Alcohols, secondary, oxidation chlorochromate

Secondary alcohols are oxidized easily and in high yield to give ketones. For large-scale oxidations, an inexpensive reagent such as Na2Cr207 aqueous acetic acid might be used. For a more sensitive or costly alcohol, however, pyridinium chlorochromate is often used because the reaction is milder and occurs at lower temperatures. 

After the known intermediate 79 (contaminated with ca. 6 % < /.v isomer) [39] was prepared from Hajos-Parrish ketone [40] 78, the tert-butyl ether was cleaved (quant.) and the ketone protected as the acetal (96 %). The secondary alcohol was oxidized by pyridinium chlorochromate (PCC) to provide ketone 80 in good yield (71 %) and after fractional crystallization afforded material absent of any m-hydrindane (Scheme 10.6). [NOTE All compounds shown in Schemes 10.6 and 10.7 are shown in the ent-configuration, as published]. The oxidation of protected hydrindane 80 under Saegusa-Ito conditions [41, 42] gave enone 81 (82 %), confirmed by X-ray crystallography. 

Secondary alcohols are oxidized by H5IO6 in the presence of various chromium catalysts to ketones [1321-1325], while primary alcohols can be oxidized to aldehydes or to carboxylic acids depending on the catalyst. Primary alcohols in the presence of pyridinium chlorochromate (PCC)[1322] or chromium (III) acetylacetonate, Cr(acac)3 [1321] are oxidized to aldehydes or ketones in excellent yields, while the use of CrOj [1326,1327] or pyridinium fluorochromate [1323] as catalysts results in the oxidation to carboxylic acids. The periodic acid promoted oxidation of primary and secondary alcohols to carbonyl compounds can also be catalyzed by Cu(II) derivatives [1328,1329], by bromide anion [1330] and by TEMPO [1331]. 

Hajipour, A. R. Mallakpour, S. E. Malakoutikhah, M. Efficient and highly selective oxidation of primary and secondary alcohols by butyltriphenylphosphonium chlorochromate under non-aqueous conditions. Indian J. Chem., Sect. B 2003, 42B, 195-198. 

Perhaps the most important reaction of alcohols is their oxidation to carbonyl compounds. Primary alcohols yield either aldehydes or carboxylic acids, secondary alcohols yield ketones, but tertiary alcohols are not normally oxidized. Pyridinium chlorochromate (PCC) in dichloromethane is often used for oxidizing primary alcohols to aldehydes and secondary alcohols to ketones. A solution of Cr03 in aqueous acid is frequently used for oxidizing primary alcohols to carboxylic acids and secondary alcohols to ketones. 

Quinolinium dichromate (QDC) oxidations of primary and secondary alcohols both proceed via a cyclic chromate ester. Acrylonitrile polymerization was observed in the oxidation of several para- and meffl-substituted benzaldehydes to the corresponding benzoic acids by quinolinium chlorochromate (QCC). QCC oxidations of diphenacyl sulfide and of aromatic anils have been studied. 

Interestingly, the little studied inorganic chlorochromates, potassium397 and magnesium398 chlorochromates are very easy to prepare and are soluble in polar organic solvents, like acetone or acetonitrile. They are able to efficiently oxidize secondary alcohols to ketones, although they provide only low yields of aldehydes on the oxidation of primary alcohols. 

The secondary alcohol is readily oxidized with pyridinium chlorochromate (PCC)25 (65) which is commercially available or easily prepared by addition of pyridine to a solution of chromium(VI)-oxide in hydrochloric acid. 

Primary alcohols can be oxidized to carboxylic acids using Jones reagent, whereas secondary alcohols give ketones. Primary alcohols can be oxidized to aldehydes using pyridinium chlorochromate (PCC). 

Primary alcohols give aldehydes. The best oxidant is pyridinium chlorochromate (PCC). Secondary alcohols give ketones the reagent is usually Cr03, H+. 

Corey, E. J. and W. Suggs (1975) Pyridinium chlorochromate. An efficient reagent for oxidation of primary and secondary alcohols to carbonyl compounds. Tetrahedron Letters 16,2647-2650. 

A better reagent for the limited oxidation of primary alcohols to aldehydes is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HC1. PCC oxidizes most primary alcohols to aldehydes in excellent yields. Unlike most other oxidants, PCC is soluble in nonpolar solvents such as dichloromethane (CH2C12), which is an excellent solvent for most organic compounds. PCC can also serve as a mild reagent for oxidizing secondary alcohols to ketones. 

Pyridinium chlorochromate (PCC), C5H6NOO3CI Oxidizes primary alcohols to yield aldehydes and secondary alcohols to yield ketones (Section 17.7). 

Poly(vinylpyridinium chlorochromate) (PVPCC) is a mild oxidant for primary, secondary, allylic and benzylic alcohols. Unfortunately, optimum conditions require the use of very nonpolar solvents (best is cyclohexane) at 80 C. More polar solvents (that would be more generally useful in synthesis) severely retard the rate of oxidation, thus necessitating an increase in the amount of oxidant used. Oxidations were found to have high inital rates, but were very slow to go to completion due to the inaccessibility of the chromium. This can be overcome by using a lower loading of oxidant or by an alternative preparation of the polymer, where the addition of 1-5% divinylbenzene gives a much more porous resin. 

Pyridinium chlorochromate (PCC) Corey and Suggs prepared PCC by mixing CrOs with pyridine in HCl. PCC is used for the oxidation of primary and secondary alcohols in CH2CI2. This reagent is less efficient than Collins reagent for the oxidation of allyl alcohols. 

Oxidations by pyridinium chlorochromate resemble those by dipyridine chromium(VI) oxide, both in scope and the mild conditions required. At room temperature, primary alcohols give aldehydes [604, 605], secondary alcohols afford ketones [605], allylic and benzylic methylene groups are oxidized to carbonyl groups [606, 6d7], enol ethers are converted into esters [608] or lactones [609], trimethylsilyl ethers of diphenols are transformed into quinones [610], and alkylboranes are converted into aldehydes (yll]. 

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