Chromium reagents acidic

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 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. 

All of the chromium reagents produce by-products and washings that contain hazardous chromium salts and must be collected as hazardous waste. In many cases, simple oxidants such as household bleach (sodium hypochlorite, NaOCl) can accomplish the same oxidations as chromic acid without using heavy metals, and without generating hazardous waste. Oxidations using sodium hypochlorite involve mildly acidic or basic conditions that may be better than chromic acid for acid-sensitive compounds. 

Many reagents are available to oxidize a simple secondary alcohol to a ketone. Most labs would have chromium trioxide or sodium dichromate available, and the chromic acid oxidation would be simple. Bleach (sodium hypochlorite) might be a cheaper and less polluting alternative to the chromium reagents. DMP and the Swem oxidation would also work. 

Finally, Cr(VI)-reagents should be mentioned for example the Jones reagent (H2S04/Cr03 in acetone) for the oxidation of primary alcohols to carboxylic acids and the oxidation of secondary alcohols to the corresponding ketones. The main problem with these chromium reagents is their high toxicity. 

A common way to change reaction conditions for the oxidation of alcohols is to modify the acid that is added to the medium. Indeed, chromium trioxide will have different oxidizing abilities in different acids. Since most organic compounds are insoluble in water, a cosolvent is usually required to dissolve not only the chromium reagent but also the alcohol substrate. This solvent must be resistant to oxidation, and acetic acid or acetone are commonly used. For the alcohol - carbonyl conversion several Cr(VI) reagents can be used, including chromium trioxide in water or aqueous acetic acid catalyzed by mineral acid, sodium dichromate in aqueous acetone catalyzed by mineral acid, sodium dichromate in acetic acid, the Cr03 pyridine complex, and err-butyl chromate.Both primary and secondary alcohols can be oxidized to the aldehyde or ketone, respectively. Aldehydes may be oxidized to the carboxylic acid under some conditions. 

Oxidative Methods. - Corey et al. have reported on the use of the cyclic chromate ester (1) as a catalyst for the peracetic acid oxidation of secondary alcohols. The reagent oxidises primary alcohols very slowly, and is compatible with tetrahydropyranyl protection of alcohols furthermore, it has the great advantage of avoiding the toxicological and disposal problems associated with stoichiometric chromium reagents. The selective oxidation of benzylic alcohols in the presence of... 

Under certain circumstances, organochromium(ni) compoimds transfer their allaryl groups to aldehydes and, less frequently, to ketones. The particular selectivity and tolerance of this reaction make it particularly useful in organic synthesis. The use of these chromium reagents may be advantageous for use with acid-sensitive substrates, because of their reduced Lewis acidity relative to other transfer reagents [e.g., trichloromethyl-titanium(IV) or all[Pg.35]

In water, primary alcohols tend to overoxidize to carboxylic acids, as shown for 1-propanol. The reason is that aldehydes in water are in equilibrium with the corresponding diols, derived by addition of water. One of the hydroxy functions of the diol then reacts further with the chromium reagent to the carboxylic acid. We shall discuss the hydration of aldehydes and ketones in Chapter 17. 

A quaternary ammonium trifluoroacetochromate(VI) polymer (26), prepared from Amberlyst A-26 with Cr03 and trifluoroacetic acid, showed greater activity than (25) for oxidation of secondary alcohols. Reaction of 2-octanol in cyclohexane at 70 °C with 3.8 molar equivalents of (26) gave 82% yield of 2-octanone in 4 h as shown in equation (11). A major advantage of the polymeric chromium reagents is the ease of isolation of the oxidation product from chromium salts. The major drawbacks are the initial expense of the polymer support and the relatively large amounts of polymer that must be used. 

---