Langmuir
Article
The self-assembled nanoflowers showed a delicate 3D
structure with a superior specific surface area, fully exposed
catalytic sites, and chiral molecular configuration, which is
expected to be used to catalyze the direct aldol reaction in
aqueous media, with excellent catalytic performance, achieving
both high yield (>99%) and enantioselectivity (>99%). The
results can be used as a feasible new strategy for constructing
nanocatalysts for asymmetric reactions through the combina-
tion of molecular design and supramolecular self-assembly.
Fc-CO-NH-P was mixed with 1 mL of phosphate buffers and sodium
hydroxide solution (50 mM; pH 6.0, 7.0, 9.0, 12.0) for 5 min,
respectively. The mixtures were regarded as the catalysts for the
control group, which were used for the aldol reaction directly.
Preparation of the L-Prolinamide Catalyst. We also dissolved
1
mg of L-prolinamide in 1 mL of phosphate buffer (50 mM, pH 12.0)
and sodium hydroxide solution (50 mM, pH 12.0) at 60 °C,
respectively. The samples were moved to a 25 °C water bath to
incubate for 24 h for full self-assembly.
Aldol Reaction of p-Nitrobenzaldehyde with Cyclohexa-
none. An amount of 4.36 mg of p-nitrobenzaldehyde (0.031 mmol, 1
equiv) and 31.7 μL cyclohexanone (0.31 mmol, 10 equiv) were mixed
in a vial. After adding the Fc-CO-NH-P catalysts (0.0031 mmol, 0.1
equiv), the mixtures were incubated at 25 °C for 72 h with stirring.
EXPERIMENTAL SECTION
Materials and Methods. Ferrocenecarboxylic acid (Fc-COOH,
8.5% in purity), L-prolinamide (98% in purity), and anhydrous
■
9
After reaction, 1 mL of saturated NH Cl solution was added into the
4
CH Cl2 were obtained from Aladdin (Shanghai, China). Other
2
mixtures to quench the reaction, and then the mixtures were washed
chemicals were of analytical grade and were purchased from
commercial sources.
thesis of chlorocarbonyl ferrocene (Scheme S1I): The reaction
proceeded under anhydrous conditions. Ferrocenecarboxylic acid
with 1 M HCl, 1 M NaHCO , and pure water, progressively. The
3
organic phase was then mixed with anhydrous sodium sulfate, and
then CH Cl was taken away in a vacuum. To determine the
2
2
1
conversion of the aldol reaction, H NMR was used. The ee% was
measured by HPLC with a Daicel Chiralcell AD-H column using the
following conditions: eluent: hexane/2-PrOH = 80:20, flow rate: 0.5
mL/min, and detection λ = 254 nm.
(
300 mg, 1.3 mmol) was suspended in 100 mL of CH Cl , and after
2 2
being fully dissolved, we added oxalyl chloride (220 μL, 2.6 mmol)
into the solution slowly at 0 °C. After adding three drops of DMF, the
mixtures continued to react at room temperature with continuous
stirring for 12 h. With the increase of the reaction time, the color of
the solution changed from light orange to wine red. When the
reaction was completed, the sample was washed successively with
HCl, NaHCO , and pure water, and CH Cl in the organic phase was
For the control experiment, the self-assembled L-prolinamide was
added into a mixture of 1.53 mg of p-nitrobenzaldehyde (0.011 mmol,
1
equiv) and 11.10 μL of cyclohexanone (0.11 mmol, 10 equiv) and
then incubated at 25 °C for 72 h with stirring. After reaction, 1 mL of
saturated NH Cl solution was added to the mixtures to quench the
4
3
2
2
reaction, and then the mixtures were washed with 1 M HCl, 1 M
removed by mixing with anhydrous sodium sulfate and by rotary
evaporation. The product with an orange color was used in the next
reaction.
NaHCO , and pure water, progressively. The organic phase was then
3
dried with anhydrous sodium sulfate, and then taken away in a
1
vacuum. The yield and dr values were detected by H NMR analysis.
Synthesis of Ferrocene-L-Prolinamide (Fc-CO-NH-P)
Cyclic Performance Test of P12. After completing the last batch
of aldol reactions, the mixtures were washed with 1 M HCl, 1 M
L-Prolinamide (120 mg, 1.05 mmol) was dissolved in 50 mL of
CH Cl , and 1.3 equiv of Et N (222 μL, 1.7 mmol) was added into
NaHCO , and pure water, progressively, and the aqueous phase was
3
2
2
3
the solution dropwise; the reactants were mixed at room temperature
with stirring for 10 min and then cooled down to 0 °C in an ice bath.
The previously synthesized chlorocarbonyl ferrocene was dissolved in
collected and centrifuged at 15 000 rpm for 20 min at 4 °C. After
centrifugation, the yellow catalysts were separated at the bottom of
the tube, followed by washing with pure water three times, and the
final solid was used to catalyze the next batch of aldol reactions.
1
0 mL of anhydrous CH Cl and then added to the mixtures
2 2
dropwise; this procedure was continued for about 30 min. The
reaction proceeded for 24 h at room temperature with stirring. When
the reaction was completed, the sample was washed successively with
HCl, NaHCO , and pure water, and CH Cl in the organic phase was
CHARACTERIZATION
■
3
2
2
Nuclear Magnetic Resonance Hydrogen Spectrosco-
1
removed by rotary evaporation. Then, the product was further
py ( H NMR). The samples were dissolved in CDCl with
3
purified using column chromatography with 10:1 of CH Cl /
1
2
2
Si(CH ) as the reference, and then the H NMR spectrum
3
4
CH OH. The yield of the two-step reaction was about 55%.
3
was measured on an NMR spectrometer (AV-400, Bruker).
Scanning Electron Microscopy (SEM). We placed 10 μL
of self-assemblies onto a glass coverslip with a diameter of 0.5
mm, and excess samples were removed. Then, the samples
were air-dried, which were sputtered with platinum (E1045 Pt-
coater, Hitachi High-Technologies CO., Japan). The morphol-
ogy of self-assemblies was observed by SEM (S-4800, Hitachi
High-Technologies CO., Japan) with an acceleration voltage of
Preparation of Different pH Solutions. NaH PO (600 mg)
2
4
and Na HPO ·12 H O (1.79 g) were dissolved in 100 mL of H O,
2
4
2
2
resulting in a 50 mM NaH PO and Na HPO solution. Phosphate
2
4
2
4
buffers with a pH of 6.0, 7.0, and 9.0 were prepared by mixing a
certain volume of NaH PO and Na HPO solution. The pH 12.0
2
4
2
4
phosphate buffer was prepared by adding a certain volume of 50 mM
NaOH solution into the Na HPO solution. The 50 mM NaOH
2
4
solution was prepared by dissolving 200 mg of NaOH in 100 mL of
water. By adding different volumes of HCl, the pH of the solution
could be set to 6.0, 7.0, 9.0, and 12.0.
3
keV.
Transmission Electron Microscopy (TEM). An aliquot
Preparation of Fc-CO-NH-P Catalysts. Different catalysts were
prepared by self-assembly of Fc-CO-NH-P in various solutions. In
general, we dissolved 1 mg of Fc-CO-NH-P in 1 mL of phosphate
buffers (50 mM; pH 6.0, 7.0, 9.0, 12.0) at 60 °C, respectively, and the
samples were moved to a 25 °C water bath to incubate for 24 h for
full self-assembly. The formed catalysts were named P6, P7, P9, and
P12. The same method was used to prepare catalysts in NaOH
solution. We also dissolved 1 mg of Fc-CO-NH-P in 1 mL of sodium
hydroxide solution (50 mM; pH 6.0, 7.0, 9.0, 12.0) at 60 °C,
respectively. Then the samples were moved to a 25 °C water bath to
incubate for 24 h for full self-assembly. The formed catalysts were
named N6, N7, N9, and N12.
(
10 μL) of the prepared self-assemblies was put on a copper
grid (200 meshes), excess self-assemblies were removed using a
pipette, and then the samples were air-dried. We then
investigated the morphology of the self-assemblies by TEM
(JEOL 100CX-II, JEOL Ltd., Japan) with an operating voltage
of 80 keV.
Atomic Force Microscopy (AFM). We extracted 100 μL
aliquot of self-assemblies using a pipette, then the samples
were dispersed on a mica sheet, and the samples were dried
using nitrogen purging. The morphology of the self-assemblies
was recorded with an atomic force microscope (AFM, Agilent
5500, Agilent).
To probe the effect of the self-assembly time on the catalytic
activity, we prepared another group of catalysts. In general, 1 mg of
E
Langmuir XXXX, XXX, XXX−XXX
Yang, Xuejiao
