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Regioselectivity and Stereochemistry of Hydroboration

Regioselectivity 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-Oxidation of Alkenes

Hydroboration-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.

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

Alkynes 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.

Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation

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Robinson annulation is a base-catalyzed reaction for the synthesis of 2-cyclohexenone derivatives from 1,3-dicarbonyl donors (such as cyclic diketones, β-ketoesters, or β-diketones) and α,β-unsaturated carbonyl acceptors. Named after Sir Robert Robinson, who discovered it, this reaction yields a six-membered ring with three new C–C bonds (two σ bonds and one π bond).

Thermal Electrocyclic Reactions: Stereochemistry

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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide

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Alkenes are converted to 1,2-diols or glycols through a process called dihydroxylation. It involves the addition of two hydroxyl groups across the double bond with two different stereochemical approaches, namely anti and syn. Dihydroxylation using osmium tetroxide progresses with syn stereochemistry.

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Un homoacoplamiento oxidativo operacionalmente simple y suave de ésteres borónicos de arilo para acceder a

Evan R Darzi¹, Brittany M White¹, Lance K Loventhal¹

¹Department of Chemistry and Biochemistry & Materials Science Institute, University of Oregon , Eugene, Oregon 97403, United States.

Journal of the American Chemical Society

|February 3, 2017

Resumen

Este estudio introduce un nuevo homoacoplamiento oxidativo catalizado por el paladio de ésteres borónicos para sintetizar macrociclos tensados. Este método suave a temperatura ambiente ofrece una alternativa más simple a las macrociclizaciones de acoplamiento cruzado tradicionales.

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Área de la Ciencia:

Química orgánica
Química sintética
Catálisis

Sus antecedentes:

Los andamios macrocíclicos restringidos son sintéticamente difíciles debido a los métodos de macrocíclicación limitados.
El homoacoplamiento oxidativo catalizado por paladio de ácidos y ésteres de borilo es un subproducto conocido en los acoplamientos cruzados Suzuki-Miyaura, pero no se utiliza ampliamente para la síntesis compleja.

Objetivo del estudio:

Desarrollar un método suave y eficiente para la síntesis de macrociclos tensados y conformacionalmente restringidos.
Para aprovechar el homoacoplamiento oxidativo catalizado por el paladio de los ésteres de boro para la macrociclización.

Principales métodos:

Se empleó una reacción de homoacoplamiento de éster borónico oxidativo intramolecular.
Se realizó una comparación con los acoplamientos cruzados intramoleculares Suzuki-Miyaura y los homoacoplamientos reductivos Yamamoto.
Las condiciones de reacción incluyen temperatura ambiente, atmósfera abierta y tolerancia al agua.

Principales resultados:

El método de homoacoplamiento oxidativo desarrollado sintetizó eficientemente macrociclos tensados, incluidos los macrociclos de polifenileno y las cicloalquinas tensadas.
Se observaron mayores rendimientos y eficiencias en comparación con los homoacoplamientos Suzuki-Miyaura y Yamamoto.
La reacción se produjo en condiciones suaves y de funcionamiento sencillo.
La investigación sugirió que las nanopartículas de paladio de 1-5 nm pueden ser las especies catalíticas activas.

Conclusiones:

El homoacoplamiento del éster borónico oxidativo catalizado por paladio es un método versátil y suave para construir andamios macrocíclicos desafiantes.
Este enfoque ofrece una alternativa práctica a las estrategias de macrociclado existentes.
La simplicidad operativa y la eficiencia de la reacción la hacen valiosa para sintetizar moléculas complejas, incluidos los productos intermedios naturales.