Fabrication of covalently functionalized mesoporous silica core-shell magnetite nanoparticles with palladium(II) acetylacetonate: Application as a magnetically separable nanocatalyst for Suzuki cross-coupling reaction of acyl halides with boronic acids

Article abstract of DOI:10.1002/aoc.3280

We report the preparation of supported palladium(II) acetylacetonate, Pd(acac)₂, coordinated by pendant acac groups, by reacting palladium acetate with acac-functionalized doubly silica-coated magnetic nanoparticles. The solid support consists of an amorphous silica-coated (as magnetite protecting layer) magnetite core and a mesoporous silica shell. The magnetically separable palladium nanocatalyst is active for Suzuki cross-coupling reaction of acyl halides with boronic acids. The catalyst is simply isolated from the reaction mixture that allows fast and efficient isolation of product and catalyst compared to traditional methods that generally make use of time- and solvent-consuming procedures.

Full text of DOI:10.1002/aoc.3280

Full Paper
Received: 19 October 2014
Revised: 30 December 2014
Accepted: 31 December 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3280
Fabrication of covalently functionalized
mesoporous silica core–shell magnetite
nanoparticles with palladium(II)
acetylacetonate: application as a magnetically
separable nanocatalyst for Suzuki cross-
coupling reaction of acyl halides with
boronic acids
Abdolreza Hajipoura,b* and Ghobad Azizia
We report the preparation of supported palladium(II) acetylacetonate, Pd(acac)2, coordinated by pendant acac groups, by reacting
palladium acetate with acac-functionalized doubly silica-coated magnetic nanoparticles. The solid support consists of an amor-
phous silica-coated (as magnetite protecting layer) magnetite core and a mesoporous silica shell. The magnetically separable pal-
ladium nanocatalyst is active for Suzuki cross-coupling reaction of acyl halides with boronic acids. The catalyst is simply isolated
from the reaction mixture that allows fast and efficient isolation of product and catalyst compared to traditional methods that
generally make use of time- and solvent-consuming procedures. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: core–shell magnetic nanoparticle; ketones; acyl halides; boron compounds; Suzuki cross-coupling reaction
yields.[25] Acylation of benzyl halides offers another direct approach
for the preparation of aromatic ketones. In this regard, Inaba and
Introduction
Ketones represent an important family of carbonyl compounds,
and are distinct building blocks in organic synthesis.[1] The
traditional method for the synthesis of aromatic ketones involves
Friedel–Crafts acylation of aromatic compounds in the presence
of a Lewis acid, which is limited by problems associated with
regioselectivity and fails with electron-deficient arenes.[2]
The nucleophilic addition of acyl halides with organometallic
reagents such as organomagnesium, organozinc, organocadmium
and organocopper compounds is an extremely useful tool for the
synthesis of ketones.[3] These protocols often proceed in low yield
because of the addition of the organometallic reagent to the
ketone to form tertiary alcohols.[4–6]
The transition metal-catalyzed cross-coupling of acyl chlorides
with organometallic reagents is a convenient alternative for
the synthesis of ketones. There are few reports on transition
metal-catalyzed coupling reactions of acyl halides with organo-
metallic reagents such as organotin,[7–11] organozinc,[12,13]
organobismuth[14,15] and organomagnesium compounds.[16]
Hydroacylation reaction between aldehydes and olefins,[17] acyla-
tion of aryl halides with aldehydes[18–21] and acylation of aryl
boronate salts with aldehydes[22–24] are other ways to obtain
aromatic ketones. The cross-coupling reaction between arylboronic
acids, carbon monoxide and aryl iodides in the presence of
palladium catalyst and base also provides aromatic ketones in high
Rieke have reported the reaction between acyl halides and benzyl
halides in the presence of metallic nickel for the preparation of
ketones.[3]
The Suzuki–Miyaura reaction is playing an ever increasing role
in modern organic synthesis.[26] The introduction of acyl halide
instead of aryl halide in the Suzuki–Miyaura reaction extends
the application spectrum of this reaction, since a number of
organoboronic acids and acid chlorides are commercially available.
This method provides convenient access to ketones that often
cannot be prepared otherwise, for example, via Friedel–Crafts
acylation methods.[27] This is a superior methodology because of
the mild conditions, functional group tolerance, effectiveness and
regioselectivity. Furthermore, boronic acids are compounds of low
toxicity and great stability.[28]
*
Correspondence to: Abdolreza Hajipour, Pharmaceutical Research Laboratory,
College of Chemistry, Isfahan University of Technology, Isfahan 84156, Islamic
Republic of Iran. E-mail: haji@cc.iut.ac.ir
a Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of
Technology, Isfahan 84156, Islamic Republic of Iran
b Department of Neuroscience, University of Wisconsin Medical School, 1300
University Avenue, Madison, WI 53706-1532, USA
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

A. Hajipour and G. Azizi
In earlier studies, Negishi et al. reported that lithium
tetraorganoborates will react with acyl halides to form ketones.[29]
Subsequently, Uemura and co-workers developed a reaction
involving the use of sodium tetraphenylborates with acyl halides
in the presence of palladium–ligand catalyst for preparing aryl ke-
tones.[30] This procedure was later modified and improved under
ligand-less conditions by Bumagin and Korolev.[31] Arylboronic
acids were used for this reaction by Haddach and McCarthy.[32]
Finally, Kabalka et al. used trialkylboranes in the presence of
palladium to generate alkyl and aryl ketones in good yields.[1] Since
these pioneer researches, other efficient palladium catalysts
such as (triphenylphosphine)palladium complex,[1,33,34] palladium–
(Fe3O4@nSiO2, where n means nonporous). Third, through a
surfactant-templating method with tetradecyltrimethylammonium
bromide (C14TAB) as template, a C14TAB–silica composite was de-
posited on the Fe3O4@nSiO2 nanoparticles (Fe3O4@nSiO2@C14TAB/
SiO2). Fourth, C14TAB was extracted using acetone to form a meso-
porous SiO2 shell, resulting in Fe3O4@nSiO2@pSiO2 nanoparticles
(p means porous). This second mesoporous silica shell offers a
high surface area for functionalization with the acac group. The
Fe3O4@nSiO2@pSiO2 solid support was reacted with 3-iodopropyl-
trimethoxysilane followed by reaction with acetylacetone (acacH)
in the presence of K2CO3 to give Fe3O4@nSiO2@pSiO2–acac parti-
cles. Finally, these acac-functionalized nanoparticles were reacted
with Pd(OAc)2 in refluxing acetone to give Fe3O4@nSiO2@pSiO2–
Pd(acac)2 as the catalyst (Scheme 3). The content of Pd in the cata-
lyst was found to be 0.0965 mmol gꢀ1 (determined using induc-
tively coupled plasma (ICP) analysis).
Figure 1 shows the powder X-ray diffraction (XRD) pattern of the
Fe3O4@nSiO2 nanoparticles compared with the XRD patterns of
pure silica and naked magnetite nanoparticles. Eleven diffraction
peaks are observed at 2θ values of 18.39°, 30.32°, 35.71°, 43.15°,
53.67°, 57.28°, 62.82°, 71.35°, 74.48°, 79.38° and 87.16°, and they
are consistent with the reference values (e.g. no. 00-001-111) of
Fe3O4 and patterns reported elsewhere.[39] The broad peak at
23.5° indicates the presence of SiO2 shell around the Fe3O4
nanoparticles.[39] The low-angle powder XRD pattern of the
Fe3O4@nSiO2@pSiO2 nanoparticles reveals that the second silica
layer has hexagonal mesopore symmetry (Fig. S1 in supporting in-
formation) which is consistent with previous results.[38,40] The
acac/Pd ratio was calculated using a combination of ICP and
elemental analysis methods. As can be seen in Table 1, the
acac/Pd ratio is 2.12, indicating that palladium forms a complex
with two acac ligands present on the surface of the nanoparticles.
Therefore, we expect that Pd(OAc)2 reacts with two acac ligands
grafted on the surface of the silica pores forming a Pd(acac)2
complex, as shown in Scheme 3.
The fabrication of Fe3O4@nSiO2@pSiO2–Pd(acac)2 using iron
oxide nanoparticles as a core, two layers of silica as shells,
as well as the surface modification of the nanoparticles was
analyzed using FT-IR spectroscopy. The FT-IR spectra of
naked Fe3O4 nanoparticles, Fe3O4@nSiO2 compared with
Fe3O4@nSiO2@C14TAB/SiO2, Fe3O4@nSiO2@C14TAB/SiO2 com-
pared with C14TAB, Fe3O4@nSiO2@pSiO2, Fe3O4@nSiO2@pSiO2–
acac and Fe3O4@nSiO2@pSiO2–Pd(acac)2 are illustrated in Figs S5–
S10, respectively. The characteristic peak of Fe3O4 nanoparticles is
Fe–O bond vibration at 569 cmꢀ1 shown in Fig. S5. For
Fe3O4@nSiO2, peaks centered at 800 and 1083 cmꢀ1 correspond
to asymmetric and symmetric stretching vibrations of the Si–O–Si
bond, respectively. A band at 452 cmꢀ1 corresponds to Si–O bond
vibration. A broad prominent peak at 3400 cmꢀ1 clearly reveals the
presence of numerous surface hydroxyl groups on the surface of
Fe3O4@nSiO2 nanoparticles. These bands clearly suggest the
copper,[35] palladium–phosphinous acid,[27] palladium chloride[36]
[28]
and Pd2dba3
have been applied for coupling of aroyl halides
and boronic acids.
Recently, we reported the functionalization of silica particles with
iron(III) acetylacetone and their application in C–C cross-coupling
reaction.[37] The main goal of the work presented here was to use
palladium(II) acetylacetonate (Pd(acac)2) supported on magnetite
nanoparticles doubly coated with dense and mesoporous silica
layers in Suzuki–Miyaura coupling of arylboronic acids with acyl
chlorides. Herein we disclose an efficient and mild palladium-
catalyzed direct acylation reaction of acyl halides with arylboronic
acids, affording ketones in one step (Scheme 1).
Results and Discussion
Typical procedures for the preparation of doubly coated magnetite
nanoparticles and palladium supported on the nanoparticles are
shown in Schemes 2 and 3, respectively. These doubly coated mag-
netite spheres were previously reported by Deng et al.[38] They used
the solvothermal method for the synthesis of magnetite micro-
spheres with a size of about 500 nm. In the present case, we used
co-precipitation instead of the solvothermal method to obtain
smaller particles.
First, magnetite nanoparticles were prepared via
a co-
precipitation method. Ammonia was used as precipitating agent.
To obtain smaller nanoparticles, ultrasound and a mechanical stirrer
(900 rpm) were used simultaneously. Second, the magnetite
nanoparticles were coated with a thin and dense silica layer by
polycondensation of tetraethyl orthosilicate (TEOS) through the
sol–gel approach to obtain nonporous silica–Fe3O4 composites
Scheme 1. Suzuki reaction between acyl chlorides and boronic acids.
Scheme 2. Preparation of Fe3O4@nSiO2@pSiO2 nanospheres.
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Suzuki cross-coupling with magnetically separable nanocatalyst
range 400–650°C. This is due to loss of the organic groups from the
surface of the catalyst and also confirms the presence of organic
moieties on the surface of the support.
The morphology and size of the nanoparticles were obtained
from field-emission scanning electron microscopy (FESEM) and
transmission electron microscopy (TEM) analyses. A TEM image of
Fe3O4@nSiO2@pSiO2–Pd(acac)2 nanoparticles is shown in Fig. 2.
This image shows two isolate particles of the catalyst. As can be
seen, the dark inner core that consists of several Fe3O4 nanoparti-
cles is coated with the silica shell of 13–18 nm in thickness. Unfor-
tunately, the first thin layer of silica is not clearly observed in this
image due to instrumental limitations. However, the presence of
this thin layer is confirmed from FT-IR and powder XRD analyses
(Figs S4 and S6). From FESEM images (Figs. 3 and S13), it is
observed that the catalyst is made from several agglomerated par-
ticles. Figure 4 (and S12) also shows a TEM image of agglomerated
particles of the catalyst. X-ray photoelectron spectroscopic analysis
was performed on Fe3O4@nSiO2@pSiO2–Pd(acac)2. The metal 3d5/2
and 3d3/2 electronic states for the Pd(II) complex are observed at
337.86 and 343.3 eV, respectively (Fig. S15).
Scheme 3. Preparation of the catalyst: Fe3O4@nSiO2@pSiO2–Pd(acac)2.
Reactivity of the prepared catalyst was tested in the cross-
coupling reaction of benzoyl chloride and phenylboronic acid with
various bases and solvents at room temperature for 3 h in the pres-
ence of Fe3O4@nSiO2@pSiO2–Pd(acac)2 (0.5 mg, 0.005 mol% Pd) as
the catalyst. Various reaction conditions were examined with the
aim of investigating their influence on the selectivity towards bi-
phenyl as the major byproduct. The results obtained are summa-
rized in Table 2. In general, in all cases, biphenyl is formed as
byproduct presumably derived from homocoupling of the boronic
acid or decarbonylative coupling between acyl chloride and bo-
ronic acid.[28] But, by optimization of the reaction conditions, better
selectivities towards benzophenone can be achieved. Formation of
biphenyl is favored in DMF, dioxane and methanol as solvents
(Table 2, entries 2, 3 and 6). When methanol is used, methyl
benzoate is formed predominantly (entry 6). The reaction pro-
ceeds in chloroform (entry 5) and toluene (entry 1) with sim-
ilar selectivity, but the conversion in toluene is higher. We
also investigated the effect of various bases in the reaction
between benzoyl chloride and phenylboronic acid catalyzed
by Fe3O4@nSiO2@pSiO2–Pd(acac)2 (0.5 mg, 0.005 mol% Pd)
by using toluene as optimum solvent (entries 7–9). Among
the bases studied, better results were obtained with K2CO3
(entry 9; 90% selectivity towards benzophenone and 89% con-
version). Although the selectivities towards benzophenone in
the presence of various amounts of K2CO3 are similar (be-
tween 89 and 92%), the conversion can be improved by in-
creasing the amount of K2CO3 from 1 eq. (89%) to 3 eq.
(94%) (entries 9–11).
By increasing the amount of catalyst to 1 mg (0.01 mol% Pd),
99% conversion is obtained although selectivity remains at 92%
similar to the result with 0.5 mg of catalyst (entry 12). Using no
catalyst in the reaction mixture yields no benzophenone,
confirming the requirement for palladium catalyst (entry 13).
We also investigated the reaction between benzoyl chloride
and phenylboronic acid catalyzed by homogeneous Pd(acac)2 (en-
try 14). The reaction proceeds faster and is slightly more selective in
the presence of unsupported Pd(acac)2.
To further comprehend the scope and limitations of the cross-
coupling reaction between acid chlorides and boronic acids
catalyzed by Fe3O4@nSiO2@pSiO2–Pd(acac)2, a variety of alkyl/aryl
and alkenyl acid chlorides and boronic acids were tested (Table 3).
The reactions of phenylboronic acid with acid chlorides bearing
Figure 1. Wide-angle powder XRD patterns of Fe3O4 nanoparticles (top),
Fe3O4@nSiO2 nanoparticles (middle) and pure silica (bottom).
Table 1. Elemental analysis data for the prepared catalyst and its
precursors
Catalyst
(mmol gꢀ1
acac
(mmol gꢀ1
Pd
acac/Pd
)
)
(mmol gꢀ1
)
MNP-R-Ia
7.1275
—
—
—
MNP-R-acacb
7.3307
0.205
—
—
MNP-R-acac-Pdc
—
—
0.0965
2.12
aMNP-R-I = Fe3O4@nSiO2@pSiO2–(CH2)3–I.
bMNP-R-acac = Fe3O4@nSiO2@pSiO2–(CH2)3–acac.
cMNP-R-acac-Pd = Fe3O4@nSiO2@pSiO2–(CH2)3–Pd(acac)2.
presence of SiO2 shell on the Fe3O4 nanoparticles (Fig. S6). The char-
acteristic peaks of the C14TAB template at 833, 909, 1406, 1482,
2855 and 2919 cmꢀ1 (Figs S6 and S7) disappear after extraction
with acetone (Fig. S8). Further surface modification with (3-
iodopropyl)trimethoxysilane is indicated by peaks at 2850 and
2940 cmꢀ1, corresponding to C–H stretching vibration of propyl
groups (data not shown). The peak at 1703 cmꢀ1 is attributed to
acac moiety and the peak at 1622 cmꢀ1 (hydrogen-bonded C¼O
group) is covered by the strong peak of silica at 1636 cmꢀ1 (Fig.
S9). Further, the bonding of acac moieties to the palladium is ev-
ident from Fig. S10. This coordination can be characterized by
theꢀd1isappearance of the free ligand absorption bands at 1703
cm
and the appearance of new bands at 1566 and 1549
cmꢀ1 attributed to the coordination of palladium to C¼O bonds
of acac that weakens this bonds by π-back-donation.
The thermogravimetric analysis trace of Fe3O4@nSiO2@pSiO2–Pd
(acac)2 is shown in Fig. S16. This shows one main weight loss in the
Appl. Organometal. Chem. (2015)
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A. Hajipour and G. Azizi
electron-donating (entries 4 and 13) and electron-withdrawing
groups (entries 2, 3, 5, 6, 12, 14 and 15) proceed in good to excellent
yields. Both alkyl and alkenyl chlorides yield the corresponding
ketones in good yields. For example, acetyl, phenylacetyl,
chloroacetyl and cinamyl chlorides undergo the reactions to give
the corresponding ketones in high yields (Table 3, entries 18, 9,
10, 16, 8 and 17).
Heteroaromatic acid chlorides such as 2-furoyl chloride also
give good yields of the corresponding ketone at 82% (Table 3, entry
7). Acid chlorides with 4-methoxyphenylboronic acid similarly
undergo the cross-coupling reactions in up to 96% yield (Table 3,
entries 11–18).
It is important to note that the synthesis of benzophenones
required only 1–2 min flushing of the reaction mixture with
nitrogen. But, for the synthesis of acetophenones, the reaction
mixtures required extensive flushing with nitrogen (>10 min)
before the addition of acid chloride for optimum yields.
As an important characteristic for heterogeneous catalysts,
recyclability of the catalyst was investigated. Spent catalyst is
easily recovered from the reaction mixture (diluted with ethyl ac-
etate) by means of a permanent magnet and re-used after the
first run (Fig. 5). The separated catalyst was washed with copious
amounts of acetone and water, and finally dried at 60°C for 1 h.
The regenerated catalyst was used for the cross-coupling reac-
tion of benzoyl chloride and phenylboronic acid in the next
run. No significant drop in yield of benzophenone is observed
after recycling after the fifth reuse. The data for reusability of the
catalyst for the preparation of benzophenone are shown in Table 3
under entry 1. The comparison of our catalyst activity with other
catalysts with the same aryl halide are shown in Table 4.
We propose the reaction mechanism shown in Scheme 4.
Oxidative addition of acid chloride to the Pd(0) on the solid support
generates an acylpalladium(II) chloride complex. Subsequent
transmetalation leads to the formation of Ph–Pd(II)–COR. Finally,
reductive elimination leads to the formation of the ketone and
regeneration of Pd(0).[35]
Figure 2. TEM image of two isolated particles of Fe3O4@nSiO2@pSiO2–Pd
(acac)2.
Figure 3. FESEM image of agglomerated Fe3O4@nSiO2@pSiO2
nanospheres.
Experimental
General
Iron(III) chloride hexahydrate (99%), iron(II) sulfate heptahydrate
(98%), ammonium hydroxide (28%), (3-chloropropyl)trimethoxysilane
(>98%), TEOS (98%), polyethylene glycol 200 (PEG-200) and all the
acyl halides and boronic acids were purchased from Merck. C14TAB
was prepared in our laboratory. Ethanol (99%) was purchased
from Bidestan. Palladium acetate (99.8%) was purchased from
Sigma Aldrich. Toluene and THF were dried over sodium
metal prior to use.
The phase analysis of the as-prepared magnetite nanoparti-
cles and silica-coated magnetic nanoparticles was performed
using a diffractometer (XRD) with Ni-filtered Cu Kα radiation
(λ = 1.54 Å). The surface chemistry of nanoparticles was deter-
mined from FT-IR spectra. Samples for FT-IR spectroscopy
were prepared in KBr and measured in the range 400–4000
cmꢀ1. The size and morphology of the nanoparticles were ob-
served using a TEM instrument (Hitachi H-700H) operated at
150 kV. The nanoparticles were thoroughly dispersed in etha-
nol using ultra-sonication and a drop of the solution was
placed on a carbon-coated copper grid and a Phillips X’PERT
PRO was used for performing FESEM. GC experiments were
performed with a BEIFEN 3420 gas chromatograph (BP5
Figure 4. TEM image of agglomerated Fe3O4@nSiO2@pSiO2 nanospheres.
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Suzuki cross-coupling with magnetically separable nanocatalyst
Table 2. Optimization of reaction conditions for the reaction of phenylboronic acid and benzoyl chloride in the presence of Fe3O4@nSiO2@pSiO2–Pd
(acac)2 nanospheres as catalysta
Entry
Solvent
Toluene
Base
a (%)
82
b (%)
18
GC conversion (%)
87
NaHCO3
1
2
DMF
NaHCO3
8
30
0
92
70b
0b
19
100c
36
40
10
11
8
99
50
0
3
Dioxane
Cyclohexane
CHCl3
NaHCO3
4
NaHCO3
5
NaHCO3
81
0
81
43
35
70
89
94
94
99
0
6
Methanol
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
NaHCO3
7
t-BuOK
64
60
90
89
92
92
0
8
Na2CO3
9
K2CO3 (1 eq.)
K2CO3 (2 eq.)
K2CO3 (3 eq.)
K2CO3 (3 eq.)
K2CO3 (3 eq.)
K2CO3 (3 eq.)
10
11
12d
13e
14f
8
0
95
5
99
aReaction conditions: boronic acid (0.14 mmol, 1.4 eq.), acid chloride (0.1 mmol, 1 eq.), catalyst (0.5 mg, 0.048 mol% Pd, 1 mg for entry 12) and base in
toluene (2 ml) at room temperature for 3 h.
bByproducts were formed.
cMethyl benzoate was formed in 57% conversion.
d1 mg catalyst (9.6 × 10ꢀ5 mmol, 0.1 mol% Pd) was used.
eNo catalyst was added.
fHomogeneous Pd(acac)2 was used as catalyst (reaction was completed after 2 h).
column, 30 m × 0.25 mm × 0.25μm). Nitrogen was used as a
carrier gas and the injection split ratio was 1:20. Separation
was achieved using the following temperature program: 20°C
minꢀ1 from 90°C (hold time 1 min) to 260°C (hold time 5
min). Injector and detector temperatures were 260 and
300°C, respectively. Conversions were based on the consump-
tion of the acyl halide by normalization of areas.
in a mixture of ethanol (80 ml) and distilled water (20 ml)
followed by 10 min of ultrasonication. Then, ammonium hydrox-
ide solution (1.0 ml, 28 wt%) followed by TEOS (100 μl) were
added. After stirring at room temperature for
6 h, the
Fe3O4@nSiO2 nanospheres were magnetically separated and
washed with copious amounts of ethanol and water, and
redispersed in a mixed solution containing C14TAB (0.30 g), eth-
anol (60 ml) and distilled water (80 ml). After 5 min of
ultrasonication, ammonium hydroxide solution (2 ml, 28 wt%)
was added followed by 5 min of mechanical stirring. Then, 400
μl of TEOS was added dropwise to the dispersion with continu-
ous stirring at 800 rpm. After reacting for 6 h, the product was
magnetically collected and washed repeated with ethanol and
water to remove nonmagnetic materials. Finally, the purified
nanospheres were redispersed in 60 ml of acetone and refluxed
for 3 × 48 h to remove the C14TAB template. Nanospheres were
washed with distilled water and ethanol and finally dried under
vacuum at 70°C overnight.
Synthesis of Magnetite (Fe3O4) Nanoparticles
Magnetite nanoparticles were prepared by co-precipitation.
Amounts of 2.02 g of FeCl3ꢁ6H2O (7 mmol), 1 g of FeSO4ꢁ7H2O
(3.5 mmol) and 2 ml of PEG-200 were added to 300 ml of dis-
tilled water in a 500 ml three-necked round-bottom flask
equipped with a mechanical stirrer. The solution was vigorously
stirred (800 rpm) and heated to 40°C for 5 min under an argon
stream and ultrasonic irradiation. Subsequently, 15 ml of ammonia
solution (28%) was added dropwise into the solution and stirring
was continued for another 30 min to allow the uniform formation
of the nanoparticles. The resulting particles were subjected to mag-
netic decantation followed by repeated washing with distilled water
and ethanol and finally dried in a vacuum oven at 70°C.
Synthesis of Fe3O4@nSiO2@pSiO2–acac Nanospheres
(3-Chloropropyl)trimethoxysilane (0.5 mmol) was added to a solu-
tion of anhydrous sodium iodide (0.5 mmol) in dry acetone (50
ml), and the mixture was heated under reflux for 12 h. The solvent
was removed under reduced pressure and the flask charged with 7
ml of dry toluene without removing the NaCl. The as-prepared
Fe3O4@nSiO2@pSiO2 was added to this toluene flask. The solution
was refluxed for 12 h. The solvent was magnetically decanted and
the nanospheres were washed thoroughly with dichloromethane.
Synthesis of Fe3O4@nSiO2@pSiO2 Nanospheres
Double-layer silica-coated magnetic nanoparticles were prepared
according to a previously reported method.[38] Briefly, 100 mg of
as-prepared Fe3O4 nanoparticles were homogeneously dispersed
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A. Hajipour and G. Azizi
Table 3. Fe3O4@nSiO2@pSiO2–Pd(acac)2 nanosphere-catalyzed Suzuki
cross-coupling of acyl chlorides and boronic acidsa
Entry
R1
R
Time (h)
3
Yield (%)b
90
H
Phenyl
1
run 2
3
3
3
3
3
3
2
2
5
2
2
2
2
2
2
1
1
3
2
2
2
2
2
88
85
80
78
71
66
83
80
80
90
75
82
90
92
84
96
90
68
92
80
88
89
92
run 3
run 4
run 5
run 6
run 7
2
H
H
H
H
H
H
H
H
H
4-Nitrophenyl
3,5-Dinitrophenyl
4-Methoxyphenyl
4-Fluorophenyl
3,5-Dichlorophenyl
Furyl
Scheme 4. Proposed mechanism for Suzuki coupling of phenylboronic
acid and halides catalyzed by Fe3O4@nSiO2@pSiO2–Pd(acac)2 as pre-
catalyst.
3
4
5
6
7
Synthesis of Fe3O4@nSiO2@pSiO2–Pd(acac)2 Nanospheres
8
Ph–CH¼CH
Ph–CH2
To a solution containing the acac-functionalized nanospheres in dry
acetone was added 2.2 mg of Pd(OAc)2 (0.01 mmol Pd). After stir-
ring overnight at room temperature, the magnetic material was re-
covered from the solution with a permanent magnet and washed
with acetone repeatedly and dried under vacuum for 8 h. The pal-
ladium content was 0.0965 mmol gꢀ1 determined using ICP
analysis.
9
10
11
12
13
14
15
16
17
18
ClCH2
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
Ph
4-Nitrophenyl
4-Methoxyphenyl
3,5-Dinitrophenyl
3,5-Dichlorophenyl
ClCH2
Ph–CH¼CH
CH3
Representative Procedure for Preparation of Benzophenone
aReaction conditions: boronic acid (1.4 mmol), acid chloride (1 mmol),
catalyst (1 mg, 0.096 mol% Pd), K2CO3 (3 mmol) in toluene (2 ml).
To a mixture of phenylboronic acid (121 mg, 1 mmol, 1.4 eq.) and
anhydrous potassium carbonate (290 mg, 3 eq.) in anhydrous tolu-
ene (2 ml) under nitrogen was added Fe3O4@nSiO2@pSiO2–Pd
(acac)2 nanospheres (1 mg, 0.096 mol% Pd) followed by injection
of benzoyl chloride (80 μl, 0.7 mmol, 1 eq.). The reaction mixture
was stirred vigorously (700 rpm) at room temperature for 3 h. The
progress of the reaction was monitored using TLC (1:10 ethyl
acetate–hexane). The reaction mixture diluted with ethyl acetate
(10 ml) and washed three times with a saturated solution of sodium
bicarbonate and brine, dried over sodium sulfate and concentrated
in vacuum to give crude benzophenone. After purification by col-
umn chromatography (1:10 ethyl acetate–hexane) benzophenone
was obtained in 90% isolated yield.
bIsolated yield.
3-Iodopropyl-functionalized nanospheres were added to a suspen-
sion of K2CO3 (0.5 mmol) and acetylacetone (0.5 mmol) in dry THF.
The mixture was refluxed for 24
h to give the crude
Fe3O4@nSiO2@pSiO2–acac nanospheres. Finally, the nanospheres
were washed with copious amounts of acetone and water and
dried under vacuum at 50°C overnight. The content of acac was
0.2050 mmol gꢀ1 based on elemental analysis.
Figure 5. Magnetic separation of catalyst from reaction mixture.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)

Suzuki cross-coupling with magnetically separable nanocatalyst
Table 4. Comparison of various conditions for the preparation of benzophenone from phenylboronic acid and benzoyl chloride
Entry
Yield
97
Catalyst
Pd2(dba)3
Amount (mol%)
1
Base
Solvent
Toluene
Temp. (°C)
Time
Ref.
[28]
K2CO3
120 (MW, 250 W)
5 min
1
2
91
90
80
68
78
PdCl2(PPh3)2
PdCl2
2
3.3
5
K3PO4
Na2CO3
Cs2CO3
Cs2CO3
—
Toluene
—
110
4 h
[41]
[36]
[32]
[33]
[35]
3
4
5
6
Room temp.
100
5 min
16 h
10 min
3 h
Pd(PPh3)4
Pd(PPh3)4
Pd(dba)2
PPh3
CuTCa
PdPOb
Toluene
Toluene
Et2O
5
MW, 260 W
Room temp.
5
10
100
2.5
7
8
93%
90%
K2CO3
K2CO3
Toluene
Dioxane
Toluene
80
60 min
3 h
[27]
Supported Pd(acac)2
0.096
Room temp.
This work
aCuTC: copper(I) thiophene-2-carboxylate.
bPdPO: palladium–phosphinous acid.
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The synthesis of ketones from acid chlorides and boronic acids un-
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mesoporous silica core–shell magnetite nanosphere-supported
palladium(II) acetylacetonate and potassium carbonate in toluene
is reported.
Acknowledgments
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We gratefully acknowledge the funding support received for this
project from the Isfahan University of Technology, IR Iran. Further
financial support from the Center of Excellence in Sensor and Green
Chemistry Research, Isfahan University of Technology, is gratefully
acknowledged.
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