An inexpensive and highly stable palladium(II) complex for room temperature Suzuki coupling reactions under ambient atmosphere

Article abstract of DOI:10.1016/j.tetlet.2009.02.058

A new air- and moisture-stable Pd(II) complex 3, which is a highly efficient catalyst for Suzuki reaction with low Pd-catalyst loading (0.01%), has been synthesized and characterized by single-crystal X-ray crystallography. The corresponding Suzuki coupling products were obtained in satisfactory to excellent yields at room temperature in aqueous media under ambient atmosphere.

Full text of DOI:10.1016/j.tetlet.2009.02.058

Tetrahedron Letters 50 (2009) 1965–1968
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An inexpensive and highly stable palladium(II) complex for room temperature
Suzuki coupling reactions under ambient atmosphere
Mengping Guo a,b,*, Qiaochu Zhang b
a
Institute of Coordination Catalysis, Yichun University, Yichun 336000, PR China
College of Chemistry and Bio-engineering, Yichun University, Yichun 336000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A new air- and moisture-stable Pd(II) complex 3, which is a highly efficient catalyst for Suzuki reaction
with low Pd-catalyst loading (0.01%), has been synthesized and characterized by single-crystal X-ray
crystallography. The corresponding Suzuki coupling products were obtained in satisfactory to excellent
yields at room temperature in aqueous media under ambient atmosphere.
Received 30 September 2008
Revised 2 December 2008
Accepted 10 February 2009
Available online 13 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Dibenzyl diisopropylphosphoramidite
ligand
Palladium complex
Crystal structure
Suzuki reaction
Mild reaction condition
Suzuki cross-coupling reaction has become one of the most
powerful methods to construct C–C bonds.1 The recent develop-
ment of C–C bond formation methods involving aryl halides have
largely focused on palladium catalysts containing electron-rich,
sterically bulky phosphanes. In particular, tri-tert-butylphosphane,
dialkyl(biphenyl)phosphanes, pentaarylferrocenylphosphane, and
coupling reaction, and found that the cross-coupling reaction pro-
ceeded smoothly to provide the desired products in satisfactory to
excellent yields.
Dibenzyl diisopropylphosphoramidite 2, which contains one
P–N bond and two P–O bonds, has not been used as ligand in tran-
sition-metal-catalyzed Suzuki cross-coupling reaction. Since ligand
2 exists as liquid and is moisture-sensitive at room temperature, it
is not easy to handle. However, palladium complex 3, achieved by
(
2,2,2-triferrocenylethyl)diphenylphosphane have proved to be
unique, highly efficient ligands for Suzuki cross-coupling reac-
2
tions. The extraordinary activity of these palladium catalysts in
Suzuki cross-coupling reactions has been explained by an in-
creased propensity of the more electron-rich catalyst to an oxida-
tive addition of the aryl halide and an easier decomplexation of
3
the catalytically active Pd(0)L species (L = phosphane). Conse-
quently, the development of structural diversity of electron-rich
and sterically bulky phosphanes is still of considerable impor-
4
tance. During our continuous research on Suzuki coupling reac-
5
tion, we have been interested in the development of new, high
activity, air-stable palladium catalysts that can be used in room
temperature Suzuki cross-coupling reaction in aqueous media un-
der ambient atmosphere, since such catalysts have potential appli-
cations in industry. Recently, for the first time, we synthesized the
palladium complex 3 with electron-rich, sterically bulky dibenzyl
diisopropylphosphoramidite ligand for palladium-catalyzed Suzuki
*
Corresponding author. Tel.: +86 0795 3200535; fax: +86 0795 3201985.
E-mail address: guomengping65@163.com (M. Guo).
Figure 1. Molecular structure of the complex 3 with the atomic numbering scheme.
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.058

1
966
M. Guo, Q. Zhang / Tetrahedron Letters 50 (2009) 1965–1968
N
P
O
O
O
THF
N
p
Na2PdCl4
+
p N
O
O
O
Pd
rt, 4h
Cl
Cl
1
2
3
Scheme 1.
reacting Na
2
PdCl
4
1 with 2 equiv of the ligand 2 in THF at room
highest yield (Table 1, entry 1) among the tested aqueous–organic
solvents.
Under the optimized reaction conditions, a series of aryl bro-
mides were coupled with phenylboronic acid with 0.01 mol % of
temperature (Scheme 1), is insensitive to air and moisture. The
X-ray crystal structure of 3 is shown in Figure 1. Single-crystal
crystallographic analysis clearly shows that the unit cell of the
complex 3 is built up of mononuclear {[(PhCH
2
O)
2
P(CH
3
)
2
CHNCH
2 3
catalyst, Na CO as the base additive at room temperature in aque-
8
(
3
CH )
2
]
2
PdCl
2
} units. The Pd(II) ion has a square planar coordina-
ous media under ambient atmosphere. As shown in Table 2, the
cross-coupling reaction displayed remarkable tolerance toward
the electronic properties of the substrates. For examples, elec-
tron-rich, as well as electron-deficient, aryl bromides coupled effi-
ciently with phenylboronic acids to provide the desired products in
satisfactory to excellent yields (entries 1–4). No significant differ-
ence was observed in yield or in the reaction time when the effect
of various aryl boronic acids were investigated (entries 7–8 and
12). In particular, aryl bromides with sterically encumbering sub-
stituents such as o-methyl, 2,6-dimethyl also coupled effectively
with phenylboronic acids to give the desired sterically demanding
biaryl products in moderate to good yields (entries 5–6). We sub-
sequently investigated the application of this catalytic system in
the coupling reaction between hetero-aryl bromides and aryl boro-
nic acids, but the results were disappointing (entries 15–16). It
could be that hetero-aryl halides are less reactive in Suzuki
tion geometry, which is coordinated by two chlorine atoms and
two phosphorus atoms from two ligands of dibenzyl dii-
sopropylphosphoramidite. Both chlorine atoms and both phospho-
rus atoms are in cis-position, with bond angles of Cl(2)–Pd(1)–Cl(1)
being 89.42(4)° and P(1)–Pd(1)–P(2) being 98.90(3)°. Regarding
the bond lengths of Pd(1)–Cl(1) [2.3659(9) Å] and Pd(1)–Cl(2)
[
2.3374(10) Å], the former is slightly longer and the latter is
slightly shorter than those found in the similar structures, where
6
the Pd–Cl bond lengths are in the range of [2.341(2)–2.353(3) Å]
and [2.339(2)–2.349(2) Å].7 Both the Pd–P bond distances
2
.2515(9) Å and 2.2570(8) Å are longer than or in accordance with
6
those in the above-cited structures [2.193(2)–2.207(2) Å] and
[
2.245(2)–2.257(2) Å].7 All the bond lengths and bond angles in
the ligand of dibenzyl diisopropylphosphoramidite are in the nor-
mal range.
1
0
Initial catalytic studies with 0.01 mol % of complex 3 were per-
formed on the Suzuki cross-coupling of 4-bromo anisole with
phenylboronic acid in acetone/water (1:1) as a model reaction at
room temperature in air and 2 equiv of various bases.9 As shown
in Table 1, the base additives strongly affected the coupling reac-
cross-coupling reactions. In addition, the coupling reaction of
the relatively unactivated aryl chlorides with phenylboronic acid
was also tested. Only moderate yields of 37–50% were achieved
(entries 17–18).
In summary, we have synthesized and characterized an easily
accessible palladium complex 3 for Suzuki cross-coupling of aryl
halides in low Pd-catalyst loading (0.01 mol %). The advantages of-
fered by this catalytic system are mild conditions, fast reactions at
room temperature in aqueous media under ambient atmosphere,
and excellent yields of products.
tion. The common and inexpensive inorganic bases, such as Na
and K CO or an organic base, Et N, are more effective (entries 1–2
and 4), while Na PO or Cs CO (entries 3 and 9) gave slightly low-
er yields. In addition, the effect of solvents on the coupling reaction
2 3
CO
2
3
3
3
4
2
3
3 3 2
was also examined. CH COCH /H O (1:1) system afforded the
Table 1
Screening of solvents and bases for Suzuki cross-coupling of 4-bromo anisole with phenylboronic acid using catalyst 3
0
.01mol% cat. 3
MeO
Br + (HO)2B
OMe
base, solvent
Entry
Base
Na CO
CO
Solvent (1:1)
Isolated yielda (%)
1
2
3
4
5
6
7
8
9
2
3
CH
3
3
3
3
3
3
3
3
3
COCH
COCH
COCH
COCH
COCH
COCH
COCH
COCH
COCH
3
/H
3
/H
3
/H
3
/H
3
/H
3
/H
3
/H
3
/H
3
/H
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
99
99
79
98
Trace
35
52
Trace
73
K
2
3
CH
CH
CH
CH
CH
CH
CH
CH
Na
NEt
3
PO
4
Á12H
2
O
3
NaOAcÁ3H
NaOH
KOH
NaF
2 3
Cs CO
2
O
10
11
12
13
14
Na
Na
Na
Na
Na
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
THF/H
DMF/H
EtOH/H
PhMe/H
CH OH/H
2
O
54
71
94
44
2
O
2
O
2
O
3
2
O
91
a
Reaction conditions: 4-bromo anisole (1.0 mmol), phenylboronic acid (1.2 mmol), base (2 equiv), solvent (4 ml), room temperature, 4 h.

M. Guo, Q. Zhang / Tetrahedron Letters 50 (2009) 1965–1968
1967
Table 2
a
Complex 3 catalyzed Suzuki cross-coupling of aryl halides and phenylboronic acid
R
+
R
0
.01 mol% Cat. 3
ArBr
B(OH)2
Ar
Na2CO3, CH3COCH3/H2O
rt.
Entry
ArBr
R
Product
Time (h)
Isolated yield (%)
B(OH)2
1
2
3
4
Br
Br
Br
COCH3
CH3
B(OH)2
B(OH)2
B(OH)2
B(OH)2
COCH3
2
99
99
99
99
CH3
2.5
4
OCH3
OCH3
Br
CH3
Br
CH3
Br
CH3
3
CH3
CH3
5
6
B(OH)2
B(OH)2
4
4
89
72
CH3
7
8
Br
Br
COCH3
COCH3
H CO
B(OH)2
H CO
COCH3
COCH3
2
2
92
95
3
3
H C
3
B(OH)2
B(OH)2
B(OH)2
3
H C
CH3
Br
CH3
9
3
H C
4
4
4
90
93
95
CH3
1
0
1
Br
Br
OCH3
3
H CO
3
H CO
OCH3
Br
1
B(OH)2
F
F
1
1
1
1
1
2
3
4
5
6
COCH3
4
4
4
8
8
92
F
B(OH)2
F
COCH3
O
O
B(OH)2
B(OH)2
B(OH)2
B(OH)2
87
Br
Br
92
COOH
COOH
Trace
Trace
Br
Br
S
N
S
N
1
7b
8b
CI
3
H CO
B(OH)2
OCH3
16
16
37
50
1
H COC
CI
B(OH)2
COCH3
3
a
Reaction conditions:1.0 mmol of aryl bromide, 1.2 mmol of aryl boronic acid, 2.0 mmol of Na
2
CO
CO
3
, complex 3 (0.0001 mmol), CH
3
COCH
3
+ H
2
O = 4 ml (1:1).
b
Reaction conditions:1.0 mmol of aryl chloride, 1.2 mmol of aryl boronic acid, 2.0 mmol of Na
2
3
, complex 3 (0.01 mmol), CH COCH
3
3
+ H
2
O = 4 ml (1:1), 50 °C.

1
968
M. Guo, Q. Zhang / Tetrahedron Letters 50 (2009) 1965–1968
8.
The complex {[(PhCH
following procedure: A solution of (PhCH
mmol in THF (2.0 ml) was added dropwise to a suspension of Na
0.147 g (0.5 mmol) in THF (20.0 ml) and the reaction mixture was stirred at
ambient temperature for 4 h. The volume was reduced to ca. 5.0 ml and diethyl
ether was added to precipitate a yellow powder which was then filtered off and
washed with diethyl ether. The complex 3 was obtained in 92% yield. Single
crystals of 3 were obtained by slow evaporation of a CH Cl solution of 3 at
2 2
ambient temperature. X-ray crystallographic analysis was carried out on a
Bruker P4 diffractometer using a rotating anode with graphite monochromated
2
O)
2
P(CH
3
)
2
CHNCH(CH
3
)
O)
2
]
2
2
PdCl
P(CH
2
} (3) was prepared by the
CHNCH(CH (0.345 g),
PdCl of
Acknowledgments
2
3
)
2
3 2
)
1
2
4
We thank the Natural Science Foundation of China (No.
2
0763008), the Science and Technology plan of the Department
of Education of Jiang-xi Province (No. [2007] 310), and the Key
Science and Technology plan of Yichun City (No. [2006] 56) for
financial support.
Mo K
2 2 4 2
a radiation (k = 0.71073 Å). Crystal data for 3: C40H56Cl N O P Pd, M =
References and notes
868.11, space group: monoclinic, P2(1)/c, a = 15.0614(15) Å, b = 15.3221(13) Å,
3
c = 18.6924(17) Å,
T = 153(2) K, Z = 4, D
[I > 2 (I)] = 0.0505, wR
a
= 90.00(0)° b = 98.376(4)°,
c
= 90.00(0)°, V = 4267.7(7) Å ,
À3
À1
1
.
(a) Niyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457; (b) Suzuki, A. J. Organomet.
Chem. 1999, 576, 147; (c) Bellina, F.; Carpita, A.; Rossi, R. Synthesis 2004, 15,
c
= 1.351 g cm
,
l
= 0.675 mm , goodness of fit = 1.073,
R
1
r
2
= 0.1326. Selected bond distances (Å) and angles (°)
2
1
419; (d) Xu, Q.; Duan, W.-L.; Lei, Z.-Y.; Zhu, Z.-B.; Shi, M. Tetrahedron 2005, 61,
1225; (e) Zapf, A.; Beller, M. Chem. Commun. 2005, 431; (f) Chen, T.; Gao, T.;
are shown as follows: Pd(1)–P(1) 2.2515(9), Pd(1)–P(2) 2.2570(8), Pd(1)–Cl(1)
2.3374(10), Pd(1)–Cl(1) 2.3659(9), P(1)–Pd(1)–P(2) 98.90(3), P(1)–Pd(1)–Cl(2)
174.00(4), P(2)–Pd(1)–Cl(2) 87.09(4), P(1)–Pd(1)–Cl(1) 84.59(3), P(2)–Pd(1)–
Cl(1) 176.49(3), Cl(2)–Pd(1)–Cl(1) 89.42(4). Atomic coordinates, bond lengths,
and angles and the other important parameters have been deposited to
Cambridge Crystallographic Data Center as supplementary publication, CCDC
No. 702614. Copies of this information can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1233
336033 or e-mail: deposit@ccdc.cam.ac.uk).
Shi, M. Tetrahedron 2006, 62, 6289; (g) Barder, T. E. J. Am. Chem. Soc. 2006, 128,
98; (h) Zhang, T.; Wang, W.-F.; Gu, X.-X.; Shi, M. Organometallics 2008, 27, 753.
8
2
.
(a) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 1998, 37, 3387; (b) Nishiyama,
M.; Yamamoto, T.; Koie, Y. Tetrahedron Lett. 1998, 39, 617; (c) Hartwig, J. F.;
Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-Roman, L. M. J. Org.
Chem. 1999, 64, 5575; (d) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000,
122, 4020; (e) Littke, A. F.; Fu, G. C. Angew. Chem. 2002, 114, 4350; (f) Littke, A.
F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176.
9.
A
Na
mixture of aryl bromide (1.0 mmol), phenylboronic acid (1.2 mmol),
CO (2.0 mmol), CH COCH /H O (2 ml/2 ml), and catalyst 3 (0.01%) was
3.
4.
5.
(a) Christmann, U.; Vilar, R. Angew. Chem. 2005, 117, 370; (b) Christmann, U.;
Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366; (c) Jose, R.; Garabatos-Perera;
Holger, B. J. Organomet. Chem. 2008, 693, 357.
2
3
3
3
2
stirred at room temperature under air. The reaction mixture was stirred for
4 h, and then quenched with water. The mixture was diluted with diethyl
ether. The organic layer was separated, and the aqueous layer was extracted
with diethyl ether for three times. The combined organic phase was dried
(a) Urgaonkar, S.; Nagarajan, M.; Verkade, J. G. Tetrahedron Lett. 2002, 43, 8921;
(
b) Yu, S.-B.; Hu, X.-P.; Deng, J.; Huang, J.-D.; Wang, D.-Y.; Duan, Z.-C.; Zheng, Z.
Tetrahedron Lett. 2008, 49, 1253.
4
with MgSO , filtrate, solvent was removed on a rotary evaporator, and the
(a) Guo, M.-P.; Jian, F.-F.; He, R. Tetrahedron Lett. 2005, 46, 9017; (b) Guo, M.-P.;
Jian, F.-F.; He, R. Tetrahedron Lett. 2006, 47, 2033; (c) Guo, M.-P.; Jian, F.-F.; He,
R. J. Fluorine Chem. 2006, 127, 177.
product was isolated by thin layer chromatography. The purfied products
were identified by 1H NMR, C NMR spectroscopy and melting points with
the literature data.
13
6
.
.
Slawin, A. M. Z.; Wainwright, M.; Woollins, J. D. J. Chem. Soc., Dalton Trans. 2002,
10. (a) Gong, J.-F.; Liu, G.-Y.; Du, C.-X.; Zhu, Y.; Wu, Y.-J. J. Organomet. Chem. 2005,
690, 3963; (b) Xu, C.; Gong, J.-F.; Yue, S.-F.; Zhu, Y.; Wu, Y.-J. Dalton Trans. 2006,
4730.
5
13.
7
Mikhel, I. S.; Bernardinelli, G.; Alexakis, A. Inorg. Chim. Acta 2006, 359, 1826.