We gratefully acknowledge the Chinese Academy of Sciences
for generous financial support. We thank Professor Yong Tang for
helpful discussions.
Notes and references
1
(a) B. T. Grobel and D. Seebach, Synthesis, 1977, 357–402; (b)
D. Seebach, Angew. Chem., Int. Ed. Engl., 1979, 18, 239–258.
2
For reviews on stable carbenes as organic catalysts: (a) D. Enders and
T. Balensiefer, Acc. Chem. Res., 2004, 37, 534–541; (b) J. S. Johnson,
Angew. Chem., Int. Ed., 2004, 43, 1326–1328; (c) V. Nair, S. Bindu and
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K. Zeitler, Angew. Chem., Int. Ed., 2005, 44, 7506–7510.
3
Recent examples: (a) A. E. Mattson and K. A. Scheidt, Org. Lett., 2004,
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A. Chan and K. A. Scheidt, J. Am. Chem. Soc., 2006, 128, 4558–4559;
(
f) S. Csihony, D. A. Culkin, A. C. Sentman, A. P. Dove,
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4
Scheme 2 A plausible catalytic cycle for the cross-coupling reaction.
4 (a) R. Breslow, J. Am. Chem. Soc., 1958, 80, 3719–3726; (b) J. Sheehan
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substrates in the cross-coupling reactions. The latter could mean
that the NHC catalyst may react with acylimines at a high
temperature, which is necessary for reversing benzoin formation in
the presence of the NHC catalyst. In our case, Schiff bases do not
react with the NHC catalyst even at high temperature which
warrants the existence of the NHC in the reaction mixture.
A plausible catalytic cycle was proposed, as illustrated in
Scheme 2. Carbene I is generated by deprotonation of thiazolium
1743–1745.
5
(a) H. Stetter, Angew. Chem., Int. Ed. Engl., 1976, 15, 639–647; (b)
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and T. Rovis, J. Am. Chem. Soc., 2004, 126, 8876–8877; (f)
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S. J. Miller, Chem. Commun., 2005, 195–197; (g) J. Read de Alaniz and
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3
salt 1a in the presence of Et N. I then reacts with benzaldehyde to
6
(a) Y. Hachisu, J. W. Bode and K. Suzuki, J. Am. Chem. Soc., 2003,
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give the Breslow intermediate II, which could lead to benzoin, 5, or
intermediate III by reacting with benzaldehyde or imine 3a,
respectively. Upon proton transfer, intermediate III will give
intermediate IV, which could further release the carbene catalyst I
and product 4a to finish the catalytic cycle.
Chem., Int. Ed., 2006, 45, 1463–1467; (c) H. Takikawa, Y. Hachisu,
J. W. Bode and K. Suzuki, Angew. Chem., Int. Ed., 2006, 45, 3492–3494.
Y.-K. Liu, R. Li, L. Yue, B.-J. Li, Y.-C. Chen, Y. Wu and L.-S. Ding,
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7
8
In summary, we have found that the readily available thiazolium
salt in the presence of triethylamine catalyzes cross-coupling of
aromatic aldehydes with unactivated imines. This organocatalytic
process affords a-aminoketones smoothly, using a cheap and
commercially available catalyst and mild reaction conditions. A
preliminary study shows that the reaction is probably under a
thermodynamic control. This strategy provides a novel access to
the acyl anion addition reaction involving a less reactive acyl
receptor. Further development of new acyl anion receptors and
related mechanistic investigations are currently under way.
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8
54 | Chem. Commun., 2007, 852–854
This journal is ß The Royal Society of Chemistry 2007