Alcohol oxidation with chromium Jones reagent

Primary aliphatic alcohols produce the corresponding carboxylic acid on oxidation with chromium trioxide. Secondary aliphatic alcohols are oxidized with either Jones reagent or with pyridinium chlorochro-mate (PyClCr04) to the corresponding ketones (see Scheme 11). 

The main applications of oxidation with chromium trioxide are transformations of primary alcohols into aldehydes [184, 537, 538, 543, 570, 571, 572, 573] or, rarely, into carboxylic acids [184, 574], and of secondary alcohols into ketones [406, 536, 542, 543, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584]. Jones reagent is especially successful for such oxidations. It is prepared by diluting with water a solution of 267 g of chromium trioxide in a mixture of 230 mL of concentrated sulfuric acid and 400 mL of water to 1 L to form an 8 N CrOj solution [565, 572, 579, 581, 585, 556]. Other oxidations with chromic oxide include the cleavage of carbon-carbon bonds to give carbonyl compounds or carboxylic acids [482, 566, 567, 569, 580, 587, 555], the conversion of sulfides into sulfoxides [541] and sulfones [559], and the transformation of alkyl silyl ethers into ketones or carboxylic acids [590]. 

When a solution of chromic and sulfuric acids in water is added at 0-20° to an alcohol or formate dissolved in acetone, a rapid oxidation takes place with the separation of the green chromium III reduction product as a separate layer. This system is commonly known as Jones reagent. The rate of oxidation is so fast that it is often possible to run the reaction as a titration to an... 

A solution of chromium trioxide in dilute sulfuric acid used in aqueous acetone is called Jones reagent [572]. Other solvents of chromium trioxide are ether [535] and hexamethylphosphoric triamide (HMPA) [543. Oxidations are also carried out with chromium trioxide adsorbed on Celite (diatomaceous earth) [53S], silica gel [537], or an ion exchanger such as Amberlyst A26 (a macroreticular quaternary ammonium salt anion exchanger) [571, 617]. Such oxidations often take place at room temperature and can be used not only for saturated alcohols but also for unsaturated and aromatic alcohols (equations 208 and 209). 

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. 

Mild acid then cleaves off the silyl protecting group. The alcohol at position 17 is then oxidized with Jones reagent (chromium trioxide in acetone) to afford 14a-hydroxyestrone 3-methyl ether (32-5). 

Ando s synthesis of hymenolin. xhis chromium trioxide/acetone/sulfuric acid reagent is often referred to as the Jones reagent, and oxidation of alcohols with this reagent is called Jones oxidation. Direct conversion of a primary alcohol to an acid is seen in the oxidation of 20 to 21, quantitatively, in Omura, Smith and coworker s synthesis of (+)-lactacystin.5 Jones oxidation is especially useful for molecules that contain alkenyl or alkynyl groups. 2 Oxidation of alcohols is usually faster in acetone than in acetic acid, and using a large excess of acetone protects the ketone product from further oxidation. - Many primary alcohols are oxidized... 

Chromium VI) reagents are powerful oxidants. The reaction of a secondary alcohol with chromium trioxide and acid in aqueous acetone is called Jones s oxidation, and the product is a ketone. Chromium oxidation of an alcohol proceeds by formation of a chromate ester, followed by loss of the a-hydrogen to form the C=0 unit. 

The sequence given in Scheme 10.13 shows the oxidation of an alcohol by CrOs in aqueous acid (Jones reagent). First, CrOs abstracts a proton from the acid (step 1). The alcohol oxygen then acts as a nucleophile and adds to the partially positive chromium (step 2). This reaction is followed by a proton abstraction by the solvent from the alcohol to give a chromate ester (step 3). These steps are analogous to the acid-catalyzed addition of an alcohol to a carbonyl. With CrOs oxidation, the O-Cr chromium metal (step 4). 

---