Cyclopalladated complexes of 2-(m-nitrophenyl)imidazolines: Synthesis, characterization and catalytic activity in the Suzuki reaction under mild conditions

Article abstract of DOI:10.1007/s11243-009-9323-8

Three cyclopalladated complexes of 2-(m-nitro-phenyl)imidazolines have been easily prepared and characterized by spectroscopic analysis. The structure of one of the complexes has been determined by single-crystal X-ray analysis. The complexes are effective catalysts for the Suzuki reaction of aryl bromides with phenylboronic acid in aqueous solution at room temperature under air. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11243-009-9323-8

Transition Met Chem (2010) 35:271–277
DOI 10.1007/s11243-009-9323-8
Cyclopalladated complexes of 2-(m-nitrophenyl)imidazolines:
synthesis, characterization and catalytic activity in the Suzuki
reaction under mild conditions
•
Xin-Qi Hao Fang Liu Bi Zhang Mei-Ling Jiang
•
•
•
•
Jun-Fang Gong Mao-Ping Song
Received: 9 October 2009 / Accepted: 11 December 2009 / Published online: 12 January 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Three cyclopalladated complexes of 2-(m-nitro-
phenyl)imidazolines have been easily prepared and char-
acterized by spectroscopic analysis. The structure of one of
the complexes has been determined by single-crystal X-ray
analysis. The complexes are effective catalysts for the
Suzuki reaction of aryl bromides with phenylboronic acid in
aqueous solution at room temperature under air.
found that cyclopalladated ferrocenylimine complexes
with tricyclohexylphosphine (PCy3) or dicyclohexylpho-
sphinobiphenyl ligands exhibit excellent activity at low
catalyst loadings in the coupling of aryl chlorides with
phenylboronic acid [13–15]. Some palladacyclic com-
pounds can even promote the coupling of unactivated
and sterically hindered aryl chlorides at room temperature
although the catalyst loadings are relatively high
(1–2 mol%) [7, 11, 12]. Furthermore, compared with other
palladium catalytic systems, especially those with bulky
alkylphosphines, the cyclopalladated complexes have some
obvious advantages in their usually high stability toward
air, moisture and heat. These properties allowed for cata-
lytic experiments to be done in the presence of air and
water [16–20]. Connecting with our interest in the palla-
dacycles and their applications [13–15, 21–25], herein, we
would like to report the synthesis and characterization of
three cyclopalladated complexes with 2-(m-nitrophenyl)
imidazolines (2a–2c) (Schemes 1, 2). It should be pointed
out that the imidazoline-NH group in ligand (1a) was
acetylated during the palladation reaction, giving the
N-acetylated cyclopalladated complex (2a) (Scheme 2).
The three palladium complexes were successfully applied
in the Suzuki reactions of aryl bromides with phenylbo-
ronic acid in aqueous solution at room temperature under
air. The results are presented in this paper.
Introduction
The palladium-catalyzed Suzuki reaction is one of the most
powerful methods to construct C–C bonds, particularly for
the preparation of biaryl compounds [1, 2]. Recent devel-
opments in this area have been largely focused on the
palladium catalysts that can activate notoriously unreactive
aryl chlorides, and/or function under mild reaction condi-
tions such as at room temperature, under air or in aqueous
media. Substantial progress has been made by using simple
palladium salts with bulky, electron-rich phosphines [3, 4]
or N-heterocyclic carbenes (NHC) [5, 6]. The cyclopalla-
dated compounds or palladacycles containing these ligands
have also been reported to be efficient for the activation of
aryl chlorides under relatively mild reaction conditions
[7–12]. We have systematically studied the cyclopallada-
tion reactions of Schiff base type ferrocenylimines and
Experimental
X.-Q. Hao ꢀ F. Liu ꢀ B. Zhang ꢀ M.-L. Jiang ꢀ J.-F. Gong ꢀ
M.-P. Song (&)
Department of Chemistry, Henan Key Laboratory of Chemical
Biology and Organic Chemistry, Zhengzhou University,
450052 Zhengzhou, People’s Republic of China
e-mail: mpsong9350@zzu.edu.cn
Glacial acetic acid was dried by distilling from P2O5.
m-Nitrobenzaldehyde, m-nitrobenzoic acid, m-nitrobenzoyl
chloride and compound (1a) were prepared according to
literature methods. All other chemicals were used as pur-
chased. Melting points were measured using a WC-1
J.-F. Gong
e-mail: gongjf@zzu.edu.cn
123

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Transition Met Chem (2010) 35:271–277
HN
triethylamine (0.6 mL), followed by p-methoxyaniline or
p-toluidine (2.7 mmol). After stirring for 4 h at room
temperature, 10% NaOH (6 mL) was added and the
mixture was stirred overnight. The aqueous mixture was
extracted with dichloromethane and the combined organic
layers were washed with saturated NaHCO3, brine, dried
over MgSO4 and evaporated. The crude product was
purified by preparative TLC on silica gel plates eluting
with ethyl acetate/petroleum ether to afford compounds
(1b–1c) as pale yellow oils.
O2N
O2N
CHO
CHO
H2N
NH2
N
I2, K2CO3
1a
O2N
O2N
O2N
COCl
1) KMnO4
2) SOCl2
H2N
N
OH
Ar
O
C
O2N
i) SOCl2
N
N
1b 56% yield. IR (KBr, cm-1): v 2935, 2868, 1606,
1531, 1514, 1349, 1246, 1180, 1143, 1040, 997, 908, 833,
739, 696. 1H NMR (400 MHz, CDCl3): d 8.38 (s, 1H,
ArH), 8.16 (d, J = 8.1 Hz, 1H, ArH), 7.77 (d, J = 7.7 Hz,
1H, ArH), 7.42 (t, J = 8.0 Hz, 1H, ArH), 6.84 (d, J =
8.8 Hz, 2H, NAr), 6.75 (d, J = 8.8 Hz, 2H, NAr), 4.11–
4.06 (m, 2H, CH2), 3.99–3.94 (m, 2H, CH2), 3.74 (s, 3H,
OCH3); 13C NMR (100 MHz, CDCl3): d 161.6 (C=N),
156.9, 147.7, 135.8, 134.3, 132.7, 129.0, 125.7, 124.3,
123.7, 114.4, 55.4, 55.3, 53.2. MS (m/z, ESI?): 298
(M ? H). ESI-HRMS: M ? H Calc. for C16H16N3O3:
298.1192, found: 298.1186. M ? Na Calc. for C16H15
N3NaO3: 320.1011, found: 320.1047.
1c 52% yield. IR (KBr, cm-1): v 2928, 2869, 1609,
1525, 1349, 1141, 998, 909, 861, 816, 701. 1H NMR
(400 MHz, CDCl3): d 8.39 (s, 1H, ArH), 8.18 (d,
J = 8.4 Hz, 1H, ArH), 7.77 (d, J = 7.7 Hz, 1H, ArH),
7.43 (t, J = 8.0 Hz, 1H, ArH), 7.00 (d, J = 8.0 Hz, 2H,
NAr), 6.74 (d, J = 8.1 Hz, 2H, NAr), 4.10–3.99 (m, 4H,
CH2CH2), 2.26 (s, 3H, CH3); 13C NMR (100 MHz,
CDCl3): d 161.2 (C=N), 147.9, 140.1, 134.5, 134.4, 132.9,
129.8, 129.1, 124.5, 123.8, 123.6, 54.8, 53.3, 20.8. MS
(m/z, ESI?): 282 (M ? H). ESI-HRMS: M ? H Calc. for
C16H16N3O2: 282.1243, found: 282.1242. M ? Na Calc.
for C16H15N3NaO2: 304.1062, found: 304.1076.
H
ii) ArNH2
OH
iii) 10% NaOH
1b Ar = p-CH3OC6H4
1c Ar = p-CH3C6H4
Scheme 1
microscopic apparatus and are uncorrected. IR spectra were
collected on a Bruker VECTOR22 spectrophotometer in
KBr pellets. H, 13C NMR, and 31P NMR spectra were
1
recorded on a Bruker DPX-400 spectrometer in CDCl3
with TMS as an internal standard for 1H and 13C NMR and
85% H3PO4 as external standard for 31P NMR. Mass
spectra were performed on an Agilent LC/MSD Trap XCT
instrument. HRMS were measured on a Micromass Q-TOF
mass spectrometer (Waters, Manchester, UK) with an ESI
source.
Synthesis of 2-(m-nitrophenyl)imidazolines (1b–1c)
To
a stirred solution of 2-aminoethanol (0.15 mL,
2.5 mmol) and Et3N (0.43 mL) in THF (10 mL) was added
dropwise a solution of m-nitrobenzoyl chloride (371 mg,
2.0 mmol) in THF (10 mL) at room temperature. After
stirring overnight, the reaction mixture was filtered and
evaporated. The residue was purified by column chroma-
tography on silica gel with ethyl acetate/petroleum ether
(1:1) as eluent, giving white solids of the corresponding
amido alcohol 377 mg (90%). The obtained amido alcohol
(377 mg, 1.8 mmol) was then reacted with thionyl chloride
(5 mL) under reflux for 8 h. Excess thionyl chloride was
evaporated. The residue was dissolved in dry diethyl ether
(10 mL) and filtered. To this solution was added dry
Synthesis of cyclopalladated complexes (2a–2c)
A mixture of the appropriate 2-(m-nitrophenyl)imidazoline
(1a–1c) (0.2 mmol) and Pd(OAc)2 (54 mg, 0.24 mmol) in
dry HOAc (60 mL) was refluxed for 12 h under a nitrogen
atmosphere. The solvent was removed under reduced
Scheme 2
R
R
N
N
i) Pd(OAc)2, HOAc
PPh3
CH2Cl2
rt, 3 h
reflux, 12 h
ii) LiCl, acetone,
H2O, rt, 48 h
O2N
O2N
N
N
Pd
Ph3P
2a R = COCH3
2b R = p-CH3OC6H4
2c R = p-CH3C6H4
Cl
1a R = H
1b R = p-CH3OC6H4
1c R = p-CH3C6H4
123

Transition Met Chem (2010) 35:271–277
273
pressure and a solution of lithium chloride (102 mg,
2.4 mmol) in acetone/water (3:2, 35 mL) was added. The
resulting solution was stirred at room temperature for 48 h
then extracted with dichloromethane. The organic layer
was washed with brine, dried over MgSO4 and filtered.
Then to this filtrate was added Ph3P (105 mg, 0.4 mmol).
After stirring at room temperature for 3 h, CH2Cl2 was
evaporated and the residue was purified by preparative
TLC on silica gel plates eluting with CH2Cl2 to afford the
cyclopalladated complexes (2a–2c) as orange solids.
2a m.p.: 186 °C. 21% yield. IR (KBr, cm-1): v 3447,
2924, 1709, 1595, 1511, 1437, 1382, 1341, 1305, 1222,
1096, 1060, 1024, 860, 749, 697. 1H NMR (400 MHz,
CDCl3): d 8.60 (d, J = 2.8 Hz, 1H, ArH), 7.73–7.70
(m, 6H, PPh3), 7.48–7.45 (m, 3H, PPh3), 7.42–7.37 (m, 7H,
PPh3 and ArH), 6.62 (dd, J = 5.2, 8.8 Hz, 1H, ArH), 4.34
(t, J = 8.4 Hz, 2H, CH2), 4.23 (t, J = 8.4 Hz, 2H, CH2),
2.42 (s, 3H, COCH3); 13C NMR (100 MHz, CDCl3): d
170.2, 168.9, 166.2, 144.1, 138.4, 137.9, 137.8, 135.2,
135.1, 131.1, 130.2, 129.6, 128.4, 128.3, 124.0, 123.9,
51.5, 49.3, 25.1. 31P NMR (162 MHz, CDCl3): d 41.81.
MS (m/z, ESI?): 600 (M - Cl). ESI-HRMS: M - Cl Calc.
for C29H25N3O3PPd: 600.0668, found: 600.0604.
(2 mL, v/v = 1/1) under air. The reaction mixture was
stirred at room temperature for 12 h (time not optimized).
Then water was added, and the aqueous phase was
extracted with dichloromethane. The combined organic
layers were washed with water, dried over MgSO4, filtered
and evaporated. The products were isolated by flash chro-
matography on silica gel (the purified products were
identified by comparison of melting points with the liter-
1
ature values or by their H NMR spectra).
X-ray crystallography
The crystals of cyclopalladated complex (2b) were obtained
by recrystallization from acetone/petroleum ether at room
temperature. Crystallographic data for 2bꢀ0.5 CH3COCH3:
C34H29ClN3O3PPdꢀ0.5CH3COCH3, 0.20 9 0.17 9 0.17
mm3, triclinic, P-1, a = 9.6401(19) A, b = 17.292(4)
˚
˚
˚
A, c = 20.719(4) A, a = 78.71(3)°, b = 76.68(3)°, c =
3
˚
81.77(3)°, V = 3278.4(11) A , Z = 4, DCalc = 1.478 Mg
m-3, l = 0.738 mm-1, F(000) = 1488. All diffraction data
of the complex (2b) were collected with a Rigaku-Raxis-IV
imaging plate area detector using graphite-monochromated
˚
Mo Ka radiation (k = 0.71073 A) at 291(2) K. The diffrac-
2b m.p.: 168 °C. 36% yield. IR (KBr, cm-1): v 3426,
2928, 1712, 1590, 1510, 1435, 1252, 1095, 1023, 837, 749,
tion data were corrected for Lorentz and polarization factors.
The structure was solved by direct methods [26] and expanded
using Fourier techniques and refined by full-matrix least-
squares methods using the SHELXTL-97 program package
[27] giving a final R1 = 0.0609, wR2 = 0.1171 and 10450
unique reflections with I [2r (I) for complex (2b). Non-
hydrogen atoms were refined anisotropically, and the hydro-
gen atoms were included but not refined. CCDC reference
number 741291.
1
696. H NMR (400 MHz, CDCl3): d 7.81–7.71 (m, 6H,
PPh3), 7.47–7.46 (m, 3H, PPh3), 7.40–7.39 (m, 6H, PPh3),
7.23–7.20 (m, 4H, ArH and NAr), 7.02–7.00 (m, 2H, NAr),
6.66–6.61 (m, 1H, ArH), 4.34 (t, J = 10.2 Hz, 2H, CH2),
4.11 (t, J = 10.2 Hz, 2H, CH2), 3.87 (s, 3H, OCH3). 31P
NMR(162 MHz, CDCl3): d 41.35. MS (m/z, ESI?): 664
(M - Cl). ESI-HRMS: M - Cl Calc. for C34H29N3O3PPd:
664.0981, found: 664.0944.
2c m.p.: 153 °C. 34% yield. IR (KBr, cm-1): v 3433,
3054, 2924, 2856, 1590, 1511, 1435, 1339, 1158, 1098,
Results and discussion
1
1024, 868, 826, 749, 698. H NMR (400 MHz, CDCl3): d
7.76–7.71 (m, 6H, PPh3), 7.47–7.44 (m, 3H, PPh3), 7.40–
7.36 (m, 6H, PPh3), 7.30–7.26 (m, 4H, ArH and NAr), 7.21
(d, J = 8.2 Hz, 2H, NAr), 6.63 (dd, J = 4.7, 8.5 Hz, 1H,
ArH), 4.35 (t, J = 10.2 Hz, 2H, CH2), 4.13 (t, J =
10.2 Hz, 2H, CH2), 2.43 (s, 3H, CH3); 13C NMR
(100 MHz, CDCl3): d 169.7, 166.2, 143.5, 138.5, 138.4,
137.9, 137.4, 135.3, 135.2, 130.9, 130.7, 130.2, 128.2,
128.1, 126.4, 123.3, 123.2, 120.7, 56.5, 50.7, 21.2. 31P
NMR (162 MHz, CDCl3): d 41.33. MS (m/z, ESI?): 648
(M - Cl). ESI-HRMS: M - Cl Calc. for: C34H29N3O2
PPd: 648.1032, found: 648.1005.
Synthesis and spectroscopic characterization
The three 2-(m-nitrophenyl)imidazolines (1a–1c) were
synthesized starting from m-nitrobenzaldehyde as shown in
Scheme 1. The aldehyde group in m-nitrobenzaldehyde
was directly converted to 2-imidazoline to yield (1a),
according to the published procedure [28]. Compounds
(1b) and (1c) were prepared by oxidation of m-nitrobenz-
aldehyde to m-nitrobenzoic acid with aqueous KMnO4,
followed by the reaction with thionyl chloride to give
m-nitrobenzoyl chloride. Compounds (1b) and (1c) were
then synthesized according to Scheme 1 [29]. The fol-
lowing cyclopalladation was carried out with the ligands
(1a–1c) and 1.2 equivalent of Pd(OAc)2 in refluxing acetic
acid for 12 h, followed by the addition of LiCl to give
the proposed chloride-bridged palladacyclic dimers. The
dimers were subjected to a bridge-splitting reaction with
General procedure for the Suzuki reaction
A tube was charged with aryl bromide (0.5 mmol), phen-
ylboronic acid (0.6 mmol), K2CO3 (1.0 mmol), the catalyst
(2) (0.0025 mmol), n-Bu4NBr (0.5 mmol) and EtOH-H2O
123

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Transition Met Chem (2010) 35:271–277
triphenylphosphine to afford the expected monomers
(2a–2c). It was found that the imidazoline-NH group in
ligand (1a) was acetylated during the palladation reaction.
A possible reason for the acetylation was that small
amounts of Ac2O might be formed by the dehydration of
HOAc with P2O5. Another possibility was that the –NH
was acetylated by HOAc directly, promoted by Pd(OAc)2,
since the condensation of secondary amines with carbox-
ylic acids is usually difficult under heating. Similar acet-
ylations were observed in the platination and palladation
reactions of bis(imidazoline)benzene ligands containing
phenolic-OH groups [25]. All the new compounds were
well characterized by 1H NMR, 13C NMR, 31P NMR, ESI–
MS and IR spectra. The IR spectra of the palladium
complexes (2a–2c) show mC=N around 1,595 cm-1, which
is shifted to lower energy in comparison with the free
2-imidazolines (1a–1c) (around 1,609 cm-1), indicating
the existence of N–Pd coordination. In the 1H NMR
spectra, the signals of the cyclopalladated aryl ring protons
in complexes (2) were significantly shifted upfield, while
those of the N-aryl and imidazoline rings were shifted
downfield relative to the corresponding signals in ligands
(1). These shifts are indicative of the N–Pd coordination
and C–Pd bond formation in the Pd complexes.
Fig. 1 Molecular structure of complex (2b) (representation of one of
the two independent molecules in the asymmetric unit). Hydrogen
atoms and solvent molecules have been omitted for clarity. Selected
˚
bond lengths (A) and angles (°) are as follows (corresponding values
for the unshown second structure are given in brackets): Pd(1)–C(1)
2.042(4) [2.023(4)], Pd(1)–N(2) 2.044(4) [2.064(4)], Pd(1)–P(1)
2.2783(12) [2.2675(12)], Pd(1)–Cl(1) 2.3744(15) [2.3726(13)] and
C(1)–Pd(1)–N(2) 80.22(16) [80.45(16)], C(1)–Pd(1)–P(1) 97.58(13)
[93.08(12)], N(2)–Pd(1)–P(1) 173.56(13) [172.45(11)], C(1)–Pd(1)–
Cl(1) 166.27(12) [169.11(13)], N(2)–Pd(1)–Cl(1) 88.40(11) [90.73
(11)], P(1)–Pd(1)–Cl(1) 94.53(6) [95.24(5)]
˚
˚
N–Ar (C(15)ꢀꢀꢀPd(1) 3.753 A, H(15)ꢀꢀꢀPd(1) 3.188 A,
C(15)–H(15)ꢀꢀꢀPd(1) 121°), one molecule (A) exists as a
dimer. The other molecule (A0) also has a dimeric structure
formed by two types of weak intermolecular hydrogen bonds
between the 0Pd0 or Cl0 atom and the adjacent C–H group of
Crystal structure of complex (2b)
The structures of the cyclopalladated complexes were
further confirmed by a single-crystal X-ray analysis of
complex (2b). The molecule is shown in Fig. 1. There are
two molecules in the asymmetric unit along with one
molecule of acetone. As depicted in Fig. 1, the Pd atom is
in a slightly distorted square-planar environment bonded to
the C1 atom of the cyclopalladated aryl ring, the chlorine
atom, the imidazolinyl imino-nitrogen atom and the P atom
of PPh3. A tricyclic system is thus formed by the cyclo-
palladated aryl ring, the five-membered palladacycle and
the imidazoline ring. The three rings are approximately
coplanar, with the interplanar angles between the cyclo-
palladated aryl ring or imidazoline ring on one hand, and
the palladacycle on the other, being equal to 6.0 (10.4) and
5.2° (3.5°), respectively. However, the N-aryl ring is not
coplanar with the three rings and the dihedral angle between
N-aryl ring with the attached imidazoline ring is 66.4°
(79.0°). All the bond distances and angles around the Pd(II)
center are similar to those observed in the related cyclo-
palladated complexes with 2-oxazolines, of which the
N–Pd–C bond angle (around 80°) is essentially identical
[30, 31]. The coordinated PPh3 is trans to the imino-nitro-
gen of imidazoline with an almost linear P–Pd–N angle.
Weak intermolecular hydrogen bonds are found in the
crystal of 2b. Due to the weak intermolecular hydrogen
bonds between the Pd atom and the adjacent C–H group of
0
0
˚
˚
N–Ar (C(11 )ꢀꢀꢀPd(1 ) 3.753 A, H(11B)ꢀꢀꢀPd(1 ) 3.037 A,
C(110)–H(11B)ꢀꢀꢀPd(10) 135°; C(120)ꢀꢀꢀCl(10) 3.637 A,
˚
0
0
0
˚
H(12B)ꢀꢀꢀCl(1 ) 2.863 A, C(12 )–H(12B)ꢀꢀꢀCl(1 ) 142°)
[32–36]. It is noteworthy that there are also weak intermo-
lecular hydrogen bonds between the two dimeric structures.
One hydrogen bond is between O2 of the nitro group and
0
˚
adjacent C–H group of PPh3 (C(22 )ꢀꢀꢀO(2) 3.367 A,
0
˚
H(22B)ꢀꢀꢀO(2) 2.704 A, C(22 )–H(22B)ꢀꢀꢀO(2) 129°). The
other hydrogen bond involves the O30 atom of the methoxy
group and adjacent C–H group of C–Ar (C(3)ꢀꢀꢀO(30)
0
0
˚
˚
3.313 A, H(3A)ꢀꢀꢀO(3 ) 2.606 A, C(3)–H(3A)ꢀꢀꢀO(3 ) 133°)
[37]. A 2D architecture of complex (2b) is thus constructed
(Fig. 2).
Suzuki reaction
To evaluate the effectiveness of the cyclopalladated com-
plexes (2a–2c) in the Suzuki reaction under mild reaction
conditions, the coupling of bromobenzene with phenylbo-
ronic acid was first chosen as a model reaction. The reac-
tion was performed using complex (2a) as the catalyst in
the presence of n-Bu4NBr under air in aqueous media at
room temperature for 12 h. As shown in Table 1, the base
and the solvent strongly affected the coupling reaction.
When CH3OH-H2O was used as solvent, K2CO3 was found
123

Transition Met Chem (2010) 35:271–277
275
Fig. 2 2D architecture of
complex (2b) formed by
hydrogen bonds. Non-hydrogen
bonding H atoms are omitted for
clarity
to be the most effective among the tested bases such as
Table 1 Suzuki coupling of bromobenzene with phenylboronic acid:
reaction conditions study
K2CO3, KOH, KFꢀ2H2O, K3PO4, Na2CO3, giving the
Complex 2a
biphenyl in
a 86% yield with a catalyst loading
Br
+
B(OH)2
n-Bu4NBr, base
of 0.5 mol% (entries 1–6). Several other solvents using
K2CO3 as the base were also examined. Among them,
EtOH-H2O was the most productive and afforded the
biphenyl in an almost quantitative yield (entry 7). CH3OH
and DMF-H2O gave high yields (entries 8–9), while
THF-H2O and toluene-H2O proved to be ineffective
(entries 10–11).
Solvent, rt, 12 h
Entry
Base
Solvent
Yielda (%)
1
K2CO3
K2CO3
KOH
CH3OH-H2O
CH3OH-H2O
CH3OH-H2O
CH3OH-H2O
CH3OH-H2O
CH3OH-H2O
EtOH-H2O
CH3OH
99b
86
45
Trace
55
57
99
94
82
4
2
3
4
KFꢀ2H2O
K3PO4
With the appropriate combination of base and solvent
(K2CO3 and EtOH-H2O) in hand, the relative activities of
three palladacycles (2) in the coupling of 3-bromotoluene
with phenylboronic acid under the same conditions were
compared. Palladacycle (2c) was found to be the most
active among the three catalysts, giving the coupled
product in 95% yield with a catalyst loading of 0.5 mol%
(Table 2, entries 1–3). In the following experiments,
therefore, the Suzuki coupling reactions of a variety of
electronically and structurally diverse aryl bromides with
phenylboronic acid were investigated using complex (2c)
as the catalyst. Similar to the result obtained for 3-bromo-
toluene, excellent yields (99%) were also obtained in the
case of other electron-neutral aryl bromides such as bro-
mobenzene and 1-bromonaphthalene (entries 4–5). Several
electron-deficient aryl bromides such as 4-chlorobromo-
5
6
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
7
8
9
DMF-H2O
10
11
THF-H2O
Toluene-H2O
9
Reaction conditions: bromobenzene 0.5 mmol, PhB(OH)2 0.6 mmol,
base 1.0 mmol, n-Bu4NBr 0.5 mmol, solvent 2 mL (v/v = 1/1),
0.5 mol% of complex (2a), at room temperature under air for 12 h
a
Isolated yields based on bromobenzene
b
1 mol% mol of complex (2a)
benzene, 2-bromonitrobenzene and 4-bromobenzaldehyde
were also efficiently converted to the corresponding biaryl
products (entries 6–8). Ortho-substituents were tolerated
123

276
Transition Met Chem (2010) 35:271–277
Table 2 Suzuki coupling reac-
tionofarylbromideswithphenyl-
boronic acid catalyzed by 2
Complex 2
+
ArBr
Ar
B(OH)2
n-Bu4NBr, K2CO3
EtOH-H2O, rt, 12 h
Entry
1
ArBr
Product
Yield a (%)
82
Complex 2
2a
Br
Br
Br
2
3
90
95
2b
2c
4
5
99
99
2c
2c
Br
Br
6
7
98
96
2c
2c
Cl
Br
Cl
Br
NO2
NO2
8
9
88
84
2c
2c
OHC
Br
OHC
Reaction conditions: aryl
bromides 0.5 mmol, PhB(OH)2
0.6 mmol, K2CO3 1.0 mmol,
n-Bu4NBr 0.5 mmol, EtOH-
H2O 2 mL (v/v = 1/1),
0.5 mol% of complex (2), at
room temperature under air for
12 h
Br
10
11
70
15
2c
2c
N
N
S
Br
a
Isolated yields based on aryl
Br
S
bromide
Acknowledgments We are grateful to the National Natural Science
Foundation of China (20872133), the Innovation Fund for Out-
standing Scholar of Henan Province (074200510005) and the Natural
Science Foundation of Henan Education Department (2009A150027)
for financial support of this work.
and even the very sterically hindered 2-bromo-m-xylene
could provide the product in a good isolated yield (84%,
entry 9). A moderate yield was obtained in the coupling of
2-bromopyridine with phenylboronic acid (70%, entry 10),
while 2-bromothiophene was found to be a poor coupling
partner in this system giving only a 15% yield (entry 11).
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In conclusion, we have synthesized and characterized three
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