Arylsilylation of aryl halides using the magnetically recyclable bimetallic Pd-Pt-Fe3O4 catalyst

Article abstract of DOI:10.1039/c7cc09926f

Transition metal-catalyzed silylations have typically involved the use of homogeneous non-recyclable catalytic systems. In this work, the first example of a recyclable catalytic system for the synthesis of arylsilanes has been reported, which utilizes the

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Arylsilylation of aryl halides using the magnetically recyclable
bimetallic catalyst Pd−Pt−Fe3O4
Jisun Jang, †a Sangmoon Byun, †b,c B. Moon Kimb* and Sunwoo Leea*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Transition metal-catalyzed silylations have typically involved the other nucleophiles because of the strong reducing power of
use of homogeneous non-recyclable catalytic systems. In this hydrosilane.9 Apart from using aryl halides, transition metal-
work, the first example of a recyclable catalytic system for the catalyzed silylations with pivalate,10 phenolic esters,11 or cyano
synthesis of arylsilanes has been reported, which utilizes the arenes12 have also been reported.
bimetallic complex Pd−Pt−Fe3O4 nanoparticles. Various arylsilanes
The direct C-H silylation reaction is an ideal methodology and an
were prepared from the reaction of aryl iodides (or bromides) attractive alternative to above-mentioned strategies because it is
with hydrosilanes. This methodology showed good functional the best atom-economical tool. Although direct C-H silylation has
group tolerance toward ester, ketone, aldehyde, nitro, and cyano been reportedly achieved with Rh-,13 Ru-,14 Pt-,15 and Ir-16 based
groups. The bimetallic Pd−Pt−Fe3O4 catalytic system showed catalysts, the substrate scope is limited to heteroarenes and arenes
better activity than monometallic catalysts Pt−Fe3O4 and bearing an ortho-directing group. Moreover, all these catalysts are
Pd−Fe3O4. In addition, the bimetallic Pd−Pt−Fe3O4 catalytic system based on precious metals. Generally, transition metal-catalyzed
could be easily recovered and reused for over twenty cycles.
silylations of aryl halides or C-H activations proceed through
homogeneous catalytic systems.
Owing to their unique properties, arylsilanes are valuable synthetic
building blocks in pharmaceutical and material chemistry.1 These
compounds have also received significant attention for their role in
electrophosphorescent devices.2 In addition, they have been widely
To the best of our knowledge, a reusable catalytic system has not
been developed for silylation reaction even though most of the
catalysts utilized for this transformation are expensive metals.
Recently, a few metal-free silylation methods have been developed,
however they can be applied only to a limited number and type of
substrates.17
used as a coupling partner in the Hiyama coupling reaction.3
A
number of synthetic methods have been developed and widely
used. Classically, silyl groups have been introduced in organic
molecules through the use of organolithium or Grignard reagents
and a silicon electrophile. However, this method lacks functional
group tolerance toward base-sensitive groups.4 To overcome this
shortcoming, transition metal-catalyzed coupling reactions of aryl
halide with disilanes or hydrosilane have been widely reported.5
Several coupling reactions of aryl halides with organosilanes
catalyzed by Pd-,6 Rh-,7 and Pt-8 based complexes have been
reported. However, the coupling reactions with hydrosilane as the
silylating reagent have not been studied as much as those with
Bimetallic catalysis has received significant attention for the
synthesis of target molecules that are difficult to prepare through
the use of conventional monometallic catalysts.18 Recently, we have
reported the synthesis of bimetallic Pd−Pt−Fe3O4 nanoflake-shaped
alloy nanoparticles as catalysts for the reduction of nitroarenes.19
The magnetically recoverable nanoparticles provided nearly
quantitative conversions and yields in the aforementioned reaction,
and could be reused for up to 250 catalytic cycles. It has also been
reported that bimetallic nanoparticles show extraordinary catalytic
activities that are better than the parent metals.
Therefore, it was envisioned that the bimetallic Pd−Pt−Fe3O4
catalyst might exhibit better activity than a monometallic catalytic
system for the silylation reaction. Homogeneous palladium and
platinum-catalyzed silylation reactions suffer from several
drawbacks. Palladium-based catalytic systems are known to require
specific phosphine ligands in some cases. These ligands are
generally unstable and quite expensive. Similarly, the use of
platinum is undesirable as it is an expensive metal even though it
may not require special ligands. Therefore, the employment of the
bimetallic nanoparticle Pd
−Pt
−
Fe3O4 as a catalyst is expected to
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yield, respectively (entries 10 and 11). Finally, the reaction with i-
Pr2EtN resulted in 88% yield of the desired product (entry 12).
When the catalyst was reduced to 1 mol%, the yield of the product
also decreased to 45% (entry 13). When the reaction was
address the abovementioned issues and those of reusability and
easy recovery.
Pd−Pt−Fe3O4 nanocatalysts were synthesized via the one-pot
solution phase reduction process. To obtain a better morphology
for better catalytic activity compared to previously reported
Pd−Pt−Fe3O4 NPs, we prepared Pd−Pt−Fe3O4 nanocatalyst from
Fe3O4 NPs possessing sphere morphology and decorated more well-
dispersed Pd−Pt NPs on Fe3O4 support by changing the amount of
polyvinylpyrrolidone (PVP). As shown in Fig S1, various PVP
conditions were tried, good morphology and dispersity of NPs were
obtained at 4 times PVP equivalent to Fe3O4 NPs. The detailed
structure and morphology of Pd−Pt−Fe3O4 NPs were characterized
with high resolution transmission electron microscopy (HR-TEM),
scanning electron microscopy-energy dispersive spectroscopy (SEM-
EDS), high bright-field scanning TEM (BF-STEM), high-angle annular
dark-field scanning TEM (HAADF-STEM) and the elemental analysis
mapping by Cs-STEM-EDS (Fig. 1 and Fig. S2-S7). Pd−Pt alloy NPs
were well immobilized and distributed on the Fe3O4 NPs surface (Fig.
1 and Fig S4). EDS mapping image of Pd−Pt−Fe3O4 showed that
Pd(red) and Pt(blue) points excellently decorated Fe(yellow) surface
(Fig. S4). As shown in HAADF-STM and BF-STEM images (Fig. S5), it
was confirmed that Pd−Pt alloy nanocarystals spread well on Fe3O4
support. Randomly homogeneous Pd−Pt alloy phase was confirmed
by Cs-STEM-EDS (Fig. S6–7). Pd−Pt alloy NPs were very evenly
dispersed on the Fe3O4 NPs with an average size of 4.8 nm (Fig. S8).
conducted with monometallic Pt−Fe3O4 or Pd−Fe3O4, the obtained
yields were lower than those obtained from the reaction employing
the bimetallic Pd-Pt-Fe3O4 catalyst (entries 14 and 15). Moreover,
the reaction employing a combination of Pd−Fe3O4 and Pt−Fe3O4
afforded a lower yield of the product than that of the bimetallic
catalytic system (entry 16). Instead of Pd−Pt−Fe3O4 as a catalyst,
several Pd- and Pt-based homogeneous catalysts were employed.
These Pd and Pt catalysts were ineffective in yielding the desired
product (entries 17 and 18). Electronic effect20 could be an evidence
of the high catalytic activity of the alloy Pd−Pt−Fe3O4 NPs in
hydrosilylation of aryl halides. For understanding of the unique
effects in Pd
−Pt−Fe
3O4 NPs, it is necessary to compare the
electronic structure of monometallic NPs (Pd− or Pt− Fe3O4) and
bimetallic ones (Pd−Pt−Fe3O4) by using XPS. It can be confirmed
that both Pd 3d (340.2 eV and 334.9 eV) and Pt 4f (73.6 eV and 70.3
eV) peaks slightly shift toward lower binding energy in Pd−Pt−Fe3O4
NPs compared with each monometallic Pd−Fe3O4 (340.3 eV and
335.0 eV) , Pt−Fe3O4 NPs (74.6 eV and 71.3 eV) (Fig S10-12).
Surprisingly, about 1.0 eV of negative shift was detected in
Pd−Pt−Fe3O4 NPs with Pt 4f peaks, as Pt gains electrons from Pd.
The electronic shift effect induced by Pd−Pt metal alloying,
electronically rich Pt can be responsible for the improved
hydrosilylation reaction reactivity.
Table 1 Optimization of arylsilylation reaction conditions with
methyl 4-iodobenzoate and triethylsilane as substratesa
cat. PdꢀPtꢀFe3O4
MeO2C
I
+
HSiEt3
MeO2C
SiEt3
base, solvent
70 oC, 15 h
1a
2a
3a
1a
Conv. (%)b
86
3a
Entry
Catalyst
Base
Solvent
Yield (%)b
39
1
2
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pd−Pt−Fe3O4
Pt−Fe3O4
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
KOAc
DMIh
DMF
Fig. 1 (a) and (b) HR-TEM images of fresh Pd-Pt-Fe3O4 NPs
66
4
The Pd−Pt−Fe3O4 NPs were made up with 4.10 wt% palladium and
3
DMSO
toluene
NMP
NMP
NMP
NMP
NMP
NMP
NMP
67
10
9.60 wt% platinum according to inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) analysis. The alloy status of the Pd-
Pt on F3O4 NPs was confirmed by X-ray diffraction (XRD) pattern and
X-ray photoelectron spectroscopy (XPS) analysis (Fig. S9-11). The
Pd−Pt−Fe3O4 NPs can be effectively separated form reaction
solution through the use of an external magnet and collected within
1 minute (Fig. S15).
4
5
40
94
6
44
6
90
75
7
8
LiOAc
CsOAc
Cs2CO3
DBU
94
81
18
74
9
92
75
75
53
10
11
12
13c
14
15
DBN
100
94
71
To find the optimal conditions for the silylation reaction using
Pd−Pt−Fe3O4 NPs, we chose methyl 4-iodobenzoate (1a) and
i-Pr2EtN
i-Pr2EtN
i-Pr2EtN
i-Pr2EtN
NMP
NMP
NMP
NMP
88
67
45
triethylsilane (2a) as substrates (Table 1). Based on the previously
reported conditions for the Pt-catalyzed silylation reaction, sodium
acetate (NaOAc) was employed as a base. First, various solvents
were tested for the coupling reaction and it was found that using
NMP (N-methyl-2-pyrrolidone) resulted in the best yield of the
product (entry 5). Next, with NMP as the solvent of choice, different
bases were tested. Metal acetates such as KOAc and CsOAc
employed as bases in the silylation reaction showed higher yields of
the desired product, 75% and 74%, respectively (entries 6 and 8),
compared to other acetates. The reaction with Cs2CO3 showed 75%
yield (entry 9). When organic amine bases were employed, 1,8-
diazabicyclo(5.4.0)undec-7-ene (DBU), and 1,5-diazabicyclo(4.3.0)
non-5-ene (DBN) afforded the desired product in 53%, and 71%
91
63
70
21
Pd−Fe3O4
Pt−Fe3O4
16
i-Pr2EtN
NMP
84
62
/Pd−Fe3O4
17
18
Homo Pdd
i-Pr2EtN
i-Pr2EtN
NMP
NMP
0-20
21-23
0
Homo Pte
trace
a Reaction conditions: A mixture of 1a (0.2 mmol), 2a (0.3 mmol), Pd−Pt−Fe3O4 (20 mg,
b
5 mol% of Pd−Pt) and base (0.6 mmol) was stirred in 1.0 mL solvent at 70 °C for 15 h.
Determined by 1H NMR using an internal standard. 1 mol% catalyst was employed.
c
d
Homogeneous palladium catalysts include Pd(OAc)2, Pd(CH3CN)2Cl2, Pd(PPh3)4, and
Pd(PPh3)2Cl2. Homogeneous platinum catalysts include PtCl2 and PtO2. DMI = N,N-
e
h
dimethylimidazolin-2-one.
To optimize the reaction time, the standard reaction was
monitored by 1H NMR. As shown in Fig. S16, when the reaction was
conducted at 70 °C, the yield of the product reached 51% and 88%
2 | J. Name., 2012, 00, 1-3
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in 12 h and 15 h, respectively, and thereafter did not show any
increase. The reaction performed at lower temperatures of 25 °C
and 50 °C did not give satisfactory results even after 24 h, and the
maximum yields at these temperatures were 13% and 36%,
respectively. The reaction carried out at 100 °C showed 83% yield in
10 h, although this yield decreased to 67% at 13 h and slowly
decreased further to 52% as a result of thermal decomposition of
the arylsilane substrate. From these results, it was concluded that
this catalytic system showed optimal yields at 70 °C and the
reaction was complete in 15 h.
reaction methodology. As expected, all aryl bromides were suitable
substitutes of the tested aryl iodides and yielded the desired
arylsilanes in good to moderate yields, which were slightly lower
than those obtained from aryl iodides. However, 4-haloanisoles
afforded desired product 3m with low yields. These result implied
that aryl halides bearing electron-donating group showed low
activity in this arylsilylation. Trihexylsilane (2b) was also coupled
with aryl iodides and bromides to give the desired products 3n, 3o,
3p, 3q, 3r, 3s and 3t in moderate to good yields.
Dimethylphenylsilane (2c) coupled with aryl iodides and bromides
to provide 3u and 3v in moderate yields.
Scheme 1 Arylsilylation of aryl iodides and bromides.a
R1
Ar Si R2
R1
R1
To evaluate the recyclability of the Pd−Pt−Fe3O4 catalyst in the
cat. PdꢀPtꢀFe3O4
H
R2
Ar
X
+
Si
silylation of methyl 4-iodobenzoate, we recovered the catalyst and
reused after each run. After the reaction was complete, the catalyst
was separated by using an external magnet and reused for the
silylation of the next batch of aryl iodide and triethylsilane in the
presence of fresh i-Pr2EtN. This procedure was repeatedly
conducted with the recovered catalyst and the yield of the product
was monitored in every cycle. As shown in Fig. 2, the products were
isolated in 84-90% yields consistently for the reactions performed
over twenty times. In contrast, the yields obtained from the
R1
iꢀPr2EtN (3.0 equiv)
NMP, 70 o
C
2a: R1, R2 = Et
1
3
1
: R , R2 = Hex
X = I, Br
2b
2c
: R1 = Me, R2 = Ph
O
O
H
O
SiEt3
SiEt3
O2N
SiEt3
SiEt3
Me
MeO
3a
3b
3d (63%, 55%)
F
(88%, 68%)
(84%, 79%)
3c
(81%, 61%)
SiEt3
F3C
SiEt3
3f (45%, 20%)
Cl
SiEt3
NC
SiEt3
F
3e (74%, 63%)
3g (84%, 73%)
3h (88%, 68%)
reaction with the monometallic catalyst Pt
after the tenth recycle. Similarly, the reaction employing
monometallic Pd Fe3O4 afforded the desired product in only 12%
−Fe3O4 slowly decreased
S
SiEt3
SiEt3
MeO
SiEt3
SiEt3
N
N
SiEt3
−
N
3i
3j (77%, 61%)
Si(n-Hex)3
3l (ꢀ, 39%)
(79%, 55%)
O
3k (81%, 77%)
3m (18%, 7%)
Si(n-Hex)3
yield after recycling nine times and its catalytic activity did not
recover in the tenth run. From these results, it was concluded that
the bimetallic catalyst showed pronouncedly better catalytic activity
and recyclability in the hydrosilylation of aryl substrates compared
to the corresponding monometallic catalysts.
O
Si(n-Hex)3
F3C
Me
MeO
3p (41%, 20%)
3n
(84%, 63%)
F
3o (81%, 70%)
Si(n-Hex)3
Cl
Si(n-Hex)3
Si(n-Hex)3
Si(n-Hex)3
N
N
F
3q
3t
(ꢀ, 40%)
(77%, 69%)
3s (68%, 52%)
3r (76%, 61%)
Me
Si
Me
Ph
F3
C
Cl
Si Ph
Me
Me
3v (62%, 57%)
3u
(40%, 19%)
a
Reaction condition: A mixture of 1 (0.7 mmol), 2 (1.05 mmol), i-Pr2EtN (1.05 mmol)
and Pd-Pt-Fe3O4 (70 mg) was stirred in NMP (4.0 mL) at 70 oC for 15 h. Yields from aryl
iodides are at the left and yields from aryl bromides are at the right in the parentheses.
Under these optimized reaction conditions, we employed a variety
of aryl iodides and bromides to evaluate the substrate scope
(scheme 1). Aryl iodides (1) having carbonyl groups such as methyl
ester, acetyl, and aldehyde reacted with triethylsilane in the
presence of Pd-Pt-Fe3O4 NPs to provide the corresponding aryl
silanes 3a, 3b, and 3c in 88%, 84%, and 81% yields, respectively.
Aryl iodides substituted with electron-withdrawing groups such as
Fig.
2 Recyclability of Pd−Pt−Fe3O4, Pd−Fe3O4, and Pt−Fe3O4
catalysts in the arylsilylation reaction.
In the case of Pd−Pt−Fe3O4, the catalytic activity was maintained
even after 20 times of reuse, however, the catalytic activity of
Pd−Fe3O4 or Pt−Fe3O4 was reduced at 15th recycle. For the
comparison of the two different cases, HR-TEM, SEM-EDS, ICP-AES,
XRD and XPS of the fresh and spent nanocatalysts were examined.
The oxidation state of the nanocatalysts after the reactions did not
change much (Fig S17-18). However, HR-TEM of Pd−Pt−Fe3O4 NPs
after 20 recycles showed some agglomeration of Pd−Pt NPs (Fig.
S19). Some detachment of Pd−Pt NPs form Fe3O4 support and
inclusion of 0.80 wt% Si were confirmed by SEM-EDS mapping
images and pattern (Fig. S20-22). In addition, metal contents of
spent Pd−Pt−Fe3O4 decreased from their fresh state, e.g. from 3.30
wt% to 2.70 wt% for Pd, and from 5.54 wt% to 3.94 wt% for Pt
(Table S1). As a result, after 20 recycles of Pd−Pt−Fe3O4, it was
1-iodo-4-nitrobenzene
and
4-iodobenzonitrile
gave
the
corresponding products 3d and 3e in 63% and 74% yield,
respectively. 1-Iodo-4-(trifluoromethyl)benzene produced 3f in a
slightly lower yield. However, the chloro- and fluoro-substituted
iodobenzenes formed 3g and 3h in good yields. Heteroaryl iodides
such as 2-iodothiophene, 3-iodopyridine, and 6-iodoquinoline
afforded the corresponding heteroaryl silanes 3i, 3j, and 3k in 79%,
77%, and 81% yields, respectively. As 3-iodoquinoline is not
commercially available, 3-bromoquinoline was employed as
a
coupling partner instead to yield, unexpectedly, 3-
(triethylsilyl)quinoline (3l) in 39% yield. This result prompted an
investigation of aryl bromides as potential substrates for this
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lost upon each cycle. On the other hand, in the cases after 15
recycles using the monometallic catalysts, Pd−Fe3O4, Pt−Fe3O4 NPs
were shown to be highly aggregated and a large degree of metal
loss was confirmed (Fig. S23-26, Table S2 and S3). The remaining Pd
and Pt contents after 15 times recycles of Pd−Fe3O4 and Pt−Fe3O4
catalysts were greatly reduced, i.e. 3.44 wt% from 7.47 wt% (fresh)
and 2.51 wt% from 10.76 wt% (fresh), respectively. In case of
Pd−Fe3O4 and Pt−Fe3O4 recycle test, on average 0.26 wt% of Pd and
0.55 wt% of Pt leached out from each run. It is surprising to note
that Pd−Pt−Fe3O4 NPs showed considerably less liberation of the
transition metal contents compared to the monometallic catalysts.
On the other hand, XRD peaks of spent nanocatalysts were not
precisely identified due to the leaching of a transition metal. (Fig.
S29). The Pd−Pt alloy has very excellent crystallinity as shown BF-
STEM image of Pd−Pt−Fe3O4 (Fig. S30). We may think that Pd−Pt
alloy is structurally and morphologically very stable, its durability in
catalytic reactions is also outstanding. Thus, the bimetallic
Pd−Pt−Fe3O4 exhibits excellent synergistic effect in both reactivity
of silylation reactions and catalyst durability. Based on previous
report6a and our experimental results, we proposed the reaction
mechanism (Fig S31).
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Conclusions
In summary, a recyclable bimetallic catalytic system for the
silylation of aryl halides has been developed. The reaction with aryl
iodides (or bromides) and hydrosilane in the presence of i-Pr2EtN
and Pd−Pt−Fe3O4 catalyst provided the corresponding aryl silanes in
good to moderate yields. In addition, this catalytic system showed
good tolerance toward the ester, ketone, aldehyde, nitro, and
nitrile functional groups. This is the first report of a recyclable
catalytic system for the arylsilylation reaction. The bimetallic
Pd
shows better activity and durability compared to the monometallic
Pd Fe3O4 and Pt Fe3O4 catalysts. The catalyst was recycled up to
−Pt−Fe3O4 catalyst can be readily recovered and reused, and
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−
−
twenty times and the product yields were found to be consistently
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Acknowledgements
This research was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-
2015R1A4A1041036) and the Nano Material Development Program
(NRF-2012M3A7B4049655) through the NRF funded by the Ministry
of Education, Science and Technology. The spectral data were
obtained from the Gwangju center and HRMS data from the Daegu
center of Korea Basic Science Institute.
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