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Nucleophilic Aromatic Substitution: Elimination–Addition01:11

Nucleophilic Aromatic Substitution: Elimination–Addition

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 confirmed through isotopic...
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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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This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

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Substituents on the benzene ring that direct an incoming electrophile to undergo substitution at the meta position are called meta directors. All meta directors either have a positive charge on the atom directly bonded to the ring or a partial positive charge. These groups function by withdrawing electrons from the ring through inductive and resonance effects. Consider the carbocation intermediates formed upon the addition of an electrophile on nitrobenzene at the ortho, meta, and para...

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Los calixarenos ópticamente activos conducidos por la sustitución del metileno.

Vijay Gopalsamuthiram1, Alexander V Predeus, Rui H Huang

  • 1Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.

Journal of the American Chemical Society
|November 26, 2009
PubMed
Resumen

Los investigadores desarrollaron una nueva síntesis para calix[4]arenas ópticamente activas, creando quiralidad a través de la sustitución de puentes de metileno. Este método utiliza una reacción clave de triple anulación para una eficiente construcción de macrociclos.

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Área de la Ciencia:

  • Química orgánica es la química orgánica.
  • Química supramolecular de las moléculas.
  • Síntesis asimétrica de síntesis.

Sus antecedentes:

  • Las arenas calix[4] son compuestos macrocíclicos con diversas aplicaciones.
  • La quiralidad en calix[4]arenas se logra típicamente a través de sustituyentes en los anillos aromáticos o en los bordes superior/inferior.
  • El desarrollo de métodos para la quiralidad originada en los puentes de metileno es un área poco explorada.

Objetivo del estudio:

  • Para describir el primer método sintético para calix ópticamente activo[4]arenas quirales debido a la sustitución en los puentes de metileno.
  • Para demostrar una nueva reacción de triple anulación para la síntesis de calixareno.
  • Para mostrar la síntesis estereoselectiva de los diastereómeros.

Principales métodos:

  • Un paso clave implica la reacción de un complejo de biscarbenos con una diena.
  • Esta reacción forma dos anillos de benceno y el anillo macrocíclico en una sola transformación.
  • La síntesis estereoselectiva se logra mediante el control de las configuraciones absolutas de los precursores.

Principales resultados:

  • Se logró la síntesis de arenas de calix[4] ópticamente activas con quiralidad en los puentes de metileno.
  • El proceso de triple anulación se aplicó con éxito para sintetizar di- y tetrametoxicalix[4]arenos.
  • Dos diastereómeros de las arenas tetrametoxicalix[4] fueron sintetizados estereoselectivamente.

Conclusiones:

  • Se ha establecido una ruta sintética novedosa y flexible para las arenas de calix[4] ópticamente activas.
  • El método descrito proporciona acceso a calix[4]arenas quirales con patrones de sustitución únicos.
  • Este trabajo abre nuevas vías para el diseño de huéspedes quirales macrocíclicos.