[70355-53-2]  · C5H11MoNO7  · (S)-(2-Hydroxy-N,N-dimethylpropanamide-O,O)oxodiperoxymolybdenum(VI)  · (MW 293.11)

(enantioselective epoxidation of unfunctionalized simple alkenes1)

Physical Data: mp 149 °C (dec). According to X-ray structural analysis, the molecule has the pentagonal bipyramidal geometry with sevenfold coordinated molybdenum.

Solubility: sol ethanol; insol ether.

Form Supplied in: yellow microcrystalline powder. Drying: dried under vacuum.1

Preparative Method: to 5mL aqueous hydrogen peroxide solution (30%) is added, in portions, 1 g molybdenum(VI) oxide at 20 °C. The mixture is stirred at 20 °C for 20 min and at 40 °C for 4 h. After most of the molybdenum oxide has dissolved the mixture is filtered. The yellow solution is treated at 10 °C with 1 equiv of ligand dissolved in approx. 4 mL methanol. The mixture is stirred for 30 min at 20 °C, concentrated and kept 12 h at 0 °C. When the complex does not crystallize after addition of dichloromethane or benzene, the mixture is diluted with ethanol and carefully (shield protection) concentrated on a rotary evaporator. The procedure is repeated five times. The mixture is then further concentrated. The complex is usually obtained as a yellow microcrystalline powder, but if it is obtained as an oil the addition of diethyl ether with stirring at 5 °C yields a yellow powder.

Handling, Storage, and Precautions: necessary care should be taken when preparing and handling peroxometal compounds. Fast and complete concentration of the mixture containing the compound may lead to deflagration of the complex. The complex decomposes at temperatures above 100 °C, changing color from yellow to blue and black. The complex is stored at 5 °C in <0.5 g portions.

Asymmetric Epoxidation.2

Methods3 are known for asymmetric epoxidation of prochiral alkenes having activated C=C double bonds (styrene, allyl alcohol, conjugated ketones, quinone). The title optically active metal-peroxo complex (1) is capable of asymmetric epoxidation of simple unfunctionalized alkenes. Simple prochiral alkenes such as propene or trans-2-butene are epoxidized stoichiometrically to optically active oxiranes by (1) in nitrobenzene at 20 °C/1 bar. The chemical yield is about 70%. The enantiomeric excess is around 30% and the configuration of the dominant oxirane enantiomer is (R). A marked increase in enantiomeric yield for trimethyloxirane from 2-methyl-2-butene is observed on reducing the reaction rate by lowering the temperature. The increasing steric hindrance of the alkyl group in 3-methyl-1-butene surprisingly leads to a decrease in the asymmetric induction. The continuously monitored enantiomeric composition of the oxiranes during the reaction remained constant within experimental accuracy. This shows that the alkene epoxidation is asymmetrically induced and that no enrichment of the enantiomers of the epoxide that is formed takes place by kinetic resolution. The asymmetric induction decreases in the order of propene > 1-butene > 3-methyl-1-butene with the preferential formation of (R)-alkyloxiranes.1 The ee increases with inversion of prochiral recognition for 3,3-dimethyl-1-butene, resulting in the preferential formation of (S)-t-butyloxirane. trans-2-Butene undergoes higher asymmetric induction than does trans-2-pentene, whereby (2S,3S)-trans-2-methyl/ethyl-3-methyloxiranes are preferentially formed. The asymmetric epoxidation of cis-2-pentene leads to the preferential formation of (2S,3R)-2-ethyl-3-methyloxirane. Geminal ethyl/methyl disubstitution at the double bond in 2-methyl-1-butene shows no enantiofacial discrimination and, consequently, only racemic 2-ethyl-2-methyloxirane is formed. In connection with the synthesis of aranciamycinone,4 complex (1) has been used for the asymmetric epoxidation of the nonfunctionalized alkene (2).5 The alkene (2) (eq 1) is treated with diluted solutions (1 mmol in 300 mL of dichloromethane) of the molybdenum(VI)-oxodiperoxo complex. Decreasing the temperature to 0 °C raises the enantiomeric yield to 53% ee. The absolute configuration of the predominantly formed enantiomer (3) was determined by an unequivocal sequence of reactions to a known glycoside.

1. Schurig, V.; Hintzer, K.; Leyrer, U.; Mark, C.; Pitchen, P.; Kagan, H. B. JOM 1989, 370, 81.
2. Kagan, H. B.; Mimoun, H.; Mark, C.; Schurig, V. AG(E) 1979, 18, 485.
3. (a) Ewins, R. C.; Henbest, H. B.; McKervey, M. A. CC 1967, 1085. Montanari, F.; Moretti, I.; Torre, G. CC 1969, 135. (b) Yamada, S.; Mashiko, T.; Terashima, S. JACS 1977, 99, 1988. Michaelson, R. C.; Palermo, R. E.; Sharpless, K. B. JACS 1977, 99, 1990. (c) Helder, R.; Hummelen, J. C.; Laane, R. W. P. M.; Wiering, J. S.; Wynberg, H. TL 1976, 1831.
4. Krohn, K.; Broser, E. LA 1982, 1907.
5. Krohn, K.; Broser, E. TL 1984, 25, 2463.

Tapan Ray

Sandoz Research Institute, East Hanover, NJ, USA

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