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

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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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ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

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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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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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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.
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Nucleophilic Aromatic Substitution: Elimination–Addition01:11

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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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Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

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Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...
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Actividad arilativa enantioselectiva catalizada por níquel del enlace aromático C-O

Jintong Zhang1, Tingting Sun1, Zishuo Zhang1

  • 1Anhui Agricultural University, Hefei, Anhui 230036, China.

Journal of the American Chemical Society
|October 27, 2021
PubMed
Resumen

Este estudio presenta la primera activación enantioselectiva de enlaces aromáticos C-O mediante catálisis de níquel. Este método produce eficientemente esqueletos quirales binafílicos valiosos con alto rendimiento y pureza.

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Área de la Ciencia:

  • Química orgánica
  • Catálisis
  • Síntesis asimétrica

Sus antecedentes:

  • El acoplamiento cruzado catalizado por níquel de electrófilos CO fue establecido por Wenkert en la década de 1970.
  • La activación asimétrica de enlaces aromáticos C-O mediante catálisis de metales de transición sigue siendo un desafío no reportado.

Objetivo del estudio:

  • Para lograr la primera activación enantioselectiva de un enlace aromático C-O.
  • Desarrollar un método catalítico para la síntesis de esqueletos quirales de 2-arilo-2'-hidroxi-1,1'-binaftil (ArOBIN).

Principales métodos:

  • Acoplamiento cruzado catalítico arilativo de apertura de anillos de diarilfuranos.
  • Catalización de níquel mediante el uso de ligandos quirales de N-heterocíclicos de carbeno.

Principales resultados:

  • Se demostró la primera activación enantioselectiva de enlaces aromáticos C-O.
  • Se han sintetizado con éxito esqueletos quirales de ArOBIN en altos rendimientos con un alto exceso enantiomérico (ee).
  • Mostró la versatilidad del esqueleto ArOBIN para futuras modificaciones sintéticas.

Conclusiones:

  • Esta nueva reacción catalizada por níquel proporciona una ruta eficiente a los compuestos de binaftil axialmente quirales.
  • La metodología desarrollada ofrece condiciones de reacción suaves y una buena economía atómica.
  • Los esqueletos quirales sintetizados sirven como intermediarios valiosos para crear moléculas complejas, catalizadores y ligandos.