528
W.-B. Yi, C. Cai / Journal of Fluorine Chemistry 129 (2008) 524–528
Table 6
a
Aldol condensation with A-21-Yb(OPf)3
Entry
R1
R2
Time (h)
Yield (%)b
1
2
3
4
5
6
7
a
H
H
12
16
8
84, 84, 82
4-CH3O
H
68
4-NO2
H
94, 93
87
H
H
H
H
4-Cl
4-CH3
4-CH3O
4-NO2
12
16
24
6
75
62
95
The reaction condition: benzaldehydes, 12 mmol; acetophenones, 20 mmol; toluene, 10 ml; A-21-Yb(OPf)3, 0.62 g (0.4 mol% of Yb); reflux.
Isolated yields based on benzaldehyde.
b
control experiment elucidated that <1% product of acylation was
Crafts acylation, and aldol condensation. The catalyst performed
is easily separated and can be reused several times without a
noticeable change in activity under fluorous solvent free
conditions. Further investigations of more favourable strategy
and conditions for recycle of Yb(OPf)3, as well as synthetic
applications, are currently in progress and will be reported in due
course.
obtained in the absence of catalyst (Table 5, entry 2). In addition,
the activity of AlCl3, a traditional industrially catalyst for Fridel-
Crafts acylation, was significantly low under the present mild
reaction conditions (Table 5, entry 3).
Inspired by the work on the Ln(NPf2)3 catalysts without
supports [17], we tried the Yb(OPf)3 Lewis acid as solids catalyst
for the acylation directly (Table 5, entry 4). However, the reaction
gave a low yield of the aromatic ketones because of low solubility
of the solid catalyst in the reaction mixture. In addition, it was
difficult to collect or recover Yb(OPf)3 by filtration, which led to
unavoidable and significant loss of Yb(OPf)3. From these observa-
tions it can be concluded that the presence of A-21 as support is
essential to obtain good yields of acylation and robustness of the
catalytic system for recycling using without fluorous solvents.
Thus, the acylation was extended to other anhydride such as
benzoic anhydride under the above reaction conditions. Although
the reaction became slower, benzoic anhydride gave 92% of the
corresponding aromatic ketones after 3 h (Table 5, entry 5).
However, the less electron rich toluene (Table 5, entries 6-7) and p-
xylene (Table 5, entry 8) underwent acylation much less smoothly
than anisole, and produced ketones in poor yields.
Acknowledgments
We thank the Nature and Science Foundation of Jiangsu
Province (BK2007592) and the National Defense Committee of
Science and Technology of China (40406020301) for financial
support.
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identified as
a,b-unsaturated ketones, and no side reactions were
observed. Based on 1H NMR and GC–MS data, the reaction was
found to give E-stereoisomer as sole product. The purity of the
products thus obtained was consistently high, probably because
the perflates-catalyzed reactions took place under neutral condi-
tions. In general, reactions between aromatic aldehydes and
ketones gave good results. The condensations of p-substituted
benzaldehyde with p-substituted acetophenones yielded the
corresponding chalcones and the product yields were remarkably
affected by the substituent groups of either aldehydes or ketones:
reactants having electron-withdrawing substituents gave high
yields and those having electron-donating ones gave low yields.
4. Conclusion
In conclusion, A-21-Yb(OPf)3 was found to be an interesting
and highly efficient catalyst for esterification, nitration, Fridel-