A solution to achieve good reusability of MNPs Fe3O4-supported (S)-diphenylprolinoltrimethylsilyl ether catalysts in asymmetric Michael reactions

Article abstract of DOI:10.1039/c6ra01051b

A new supported (S)-diphenylprolinol trimethylsilyl ether (Fe₃O₄/PVP@SiO₂/ProTMS) using polyvinylpyrrolidone (PVP)-modified MNPs Fe₃O₄ as a support was prepared via one-pot surface-modification, and e

Full text of DOI:10.1039/c6ra01051b

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DOI: 10.1039/C6RA01051B
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A solution to achieve good reusability of MNPs Fe3O4-supported
(S)-Diphenylprolinoltrimethylsilyl ether catalysts in asymmetric
Michael reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Tao Wu,a Dandan Feng,a Bing Xie*b and Xuebing Ma*a
DOI: 10.1039/x0xx00000x
A new supported (S)-diphenylprolinol trimethylsilyl ether (Fe3O4/PVP@ProTMS) using polyvinylpyrrolidone (PVP)-modified
MNPs Fe3O4 as a support was prepared via one-pot surface-modification, and exhibited the high yield (93-99%), excellent
diastereoselectivities (syn/anti = 81–96:19 – 4) and enantioselectivities (95– 98% ee) in the asymmetric Michael addition of
propanal to various nitroalkenes. TGA, XRD, IR, SEM, TEM, elemental analysis and N2 adsorption-desorption isotherm
demonstrated that the adsorption of PVP onto MNPs Fe3O4 resulted in the spectacular change in the chemical composition,
surface morphology and pore structure of Fe3O4/PVP@SiO2/ProTMS. The catalyst could be easily separated from the
reaction by an external magnet and reused for ten times with the high yields (77–99%) and unchanged excellent
steroselectivities (97–98% ee and syn/anti = 96/4) in the Michael addition for the first time.
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tion, the magnetic nanoparticles (MNPs) of Fe3O4 had emerged
recently as a new type of catalyst support in heterogeneous asy-
mmetric organocatalysis with good catalytic performances, esp-
Introduction
ecially in Aldol addition.10 However, in the asymmetric Micha-
el addition of aldehyde to nitroalkene, all the reported MNPs
Fe3O4-supported Jørgensen–Hayashi catalysts gave the unsatis-
factory reusability. In 2010, W. Wang et al. developed Fe3O4@
SiO2-supported Jørgensen–Hayashi catalyst, whose yield and
stereoselectivity significantly decreased to 54%, syn/anti = 83:
17 and 85% ee in the third run.11 One year later, A. Pericás et al.
immobilized Jørgensen–Hayashi catalyst onto azide functiona-
lized MNPs Fe3O4@SiO2. Unfortunately, the similar sharp drop
in catalytic performance was found in the fourth run (57%, syn/
anti = 72:28 and 92% ee).12 Recently, A. Ouali et al. also repor-
ted on a poly(benzylazide)styrene-functionalized magnetic Co/
C-supported (S)-α, α-diphenylprolinoltrimethylsilyl ether with a
sharp drop in the activity in the 3rd run.13 Untill now, the good
reusability of MNPs-supported Jørgensen–Hayashi catalyst
could not be achieved in the asymmetric Michael addition of
aldehyde to nitroalkene.
The Jørgensen–Hayashi organocatalysts, i.e., α, α-diarylprolin-
oltrimethylsilyl ethers,1 which activate aldehyde via an enami-
2
ne and α, β-unsaturated carbonyl compound via an iminium
ion,3 can be considered as one of the most famous leading act-
ors among the versatile organocatalysts in the asymmetric reac-
tions such as Aldol, Michael, cycloaddition, epoxidation and
desymmetrization.4 Moreover, Jørgensen–Hayashi catalysts had
been utilized in asymmetric tandem and multi-component react-
ions, through which large quantities of enantiopure compounds
with multiple stereogenic carbon centers can be conveniently
constructed.5 However, the usage of organocatalyst, typically
10‒30 mol%, was needed to achieve desirable catalytic perfor-
mance in most cases of organocatalyzed reactions. From the
viewpoint of green chemistry, several efficient and economical
protocols using polymer,6 inorganic material,7 ionic liquid 8 and
graphene oxide 9 as catalyst supports were developed to achieve
the reuse of expensive Jørgensen–Hayashi catalysts.
On the other hand, due to the potential advantages of inexp-
ensive, non-toxic, chemically stable properties and re-collection
by simple magnetic decantation without filtration or centrifuga-
In our attempts to solve the reusability of MNPs-supported
Jϕrgensen-Hayashi catalyst, we report the supporting of (S) -α,
α-diphenylprolinoltrimethylsilyl ether onto polyvinylpyrrolido-
ne (PVP)-modified MNPs of Fe3O4 through the hydrolysis of –
Si(OCH3)3 by the facile one-pot surface-modification (Scheme
1). Due to the vital function of PVP, the as-synthesized Fe3O4/
PVP@ProTMS possessed the excellent catalytic performances
a.Key Laboratory of Applied Chemistry of Chongqing Municipality, School of
Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715,
P. R. China. E-mail: zcj123@swu.edu.cn; Fax: +86 2368253237; Tel: +86 23
68253237.
(93‒99% yield, syn/anti = 81‒96:19‒4, 95‒98 %ee) and good
b.School of Chemistry and environmental science, Guizhou Minzhu University,
Guiyang, 550025, P. R. China. E-mail: bing_xie1963@hotmail.com;. Fax: (+86)851-
3610278; Tel: (+86)851-3610278.
reusability with the unchanged excellent steroselectivities (syn/
anti = 96:4, 98% ee) and moderate yield (82%), even in the 10th
run.
Electronic Supplementary Information (ESI) available: TGA, TEM and N2 adsorption-
desorption isotherm of catalysts, 1H, 13C NMR and HPLC spectra of intermediates
and products. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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DOI: 10.1039/C6RA01051B
Scheme 2. The synthetic route to the Jørgensen–Hayashi organocatalyst 5.
Scheme 1 The one-pot preparation of PVP-modified MNPs Fe3O4-supported
Jørgensen–Hayashi organocatalysts
ganic layers were dried over anhydrous Na2SO4 and concentra-
ted under reduced pressure. The residue was purified by silica
gel gradient column chromatography using petroleum ether/eth-
yl acetate (v/v = 4/1 to 1/2) as eluents to afford yellow oil
1
(2.5
1.47 (m,
3.73 (m, 2H,
4.53 (m, 3H, OCH,
OCH2), 5.24 (d, J = 10.8 Hz, 1H, =CH2), 5.74 (d, J = 17.6 Hz,
1H, =CH2), 6.70 (dd, J = 10.9, 17.6 Hz, 1H, =CH), 7.26 7.39
Experimental
1
g, 80%). H NMR (600 MHz, CDCl3, TMS) δ 1.42
9H, CH3), 2.10 2.46 (m, 2H, NCHCH2), 3.51
NCH2), 4.17 4.17 (m, 1H, NCH), 4.36
‒
‒
‒
Characterization
‒
‒
1H and 13C NMR spectra were conducted using a Bruker av-600
NMR instruments, in which all chemical shifts were report-
ed down-field in ppm relative to the hydrogen and carbon
resonances of TMS. FT-IR spectroscopy was performed on a
Perkin-Elmer model GX spectrometer using the KBr pellet.
Thermogravimetry-differential thermal analysis was carried out
on a SBTQ600 thermal analyzer at a heating rate of 10 °C min
−1 from 40 to 800 °C using N2 as protective gas (100 mL min−1).
Elemental analysis was performed using a vario Micro cube
elemental analyzer instrument. The surface morphologies of the
samples were observed by JSM-6510LV scanning electron mi-
croscopy and Tecnai G2 F20 transmission electron microscope,
operated at 20 Kv/15mA and 200 kV respectively. X-ray powd-
er diffraction patterns were detected on an XRD-7000 S/L instr-
ument: Cu-Kα radiation, X-ray tube settings of 40 kV/30 mA
and a step size of 2° min−1 in the 10–100° (2θ) range. N2 adsor-
ption–desorption isotherm was carried out at 77.4 K using an
Autosorb-1 apparatus (Quantachrome), in which the sample
was degassed at 120 °C for 12 h before measurement, and the
specific surface area and the pore diameter were calculated by
the BET method and BJH model, respectively. Magnetization
curve was determined by HH-15 vibrating sample magneto-
meter. The syn/anti ratios and % ee values of products were de-
termined by 1H NMR and Agilent LC-1200 HPLC using Daicel
Chiralpak OD-H 4.6 mm × 25 cm column under 20 °C, 220 nm
and 1.0 mL min−1 conditions, respectively.
‒
(m, 4H, Ar-H), 8.90 (s, 1H, COOH) ppm; 13C NMR (151 MHz,
CDCl3): δ 28.2 (CH3), 36.7 (NCHCH2), 52.0 (NCH2), 57.9
(NCH), 71.0, 75.9, 80.7 (OCH2, OCH and OC(CH3)3), 114.0
(=CH2), 126.3, 127.7, 127.8, 136.4, 137.2 (Ph, =CH), 153.9
(NC=O), 178.2 (C=O) ppm.
Synthesis of compound 2: The reaction mixture of 1 (1.0 g,
2.88 mmol) and K2CO3 (1.59 g, 11.53 mmol) in 10 mL of DMF
was added CH3I (0.49 g, 3.47 mmol) and stirred at room tem-
perature for 1 h. Subsequently, after being quenched by water
(50 mL), the resulting mixture was extracted with ethyl acetate
(50 mL × 3). The combined organic layers were washed with
water (50 mL × 4), dried over anhydrous Na2SO4 and concen-
trated under reduced pressure to afford the yellow oil
2
(0.99 g,
1.47 (m, 9H,
3.73 (m, 2H, NCH2),
4.53 (m, 3H, OCH, OCH2),
5.24 (d, J = 10.8 Hz, 1H, =CH2), 5.74 (d, J = 17.6 Hz, 1H,
=CH2), 6.70 (dd, J = 10.9, 17.6 Hz, 1H, =CH), 7.26 7.39 (m,
1
95%). H NMR (600 MHz, CDCl3, TMS) δ 1.42
CH3), 2.10 2.46 (m, 2H, NCHCH2), 3.51
4.17 4.17 (m, 1H, NCH), 4.36
‒
‒
‒
‒
‒
‒
4H, Ar-H), 8.90 (s, 1H, COOH) ppm; 13C NMR (151 MHz,
CDCl3): δ 28.2 (CH3), 36.7 (NCHCH2), 51.3 (NCH2), 57.9
(NCH), 71.0, 75.9, 80.7 (OCH2, OCH and OC(CH3)3), 114.0
(=CH2), 126.3, 127.7, 127.8, 136.4, 137.2 (Ph, =CH), 153.9
(NC=O), 178.2 (C=O) ppm.
Synthesis of compound 3
charged with magnesium chips (1.08 g, 45.0 mmol), bromoben-
zene (4.71 g 30.0 mmol) and anhydrous THF (45 mL), reflu-
xed for 3 h and cooled to room temperature. To the obtained re-
action mixture was added anhydrous THF solution containing
:
A N
2-filled three-necked flask was
Synthesis of Jørgensen–Hayashi catalyst
5
，
Synthesis of compound 1: To a anhydrous THF (20 mL) sus-
pension of NaH (0.75 g, 18.8 mmol) was added in turn 30 mL
of THF solution containing Boc-L-hydroxyproline (2.0 g, 8.7
mmol), 18-crown-6 (0.23 g, 0.86 mmol) and 4-vinylbenzyl chl-
oride (90 %, 3.66 g, 21.6 mmol) at room temperature. The rea-
ction mixture was warmed to 50 °C for 12 h. The resulting mix-
ture was added water (30 mL), extracted with cyclohexane (50
mL × 2) to remove residual 4-vinylbenzyl chloride, adjusted to
2
(30 mL, 3.0 g, 8.3 mmol) dropwise at 0 °C and stirred at room
temperature for 3 h. After being quenched with aqueous NH4Cl
solution (50 mL), the reaction mixture was extracted with dieth-
yl ether (100 mL × 3). The combined organic phases were dried
over anhydrous Na2SO4 and concentrated to afford the residue,
which was purified by silica gel column chromatography using
petroleum ether/ethyl acetate (v/v = 50/1 to 20/1, 10/1 and 1/1)
pH in the range of 2‒3 by saturated aqueous KHSO4 solution
and extracted with ethyl acetate (50 mL × 3). The combined or-
as eluents to afford the white powder
3
(0.75 g, 77%); 1H NMR
2 | J. Name., 2012, 00, 1-3
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(600 MHz, CDCl3, TMS) δ 1.38 (s, 9H, CH3), 2.08
2H, NCHCH2), 2.86 (s, 1H, NCH), 3.35
3.52 (m, 2H, NCH2), (10.0 mL, 25 g L-1). After being stirred atDrOoIo:m10.t1e0m39p/Cer6aRtAu0r1e05fo1Br
4.28 4.34 (m, 2H, OCH2), 5.02 5.05 (m, 1H, OCH), 5.22 (d, J 12 h, the reaction mixture was added acetone (250 mL) and stir-
= 10.9 Hz, 1H, =CH2), 5.72 (d, J = 17.6 Hz, 1H, =CH2), 6.69 red for 10 min. The MNPs Fe3O4/PVP were obtained by magn-
‒2.18 (m, mL) and added aqueous polyvinylpyrrolidone (PVViePw)ArtsicoleluOtniloinne
‒
‒
‒
(dd, J = 10.9, 17.6 Hz, 1H, =CH), 7.16‒7.49 (m, 14H, Ar-H) etic decantation, washed with ethanol (50 mL × 2) and dried
ppm; 13C NMR (151 MHz, CDCl3): δ 28.2 (CH3), 36.5 under vacuum at 55 °C for 24 h. Anal. Calcd for Fe3O4/PVP
(NCHCH2), 52.9 (NCH2), 65.1 (NCH), 70.6, 76.4, 80.7 (OCH2, Found: C, 0.01; H, 0.23; N, 0.05.
OCH, OC(CH3)3), 81.6 (COH), 113.8 (=CH2), 126.2, 127.1,
Preparation of supported Jørgensen–Hayashi catalysts
Fe3O4@SiO2/ProTMS and Fe3O4/PVP@SiO2/ProTMS
127.2, 127.5, 127.8, 127.9, 136.6, 137.1, 137.5 (Ph, =CH),
145.7 (NC=O) ppm.
Synthesis of compound 4
:
The reaction mixture of
3
(1.47 g, A mixture of 3-mercaptotrimethoxysilane (MPTMS, 0.25 mL,
(0.23 g, 0.5
3.04 mmol) and KOH (1.68 g, 0.03 mol) in the mixed MeOH/ 1.14 mmol), AIBN (41.0 mg, 0.25 mmol) and
5
DMSO (20 mL, v/v = 2/7) was stirred at 60 °C for 6 h, added mmol) in 15 mL of CHCl3 was stirred at 80 °C for 12 h under a
water (20 mL) and extracted with cyclohexane (20 mL × 5). N2 atmosphere and the solvent was removed under reduced pre-
The combined organic layer was dried over anhydrous Na2SO4 ssure. The residues containing MPTMS and ProTMS were add-
and concentrated under reduced pressure to afford white solid
4
ed slowly to ethanol solution containing Fe3O4 or Fe3O4/PVP
1.68 (25 mL, 125.0 mg) pretreated with aqueous ammonia (0.75 mL,
1.83 (m, 1H, NCHCH2), 3.09 (s, 2H, 23%) and tetraethyl orthosilicate (0.13 mL, TEOS) for 2 h. Aft-
4.02 (m, 1H, NCH), 4.38 4.50 (m, 2H, OCH2), er the resulting mixture was stirred at room temperature for 24
1
(1.08 g, 92%). H NMR (600 MHz, CDCl3, TMS) δ1.64
(m, 1H, NCHCH2), 1.76
NCH2), 4.01
‒
‒
‒
‒
4.57 (dd, J = 9.6, 6.7 Hz, 1H, OCH), 5.23 (d, J = 10.9 Hz, 1H, h, the pale yellow MNPs-supported Jørgensen–Hayashi Fe3O4
=CH2), 5.73 (d, J = 17.6 Hz, 1H, =CH2), 6.70 (dd, J = 10.9, @SiO2/ProTMS (0.46 g) and Fe3O4/PVP@SiO2/ProTMS (0.57
17.6 Hz, 1H, =CH), 7.13‒
7.57 (m, 14H, Ar-H) ppm; 13C NMR g) catalysts were separated magnetically, washed with CHCl3
(151 MHz, CDCl3): δ 33.0 (NCHCH2), 52.4 (NCH2), 63.4 (25 mL × 3), ethanol (25 mL × 3) and dried under vacuum for 4
(NCH), 70.5, 76.9, 79.5 (OCH2, OCH and OC(CH3)3), 113.8 h. Anal. Calcd for Fe3O4@SiO2/ProTMS Found: C, 31.51; H,
(=CH2), 125.4, 126.0, 126.3, 126.4, 126.6, 127.8, 128.0, 128.3, 4.10; N, 1.02; S, 6.15 and Fe3O4/PVP@SiO2/ProTMS Found: C,
136.5, 137.1, 137.9, 144.9, 147.3 (Ph, =CH) ppm.
Synthesis of compound 5 Triethylamine (0.19 g, 1.9 mmol)
and trimethylsilyl trifluoromethanesulfonate (TMSOTf, 0.42 g,
31.56; H, 3.22; N, 0.90; S, 5.42.
:
General procedure for the Michael reaction
1.9 mmol) were added in turn at 0 °C to anhydrous CH2Cl2 so- A reaction mixture of (E)-nitrostyrene (40.0 mg, 0.27 mmol),
lution containing (15 mL, 0.39 g, 1.0 mmol) under a N2 atmo- Fe3O4/PVP@SiO2/ProTMS (30.0 mg, 7 mol%) and CHCl3 (2.0
4
sphere. After being stirred at room temperature for 3 h, the re- mL) was stirred at 0°C for 10 min, added propanal (94.0 mg,
sulting mixture was quenched with water (20 mL) and extracted 1.62 mmol) by syringe and monitored by TLC until completion.
with CH2Cl2 (20 mL × 3). The combined organic phases were The catalyst was separated using an external magnetic field,
dried over anhydrous Na2SO4 and concentrated under reduced washed with CHCl3 (2.0 mL × 3) and reused directly for the
pressure. The crude product was purified by silica gel column next catalytic cycle. The combined organic layers were concen-
chromatography using petroleum ether/ethyl acetate (v/v = 20/1 trated and purified by silica gel column chromatography using
1
to 10/1) as eluents to afford yellow oil
NMR (600 MHz, CDCl3, TMS) δ-0.19 (s, 9H, CH3), 1.59
(m, 2H, NCHCH2), 1.99 (s, 1H, NH), 2.73 (dd, J = 4.9, 11.6 Hz, tioselectivities (% ee) of products were determined by H NMR
1H, NCH2), 2.90 (dd, J = 2.1, 11.6 Hz, 1H, NCH2), 3.69 3.71 and HPLC on a chiral stationary phase (Daicel Chiralpak OD-H,
(m, 1H, NCH), 4.27 4.38 (m, 3H, OCH, OCH2), 5.13 (d, J = AD-H, AS-H or OJ-H, 4.6 mm × 25 cm column, see ESI†), res-
11.0 Hz, 1H, =CH2), 5.64 (d, J = 17.6 Hz, 1H, =CH2), 6.61 (dd, pectively.
J = 10.9, 17.6 Hz, 1H, =CH), 7.10 7.39 (m, 14H, Ar-H) ppm;
5
(0.36 g, 78%); H petroleum ether/ethyl acetate (v/v = 5/1) as eluents to afford the
‒
1.66 Michael adduct. The diastereoselectivities (syn/anti) and enan-
1
‒
‒
‒
13C NMR (151 MHz, CDCl3): δ 2.1 (CH3), 34.3 (NCHCH2),
52.9 (NCH2), 63.7 (NCH), 70.5, 79.2, 82.9 (OCH2, OCH,
OC(CH3)3), 113.7 (=CH2), 126.2, 126.8, 126.9, 127.4, 127.5,
127.6, 127.8, 128.4, 136.5, 136.9, 138.1, 145.3, 146.6 (Ph, =CH)
ppm.
Results and discussion
Preparation of MNPs-supported organocatalyst
As described in Scheme 2, Jørgensen–Hayashi catalyst
5 could
be synthesized using Boc-L-hydroxyproline as starting material
with the moderate to good yields in each step (77–95%).14 The
Preparation of MNPs Fe3O4 and Fe3O4/PVP
FeCl3·6H2O (11.0 g, 40.7 mmol) and FeCl2·4H2O (4.0 g, 20.4
mmol) in water (50 mL) were mixed well at room temperature
under argon. The reaction mixture was heated to 85 °C and then
TEM image showed that the spherical MNPs of Fe
3
O with the
4
20–30 nm dimeters could be generated at a pH of 8–9 by simple
co-precipitation of FeCl3 and FeCl2 with molar ratio of 2 in the
presence of aqueous ammonia (23%). After being modified for
12 h at room temperature in aqueous PVP solution, TEM image
added aqueous ammonia (23%) dropwise to a pH of 8‒9. After
being aged at 85 °C for 4 h, the MNPs Fe3O4 were separated by
magnetic decantation and washed with water to pH = 7. Subse-
quently, MNPs Fe3O4 (4.0 g) were well-dispersed in water (50
revealed that MNPs Fe
3
O /PVP had no significant change in
4
the surface morphology and particle diameter. According to the
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the content of nitrogen (0.05%) detected by elemental analysis, C-S and Si-C bonds at 1249 and 702 cm−1, 17 (3) the various C–
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the loading of PVP onto the backbone of Fe
to be very low (about 0.4%).
3O
4
was calculated H stretching vibrations and characteristicDpOhI:e1n0y.1l03r9in/Cg6RatA031005516B,
3025, 2930 cm−1 and in the 1632–1448 cm−1 range, respectively.
As described in Scheme 1, the Jørgensen–Hayashi catalyst After being modified by PVP (0.4%), there was no significant
5
attached to ProTMS could be immobilized by one-pot to pre- difference in IR spectra between Fe3O4/PVP and bare Fe3O4.
pare the supported organocatalysts Fe3O4@SiO2/ProTMS and Furthermore, all Fe3O4/PVP@SiO2/ProTMS and Fe3O4@SiO2/
Fe3O4/PVP@SiO2/ProTMS through the radical addition of sulf- ProTMS had a strong stretching vibration of C-O bond at 1134
ydryl (-SH) to C=C double bond in
5
and subsequent surface- cm−1, which implied that a large amount of –SiOCH3 moieties
modification taking advantage of the hydrolysis of Si(OCH3)3 existed on the surface of Fe3O4/PVP@SiO2/ProTMS and Fe3O4
on the surface of MNPs Fe3O4 and Fe3O4/PVP. Based to the @SiO2/ProTMS.
nitrogen contents by elemental analysis (1.02% and 0.90%), the
loading capacities of Jørgensen–Hayashi catalyst
5 in Fe3O4@
SiO2/ProTMS and Fe3O4/PVP@SiO2/ProTMS were calculated
to be 0.73 mmol g-1 and 0.64 mmol g-1, respectively. Moreover,
the loadings of MPTMS onto Fe3O4@SiO2/ProTMS and Fe3O4/
PVP@SiO2/ProTMS were also determined by elemental analy-
sis of sulphur (6.15% and 5.42%) to be 1.92 mmol g-1 and 1.69
mmol g-1. Owing to the partial radical addition of sulfydryl (-
SH) to C=C double bond in 5, the free MPTMS were found to
be 1.19 mmol g-1 and 1.05 mmol g-1. Based on the loaded
MPTMS and ProTMS, it was found that Fe3O4@SiO2/ProTMS
and Fe3O4/PVP@SiO2/ProTMS possessed the same molar ratio
(1.6/1) of free MPTMS to ProTMS. Furthermore, the superpa-
ramagnetic behavior of Fe3O4@SiO2/ProTMS (21.5 emu g-1)
and Fe3O4/PVP@SiO2/ProTMS (10.4 emu g-1) at 298 K was
evidenced by the zero coercivity and resonance of each magne-
tization loop in the magnetization curves (Fig.1).
Fig.2 IR spectra of Fe3O4 MNPs (a), Fe3O4/PVP MNPs (b), Jørgensen–Hayashi
catalyst 5 (c), Fe3O4/PVP@SiO2/ProTMS (d) and Fe3O4@SiO2/ProTMS (e)
Fig.1 Magnetization curves for Fe3O4/PVP (a), Fe3O4@SiO2/ProTMS (b), 10th-
recycled Fe3O4/PVP@SiO2/ProTMS (c) and Fe3O4/PVP@SiO2/ProTMS (d).
The role of polyvinylpyrrolidone (PVP)
Poly(vinylpyrrolidone) (PVP, K29‒32), amphiphilic and noni-
onic polymer, was widely used in science and technology, and
could be adsorbed onto a broad range of different materials as
stabilizing agent.15 Herein, we endeavoured to elucidate where
PVP was adsorbed onto the backbone of MNPs Fe3O4 and what
changes in chemical and physical properties were resulted from
PVP.
Fig.3 Thermogravimetric curves of Fe3O4@SiO2/ProTMS (a), 10th-recycled
Fe3O4/PVP@SiO2/ProTMS (b) and Fe3O4/PVP@SiO2/ProTMS (c).
The effect of PVP on the hydrolysis degree of -Si(OCH3)3
attached to ProTMS and MPTMS onto MNPs Fe3O4/PVP and
Fe3O4 could be further monitored by TGA. From the TG and
DTG curves (Fig. 3), the thermal decomposition of organic
moieties in Fe3O4/PVP@SiO2/ProTMS occurred in two distinct
steps: the first one in the temperature 200–500 °C range and the
The covalent attachment of Jørgensen–Hayashi catalyst
5 to
the backbone of Fe3O4 MNPs is clearly corroborated by FT-IR
spectroscopy (Fig. 2), which confirmed by (1) the asymmetrical,
symmetrical stretching vibration and flexural vibration of Si-O-
Si at 1067, 807 and 459 cm−1, 16 (2) the stretching vibrations of
4 | J. Name., 2012, 00, 1-3
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second one in the 500–600 °C range, accompanied by a conti-
nuous endothermic peak (see ESI†). It is noteworthy that Fe3O4
/PVP@SiO2/ProTMS exhibited the 15.2% higher weight loss
than Fe3O4@SiO2/ProTMS in the temperature range of 200–
600 °C, although Fe3O4/PVP@SiO2/ProTMS contained a lower
View Article Online
DOI: 10.1039/C6RA01051B
amount of Jørgensen–Hayashi catalyst 5. Another phenomenon
was that a sharp weight loss of Fe3O4/PVP@SiO2/ProTMS
(30.01%) was newly emerged in the 500–600 °C range (Fig. 3c).
Based on the two points mentioned above, it was demonstrated
that the hydrolysis efficiency of -Si(OCH3)3 decreased upon the
attachment of ProTMS and MPTMS onto Fe3O4/PVP, and resu-
lted in the increased SiOCH3 moiety due to the package of PVP,
which was in accord with the stronger C-O stretching vibration
at 1134 cm−1 in IR spectra of Fe3O4/PVP@SiO2/ProTMS and
verified by the weakened C-O stretching vibration of 10th-recy-
cled Fe3O4/PVP@SiO2/ProTMS. Furthermore, besides the same
typical peaks of Fe3O4 at 30.3º, 35.6º, 43.3º, 53.8º, 57.4º and
63.0º in the XRD pattern, Fe3O4@SiO2/ProTMS and Fe3O4/
PVP@SiO2/ProTMS also displayed the difference in the typical
peak of SiO2 in the 16–25 ° range (Fig. 4a, b), which was close-
ly evidenced to the hydrolysis degree of -Si(OCH3)3.
Fig.5 The surface morphologies for TEM of MNPs Fe3O4 (a), SEM of Fe3O4@
SiO2/ProTMS (b), TEM of Fe3O4@SiO2/ProTMS (c, d), TEM of MNPs Fe3O4/PVP (e),
SEM of Fe3O4/PVP@SiO2/ProTMS (f), TEM of of Fe3O4/PVP@SiO2/ProTMS (g, h).
(10.0 mL, 25 g L-1) for 12 h, it was found that MNPs Fe3O4/
PVP possessed the higher BET-specific surface area (65.8 m2 g-
1), average pore diameter (11.7 nm) and pore volume (0.39 cc g-
1) than bare Fe3O4 (7.9 m2 g-1, 5.2 nm and 0.021cc g-1). Espe-
cially, Fe3O4/PVP had five characteristic cavities centered at
0.9, 1.4, 2.1, 3.4 and 6.6 nm, accompanied by some additional
pores in the different sizes with the nitrogen desorption (Dv) (<
1.0
×
10-3 mL Å-1 g-1) (Fig. 6c), whereas bare MNPs Fe3O4
displayed a broad pore size distribution (Fig. 6a). Interestingly,
except for these five characteristic cavities, all the other pores
in Fe3O4/PVP disappeared upon the immobilization of ProTMS
and MPTMS through the hydrolysis of -Si(OCH3)3, which indi-
cated that Jørgensen–Hayashi catalyst
5 was entrapped inside
these disappeared pores. To our delight, the remained four char-
acteristic pores of Fe3O4/PVP@SiO2/ProTMS with the high Dv
values (> 1.0
×
10-3 mL Å-1 g-1) provided the crucial channels
for reactants to access catalytic sites in catalytic process. Unfor-
tunately, Fe3O4@SiO2/ProTMS exhibited the non-uniform pore
size distribution in the 1–10 nm range without the modification
of PVP.
Fig.4 Powder X-ray diffraction patterns of Fe3O4/PVP@SiO2/ProTMS (a), Fe3O4@
SiO2/ProTMS (b), 10th-recycled Fe3O4/PVP@SiO2/ProTMS (c) and Fe3O4 MNPs (d).
The presence of PVP not only altered the chemical compo-
sition but also affected the morphology of MNPs-supported cat-
alyst. The grafting of Jørgensen–Hayashi catalyst
5 onto MNPs
Fe3O4 and Fe3O4/PVP led to a serious agglomeration due to the
coated SiO2 generated from the hydrolysis of -Si(OCH3)3 moie-
ty. However, seen from the SEM images, Fe3O4/PVP@SiO2/
ProTMS exhibited the smaller even-textured and granular parti-
cles with the diameters ranging from 0.2 to 0.3 um (Fig. 5f),
compared with irregular Fe3O4@SiO2/ProTMS (Fig. 5b). The
further investigation by the TEM image showed the densely
agminated Fe3O4@SiO2/ProTMS (Fig. 5d) and nano-structured
Fe3O4/PVP@SiO2/ProTMS (Fig. 5h) in an accumulative state
upon being coated with SiO2 shell, respectively due to the pro-
found and weakened hydrolysis of -Si(OCH3)3 in the absence or
presence of PVP.
The nitrogen adsorption–desorption isotherm plots of all sa-
mples, obtained at 77 K, were shown in ESI†. After being stirr-
ed at room temperature polyvinylpyrrolidone (PVP) solution
Fig.6 Pore size distributions (PSDs) of Fe3O4 (a), Fe3O4@SiO2/ProTMS (b),
Fe3O4/PVP (c) and Fe3O4/PVP@SiO2/ProTMS (d).
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ARTICLE
Journal Name
Table 2 Asymmetric Michael addition of propanal to various nitroalkenes catalyzed by
View Article Online
DOI: 10.1039/C6RA01051B
Fe3O4/PVP@SiO2/ProTMS a
Asymmetric Michael addition
To evaluate influence of PVP on the catalytic performance and
reusability of Fe3O4@SiO2/ProTMS and Fe3O4/PVP@SiO2/
ProTMS, the asymmetric Michael addition of propanal to trans-
β-nitrostyrene was selected as a model reaction.
Entry t (h) Product
Yield (%)b
98
Syn/antic % eed
In the Michael addition of propanal to trans-β-nitrostyrene,
the corresponding syn-adducts, whose absolute stereochemistry
was determined to be 2R, 3S-configurations by comparing its
optical rotation and HPLC spectra with literature values,18 were
obtained as major steroisomers in the high yields and excellent
enantioselectivities (Table 1). To the best of our knowledge, it
was found that Fe3O4/PVP@SiO2/ProTMS appeared as the
most efficient MNPs–supported Jørgensen–Hayashi catalyst to
date in terms of catalytic activity, steroselectivity and reusabi-
lity among the ever-reported Michael additions.11–-13 Meanwh-
ile, Fe3O4 /PVP@SiO2/ProTMS (99%, syn/anti = 92/8, 98% ee)
presented the superior catalytic performances to Fe3O4@SiO2/
ProTMS (89%, syn/ anti = 89/11 and 95% ee) (entry 18), which
might be rationalized in terms of catalytic accessibility and ster-
ic confinement owing the above-mentioned structural texture
and morphology resulted from the vital function of PVP. In the
following catalytic reactions, Fe3O4/PVP@SiO2/ProTMS was
chosen for the further optimization of experimental conditions.
1
2
8
7
88/12
87/13
97
97
99
95
3
9
86/14
96
4
5
24
48
98
99
88/12
92/8
96
97
Table 1 Screening of reaction conditions of Fe3O4/PVP@SiO2/ProTMS for the Michael
addition of propanal to trans-β-nitrostyrene a
6
7
12
48
97
93
89/11
92/8
96
96
Entry Catalyst
(mg/mol%)
30/7
Addictive
Solvent T/h
Yield Syn/antic ee
(°C / h) (%)b
(%)d
97
n.d.
97
97
98
97
95
96
n.d.
97
98
97
98
96
97
98
98
97
1
2
3
4
5
6
7
8
9
-
CHCl3 0/17
CHCl3 0/8
CHCl3 0/12
CHCl3 0/12
99
89/11
30/7
30/7
30/7
30/7
30/7
30/7
30/7
30/7
TFA
PhCO2H
2-FPhCO2H
3-NO2PhCO2H CHCl3 0/8
3-NO2PhCO2He CHCl3 0/24
3-NO2PhCO2H C6H5CH3 0/8
3-NO2PhCO2H CH2Cl2 0/8
3-NO2PhCO2H EtOH 0/8
trace n.d.
93
95
99
93
87
94
92/8
90/10
96/4
93/7
88/12
90/10
trace n.d.
8
9
12
48
98
95
89/11
90/10
96
97
10 60/14
11 60/14
12 15/3.5
13 15/3.5
14 30/7
15 30/7
16 30/7
17 30/7
18f 26/7
-
CHCl3 0/8
3-NO2PhCO2H CHCl3 0/4
CHCl3 0/32
98
98
76
96
99
99
91/9
91/9
92/8
89/11
87/13
90/10
96/4
98/2
93/7
-
3-NO2PhCO2H CHCl3 0/22
3-NO2PhCO2H CHCl3 20/5
3-NO2PhCO2H CHCl3 10/7
3-NO2PhCO2H CHCl3 -10/20 86
3-NO2PhCO2H CHCl3 -20/32 82
3-NO2PhCO2H CHCl3 0/8
89
10 11
98
81/19
95
a Reaction conditions: trans-β-Nitrostyrene (40 mg, 0.27 mmol), propanal (94 mg, 1.62
mmol), additive (10 mol%), solvent (2.0 mL). b Isolated yield. c Determined by 1H NMR. d
Determined by chrial HPLC. e 20 mol% of additive. f Fe3O4@SiO2/ProTMS used.
Firstly, it was found that the acid strengths of brøsted acids
such as TFA (pKa = 0.23), 3-NO2C6H4CO2H (pKa = 2.45), 2-
FC6H4CO2H (pKa = 3.27) and C6H4CO2H (pKa = 4.21) had a
6 | J. Name., 2012, 00, 1-3
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was completed, an external magnet was put nearby the reaction
View Article Online
DOI: 10.1039/C6RA01051B
flask to concentrate Fe3O4/PVP@SiO2/ProTMS, and the solu-
11 34
97
86/14
96/4
97
98
tion could be easily decanted. The recovered catalyst was direc-
tly reused for the next run after being washed with CHCl3 (2.0
mL × 3).
Fig. 7 showed the comparative reusabilities of Fe3O4/PVP@
SiO2/ProTMS and Fe3O4@SiO2/ProTMS including the yields
and steroselectivities. From Fig.6, Fe3O4/PVP@SiO2/ProTMS
displayed a better reusability than Fe3O4@SiO2/ProTMS. Unfo-
rtunately, the decreasing yields and diasteroselectivity cataly-
zed by Fe3O4@SiO2/ProTMS after the second run like the ever-
reported Jørgensen–Hayashi catalysts using MNPs as a supp-
ort.11–-13 On the contrary, the yields catalyzed by Fe3O4/PVP@
SiO2/ProTMS remained at the very high level (>94%) in the
first five cycles. Even in 10th run, the 77% yield could be achie-
ved after prolonging the reaction time to 48 h. More excitedly,
the constant excellent stereoselectivity (98% ee and 96/4 dr)
could be maintained in the ten consecutive cycles. To the best
of our knowledge, Fe3O4/PVP@ProTMS exhibited the most
effective reusability among the reported MNPs-supported
Jørgensen–Hayashi catalysts.
12
8
>99
a Reaction conditions: Nitrolefin (40 mg, 0.27 mmol), propanal (94 mg, 1.62 mmol), 3-
NO2C6H4CO2H (10 mol%), CHCl3 (2.0 mL), 0°C. b Isolated yield. c Determined by 1H NMR.
d Determined by chrial HPLC.
great effect on catalytic performances (entries 2-6). Disappoint-
edly, TFA with the stronger acid strength only afforded a trace
of Michael adduct (entry 2). Fortunately, the other weak brøst-
ed acids could effectively improve catalytic rate and stereosele-
ctivity. Among them, 3-NO2C6H4CO2H (10 mol%) was proved
to be the best effective co-catalyst with the excellent performa-
nces (99%, syn/anti = 96/4, 98% ee, entry 5). It was worthwhile
to note that Fe3O4/PVP@SiO2/ProTMS gave the less catalytic
performances (93%, syn/anti = 93/7, 97% ee, entry 6) after 24 h
upon doubling the dosage of 3-NO2C6H5CO2H. Next, the
effects of temperature, solvent and used amount of catalyst on
catalytic performance were further screened out in detail. Our
attempts to further reduce catalyst loading led to the decreased
yield and stereoselectivity (entries 12, 13), and ethanol as a
solvent resulted in no activity (entry 9). Furthermore, although
the decrease in temperature to -20 °C led to the product with the
higher steroselectivity (syn/anti = 98/2, 98% ee, entry 17), it
took place at a considerably lower rate, requiring up to 32 h for
the achievement of 82% yield.
The optimized protocol was expanded to a wide variety of
trans-β-nitrostyrenes bearing electron-donating (-CH3, OCH3)
and electron-withdrawing (-X) substituents at the o, m, p-posi-
tions. From Table 2, all trans-β-nitrostyrenes bearing electron-
donating and electron-withdrawing substituents afforded the ex-
cellent catalytic performances at the same level (93–99%, syn/
anti = 86–92:14–8, 96–98% ee, entries 1–9). It was worthwhile
to note that the nitrostyrenes with o, p-OCH3 and o-CH3
electron-donating (entries 5, 7 and 9) and o-Cl electron-with-
drawing substituent (entry 4) required respective 48 h and 24 h
to achieve the 93–99% yields resulted from the electron-donat-
ing effect and vicinal hidrance of the substituents. A nitroalkene
bearing a heterocyclic 2-furyl substituent also afforded a high
98% yield and 95% ee but gave a moderate diasteroselectivity
(syn/anti = 81/19) (entry 10). Additionally, the Michael addi-
tion of propanal to aliphatic nitroalkene gave the good catalytic
performances (97%, 97% ee, 86:14 dr) but needed a prolonged
reaction period (34 h, entry 11).
Fig. 7 The comparative reusability of Fe3O4@SiO2/ProTMS and Fe3O4/PVP@
SiO2/ProTMS under the optimized reaction conditions.
To understand the reason for the decreased yield, the struct-
ural change of the 10th-recycled Fe3O4/PVP@SiO2/ProTMS
was monitored by TEM, TGA, elemental analysis and N2 adso-
rption–desorption isotherm. Above all, the increased carbon,
hydrogen and nitrogen contents, especially the doubled nitrogen
content (0.90%→1.76%) stemmed from nitrogenous compo-
unds such as nitroalkene, elucidated that the reactants and
products were adsorbed on the porous backbone of the catalyst,
which also verified by the decreased average pore size and pore
volume from 15.7 nm and 0.031 cc g-1 to 2.5 nm and 0.021 cc
g-1 respectively. Moreover, the structural change of SiO2 attach-
ed on the core of Fe3O4/PVP was elucidated by XRD and TGA,
in which a wide and round XRD peak of SiO2 in the range of
16–25 ° (Fig. 3b) and a disappeared weight loss in the 500–
600 °C range demonstrated the further hydrolysis of -SiOCH3.
Meanwhile, it was found that the 10th-recycled Fe3O4/PVP@
SiO2/ProTMS maintained its original magnetic property (Fig.
1b) and no agglomeration was observed by TEM (Fig. S10,
ESI†). In conclusion, it was speculated that the adsorbed impu-
Reusability of MNPs-supported catalyst
One of the main advantages associated with the supporting of a
catalyst onto MNPs is the facile separation of a catalyst from
reaction mixture with an external magnet. When the reaction
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Journal Name
Calogero, D. Lanari, M. Orru, O. Piermatti, F. Pizzo and L.
rities and further hydrolysis of -SiOCH3 resulted in the stru-
ctural changes such as pore volume and pore size distribution
and was ultimately responsible for the decreased catalytic
activity of Fe3O4/PVP@SiO2/ProTMS.
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In summary, we had immobilized the homogeneous Jørgensen
–Hayashi catalyst of (S)-diphenylprolinoltrimethylsilyl ether on-
to the backbone of Fe3O4/PVP via facile one-pot surface-modi-
fication, and successfully applied the MNPs Fe3O4-supported
catalyst Fe3O4 /PVP@SiO2/ProTMS in the asymmetric Michael
addition of propanal to various nitroalkenes with excellent cat-
alytic performances. Due to the adsorbed poly (vinylpyrrolido-
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chemical composition, surface morphology and pore structure
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Table of contents entry
PVP-modified MNPs Fe3O4-supported Jøgensen-Hayashi catalyst provided suitable
microenvironments to achieve the good reusability with the high yields and unchangeable
excellent steroselectivities in the asymmetric Michael addition of propanal to nitroalkenes.