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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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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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A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
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By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...
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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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The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
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Activación del enlace C-H mono/disectivo controlado por la superficie

Qing Li1, Biao Yang1, Haiping Lin1

  • 1Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University , 199 Ren'ai Road, Suzhou, 215123, Jiangsu P. R. China.

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Resumen

Los investigadores exploraron la activación selectiva del enlace C-H en las superficies Au{11} y Ag{11} utilizando derivados de fenol. Se observaron diferentes selectividades superficiales, que ofrecen nuevas vías para la síntesis orgánica asistida por superficie.

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Área de la Ciencia:

  • Química orgánica
  • Ciencias de la superficie
  • Catálisis

Sus antecedentes:

  • La activación selectiva de enlaces C-H es un desafío significativo en la química orgánica debido a las altas entalpias de disociación de enlaces y la escasa selectividad de la reacción.
  • Los derivados de fenol presentan sustratos complejos para la activación de C-H debido a las vías de reacción competitivas.

Objetivo del estudio:

  • Investigar la funcionalización orto-C-H selectiva y los acoplamientos orto-ortho de los derivados del fenol en las superficies Au{111) y Ag{111).
  • Comprender la influencia de la deshidrogenación y la desoxigenación en las vías de reacción y la selectividad.

Principales métodos:

  • Se utilizan las superficies metálicas Au{11} y Ag{11} como plataformas de reacción.
  • Se utiliza el microscopio de túnel de barrido (STM) para la obtención de imágenes de superficie.
  • Teoría funcional de densidad aplicada (DFT) para cálculos mecanicistas.
  • Se utilizó la espectroscopia de fotoelectrones de rayos X (XPS) para el análisis de la superficie.

Principales resultados:

  • Se ha observado la activación deselectiva de enlaces orto-C-H en las superficies Au{11}.
  • Se observa la activación de enlaces C-H orto monoselectivos en las superficies Ag{111}.
  • Se ha demostrado que la competencia entre la deshidrogenación y la desoxigenación dicta las vías de reacción.

Conclusiones:

  • El estudio revela distintas vías de activación mono y deselectiva de C-H en diferentes superficies metálicas.
  • Los hallazgos ofrecen nuevas estrategias para la síntesis orgánica asistida por superficie a través de la activación controlada de enlaces C-H.
  • El trabajo proporciona información fundamental sobre los mecanismos que rigen las reacciones de derivados de fenol en superficies metálicas.