Pyridinium chlorochromate

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Pyridinium chlorochromate
Chemical structure of the Pyridinium Chlorochromate
Pyridinium-3D-balls.png Ball-and-stick model of the chlorochromate anion
Identifiers
CAS number 26299-14-9 N
ChemSpider 10608386 N
Jmol-3D images Image 1
Properties
Molecular formula C5H5NHClCrO3
Molar mass 215.56 g/mol
Appearance orange crystalline powder
Melting point 205 °C (401 °F; 478 K)
Solubility in other solvents soluble in dichloromethane,
benzene, diethyl ether,
acetone, acetonitrile,
THF
Hazards
MSDS external MSDS sheet
R-phrases R49, R8, R43, R50/53
S-phrases S53, S45, S60, S61
Main hazards Poison, contact hazard, inhalation hazard, environmental hazard, carcinogenic, irritant
NFPA 704
Flammability (red): no hazard code Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 3: Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked. E.g., fluorine Special hazards (white): no codeNFPA 704 four-colored diamond
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Pyridinium chlorochromate (PCC) is a red–orange salt with the formula C5H5NH[CrO3Cl]. It is a reagent in organic synthesis used primarily for the oxidation of alcohols to form carbonyls. A variety of related compounds are known with similar reactivity. Although no longer widely used, PCC offered the advantage of the selective oxidation of alcohols to aldehydes, whereas many other reagents were less selective.[1] Disadvantages to using PCC are the tedious reaction workup and its toxicity. The chemical was first synthesized and used by E.J. Corey and J. William Suggs.

Preparation and structure[edit]

The original preparation, which was discovered by accident,[2] involves the reaction of pyridine with chromium trioxide and concentrated hydrochloric acid:[3]

C5H5N + HCl + CrO3 → [C5H5NH][CrO3Cl]

Alternative syntheses have been developed.[4]

The compounds consists of the cation pyridinium ([C5H5NH+]) and the tetrahedral chlorochromate anion (CrO3Cl). Related salts are also known, such as [C5H5N(C4H9)]+[CrO3Cl], 1-butylpyridinium chlorochromate.

Properties and uses[edit]

Oxidation of alcohols[edit]

PCC is 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, unlike the related Jones reagent. A typical PCC oxidation involves addition of the alcohol to a suspension of PCC in dichloromethane.[5][6][7] A sample reaction would be:

C5H5NH⋅CrO3Cl + R2CHOH → C5H5NHCl + H2CrO3 + R2C=O

In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate workup. 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.

Other reactions[edit]

In addition to simple oxidations of hydroxyl groups, rearrangements are possible. For example, tertiary alcohols cannot be oxidized directly. However, in the Babler oxidation, the chromate ester formed with PCC and an allylic tertiary alcohol can isomerize via a [3,3]-sigmatropic reaction before the carbonyl-forming oxidation step. Other common oxidants usually lead to simple dehydration rather than any oxidation reaction at tertiary hydroxyl centers.

Another oxidative reaction of PCC is its conversion of unsaturated alcohols or aldehydes to cyclohexenones. This pathway, an oxidative cationic cyclization, is illustrated by the conversion of (−)-citronellol to (−)-pulegone. PCC also effects allylic oxidations, e.g. for the conversion dihydrofurans to the lactones.[1]

Related reagents[edit]

Other reagents for oxidizing alcohols using more convenient or less toxic reagents include DMSO-based oxidations (Swern oxidation, Moffatt oxidation) and hypervalent iodine based oxidation (such as the Dess–Martin periodinane).

References[edit]

  1. ^ a b Giovanni Piancatelli, Frederick A. Luzzio, "Pyridinium Chlorochromate" Encyclopedia of Reagents for Organic Synthesis, 2007, John Wiley. doi:10.1002/047084289X.rp288.pub2
  2. ^ http://pipeline.corante.com/archives/2005/10/11/the_old_stuff.php#44460
  3. ^ Corey, E.J.; Suggs, W. (1975). "Pyridinium Chlorochromate. An Efficient Reagent for Oxidation of Primary and Secondary Alcohols to Carbonyl Compounds". Tetrahedron Lett. 16 (31): 2647–2650. doi:10.1016/S0040-4039(00)75204-X. 
  4. ^ 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 (12): 4417–4420. doi:10.1016/S0040-4020(01)86776-4. 
  5. ^ Paquette, L. A.; Earle, M. J.; Smith, G. F. (1998), (4R)-(+)-tert-Butyldimethylsiloxy-2-cyclopenten-1-one, Org. Synth. ; Coll. Vol. 9: 132 
  6. ^ 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 
  7. ^ White, J. D.; Grether, U. M.; Lee, C.-S. (2005), (R)-(+)-3,4-Dimethylcyclohex-2-en-1-one, Org. Synth. 82: 108 

Further reading[edit]

  • G. Tojo and M. Fernâandez (2006). Oxidation of alcohols to aldehydes and ketones : a guide to current common practice. New York: Springer. ISBN 0-387-23607-4. 

External links[edit]