[23319-73-5]  · C6H14MoN2O7  · Bis(N,N-dimethylformamido)oxodiperoxomolybdenum(VI)  · (MW 322.13)

(useful as an aprotic oxidant in the oxidation of trimethylsilylated amides to the corresponding hydroxamic acids;1 catalyst for the epoxidation of alkenes2,3)

Physical Data: mp 100-102 °C.

Solubility: sol water, CHCl3, CH2Cl2; insol ether.

Form Supplied in: yellow powder. Drying: the hydrated complex is dried in vacuo over P2O5.

Preparative Method: molybdic acid (20 g, 0.15 mol) is dissolved in 30% hydrogen peroxide (100 mL) at 35 °C. The yellow solution is cooled to 15 °C and DMF (21.9 g, 0.3 mol) added. The aqueous solution is partially evaporated under reduced pressure, maintaining the temperature below 35 °C. The resultant yellow crystals are collected by filtration, and washed thoroughly with ether and a small volume of methanol (2 × 25 mL).

Handling, Storage, and Precautions: the complex is stable under storage.

Hydroxamic Acids.

A variety of natural products containing hydroxamic acid functions possess a wide range of biological activities, including the important role of hydroxamic acids in certain iron-transporting systems.4 Trimethylsilylation of secondary amides activates them towards oxidation to hydroxamic acids by the peroxo-molybdenum complex MoO5.HMPA.5 The trimethylsilylation of amides is a well documented process.6 The derivatives are sensitive to water and protic solvents, being solvolyzed to the parent amide. In order to oxidize trimethylsilylated amides, aprotic oxidants are required. Since molybdenum is known to form stable complexes with hydroxamic acids, there existed the possibility that, after oxidation, the product hydroxamic acid would become complexed with the molybdenum ion, thus avoiding further overoxidation during the reaction. Molybdenum pentoxide complexes are stable, covalent diperoxo species.7 Several such complexes are known. Two useful ones are MoO5.HMPA and (see Oxodiperoxymolybdenum(pyridine)(hexamethylphosphoric triamide)). These HMPA complexes are more easily prepared because they are insoluble in water, but released HMPA can interfere with workup. The bis(dimethylformamide) complex is the preferred oxidant. It is water soluble and more stable on storage and there is no residual water ligand to interfere with the subsequent oxidation. Furthermore the released DMF ligands interfere less in the workup procedure than HMPA. Since oxidations with these peroxo complexes are thought to involve substrate-molybdenum complex formation,8 competing strong donor ligands or very polar solvents competitively inhibit the reaction and oxidations are best carried out in chloroform or, routinely, dichloromethane solutions. Liberation of the uncomplexed hydroxamic acids is accomplished by extraction of the molybdenum ion with warm EDTA at pH 9. Since many of the hydroxamic acids are water soluble, their efficient recovery requires continuous extraction with either chloroform or dichloromethane after adjustment of the pH of the solution to 7.5. Although the yield on oxidation of silylated secondary aliphatic amides is usually only moderate (15-40%), hydroxylation of acetanilides proceeds consistently with yields of 40-50%. The reaction is efficient for oxidation of benzoxazinones (eq 1). The products are useful herbicides because of inhibition of auxin.

Some other substrates are 2-(trimethylsilyloxy)pyridine (eq 2) and 4-quinolone (eq 3) as its trimethylsilyl derivative, giving the corresponding hydroxamic acids in 90% and 55% yield, respectively. In contrast, silylated ureas and carbamates fail to give the desired oxidation products.

Epoxidation Catalyst.2,3

A UV and IR examination of the epoxidation of cyclohexene by 1,1-Di-t-butyl Peroxide, catalyzed by the complexes of MoO5 with DMF or HMPA, showed that the mechanism involves the initial, rapid, reaction of the catalyst with t-BuOOH to give the Mo peroxo complex which then epoxidizes the alkene via an O transfer. This latter step involves the formation and subsequent cleavage of a fused molybdenadioxacyclopentane ring.

1. Matlin, S. A.; Sammes, P. G.; Upton, R. M. JCS(P1) 1979, 2481.
2. Xingkai, Y.; Shuhua, Q.; Yue, W. Huaxue Xuebao 1985, 43, 572 (CA 1986, 104, 19 254y).
3. Bocard, C.; Gadelle, C.; Mimoun, H.; de Seree, R. I. Fr. Patent 2 044 007, 1971, (CA 1971, 75, 151 660q).
4. (a) Sentmyer, G. A. Science 1944, 100, 294. (b) Young, C. W.; Schochetenan, G. S.; Hodas, S.; Balis, M. E. Cancer Res. 1967, 27, 535.
5. Matlin, S. A.; Sammes, P. G. CC 1972, 1222.
6. (a) Birkofer, L.; Ritter, A. Newer Methods Prep. Org. Chem. 1968, 5, 211. (b) Klebe, J. F. Adv. Org. Chem. 1972, 8, 97.
7. Mimoun, H.; de Roche, I. S.; Sajus. L. BSF(2) 1969, 1481.
8. Mimoun, H.; de Roche, I. S.; Sajus, L. T 1970, 46, 268.

Tapan Ray

Sigma Radiochemical Co, St Louis, MO, USA

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