Chromium acetate, reduction

Divalent chromium salts show very strong reducing properties. They are prepared by reduction of chromium(III) compounds with zinc [187] or a zinc-copper couple and form dark blue solutions extremely sensitive to air. Most frequently used salts are chromous chloride [7SS], chromous sulfate [189], and less often chromous acetate. Reductions of organic compounds are carried out in homogeneous solutions in aqueous methanol [190], acetone [191], acetic acid [192], dimethylformamide [193] or tetrahydrofuran [194] (Procedure 37, p. 214). 

Chromium contributes to organic transformations as reagents. Some examples include chromic acid Qones reagent), chromium acetate for the reduction of carbon-halogen bonds and conversion of ketoximes to ketones, chromyl acetate, chromium perchlorate, chromium sulfate (reduction of allq nes to alkenes), and Jacob s reagent (Cr02Cl2). The catalytic... 

The procedure and apparatus described by Balthis and Bailar have been modified to eliminate certain handling difficulties and to give increased yields of chromium(II) acetate. Reduction of the chromium(III) ion is effected by using a Jones reductor. This makes it unnecessary to filter the chromium (II) chloride solution. The reductor may be used for a number of preparations without replenishing the zinc. The chromium(II) chloride solution is run from the reductor directly into the sodium acetate solution to ensure complete use of all the chromium(II) salt. Transfer of the reaction slurry from the bottom of the reaction flask to a filter makes it possible to dry completely and process the precipitate under nonoxidizing conditions. 

The anhydrous hahdes, chromium (II) fluoride [10049-10-2], chromium (II) bromide [10049-25-9], CrBr2, chromium (II) chloride [10049-05-5], CrCl2, and chromium (II) iodide [13478-28-9], 03x1, are prepared by reaction of the hydrohaUde and pure Cr metal at high temperatures, or anhydrous chromium (II) acetate [15020-15-2], Cr2(CH2COO)4, atlower temperatures, or by hydrogen reduction of the Cr(III) hahde at about 500—800°C (2,12). 

A similar procedure may be used for the preparation of /)-cyanobenzaldiacetate from -tolunitrile. Information submitted by Rorig and Nicholson, of G. D. Searle and Company, indicates that the critical step in this preparation is to maintain the reaction temperature below 10° throughout the process. Exposure of -cyanobenzaldiacetate to excess chromic, acetic, and sulfuric acids causes a reduction in yield. During the oxidation care should be taken to prevent chromium trioxide from adhering to the walls of the flask above the reaction mixture and then dropping in large amounts into the solution. 

Steroidal 17-cyanohydrins are relatively stable towards chromium trioxide in acetic acid (thus permitting oxidation of a 3-hydroxyl group ) and towards ethyl orthoformate in ethanolic hydrogen chloride (thus permitting enol ether formation of a 3-keto-A system ). Sodium and K-propanol reduction produces the 17j -hydroxy steroid, presumably by formation of the 17-ketone prior to reduction. ... 

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. 

Two polarographic methods have been developed for the determination of cohalt(II) at concentrations ranging from approximately 1 to 80 mM in an aqueous sample. For the first method [15], which is suitable for samples containing large amounts of nickel]11), the cobalt(II) is oxidized to Co(NH3)6 in an ammoniacal medium with the aid of sodium perborate, after which the cobalt(III) species is determined. A second procedure [16] entails the use of lead dioxide in an acetic acid-acetate buffer containing oxalate to convert cobalt(II) to the 0(0204)3 ion, which can be subjected to polarographic reduction. This latter approach is well suited to the determination of cobalt in the presence of copper(II), iron(III), nickel(II), tin(IV), and zinc(II), whereas the chief interferences are cerium, chromium, manganese, and vanadium. 

The methyl substituent of 2-methyl-4,8-dihydrobenzo[l,2- 5,4-. ]dithiophene-4,8-dione 118 undergoes a number of synthetic transformations (Scheme 8), and is therefore a key intermediate for the preparation of a range of anthraquinone derivatives <1999BMC1025>. Thus, oxidation of 118 with chromium trioxide in acetic anhydride at low temperatures affords the diacetate intermediate 119 which is hydrolyzed with dilute sulfuric acid to yield the aldehyde 120. Direct oxidation of 118 to the carboxylic acid 121 proceeded in very low yield however, it can be produced efficiently by oxidation of aldehyde 120 using silver nitrate in dioxane. Reduction of aldehyde 120 with sodium borohydride in methanol gives a 90% yield of 2-hydroxymethyl derivative 122 which reacts with acetyl chloride or thionyl chloride to produce the 2-acetoxymethyl- and 2-chloromethyl-4,8-dihydrobenzo[l,2-A5,4-3 ]-dithiophene-4,8-diones 123 and 124, respectively. 

Dehalogenation. Barton et at. (1, 148) effected dehalogenation of steroidal /i-hydroxy halides with chromium(II) acetate and butancthiol as the proton donor in DMSO. The method is only useful with tertiary halides. A recent improvement that permits reduction of halides of all types uses the ethylenediamine complex of CrtCIOzh and the tetrahydropyranyl ethers of the /J-hydroxy halide. Catalytic amounts of the reducing agent can be used in "indirect electrolysis." The reaction is convenient for preparation of deoxynucleosides.1... 

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