Effect of Alkyl Substituents and Ring Size on Alkoxy Radical Cleavage Reactions

Wilsey, Sarah; Dowd, Paul; Houk, K. N.

doi:10.1021/jo990652+

Cited by 109 publications

(82 citation statements)

References 23 publications

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“…We propose that, in cycloalkane oxidations, ring-opened products are formed via b-cleavage of intermediate cycloalkoxy radicals formed via (metal-catalysed) decomposition of the corresponding hydroperoxide (Scheme 4). The formation of carbonyl compounds via b-cleavage is a well-documented reaction of alkoxy radicals, [15] which in the case of cyclic alkoxy radicals leads to ring opening. In the presence of NHS (or NHPI) efficient scavenging of alkoxy radicals suppresses b-cleavage in favour of alcohol formation leading to a higher selectivity to alcohol ketone (Scheme 4).…”

Section: By-product Formation In Cycloalkane Oxidationmentioning

confidence: 99%

Aerobic Oxidation of Cycloalkanes, Alcohols and Ethylbenzene Catalyzed by the Novel Carbon Radical Chain Promoter NHS (N‐Hydroxysaccharin)

et al. 2004

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Replacement of Ishii×s N-hydroxyphthalimide (NHPI) with the novel carbon radical chain promoter N-hydroxysaccharin (NHS) affords, in combination with metal salts, notably Co, or other additives, selective catalytic autoxidation of hydrocarbons, alcohols and alkylbenzenes under mild conditions (25 ± 100 8C, O 2 1 atm). The effects of solvent, temperature and the nature of the additives were investigated to give an optimised oxidation protocol for the various systems. The NHS/Co combination was more reactive than NHPI/Co in the autoxidation of cycloalkanes. In contrast, the opposite order of reactivity was observed in the autoxidation of ethylbenzene and alcohols. It is suggested, on the basis of bond dissociation energy (BDE) considerations, that this is a result of a change in the rate-limiting step with the more reactive ethylbenzene and alcohol substrates. In the autoxidation of the model cycloalkane, cyclododecane, the best results (90% selectivity to a 4 : 1 mixture of alcohol and ketone at 24% conversion) were obtained with NHS/Co(acac) 3 in PhCF 3 at 80 8C. Competition experiments revealed that, in contrast to what is commonly believed, formation of the dicarboxylic acid by ring opening is not a result of further oxidation of the ketone product. It is suggested that ring opened products are a result of˚-scission of the cycloalkoxy radical formed via (metal-catalysed) decomposition of the hydroperoxide. This is suppressed in the presence of NHS (or NHPI) which efficiently scavenge the alkoxy radicals.

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Section: By-product Formation In Cycloalkane Oxidationmentioning

confidence: 99%

Aerobic Oxidation of Cycloalkanes, Alcohols and Ethylbenzene Catalyzed by the Novel Carbon Radical Chain Promoter NHS (N‐Hydroxysaccharin)

et al. 2004

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“…Alkoxy radicals normally cleave to give a ketone (or an aldehyde) and the most stable possible alkyl radical. [33][34][35] …”

Section: Methodsmentioning

confidence: 99%

A New Insight into the Oxidation of Cyclododecane with Hydrogen Peroxide in the Presence of Iron-Substituted Polyoxotungstates

Santos¹,

Simões²,

Balula³

et al. 2008

Synlett

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The catalytic homogeneous oxidation of cyclododecane with hydrogen peroxide in the presence of tetrabutylammonium salts of iron-substituted Keggin-type polyoxotungstates of general formula (TBA) 4 H z XW 11 Fe(H 2 O)O 39 ·nH 2 O (where X = P, Si, B, and z = 0-2) is described. In the reaction conditions reported, the corresponding alcohol, ketone, and hydroperoxide are obtained as the main reaction products. The catalytic activity of the anions with phosphorous, silicon, and boron is compared in different reaction conditions. These catalytic oxidation reactions seem to be radical processes, since they are totally inhibited in the presence of I 2 , a well-known radical scavenger.Selective oxidation of saturated hydrocarbons is one of the most challenging subjects in oxidation chemistry and many catalytic systems for these transformations have been extensively investigated over the last decades. 1-5 Among others, some systems have been described in which the well-known transition-metal-monosubstituted Keggin-type polyoxometalates, 6 like a-[XW 11 M(H 2 O)O 39 ] n-(X = P, Si, M = first-row transitionmetal ion), have been employed as catalysts or catalyst precursors for several types of alkane oxidative reactions, using different oxidants (e.g. PhIO, O 2 , TBHP) and diverse reaction conditions. 7-10 Alcohols and ketones have been the prominent reaction products obtained in these studies.The use of aqueous H 2 O 2 in the oxidation of organic substrates is very attractive from the point of view of synthetic organic chemistry, since aqueous H 2 O 2 is considered an inexpensive, environmentally clean and easy to handle reagent. 11,12 To our knowledge, the first report concerning the oxidation of alkanes with H 2 O 2 in the presence of a transition-metal-substituted polyoxotungstate was published by Mizuno et al., 13 who used {g-SiW 10 [Fe(OH 2 )] 2 O 38 } 6-in the catalytic oxidation of cyclohexane, n-hexane, n-pentane, and adamantane. Our studies on cyclohexane and cyclooctane oxidation with hydrogen peroxide in acetonitrile have shown that considerable amounts of the corresponding alkyl hydroperoxides could be formed in the presence of several iron(III)-substituted heteropolytungstates. 14-19 The formation of alkyl hydroperoxides may be explored in several ways, as they may be selectively decomposed to ketone(aldehyde)-alcohol mixtures 20,21 or reduced to the corresponding alcohols.The oxidation of cyclododecane is important from the industrial point of view as it provides precursors for the synthesis of nylon-12 polymers. 22 This reaction is usually carried out using O 2 and Co(II) acetate in the presence of boric acid until attaining 30-35% conversion, originating 80% of cyclododecanol, 10% of cyclododecanone, and 10% of byproducts. The catalytic dehydrogenation of the cyclododecanol-cyclododecanone mixture at 220 °C originates pure cyclododecanone. 23 Despite its significance, the catalytic oxidation of cyclododecane is much less studied than related processes concerning cyclohexane. Cyclododecane is much less reacti...

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“…[19] and [20]). [54][55][56] These radicals were further oxidized to produce AA and other decarboxylated byproducts.…”

Section: Page 12 Of 20mentioning

confidence: 99%

Synergistic effect of co-reactant promotes one-step oxidation of cyclohexane into adipic acid catalyzed by manganese porphyrins

She

et al. 2015

Can. J. Chem.

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The synergistic effect of cyclohexane and cyclohexanone promoted synthesis of adipic acid catalyzed by [Mn III T(p-Cl)PP]Cl with cyclohexane and cyclohexanone as co-reactants. The results showed that the conversions of cyclohexane and cyclohexanone were significantly enhanced because of the cyclohexanone synergistic effect, and the higher selectivity to adipic acid was obtained with dioxygen as an oxidant. The studies indicated that the co-oxidation of cyclohexane and cyclohexanone was influenced by the initial molar ratio of cyclohexanone and cyclohexane, catalyst structure, catalyst concentrations and reaction conditions. The preliminary mechanism of the co-oxidation reaction of cyclohexane and cyclohexanone using [Mn III T(p-Cl)PP]Cl as the catalyst was proposed.

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Effect of Alkyl Substituents and Ring Size on Alkoxy Radical Cleavage Reactions

Cited by 109 publications

References 23 publications

Aerobic Oxidation of Cycloalkanes, Alcohols and Ethylbenzene Catalyzed by the Novel Carbon Radical Chain Promoter NHS (N‐Hydroxysaccharin)

Aerobic Oxidation of Cycloalkanes, Alcohols and Ethylbenzene Catalyzed by the Novel Carbon Radical Chain Promoter NHS (N‐Hydroxysaccharin)

A New Insight into the Oxidation of Cyclododecane with Hydrogen Peroxide in the Presence of Iron-Substituted Polyoxotungstates

Synergistic effect of co-reactant promotes one-step oxidation of cyclohexane into adipic acid catalyzed by manganese porphyrins

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