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DOI: 10.1055/s-0030-1260545
Metal-Free Chiral Phosphoric Acid or Chiral Metal Phosphate as Active Catalyst in the Activation of N-Acyl Aldimines
Publication History
Publication Date:
29 April 2011 (online)
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Abstract
Whether metal-free chiral phosphoric acid or chiral metal phosphate functions as an active catalyst was confirmed in three reactions. In the aza-Friedel-Crafts and aza-ene-type reactions, a metal-free chiral phosphoric acid, namely, a chiral Brønsted acid, was verified to be the active catalyst. In contrast, the substitution reaction of α-diazoacetate with aldimine was accelerated by a salt-containing chiral phosphoric acid and hence chiral metal phosphate presumably functioned as an active catalyst.
Key words
asymmetric catalysis - enantioselectivity - Friedel-Crafts reaction - organocatalysis - substitution
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References and Notes
Sorimachi, K.; Terada, M. unpublished results.
14List and co-workers reported that silica gel purified chiral phosphoric acid 1a contained substantial amounts of alkali and alkaline-earth metals along with several metal impurities as ascertained by ICP-OES elemental analysis. See ref. 10a.
18
For the Preparation
of Acid-Washed 1b (Method A)
Silica gel purified 1b was dissolved in Et2O. The
resultant solution was washed with HCl aq solution (2 M) in a separatory
funnel. The resultant ether layer was dried over Na2SO4 and
evaporated to remove organic solvents. The resultant residue was
dried under reduced pressure for more than 12 h to eliminate organic
solvents completely.
For the Preparation
of Acid-Washed 1c (Method A)
Silica gel purified 1c was dissolved in MeOH. Then HCl aq solution
(2 M) was added to the resultant MeOH solution to give a white suspension.
The resultant suspension was extracted with CH2Cl2 (twice
or more), and the combined organic layer was dried over Na2SO4.
This was followed by the procedure as shown in the preparation of
acid-washed 1b (method A).
Extra pure silica gel (Silica gel 60 extra pure for column chromatography: Catalogue No. 1.07754) was purchased from Merck KgaA. Short-path column chromatography was performed using CH2Cl2-MeOH (10:1) mixtures as eluent.
22A silica gel purified chiral phosphoric acid contains considerable amounts of alkali and alkaline-earth metals with other metal impurities. Therefore it should be considered that a problem of reproducibility in yield and selectivity would happen, because the composition of metals is dependent on the conditions of silica gel column chromatography conducted.