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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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Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
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Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
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In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
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Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...
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Hidrogenación ir catalizada enantio y regioselectiva de cicloalquenos di y trisubstituidos

Byron K Peters1, Jianguo Liu1, Cristiana Margarita1

  • 1Department of Organic Chemistry, Stockholm University, Arrhenius-laboratory , 10691 Stockholm, Sweden.

Journal of the American Chemical Society
|August 23, 2016
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Resumen

Los nuevos catalizadores de iridio logran una excelente enantioselectividad en la hidrogenación asimétrica de diversas olefinas cíclicas. Estos catalizadores hidrogenan efectivamente sustratos funcionalizados y demuestran regioselectividad, incluso distinguiendo entre olefinas trisubstituidas y tetrasubstituidas.

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Área de la Ciencia:

  • Química orgánica
  • Catálisis
  • Síntesis asimétrica

Sus antecedentes:

  • La hidrogenación asimétrica es crucial para la síntesis de moléculas quirales.
  • El desarrollo de catalizadores eficientes para diversos sustratos sigue siendo un desafío.
  • Los catalizadores de iridio ligado N,P ofrecen potencial para transformaciones selectivas.

Objetivo del estudio:

  • Evaluar nuevos catalizadores basados en imidazol de iridio N,P ligado para la hidrogenación asimétrica.
  • Evaluar el rendimiento del catalizador con una amplia gama de sustratos de olefinas cíclicas.
  • Investigar la enantioselectividad y la regioselectividad de las reacciones de hidrogenación.

Principales métodos:

  • Síntesis de varias olefinas cíclicas con diversas funcionalidades.
  • Aplicación de catalizadores basados en imidazol de iridio ligado N,P (tipo Crabtree).
  • Optimización de las condiciones de reacción para controlar la selectividad.

Principales resultados:

  • Se obtienen excelentes enantioselectividades (hasta > 99% ee) para las olefinas cíclicas no funcionalizadas y funcionalizadas.
  • Se ha observado un exceso enantiomérico elevado en sustratos con grupos de coordinación cerca del enlace doble.
  • Se ha demostrado la hidrogenación regioselectiva, incluida la reducción selectiva de las olefinas trisubstituidas frente a las tetrasubstituidas.

Conclusiones:

  • Los catalizadores basados en imidazol de iridio N,P ligado son altamente eficaces para la hidrogenación asimétrica de las olefinas cíclicas.
  • El sistema catalizador exhibe un amplio alcance de sustrato y una alta selectividad.
  • Este trabajo proporciona una herramienta valiosa para la síntesis enantioselectiva de moléculas quirales complejas.