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E. Obijalska et al. / Tetrahedron Letters 54 (2013) 2462–2465
O
P
OH
*
Ph
O
P
OH
*
Ph
.
t -Bu
t-
.
.
Bu
Ph
NH
t-Bu
t -
F3
C
NH
Bu
Ph
SH
F3C
SH
.
(
enantiomerically enriched) -4a (S )-6
4 a
6
(rac)-
(S)-
Figure 1. 1H NMR spectra of racemic and enantiomerically enriched 4a recorded in the presence of (S)-6.
order to find the best co-catalyst, potassium fluoride, potassium
carbonate, and potassium hydroxide were tested. Comparable re-
sults were achieved for KF and K2CO3, and the latter was chosen
for further experiments. It was noticed that lowering the tempera-
ture of the reactions caused a slight increase in the ee value of
product 4a. Also, decreasing the loading of the catalyst CD-
(8S,9R)-5f to 10 mol % resulted in a slight decrease in the ee value.
However, product 4a was not observed when the reactions were
carried out in the presence of 5 mol % of catalyst CD-(8S,9R)-5f.
Next, imino ketones 2b–h, which contain different Ar1 and R1
substituents were tested under the optimized conditions using
CD-(8S,9R)-5f as the catalyst and K2CO3 as the co-catalyst, in tolu-
ene at À40 °C.14 It was found that replacement of the t-Bu substi-
tuent with i-Pr on the N-atom only caused a slight change in the ee
values of the products 4b–h. On the other hand, the stereochemical
outcomes of the reactions were highly influenced by the type of
aryl substituent. The presence of an electron-withdrawing group
on the phenyl ring caused a noticeable decrease in the ee values.
Therefore it is possible that the enantioselectivity of the process
depends on electronic effects more than on steric factors.
It is worth mentioning that the enantiomeric excesses of the fi-
nal products 4a–h were determined on the basis of 1H and/or 19F
NMR spectra recorded in the presence of chiral solvating agents,
for example, (S)-(tert-butyl)phenylthiophosphinic [(S)-6] or (R)-
mandelic acid. An example of a characteristic fragment of the 1H
NMR spectra for racemic and enantiomerically enriched product
4a recorded with (S)-6 is presented in Figure 1. In these studies
the absolute configurations of the main enantiomers of the prod-
ucts 4a–h were not determined.
Acknowledgments
We gratefully acknowledge the National Science Center of Po-
land for the financial support (Grant ‘SONATA’ # DEC-2011/03/D/
ST5/05231) (E.O.).
Supplementary data
Supplementary data (experimental details and the characteriza-
tion data for compounds 4a–h) associated with this article can be
References and notes
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Obijalska, Emilia
