Suzuki cross-coupling catalyzed by palladium (II) complexes bearing 1-aryl-3,4,5,6-tetrahydropyrimidine ligands

Article abstract of DOI:10.1016/j.jorganchem.2012.01.015

A novel highly efficient monodentate imine palladium catalyst system has been tested for Suzuki cross-coupling reaction. Under the standard conditions, a series of aryl halides, including phenyl iodide, aryl bromides and phenyl chloride, were coupled with phenylboronic or 1-naphthylboronic acid, producing the corresponding biaryls in good to excellent yields. High-resolution transmission electron microscopy (HR-TEM) study of the recycled catalyst showed the formation of palladium nano particles in the catalytic system.

Full text of DOI:10.1016/j.jorganchem.2012.01.015

Journal of Organometallic Chemistry 705 (2012) 39e43
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Suzuki cross-coupling catalyzed by palladium (II) complexes bearing 1-aryl-
3,4,5,6-tetrahydropyrimidine ligands
,
,
b
b
b
a,
Pu Mao a b, Liangru Yang b , Yongmei Xiao , Jinwei Yuan , Xiujun Liu , Maoping Song
*
*
a Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China
b School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel highly efficient monodentate imine palladium catalyst system has been tested for Suzuki cross-
coupling reaction. Under the standard conditions, a series of aryl halides, including phenyl iodide, aryl
bromides and phenyl chloride, were coupled with phenylboronic or 1-naphthylboronic acid, producing
the corresponding biaryls in good to excellent yields. High-resolution transmission electron microscopy
(HR-TEM) study of the recycled catalyst showed the formation of palladium nano particles in the catalytic
system.
Received 23 August 2011
Received in revised form
16 January 2012
Accepted 17 January 2012
Keywords:
Suzuki cross-coupling
Catalyst
Ó 2012 Elsevier B.V. All rights reserved.
Palladium nano particle
Tetrahydropyrimidine
Imine ligand
1. Introduction
extensively investigated and successfully employed in catalytic
transformations, while research on monodentate imine ligand
The palladium-catalyzed Suzuki cross-coupling between orga-
noboronic acid and halides has been evolved into one of the most
important and powerful methods for carbonecarbon bond forma-
tion because of the versatility and the potentiality of the products
formed [1e4]. The area that has perhaps received most research is
the development of new catalysts and ligands for Suzuki cross-
coupling reaction, and the use of these catalytic systems in the
synthesis of pharmaceutical agents, organic materials, and natural
products [5e13]. Until now, phosphine-based ligands have
remained to be the most popular selection in the palladium-
catalyzed cross-coupling reactions [14e21]. However, their sensi-
tivity to air and moisture is known to limit their stability and shelf-
life. Recent application of alternative ligands such as N-heterocyclic
carbenes (NHCs) [22e28], as well as ligand free systems [29e32], in
the coupling reactions has opened new opportunities in catalysis.
Nitrogen ligands, such as amine [33e36], diazabutadienes [37],
oxazolines [38e40], and hydrazones [41] have attracted consider-
able interest due to their stability and excellent activity. Examples
of Schiff base derived systems, such as cyclopalladated imine
[42,43], multi- or bi-dentate imine ligands [44e49] have been
remains scarce to date [50]. Here we report a highly efficient
catalyst system for Suzuki cross-coupling using simple mono-
dentate imine palladium complexes.
2. Results and discussion
Our group is interested in the preparation of new N-hetero-
cyclic carbene (NHC) ligands based on substituted 1,4,5,6-
tetrahydropyrimidine and their palladium complexes. In the
course of preparing the palladium complexes of some bi-dentate
NHC ligand, we observed that the reaction of the methylene
bridged tetrahydropyrimidine salts with Pd(OAc)2, under unopti-
mized reaction conditions, afforded novel monodentate imine
coordinated palladium complexes, trans-bis-(1-aryl-3,4,5,6-
tetrahydropyrimidine)dibromopalladium(II) (1 and 2) (Scheme 1)
[51]. Aiming at finding versatile, robust, and easy-to-prepare
catalytic systems suitable for a wide range of carbonecarbon
bond forming processes, we investigated the activity of the new
imine palladium complexes toward the Suzuki cross-coupling.
Initially, we conducted a brief base and solvent screening by
running the cross-coupling of bromobenzene and phenylboronic
acid as a model (Scheme 2). Among the bases tested, NEt3, K2CO3,
Na2CO3, NaHCO3, and t-BuOK all produced good yields (Table 1,
entries 1e5). Tests of different solvents showed that toluene,
dioxane, DMF or a 1:1 mixture of DMF/H2O are all efficient system
* Corresponding authors. Tel.: þ86 371 67756712; fax: þ86 371 67756718.
E-mail addresses: lryang@haut.edu.cn (L. Yang), henangongda@yahoo.com
(M. Song).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.01.015

40
P. Mao et al. / Journal of Organometallic Chemistry 705 (2012) 39e43
Table 1
Base and solvent effect on the cross-coupling of bromobenzene with phenylboronic
acid.
Entrya Catalyst loading Base
Solvent
Reaction GC yield TON
(mol%)
time (h)
(%)
1
2
3
4
5
6
7
8
9
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
NEt3
K2CO3
Na2CO3
DMF/H2O
DMF/H2O
DMF/H2O
2
2
2
2
2
2
2
2
2
74
88
87
85
90
86
89
88
60
148
176
174
170
180
172
178
176
120
Scheme 1. Synthesis of complexes 1 and 2.
NaHCO3 DMF/H2O
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
DMF/H2O
Toluene
Dioxane
DMF
for the cross-coupling, producing the product biphenyl in yields
around 90%, while the reaction in pure water resulted in a relatively
low yield of 60% (Table 1, entries 5e9).
H2O
The effect of catalyst loading on the model reaction was then
investigated by running the cross-coupling of bromobenzene and
phenylboronic acid in a 1:1 mixture of DMF/H2O, using complex 2
as catalyst and t-BuOK as base. As shown in Table 2, compared to
the blank reaction, addition of 0.5 mol% complex 2 accelerated the
cross-coupling efficiently, improving the biphenyl yield from 16% to
90% (Table 2, entries 1, 5). Increasing catalyst loading to 1.0 mol%,
1.5 mol% or 2.0 mol% increased the reaction rate, lead to more than
50% conversion within 0.25 h and high TOF (Turn Over Frequency),
but did not improve the yield significantly (Table 2, entries 2e17).
While decreasing the catalyst loading resulted in relatively low
yields and slow reaction rate, as long as 24 h were needed to obtain
high yields, either for bromobenzene or iodobenzene (Table 2,
entries 18e21). These results indicated that the proper catalyst
loading is 0.5 mol% to phenylhalides and the catalyst is stable under
the reaction conditions for long time.
a
Reaction condition: 1.0 mmol bromobenzene, 1.0 mmol phenylboronic acid,
1.5 mmol base, 3 mL solvent, 100 ꢀC. All reactions were monitored by GC.
opposite effect of electronegativity and space effect, the overall
effect is negligible (Table 3, entries 1, 11).
As there are many examples of the formation of palladium nano
particles from the palladium complexes with nitrogen ligands [52].
We studied the recycled catalyst through electron microscopic
techniques to determine the possibility of palladium nano particles
formation in our system. Indeed high-resolution transmission
electron microscopy (HR-TEM) study of the recycled catalyst
showed the formation of palladium nano particles (diameter
around 20 nm) (Fig. 1), which maybe the true active species and
responsible for the similar activities of complexes 1 and 2.
3. Conclusion
Under the standard conditions, using t-BuOK as base and a 1:1
mixture of DMF/H2O as solvent, the catalytic activity of complexes 1
and 2 toward Suzuki cross-coupling of series aryl halides, including
phenyl iodide, aryl bromides and phenyl chloride, with phenyl-
boronic acid and 1-naphthylboronic acid was investigated and the
results were summarized in Table 3 (Scheme 3). The results showed
that complexes 1 and 2 presented almost equally high catalytic
efficiency, producing the target biaryls in high yields in nearly all
the cases.
In conclusion, we presented here the novel monodentate imine
palladium complexes as a new and highly efficient catalyst system
for Suzuki cross-coupling reactions. Under the standard conditions,
using t-BuOK as base, 1:1 mixture of DMF/H2O as solvent, the
palladium complexes showed high catalytic activity for the cross-
coupling of series of phenylboronic acids and 1-naphthylboronic
acid with aryl halides (including phenyl iodide, aryl bromides and
phenyl chloride). HR-TEM study of the catalyst residue showed that
Due to the bulky space effect of 1-naphthyl, the cross-coupling
of arylbromides with 1-naphthylboronic acid resulted in rela-
tively low yields than the corresponding cross-coupling with
phenylboronic acid (Table 3, entries 2e3, 5e6, 9e10, 13e14, 17e18).
In the cross-coupling with iodobenzene, the difference is even
more obvious, which can be tentatively attributed to the bulkiness
of both iodo and 1-naphthyl (Table 3, entries 21e22). Due to the low
activity of chlorobenzene, prolonging the cross-coupling with 1-
naphthylboronic acid to 48 h only resulted in a moderate yield of
50%.
From Table 3, it can be seen that as expected, the effect of the
substituent on the para-position of the arylhalides is mainly
depending on the electronegativity of the substituent, electron-
withdrawing substituents leading to relatively high yields in the
cross-coupling reaction (Table 3, entries 4, 5, 6, 8, 9, 10), while
electron-donating substituents resulting in relatively low yields
(Table 3, entries 12, 13, 16, 17). While effect of the substituent on the
ortho-position of the arylbromide is mainly depending on the size
of the substituent, bulky ortho-substituted arylbromide afforded
biaryls in relatively low yields, and the yields decreased according
to the size increment of the substituent (Table 3, entries 7, 15, 19). In
the case of 2-COCH3 substituted phenyl bromide, due to the
Table 2
Catalyst loading effect on the cross-coupling of bromobenzene with phenylboronic
acid.
Entrya
Catalyst
loading (mol%)
Reaction
time (h)
GC Yield (%)
TON
TOF (hꢁ1
)
1
2
3
4
5
6
7
8
0
2
0.25
0.5
1
16
40
60
90
90
55
70
90
91
62
78
89
92
64
80
90
92
81
77
75
86
/
/
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
1.5
1.5
1.5
1.5
2.0
2.0
2.0
2.0
0.25
0.10
0.05
0.05
80
320
240
180
90
220
140
90
120
180
180
55
70
90
91
41
52
59
2
0.25
0.5
1
9
2
46
10
11
12
13
14
15
16
17
18
19
20
21b
0.25
0.5
1
164
104
59
31
128
80
45
23
14
32
2
61
32
40
45
0.25
0.5
1
2
46
24
24
24
24
324
770
1500
1720
63
72
a
Reaction condition: 1.0 mmol bromobenzene, 1.0 mmol phenylboronic acid,
1.5 mmol t-BuOK, 3 mL DMF/H2O (1/1, V/V) 100 ꢀC. All reactions were monitored by
GC.
b
Scheme 2. Base and solvent screening of the catalytic system.
Iodobenzene was used as aryl halide.

P. Mao et al. / Journal of Organometallic Chemistry 705 (2012) 39e43
41
Table 3
filtered. Evaporation of the filtrate afforded orange solids. Recrys-
tallization from acetonitrile/diethyl ether gave the analytically pure
complexes 1 and 2, respectively.
Suzuki cross-coupling of aryl halides with phenylboronic acid and 1-
naphthylboronic acid.
Entrya Ar-X
Ar0
Cat.
Reaction GC yield (%)
(0.5 mol%) time (h)
4.2.1. Bis-[1-(2,4,6-trimethyl-phenyl)-3,4,5,6-
1
C6H5Br
Phenyl
1
2
2
1
2
2
2
1
2
2
2
1
2
2
2
1
2
2
2
1
2
2
1
2
2
1
1
1
1
1
1
2
1
1
1
3
1
1
1
2
1
1
88
90
74
94
97
87
78
94
98
84
89
87
86
76
82
88
87
75
91
94
98
79
75
71
50
tetrahydropyrimidine) dibromopalladium (1)
2
C6H5Br
Phenyl
Orange solid; yield, 56%. 1H NMR (400 MHz, CDCl3):
d 1.97 (m,
3
4
5
6
C6H5Br
1-Naphthyl
Phenyl
Phenyl
1-Naphthyl
Phenyl
4H, H5), 2.20 (s, 12H, CH3), 2.26 (s, 6H, CH3), 3.28 (t, J ¼ 5.78 Hz, 4H,
4-NO2-C6H4-Br
4-NO2-C6H4-Br
4-NO2-C6H4-Br
2-NO2-C6H4-Br
H6), 3.47 (t, J ¼ 5.62 Hz, 4H, H4), 6.87 (s, 4H, ArH), 7.27 (s, 2H, H2).
13C NMR (100 MHz, CDCl3):
d 17.9 (CH3), 20.9 (CH3), 21.5 (C5), 45.4
7
(C6), 47.4 (C4), 129.4, 135.9 138.2, 147.2 (ArC), 155.4 (C2). Anal. Calcd
for C26H36Br2N4Pd(H2O)0.5: C, 45.93; H, 5.49; N, 8.24. Found : C,
45.96; H, 5.30; N, 8.10.
8
9
4-COCH3-C6H4-Br Phenyl
4-COCH3-C6H4-Br Phenyl
4-COCH3-C6H4-Br 1-Naphthyl
2-COCH3-C6H4-Br Phenyl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
4-OCH3-C6H4-Br
4-OCH3-C6H4-Br
4-OCH3-C6H4-Br
2-OCH3-C6H4-Br
4-CH3-C6H4-Br
4-CH3-C6H4-Br
4-CH3-C6H4-Br
2-CH3-C6H4-Br
C6H5I
Phenyl
Phenyl
1-Naphthyl
Phenyl
Phenyl
Phenyl
1-Naphthyl
Phenyl
4.2.2. Bis-[1-(2,6-diisopropyl-phenyl)-3,4,5,6-
tetrahydropyrimidine) dibromopalladium (2)
Orange solid; yield, 60%. 1H NMR (400 MHz, CDCl3):
d 1.20 (d,
J ¼ 5.48 Hz, 12H, CH3), 1.26 (d, J ¼ 6.76 Hz, 12H, CH3), 1.99 (m, 4H,
H5), 2.96(m, 4H, CH(CH3)2), 3.31 (t, J ¼ 5.68 Hz, 4H, H6), 3.51 (t,
J ¼ 5.70 Hz, 4H, H4), 7.15 (m, 6H, ArH), 7.30 (t, J ¼ 7.76 Hz, 2H, H2).
1
1
1
1
13C NMR (100 MHz, CDCl3):
d 24.3 (CH3), 24.8 (CH3), 28.2
Phenyl
(CH(CH3)2), 29.7 (C5), 46.8(C6), 47.5 (C4), 124.3, 129.3, 138.2, 147.2
(ArC), 155.4 (C2). Anal. Calcd for C32H48Br2N4Pd(Et2O)0.25: C, 51.24;
H, 6.58; N, 7.24; Found : C, 51.47; H, 6.36; N, 7.21.
C6H5I
Phenyl
C6H5I
C6H5Cl
1-Naphthyl
Phenyl
1
24
24
48
C6H5Cl
Phenyl
C6H5Cl
1-Naphthyl
4.3. General procedure for Suzuki cross-coupling
a
Reaction condition: 1.0 mmol aryl halide, 1.5 mmol arylboronic acid, 1.5 mmol
t-BuOK, 0.005 mmol catalyst, 3 mL DMF/H2O (1/1, V/V), 100 ꢀC. All reactions were
monitored by GC.
A Schlenk tube was charged with the appropriate aryl halide,
arylboronic acid and base under nitrogen. The required amount of
the catalyst stock solution and additional solvent were added to
obtain a total volume of 3 mL. The reaction mixture was heated at
100 ꢀC for specified time, and then allowed to cool. The reaction
mixture was extracted three times with diethyl ether, and the
combined organic extracts were washed with water, dried (MgSO4),
and evaporated to dryness. Internal standard was added to the
residue and the yield was then measured by GC. The products were
isolated by flash chromatography on silica gel, and characterized by
m.p. and 1H NMR.
the true active species maybe palladium nano particles formed in
the catalytic system. Further studies on the applicability of this
catalyst system in other coupling reactions such as Heck, Sonoga-
shira and amination are currently under investigation in our
laboratory.
4. Experimental
4.1. Materials and methods
The N,N-methylene-N0,N0-bis-aryl-1,4,5,6-tetrahydropyrimidi
nium dibromides were prepared in our lab. All other chemicals
were used as purchased. Solvents were dried and freshly distilled
prior to use. Suzuki cross-coupling reactions were carried out
under a dry nitrogen atmosphere using standard Schlenk tech-
niques. Melting points were measured using a WC-1 microscopic
apparatus and were uncorrected. Elemental analyses were ob-
4.3.1. Biphenyl
White plate, m.p. 70 ꢀC, Lit. 70e71 ꢀC [53]; 1H NMR (400 MHz,
CDCl3):
d 7.60 (m, 4H), 7.46e7.42 (m, 4H), 7.35e7.32 (m, 2H).
4.3.2. 1-Phenylnaphthalene
Colorless oil; 1H NMR (400 MHz, CDCl3):
d
8.19 (d, J ¼ 8.4 Hz,
1H), 8.12 (d, J ¼ 8.0 Hz, 1H), 8.07 (d, J ¼ 8.0 Hz, 1H), 7.75e7.62 (m,
tained from a Thermo Flash EA 1112 elemental analyzer. 1H and 13
C
9H).
NMR (400 and 100 MHz) spectra were recorded on a Bruker DPX-
400 spectrometer using TMS as an internal standard. The catalyst
stock solution for Suzuki cross-coupling was prepared by dissolving
exactly weighed complexes 1 or 2 in 5 mL DMF.
4.3.3. 4-Nitrobiphenyl
Yellow needle, m.p. 113e114 ꢀC, Lit. 112e114 ꢀC [53]; 1H NMR
(400 MHz, CDCl3):
7.62 (d, J ¼ 7.5 Hz, 2H), 7.49 (t, J ¼ 7.5 Hz, 2H), 7.44 (t, J ¼ 7.5 Hz, 1H).
d
8.29 (d, J ¼ 8.5 Hz, 2H), 7.73 (d, J ¼ 8.5 Hz, 2H),
4.2. General procedure for the synthesis of complexes 1 and 2
4.3.4. 2-Nitrobiphenyl
Yellow solid, m.p. 37e38 ꢀC; 1H NMR (400 MHz, CDCl3):
d 7.85
Pd(OAc)2 (101 mg, 0.45 mmol) was added to a solution of N,N-
methylene-N0,N0-bis-aryl-1,4,5,6-tetrahydropyrimidinium dibro-
mides (0.45 mmol) in DMSO (3 mL). The mixture was then heated
at 50 ꢀC for 5 h. After cooling, the solvent was removed completely
under vacuum. The residue was then dissolved in CHCl3, and
(m, 1H), 7.65e7.58 (m, 1H), 7.47e7.43 (m, 5H), 7.33e7.29 (m, 1H).
4.3.5. 1-(4-Nitrophenyl)naphthalene
White solid, m. p. 131 ꢀC; 1H NMR (400 MHz, CDCl3):
d 8.39 (d,
J ¼ 8.4 Hz, 2H), 7.98e7.96 (m, 2H), 7.81 (d, J ¼ 8.4 Hz, 1H), 7.71 (d,
J ¼ 8.4 Hz, 2H), 7.61e7.45 (m, 4H).
4.3.6. 4-Acetylbiphenyl
White solid, m.p. 118 ꢀC, Lit. 116e118 ꢀC [53]; 1H NMR (400 MHz,
Scheme 3. Suzuki cross-coupling of aryl halides with arylboronic acids.
CDCl3):
d
8.06 (d, J ¼ 8.4 Hz, 2H), 7.72 (d, J ¼ 8.4 Hz, 2H), 7.66 (d,

42
P. Mao et al. / Journal of Organometallic Chemistry 705 (2012) 39e43
Fig. 1. HR-TEM micrographs of palladium nano particles formed in the Suzuki reaction.
J ¼ 7.2 Hz, 2H), 7.50 (t, J ¼ 7.6 Hz, 2H), 7.44 (t, J ¼ 7.2 Hz, 1H), 2.67 (s,
References
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13662e13663.
4.3.7. 2-Acethylbiphenyl
Colorless oil; 1H NMR (400 MHz, CDCl3):
d
7.65 (m, 1H),
7.47e7.44 (m, 1H), 7.38e7.32 (m, 5H), 7.30e7.28 (m, 1H).
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4.3.8. 1-(4-Acetylphenyl)naphthalene
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White solid, m.p. 101 ꢀC; 1H NMR (400 MHz, CDCl3):
d 8.27 (d,
J ¼ 8.4 Hz, 1H), 8.14 (d, J ¼ 8.0 Hz, 1H), 8.09 (d, J ¼ 8.0 Hz, 1H),
7.78e7.61 (m, 6H), 7.55 (d, J ¼ 7.6 Hz, 2H), 2.71 (s, 3H, COCH3).
4.3.9. 4-Methoxybiphenyl
White solid, m.p. 90e91 ꢀC, Lit. 91e92 ꢀC [53]; 1H NMR
(400 MHz, CDCl3):
d 7.56e7.52 (m, 4H), 7.50e7.35 (m, 2H), 7.30 (t,
J ¼ 7.5 Hz, 1H), 7.00 (d, J ¼ 8.5 Hz, 2H), 3.85 (s, 3H, OCH3).
4.3.10. 2-Methoxybiphenyl
Colorless oil; 1H NMR (400 MHz, CDCl3):
d 7.53e7.50 (m, 2H),
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7.39e7.36 (m, 2H), 7.30e7.28 (m, 3H), 7.04e6.94 (m, 2H), 3.77 (s, 3H).
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4.3.11. 1-(4-Methoxyphenyl)naphthalene
White solid, m.p. 112e113 ꢀC; 1H NMR (400 MHz, CDCl3):
d
7.99e7.94 (m, 2H), 7.89 (d, J ¼ 8.4 Hz, 1H), 7.56e7.45 (m, 6H), 7.08
(d, J ¼ 8.8 Hz, 2H), 3.94 (s, 3H, OCH3).
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4.3.12. 4-Methylbiphenyl
White solid, m.p. 46e47 ꢀC; 1H NMR (400 MHz, CDCl3):
d 7.58 (d,
J ¼ 7.6 Hz, 2H), 7.49 (d, J ¼ 8.0 Hz, 2H), 7.45e7.40 (m, 2H), 7.35e7.30
(m, 1H), 7.25 (d, J ¼ 7.6 Hz, 2H), 2.40 (s, 3H).
4.3.13. 2-Methylbiphenyl
Light yellow oil; 1H NMR (400 MHz, CDCl3):
d 7.40e7.35 (m, 2H),
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4.3.14. 1-(4-Methylphenyl)naphthalene
Colorless oil; 1H NMR (400 MHz, CDCl3):
d
8.12 (d, J ¼ 8.0 Hz,
2H), 7.94 (t, J ¼ 8.2 Hz, 2H), 7.87 (d, J ¼ 8.4 Hz, 1H), 7.64 (d,
J ¼ 8.0 Hz, 2H), 7.59e7.45 (m, 4H), 2.18 (s, 3H, CH3).
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Financial support from the National Natural Science Foundation
of China (No. 20902017 & No. 21172055) and the Scientific Research
Foundation for the Returned Overseas Chinese Scholars, State
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