Name: pinacol coupling and solvent effects
Uploaded: 2023-04-30T11:22:57.000Z
Duration: 1 min 27 s
Channel: JoVE

Die Pinacol-Kupplung ist eine radikalische Dimerisierungsreaktion und beinhaltet die reduktive Kopplung von Aldehyden oder Ketonen in einem Kohlenwasserstofflösungsmittel, um vicinale Diole zu erhalten.
Ein einzelner Elektronentransfer von einem Metall auf ein spinpaariges Molekül wie Keton löst die Reaktion aus und bildet ein Ketyl – ein radikales Anion. Das Ketyl kann mit dem Radikal auf Kohlenstoff oder Sauerstoff und der Ladung auf der anderen Seite dargestellt werden.
Dabei fungieren Metalle wie Natrium und Magnesium als Elektronenquelle.
Das Schicksal des Ketyls hängt von dem Lösungsmittel ab, das für die Reaktion verwendet wird.
In protischen Lösungsmitteln wie Ethanol wird das Ketyl protoniert, gefolgt von einem zweiten Elektronentransfer, um ein Alkoxid-Anion zu erhalten, das bei Ansäuerung Alkohol bildet.
In aprotischen Lösungsmitteln wie Benzol ermöglicht das Fehlen von Protonen jedoch die Akkumulation von Ketyl, was zu seiner Dimerisierung zu einem Diol führt. Dies bildet die Grundlage der Pinacol-Kopplung.
Bemerkenswert ist, dass die elektrostatische Abstoßung zwischen den negativen Ladungen die Dimerisierung nicht behindert, da Metalle wie Magnesium sich mit Ketylen koordinieren, indem sie kovalente Metall-Sauerstoff-Bindungen bilden, die es ihnen ermöglichen, schnell zu reagieren.

vicinale diole durch reduktive kopplung von aldehyden oder ketonen: pinacol-kopplung Übersicht

Vicinal Diols via Reductive Coupling of Aldehydes or Ketones: Pinacol Coupling Overview

Wilhelm Rudolph Fittig entdeckte die Pinakol-Kupplungsreaktion im Jahr 1859. Es handelt sich dabei um eine radikalische Dimerisierungsreaktion, bei der Aldehyde oder Ketone in Gegenwart eines Kohlenwasserstofflösungsmittels reduktiv gekoppelt werden, um vicinale Diole herzustellen.

<img alt="Figure1" src="/files/ftp_upload/12905/12905_PT_Figure_1.svg" style="text-align: center; height: 160px;">

Die radikalische Reaktion wird durch die Übertragung eines einzelnen Elektrons von Metallen wie Natrium und Magnesium auf ein Molekül mit Spin-Paarung wie Aldehyde oder Ketone eingeleitet, wobei ein Ketyl - ein radikalisches Anion - entsteht. Das Ketyl hat einen radikalischen Charakter am Kohlenstoffatom und eine Ladung am Sauerstoffatom. Seine Resonanzform hat eine radikalische Position am Sauerstoffatom und eine Ladung am Kohlenstoffatom.

Das Verhalten des Ketyls wird durch das Lösungsmittel, in dem die Reaktion durchgeführt wird, stark beeinflusst.

Protische Lösungsmittel, wie Ethanol, protonieren das Ketyl. Es folgt eine zweite Elektronenübertragung vom Metall, wodurch ein Alkoxidanion entsteht. Bei der Ansäuerung eines Alkoxidanions bildet sich Alkohol. Dabei löst sich das Metall mit der Reaktion auf und dient als Quelle für freie Elektronen.

In Anwesenheit von aprotischen Lösungsmitteln wie Benzol oder Ether werden Ketylradikalanionen nicht protoniert, da keine Protonen vorhanden sind. Dies fördert die Ketyl-Dimerisierung und ist Teil der berühmten Pinacol-Kopplungsreaktion.

Interessanterweise begünstigen Metalle wie Magnesium, Aluminium oder Natrium die Reaktion, indem sie kovalente Metall-Sauerstoff-Bindungen bilden, die sich mit mehreren Ketylradikalen koordinieren und schnell zu einem Diol reagieren. Die elektrostatischen Abstoßungen zwischen den negativen Ladungen der Ketylgruppen behindern die Dimerisierung nicht. Zusammenfassend lässt sich sagen, dass protische Lösungsmittel die Bildung von Alkoholen begünstigen, während aprotische Lösungsmittel vicinale Diole begünstigen.

Vicinale Diole durch reduktive Kopplung von Aldehyden oder Ketonen: Pinacol-Kopplung Übersicht

Wilhelm Rudolph Fittig entdeckte 1859 die Pinacol-Kupplungsreaktion. Es handelt sich um eine radikalische Dimerisierungsreaktion, bei der es sich um die reduktive Kopplung von ...

vicinal-diols-via-reductive-coupling-aldehydes-or-ketones-pinacol

Education

Lehre

JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Radical Chemistry

Radikalchemie

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

pinacol coupling and solvent effects

Pinacol coupling is a radical dimerization reaction that joins aldehydes or ketones to form vicinal diols. Wilhelm Rudolph Fittig first discovered this reaction in 1859. It is a reductive coupling, which means the molecules are joined after they gain electrons.

The reaction begins with a single electron transfer from metals such as sodium or magnesium to an aldehyde or ketone. This forms a ketyl, a radical anion. In a ketyl, the carbon atom can carry radical character while the oxygen atom carries a negative charge. A resonance form also places the radical on oxygen and the charge on carbon.

The solvent has a major effect on the outcome. In protic solvents, such as ethanol, the ketyl is protonated first. A second electron transfer from the metal then gives an alkoxide anion. After acidification, this alkoxide forms an alcohol. In this way, the metal acts as a source of free electrons as it dissolves during the reaction.

In aprotic solvents, such as benzene or ether, protonation does not occur because there are no protons available. This favors ketyl dimerization and leads to pinacol coupling. Metals such as magnesium, aluminum, or sodium can help by forming covalent metal-oxygen bonds that coordinate several ketyl radicals. Even though the ketyls carry negative charge, electrostatic repulsion does not stop them from dimerizing. As a result, aprotic solvents favor vicinal diols, while protic solvents favor alcohol formation.

Pinacol Coupling and Solvent Effects

radicals: electronic structure and geometry

radicals-electronic-structure-and-geometry

Radical Geometry: Planarity and Inversion

Radical geometry depends on the electronic structure around the radical center. An unpaired electron, like a lone pair, affects the shape of the molecule. For a trivalent radical, the structure falls between the geometries of carbocations and carbanions. A carbocation with sp2 hybridization is trigonal planar. A carbanion with sp3 hybridization is trigonal pyramidal. The difference comes from the number of nonbonding electrons. The carbocation has none, while the carbanion has two. A carbon...

electron paramagnetic resonance (epr) spectroscopy: organic radicals

electron-paramagnetic-resonance-epr-spectroscopy-organic-radicals

EPR Hyperfine Splitting in Organic Radicals

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radical formation: overview

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Radicals are generated when bonds break in specific ways or when one radical leads to another. In homolytic bond cleavage, a bond splits evenly and forms radicals. In heterolytic bond cleavage, the bond breaks to form ions instead. A fishhook arrow shows the movement of a single electron during homolytic cleavage. Radicals can come from spin-paired molecules by homolysis or by electron transfer. Homolysis forms two radicals. Electron transfer adds an electron to the molecule, and this process...

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Homolysis and Radical Bonds

Homolysis is a bond-breaking process that splits a shared electron pair evenly. When a bond is broken with enough energy, the two electrons can also move to one atom in heterolysis, which forms ions. Homolysis gives one electron to each atom and creates radicals, which are highly reactive particles with an unpaired electron. HCl shows the difference between these bond cleavages. In solution, HCl can break into H+ and Cl− ions, and chlorine takes both bonding electrons, leaving a naked proton.

radical formation: abstraction

Radical Abstraction in Chain Reactions

Radical abstraction helps drive radical chain reactions. In this process, a relatively unstable radical removes an atom from a compound and forms a new radical that is more stable. The atom taken in this step can be hydrogen or a halogen. A radical initiator can first form radicals by homolysis, which is the breaking of a spin-paired molecule into two radicals. That is different from abstraction. In abstraction, a radical reacts with a spin-paired molecule and produces one new radical plus one...

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Radical Addition and Spin Conservation

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radical formation: elimination

Radicals from Elimination Reactions

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radical reactivity: overview

Radical Reaction Pathways and Stability

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Steric Shielding in Stable Radicals

Steric shielding helps make some radicals more stable and less reactive. A radical is a species with an unpaired electron at its center. In many cases, nearby groups can also give electronic stabilization, including electron-donating, electron-withdrawing, or conjugating groups. Several radicals are known as persistent radicals because they are remarkably stable. Examples include triphenylmethyl, tetramethylpiperidine-N-oxide, and 2,2-diphenyl-1-picrylhydrazyl. Some of these persistent...

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radical-reactivity-electrophilic-radicals

Electrophilic Radicals and Alkene Reactions

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radical-reactivity-intramolecular-vs-intermolecular

Radical Cyclization in One Molecule

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radical-oxidation-of-allylic-and-benzylic-alcohols

Selective MnO2 Oxidation of Alcohols

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radical substitution: allylic chlorination

radical-substitution-allylic-chlorination

Allylic Chlorination by Radical Chain Reaction

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radical substitution: allylic bromination

Allylic Bromination with NBS

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Alkyl Halide Reduction with Tributyltin Hydride

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radical anti-markovnikov addition to alkenes: overview

radical-anti-markovnikov-addition-to-alkenes-overview

Peroxide Effect in HBr Addition to Alkenes

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radical anti-markovnikov addition to alkenes: mechanism

radical-anti-markovnikov-addition-to-alkenes-mechanism

Hydrogen Bromide Addition to Alkenes by Radicals

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radical anti-markovnikov addition to alkenes: thermodynamics

radical-anti-markovnikov-addition-to-alkenes-thermodynamics

HBr Selectivity in Alkene Addition Thermodynamics

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vicinal diols via reductive coupling of aldehydes or ketones: pinacol coupling overview

Pinacol coupling is a radical dimerization reaction that joins aldehydes or ketones to form vicinal diols. Wilhelm Rudolph Fittig first discovered this reaction in 1859. It is a reductive coupling, which means the molecules are joined after they gain electrons. The reaction begins with a single electron transfer from metals such as sodium or magnesium to an aldehyde or ketone. This forms a ketyl, a radical anion. In a ketyl, the carbon atom can carry radical character while the oxygen atom...

alkenes via reductive coupling of aldehydes or ketones: mcmurry reaction

alkenes-via-reductive-coupling-aldehydes-or-ketones-mcmurry

Titanium-Coupled Alkenes from Carbonyls

The McMurry reaction turns aldehydes or ketones into alkenes with titanium metal. It first forms a diol-like intermediate through a coupling step, and then removes oxygen to make the double bond. In some cases, this process can be compared to pinacol coupling, which also joins carbonyl compounds. Titanium acts as a source of electrons in the first step. It transfers a single electron to the carbonyl group and forms a ketyl intermediate. That intermediate can lead to a vicinal diol, which is a...

&alpha;-hydroxy ketones via reductive coupling of esters: acyloin condensation overview

hydroxy-ketones-via-reductive-coupling-esters-acyloin-condensation

Acyloin Condensation: Ester Coupling Pathway

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key terms and concepts

Learn how pinacol coupling joins aldehydes and ketones into vicinal diols, and how protic versus aprotic solvents change the product.

Pinacol Coupling and Solvent Effects | JoVE K12

organic chemistry

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Pinacol Coupling and Solvent Effects

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