Alcohols, secondary, oxidation with Jones reagent

A special category of ethers are trimethylsilyl ethers. Trimethylsilyl ethers of primary alcohols, on treatment with Jones reagent, give acids [590]. On treatment with A-bromosuccinimide under irradiation, trimethylsilyl ethers yield esters [744]. Secondary alkyl trimethylsilyl ethers are converted into ketones by oxidation with both reagents [590, 744, 981]. Oxidation with Jones reagent is regiospecific the 2-ferf-butyldimethylsilyl 11-Krf-butyldiphenylsilyl ether of 2,11-dodecanediol is oxidized only in the sterically less hindered position [590]. Trimethylsilyl ethers of tertiary alcohols are degraded by periodic acid to carboxylic acids with shorter chains [755] (equations 336-339). 

One of die most common methods for the preparation of ketones is by the oxidation of secondary alcohols. The use of chromic acid (Jones reagent) is easy, safe, and effective for the oxidation of secondary alcohols to ketones. Furthermore Jones reagent gives a nearly neutral solution and thus can be used with a variety of acid-sensitive functional groups. 

Primary alcohols are cleanly oxidized to aldehydes (not to carboxylic acids as with Jones reagent), secondary alcohols yield ketones, and tertiary alcohols are again unre-active. Therefore, a Swem oxidation accomplishes the same transformations as PCC. Hence, in Example 10.11, the same products would be obtained with a Swern oxidation as with the application of PCC. 

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). 

Other attempts to prepare 2,4,6-tris(trifluoromethyl ) acetophenone have also been met only with very limited success (28)i 1) No reaction was observed when 1,3,5-tris trifluoromethyl)-benzene was treated with acetyl chloride in the presence of anhydrous AICI3 ( vide supra), 2) The secondary alcohol RfCH(OH)CH3 can be prepared from RpLi and acetaldehyde. However, it was found impossible to oxidize this alcohol with Jones reagent (Cr ). Although IR spectra showed some evidence... 

Selective oxidation of secondary alcohols to ketones is usually performed with CrOj/HjSO, I I in acetone (Jones reagent) or with CrOjPyj (Collin s reagent) in the presence of acid-sensitive groups (H.G. Bosche, 1975 C. Djerassi, 1956 W.S. Allen, 1954). As mentioned above, a,)S-unsaturated secondary alcohols are selectively oxidized by MnOj (D.G. Lee, 1969 D. Arndt, 1975) or by DDQ (D. Walker, 1967 H.H. Stechl, 1975). 

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]. 

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. 

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 reagent also reacts with primary and secondary alcohols (see Chromic Anhydride Test Jones Oxidation, p. 640). 

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