Mechanism oxidation with chromium

Adam and Lohray122 have used thianthrene 5-oxide (88) as a mechanistic probe in oxidations with transition metal peroxides. They oxidized 88 with various diperoxo complexes of chromium, molybdenum and tungsten and formulated a plausible mechanism on the basis of the products formed, 89 and 90. 

Many types of oxide layers have a certain, not very high electrical conductivity of up to 10 to 10 S/cm. Conduction may be cationic (by ions) or anionic (by or OH ions), or of the mixed ionic and electronic type. Often, charge transport occurs by a semiconductor hole-type mechanism, hence, oxides with ionic and ionic-hole conduction are distinguished (in the same sense as p-type and n-type conduction in the case of semiconductors, but here with anions or cations instead of the electrons, and the corresponding ionic vacancies instead of the electron holes). Electronic conduction is found for the oxide layers on iron group metals and on chromium. 

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

Witt, D. R., Reactivity and Mechanism with Chromium Oxide Polymerization Catalysts, Chap. 13 in Reactivity, Mechanism, and Structure in Polymer Chemistry, A. D. Jenkins and A. Ledwith, eds., Wiley, New York, 1974. 

The Jones reagent851 and < rt-BuOOH in the presence of chromium(VI) complexes852,853 were found to be particularly useful in the oxidation of tetralins and indans. Oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) occurs with an exceptional mechanism.854 In contrast with the radical processes observed in other oxidations DDQ generates a carbocation by hydride abstraction that is trapped by water to form an alcohol ... 

The data on the rate of reaction of o-, m-, and p-xylene over vanadium oxide catalyst and of m-xylene over mixed vanadium oxide catalysts (chromium-vanadium and antimony-vanadium) were correlated with the reaction scheme below by the following rate expressions, which are based on the Langmuir-Hinshelwood mechanisms where the adsorption of m-xylene is strong. 

The active site concentration on the organochromium catalysts may be higher than that of the oxide catalysts. The activity usually assumes a more linear increase with chromium loading than on the oxide catalysts, at least up to 2% Cr. Yermakov and Zakharov, studying allyl-Cr(III)/silica catalysts, stopped the polymerization with radioactive methanol, and found that the kill mechanism is different from that on the oxide catalysts (59). The proton of the methanol, and not the alkoxide, became attached to the polymer. This suggests a polarity opposite to that of the oxide catalysts, with the site being more positive than the chain. 

Other chromium-based reagents are also found to oxidize alcohols, following a mechanism like the one depicted above for oxidation with chromic acid.4... 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Electrocatalytic ETC substitution initiated by oxidation has been reported with chromium triad carbonyl derivatives.113 Several of the reactions studied, and ones that give little or no product when uncatalyzed, are shown in Eqs. (24)—(26). The mechanism described earlier for the reactions of [(MeCp)Mn(CO)2(MeCN)] applies to these reactions. The... 

Complex 6.37 has been shown to be an active catalyst for the manufacture of HDPE. Note that 6.37 is a 15-electron complex with chromium in the formal oxidation state of 3 +. The mechanism of polymerization involves generation of coordinative unsaturation through the dissociation of a THF molecule from 6.37. The evidence for an oxidation state of 3+ in the commercial catalyst comes from the fact that complex 6.38 is active for polymerization. However, complex 6.39, identical to 6.38 in every respect except the oxidation state of the metal ion, is inactive. Note that the oxidation state of chromium in 6.39 is 2+. 

Mechanism The mechanism of the Cr( VI) oxidation of aldehydes has been studied in detail in Scheme 7.14. A hydrate of aldehyde A is formed first, which reacts with chromium species to form a chromate ester B. A base abstracts a proton from the chromate ester B and Cr species leaves (E2 elimination) to give carboxylic acid. 

Chromyl chloride, Cr202Cl2, a dark-red liquid (mp -96.5 °C, bp 117 °C, d 1.911), is prepared from chromium trioxide or sodium dichromate, hydrochloric acid, and sulfuric acid [665]. The reagent is used in solutions in carbon disulfide, dichloromethane, acetone, tert-butyl alcohol, and pyridine. Oxidations with chromyl chloride are often complicated by side reactions and do not always give satisfactory yields. The mechanism of the oxidation with chromyl chloride, the Etard reaction, is probably of free-radical nature [666]. Complexes of chromyl chloride with the compounds to be oxidized have been isolated [666, 667, 668]. 

HCr03 ion to form the ketone. It is possible that the proton is lost to an oxygen of the ester group in a cyclic mechanism (2a). Additional alcohol is then oxidized, evidently by reactions (3)-(5), with chromium finally reaching the Cr(III) state. 

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