Pyridinium Fluorochromate1

[83042-08-4]  · C5H6CrFNO3  · Pyridinium Fluorochromate  · (MW 199.12)

(mild oxidizing agent for primary and secondary alcohols;2 can oxidize activated C-H bonds2b)

Alternate Name: PFC.

Physical Data: mp 106-108 °C.

Solubility: sol DMF, acetonitrile, acetone; less sol dichloromethane; sparingly sol benzene, carbon tetrachloride, chloroform, hexane.

Form Supplied in: not commercially available.

Preparative Method: prepared from Pyridine, aqueous 40% Hydrofluoric Acid, and Chromium(VI) Oxide in a molar ratio 1:1.5:1. The bright orange, crystalline reagent is isolated by filtration and dried in vacuo; yield 93-94%. The procedure can be performed on a 200 g scale.2

Handling, Storage, and Precautions: the dry orange crystalline reagent can be stored in sealed polythene bags for long periods without decomposition. Reaction solvents must be anhydrous and free of reducing impurities. Chromium(VI) derivatives are reputed to be toxic agents. The reagent should be used in a fume hood.

Oxidation of Primary and Secondary Alcohols.

PFC is an effective reagent for the oxidation of a wide range of alcohols. With 1.5 equiv of reagent in dichloromethane at rt for 1-3 h, primary and secondary alcohols are converted to aldehydes and ketones in yields ranging from 77 to 98% (eqs 1-3).2,3 (E)-(Z) isomerization has been observed during the oxidation of some allylic alcohols.3

PFC seems to have certain advantages over similar oxidizing agents in terms of the amount of oxidant and solvent required, shorter reaction times, and improved yields of products. Since its acidity is less pronounced than that of Pyridinium Chlorochromate, compounds with acid-sensitive groups can be oxidized without buffering the reaction mixture. PFC can be used in the oxidation of a secondary alcohol in the presence of a primary alcohol protected as a silyl ether (eq 4).3 The more acidic PCC does attack silyl ether groups.

The fluorochromate anion has been supported on a polymeric matrix, the cross-linked Poly(4-vinylpyridine). Poly(4-vinylpyridinium fluorochromate) oxidizes primary and secondary alcohols to aldehydes and ketones in very good yield in both hexane or benzene. It is essential that the reagent be wet; use of dry reagent does not afford the desired product.4 No products of overoxidation are found in the reaction mixture. In addition, the wet supported oxidant can be employed as an efficient reagent for the cleavage of aldoximes and ketoximes to the corresponding parent carbonyl compounds in excellent yield.4

Oxidation of Activated C-H Bonds.

PFC can be utilized for selective oxidation of certain aromatic hydrocarbons. Anthracene and phenanthrene are converted by the oxidant (2.5 equiv) at rt into the corresponding anthraquinone (eq 5) and phenanthrene-9, 10-quinone in 68% and 52% yield. The yields may be raised to 98% and 72% by using Acetic Acid as the reaction medium.2

Several reports have suggested that PFC is a suitable oxidant for both allylic5 and benzylic C-H bonds,6 although no yields have been reported. Oxidation of D3-carene in aqueous acetic acid containing perchloric acid gives rise to a variety of ketonic products.5 By the same procedure, oxidation of substituted toluenes afford the corresponding aldehydes as the main products.6c Diphenylmethane and fluorene are converted mainly into benzophenone and fluorenone.6b PFC can oxidize a tertiary benzylic C-H bond: triphenylmethanol is obtained in 90% yield from triphenylmethane by reaction with the oxidant in acetic acid containing perchloric acid (eq 6).7

PFC in DMF has the capability to oxidize selectively organic sulfides into sulfoxides. No yields are reported.8

1. (a) Cainelli, G.; Cardillo, G. Chromium Oxidation in Organic Chemistry; Springer: Berlin, 1984; pp 178-180. (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, pp 38-39. (c) Ley, S. V.; Madin, A. COS 1991, 7, 267.
2. (a) Bhattacharjee, M. N.; Chaudhuri, M. K.; Dasgupta, H. S.; Roy, N.; Khathing, D. T. S 1982, 588. (b) Luzzio, F. A.; Guziec, F. S., Jr. OPP 1988, 20, 533.
3. Nonaka, T.; Kanemoto, S.; Oshima, K.; Nozaki, H. BCJ 1984, 57, 2019.
4. Narayanan, N.; Balasubramanian, T. R. JCR(S) 1992, 132.
5. Varadarajan, R.; Dhar, R. K. IJC(B) 1986, 25B, 971.
6. (a) Varadarajan, R.; Dhar, R. K. IJC(B) 1986, 25B, 746. (b) Bhattacharjee, U.; Bhattacharjee, A. K. IJC(A) 1990, 29A, 1187. (c) Bhattacharjee, B.; Bhattacharjee, M. N.; Bhattacharjee, M.; Bhattacharjee, A. K. BCJ 1986, 59, 3217.
7. Bhattacharjee, U. B.; Bhattacharjee, A. K. Oxid. Commun. 1991, 14, 66.
8. Banerji, K. K. JCS(P2) 1988, 2065.

Giovanni Piancatelli

University of Rome La Sapienza and CNR, Rome, Italy

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