Dirhodium-Catalyzed Phenol and Aniline Oxidations with T-HYDRO. Substrate Scope and Mechanism of Oxidation

Dirhodium caprolactamate, Rh2(cap)4, is a very efficient catalyst for the generation of the tert-butylperoxy radical from tert-butyl hydroperoxide, and the tert-butylperoxy radical is a highly effective oxidant for phenols and anilines. These reactions are performed with 70% aqueous tert-butyl hydro...

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Bibliographic Details
Published in:Journal of organic chemistry 2011-04, Vol.76 (8), p.2585-2593
Main Authors: Ratnikov, Maxim O, Farkas, Linda E, McLaughlin, Emily C, Chiou, Grace, Choi, Hojae, El-Khalafy, Sahar H, Doyle, Michael P
Format: Article
Language:English
Subjects:
Amines - chemistry
Aniline Compounds - chemistry
Catalysis
Catalysts: preparations and properties
Chemistry
Cyclohexenes - chemical synthesis
Electrons
Estrone - chemistry
Exact sciences and technology
Free radicals chemistry
General and physical chemistry
Kinetics and mechanisms
Noncondensed benzenic compounds
Organic chemistry
Oxidants - chemistry
Oxidation-Reduction
Oxygen
Phenols - chemistry
Preparations and properties
Protons
Reactivity and mechanisms
Rhodium - chemistry
Solvents - chemistry
Stereoisomerism
Substrate Specificity
tert-Butylhydroperoxide - chemistry
Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
Water
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Summary:Dirhodium caprolactamate, Rh2(cap)4, is a very efficient catalyst for the generation of the tert-butylperoxy radical from tert-butyl hydroperoxide, and the tert-butylperoxy radical is a highly effective oxidant for phenols and anilines. These reactions are performed with 70% aqueous tert-butyl hydroperoxide using dirhodium caprolactamate in amounts as low as 0.01 mol % to oxidize para-substituted phenols to 4-(tert-butyldioxy)cyclohexadienones. Although these transformations have normally been performed in halocarbon solvents, there is a significant rate enhancement when Rh2(cap)4-catalyzed phenol oxidations are performed in toluene or chlorobenzene. Electron-rich and electron-poor phenolic substrates undergo selective oxidation in good to excellent yields, but steric influences from bulky para substituents force oxidation onto the ortho position resulting in ortho-quinones. Comparative results with RuCl2(PPh3)3 and CuI are provided, and mechanistic comparisons are made between these catalysts that are based on diastereoselectivity (reactions with estrone), regioselectivity (reactions with p-tert-butylphenol), and chemoselectivity in the formation of 4-(tert-butyldioxy)cyclohexadienones. The data obtained are consistent with hydrogen atom abstraction by the tert-butylperoxy radical followed by radical combination between the phenoxy radical and the tert-butylperoxy radical. Under similar reaction conditions, para-substituted anilines are oxidized to nitroarenes in good yield, presumably through the corresponding nitrosoarene, and primary amines are oxidized to carbonyl compounds by TBHP in the presence of catalytic amounts of Rh2(cap)4.
ISSN:0022-3263
1520-6904
DOI:10.1021/jo1024865