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01:13Relative Reactivity of Carboxylic Acid Derivatives
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Carboxylic acid derivatives such as acid halides, anhydrides, esters, and amides undergo nucleophilic acyl substitution reactions with varying degrees of reactivity.
A key factor in assessing the reactivity of the acid derivatives is the basicity of the substituent or the leaving group. The lower the basicity of the leaving group, the higher the reactivity of the derivative. The basicity of the leaving group follows this order:
Halide ions < Acyloxy ions < Alkoxy ions < Amine ions
A key factor in assessing the reactivity of the acid derivatives is the basicity of the substituent or the leaving group. The lower the basicity of the leaving group, the higher the reactivity of the derivative. The basicity of the leaving group follows this order:
Halide ions < Acyloxy ions < Alkoxy ions < Amine ions
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02:14Leaving Groups
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The nature of leaving groups strongly influences the outcome of a nucleophilic substitution reaction.
In general, in a nucleophilic substitution reaction, a nucleophile displaces a functional group, called the leaving group, from the substrate to give a substituted product. A leaving group departs the substrate molecule through heterolytic cleavage, taking the pair of electrons with it to become a relatively stable weak base in the form of an anion or a neutral molecule.
In a...
In general, in a nucleophilic substitution reaction, a nucleophile displaces a functional group, called the leaving group, from the substrate to give a substituted product. A leaving group departs the substrate molecule through heterolytic cleavage, taking the pair of electrons with it to become a relatively stable weak base in the form of an anion or a neutral molecule.
In a...
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02:23SN2 Reaction: Stereochemistry
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In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not...
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not...
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02:43E2 Reaction: Stereochemistry and Regiochemistry
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Elimination reactions of alkyl halides can yield one or more alkenes depending on the specific regiochemical and stereochemical considerations. While the regiochemistry of the reaction governs the location of the double bond in the product, the stereochemical requirements often influence the geometry.
When a substrate with two different β hydrogens undergoes an E2 elimination, the presence of a strong base can yield two regioisomeric alkenes. The more-substituted alkene is the major...
When a substrate with two different β hydrogens undergoes an E2 elimination, the presence of a strong base can yield two regioisomeric alkenes. The more-substituted alkene is the major...
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02:46E1 Reaction: Kinetics and Mechanism
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Here, in contrast to the E2 reaction mechanism, we delve into the aspects of the E1 reaction mechanism, which has two steps: rate-limiting loss of the leaving group and abstraction of the beta hydrogen by a weak base. Typically, the experimental proof for the E1 mechanism is via kinetic studies or isotope studies. While the former demonstrates the first-order kinetics—the dependence of the reaction solely on substrate concentration—the latter proves the abstraction of hydrogen only...
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02:26Preparation and Reactions of Sulfides
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Sulfides are the sulfur analog of ethers, just as thiols are the sulfur analog of alcohol. Like ethers, sulfides also consist of two hydrocarbon groups bonded to the central sulfur atom. Depending upon the type of groups present, sulfides can be symmetrical or asymmetrical. Symmetrical sulfides can be prepared via an SN2 reaction between 2 equivalents of an alkyl halide and one equivalent of sodium sulfide.
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Regioselective O-Glycosylation of Nucleosides via the Temporary 2',3'-Diol Protection by a Boronic Ester for the Synthesis of Disaccharide Nucleosides
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Diferencias notables de reactividad entre glucósidos con grupos de salida idénticos
Tianmeng Duo1, Kyle Robinson1, Ian R Greig1
1Department of Chemistry, University of British Columbia , 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada.
Journal of the American Chemical Society
|October 17, 2017
Resumen
Dos glucósidos fluorados isoméricos muestran una diferencia de un millón de veces en las tasas de reacción enzimática, derivadas de propiedades moleculares inherentes, no de interacciones enzimáticas. La hidrólisis no enzimática también reveló variaciones significativas de la velocidad.
Área de la Ciencia:
- La bioquímica
- Enzimología
- Química orgánica
Sus antecedentes:
- Las β-glucosidasas son enzimas cruciales en el metabolismo de los carbohidratos.
- Comprender la especificidad del sustrato y los mecanismos de reacción es clave para la función de la enzima.
- Los compuestos isoméricos pueden exhibir perfiles de reactividad muy diferentes.
Objetivo del estudio:
- Investigar las diferencias significativas de velocidad en la hidrólisis enzimática de dos isómeros de arilo 2-deoxy-2-fluoro-β-glucósidos.
- Para aclarar los orígenes de estas diferencias de velocidad mediante el examen de la hidrólisis no enzimática y las propiedades moleculares.
- Determinar la contribución de los efectos del estado fundamental y del estado de transición a la reactividad observada.
Principales métodos:
- Cinética de la hidrólisis enzimática mediante el uso de β-glucosidasa.
- Estudios de hidrólisis no enzimática (espontánea).
- Experimentos de etiquetado con O para sondear los mecanismos de reacción.
- Cristalografía de rayos X y cálculos químicos cuánticos.
Principales resultados:
- Se observó una diferencia de 10 veces en las tasas de reacción entre los dos sustratos isoméricos con β-glucosidasa.
- La hidrólisis no enzimática mostró una diferencia de velocidad de 10 veces, lo que indica que las propiedades inherentes del sustrato son responsables.
- El etiquetado con O excluyó un mecanismo alternativo de sustitución de arilo nucleófilo.
- Los estudios de rayos X y computacionales revelaron que la desestabilización del estado fundamental y la estabilización del estado de transición contribuyen igualmente a las diferencias de reactividad.
Conclusiones:
- Las diferencias dramáticas en la actividad de la β-glucosidasa se deben principalmente a las propiedades inherentes de los sustratos isoméricos.
- Tanto la desestabilización del estado fundamental como la estabilización del estado de transición juegan un papel significativo y casi igual en la modulación de la reactividad.
- Las medidas simples de equilibrio como pKa son insuficientes para predecir la capacidad de salida del grupo en reacciones enzimáticas complejas.