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La hidroaminación enantioselectiva de tipo cope catalizada por la tiureourea es una hidroaminación intramolecular
Adam R Brown1, Christopher Uyeda, Carolyn A Brotherton
1Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
Journal of the American Chemical Society
|April 20, 2013
Resumen
Los catalizadores de tiurea quiral permiten reorganizaciones tipo Cope de las hidroxilaminas. Esta reacción produce de manera eficiente pirrolidinas enriquecidas con enantio, ofreciendo una alternativa libre de metales para la síntesis de compuestos valiosos.
Área de la Ciencia:
- Química orgánica es la química orgánica.
- Catalización asimétrica por catálisis asimétrica.
- Metodología sintética de la metodología sintética.
Sus antecedentes:
- Los reordenamientos tipo Cope son transformaciones valiosas en la síntesis orgánica.
- El acceso a las pirrolidinas enriquecidas con enantio es crucial para el desarrollo farmacéutico.
- Las reacciones de hidroaminación catalizadas por metales son comunes, pero pueden tener limitaciones.
Objetivo del estudio:
- Desarrollar un nuevo sistema catalítico para reordenamientos tipo Cope.
- Para lograr una alta enantioselectividad en la síntesis de las pirrolidinas α-sustituidas.
- Para establecer una alternativa sin metales a los métodos sintéticos existentes.
Principales métodos:
- Utilizó derivados quirales de tiourea como organocatalisadores.
- Investigó el reordenamiento tipo Cope de las hidroxilaminas bis-homoalílicas.
- Se empleó la catálisis asimétrica para controlar la estereoquímica.
Principales resultados:
- Catalizó con éxito la reorganización tipo Cope de las hidroxilaminas bis-homoalilicas.
- Se obtienen productos de pirrolidina alfa sustituida altamente enriquecida con enantio.
- Demostró la eficacia de los catalizadores quirales de tiourea en esta transformación.
Conclusiones:
- Los derivados de tiurea quiral catalizan eficazmente las reorganizaciones tipo Cope.
- Este método proporciona una nueva ruta a las pirrolidinas enriquecidas con enantio.
- La reacción representa un valioso enfoque libre de metales en la síntesis asimétrica.
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Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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The Hofmann and Curtius rearrangement reactions can be applied to synthesize primary amines from carboxylic acid derivatives such as amides and acyl azides. In the Hofmann rearrangement, a primary amide undergoes deprotonation in the presence of a base, followed by halogenation to generate an N-haloamide. A second proton abstraction produces a stabilized anionic species, which rearranges to an isocyanate intermediate via an alkyl group migration from the carbonyl carbon to the neighboring...
02:36Regioselectivity and Stereochemistry of Hydroboration
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.
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.
01:16Amines to Alkenes: Hofmann Elimination
Alkenes can be obtained from amines via an E2 elimination. The amine is first converted into a good leaving group, such as a quaternary ammonium salt. This is accomplished by treating the amine with an excess of alkyl halide, which results in a halide salt. Next, the halide salt is transformed into a hydroxide salt that functions as a base to enable elimination.
Under thermal conditions, the hydroxide can abstract a proton from the β carbon; this generates an alkene with the simultaneous...
Under thermal conditions, the hydroxide can abstract a proton from the β carbon; this generates an alkene with the simultaneous...