Chromium reagents oxidants

A.i. Chromium Reagents. Oxidation of an alcohol is accompanied by reduction of the chromium [Cr (VI) Cr(III)]. As implied by the reduction potentials of the three chromium species shown, there are many Cr(VI) reagents,22.23 jjjg most common is the inorganic reagent chromium trioxide, a polymeric species... 

Further oxidation of an aldehyde product to the corresponding carboxylic acid does not take place. Moreover, the SM>ern oxidation reaction does not require the use of toxic and pollutant chromium reagents. The activated DMSO species, however, are stable only at low temperature, which might in some cases be a drawback of this method. 

A variety of chromium (VI) oxidizing systems have been developed which allow for the oxidation of a wide range of sensitive compounds. One of the most widely used chromium (VI) reagents is the Jones reagent (/), whose use is detailed in the procedure. A related system employs acetic acid as the solvent, and an example of this reagent is also given. 

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... 

Oxidations are often carried out using stoichiometric amounts of oxidants, such as permanganate, manganese dioxide or chromium reagents. Catalytic alternatives, therefore, hold great promise to substitute such processes. 

An interesting alternative to the use of chromium(VI) oxidants for the conversion of 1 to 2 involves the use of a low-valent iron reagent prepared in situ by the action of hydrogen peroxide on an iron(II) complex of 1 (73). Vinblastine (as the free base) is treated with 2 equiv of perchloric acid in acetonitrile at -20°C. Ferrous perchlorate is then added, followed by the addition of excess 30% hydrogen peroxide. Work-up of the reaction mixture with a saturated solution of ammonium hydroxide gives 2 in yields of 35-50% after chromatography. 

I )ipyridine chromium(VI) oxide is available from East-imni (Irganic Chemicals. To be an effective reagent, it must be ttnliydi oiis. It should form a red solution on dissolution in mdiydroiis methylene chloride. 

Each substrate was oxidized to the corresponding carbonyl compounds in good yields. Moreover, the coexisting olefin linkage remained intact upon treatment with the oxodiperoxochromium complex and no epoxy compounds were observed in the reaction mixture. Hexavalent chromium reagents such as anhydrous chromium trioxide and pyridinium... 

Aqueous 1 M chromium trioxide (12) does not react with metallic polonium, but with polonium(IV) hydroxide or tetrachloride yields an orange-yellow solid, thought to be Po(Cr04)2. This is insoluble in an excess of the reagent and is easily hydrolyzed by water or wet acetone to a dark brown basic chromate with a composition close to 2Po02 Cr()3 (cf., the basic sulfate and selenate). On long standing in an excess of aqueous chromium trioxide, oxidation to polonium(VI) may occur (Section VI,A). 

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... 

For a review of this oxidation with chromium reagents, sec Cainclli Cardillo, Ref. 10. pp. 92-117. 

A selection of cases in which 2 has been found to be particularly efficacious is given in the Table. Additional examples are cited in references 1 and 5. Particularly noteworthy examples include the oxidation of acid- and base-sensitive systems, systems containing sulfur and selenium, and 1,3-diols to 1,3-dicarbonyl compounds. Use of chromium reagents in these latter cases often leads to fragmentation products. 

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). 

Trifluoro-l-hydroxy-1-methylethyl)cyclohexene is monooxidized at both allylic positions in the ring with different reagents to give a sole product 66 or a product mixture of 67 and 68. The selectivity of the reaction depends on the oxidizing agent used. The rearranged product 68 is the major one when oxidation is accomplished with chromium(VI) oxide in dichloromethane.103... 

Chromium-based oxidants tend to react quicker with unsaturated alcohols, although the difference of oxidation speed with saturated alcohols is normally not sufficient for synthetic purposes. Nevertheless, the chromium-based reagent pyridinium dichromate (PDC) possesses a mildness and, therefore, a relative greater selectivity that allows its occasional employment for selective oxidations of allylic and benzylic alcohols.134... 

The periodinane (10) may also be prepared from o-iodobenzoic acid by oxidation with potassium bromate and then treatment with acetic anhydride18 (see Expt 6.36 for detailed formulation). It should be noted that the organic derivatives of pentacoordinate iodine(v) are termed periodinanes.18b This compound (the systematic name is l,l,l-triacetoxy-2,l-benzoxiodol-3(3//)-one) has found use as an oxidant of primary alcohols to aldehydes and alicyclic ketones to secondary alcohols it is claimed to have advantages over the chromium-based oxidation reagents. 

Oxidizing reagent To a solution of 500 g (0.5 mol) of chromium (VI) oxide and 300 mL of water 1s added 250 g of silica gel (silica gel 60, E. Merck, Darmstadt, West Germany). The mixture is shaken at 30-35°C for 1 hr. After this, the water is removed on a rotary evaporator to yield a yellow-orange, free-flowing powder. 

In the second step the alcohol is transformed to an aldehyde by means of the Swem oxidation. Other reagents to oxidize alcohols to aldehydes are e. g. Dess-Martin-periodinane and chromium reagents like PCC or PDC. 

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