Magnetically recoverable Pd/Fecatalyzed hiyama cross-coupling of aryl bromides with aryl siloxanes

Article abstract of DOI:10.1055/s-0030-1259927

This letter describes a simple and efficient method for the synthesis of biaryls by fluoride-free Hiyama cross-coupling of bromoarenes with aryl siloxanes using magnetically separable Pd/Feas the catalyst under aqueous conditions. This methodology is applicable to wide range of aryl bromides and aryl siloxanes. High catalytic activity, ease of recovery using an external magnetic field and use of water as the solvent are additional eco-friendly attributes of this catalytic system. The catalyst was recycled five times without significant loss of catalytic activity. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1259927

LETTER
1081
Magnetically Recoverable Pd/Fe3O4-Catalyzed Hiyama Cross-Coupling of
Aryl Bromides with Aryl Siloxanes
C
ross-Coupling o
.
f
Aryl Brom
S
ides
w
ith
A
ryl
r
Siloxan
e
es edhar,* A. Suresh Kumar, Divya Yada
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (Council of Scientific and Industrial Research),
Hyderabad 500007, India
Fax +91(40)27160921; E-mail: sreedharb@iict.res.in
Received 2 December 2010
ligand free Hiyama reaction was reported that used palla-
Abstract: This letter describes a simple and efficient method for
dium salts and palladium nanoparticles under aqueous
the synthesis of biaryls by fluoride-free Hiyama cross-coupling of
bromoarenes with aryl siloxanes using magnetically separable Pd/
Fe3O4 as the catalyst under aqueous conditions. This methodology
conditions with significant success.13 Although these re-
sults are encouraging, there is a considerable room for im-
is applicable to wide range of aryl bromides and aryl siloxanes. provement in terms of substrate scope, reaction conditions
High catalytic activity, ease of recovery using an external magnetic
and reusability of the catalyst. In view of the above, it con-
field and use of water as the solvent are additional eco-friendly at-
tinues to be desirable to develop a ligand-free and reus-
tributes of this catalytic system. The catalyst was recycled five
able catalytic system for the Hiyama cross-coupling with
times without significant loss of catalytic activity.
wide substrate scope.
Key words: Hiyama cross-coupling, palladium, aryl siloxanes, aryl
bromides, biaryls
Heterogeneous catalysis is particularly attractive because
it allows the production and ready separation of large
quantities of products with the use of a small amount of
catalyst. Magnetic nanoparticles are a class of nanostruc-
tured materials of current interest, due largely to their ad-
vanced technological and medical applications, either
envisaged or realized.14 Among the various magnetic
nanoparticles under investigation, Fe3O4 nanoparticles are
arguably the most extensively studied14a,e,15 and have re-
cently emerged as promising supports for immobilization
of metal nanoparticles. Fe3O4-supported metal catalysts
can be separated from the reaction medium by an external
permanent magnet.
The biaryl moiety is an important structural motif in a
great number of biologically active compounds and func-
tional molecules.1 Such molecules include the natural
product biphenomycin,2 the pharmaceuticals valsartan,
telmisartan, felbinac, losartan, and imatinib,3 the agro-
chemical boscalid,4 and liquid crystals for LCD screens.5
Biaryls can be synthesized by using Pd-catalyzed Suzuki,6
Negishi,7 Kumada,8 Hiyama,9 and Stille10 reactions. How-
ever, the potentially most attractive of the aforementioned
methodologies is Hiyama cross-coupling because organo-
silicon reagents are commercially available at relatively
low cost or can be easily prepared. These reagents are also
nontoxic and quite stable to the presence of other func-
tionalities and to a variety of reaction conditions. Howev-
er, because organosilanes are comparatively unreactive
nucleophiles and because the corrosive fluoride anion is
often needed to activate the organosilanes, Hiyama cou-
pling has not yet replaced Stille and Suzuki methodolo-
gies.6,9,10
As part of our research program directed towards the util-
ity of Fe3O4 magnetic nanoparticles,16 in the present
study, we have exploited Fe3O4-supported palladium
nanoparticles (Pd/Fe3O4) for the practical and fluoride-
free Hiyama cross-coupling reaction of aryl halides with
aryl siloxanes under aqueous conditions (Scheme 1).
Br
R1
Pd/Fe3O4
The potential of the aqueous Hiyama reaction was recog-
nized with the development of sodium hydroxide as an ef-
ficient promoter, and several examples of the Hiyama
reaction in aqueous medium have recently been demon-
strated.11 Generally, phosphine ligands are used to com-
plex and activate the palladium species, and excellent
results have been reported for the palladium-catalyzed
Hiyama cross-coupling reaction.12 The use of such ligands
is undesirable because of their toxicity and because of
their air- and moisture-sensitivity and consequent conver-
sion into phosphine oxide species. Recently, a phosphine
+
NaOH, H2O
90–100 °C
R2
R1
Si(OR3)3
R2
Scheme 1
The Pd/Fe3O4 catalyst was prepared according to the liter-
ature procedure17 and characterized with XRD, TEM,
XPS, and ICP-AES. The palladium content in the catalyst
was measured as 0.023 mmol·g–1 using ICP-AES. The
XRD peaks for Pd/Fe3O4 were similar to those of the par-
ent Fe3O4, and the appearance of a peak at 39.86° con-
firms the presence of palladium in the metallic state
(Figure 1). In the transmission electron microscopy im-
ages, iron oxide nanoparticles of 25–50 nm in diameter
SYNLETT 2011, No. 8, pp 1081–1084
x
x.
x
x.
2
0
1
1
Advanced online publication: 29.03.2011
DOI: 10.1055/s-0030-1259927; Art ID: D31410ST
© Georg Thieme Verlag Stuttgart · New York

1082
B. Sreedhar et al.
LETTER
and palladium nanoparticles of 5 nm entrapped in iron ox- Table 1 Screening of Bases in the Hiyama Cross-Coupling between
4-Bromotoluene and Trimethoxyphenylsilanea
ide are observed; such measurements also clearly con-
firmed that no change in the morphology took place even
Entry
Base
Yield (%)
after five cycles (Figure 2). X-ray photoelectron spectro-
scopic investigation of the Pd/Fe3O4 at the Pd 3d level
shows peaks at 335.4 and 340.7 eV for Pd 3d5/2 and Pd
3d3/2, respectively, which clearly indicates that the palla-
dium is in the zero oxidation state (Figure 3).
1
2
3
4
5
6
7
NaOH
91
82
55
28
16
31
–
KOH
LiOH
K2CO3
Cs2CO3
KF
K3PO4, NaHCO3, or NaOAc
a Reaction conditions: 4-bromotoluene (1 mmol), trimethoxyphenyl-
silane (1.2 mmol), Pd/Fe3O4 (50 mg, 0.2 mol% Pd), NaOH (3 equiv),
H2O (3 mL), stirred at 90 °C for 6 h.
Figure 1 XRD patterns of pure Fe3O4 and Pd/Fe3O4 catalyst
Pd/Fe3O4 gave no coupled product, despite a prolonged
reaction time.
Under the optimized reaction conditions, a variety of
structurally divergent bromoarenes and a wide range of
aryl siloxanes were selected to assess the scope and gen-
erality of the Pd/Fe3O4-promoted Hiyama cross-coupling
reaction to afford biaryls, as presented in Table 2.
Unsubstituted as well as 4-methyl- and 4-methoxy-substi-
tuted bromobenzenes underwent smooth reaction with
good yields when compared to their 2-substituted counter-
parts (Table 2, entries 1–8). Furthermore, the process
could be extended to other aryl halides, such as 4-fluoro,
4-acetyl- and 3-methoxy-bromobenzenes to generate bi-
aryl products in good to excellent yields (Table 2, entries
9–11). We next turned our attention to Hiyama coupling
of substituted aryl siloxanes and found that, irrespective
of the electronic nature of the substituent, all underwent
the reaction smoothly to give the corresponding products
in good yields (Table 2, entries 3, 4, 6, 8, and 12–14). Giv-
en the remarkably high levels of catalytic activity dis-
played with Pd/Fe3O4, we investigated its use in the
preparation of hindered biaryls. To our satisfaction, we
found that hindered aryl bromides such as 2-bromomesi-
tylene reacted in moderate to good yields (Table 2, entries
15–17). On the other hand, 1-bromonaphthalene under-
went cross-coupling reaction with trimethoxyphenylsi-
lane smoothly and gave the desired product in good yield
(Table 2, entry 18).
Figure 2 TEM images of Pd/Fe3O4 nanoparticles before (a) and af-
ter (b) catalysis observed at 120 kV
Figure 3 XPS spectra of Pd/Fe3O4 catalyst
To explore the potential activity of this Pd/Fe3O4 catalyst,
we used it to catalyze Hiyama cross-coupling of bro-
moarenes with various aryl siloxanes. In an effort to de-
velop a better catalytic system, initially the coupling of 4-
bromotoluene with trimethoxyphenylsilane in pure water
at 90 °C was investigated under open air by using differ-
ent bases (NaOH, KOH, LiOH, K2CO3, K3PO4, Cs2CO3,
KF, NaHCO3 and NaOAc); the results are summarized in
Table 1. The base was found to have a pronounced effect
in this reaction, and NaOH was found to be optimal, with
KOH being almost as good (Table 1, entries 1 and 2). Al-
ternative bases such as LiOH, K2CO3, Cs2CO3 and KF
gave the product in low to moderate yields (Table 1, en-
tries 3–6), and no product was formed with K3PO4,
NaHCO3 or NaOAc (Table 1, entry 7). The control reac-
tion conducted under identical conditions and devoid of
The heterogeneity of the catalyst was evaluated to study
whether the reaction using solid Pd catalysts occurred on
the Fe3O4 surface or was catalyzed by Pd species in the
liquid phase. To address this issue, two separate experi-
ments were conducted with bromobenzene and trimethoxy-
phenylsilane. In the first experiment, the reaction was
terminated after two hours; at this juncture, the catalyst
was separated from the reaction mixture and the reaction
was continued with the filtrate for an additional four
hours. In the second experiment, the reaction was termi-
nated after two hours. In both cases, the desired product
Synlett 2011, No. 8, 1081–1084 © Thieme Stuttgart · New York

LETTER
Cross-Coupling of Aryl Bromides with Aryl Siloxanes
1083
phenylsilane under the optimized reaction conditions. The
catalyst was separated from the reaction mixture and re-
used five times without significant loss of catalytic activ-
ity.
Table 2 Pd/Fe3O4-Catalyzed Hiyama Cross-Coupling of Aryl Bro-
mides with Aryl Siloxanesa
Si(OR3)3
Br
R1
R2
+
R1
R2
Ar1 Ar2
Ar1
1
X
1
2
3
3
oxidative addition
reductive
Pd/Fe3O4
Entry R1
R2
R3
Me
Me
Et
Product 3 Yield (%)
elimination
Ar1
Ar2
1
2
H
H
3a
3b
3c
3d
3e
3f
91
85
87
81
85
81
78
83
72
90
92
87
81
80
65
68
73
92
X
Ar1
4-Me
H
3
4-Me
4-Me
4-Me
H
C
A
Ar2 Si(OR)3
4
2-Me
Et
Na+
2
+
OH
RO
RO
Si– Ar2
HOSi(OR)3 + NaX
5
4-OMe
4-OMe
2-OMe
2-OMe
3-OMe
4-F
Me
Et
transmetalation
NaOH
OR
B
6
4-Me
H
X = Br, R = Me, Et
7
Me
Et
3g
3h
3i
Scheme 2 Possible mechanism for the Pd/Fe3O4-catalyzed Hiyama
cross-coupling reaction
8
4-Me
H
9
Me
Me
Me
Et
As outlined in Scheme 2, plausibly, the Pd/Fe3O4-cata-
lyzed Hiyama cross-coupling reaction takes place in three
steps: (1) oxidative addition, (2) transmetalation, and (3)
reductive elimination.18 The initial oxidative addition of
aryl bromide to Pd/Fe3O4 generates the arylpalladium in-
termediate A. Sodium hydroxide works here as an alterna-
tive promoter to fluoride ions used in conventional
procedures and forms the pentacoordinate arylsilicate
anion B, facilitating transmetallation to afford the
(aryl)(aryl)-Pd/Fe3O4 species C.11 Subsequent reductive
elimination liberates the biaryl product with the regenera-
tion of the active Pd/Fe3O4 to complete the catalytic cycle.
10
11
12
13
14
15b
16b
17b
18
H
3j
4-COMe
H
H
3k
3e
3g
3l
4-OMe
2-OMe
2-Me
H
H
Et
H
Et
2,4,6-Me3
2,4,6-Me3
2,4,6-Me3
1-naphthyl
Me
Et
3m
3n
3o
3p
4-OMe
4-Me
Me
Et
In conclusion, we have developed a simple and efficient
method for the synthesis of biaryls via fluoride-free aque-
ous Hiyama cross-coupling of bromoarenes with aryl
siloxanes using magnetically separable Pd/Fe3O4 as the
catalyst under mild reaction conditions.19 This protocol
can be used to generate a diverse range of biaryls in good
to excellent yields. The catalyst is completely magnetical-
ly recoverable because of the superparamagnetic behavior
of Fe3O4 and the efficiency of the catalyst remains unal-
tered even after five cycles.
Et
a Reaction conditions: aryl halide (1 mmol), aryl siloxane (1.2 mmol),
Pd/Fe3O4 (50 mg, 0.2 mol% Pd), NaOH (3 equiv), H2O (3 mL), stirred
at 90 °C for 6 h.
b Reaction carried out at 100 °C for 15 h.
was obtained in the same yield (30%). Pd was not detected
in the filtrate in either experiment by ICP-AES. These
studies demonstrate that only the Pd bound to Fe3O4 dur-
ing the reaction is active, and the reaction proceeds on the
heterogeneous surface.
Acknowledgment
To check the reusability of the catalyst (Table 3), the re-
action was performed with bromobenzene and trimethoxy-
A.S.K. thanks Council of Scientific and Industrial Research, New
Delhi for the award of Senior Research Fellowship.
Table 3 Recovery and Reuse of Pd/Fe3O4 Catalyst for the Hiyama
Cross-Coupling of Bromobenzene with Trimethoxyphenylsilanea
References and Notes
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Yield (%) for each run
Average yield (%)
1st
91
2nd
90
3rd
86
4th
82
5th
80
86
a Reaction conditions: bromobenzene (1 mmol), trimethoxyphenyl-
silane (1.2 mmol), Pd/Fe3O4 (50 mg, 0.2 mol% of Pd), NaOH (3 equiv),
H2O (3 mL), stirred at 90 °C for 6 h.
Synlett 2011, No. 8, 1081–1084 © Thieme Stuttgart · New York

1084
B. Sreedhar et al.
LETTER
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Synthesis of the Pd/Fe3O4 catalyst: Fe3O4 nanoparticles
were impregnated with Na2PdCl4 (1.0%) aqueous solution
and stirred for 1 h. After impregnation, the suspension was
adjusted to pH 12 by adding NaOH (1 M) and stirred for 6 h.
The solid was washed with distilled H2O. The catalyst
precursors were reduced by adding 0.2 M NaBH4 solution
dropwise under gentle stirring in an ice–water bath for
30 min until no obvious bubbles were observed in the
solution. The resulting Pd/Fe3O4 was washed thoroughly
with distilled H2O and subsequently with EtOH. The
palladium content in the catalyst was measured as
0.023 mmol·g–1 using ICP-AES.
General procedure for the Hiyama reaction: A mixture
of aryl bromide (1 mmol), aryl siloxane (1.2 mmol), NaOH
(3 mmol), Pd/Fe3O4 catalyst (50 mg, 0.2 mol% of Pd) and
distilled H2O (3 mL) was taken in a round-bottomed flask
and stirred at 90 °C for 6 h. After completion of the reaction
(monitored by TLC) the catalyst was easily separated from
the reaction mixture with an external magnet. After
removing the solvent, the crude material was purified by
chromatography on silica gel to afford the pure product. The
spectroscopic data of all known compounds were identical to
those reported in the literature.
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2¢-Methoxy-4-methylbiphenyl (Table 2, entry 8): 1H NMR
(300 MHz, CDCl3): d = 2.35 (s, 3 H), 3.78 (s, 3 H), 6.88–
6.99 (m, 2 H), 7.22–7.28 (m, 2 H), 7.15 (d, J = 8.0 Hz, 2 H),
7.35 (d, J = 8.0 Hz, 2 H). 13C NMR (75 MHz, CDCl3): d =
21.1, 55.4, 111.1, 120.7, 128.3, 128.6, 129.3, 130.7, 131.4,
131.5, 136.4, 156.4. MS (EI): m/z = 198 [M]+.
4¢-Methoxy-2,4,6-trimethylbiphenyl (Table 2, entry 16):
1H NMR (300 MHz, CDCl3): d = 2.03 (s, 6 H), 2.35 (s, 3 H),
3.86 (s, 3 H), 6.95 (s, 2 H), 7.07 (d, J = 8.6 Hz, 2 H), 7.49 (d,
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(EI): m/z = 226 [M]+.
4¢-tert-Butyl-2,4,6-trimethylbiphenyl (Table 2, entry 17):
1H NMR (300 MHz, CDCl3): d = 1.39 (s, 9 H), 1.98 (s, 6 H),
2.30 (s, 3 H), 6.85 (s, 2 H), 7.01 (d, J = 8.3 Hz, 2 H), 7.41 (d,
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MHz, CDCl3): d = 2.45 (s, 3 H), 7.25 (d, J = 8.3 Hz, 2 H),
7.33–7.38 (m, 3 H), 7.39–7.49 (m, 3 H), 7.77–7.92 (m, 3 H).
13C NMR (75 MHz, CDCl3): d = 21.2, 125.3, 125.6, 125.8,
126.0, 126.8, 127.4, 128.2, 128.9, 129.9, 131.6, 133.7,
136.8, 137.7, 140.2. MS (ESI): m/z = 218 [M]+.
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