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Reactividad de los borilenos hacia el etino, eteno y metano
Małgorzata Krasowska1, Holger F Bettinger
1Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany.
Journal of the American Chemical Society
|October 6, 2012
Resumen
Este estudio investigó computacionalmente la reactividad del borileno, encontrando que BH es el más reactivo y electrófilo, mientras que BF es el menos reactivo y el más nucleófilo. Estos hallazgos ofrecen información sobre las reacciones de adición de borileno.
Área de la Ciencia:
- Química computacional es la química computacional.
- Química cuántica es la química cuántica.
- Química organometálica Química orgánica de los metales.
Sus antecedentes:
- Los borilenos, análogos de silicio de los carbenos, son intermedios reactivos.
- Comprender la estructura electrónica y la reactividad del borileno es crucial para las aplicaciones sintéticas.
Objetivo del estudio:
- Para investigar la estructura electrónica y geométrica de los borilenos sustituidos.
- Examinar la reactividad de los borilenos hacia los hidrocarburos.
- Para dilucidar el mecanismo y la energética de las reacciones de adición e inserción de borileno.
Principales métodos:
- La teoría funcional de densidad híbrida (B3LYP) y la teoría de perturbaciones de Møller-Plesset (MP2).
- Computaciones de clúster acopladas [CCSD(T) ].
- Se utilizan conjuntos de base 6-311+G**, cc-pVTZ y aug-cc-pVTZ.
Principales resultados:
- BH es el borileno más electrófilo y reactivo, exhibiendo reacciones de adición exotérmica sin barreras de energía o de baja energía.
- BF es el borileno más nucleófilo, con mayores barreras de reacción y menor exotermicidad.
- Se investigaron las reacciones de los borilenos con hidrocarburos insaturados (eteno, etina) y la inserción de enlaces C-H en el metano.
Conclusiones:
- La reactividad del borileno está fuertemente influenciada por el sustituto (grupo R).
- BH demuestra la mayor reactividad, mientras que BF muestra la más baja.
- Las ideas computacionales guían la comprensión de los mecanismos de reacción del borileno y sus posibles aplicaciones.
Videos de Conceptos Relacionados
03:08Hydroboration-Oxidation of Alkenes
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
02:38Electrophilic 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 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.
01:17Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.
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.
02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
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.
02:42Acidity of 1-Alkynes
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.