Pyridinium Chlorochromate-Alumina1


[26299-14-9]  · C5H6ClCrNO3  · Pyridinium Chlorochromate-Alumina  · (MW 215.57) (Al2O3)

[1344-28-1]  · Al2O3  · Pyridinium Chlorochromate-Alumina  · (MW 101.96) (SiO2)

[7631-86-9]  · O2Si  · Pyridinium Chlorochromate-Silica Gel  · (MW 60.09)

(mild and selective oxidizing agent for primary and secondary alcohols;2 can aromatize 1,4-dihydropyridines5)

Solubility: insol n-hexane, benzene, dichloromethane.

Form Supplied in: PCC-alumina: yellow orange solid; not commercially available. PCC-silica: light orange solid; not commercially available.

Preparative Methods: PCC-alumina:2a Pyridine (60 mmol) is added to a solution of Chromium(VI) Oxide (60 mmol) in 11 mL of 6 N Hydrochloric Acid over 10 min at 40 °C. The mixture is then kept at 10 °C until a yellow-orange solid forms. Reheating to 40 °C gives a solution. Neutral Alumina (50 g) is then added to the solution with stirring at 40 °C. After concentration (rotary evaporator), the resulting orange solid is dried in vacuum for 2 h at rt. Care must be taken not to evaporate pyridine during this process; some workers recommend that additional pyridine should be added to ensure that the reagent is not acidic. The average capacity of the dried reagent is 1 mmol g-1 of alumina. PCC-silica:2c commercial grade PCC (12 mmol) is ground with 70-240 mesh silica gel (1 wt equiv) in a mortar; the resultant free-flowing light orange solid is ready for use in oxidations.

Handling, Storage, and Precautions: PCC-alumina: the reagent can be stored for several weeks under vacuum in the absence of light without losing its activity. PCC-silica: no data are available. Pyridinium chlorochromate is reputed to be a cancer suspect agent. The reagent should be used in a fume hood.

Oxidation of Primary and Secondary Alcohols.

Pyridinium Chlorochromate oxidizes a wide variety of alcohols to carbonyl compounds with high efficiency. However, workup and removal of the chromium-containing byproducts are often tedious and difficult. In addition, PCC shows acidic character which can give rise to complications and side reactions. PCC adsorbed on alumina has been demonstrated to be a mild and selective reagent for the oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones (eqs 1 and 2).2

The acidic nature of PCC is diminished by alumina, which presumably acts as a buffer and enhances the reactivity of PCC as well, often resulting in cleaner oxidations. These reactions are performed by stirring excess oxidant with alcohols in solvents such as n-hexane, benzene, and dichloromethane at rt for 2-3 h. An additional advantage of using a solid support is that it acts as an in situ adsorbent of the reduced byproduct chromium tars,2 and workup then requires only filtration and then concentration. The oxidation seems compatible with many acid-sensitive groups (eq 3).3

Citronellol is directly oxidized to citronellal in 82% yield without any cationic cyclization.2 The oxidation of cholesterol to 5-cholesten-3-one is performed in 80% yield.2 No formation of 4-cholesten-3-one is observed. g,d-Unsaturated alcohols can be smoothly oxidized in good yield to the corresponding g,d-unsaturated aldehydes by using PCC in the presence of alumina (eq 4).4

Oxidative Aromatization.

Interestingly, PCC-alumina is efficient for the oxidative aromatization of 1,4-dihydropyridines into pyridines in yields exceeding 90% (eq 5).5 It is noteworthy that dealkylation (in addition to aromatization) occurs when the starting dihydropyridines bear a secondary alkyl group or a benzyl group at the 4-position.

Oxidations using PCC-Silica.

The oxidation of primary and secondary alcohols with PCC-silica employs dichloromethane as solvent and requires no added buffers, even for compounds bearing acid-sensitive groups. As with PCC-alumina, this reagent offers superior results in terms of simplicity of workup, since silica gel absorbs the byproduct chromium tars and is easily filterable. However, oxidations with this reagent require longer reaction times than the usual PCC procedure. The use of ultrasound in PCC-silica oxidations of alcohols gives rise to a great reaction rate enhancement, reducing the oxidation times generally from hours to minutes and improving the yields of the corresponding carbonyl compounds (60-94%). Acid-labile groups resist the experimental conditions. Neither (E)-(Z) isomerization nor overoxidation are observed (eq 6).6

PCC-silica was found to be the most efficient reagent system for the 1,3-oxidative rearrangement of an unsaturated secondary alcohol to the corresponding a,b-unsaturated aldehyde (eq 7).7

1. (a) Cainelli, G.; Cardillo, G. Chromium Oxidation in Organic Chemistry; Springer: Berlin, 1984; pp 201-204. (b) Haines, A. H. Methods for the Oxidation of Organic Compounds, Alcohols, Alcohol Derivatives, Alkyl Halides, Nitroalkanes, Alkyl Azides, Carbonyl Compounds, Hydroxyarenes and Aminoarenes; Academic: London, 1988; p 38. (c) Ley, S. V.; Madin, A.; Laszlo, P. COS 1991, 7, 265, 279, 841.
2. (a) Cheng, Y.-S.; Liu, W.-L.; Chen, S.-H. S 1980, 223. (b) Piancatelli, G.; Scettri, A.; D'Auria, M. S 1982, 245. (c) Luzzio, F. A.; Guziec, F. S. OPP 1988, 20, 533. (d) Guth, M.; Kirmse, W. ACS 1992, 46, 606.
3. Savoia, D.; Trombini, C.; Umani-Ronchi, A. JOC 1982, 47, 564.
4. Corey, E. J.; Tramontano, A. JACS 1984, 106, 462.
5. Vanden Eynde, J.-J.; Mayence, A.; Maquestiau, A. T 1992, 48, 463.
6. (a) Adams, L. L.; Luzzio, F. A. JOC 1989, 54, 5387. (b) Otargaliev, T. G.; Khaitbaev, Kh. Kh.; Ishbaev, A. I.; Irgasheva, G. A.; Kasymov, T. K.; Abduvakhabov, A. A. U.S.S.R. Patent 1 641 802 (CA 1992, 116, 6158t).
7. Bhaskar, K. V.; Chu, W.-L. A.; Gaskin, P. A.; Mander, L. N.; Murofushi, N.; Pearce, D. W.; Pharis, R. P.; Takahashi, N.; Yamaguchi, I. TL 1991, 32, 6203.

Giovanni Piancatelli

University of Rome La Sapienza and CNR, Rome, Italy

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