Communications
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[17] A colored LC phase can be obtained, in theory, from any
reaction mixture for 4 (% ee ꢁ % conversion) ꢃ 5000 [Eq. (1)].
However, at this point no discrimination between the ee value
and conversion can be made, but as a good chiral catalyst should
give high conversion as well as high enantioselectivity this
method can be applied in the screening of both quantities at the
same time.
substrate in catalytic reactions leading to C C bond forma-
tion. These results make substrate 3 a valuable benchmark
reagent for these reactions. Enantioselectivities can be
determined visually (by looking at the color of an aligned
liquid-crystalline film doped with the product)or spectro-
scopically (by measuring the reflection wavelength and
comparing it with a calibration curve). Both methods,
although of different accuracy, give ee values comparable to
results obtained by HPLC on a chiral stationary phase. This
new procedure involves simple filtration and mixing, and does
not require chiral auxiliaries or post-reaction derivatization.
Furthermore, microgram quantities of product are sufficient.
In conclusion, this methodology allows fast and accurate
screening of enantioselectivities in asymmetric catalysis, and
the development of simple color tests for a wide range of
asymmetric transformations is envisioned based on this
principle.
Experimental Section
General procedure: A chiral ligand (L1–L6, 15.0 mmol, 10 mol%),
Cu(OTf)2 (2.7 mg, 7.5 mmol, 5 mol%), and dichloromethane (4.2 mL)
were added to a flame-dried Schlenk flask under argon. The mixture
was stirred for 1 h at room temperature, and then substrate 3
(60.0 mg, 0.15 mmol)was added. This mixture was stirred for an
additional 15 min and subsequently cooled to ꢀ258C. Et2Zn
(0.35 mL, 1.0m solution in hexanes)was added and the mixture was
stirred for 2 days at ꢀ258C. The reaction was then quenched by the
addition of saturated aqueous NH4Cl (1 mL). The resulting mixture
was dissolved in dichloromethane (50 mL)and filtered through a 3-
cm plug of SiO2 and collected in a flask of known weight. After
evaporation of the solvent, the flask was weighed to determine the
yield of the product (typically around 54 mg). The product was
dissolved in toluene (5.0 mL)and an appropriate amount of this
solution containing 0.80 mg of product was mixed with 85.7 mL of a
stock solution of E7 in toluene (17.55 mg in 500 mL). The solution was
then poured onto a linearly rubbed, polyimide-coated glass plate. The
color appeared immediately after evaporation of the toluene in the air
at room temperature. These phases were stable for 1–3 days, depend-
ing on the cholesteric pitch. The colors of these phases could be
detected by visual inspection or by measuring the reflection wave-
length at a 458 angle by using a modified UV apparatus.
Received: May 28, 2004
Keywords: asymmetric catalysis · chirality · combinatorial
.
chemistry · enantioselectivity · liquid crystals
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Angew. Chem. Int. Ed. 2004, 43, 5013 –5016