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ortho–para-Directing Deactivators: Halogens01:24

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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...
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Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
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Catalysis

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The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed during the process. A basic comprehension of a catalysts’ role during chemical reactions can be understood from the concept of reaction mechanisms and energy diagrams.
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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Heterogeneous Catalysis

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Heterogeneous catalysis involves a catalyst in a different phase from the reactants. It is a process where the catalyst and the reactants are in distinct phases, typically solid and gas or liquid.Most heterogeneous catalysts are metals, metal oxides, or acids. The list includes transition metals like iron (Fe), cobalt (Co), nickel (Ni), palladium (Pd), platinum (Pt), chromium (Cr), manganese (Mn), tungsten (W), silver (Ag), and copper (Cu). These metals possess partially vacant d orbitals that...
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meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H01:13

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All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for...
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Modulating NHC catalysis with fluorine.

Yannick P Rey1, Ryan Gilmour2

  • 1Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany ; Department of Chemistry and Applied Biosciences, ETH Zürich, Wolfgang-Pauli-Str. 10, 8093 Zürich, Switzerland.

Beilstein Journal of Organic Chemistry
|December 25, 2013
PubMed
Summary
This summary is machine-generated.

Fluorine substitution in triazolium salts creates novel N-heterocyclic carbene organocatalysts. These catalysts efficiently drive enantioselective Steglich rearrangements, offering a new tool for organic synthesis.

Keywords:
Steglich rearrangementcatalysisenantioselectivityfluorinegauche effectorgano-fluorine

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Area of Science:

  • Organic Chemistry
  • Catalysis
  • Organofluorine Chemistry

Background:

  • Fluorination is a key strategy for modifying organocatalyst properties.
  • N-heterocyclic carbenes (NHCs) are versatile organocatalysts.
  • Triazolium salts serve as precursors to NHCs.

Purpose of the Study:

  • To investigate the impact of incorporating a β-fluoroamine motif into triazolium pre-catalysts.
  • To evaluate the effect of hydrogen-to-fluorine substitution on NHC behavior and catalytic activity.
  • To explore the enantioselective Steglich rearrangement using novel fluorinated NHC catalysts.

Main Methods:

  • Synthesis of fluorinated triazolium salts.
  • X-ray crystallographic analysis of catalyst derivatives.
  • Deprotonation to generate NHCs and subsequent catalytic reactions.

Main Results:

  • Successful synthesis of β-fluoroamine-containing triazolium salts.
  • X-ray crystallography provided insights into conformational changes.
  • Fluorinated NHCs effectively catalyzed the enantioselective Steglich rearrangement of oxazolyl carbonates.
  • Achieved enantiomeric ratios (e.r.) up to 87.0:13.0.

Conclusions:

  • Strategic fluorination of triazolium salts modulates NHC conformation and reactivity with minimal steric impact.
  • Fluorinated NHCs are effective organocatalysts for enantioselective transformations.
  • This study introduces a valuable molecular editing approach for catalyst design.