Chromium trioxide-pyridine complex preparation

OXIDATION WITH THE CHROMIUM TRIOXIDE-PYRIDINE COMPLEX PREPARED in situ ... 

Carboxylic acids, a-bromination of 55, 31 CARBOXYLIC ACID CHLORIDES, ketones from, 55, 122 CARBYLAMINE REACTION, 55, 96 Ceric ammonium nitrate [Ammonium hexa mtrocerate(IV)[, 55, 43 Chlorine, 55, 33, 35, 63 CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Cinnamomtnle, a-phenyl- [2-Propeneni-tnle 2,3-diphenyl-], 55, 92 Copper(l) iodide, 55, 105, 123, 124 Copper thiophenoxide [Benzenethiol, copper(I) salt], 55, 123 CYCLIZATION, free radical, 55, 57 CYCLOBUTADIENE, 55, 43 Cyclobutadieneiron tricarbonyl [Iron, tn-carbonyl(r)4-l,3-cyclo-butadiene)-], 55,43... 

CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Chrysene, 58,15, 16 fzans-Cinnamaldehyde, 57, 85 Cinnamaldehyde dimethylacetal, 57, 84 Cinnamyl alcohol, 56,105 58, 9 2-Cinnamylthio-2-thiazoline, 56, 82 Citric acid, 58,43 Citronellal, 58, 107, 112 Cleavage of methyl ethers with iodotri-methylsilane, 59, 35 Cobalt(II) acetylacetonate, 57, 13 Conjugate addition of aryl aldehydes, 59, 53 Copper (I) bromide, 58, 52, 54, 56 59,123 COPPER CATALYZED ARYLATION OF /3-DlCARBONYL COMPOUNDS, 58, 52 Copper (I) chloride, 57, 34 Copper (II) chloride, 56, 10 Copper(I) iodide, 55, 105, 123, 124 Copper(I) oxide, 59, 206 Copper(ll) oxide, 56, 10 Copper salts of carboxylic acids, 59, 127 Copper(l) thiophenoxide, 55, 123 59, 210 Copper(l) trifluoromethanesulfonate, 59, 202... 

A recently discovered (2) oxidizing system promises to become very important for the oxidation of acid-sensitive compounds. The reagent is chromium trioxide-pyridine complex, which may be isolated after preparation and employed in nonaqueous solvents (usually methylene chloride). A remarkable feature of the reagent is that good yields of aldehydes are obtained by direct oxidation of primary alcohols. The preparation of the reagent and its use are given. 

A 5% solution of chromium trioxide-pyridine complex in dry methylene chloride is prepared. The alcohol (0.01 mole) is dissolved in dry methylene chloride and is added in one portion to the magnetically stirred oxidizing solution (310 ml, a 6 1 mole ratio) at room temperature. The oxidation is complete in 5-15 minutes as indicated by the precipitation of the brownish black chromium reduction products. The mixture is filtered and the solvent is removed (rotary evaporator) leaving the crude product, which may be purified by distillation or recrystallization. Examples are given in Table 1.1. 

The chromium trioxide-pyridine complex affords fair yields of dicarbonyl compounds by oxidation of suitably protected precursors 2,3 4,5-di-<3-isopropylidene-(2/f/e/n f/o-/l-D-(2ra/)/rtrt-hcxos-2-ulose-2,6-pyranose was thus prepared in 53% yield from 2,3 4,5-di-0-isopropylidene-/f-D-fructo-... 

Pyridinium chlorochromate, described as a safe, stable, and readily prepared alternative to the chromium trioxide-pyridine complex, is a convenient oxidant for primary and secondary alcohols84 and should find use in steroid chemistry. Dimethyl sulphide ditriflate , prepared from DMSO and trifluoromethanesulphonic anhydride at —78°C, has been used to oxidize 3-hydroxy-steroids.85 The biological... 

An alternative to the chromium trioxide- yridine complex i provided by pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). These reagents, now ubiquitous for chromate-based oxidation of alcohols, overcome the hygroscopic nature of the chromium trioxide-pyridine complex and are prepared by a less hazardous procedure both are commercially available as are several other derivative reagents. 

Conjugated acetylenic ketones can be prepared in about 40% yield by oxidation of alkynes with chromium trioxide-pyridine complex ... 

Oxidation. The chromium trioxide-pyridine complex is conveniently and safely prepared from the components in dichloromethane this solution readily oxidizes alcohols to give aldehydes or ketones. The complex has more vigorous oxidizing properties in acetic acid. " Primary and secondary alcohols are... 

Pyridinium chlorochromate (PCC), CsHsN-HChCrOj, mp 205 C (dec), which is commercially available, is prepared by adding chromium irioxide to a solution of pyridine in hydrochloric acid [603]. The preparation i.s easier than that of the chromium trioxide-pyridine complex. 

Ratcliffe and co-workers further improved this procedure by preparing dichloromethane solutions of the chromium trioxide-pyridine complex directly.Addition of chromium trioxide to a solution of dichloromethane and pyridine was followed by the alcohol, which is the procedure typically used today, but it is still referred to as Collins oxidation. 

Pyridine forms explosive mixtures with air, with LEL and UEL values of 1.8 and 12.4% by volume in air, respectively. Its reactions with strong oxidizers can be violent. There is a report of an explosion with chromium trioxide during the preparation of a chromium trioxide-pyridine complex (MCA 1967). Many metal perchlorates, such as silver perchlorate, form solvated salts with pyridine, which can explode when struck. It undergoes exothermic reactions with concentrated acids. 

Chromium trioxide—pyridine complex (Cr03 C5H5N) may explode if the C1O3 concentration is too high. The complex should be prepared by addition of Cr03 to excess C5H5N. 

This complex, formerly called pyridine perchromate and now finding application as a powerful and selective oxidant, is violently explosive when dry [1], Use while moist on the day of preparation and destroy any surplus with dilute alkali [2], Preparation and use of the reagent have been detailed further [3], The analogous complexes with aniline, piperidine and quinoline may be similarly hazardous [4], The damage caused by a 1 g sample of the pyridine complex exploding during desiccation on a warm day was extensive. Desiccation of the aniline complex had to be at ice temperature to avoid violent explosion [4]. Pyridinium chlorochromate is commercially available as a safer alternative oxidant of alcohols to aldehydes [5], See Chromium trioxide Pyridine Dipyridinium dichromate See Other AMMINECHROMIUM PEROXOCOMPLEXES... 

In a typical Knof procedure, 3jS-hydroxyandrost-5-en-17-one acetate is epoxidized with perbenzoic acid (or m-chloroperbenzoic acid ) to a mixture of 5a,6a- and 5)5,6)5-epoxides (75) in 99 % yield. Subsequent oxidation with aqueous chromium trioxide in methyl ethyl ketone affords the 5a-hydroxy-6-ketone (76) in 89% yield. Baeyer-Villiger oxidation of the hydroxy ketone (76) with perbenzoic acid (or w-chloroperbenzoic acid ) gives keto acid (77) in 96% yield as a complex with benzoic acid. The benzoic acid can be removed by sublimation or, more conveniently, by treating the complex with benzoyl chloride and pyridine to give the easily isolated )5-lactone (70) in 40% yield. As described in section III-A, pyrolysis of j5-lactone (70) affords A -B-norsteroid (71). Knof used this reaction sequence to prepare 3)5-hydroxy-B-norandrost-5-en-17-one acetate, B-noran-... 

The deployment of chromium(VI) reagents in the oxidative transformation is compromised due to inherent toxicity, involved preparation of its various complex forms (with pyridine or acetic acid) and cumbersome workup procedures. Chromium trioxide (Cr03) immobilized on premoistened alumina enables efficient oxidation of ben -... 

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