Page 114 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

100 hydrolysis, oxidation and reduction

Materials and equipment

. Benzene (freshly distilled over sodium/benzophenone under a nitrogen at-
mosphere), 5 mL
. (E)-b-Methylstyrene (purified by simple distillation), 118 mg, 1 mmol
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. [Ru (L )O 2 ], 20±40 mg, 0.015±0.03 mmol
. Pyrazole (Hpz) (AR, Aldrich), 30 mg, 0.4 mmol
. Petroleum ether (AR) and CH 2 Cl 2 (AR)
. Silica gel (70±230 mesh ASTM)
. 10 mL Round-bottomed flask with a magnetic stirrer bar
. Magnetic stirrer plate
. Glass column for chromatography
. Rotary evaporator
. HP-UV 8543 Ultraviolet±visible spectrophotometer
. HP5890 Series II Gas Chromatograph equipped with a chiraldex G-TA
capillary column and a flame ionization detector

Procedure
1. In an ice-cooled (0 8C) 10 mL round-bottomed flask equipped with a mag-
netic stirrer bar were placed dry benzene (5 mL), (E)-b-methylstyrene
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(118 mg) and pyrazole (30 mg) under an argon atmosphere. [Ru (L )O 2 ]
(20±40 mg) was added to the mixture with stirring. The resulting solution
was stirred for overnight at 0 8C under an inert atmosphere.
2. The reaction was monitored by a UV±vis spectrophotometer, and the com-
pletion of the reaction was confirmed by the disappearance of the Soret
band at 442 nm.
3. The mixture was passed through a silica gel column and eluted initially with
petroleum ether to remove the unreacted alkene. The product epoxide was
collected by using CH 2 Cl 2 as the eluent.
The epoxide and aldehyde were identified and quantified by capillary
GLC equipped with a Chiraldex G-TA column using 4-bromochloroben-
zene as the internal standard (oven temp: 110 8C, carrier gas: He, flow rate:
ÿ1
60±65 mL min , split ratio 100:1, detector: FID at 250 8C, (1S, 2S)-(E)-b-
methylstyrene oxide: R t ˆ 6:98 min; (1R, 2R)-(E)-b-methylstyrene oxide:
R t ˆ 7:81 min.) The enantiopurity of the 1R,2R-epoxide ˆ 70 % ee, Yield
ˆ 90 % (>99 % trans).

6.3.3 CONCLUSION

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A D 2 -symmetric chiral trans-dioxoruthenium(VI) porphyrin, [Ru (L )O 2 ], bifa-
cially encumbered by four threitol units can effect enantioselective epoxidation of
(E)-b-methylstyrene in up to 70 % ee. For the asymmetric styrene oxidation, a
lower enantioselectivity of 40 % ee was obtained (c.f. 62 % ee, see Table 6.3) when