Cerium reagents oxidants

Sarel and co-workers have examined some reactions of alkynylcyclopropanes with iron carbonyl compounds [1]. Treatment of cyclopropylacetylene (5) with iron pentacarbonyl under photolytic conditions gives, after cerium(IV) oxidation, isomeric quinones 6 and 7, derived from two molecules of 5 and two carbonyls with both cyclopropane rings intact [6]. Furthermore, the photoreaction of dicyclopropylacetylene (8) with iron carbonyl gives some ten different products depending on the reagents and the reaction conditions, and some of them have the cyclopentenone skeleton formed by the opening of cyclopropane ring coupled with carbonyl insertion [7] (Scheme 2). 

Bacteria, antimicrobials against, 12, 456 Baeyer-Villiger oxidation, via tin amides, 9, 370 Barbier-Grignard-type reactions, and sonochemical metal insertions, 1, 315 Barbier-type reactions allenyl and propargyl tins, 9, 358 with allylic tins, 9, 357 with antimony(III) compounds, 9, 426 with bismuth(III) compounds, 9, 433 with cerium reagents, 10, 409 with indium compounds, 9, 685... 

The union of fragments 78 and 81 was performed by treatment of 81 with t-BuLi to induce a bromine-lithium exchange, followed by transmetal lation to the cerium reagent [49] and finally the addition of aldehyde 78 (Scheme 17). The coupled product was obtained in 85% yield as a diastereomeric mixture of alcohols. Swem oxidation [50] and reaction with MeMgBr at -78 °C provided the tertiary alcohol stereoselectively in 98% yield. A final deprotection with tosic acid gave venustatriol (2). 

Cerium is a member of the lanthanides in the Periodic Table and adopts tetra- and tripositive states in its electronic configuration. Among cerium reagents, ceric ammonium nitrate (CAN) is most widely used in organic synthesis. It is well known to convert phenol derivatives to quinones in high yields under mild conditions. An excellent review on cerium(IV) oxidation of organic compounds is available, and only a few examples will be described herein. 

Cerium(IV) has been used extensively, and the two most common reagents are ceric ammonium sulfate [Ce(S04)2 2(NH4)2S04 2 H2O] and ceric ammonium nitrate [Ce(NH4)2(N03)6].l 3 Modified cerium reagents such as Ce(OH)302Hl 4 d [(N03)3Ce]3H2l06l - have been used by Firouzabadi et al. or the oxidation of primary alcohols, especially benzylic and allylic alcohols. 

Anthranilic acid reagent (NH2-C(,H COOH) cerium(IV) oxidizes anthranilic acid to a brown compound. Cerium(III) does not react and must be oxidized first with lead dioxide and concentrated nitric acid to cerium(IV) other oxidizing agents cannot be used, since they react with the anthranilic acid. Iron(III) ions inhibit the test and must be masked by the addition of phosphoric acid. The ions of gold and vanadium, as well as chromate ion, react similarly and therefore interfere. Reducing agents must be absent. 

Although the rare earths are conveniently stored as oxalates or double sulfates, they are frequently converted to oxides for storage and, particularly, for use in reactions. However, if the sample of rare earth material contains an unusually large amoimt of cerium, it is not advisable to convert it to oxides because of the difficulty of dissolving cerium(IV) oxide in common reagents. 

Other instances included the use of ceric acetate in the synthesis of lactones, although preparation of the reagent was not trivial, and preferable alternatives were subsequently devised (Heiba and Dessau, 1971). Also, a combination of cerium(IV) oxide and hydrogen peroxide was found to oxidize phenols, possibly through the formation of singlet oxygen in situ (Barton et al., 1975). 

Cerium(IV) ammonium nitrate in methanol has been used to oxidize phenazine to the mono-N-oxide (41) in good yield (75JCS(P1)1398), but no other reports on the application of this reagent to the pyrazine or quinoxaline series have appeared. 

To determine of Ce(IV) in acid soluble single crystals, a simple and sensitive method is proposed. The method is based on the reaction of tropeoline 00 oxidation by cerium(IV) in sulfuric acid solution with subsequent measurement of the light absorption decrease of the solution. The influence of the reagent concentration on the analysis precision is studied. The procedure for Ce(IV) determination in ammonium dihydrophosphate doped by cerium is elaborated. The minimal determined concentration of cerium equal to 0.04 p.g/ml is lower than that of analogous methods by a factor of several dozens. The relative standard deviation does not exceed 0.1. 

Aravamudan and Venkappayya75 oxidized dimethyl sulphoxide in acetate buffer of pH 4 to 4.5 and with a reaction time of only 1 min. They then added potassium iodide and acid and titrated with thiosulphate the iodine liberated by unused reagent. They reported that cerium(IV) and Cr(VI) were much less effective oxidizing reagents for the sulphoxide. A very similar procedure was used by Rangaswama and Mahadevappa76 to determine dimethyl sulphoxide and numerous other compounds with chloramine B. 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

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