Manganese(III) Acetate

Dedicated to my honourable mentor Dr. Utpal Bora

Introduction

Manganese(III) acetate [Mn(OAc)₃]-mediated free-radical reactions have emerged as important synthetic methods. Manganese acetate can be used as an oxidizing and chlorinating agent[ ¹ ] and in some cases in alkylations[ ¹ ] and inter- and intramolecular cyclizations.[ ² ] Due to the poor solubility of Mn(OAc)₃ in common organic solvents, Mn(OAc)₃-mediated reactions are often carried out in acetic acid, although other solvents including toluene, alcohols, acetonitrile, dichloromethane and chlorobenzene can be used.[ ³ ] Mn(OAc)₃ promotes the synthesis of many alkaloids,[ ⁴ ] triterpenes,[ ⁵ ] and drugs.[ ⁶ ] Mn(OAc)₃ is commercially available in form of its dehydrate as a brown powder and can be prepared in the laboratory[ ⁷ ] by adding KMnO₄ to a pre-heated mixture of Mn(OAc)₂ and glacial acetic acid (Scheme [1]). Addition of acetic anhydride to the reaction produces the anhydrous form.[ ⁸ ]

Scheme 1 Preparation of manganese (III) acetate

Abstracts

(A) Catalytic amount of manganese acetate in glacial acetic acid oxidizes 3-alkyl-substituted 2,4-pyrrolidinediones to their corresponding 3-hydroperoxy derivatives. This procedure leads to a new type of heterocyclic hydroperoxides.[ 9 ]
(B) Manganese acetate-mediated oxidative cyclisation of alkyl substituted 2-[2-(N-arylamino)-2-oxoethyl]malonates yields the corresponding substituted 4,4-bis(ethoxycarbonyl)-3,4-dihydro-2(1H)-quinolinones in good to excellent yield (46–97%).[ 4c ]
(C) Manganese acetate generates aryl radicals or cations when reacted with aryl boronic acids[10a] [b] or aryl hydrazines[ 10c ] preferentially in aromatic solvents (benzene, thiophene) and subsequently leads to the formation of biaryls under microwave,[ 10a ] room temperature,[ 10b ] and reflux[ 10c ] conditions.
(D) When a catalytic amount of manganese acetate is added to a stirred solution of alkyl- or aryl-substituted alcohol and acetic acid, acetylation occurs quantitatively (99% yield) within two hours at reflux.[ 11 ]
(E) Manganese acetate can be used as an effective and mild oxidizing agent for the regeneration of carbonyl compounds from their corresponding oximes in good yield (86–96%). This reaction condition can tolerate many functional groups, e.g. alkyl and aryl oximes.[ 12 ]
(F) Oxidative ring expansion of substituted 9H-xanthene 1 in the presence of manganese acetate gives 9- or 10-dibenz[b,f]oxepincarboxylates 2 in good yield. When R1 = Me and R2 = 1-OMe, this reaction gives two regioisomers. Otherwise the reaction is highly regioselective. It was proposed that the process for the formation of the product must include a 1,2-aryl radical rearrangement followed by an oxidative decarboxylation.[ 4b ]
(G) Under manganese acetate-catalyzed aerobic oxidation conditions in glacial acetic acid, tetronic acid 1 reacts with 1,1-disubstituted alkenes 2 (R1, R2 = Alk, Ar) to yield hydroperoxyethyl peroxylactones 3, while a similar reaction using 3-alkyl-substituted tetronic acid gives stable, crystalline peroxylactone 4 in good to excellent yield.[ 13 ]
(H) Manganese acetate can be employed as an oxidant for the regeneration of 2,3-dichloro-5,6-dicyanoquinone (DDQ) from the corresponding hydroquinone (HDDQ). This DDQ-regeneration technique using manganese acetate (3 equiv) and DDQ (10 mol%), can be applied to the deprotection of p-methoxy benzyl (PMB) ethers.[ 14 ]
(I) Manganese acetate-promoted oxidative free-radical condensation reaction of phosphate ester(dialkylphosphate) with alkynes[ 15 ] yields the corresponding indenones 1 and direct phosphonylation of arenes[ 6 ] yields regioselectively dialkylphosphonates 2.

• References

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