1
24
D.-G. Crivoi et al. / Journal of Catalysis 334 (2016) 120–128
94%
condensation reaction under neat conditions. Literature reports
that when this reaction was carried out without solvent, it either
requires longer reaction times [31] and high reaction temperatures
1
00
7
4%
67%
8
6
4
2
0
0
0
0
0
67%
[
32] or leads to very low conversions [33]. On the contrary, this
49%
new nanohybrid catalyst favours the aldol condensation reaction
in the absence of solvent, affording total selectivity towards
trans-chalcone and total conversion.
3
8%
28%
36%
35%
1
5%
12%
8% 6%
3.3. One-pot Claisen–Schmidt condensation/Juliá–Colonna
30
60
80
epoxidation reaction
Temperature (°C)
Conversion
S 1a%
S1b%
S 2%
S 3%
The results presented above demonstrate that the Claisen–
Schmidt condensation reaction between acetophenone and
benzaldehyde can be carried out at a lower temperature without
solvent. To test the effectiveness of this nanohybrid catalyst, we
studied its activity in the one-pot Claisen–Schmidt condensation/
Fig. 3. Effect of temperature upon Claisen–Schmidt condensation. 1a – trans-
chalcone; 1b – cis-chalcone; 2 – 1,3,5-triphenylpentane-1,5-dione; 3 – 3-hydroxy-
,3-diphenylpropan-1-one. Reaction conditions: 100 wt% PLL with respect to
1
ketone, acetophenone and benzaldehyde 0.95 M ratio, 1 ml water, 3 h. Conversion
and selectivity from 1H NMR spectra of the crude material.
Juliá–Colonna epoxidation reaction. The protocol for
a,b-epoxy
ketone synthesis involves two steps: (i) formation of enone from
the corresponding ketone and aldehyde catalysed by the HT part
of the IPL and (ii) asymmetric epoxidation of enone with H
catalysed by the immobilised poly-amino acid. To enhance the
enantioselectivity, the length of the poly- -leucine is of crucial
importance, as Berkessel et al. showed [34]. Hence, we synthesised
poly- -leucine with a length between 30 and 45 monomers.
2 2
O ,
L
L
We studied the one-pot reaction with a Claisen–Schmidt con-
densation of benzaldehyde and acetophenone at 60 °C under neat
conditions, followed by the addition of the phase-transfer cocata-
Fig. 4. The aldol addition product obtained in the Claisen–Schmidt condensation.
2 2
lyst (TBAB), H O , NaOH and solvent. The IPL catalyst was re-
by the IPL is 60 °C. At this temperature total conversion was
obtained with very good selectivity towards the trans-chalcone.
used for 4 consecutive runs, to study its stability and selectivity
(Table 1). The material proved to be very sable and efficient in
terms of selectivity and enantioselectivity (both of which remained
constant in all the 4 runs).
To identify whether the hydrotalcite-part of the IPL plays a role
in the second part of the reaction we have carried out four control
experiments, using the same conditions as previously mentioned
3.2.2. Solvent effect
It is widely acknowledged the importance of solvent effects on
the chemical reactivity. Thus, we performed the Claisen–Schmidt
condensation reaction in different solvents (toluene, ethanol,
water) and without solvent (Fig. 5). The low conversion observed
when ethanol (a non-aqueous protic solvent) was used can be
explained through its acidic character which, during 3 h of reac-
tion, could deactivate the strong basic sites of the hydrotalcite.
The only detected product was trans – chalcone (1a). In the case
of non-protic solvents (ex. toluene), the reaction proceeded with
a better conversion than in the case of ethanol, with the formation
of both cis and trans-chalcone.
When water was used as solvent, despite attaining a good con-
version, the aldol addition product (3) was obtained, demonstrat-
ing that in an aqueous medium the formed chalcone can be
hydrolysed.
Green chemistry implies the use of safer solvents or their
elimination [30]. In this context, we have run Claisen–Schmidt
(
results presented in Table 2). The first two experiments consisted
in carrying out the epoxidation reaction with and without rehy-
drated hydrotalcite (Table 2, entries 1 and 2) and in both cases
total conversion was obtained. Knowing that rehydrated hydrotal-
cites possess basic properties, we have run the epoxidation reac-
tion without the presence of NaOH using HTrus and IPL (Table 2,
entry 3 and 4). In the former case a conversion of 5% was obtain
whilst in the latter one no epoxide was observed. This information
suggests than any basic centre found in the HTrus capable to depro-
tonate the hydrogen peroxide is deactivated when the polymer is
immobilised. All these findings demonstrate that in the second
part of the one-pot reaction only the poly-L-leucine is responsible
for the asymmetric induction and the HTrus is not active.
As mentioned in the section dedicated to the Claisen–Schmidt
condensation, there are several side products that might appear,
depending on the basicity of the environment, excess of reactants
and the solvent used. A key point in recycling the catalyst for the
one-pot reaction is washing the material after each run, to elimi-
nate the following: (i) any unreacted NaOH, (ii) any unreacted
reagent and (iii) traces of the obtained product, which may lead
to side products in the next run.
The TGA pattern of the catalyst after 4 consecutive runs resem-
bles to that of the catalyst before reaction and no difference in
weight loss is observed, ruling out the possible leaching of the cat-
alyst (Fig. III in Supporting Information).
100%
100%
100%
92%
94%
100
7
4%
80
60
40
20
0
37%
15%
12%
8%6%
8%
toluene
ethanol
water
no solvent
Solvent
Conversion S 1a%
S 1b%
S 2%
S 3%
3
.4. One-pot Claisen–Schmidt condensation/Juliá–Colonna
Fig. 5. Claisen–Schmidt condensation using different solvents: 1a – trans-chalcone;
cis-chalcone; 1,3,5-triphenylpentane-1,5-dione; 3-hydroxy-1,3-
epoxidation reaction – scope of reaction
1
b
–
2
–
3 –
diphenylpropan-1-one. Reaction conditions: 100 wt% PLL with respect to ketone,
acetophenone and benzaldehyde 0.95 M ratio, 1 ml solvent, 3 h. Conversion and
selectivity from 1H NMR spectra of the crude material.
To confirm the activity of this new nanohybrid system, we stud-
ied the one-pot reaction using acetophenone/benzaldehyde with