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Synthesis 2010(12): 2106-2109  
DOI: 10.1055/s-0029-1218783
PSP
© Georg Thieme Verlag Stuttgart ˙ New York

Gram-Scale Preparation of VAPOL Hydrogenphosphate: A Structurally Distinct Chiral Brønsted Acid

Aman A. Desaia, Li Huanga, William D. Wulff*a, Gerald B. Rowlandb, Jon C. Antillab
a Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA
Fax: +1(517)3531793; e-Mail: wulff@chemistry.msu.edu;
b Department of Chemistry, University of South Florida, Tampa, FL 33620, USA
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Publikationsverlauf

Received 10 March 2010
Publikationsdatum:
10. Mai 2010 (online)

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Abstract

A detailed gram-scale synthesis of VAPOL hydrogenphosphate, a structurally distinct chiral Brønsted acid, is presented. The reaction utilizes commercially available starting materials, proceeds with high yields and has been reproduced numerous times at scale.

Key words

VAPOL - asymmetric catalysis - Brønsted acid - ligands - phosphoric acid

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5

The VANOL and VAPOL ligands are now commercially available from Sigma-Aldrich and Strem Chemicals, Inc.

16

Commercial CHCl3 stabilized with amylene was used; and NOT the CHCl3 stabilized with EtOH. It was found that if the latter is used, it becomes extremely difficult to remove the residual EtOH from the product.

17

Sometimes it is difficult to dissolve the crude product in 35 mL of 1:1 MeOH-CHCl3. In such cases, a little pure MeOH could be added to help the dissolution. Alternately, the mixture could be heated at ca. 30 ˚C to aid the dissolution.

18

The side-product was collected, subjected by rotary evaporation to dryness and high vacuum (0.1 mmHg) for 2 h. Its weight was 21 mg, and ¹H NMR analysis showed a mixture of unidentified products.

19

The product, when spotted on a TLC and observed under short-wave UV (254 nm), is an intense purple spot.

20

Sometimes, the VAPOL hydrogenphosphate obtained after column chromatography did not dissolve in CH2Cl2 to give a clear solution. In such cases, the crude product should be left on high vacuum (0.1 mmHg) overnight again, which might solve the problem. If not, then the precipitation should be carried out with the emulsion obtained on the addition of CH2Cl2 to the crude product - it was found that it proceeded just fine even if a clear solution was not obtained.

21

It has been observed in some reactions¹0 that lower asymmetric inductions are obtained if the VAPOL hydrogenphosphate is not properly dried.