Chemistry Letters Vol.32, No.3 (2003)
235
efficiency of amine moieties and silicates separately. The high
performance of piperidine-type secondary amine in combination
with mesoporous silica is valuable in view of the simplicity of
reaction procedure.
As shown in Table 2, lower reactivities and selectivities were
observed for aromatic aldehydes other than 1a. However, the
positive effect of FSM-16 was obvious. The addition of FSM-16
increased not only conversion but also the selectivity for b-
hydroxyketone 3. In this catalytic system, aldehyde having
electron-withdrawing group seems to be preferable for the
selective formation of 3.
Table 2. Aldol reaction of 1b–e with 2 catalyzed by piperidine with or
without additivea
Temp. Conv. Yieldb/%
Entry
Ar
1
Additive
—
Figure 1. Time-course of the reaction of 1a with 2 catalyzed by various amine-
immobilized silicates: (a) MAP-FSM-16, (b) PzP-MCM-41, (c) PzP-FSM-16, (d)
AP-MCM-41, (e) AP-FSM-16, (f) AP-SiO2, and (g) DMAP-FSM-16. 120 mg of
each catalyst per milimole of 1a was used. Each catalyst contains
1:30 ꢂ 0:05 mmol gÀ1 of effective nitrogen on the basis of elemental analysis.
/ꢁC
/%
3
4
1
2
3
4
5
6
7
8
4-Chlorophenyl
4-Chlorophenyl
Phenyl
Phenyl
4-Methylphenyl
4-Methylphenyl
1b
30
7
54
5
36
8
2
26 11
3
1b FSM-16 30
1c 30
1c FSM-16 30
1d 30
1d FSM-16 30
60
3,4,5-Trimethoxyphenyl 1e FSM-16 60
—
1
20 15
3
could be assisting the increase in reactivity in the same
manner as suggested in cited Ref. 4. The effect should be
dependent on the acidic property, and the difference in the
promoting effect between mesoporous and amorphous silicas
might be due to the difference in acidity of silanols inside
them.19
—
1
6
0
6
13
0
19
0
3,4,5-Trimethoxyphenyl 1e
—
49
17 19
aThe reaction was carried out as described in the text. bIsolated yields.
We also investigated the effect of amine-immobilized
silicates (Figure 1). 3-Aminopropyl (AP)-, 3-piperazinopropyl
(PzP)-, N-methylaminopropyl (MAP)-, and N,N-dimethylamino-
propyl (DMAP)-functionalized materials were used. Immobili-
zation of basic site on silicate was carried out according to the
reported procedure with slight modification.16 Among the
immobilized amines tested, secondary amine showed the highest
activity. This is the same tendency as described above, suggesting
that enamine may be the key intermediate. The particularly high
activity shown by MAP-FSM-16 (yield of 3a and 4a after 1 h were
86% and 7%, respectively) is probably due to the facile formation
of enamine. The combined yield of 3a and 4a was only 16% when
the reaction was carried out in the presence of PrNH2 (19 mol%)
+ FSM-16, whereas it increased significantly (>50% as shown in
Figure 1) when the reaction was catalyzed by AP-FSM-16
(14 mol% of active site). This means that ‘‘immobilization’’
enhances the reactivity much better than ‘‘simple physical
mixing’’. The order of activity (AP-MCM-41, AP-FSM-
16>AP-SiO2) indicates again that the periodical structure is
advantageous than amorphous. This tendency was consistent with
the results given in Table 1, Entries 2–4. Moreover, the same
trend was observed in the case of 1,4-addition of aldehydes to
vinylketones.17
In summary, we demonstrated that the catalytic performance
of a piperidine-type secondary amine for the aldol reaction is
significantly improved by addition of a mesoporous silica to give
the aldol adduct in relatively high selectivity. Although immo-
bilization is more advantageous towards eco-benignity, this will
provide another simple and efficient way to obtain b-hydroxy-
ketones by aldol reaction. Additionally, this is another clear
example of the promoting effect of silicate support.4;17
References and Notes
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3
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Similarly, in our reaction system silanols on the silicate wall
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