stolz1,2, Marco Di Giovannantonio1, José I Urgel1

  • 1Empa, Swiss Federal Laboratories for Materials Science and Technology, nanotech@surfaces Laboratory, 8600, Dübendorf, Switzerland.

Angewandte Chemie (International Ed. in English)
|April 28, 2020
PubMed
Summary
This summary is machine-generated.

On-surface synthesis using dehalogenative coupling is key for carbon nanomaterials. This study reveals reversible dehalogenation on gold surfaces, unlike copper, impacting reaction kinetics and energetics for better material fabrication.

Keywords:
aryl-aryl couplingdehalogenationreaction mechanismsreversibilitysurface chemistry

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Area of Science:

  • Surface Science
  • Materials Chemistry
  • Nanotechnology

Background:

  • On-surface synthesis enables fabrication of covalently bonded carbon-based nanomaterials.
  • Dehalogenative aryl-aryl coupling is a primary method in this field.
  • Reaction kinetics and energetics of this process remain poorly understood.

Purpose of the Study:

  • To investigate the reaction kinetics and energetics of on-surface dehalogenative polymerization.
  • To compare the process on two different coinage metal surfaces: Cu(111) and Au(111).
  • To elucidate the mechanistic differences influencing the polymerization on these surfaces.

Main Methods:

  • Comprehensive temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) was employed.
  • The study focused on the polymerization of 4,4''-dibromo-p-terphenyl to poly(para-phenylene).
  • Kinetic models were developed incorporating observed reaction phenomena.

Main Results:

  • Clear evidence of reversible dehalogenation was observed on the Au(111) surface.
  • Dehalogenation was inhibited on the Cu(111) surface due to organometallic intermediate formation.
  • Incorporating reversible dehalogenation into rate equations provided excellent agreement with experimental data, enabling energy barrier extraction.

Conclusions:

  • The study provides a deeper mechanistic understanding of surface-confined dehalogenative aryl-aryl coupling.
  • Reversible dehalogenation is a critical factor on Au(111) but not on Cu(111).
  • These findings necessitate a reassessment of aryl-aryl coupling mechanisms on commonly used metal substrates.