Hydrogen Peroxide-Urea1

[124-43-6]  · CH6N2O3  · Hydrogen Peroxide-Urea  · (MW 94.09)

(alternative to 90% hydrogen peroxide as a source of anhydrous hydrogen peroxide for use in oxidation reactions)

Alternate Name: UHP.

Physical Data: mp 84-86 °C (dec).2

Solubility: sol water and alcohols; low solubility in organic solvents such as dichloromethane.

Form Supplied in: white crystalline powder with urea as impurity; commercially available. Drying: over calcium chloride in a desiccator.

Preparative Method: made by the recrystallization of Urea from aqueous Hydrogen Peroxide

Handling, Storage, and Precautions: the pure material should be stored at low temperature but the commericial material, which has a purity of ca. 90%, may be stored at rt. In a sufficiently forcing test, it can be made to explode; its decomposition is acceleratory above 82 °C. Conduct work with this reagent in an efficient fume hood behind a polycarbonate safety screen.

Epoxidation Reactions.

The hydrogen peroxide-urea complex, which is normally known as urea-hydrogen peroxide (UHP), has been used in anhydrous organic solvents in combination with a number of carboxylic anhydrides together with disodium hydrogen phosphate for the epoxidation of a wide range of alkenes. The carboxylic anhydride of choice depends on the electron density in the double bond.3 With electron-rich alkenes such as a-methylstyrene and a-pinene (eq 1), good yields of the expected products can be obtained by using Acetic Anhydride. Other anhydrides have also been used; for example, the epoxide from trans-stilbene is obtained in 80% yield using Maleic Anhydride.4 Monoperoxymaleic acid, prepared by the reaction of maleic anhydride with 90% aqueous hydrogen peroxide, is more reactive than other common peroxy acids with the exception of Trifluoroperacetic Acid.5 With relatively nonnucleophilic and nonvolatile terminal alkenes such as 1-octene, a good yield of the epoxide can be obtained by substituting Trifluoroacetic Anhydride for the acetic anhydride (eq 2). The addition of Imidazole increases the rate at which epoxidation reactions proceed when using UHP and acetic anhydride.6 Presumably N-acetylimidazole is formed and rapidly perhydrolyzed. A comparison of the diastereoselective epoxidations of a steroidal allylic alcohol has been carried out using a range of peroxy acids, including peracetic acid generated by the interaction of acetic anhydride with UHP.7

The epoxidation of electron-deficient alkenes like methyl methacrylate can also be achieved using the trifluoroacetic anhydride method. In the case of a,b-unsaturated ketones such as isophorone (eq 3) and nitro alkenes such as b-methyl-b-nitrostyrene (eq 4), alkaline hydrogen peroxide has been generated from UHP.

The selective epoxidation of compounds such as a-ionone can also be achieved. The result of an epoxidation reaction frequently depends on the reagent and precise reaction conditions. This is exemplified in the reactions shown in eq 5. When using peroxytrifluoroacetic acid generated conventionally, water that is always present diverts the reaction exclusively to the hemiacetal (1). However, when using the UHP method, the spiroacetal (2) predominates over the other product.8

Baeyer-Villiger and Related Reactions.

The oxidation of aldehydes and ketones to afford esters and lactones can be achieved using a variety of peroxycarboxylic acids. The ease with which the reaction occurs is related to the strength of the conjugate acid of the leaving group and so the stronger the carboxylic acid the more powerful is the peroxy acid in its oxidation reactions. Reactions involving ketones are frequently slow when using weakly acidic peroxy acids and so the majority of the reactions using UHP have been carried out with trifluoroacetic anhydride as the coreactant (eqs 6-8).

Dakin reactions, where an aromatic aldehyde has an electron releasing substituent either ortho or para to the formyl group, can be carried out using UHP-acetic anhydride as shown in eq 9. In the absence of suitable activating substituents, hydrogen migration occurs in place of aryl migration and the product is then a carboxylic acid (eq 10). This is a potentially valuable way of converting a formyl group into a carboxyl group.

Heteroatom Oxidation.

The first example of the use of UHP in organic chemistry involved the formation of the N-oxide shown in eq 11,9 and, in a modification of the UHP method, phthalic anhydride has been used to oxidize 4-t-butylpyridine to the N-oxide in 93% yield.10 The oxidation of aliphatic aldoximes using UHP-trifluoroacetic anhydride has been achieved in good yields with retention of configuration at neighboring chiral centers (eq 12).11

1. (a) Heaney, H. Top. Curr. Chem. 1993, 164, 1. (b) Heaney, H. Aldrichim. Acta 1993, 26, 35.
2. Lu, C.-S.; Hughes, E. W.; Giguère, P. A. JACS 1941, 63, 1507.
3. Cooper, M. S.; Heaney, H.; Newbold, A. J.; Sanderson, W. R. SL 1990, 533.
4. Astudillo, L.; Galindo, A.; González, A. G.; Mansilla, H. H 1993, 36, 1075.
5. White, R. H.; Emmons, W. D. T 1962, 17, 31.
6. Rocha Gonsalves, A. M. d'A.; Johnstone, R. A. W.; Pereira, M. M.; Shaw, J. JCR(S) 1991, 208.
7. Back, T. G.; Blazecka, P. G.; Krishna, M. V. CJC 1993, 71, 156.
8. (a) Ziegler, F. E.; Metcalf, C. A., III; Schulte, G. TL 1992, 33, 3117. (b) Ziegler, F. E.; Metcalf, C. A., III; Nangia, A.; Schulte, G. JACS 1993, 115, 2581.
9. Eichler, E.; Rooney, C. S.; Williams, H. W. R. JHC 1976, 13, 41.
10. Kaczmarek, L.; Balicki, R.; Nantka-Namirski, P. CB 1992, 125, 1965.
11. Ballini, R.; Marcantoni, E.; Petrini, M. TL 1992, 33, 4835.

Harry Heaney

Loughborough University of Technology, UK

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