Dipyridine chromium oxide

DPC dipyridine chromium(vi) oxide TFAA trifluoroacedc anhydride... 

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... 

The chief drawbacks to using the Collins reagent are the nuisance involved in preparing pure dipyridine chromium VI) oxide,6 its hygroscopic nature5 and its propensity to enflame during preparation.2 3 5 The present method avoids these difficulties by simply preparing diehloromethane solutions of the complex directly.7 In... 

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). 

Dipyridine chromium(VI) oxide, 52,5 DIRECT lODINATION OF POLY-... 

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. 

Collins, J. C., W. W. Hess, and F. J. Frank (1968) Dipyridine-chromium (VI) oxide oxidation of alcohols in dichloromethane. Tetrahedron Letters 9, 3363-3366. 

The reagent is comparable to the dipyridine -chromium(VI) oxide of Collins (this volume) however, pure Collins reagent is very hygroscopic. 

Dipyridine chromium(VI) oxide (Collins reagent), Cr03(C5H5N)2, 2, 74-75 3, 55-56. Mol. wt. 258.21, red crystals. Supplier Eastman. 

Deacetoxyhtion. In a synlhesis of I l-desoxy-IOz-hydroxyprostaglandins, Crabbe et al. oxidized the alcohol (1) with Collins reagent (dipyridine chromium(Vl) oxide, this volume) to give the diacetoxy aldehyde (2) treatment of this with pyridine for a few... 

Oxidations with dipyridine chromium(VI) oxide are carried out (usually at room temperature) in solutions in dichloromethane. Primary alcohols, including allylic and benzylic alcohols, are oxidized to aldehydes [594, 595, 596, 597, 598, 599, 600] and secondary alcohols to ketones [592, 595,... 

Oxidations by pyridinium chlorochromate resemble those by dipyridine chromium(VI) oxide, both in scope and the mild conditions required. At room temperature, primary alcohols give aldehydes [604, 605], secondary alcohols afford ketones [605], allylic and benzylic methylene groups are oxidized to carbonyl groups [606, 6d7], enol ethers are converted into esters [608] or lactones [609], trimethylsilyl ethers of diphenols are transformed into quinones [610], and alkylboranes are converted into aldehydes (yll]. 

In situ preparation of dipyridine-chromium trioxide is recommended as a simplification of the previous procedure for the obtention of this excellent oxidizing reagent however, the isolated reagent is preferred for the oxidation of disubstituted acetylenes to conjugated acetylenic ketones (Scheme 60). The use of acetic acid as solvent for dipyridine-chromium... 

Ratcliffe has reported that 1-decanol is oxidized to 1-decanal in 63-66% yield, using dipyridine-chromium(VI) oxide prepared directly in methylene chloride at 20°C. This procedure has the advantage over the earlier reported procedure by Sarett and co-workers and Collins in that pure dipyridine-chromium(VI) oxide does not have to be separately prepared. The latter is troublesome since it is hygroscopic and has a propensity to enllame during preparations. 

Collin s oxidation with chromium trioxide-dipyridine complex, or by use of... 

R. E. Arrick, D. C. Baker, and D. Horton, Chromium trioxide-dipyridine complex as an oxidant for partially protected sugar preparation of aldehydo and certain keto sugar derivatives, Carbohydr. Res., 26 (1973) 441 147. 

The chromium(VI) oxide-dipyridine complex also has been found to cause oxidative rearrangement of tertiary allylic alcohols to a. -epoxy aldehydes and small amounts of a. -unsaturated aldehydes (equation 6 and Table 3). This is potentially useful as a homologation sequence since the starting materials are readily available ri om vinyl metal addition to ketones. Use of pyridinium chlorochromate (PCC) for this transformation gives mosUy a. -unsaturated aldehydes. 

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