Bis(tetrabutylammonium) Dichromate1


[56660-19-6]  · C32H72Cr2N2O7  · Bis(tetrabutylammonium) Dichromate  · (MW 700.92)

(mild selective oxidizing agent for benzylic and allylic alcohols;2 can oxidize carbon-halogen bonds3)

Alternate Name: TBADC.

Physical Data: mp 139-143 °C.

Solubility: sol dichloromethane, chloroform.

Form Supplied in: orange crystalline solid; widely available.

Preparative Method: a concentrated aqueous solution (300 mL) of Potassium Dichromate (29.4 g, 0.1 mol) is added to a saturated aqueous solution (300 mL) of Tetra-n-butylammonium Bromide (64.4 g, 0.2 mol). TBADC precipitates and can be removed by filtration (62.8 g, 90% yield).

Handling, Storage, and Precautions: reported to be a toxic agent. The reagent should be handled in a fume hood.

Oxidation of Benzylic and Allylic Alcohols.

TBADC is a neutral and mild oxidizing reagent, soluble in organic solvents. It can be used for the selective oxidation of benzylic and allylic alcohols to the corresponding carbonyl compounds. The reactions are performed in refluxing dichloromethane for 1-3 h with a stoichiometric amount of oxidant; yields range from 78 to 90% (eqs 1 and 2);2 (E)/(Z) isomerization is observed in the oxidation of allylic alcohols.

Unactivated alcohols resist attack by the oxidant, being only slightly oxidized (10-20%) after 24 h at reflux.2a

Oxidation of Carbon-Halogen Bonds.

TBADC, acting as an oxidizing nucleophile, efficiently converts benzylic and allylic halides into the corresponding carbonyl compounds in boiling chloroform. The yields are generally excellent (eq 3).3,4 The reactions are faster for bromides than for chlorides, and in the presence of an excess of oxidant.3

Secondary alkyl bromides afford ketones in high yields, while primary alkyl bromides give a mixture of products with the corresponding aldehydes formed in very low yield.3 Methyl 3,5-bromomethylbenzoate undergoes selective oxidation at one of two benzylic bromomethyl groups to form the corresponding aldehyde upon treatment with TBADC (0.66 equiv) in refluxing chloroform for 1 h, albeit in a moderate yield. The dialdehyde is obtained as a byproduct (eq 4).4

1. (a) Cainelli, G.; Cardillo, G. Chromium Oxidation in Organic Chemistry; Springer: Berlin, 1984; pp 239-240. (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: Orlando, 1988; p 36. (c) Ley, S. V.; Madin, A. COS 1991, 7, 286.
2. (a) Santaniello, E.; Ferraboschi, P. SC 1980, 10, 75. (b) Luzzio, F. A.; Guziec, F. S. OPP 1988, 20, 533.
3. Landini, D.; Rolla, F. CI(L) 1979, 213.
4. Keana, J. F. W.; Pou, S. OPP 1989, 21, 303.

Giovanni Piancatelli

University of Rome La Sapienza and CNR, Italy

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