L-Proline-modified magnetic nanoparticles (LPMNP): A novel magnetically separable organocatalyst

Article abstract of DOI:10.1039/c4ra01121j

This study offers a new and efficient method for the stabilization of l-proline moieties on magnetic nanoparticles in order to prepare a novel magnetic recyclable organocatalyst for application in organic transformations. The catalytic activity of this he

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ARTICLE TYPE
L-proline-modified magnetic nanoparticles (LPMNP): A novel
magnetically separable organocatalyst
Ali Khalafi-Nezhad,* Maryam Nourisefat and Farhad Panahi*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
other supported and unsupported counterparts. There are
This study offers a new and efficient method for the
stabilization of L-proline moieties on magnetic nanoparticles
50 various reported magnetically separable nanoꢀorganocatalysts
for application in organic transformations.10 On the other
hand, Lꢀproline and its derivatives have shown considerable
catalytic efficiency in different organic transformations.11
There are widespread interest to the development of new
55 heterogeneous organocatalysts based on Lꢀproline.12 In this
communication, in continuing of our previous works on Lꢀ
prolineꢀcatalyzed reactions13,14 we would like to report a new
and easy access to a new magnetic nanoparticleꢀsupported
organocatalyst based on Lꢀproline. Our strategy for the
60 synthesis of Lꢀprolineꢀmodified magnetic nanoparticles
(LPMNPs) is shown in Scheme 1.
in order to prepare
a
novel magnetic recyclable
organocatalyst for application in organic transformations.
10 The catalytic activity of this heterogeneous organocatalyst
was evaluated in the condensation reaction of indoles and
aldehydes for the synthesis of bis(indolyl)methanes in water.
Organocatalysis has become a very considerable area of
research, because it has
emerged
as
a
powerful
15 methodology in organic synthesis.1 As well, organocatalysis
is essentially important since it affords a sustainable way to
convert materials into precious chemicals in a costꢀeffective,
proficient, and environmentally benign methods.2 Moreover, a
range of important reactions have been performed using
20 organocatalyst systems.3 This environmentally benign
methodology has been made even greener by immobilization
of the organocatalysts onto solid supports.4 However, due to
the decreasing in reactant diffusion rate to the surface of
catalyst, the reactivity and selectivity of immobilized
25 organocatalyst on a solid support are frequently decreased.5
Fortunately, the aforementioned problem can be resolved to
some extent by selecting the very small size ranges of the
support. Thus, the resulted reactivity due to the homogeneous
catalyst can be effective in the reaction progress.6
30 Accordingly, because of nanometer sizeꢀrange of
nanoparticles which allow their surface areas to increase
dramatically, these materials are reasonable potential
candidate supports. Also, nanoparticles can disperse in
solution to form emulsion and this action increases the
35 diffusion rate of reactant to the surface of catalyst.
Furthermore, reactants in solution have easy access to the
active sites on the surface of nanoparticles because of their
large surfaceꢀtoꢀvolume ratio.7 Nevertheless, when the size of
support is decreased to nanometer scale, a simple filtration
40 method cannot overcome the big obstacle of catalyst
separation from the reaction media.8 Trying to find a solution
to this problem, the efforts have been focused on magnetic
recyclable supports.9 Due to the magnetic nature of the
catalyst, it could be recovered within a few seconds using an
45 external magnetic field. Most importantly, the isolation of the
final product can be possible by a simple decantation. In
addition to these advantages, such catalysts also showed better
selectivity, and superior stability when compared to their
Si(OMe)3
TEOS
Fe3O4
SiO2
Fe3O4
Fe3O4
SiO2
Fe3O4@SiO2
VMNP
HN
O
H2O2
O
O
OH
OH
O
N
1)
HO
O
O
H
O
O
N
O
O
Fe3O4
SiO2
Boc
O
OH
O
Fe3O4
SiO2
O
O
H
2) H2O
OH
N
O
OH
NH
O
O
O
MNPO
LPMNP
NH
Scheme 1 Synthetic route for the preparation of LPMNP catalyst.
In this study, MNPs were prepared by coꢀdeposition method
65 using a procedure in the literature.15 The synthesized MNPs,
were coated by silica using a solꢀgel process to obtain coreꢀ
shell MNPs (Fe3O4@SiO2).16 The SiO2 layer can prevent
Fe3O4 core from aggregation, lead to simple surface
functionalization. Anchoring the SiO2 layer to the surfaces of
70 Fe3O4 could provide SiꢀOH groups for fine chemical
modification using available alkoxy silane materials [(Rnꢀ
Si(OR)3ꢀn]. Afterward, the Fe3O4@SiO2 substrate was treated
with trimethoxy(vinyl)silane to produce
a vinyl MNP
(VMNP) substrate. Subsequently, VMNP was oxidized using
75 H2O2 to generate the MNPꢀoxiran (MNPO) material. The
VMNP and MNPO materials are the key intermediates in our
strategy. We believed that these materials (especially MNPO)
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can open up a new direction in the synthesis of various
heterogeneous catalysis and adsorption materials (Scheme
2).14
decomposition of grafted Lꢀproline from the silica substrate.
This part of the thermogram reveals the amounts of supported
Lꢀproline on silica which is estimated to be ~6% (w/w). The
reduction in the weight percentage of the catalyst at ~300–360
40 °C is related to decomposition of carbon chain from the
surface of MNPs. So, the elevated temperature for grafted
organic group removal indicates the high thermal stability for
LPMNP catalyst which establishes the covalent bonding of
these groups to the surface of MNPs. The amount of N percent
45 in elemental analysis was 0.7, which demonstrates there is ~5
mol% of Lꢀproline in the structure of LPMNP catalyst.
5
Scheme 2 Generation of catalytic sites on the MNPs surface using MNPO
substrate
Ringꢀopening of oxiranes in the MNPO substrate with Lꢀ
proline (in this study), was resulted the production of LPMNP
catalyst. After preparation of the LPMNP catalyst, it was
10 characterized using various techniques such as FTꢀIR, TGA,
XRD, TEM, SEM, EDX, VSM and elemental analysis.
Comparison between the FTꢀIR spectra of Fe3O4,
Fe3O4@SiO2, VMNP, MNPO, NꢀBocꢀprotected Lꢀproline and
the LPMNP catalyst, reveals some absorption bands which
15 confirm the presence of Lꢀproline moieties in the structure of
the catalyst (ESI†, Fig. 1S). The TEM and SEM images of the
synthesized catalyst were recorded and represented in Fig. 1a
and 1b, respectively. According to TEM image and histogram
(Fig. 1d), the average diameter of the synthesized LPMNPs
20 based on the proposed procedure is estimated to be about 50
nm. Considering the SEM image (Fig. 1c), it is clear that the
LPMNPs are regular in shape and approved in an
approximately good arranged mode. These images also
established this point that the LPMNPs are created with near
25 sphereꢀshaped morphology. The histogram was anticipated
according to the results obtained from the TEM and SEM
images (Fig. 1d).
Fig. 2 A typical TGA curve from LPMNP catalyst (a). The XRD pattern
of LPMNP catalyst (b). The EDX analysis of the LPMNP catalyst (c).
50 The vibrating sample magnetometer (VSM) of the LPMNP catalyst (d).
According to the XRD patterns of LPMNP catalyst (Fig. 2b),
the strongest peaks of the XRD pattern correspond to SiO2,
demonstrating that the coreꢀshell structure of material. The
peaks are indexed as the (220), (311), (400), (422), (511),
55 (440), and (533) planes of the Fe3O4 nanoparticle.17 The EDX
from the obtained LPMNP catalyst (Fig. 2c) presented the
presence of the expected elements in the structure of the
catalyst. The magnetic properties of the catalyst was explored
at room temperature using a vibrating sample magnetometer
60 (VSM) (Fig. 2d). Based on magnetization curve, the
magnetization is saturated up to 50 emu g−1 at an applied field
of 8300 Oe, with an almost unimportant coercivity.
After preparation and characterization of the LPMNP catalyst,
its catalytic activity was evaluated in a condensation reaction
65 between 1Hꢀindole and aldehydes for the preparation of
bis(indolꢀ3ꢀyl)methanes18 under green conditions. To achieve
appropriate conditions for the synthesis of bis(indolꢀ3ꢀ
yl)methane derivatives using LPMNP catalyst, we tested the
reaction of benzaldehyde and indole as a simple model
70 substrate in various conditions (ESI†, Table. 1S). Thus, a
simple system including LPMNP (2.5 mol%) and H2O at 50
°C was chosen as the optimized reaction conditions (Scheme
2).
Fig. 1 A TEM images of two different positions of LPMNP catalyst
30 particles (a & b). A SEM image of the LPMNP catalyst (c). A histogram
which representing the size distribution of the LPMNP catalyst
The TGA curve of LPMNP catalyst (Fig. 2a) shows three
main weight losses. The first one is accounted for adsorbed
water in the structure of the catalyst (~4%). The seconded one
35 which is occurred at ~150–280 °C is related to the
75 Scheme 2 General strategy for the synthesis of bis(indolꢀ3ꢀyl)methanes in
the presence of LPMNP catalyst
2
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It is established that condensation between different aldehydes
catalytic activity without leaching of a significant quantity of
and 1Hꢀindole (2) in water at 50 °C (Table 1) produces 30 Lꢀproline in the reaction media.
bis(indolꢀ3ꢀyl)methanes in good to excellent yields over
LPMNP catalyst. The obtained products are characterized by
1H, 13C NMR, mass and elemental analysis and the data are
given in ESI.
In conclusion, a simple and practical synthetic strategy for the
synthesis of a novel MNPꢀsupported organocatalyst based on
Lꢀproline has been developed. The catalytic usefulness of this
magnetic recyclable organocatalyst was evaluated in the
35 condensation reaction of aldehydes and 1Hꢀindole for efficient
synthesis of bis(indolyl)methanes in water. This is the first
example of magnetic recyclable organocatalyst based on Lꢀ
proline which prepared using the reaction of Lꢀproline with
MNPꢀoxiran. We anticipate that the MNPꢀoxiran material
40 opens up a new direction for the development of new
magnetic recyclable organocatalysts. Also, the LPMNP
catalyst provides great promise toward further useful
applications in other organic transformations in the future.
The financial supports of research councils of Shiraz
45 University are gratefully acknowledged.
5
Table 1. Synthesis of bis(indolꢀ3ꢀyl)methanes using LPMNP catalysta
Time
(min)
60
90
90
Entry
R
Product
Yield (%)b
1
2
3
4
5
C6H5
3a
3b
3c
3d
3e
94
80
83
85
92
2ꢀOHꢀC6H4
4ꢀOHꢀC6H4
3ꢀOHꢀC6H4
4ꢀOMeꢀC6H4
90
60
6
4ꢀOMeꢀ3ꢀOHꢀC6H3 3f
60
81
7
8
9
10
11
12
13
14
4ꢀMeꢀC6H4
3,4ꢀFꢀC6H3
3ꢀCNꢀC6H4
3ꢀNO2ꢀC6H4
4ꢀNO2ꢀC6H4
2ꢀNO2ꢀC6H4
2ꢀClꢀC6H4
3g
3h
3i
3j
3k
3l
60
75
75
90
90
90
75
75
92
93
91
90
95
90
88
89
Notes and references
Department of Chemistry, College of Sciences, Shiraz University, Shiraz
71454, Iran. E-mail: khalafi@chem.susc.ac.ir (A.K.-N.),
panahi@shirazu.ac.ir (F.P.). Fax: +98(711)2280926.
50 † Electronic Supplementary Information (ESI) available: Experimental
procedures, recycling tests of LPMNP, and copy of 1Hand 13C NMR data
for the synthesized products along with the spectral data. See
DOI: 10.1039/b000000x/
3m
3n
4ꢀClꢀC6H4
a Reaction conditions: LPMNP (0.05 g, 2.5 mol%), H2O (2 mL), aldehyde
(1.0 mmol) and indole (2.1 mmol). b Isolated yield.
1
2
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10 In view point of mechanism, activation of the aldehyde can be
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Scheme 3 Proposed reaction mechanism for the preparation of bis(indolꢀ
15 3ꢀyl)methanes in the presence of LPMNP catalyst via iminium formation
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applications
of
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heterogeneous
organocatalyst, the level of reusability was also evaluated.
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