Q. Gao et al. / Tetrahedron Letters 52 (2011) 3779–3781
3781
Table 3
Acknowledgments
Investigation on the recyclability of the organocatalyst 5aa
This work was financially supported by the Natural Science
Foundation of China (NSFC 20921092) and the Programme Strate-
gic Scientific Alliances between China and the Netherlands
(2008DFB50130).
Supplementary data
Supplementary data associated with this article can be found, in
Entry
Recycle
Yieldb (%)
eec (%)
1
2
3
4
5
1st
87
85
85
79
60
90
89
88
89
85
2nd
3rd
4th
5th
References and notes
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Weinheim: Wiley-VCH, 2004; (b) Mahrwald, R. Aldol Reactions; Heidelberg:
Springer, 2009.
2. (a) List, B.; Lerner, R. A.; Barbas, C. F., III J. Am. Chem. Soc. 2000, 122, 2395–2396;
(b) Sakthivel, K.; Notz, W.; Bui, T.; Barbas, C. F., III J. Am. Chem. Soc. 2001, 123,
5260–5267.
3. For reviews on organocatalysis, see: (a) Berkessel, A.; Groger, H. Asymmetric
Organocatalysis; Wiley-VCH: Weinheim, 2005; (b) Dalko, P. I. Enantioselective
Organocatalysis; Wiley-VCH: Weinheim, 2007; (c) Mukherjee, S.; Yang, J. W.;
Hoffmann, S.; List, B. Chem. Rev. 2007, 107, 5471–5569.
a
Reactions were performed with neat 2-octanone (0.5 mL), 2.5 mol % catalyst 5a,
p-nitrobenzaldehyde (0.5 mmol), at rt for 48 h.
b
Isolated yields based on aldehyde.
Determined by chiral-phase HPLC.
c
4. For reviews, see: (a) List, B. Tetrahedron 2002, 58, 5573–5590; (b) List, B. Synlett
2001, 1675–1686; (c) Alcaide, B.; Almendros, P. Angew. Chem., Int. Ed. 2003, 42,
858–860; (d) List, B. Acc. Chem. Res. 2004, 37, 548–557; (e) Notz, W.; Tanaka, F.;
Barbas, C. F., III Acc. Chem. Res. 2004, 37, 580–591; (f) Saito, S.; Yamamoto, H. Acc.
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Asymmetry 2007, 18, 2249–2293.
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Zhao, J.-F.; He, L.; Jiang, J.; Tang, Z.; Cun, L.-F.; Gong, L.-Z. Tetrahedron Lett. 2008,
49, 3372–3375.
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After the reaction, catalyst 5a can be easily separated in the
form of precipitation by adding diethyl ether and other low polar
solvents. The reusability of the catalyst was evaluated using 2-
octanone (6) with p-nitrobenzaldehyde (7). The results showed
that the recovered catalyst 5a could be reused for four times with-
out obvious loss of enantioselectivity and its activity decreased
slightly (Table 3, entries 1–5). After the 4th recycle, the activity be-
gan to decrease, which could be attributed to the loss of catalyst
while recycling.
In summary, we have disclosed a direct asymmetric aldol reac-
tion of long-chain aliphatic ketones with aromatic aldehydes, pro-
viding the product in high yields and enantioselectivities. The
reactions were carried out under solvent-free conditions and the
chiral diamine–polyoxometalate acids combined organocatalysts
can be easily recovered and reused for four times with nearly un-
changed enantioselectivity and reactivity.