Pyridine-Chromium VI Oxide

Four grams of chromium (VI) oxide (0.04 mol) is dried at 110° in vacuo for 4 hours and is then cooled in an ice-salt mixture. Fifty milliliters of pyridine (0.63 mol) is also cooled in a 300-ml. Erlenmeyer flask. While the 50 ml. of pyridine is shaken in the flask immersed in an ice-salt-water mixture, the chromium(VI) oxide is slowly added. The flask is stoppered, and the mixture is agitated until all solid particles have dissolved. A larger amount of pyridine may be used to hasten solution, if desired. Cooling is necessary, since the reaction generates enough heat to bring about oxidation of the pyridine unless the temperature is controlled. 

The reaction flask is connected to a vacuum pump through a cold finger cooled in an acetone- Dry Ice mixture or in liquid air. The evacuation is continued until all the liquid has evaporated. Since the compound is photosensitive, the reaction flask should be protected from light. 

After evaporation of the excess liquid, the reaction flask is disconnected and the product removed. The product will vary from a light yellow cake to dark red crystals depending on the rate of evaporation of the pyridine. Slow evaporation is most favorable to the formation of large crystals. The yield is approximately 10.3 g. (100%). The sample may be stored indefinitely at room temperature in a tightly stoppered bottle in the dark. Anal. Calcd. for 2C5H6N-Cr03 Cr, 20.14. Pound total Cr, 20.48, 20.58 Cr6+, 20.15, 20.35. 

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Cr(NH3) ]Cl3 Hexamminechro-mium(III) chloride, 2 196 [Cr(NH3)6](N03)3 Hexamminechro-mium(III) nitrate, 3 153 Cr02Cl2 Chromyl chloride, 2 205 Cr03-2CsH6N Pyridine-chromium-(VI) oxide, 4 94... 

Pyridine forms a number of complexes and salts which are useful synthetic reagents. These include pyridinium hydrobromide perbromidc (CjHjNH Brj ), which is a convenient crystalline, easily handled, bromi-nating agent, the pyridine sulfur trioxide complex (C H NrSOj) for sul-fonations, pyridine borane (C HjNrBH,) for hydroborations and the pyridine-chromium(VI) oxide complex (CjHjNiCrOj) for oxidations. 

Dipyridiue-chromium(VI) oxide2 was introduced as an oxidant for the conversion of acid-sensitive alcohols to carbonyl compounds by Poos, Arth, Beyler, and Sarett.3 The complex, dispersed in pyridine, smoothly converts secondary alcohols to ketones, but oxidations of primary alcohols to aldehydes are capricious.4 In 1968, Collins, Hess, and Frank found that anhydrous dipyridine-chromium(VI) oxide is moderately soluble in chlorinated hydrocarbons and chose dichloro-methane as the solvent.5 By this modification, primary and secondary alcohols were oxidized to aldehydes and ketones in yields of 87-98%. Subsequently Dauben, Lorber, and Fullerton showed that dichloro-methane solutions of the complex are also useful for accomplishing allylic oxidations.6... 

Ethyl sulfate Flammable liquids Fluorine Formamide Freon 113 Glycerol Oxidizing materials, water Ammonium nitrate, chromic acid, the halogens, hydrogen peroxide, nitric acid Isolate from everything only lead and nickel resist prolonged attack Iodine, pyridine, sulfur trioxide Aluminum, barium, lithium, samarium, NaK alloy, titanium Acetic anhydride, hypochlorites, chromium(VI) oxide, perchlorates, alkali peroxides, sodium hydride... 

B. General Oxidation Procedure for Alcohols. A sufficient quantity of a 5% solution of dipyridine chromium (VI) oxide (Note 1) in anhydrous dichloromethane (Note 7) is prepared to provide a sixfold molar ratio of complex to alcohol. This excess is usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25° at 5% concentration to give a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex (Note 8). The alcohol, either pure or as a solution in anhydrous methylene chloride, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 minutes to 15 minutes at 25° with deposition of brownish-black, polymeric, reduced chromium-pyridine products (Note 9). When deposition of reduced chromium compounds is complete (monitoring the reaction by gas chromatography or thin-layer chromatography analysis is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate and the precipitate is rinsed thoroughly with dichloromethane (Note 10). 

All of the usual chromium-based oxidation reagents that have been used for the oxidation of cyclobutanols to cyclobutanones, for example, chromium(VI) oxide (Jones reagent),302 pyri-dinium chlorochromate,304 pyridinium dichromate,307 and chromium(YI) oxide/pyridine (Collins),303 are reported to do so without any serious problems. Alternatively, tetrapropylam-monium perruthenate in the presence of A-methylmorpholine A -oxide. oxalyl chloride in the presence of triethylamine in dimethyl sulfoxide (Swern),158,309,310 or phenyl dichlorophos-phate in the presence of triethylamine and dimethyl sulfoxide in dichloromethane (Pfitzner-Moffatt),308 can be used. The Pfitzner-Moffatt oxidation procedure is found to be more convenient than the Swern oxidation procedure, especially with respect to the strict temperature control that is necessary to achieve good yields in the latter, e.g. oxidation of 1 to give 2.308... 

Arylcoumarins are produced by the oxidation of the corresponding chromenes. Several reagents bring about the conversion, but chromium VI) oxide in pyridine at 55 °C appears to be the most efficient (73JCS(P1)965). 

A Preparation of Dipyridine Chromium(VI) Oxide (Note 1). t notion The reaction of chromium trioxide with pyridine is twin wely exothermic the preparation should be conducted in a hood, observing the precautions noted. 

Although Cr03 is soluble in some organic solvents, like tert-butyl alcohol, pyridine or acetic anhydride, its use in such solvents is limited, because of the tendency of the resulting solutions to explode.2,3 Nevertheless, acetone can safely be mixed with a solution of chromium trioxide in diluted aqueous sulfuric acid. This useful property prompted the development of the so-called Jones oxidation, in which a solution of chromium trioxide in diluted sulfuric acid is dropped on a solution of an organic compound in acetone. This reaction, first described by Jones,13 has become one of the most employed procedures for the oxidation of alcohols, and represents a seminal contribution that prompted the development of other chromium (VI) oxidants in organic synthesis. 

The secondary alcohol is readily oxidized with pyridinium chlorochromate (PCC)25 (65) which is commercially available or easily prepared by addition of pyridine to a solution of chromium(VI)-oxide in hydrochloric acid. 

Chromium (VI) oxide, addition compounds with pyridine and 3-and 4-picoline, 4 94, 95 analysis of, 4 95... 

Pyridine and picoline addition compounds of chromium (VI) oxide, synthesis 33 Biurea, synthesis 8 Urazine, synthesis 9 Carbohydrazide, synthesis 10... 

Sarett and coworkers discovered that the complex (1) prepared by the addition of chromium(VI) oxide to pyridine (CAUTION—reverse order of addition may cause the mixture to inflame) is an efficient oxidizing agent for the preparation of ketones from secondary alcohols. The reagent, as prepared by Sarett, is moderately soluble in pyridine, but is only sparingly soluble in standard organic solvents. Thus the normal procedure is to add a solution of the dcohol in pyridine to three equivalents of the complex, also in pyrictoe. This procedure is also useful for the preparation of aromatic and a, -unsaturated aldehydes, but the use of pyridine as solvent prohibits the oxidation of volatile, saturated primary alcohols. ... 

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