Kinetics and Mechanism of Reactions between Methyl Aromatic Compounds and the Dibromide Radical

Metelski, Peter D.; Espenson, James H.

doi:10.1021/jp010311b

Cited by 21 publications

(17 citation statements)

References 15 publications

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“…Thus, the rate constant for reaction of HBr 2 · with toluene (in HOAc) is > 10 4 times that for the benzylperoxy radical. [14,35] Variable valence metal ions, such as cobalt and manganese, serve as catalysts of autoxidations by undergoing one electron redox reactions with alkyl hydroperoxides to produce alkoxy and alkylperoxy radicals. [30] The former abstract hydrogen atoms from C À H bonds with a rate constants 10 5 -10 7 larger than for alkylperoxy radicals, [13] a reflection of the much larger BDE of RO À H (437 kJ/mol) compared to ROO À H (370 kJ/mol).…”

Section: Reviewsmentioning

confidence: 99%

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

2004

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The use of nitroxyl radicals, alone or in combination with transition metals, as catalysts in oxidation processes is reviewed from both a synthetic and a mechanistic viewpoint. Two extremes of reactivity can be distinguished: stable (persistent) dialkylnitroxyls, such as the archetypal TEMPO, and reactive diacylnitroxyls, derived from N-hydroxy imides, such as N-hydroxyphthalimide (NHPI). The different types of reactivity observed are rationalized by considering the bond dissociation energies (BDEs) of the respective N-hydroxy precursors, substrates and reaction intermediates. Reactive diacylnitroxyl radicals are generated in situ from the corresponding N-hydroxy compound. The protagonist, NHPI, catalyzes a wide variety of free radical autoxidations, improving both activities and selectivities by increasing the rate of chain propagation and/or decreasing the rate of chain termination. In the absence of metal co-catalysts improved conversions and selectivities are obtained in the autoxidation of hydrocarbons to the corresponding alkyl hydroperoxides. For example, cyclohexylbenzene afforded the 1-hydroperoxide in 97.6% selectivity at 32% conversion when the autoxidation was performed in the presence of 0.5 mol % NHPI, and the product hydroperoxide as initiator, at 100 8C. This forms the basis for a potential coproduct-free route from benzene to phenol. In combination with transition metal co-catalysts, notably cobalt, NHPI and related compounds, such as N-hydroxysaccharin NHS, afford effective catalytic systems for the effective autoxidation of hydrocarbons, e.g., toluenes to carboxylic acids, under mild conditions. In the case of the less reactive cycloalkanes, NHS proved to be a more active catalyst than NHPI which is attributed to the higher reactivity of the intermediate nitroxyl radical, resulting from the replacement of a carbonyl group in NHPI by the more strongly electron-attracting sulfonyl group. Stable dialkylnitroxyl radicals, exemplified by TEMPO, catalyze oxidations of, e.g., alcohols, with single oxygen donors such as hypochlorite and organic peracids. The reactions involve the intermediate formation of the corresponding oxoammonium cation as the active oxidant. Alternatively, in conjunction with transition metals, notably ruthenium and copper, they catalyze aerobic oxidations of alcohols. These reactions involve metal-centered dehydrogenations and the role of the TEMPO is to facilitate regeneration of the catalyst (Ru and Cu) and oxidation of the alcohol (Cu) via hydrogen abstraction or one-electron oxidation processes. Detailed mechanistic investigations, including kinetic isotope effects, revealed that oxoammonium cations are not involved as intermediates in these reactions. In contrast, oxoammonium cations are involved in the aerobic oxidation of alcohols catalyzed by the copper-dependent oxidase, laccase, in combination with TEMPO. This different mechanistic pathway is attributed to the much higher redox potential of the copper(II) in the enzyme. Similarly, N-hydroxy compounds such as ...

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Section: Reviewsmentioning

confidence: 99%

Organocatalytic Oxidations Mediated by Nitroxyl Radicals

2004

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“…[17] As a result the steady state concentrations of cobalt and manganese remain largely in their lower oxidation states. [14] With a Co/Mn catalyst, reactions 5 and 6 will occur but not 7, hence high steady states of Mn(III) are expected to occur.…”

Section: Full Papersmentioning

confidence: 99%

The Complex Synergy of Water in the Metal/Bromide Autoxidation of Hydrocarbons Caused by Benzylic Bromide Formation

Partenheimer

2004

Adv Synth Catal

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One of the most active and selective catalysts in homogeneous liquid phase oxidation using molecular oxygen (O 2 ) is a mixture of cobalt, manganese and bromide salts in acetic acid. It has been used to produce hundreds of different carboxylic acids in high yield and purity including the commercial production of terephthalic acid from pxylene. Water is normally a by-product in these reactions and it is shown here that its concentration is an important reaction variable. In anhydrous acetic acid, with reagents with sufficiently strong electronwithdrawing substitutents (toluene, 4-carboxytoluene, 4-chlorotoluene), all of the active bromide becomes inactive via benzylic bromide formation. The Co/Mn/ Br catalyst is therefore converted to a Co/Mn catalyst which is dubbed −catalyst failure× because of its undesirable characteristics of lower activity, decreased selectivity especially towards over-oxidation and color formation. For 4-chlorotoluene, increasing the water concentration to 5 weight % initially decreases the rate of reaction but eventually is more active and selective because the oxidation and hydrolysis of the benzylic bromide allows for sufficient active catalytic bromide. It is shown that benzylic bromides do not −promote× the reaction and that both oxidation and solvolysis of the benzylic bromide occurs during autoxidation. During polymethylbenzene oxidation, benzylic bromide formation occurs only with the most reactive methyl group. The complex factors during metal/bromide autoxidation ± some favored by increased water concentration and others detrimental ± are outlined.

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“…In the subsequent step, CoBr 2 (OAc) and Mn(OAc) 3 oxidize the bromide ion to dibromide radical. The formation of dibromide radical was studied directly by laser flash photolysis, and thus confirmed independently [27]. Hence, a single peroxyl radical becomes three by a sequence of pathways, as shown in Scheme 1, in which the numbering of rate constants corresponds to the prior usage [19,23].…”

Section: Discussionmentioning

confidence: 86%