Alcohols, primary with pyridinium chlorochromate

Reaction of the C-0 and O-H Bonds Primary alcohols oxidize to carboxylic acids secondary alcohols oxidize to ketones with chromium trioxide or sodium dichromate. Tertiary alcohols do not oxidize under mild conditions. With pyridinium chlorochromate (PCC) the oxidation of primary alcohols can be stopped at aldehydes. 

Triol 984 is converted to acetonide 985 and the free hydroxyl group is sequentially oxidized to an aldehyde (986) with pyridinium chlorochromate and then to an acid under Jones conditions. Acidic workup furnishes the hydroxy butyrolactone 987. Treatment of THP-protected butyrolactone 988 with methyl Grignard reagent affords diol 989. Removal of the THP group, tosylation of the primary alcohol, and cyclization under basic conditions provides epoxide 990. Opening the oxirane ring with 2-(l,3-butadienyl)magnesium chloride in the presence of... 

In order to remove the primary hydroxy group, the protected triol 24 was acet-ylated and treated with pyridinium tosylate to yield the monoester 25. Selective tosylation of the primary hydroxy group followed by Finkelstein exchange for iodine yielded the iodo derivative 26 which was transformed into the protected cyclohexene diol 27 by reductive removal of the iodine with tributyltin hydride followed by protection of the tertiary hydroxy group with dihydopyran. After removal of the benzoyl group by alkaline saponification the resulting allylic alcohol was oxidised to the desired enone 28 with pyridinium chlorochromate in buffered solution (ref. 16). 

These objectives have spurred research directed toward developing alternative synthetic methods. Take alcohol oxidation, for example. As described in Section 15.9, primary alcohols are converted to aldehydes by oxidation with pyridinium chlorochromate (PCC). 

A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl. 

The oxidation of primary alcohols to aldehydes also suffers from the problem of overoxidation of the aldehyde to a carboxylic acid. Mild methods capable of stopping die oxidation at the aldehyde oxidation level are required if aldehydes are to be obtained. The most common and effective reagent for this purpose is pyridinium chlorochromate (PCC), produced by the reaction of pyridinium hydrochloride with chromium trioxide. This reagent is soluble in dichloromethane and smoothly oxidizes primary alcohols to aldehydes in high yields. Because of die mild, neutral reaction conditions and the use of stoichiomettic amounts of oxidant, the aldehyde product is not oxidized further. 

Oxidation of several primary aliphatic alcohols with potassium dichromate, pyri-dinium dichromate, quinolinium dichromate (QDC), imidazolium dichromate, nico-tinium dichromate, isonicotinium dichromate, pyridinium fluorochromate (PFC), quinolinium fluorochromate, imidazolium fluorochromate, pyridinium chlorochromate (PCC), quinolinium chlorochromate (QCC), and pyridinium bromochromate (PBC), in aqueous acetic acid and in the presence of perchloric acid, showed similar kinetics. The values of the reaction constants did not differ significantly, indicating operation of a common mechanism.1... 

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) 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. 

Oxidation of primary alcohols carrying unsaturation at the 5 or 6 position by means of the pyridinium chlorochromate provides the cyclized products. The same products are obtained by oxidation of analogously substituted aldehydes, with this reagent. ... 

Pyridinium Chlorochromate. The need for improved oxidation of primary alcohols and greater ease for isolation of products prompted further research into the nature of Cr(VI) reagents. Corey found that addition of pyridine to a solution of chromium trioxide in aqueous HCl allowed crystallization of a solid reagent characterized as 31, pyridinium chlorochromate (PCC). This reagent was superior for the conversion of primary alcohols to aldehydes in dichloromethane but less efficient than the Collins oxidation when applied to allylic alcohols. Oxidation of 1-heptanol with PCC in dichloromethane gave 78% of heptanal, for example. As stated by Corey, PCC is an effective oxidant in dichloromethane although aqueous chlorochromate species are not very effective oxidants. Oxidation of secondary alcohols to ketones is straightforward, as in Banwell s synthesis of y-lycorane, in which 32 was oxidized by PCC to the ketone (33). ... 

With primary alcohols, oxidation can be stopped at the aldehyde stage by special reagents, such as pyridinium chlorochromate (PCC), shown in eq. 7.38. ... 

An excellent reagent to use for converting a primary alcohol to an aldehyde is pyridinium chlorochromate (abbreviated PCC), the compound formed when CrOs is dissolved in hydrochloric acid and then treated with pyridine ... 

The uses of the versatile oxidant pyridinium chlorochromate have been reviewed. Pyridinium chlorochromate has been used for the oxidation of methyl 5-hydroxypentanoate to the aldehyde, an intermediate useful in the synthesis of leukotrienes, and the same reagent oxidatively cleaves secondary vicinal diols to the corresponding aldehydes. The reactivity of the chlorochromate ion as an oxidant can be influenced by the counter-ion. For example, the 4-dimethylaminopyridinium salt is a mild, selective reagent for the oxidation of allylic and benzylic alcohols. Pyridinium fluorochromate oxidizes primary and secondary alcohols to aldehydes and ketones respectively in dichloromethane solution, and shows a less pronounced acidity compared with the chloro-... 

In Summary Reductions of aldehydes and ketones by hydride reagents constitute general syntheses of primary and secondary alcohols, respectively. The reverse reactions, oxidations of primary alcohols to aldehydes and secondary alcohols to ketones, are achieved with chromium(Vl) reagents. Use of pyridinium chlorochromate (PCC) prevents overoxidation of primary alcohols to carboxylic acids. 

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