Pyridinium chlorochromate

Pyridinium chlorochromate is a reddish orange solid reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Pyridinium chlorochromate, or PCC, will not fully oxidize the alcohol to the carboxylic acid as does the Jones reagent. A disadvantage to using PCC is its toxicity. PCC was developed by Elias James Corey and William Suggs in 1975.^[1]

Pyridinium dichromate is a similar oxidizing agent, which has the advantage of being less acidic.

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Preparation

The original preparation by Corey^[1] involves adding one equivalent of pyridine to a solution of one equivalent of chromium(VI) trioxide and concentrated hydrochloric acid:

C₅H₅N + HCl + CrO₃ → [C₅H₅NH][CrO₃Cl]

Agarwal et al. presented an alternative synthesis that avoids the harmful side product chromyl chloride (CrO₂Cl₂).^[2] Chromium(VI) oxide is treated with pyridinium chloride:

[C₅H₅NH⁺]Cl⁻ + CrO₃ → [C₅H₅NH][CrO₃Cl]

Properties and uses

PCC is primarily used as an oxidant. In particular, it has proven to be highly effective in oxidizing primary and secondary alcohols to aldehydes and ketones, respectively. Rarely does over-oxidation occur (whether intentionally or accidentally) to form carboxylic acids. A typical PCC oxidation involves addition of the alcohol to a suspension of PCC in dichloromethane. A sample reaction would be:

2 C₅H₅NHCrO₃Cl + 3 R₂CHOH → 2 C₅H₅NHCl + "CrO_1.5" + 3 R₂C=O + 3 H₂O

In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate matters. Addition of an inert adsorbent such as crushed molecular sieves or silica gel allows the sticky byproduct to adsorb to the surface, and makes workup easier.

PCC is also remarkable for its high selectivity. For example, when oxidizing tertiary allyl alcohols, unsaturated aldehydes are observed as the sole product. This reaction is known as the Babler oxidation. Otherwise such oxidations commonly afford dienes as by-products resulting from dehydration.

Another notable oxidative reaction of PCC is its efficient conversion of unsaturated alcohols or aldehydes to cyclohexenones. This particular pathway is known as oxidative cationic cyclization.

Controversy

PCC is controversial as it contains chromium(VI). More environmentally friendly oxidants listed above are therefore favored by green chemists. Other methods for oxidizing alcohols using less toxic reagents have been introduced:

• DMSO-based oxidations (Swern oxidation, Moffatt oxidation)
• hypervalent iodine based oxidation (such as the Dess-Martin periodinane)

References

1. ^ ^{a b} Corey, E.J.; Suggs, W. (1975). "Pyridinium Chlorochromate. An Efficient Reagent for Oxidation of Primary and Secondary Alcohols to Carbonyl Compounds". Tetrahedron Lett. 16: 2647-2650. doi:10.1016/S0040-4039(00)75204-X.
2. ^ Agarwal, S; Tiwari, H. P.; Sharma, J. P. (1990). "Pyridinium Chlorochromate: an Improved Method for its Synthesis and use of Anhydrous acetic acid as catalyst for oxidation reactions". Tetrahedron 46: 4417-4420. doi:10.1016/S0040-4020(01)86776-4.

• Paquette, L. A.; Earle, M. J.; Smith, G. F. (1998). "(4R)-(+)-tert-Butyldimethylsiloxy-2-cyclopenten-1-one". Org. Synth.; Coll. Vol. 9: 132. 
• Piancatelli, G. 'Pyridinium Chlorochromate', Encyclopedia of Reagents for Organic Synthesis 2001. ([http://www.mrw.interscience.wiley.com/eros/articles/rp288/frame.html}}
• Tu, Y.; Frohn, M.; Wang, Z.-X.; Shi, Y. (2003). "Synthesis of 1,2:4,5-Di-‘’O’’-Isopropylidene-D-erythro-2,3-hexodiulo-2,6-pyranose. A highly Enantioselective Ketone Catalyst for Epoxidation". Org. Synth. 80: 1. 
• White, J. D.; Grether, U. M.; Lee, C.-S. (2005). "(R)-(+)-3,4-Dimethylcyclohex-2-en-1-one". Org. Synth. 82: 108. 

Further reading

• G. Tojo and M. Fernâandez (2006). Oxidation of alcohols to aldehydes and ketones : a guide to current common practice. Springer. ISBN 0387236074.