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Videos de Conceptos Relacionados

Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Halogenation of Alkenes02:46

Halogenation of Alkenes

Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

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...
Formation of Halohydrin from Alkenes02:41

Formation of Halohydrin from Alkenes

An alkene, such as propene, reacts with bromine in the presence of water to yield a halohydrin. Halohydrins contain a halogen and a hydroxyl group attached to adjacent carbons. When the halogen is bromine, it is called a bromohydrin, while a chlorohydrin has chlorine as the halogen.
Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.
Preparation of Alkynes: Dehydrohalogenation02:34

Preparation of Alkynes: Dehydrohalogenation

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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A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis
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A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

Published on: February 16, 2020

Reactivos simples para las ciclizaciones directas de polieno inducidas por halonio.

Scott A Snyder1, Daniel S Treitler, Alexandria P Brucks

  • 1Department of Chemistry, Columbia University, Havemeyer Hall, 3000 Broadway, New York, New York 10027, USA. sas2197@columbia.edu

Journal of the American Chemical Society
|September 23, 2010
PubMed
Resumen

Una nueva clase de reactivos estables promueve efectivamente las ciclizaciones de polieno utilizando fuentes simples de halógenos. Esto permite una síntesis eficiente de marcos policíclicos complejos y productos naturales, incluido el peyssonol A.

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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes
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Accessing Valuable Ligand Supports for Transition Metals: A Modified, Intermediate Scale Preparation of 1,2,3,4,5-Pentamethylcyclopentadiene
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Área de la Ciencia:

  • Química orgánica es la química orgánica.
  • Química sintética de la química sintética.
  • Síntesis de Productos Naturales.

Sus antecedentes:

  • Las ciclizaciones de polieno son cruciales para la síntesis de moléculas complejas.
  • Las fuentes simples de halógenos electrófilos tienen limitaciones para promover estas reacciones.
  • El desarrollo de reactivos efectivos para la ciclización del polieno es un desafío continuo.

Objetivo del estudio:

  • Introducir una clase novedosa y estable de reactivos para la ciclización del polieno.
  • Para demostrar la amplia aplicabilidad de estos reactivos con varios terpenos.
  • Para permitir la síntesis de diversos halógenos que contienen marcos policíclicos.

Principales métodos:

  • Utilizó una clase de reactivos fácilmente preparados y estables.
  • Reactivos aplicados a los terpenos ricos en electrones y deficientes en electrones (geraniol, farnesol, nerol).
  • Marco policíclico sintetizado que contiene cloro, bromo y yodo.

Principales resultados:

  • Logró la primera síntesis racémica total y revisión estructural del peyssonol A.
  • Completó una eficiente síntesis total inaugural de ácido peisonoico A.
  • Habilitó la síntesis racémica total formal de aplysin-20, loliolide, K-76 y stemodin a través de rutas más cortas, de mayor rendimiento y más ecológicas.

Conclusiones:

  • Los reactivos desarrollados son ampliamente eficaces para las ciclizaciones de polieno.
  • Esta metodología proporciona un acceso eficiente a productos naturales halogenados complejos.
  • Los estudios preliminares muestran potencial para aplicaciones enantioselectivas.