Application of a dimeric ortho-palladated complex of tribenzylamine as an efficient catalyst in microwave-assisted Hiyama coupling reactions

Article abstract of DOI:10.1002/aoc.1863

The activity of [Pd{C₆H₄(CH₂N(CH ₂Ph)₂)}(Iμ-Br)]₂ complex was investigated in cross-coupling reactions of triethoxy(phenyl)silane with various aryl halides under microwave irradiation. This complex is an efficient and stable catalyst for the synthesis of substituted biphenyls that is non-sensitive to air and moisture. The combination of dimeric complex as homogenous catalyst, microwave irradiation, DMF as microwave-active polar solvent and TBAF as microwave-active additive led to excellent yields in short reaction times. Copyright

Full text of DOI:10.1002/aoc.1863

Full Paper
Received: 4 October 2011
Revised: 16 November 2011
Accepted: 16 November 2011
Published online in Wiley Online Library: 10 January 2012
(wileyonlinelibrary.com) DOI 10.1002/aoc.1863
Application of a dimeric ortho-palladated complex
of tribenzylamine as an efficient catalyst in
microwave-assisted Hiyama coupling reactions
Abdol R. Hajipoura,b* and Fatemeh Rafieea
The activity of [Pd{C6H4(CH2N(CH2Ph)2)}(m-Br)]2 complex was investigated in cross-coupling reactions of triethoxy(phenyl)silane with
various aryl halides under microwave irradiation. This complex is an efficient and stable catalyst for the synthesis of substituted
biphenyls that is non-sensitive to air and moisture. The combination of dimeric complex as homogenous catalyst, microwave
irradiation, DMF as microwave-active polar solvent and TBAF as microwave-active additive led to excellent yields in short reaction
times. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: cyclopalladated catalyst; tribenzylamine; Hiyama reaction; biaryls
completion with traditional heating. Microwave irradiation is
known as a state-of-the-art and powerful technology by which
Introduction
Palladium-catalyzed C-C coupling reactions to produce symmetri-
cal and unsymmetrical biaryls are among the most potent and
convenient tools of modern organic synthesis.[1] Biaryls are
applied as the building block of a wide range of herbicides,[2]
pharmaceuticals,[3] natural products,[4] bioactive products,[5]
microelectrode arrays,[6] conducting polymers, and liquid crystal
reactions can be brought to completion in seconds or minutes
rather than in hours or days.[29] High-speed Hiyama coupling reac-
tions have been carried out successfully under controlled micro-
wave conditions.[28,30] The use of a homogeneous metal catalyst
in conjunction with microwave irradiation has led to an increased
lifetime of the catalyst, saving time and energy, producing high
yields and decreasing discarded byproducts from thermal side
reactions.[31–34]
materials.[7]
Palladium-catalyzed
Suzuki
(organoboron
reagents),[8,9] Negishi (organozinc reagents),[10] Kumada (organo-
magnesium reagents),[11] and Stille (organotin reagents)[12]
coupling procedures are employed widely to construct biaryls.
Hiyama coupling reactions using organosilicon reagents have
been intensively developed and utilized as an important and
useful method for producing carbon–carbon bonds.[13] Silicon
compounds are more stable toward air and moisture than orga-
nomagnesium and organozinc reagents. Organoboron reagents
are difficult to purify and some are unstable, losing boron readily
to give homocoupling products,[14] while organotin compounds
release toxic tin byproducts.[15] The organosilicon reagents are
available at low cost or are very easily prepared, nontoxic, and
are very stable to other functionalities and to the reaction condi-
tions. Because of the low reactivity of organosilanes, the reaction
generally requires fluoride anions for activation of the stable sili-
con–carbon bond to facilitate the transmetallation step in the
catalytic cycle.[16] Many efforts have focused on the development
of efficient and selective catalytic systems for this transformation us-
ing various palladium catalysts and fluoride derivatives.[17–23] A few
phosphine-free Hiyama coupling reactions using N-ligand-based
palladium catalysts have been reported.[24,25] Among the new cata-
lytic systems, palladacycle complexes as active and more air-inert
catalytic candidates have been employed in cross-coupling
reactions such as Hiyama coupling reactions. The high productivity
of palladacycle catalysts is due to the slow generation of low ligated
Pd(0) complexes from a stable palladium(II) pre-catalyst.[26–28]
Transition-metal-catalyzed cross-coupling reactions typically
need long reaction times and an inert atmosphere to reach
In continuation of our recent investigations on the synthesis
and application of the palladacycle catalysts in microwave-
assisted cross-coupling reactions,[35–42] we now wish to report
the extension of the [Pd{C6H4(CH2N(CH2Ph)2)(m-Br)]2 homoge-
neous complex[43] as a thermally stable and oxygen-insensitive
catalyst for the cross-coupling of triethoxy(phenyl)silane with
various aryl halides under microwave irradiation.
Results and Discussion
We have recently applied the dimeric ortho-palladate complex
[Pd{C6H4(CH2N(CH2Ph)2)}(m-Br)]2 in the Heck coupling reaction.[40]
A proper palladium-catalyzed reaction requires high catalyst
productivity and activity. Also the availability and catalyst costs
and the price of the organic starting materials are of great impor-
tance. Tribenzylamine as an N-donor ligand is an available and in-
expensive amine. The ortho-palladation reaction of this substrate
*
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156,
Islamic Republic of Iran. E-mail: haji@cc.iut.ac.ir
a Department of Chemistry, Isfahan University of Technology, Isfahan 84156, Iran
b Department of Pharmacology, University of Wisconsin, Medical School, Madison,
WI, 53706-1532, USA
Appl. Organometal. Chem. 2012, 26, 51–55
Copyright © 2012 John Wiley & Sons, Ltd.

A. R. Hajipour and F. Rafiee
is simple and leads to an efficient catalyst in low loading for cou-
pling reactions – even aryl chlorides – as available and cheap sub-
strates. In this paper, the efficiency of this catalyst is investigated
in Hiyama coupling reactions to produce substituted biphenyls
under microwave irradiation (Scheme 1).
In order to probe the efficiency of this catalytic system and op-
timize the reaction conditions, a series of screening experiments
were carried out in cross-coupling of 4-bromoanisole with PhSi
(OEt)3 as the model substrates. The results are summarized in
Table 1. The monitoring system for reaction times, temperature
and power in the microwave reactor led to an excellent control
of reaction parameters, which generally led to rapid optimization
and more reproducible reaction conditions. We examined the
effect of various reaction parameters such as solvent,
temperature and additive on the yields of coupling products as
shown in Table 1. In general, all the couplings with organosilicon
compounds are promoted by fluoride anion, usually tetrabuty-
lammonium fluoride (TBAF), by formation of the corresponding
hypervalent fluorosilicate to facilities the transmetalation step.
According to experimental evidence pentacoordinate silicates
are more reactive than the related tetracoordinate silanes. The
fluorine substituent may be replaced by an alkoxy, hydroxyl, or
any other heteroatom substituent.[16a] We investigated the effect
of type and amount of fluoride salts such as TBAF.3H2O, KF, NaF,
CsF and also NaOH and other bases as additives. Among these
promoters, TBAF.3H2O (1.2 equiv.) was found to be the most ef-
fective. In this reaction 4,4′-dimethoxybiphenyl was formed due
to the homocoupling reaction of 4-bromoanisole as byproduct.
Scheme 1. The Hiyama coupling reaction by cyclopalladated complex of tribenzylamine.
Table 1. Optimization of reaction conditions under microwave irradiationa
Entry
Solvent
Additive
Temperature (ꢀC)
Yield (%)c
Yield (%)
of 4-methoxybiphenyl
of homocoupled byproduct
1
THF
TBAF
TBAF
NaOH
NaOH
TBAF
TBAF
TBAF
TBAF
TBAF
TBAF
TBAFd
TBAFe
KF
60
110
90
70
100
70
55
60
90
90
90
90
90
90
90
90
90
90
90
90
95
75
70
60
70
65
50
40
60
75
100
80
90
15
25
10
—
—
—
20
—
50
—
10
8
2
NMP
DMF
EtOH
p-Xylene
EtOH
MeOH
THF
3
4
5
5
10
5
6
7
5
8b
9b
10
11
12
13
14
15
16
17
18
19
20
21b
20
25
—
—
10
7
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Dioxane
CsF
5
NaF
10
30
15
20
—
10
10
K2CO3
Cs2CO3
NaOAC
NaOH
—
TBAF
aReaction conditions: triethoxy(phenyl)silane (1.2 mmol), additive (1.2equiv.), THF (2 ml), 1 h, room temperature, stirring, then 4-bromoanisole
(1 mmol), palladacycle catalyst (0.05 mmol%), solvent, MW, 500 W, 5 min.
bReaction conditions: 4-bromoanisole (1 mmol), triethoxy(phenyl)silane (1.2 mmol), additive (1.2 equiv.), solvent (2 ml), palladacycle catalyst
(0.05 mmol%), 500 W, 5 min.
cGC yield.
dTBAF (0.75 equiv.).
eTBAF (2equiv.).
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Appl. Organometal. Chem. 2012, 26, 51–55

Hiyama coupling by Ortho-palladated complex
Reducing the amount of TBAF gave lower yields of the desired
product (Table 1, entry 11), whereas increased amount of TBAF
(2 equiv.) led rather to yields of both cross- coupling and homo-
coupling products (Table 1, entry 12). In order to reduce side
product formation, as well as to further increase the desired
product yield, the reaction was carried out in two steps. First a
mixture of TBAF.3H2O and PhSi(OEt)3 in THF was stirred at room
temperature for 1 h, then THF was removed and aryl halide, cat-
alyst and solvent were added to the mixture and heated under
microwave irradiation. Under these reaction conditions, no
homocoupled side product was detected. The use of KF, NaF or
CsF to replace TBAF as an additive resulted in low product yields
(Table 1, entries 13–15). Other bases such as K2CO3, Cs2CO3, and
NaOAc failed to promote the cross-coupling reaction (Table 1,
entries 16–18), while the use of NaOH led to 20% yield (Table 1,
entry 19). No cross-coupled product was obtained without using
of additive (Table 1, entry 20). Also, we examined homocoupling
of PhSi(OEt)3 in the absence of 4-bromoanisole, and in this case
no biphenyl was observed.
Various catalyst concentrations were also tested (Table 2). The low
palladium concentration usually led to a long period of reaction, as
increasing the amount of palladium catalyst shortened the reaction
time, but did not increase the yield of 4-methoxybiphenyl.
According to results obtained, DMF as solvent, TBAF.3H2O as the
promoter and 0.1 mmol% of catalyst gave the best results (Table 2,
entry 6). As this catalytic system is not sensitive to oxygen, the
reactions were carried out under air atmosphere.
steric hindrance of the procedure was examined using 2-, 3- and
4-bromoacetophenone as hindered substituted aryls (Table 3,
entries 10–12). Increasing the hindrance in the vicinity of the
leaving group can cause a decrease in the reaction conversion.
2-Bromoacetophenone showed slower reaction times and there-
fore only a reasonable yield was obtained. The chemoselectivity
of the procedure was examined using bromochlorobenzenes
(Table 3, entries 13–15). In these reactions Br acted as a better
leaving group. In some of these cross-coupling reactions,
symmetrical biphenyls were produced in low yields (0–5%) as
byproducts due to the homocoupling reactions of aryl halides.
A general catalytic cycle for the Hiyama reaction has been
presented in Scheme 2. Initially, Pd(II) converts to Pd(0), followed
by the oxidative addition of aryl halide to Pd(0) to form aryl
palladium(II) intermediate 1. Triethoxy(phenyl)silane is activated
by TBAF and produced pentacoordinate phenylsilicate anion,
which reacts with intermediate 1. After transmetallation reaction,
intermediate 2 is obtained. Finally, reductive elimination of inter-
mediate 2 produces the desired coupling products.[44] Several stud-
ies in palladacycle catalyst cross-coupling have indicated that these
catalysts behave as a mere resource for producing Pd(0) species. To
investigate the suggested mechanism, the mercury drop test was
carried out under the indicated optimized conditions, since
mercury leads to amalgamation of the surface of a heterogeneous
catalyst. In contrast, Hg(0) cannot have a poisoning effect on homo-
geneous palladium complexes, where the Pd(II) metal center is
tightly bound to the ligand.[45,46] When a drop of Hg(0) was added
to the reaction mixtures, no catalytic activity was observed for this
catalyst. The data obtained can be rationalized as a Pd(0):Pd(II) cycle.
These optimized reaction conditions were applied in the
Hiyama cross-coupling reaction of various aryl halides under
microwave irradiation (Table 3).
We examined the electronic and steric effects on the resulting
yields and conversion times. Aryl iodides and bromides substi-
tuted with electron-withdrawing groups transformed to the
corresponding coupled products rather than electron-donating
substituent, with better conversions and shorter reaction times.
Aryl chlorides are more attractive as starting materials because
of their lower cost and wider availability compared to their
bromide or iodide counterparts. We attempted to extend this
catalytic system to aryl chlorides: activated aryl chlorides afforded
the desired products in good yields (Table 3, entries 20–23). The
Conclusion
We report here an active and efficient catalyst for promoting the
Hiyama cross-coupling reaction of various aryl halides with
triethoxy(phenyl)silane to produce the corresponding products
in excellent yields and short reaction times. The combination of
homogeneous complex as catalyst, microwave irradiation, DMF
as microwave active polar and also high dipole moment of TBAF
as additive improved the yields of the reactions, decreased
Table 2. Optimization of catalyst concentration under microwave irradiationa
TOF (hÀ1 c
)
Yield (%)b
Time (min)
Catalyst (mol%)
Entry
—
0
10
15
12
10
7
None
0.005
0.01
0.05
0.07
0.1
1
2
3
4
5
6
7
8 000
7 500
8 400
11 020
20 000
15 000
10
15
70
90
100
100
3
2
0.2
aReaction conditions: triethoxy(phenyl)silane (1.2 mmol), TBAF.3H2O (1.2 mmol), THF (2 ml), 1 h, room temperature, stirring, then 4-bromoanisole
(1 mmol), palladacycle catalyst, DMF, MW, 90ꢀC, 500 W.
bGC yield.
cEstimated values.
Appl. Organometal. Chem. 2012, 26, 51–55
Copyright © 2012 John Wiley & Sons, Ltd.
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A. R. Hajipour and F. Rafiee
Table 3. Cross-coupling reaction of triethoxy(phenyl)silane with various aryl halides under microwave irradiationa
Entry
Ar-X
Biaryl product
Ph-Ph
Time (min)
Yield (%)b
1
Ph-I
1
2
3
5
4
2
3
4
5
6
8
10
2
3
8
5
7
5
7
7
8
8
7
97
95
92
85
96
97
95
96
94
90
78
70
92
95
76
88
82
90
94
84
76
87
68
2
p-MeO-Ph-I
p-O2N-Ph-I
p-MeO-Ph-Ph
p-O2N-Ph-Ph
p-HOOC-Ph-Ph
p-PhOC-Ph-Ph
Ph-Ph
3
4
p-HOOC-Ph-I
p-PhOC-Ph-I
Ph-Br
5
6
7
p-MeO-Ph-Br
p-O2N-Ph-Br
p-NC-Ph-Br
p-MeO-Ph-Ph
p-O2N-Ph-Ph
p-NC-Ph-Ph
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
p-MeOC-Ph-Br
m-MeOC-Ph-Br
o-MeOC-Ph-Br
p-Cl-Ph-Br
p-MeOC-Ph-Ph
m-MeOC-Ph-Ph
o-MeOC-Ph-Ph
p-Cl-Ph-Ph
m-Cl-Ph-Br
m-Cl-Ph-Ph
o-Cl-Ph-Br
o-Cl-Ph-Ph
p-OHC-Ph-Br
p-HOOC-Ph-Br
1-Br-Naphthalene
9-Br-Phenanthrene
Ph-Cl
p-OHC-Ph-Ph
p-HOOC-Ph-Ph
1-Ph-Naphthalene
9-Ph-Phenanthrene
Ph-Ph
p-OHC-Ph-Cl
p-MeOC-Ph-Cl
p-O2NPh-Cl
p-OHC-Ph-Ph
p-MeOC-Ph-Ph
p-O2N-Ph-Ph
aReaction conditions: triethoxy(phenyl)silane (1.2 mmol), TBAF.3H2O (1.2 mmol), THF (2 ml), 1 h, room temperature, stirring, then 4-bromoanisole
(1 mmol), palladacycle catalyst (0.1 mmol%), DMF, MW, 90ꢀC, 500 W.
bIsolated yield.
reaction times and increased the lifetime of the catalyst. Because
of the commercial availability of aryl silicon reagents, this
methodology offers a valuable alternative to traditional methods
for biaryl formation such as Suzuki and Stille couplings.
Experimental
General
All melting points were taken on a Gallenkamp melting apparatus
and are uncorrected. 1H NMR spectra were recorded using
400 MHz in CDCl3 solutions at room temperature (tetramethylsilane
(TMS) was used as an internal standard) on a Bruker Avance 500
instrument (Rheinstetten, Germany) and a Varian 400 NMR. FT-IR
spectra were recorded on a spectrophotometer (Jasco-680, Japan).
Spectra of solids were carried out using KBr pellets. Vibrational
transition frequencies are reported as wave number (cmÀ1). We
used the Milestone microwave (Microwave Labstation MLS GmbH
ATC-FO 300) for synthesis. We also used a BEIFIN 3420 gas
chromatograph equipped with a Varian CP SIL 5CB column (30 m,
0.32 mm, 0.25 mm) for examination of reaction completion and
yields. Palladium acetate, triethoxy(phenyl)silane, aryl halides and
all chemicals were purchased from Merck and Aldrich and were
used as received.
General Procedure for the Hiyama Reaction of Aryl Halides
A mixture of TBAF.3H2O (1.2 mmol) and triethoxy(phenyl)silane
(1.2 mmol) in THF (2 ml) was stirred at room temperature for
1 h, then THF was removed and DMF, aryl halide (1 mmol) and
palladacycle catalyst (0.1 mmol %) were added to the mixture
and placed in the Milestone microwave. Initially the microwave
Scheme 2. Proposed mechanism for Hiyama coupling reaction.
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Appl. Organometal. Chem. 2012, 26, 51–55

Hiyama coupling by Ortho-palladated complex
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4-Phenylbenzophenone (Table 3, entry 5)
M.p. 98–100ꢀC, 1H NMR (400 MHz, CDCl3, TMS) d = 7.90 (d, 2H,
J = 8.1 Hz), 7.84 (d, 2H, J = 8.1 Hz), 7.71 (d, 2H, J = 8.1Hz), 7.65
(d, 2H, J =7.7 Hz), 7.60 (t, 1H, J = 8.1 Hz), 7.52–7.47 (m, 4H), 7.41
(t, 1H). 13C NMR (100 MHz, ppm, CDCl3) d =196.76, 145.67, 140.43,
138.22, 136.69, 132.74, 131.15, 130.43, 129.40, 128.74, 128.62,
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4-Biphenylcarbaldehyde (Table 3, entries 16 and 21)
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(d, 2H, J = 8.0 Hz), 7.65 (d, 2H, J = 8.0 Hz), 7.54 (d, 2H, J = 7.2 Hz),
7.38 (m, 2H), 7.31 (m, 1H). 13C NMR (100 MHz, ppm, CDCl3)
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694, 2548.
M.p. 58–60ꢀC, 1H-NMR (400 MHz, CDCl3, TMS) d = 8.80 (d, 1H,
J = 8.4 Hz), 8.74 (d, 1H, J = 8.0 Hz), 8.10 (s, 1H), 7.92 (t, 2H,
J = 8.8 Hz), 7.62–7.73 (m, 4H), 7.54–7.58 (m, 4H), 7.50 (brs d, 1H).
13C NMR (100 MHz, ppm, CDCl3) d = 140.8, 138.8, 130.1, 128.7,
128.3, 128.1, 127.9, 127.6, 127.5, 127.4, 127.3, 127.1, 126.9, 126.8,
126.6, 126.5, 126.4, 122.9, 122.6., FT-IR (KBr, cmÀ1): υ 1600.
Acknowledgments
We gratefully acknowledge the funding support received for this
project from Isfahan University of Technology (IUT), IR Iran and
Isfahan Science and Technology Town (ISTT), Iran. Further finan-
cial support from the Center of Excellence in Sensor and Green
Chemistry Research (IUT) is gratefully acknowledged.
[40] A. R. Hajipour, F. Rafiee, J. Organomet. Chem. 2011, 696, 2669.
[41] A. R. Hajipour, F. Rafiee, Appl. Organomet. Chem. 2011, 25, 542.
[42] A. R. Hajipour, K. Karami, G. Tavakoli, J. Organomet. Chem. 2011, 696,
819.
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