Name: selective mno2 oxidation of alcohols
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 21 s
Channel: JoVE

Allyl- und Benzylalkohole durchlaufen eine selektive Oxidation durch aktiviertes Mangan(IV)-dioxid über ein radikalisches Zwischenprodukt.
Primäre Allylalkohole ergeben Aldehyde, und sekundäre Allylalkohole bilden Ketone.
Aktiviertes MnO2 wird durch eine Oxidations-Reduktions-Reaktion hergestellt, wenn ein Mn+2-Salz, wie z. B. Mangansulfat, mit Kaliumpermanganat reagiert.
Die hohe Selektivität vonMnO 2 gegenüber den allylischen und benzylischen Alkoholen kann aus dem Reaktionsmechanismus verstanden werden.
Die Hydroxylgruppe des Alkohols fügt sich zunächst anMnO2 an, um einen Ester zu bilden.
Als nächstes nimmt das Mn(IV) ein Elektron auf, und ein Wasserstoffatom wird vom allylischen oder benzylischen Kohlenstoff auf den an Mangan gebundenen Sauerstoff übertragen.
Dadurch wird das Mn(IV) zu Mn(III) reduziert, wodurch ein allylischer oder benzylischer Kohlenstoffrest entsteht, der resonanzstabilisiert ist.
Schließlich reduziert eine weitere Elektronenumlagerung Mn(III) zu Mn(II) zusammen mit der Bildung eines Aldehyd- oder Ketonprodukts.
Bemerkenswert ist, dass während der radikalischen Oxidation der nicht-benzylische Alkohol unbeeinflusst bleibt.

<ol>
	<li>Loudon, M., & Parise, J. (2016). Organische Chemie. New York, NY: Macmillan Verlag, 851-852.</li>
	<li>Klein, D. (2017). Organische Chemie. New Jersey, NJ: Wiley, 462.</li>
</ol>

radikalische oxidation allylischer und benzylischer alkohole

Radical Oxidation of Allylic and Benzylic Alcohols

Aktiviertes Mangan(IV)-oxid kann selektiv Allyl- und Benzylalkohole über einen radikalischen Zwischenmechanismus oxidieren. Primäre Allylalkohole werden zu Aldehyden oxidiert, während sekundäre Allylalkohole Ketone ergeben. Die Redoxreaktion von Kaliumpermanganat mit einem Mn(II)-Salz wie Mangansulfat (entweder unter alkalischen oder sauren Bedingungen), gefolgt von einer gründlichen Trocknung, ergibt das Oxidationsmittel: aktiviertes MnO_2. Während MnO_2 in den für die Reaktion verwendeten Lösungsmitteln unlöslich ist, erfolgt die Oxidation von Allyl- und Benzylalkoholen an der MnO_2-Oberfläche. Da Wasser mit dem Alkohol um die aktiven Stellen auf dem MnO_2 konkurriert, muss es durch Trocknen entfernt werden.

Die radikalische Oxidation durch MnO_2 ist selektiv, und das Reagenz wirkt nicht auf Alkohole, die keine Benzyl- oder Allylalkohole sind. Zunächst verbindet sich die Hydroxylgruppe des Alkohols mit MnO_2 und bildet einen Ester. Anschließend nimmt das Mn(IV) ein Elektron auf und ein Wasserstoffatom wird vom Allyl- oder Benzylkohlenstoff auf den an das Mangan gebundenen Sauerstoff übertragen. Auf diese Weise wird Mn(IV) zu Mn(III) reduziert, wobei ein allylischer oder benzylischer Kohlenstoffrest entsteht, der resonanzstabilisiert ist. Die erhöhte Stabilität dieses benzylischen/allylischen Radikal-Zwischenprodukts im Vergleich zur Stabilität des Zwischenprodukts, das durch nicht benzylische oder nichtallylische Alkohole gebildet wird, erhöht die Geschwindigkeit dieses Schritts und fördert die Selektivität. Eine weitere Elektronenumlagerung führt zur Reduktion von Mn(III) zu Mn(II) und liefert das Aldehyd oder das Keton.

Radikalische Oxidation allylischer und benzylischer Alkohole

Aktiviertes Mangan(IV)-oxid kann Allyl- und Benzylalkohole über einen radikalischen Zwischenmechanismus selektiv oxidieren. Primäre allylische Alkohole sind ...

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

Education

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JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Radical Chemistry

Radikalchemie

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

selective mno2 oxidation of alcohols

Activated manganese(IV) oxide, or MnO2, is a selective oxidizing agent for allylic and benzylic alcohols. It changes primary allylic alcohols into aldehydes and secondary allylic alcohols into ketones. The reaction happens by a radical pathway, so the carbon next to the alcohol group plays a key role.

The oxidizing agent is made from the redox reaction of potassium permanganate with an Mn(II) salt such as manganese sulfate. This can be done under alkaline or acidic conditions. After that, the material must be dried well to give activated MnO2. Water competes with alcohol for the active sites on MnO2, so removing water is important.

MnO2 is insoluble in the solvents used for this reaction. Even so, the oxidation takes place on the MnO2 surface. The reagent is selective and does not affect non-benzylic or non-allylic alcohols. First, the alcohol hydroxyl group adds to MnO2 to form an ester. Then Mn(IV) accepts an electron, and a hydrogen atom moves from the allylic or benzylic carbon to the oxygen attached to manganese.

This step reduces Mn(IV) to Mn(III) and forms a carbon radical. A radical is a species with an unpaired electron. The allylic or benzylic radical is resonance-stabilized, which means its electrons can spread out over more than one structure. That extra stability makes the reaction faster than for non-benzylic or non-allylic alcohols and helps explain the selectivity. Further electron rearrangement reduces Mn(III) to Mn(II) and gives the aldehyde or ketone.

Selective MnO2 Oxidation of Alcohols

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Radical Halogenation: Chiral Products

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radical-substitution-allylic-chlorination

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radical substitution: allylic bromination

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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

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

Pinacol Coupling and Solvent Effects

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Titanium-Coupled Alkenes from Carbonyls

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

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Acyloin Condensation: Ester Coupling Pathway

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key terms and concepts

See how activated MnO2 selectively oxidizes allylic and benzylic alcohols to aldehydes or ketones through a radical surface mechanism.

Selective MnO2 Oxidation of Alcohols | JoVE K12

organic chemistry

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Selective MnO2 Oxidation of Alcohols

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