Synthesis and characterization of a series of chiral NHC-Pd complexes derived from l-phenylalanine

Article abstract of DOI:10.1007/s11243-013-9694-8

The synthesis of a series of chiral Pd(L)PyBr₂ (3a-3e) and Pd(L)PyCl₂ (4d and 4e) complexes from l-phenylalanine is presented (L = (S)-3-allyl-4-benzyl-1-(2,6-diisopropylphenyl)-imidazolin-2-ylidene (a), (S)-4-benzyl-1-(2,6-diisopropylphenyl)-3-(naphthalen-2-ylmethyl) imidazolin-2-ylidene (b), (S)-4-benzyl-3-(biphenyl-4-ylmethyl)-1-(2,6- diisopropylphenyl)imidazolin-2-ylidene (c), (S)-4-benzyl-1-(2,6- diisopropylphenyl)-3-(naphthalen-1-ylmethyl)imidazolin-2-ylidene (d) or (S)-4-benzyl-1-(2,6-diisopropylphenyl)-3-(2,4,6-trimethylbenzyl) imidazolin-2-ylidene (e). The complexes were characterized by physicochemical and spectroscopic methods, and the X-ray crystal structures of 3a-3c and 4d are reported. In each case, there is a slightly distorted square-planar geometry around palladium, which is surrounded by imidazolylidene, two trans halide ligands and a pyridine ligand. There are π-π stacking interactions in the crystal structures of these complexes. Complex 3a showed good catalytic activity in the Cu-free Sonogashira coupling reaction under aerobic conditions.

Full text of DOI:10.1007/s11243-013-9694-8

Transition Met Chem (2013) 38:367–375
DOI 10.1007/s11243-013-9694-8
Synthesis and characterization of a series of chiral NHC–Pd
complexes derived from L-phenylalanine
•
Longguang Yang Yunfei Li Qian Chen
•
•
•
•
•
Jianfeng Zhao Changsheng Cao Yanhui Shi
Guangsheng Pang
Received: 30 November 2012 / Accepted: 4 January 2013 / Published online: 12 March 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract The synthesis of a series of chiral Pd(L)PyBr2
(3a–3e) and Pd(L)PyCl2 (4d and 4e) complexes from
L-phenylalanine is presented (L = (S)-3-allyl-4-benzyl-1-(2,
6-diisopropylphenyl)-imidazolin-2-ylidene (a), (S)-4-benzyl-
1-(2,6-diisopropylphenyl)-3-(naphthalen-2-ylmethyl)imidaz-
olin-2-ylidene (b), (S)-4-benzyl-3-(biphenyl-4-ylmethyl)-1-
(2,6-diisopropylphenyl)imidazolin-2-ylidene (c), (S)-4-benzyl-
1-(2,6-diisopropylphenyl)-3-(naphthalen-1-ylmethyl)imidazo-
lin-2-ylidene (d) or (S)-4-benzyl-1-(2,6-diisopropylphenyl)
-3-(2,4,6-trimethylbenzyl)imidazolin-2-ylidene (e). The
complexes were characterized by physicochemical and
spectroscopic methods, and the X-ray crystal structures of
3a–3c and 4d are reported. In each case, there is a slightly
distorted square-planar geometry around palladium, which is
surrounded by imidazolylidene, two trans halide ligands and
a pyridine ligand. There are p–p stacking interactions in the
crystal structures of these complexes. Complex 3a showed
good catalytic activity in the Cu-free Sonogashira coupling
reaction under aerobic conditions.
Introduction
Interest in N-heterocyclic carbene (NHC) ligands has dra-
matically increased, due to their low toxicity, stability, low
cost and better electron donor properties compared to
phosphines. Thus, NHC’s have become an important class of
ligands after organic phosphine ligands [1–9]. Chiral NHC
ligands are also receiving growing attention, and a large
number of chiral NHC derivatives and their applications in
asymmetric catalysis have been reported [10–17]. There are
a few strategies for the development of chiral N-heterocyclic
carbene ligands, including those with chiral N-substituents
[18, 19], chiral N-heterocycles [20, 21] or both chiral N-
substituents and N-heterocylces [22, 23]. Construction of
chiral NHC’s is generally costly, since most chiral com-
pounds are expensive. The amino acids, as a class of easily
available natural chiral reagents, are often a good choice of
reactant to introduce chirality in the molecule. On extension
of our previous research into the development of chiral NHC
ligands and their complexes [24], we here report the syn-
thesis and characterization of a series of new NHC ligands
with chiral N-heterocycles, starting from L-phenylalanine,
and their palladium complexes.
L. Yang Á Y. Li Á Q. Chen Á J. Zhao Á C. Cao (&) Á Y. Shi (&)
College of Chemistry and Chemical Engineering and Jiangsu
Key Laboratory of Green Synthetic Chemistry for Functional
Materials, Jiangsu Normal University, Xuzhou 221116,
Jiangsu, People’s Republic of China
e-mail: chshcao@jsnu.edu.cn
Materials and measurements
Y. Shi
e-mail: yhshi@jsnu.edu.cn
(S)-4-Benzyl-1-(2,6-diisopropylphenyl)-4,5-dihydro-1H-
imidazole (1) was prepared according to the literature
procedure [25, 26]. Pyridine was distilled from calcium
hydride under argon before use. Potassium carbonate was
ground to a fine powder prior to use. All other reagents
were commercially available and were used without further
Y. Shi
State Key Laboratory of Coordination Chemistry, Nanjing
University, Nanjing 210093, People’s Republic of China
G. Pang
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun 130012,
Jilin, People’s Republic of China
1
purification. H and 13C NMR spectra were recorded on a
Bruker DPX 400 MHz spectrometer at room temperature
123

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Transition Met Chem (2013) 38:367–375
and referenced to the residual signals of the solvent. Ele-
mental analyses were performed on a EuroVektor Euro
EA-300 elemental analyzer. GC–MS was performed on an
Agilent 6890-5973 N system with electron ionization (EI)
mass spectrometry. Optical rotation was measured with
Autopol IV automatic ploarimeter in DCM solution, and
the optical rotations of products 2, 3 and 4 are reported.
124.5, 122.1, 60.5, 56.7, 48.5, 36.5, 28.6, 28.2, 25.0, 24.9,
23.9. Anal. Calc. for C25H33BrN2 (441.45 g/mol): C, 68.0;
H, 7.5; N, 6.4. Found: C, 67.9; H, 7.1; N, 6.4 %.
Synthesis of (S)-4-benzyl-1-(2,6-diisopropylphenyl)-3-
(naphthalen-2-ylmethyl)-4,5-dihydro-1H-imidazol-3-
ium bromide (2b)
Synthesis of (S)-3-allyl-4-benzyl-1-(2,6-
diisopropylphenyl)-4,5-dihydro-1H-imidazol-3-ium
bromide (2a)
A mixture of 1 (0.640 g, 2 mmol), 2-napthylmethyl bromide
(0.884 g, 4 mmol) and acetonitrile (5 mL) were placed
inside a 15-mL pressure tube. The mixture was heated to
110 °C for 5 h. The volatiles were then removed under
vacuum, and diethyl ether was added to the residue to give a
solid. The product was collected, washed with ether and
dried under high vacuum. Yield: 1.07 g (99 %). White solid,
To a solution of 1 (1.282 g, 4 mmol) in toluene (10 mL),
allyl bromide (0.968 g, 8 mmol) was added. The reaction
mixture was stirred for 10 h at 65 °C, and the product pre-
cipitated from the solvent as a white solid. The solid was
collected and washed with diethyl ether and dried under high
vacuum. Yield: 1.68 g (95 %). White solid, mp: 148–
1
mp: 225–226 °C; [a]2D5 = -86.4° (c 1.0, DCM); H NMR
(CDCl3, 400 MHz): d 10.12 (s, 1H), 7.86–7.93 (m, 4H), 7.69
(d, J = 8.0 Hz, 1H), 7.54–7.55 (m, 2H), 7.34–7.39 (m, 4H),
7.13–7.19 (m, 4H), 6.32 (d, J = 16.0 Hz, 1H), 4.88
(d, J = 16.0 Hz, 1H), 4.64–4.73 (br, 1H), 3.93 (t,
J = 12.0 Hz, 1H), 3.80 (t, J = 8.0 Hz, 1H), 3.27–3.32
(m, 1H), 3.17–3.22 (m, 1H), 2.70–2.77 (m, 1H), 2.27–2.34
(m, 1H), 1.35 (d, J = 8.0 Hz, 3H), 1.17 (t, J = 4.0 Hz, 6H),
1.11 (d, J = 8.0 Hz, 3H). 13C NMR (CDCl3, 100 MHz):
159.2, 146.3, 146.0, 134.1, 133.6, 131.4, 130.9, 130.4, 129.5,
129.3, 129.2, 129.1, 128.0, 127.8, 127.4, 126.4, 125.2, 124.7,
124.6, 123.3, 60.1, 56.8, 48.6, 37.0, 28.5, 28.4, 25.1, 24.9,
1
149 °C; [a]2D5 = -37.1° (c 1.0, DCM); H NMR (CDCl3,
400 MHz): d 9.71 (s, 1H), 7.32–7.39 (m, 4H), 7.14–7.20
(m, 4H), 6.00–6.10 (m, 1H), 5.40–5.56 (m, 3H), 4.89–4.97
(m, 1H), 4.29–4.35 (m, 1H), 4.09 (t, J = 12.0 Hz, 1H),
3.77–3.82 (m, 1H), 3.31 (dd, J = 12.0 Hz, J = 4.0 Hz, 1H),
3.04–3.09 (m, 1H), 2.80–2.87 (m, 1H), 2.31–2.38 (m, 1H),
1.28 (d, J = 8.0 Hz, 3H), 1.20 (t, J = 8.0 Hz, 6H), 1.12 (d,
J = 8.0 Hz, 3H). 13C NMR (CDCl3, 100 MHz): 158.5,
146.2, 133.5, 130.8, 130.2, 129.6, 129.4, 129.1, 127.6, 124.6,
Ph
Ph
Ph
N
N
5 steps
Ph
H2N
O
RBr
R
PdCl2, K2CO3
NaBr, Pyridine
N
N
N
N
Br Pd Br
N
R
OH
overall yield:
27%
1
Br
3a - 3e
2a - 2c
O
C
K
,
l
C
d
P
e
n
i
d
i
r
y
P
,
r
B
a
N
Ph
Ph
N
N
RCl
R
PdCl2, K2CO3
Pyridine
N
N
Cl Pd Cl
N
R
Cl
2d - 2e
4d - 4e
R =
,
,
,
,
c
a
b
d
e
Scheme 1 Synthesis of compounds of 1–4
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Transition Met Chem (2013) 38:367–375
369
Table 1 Crystal data and structure refinement details for 3a–3c and 4e
Complex
3a
3b
3c
4e
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C30H37Br2N3PdÁH2O
723.86
C38H41Br2N3Pd
805.96
C40H43Br2N3Pd
831.99
C37H45Cl2N3Pd
709.06
298(2)
298(2)
293(2)
296(2)
Monoclinic
C2/c
Orthorhombic
P2(1)2(1)2(1)
10.498(2)
17.281(4)
19.878(4)
90.00
Orthorhombic
P2(1)2(1)2(1)
9.7263(19)
9.984(2)
Orthorhombic
P2(1)2(1)2(1)
9.2382(5)
12.3080(7)
30.3968(17)
90.00
˚
a (A)
24.545(2)
11.2551(11)
23.052(2)
90.00
˚
b (A)
˚
c (A)
37.999(8)
90.00
a (°)
b (°)
c (°)
97.8900(10)
90.00
90.00
90.00
90.00
90.00
90.00
90.00
3
˚
V (A )
6307.8(11)
8
3606.3(13)
4
3690.1(13)
4
3456.2(3)
4
Z
Dcalcd (mg cm-3
)
1.523
1.484
1.498
1.363
Absorption coefficient (mm-1
)
3.148
2.760
2.700
0.721
F (000)
2912
1620
1680
1472
h range (°)
2.597–25.813
15241
3.0539–25.3491
34071
3.1147–25.3491
11519
2.68–27.86
39823
Reflection collected/unique
Data/restrains/parameters
Goodness-of-fit on F2
Final R indices [I [ 2r(I)]
5555/0/338
0.986
6353/0/397
1.144
5764/0/415
0.962
6068/0/388
1.040
R1 = 0.0528
wR2 = 0.1151
R1 = 0.1016
wR2 = 0.1296
R1 = 0.0377
wR2 = 0.0809
R1 = 0.0431
wR2 = 0.0828
R1 = 0.0637
wR2 = 0.1311
R1 = 0.0875
wR2 = 0.1430
R1 = 0.0235
wR2 = 0.0708
R1 = 0.0242
wR2 = 0.0792
R indices (all data)
23.9. Anal. Calc. for C33H37BrN2 (541.56 g/mol): C, 73.2;
H, 6.89; N, 5.2. Found: C, 72.9; H, 6.7; N, 5.4 %.
Synthesis of (S)-4-benzyl-3-(biphenyl-4-ylmethyl)-1-
(2,6-diisopropylphenyl)-4,5-dihydro-1H-imidazol-3-
ium bromide (2c)
The synthesis of 2c was carried out in the same way as that
described for 2b, but p-phenylbenzyl bromide (0.989 g,
4 mmol) was used instead of 2-napthylmethyl bromide.
Yield: 1.12 g (99 %). White solid, mp: 216–217 °C;
[a]2D5 = -62.2° (c 1.0, DCM); 1H NMR (CDCl3, 400 MHz):
d 10.01(s, 1H), 7.64 (t, J = 8.0 Hz, 4H), 7.58 (d, J = 8.0 Hz,
2H), 7.46 (t, J = 8.0 Hz, 2H), 7.34–7.39 (m, 5H), 7.13–7.21
(m, 4H), 6.16 (d, J = 12.0 Hz, 1H), 4.72–4.80 (m, 2H), 3.99
(t, J = 12.0 Hz, 1H), 3.81 (t, J = 8.0 Hz, 1H), 3.31
(d, J = 12.0 Hz, 1H), 3.16–3.21 (m, 1H), 2.72–2.79 (m, 1H),
2.27–2.34 (m, 1H), 1.33 (d, J = 4.0 Hz, 3H), 1.18 (s, br, 6H),
1.11 (d, J = 8.0 Hz, 3H). 13C NMR (CDCl3, 100 MHz):
158.4, 146.1, 145.8, 141.8, 139.9, 133.5, 131.7, 130.7, 129.7,
129.4, 129.3, 129.1, 128.6, 127.7, 127.6, 127.5, 126.8, 124.5,
60.1, 56.7, 49.1, 36.3, 28.6, 28.1, 25.1, 24.9, 23.6, 23.5. Anal.
Calc. for C35H39BrN2 (576.60 g/mol): C, 74.1; H, 6.9; N, 4.9.
Found: C, 73.8; H, 6.8; N, 5.2 %.
Fig. 1 ORTEP structure of complex 3a showing 30 % probability
ellipsoids; hydrogen atoms and solvent are omitted for clarity
123

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Transition Met Chem (2013) 38:367–375
Fig. 4 ORTEP structure of complex 4e showing 30 % probability
ellipsoids; hydrogen atoms are omitted for clarity
Fig. 2 ORTEP structure of complex 3b showing 30 % probability
ellipsoids; hydrogen atoms are omitted for clarity
Table 2 Selected bond lengths and angles for complexes 3a–3c and
4e
˚
Distance (A)
3a
1.961(6)
3b
3c
4e
C1–Pd1
N3–Pd1
1.970(4)
2.126(4)
2.4134(17) 2.4298(15) 2.3052(8)
1.939(9)
1.951(3)
2.102(5)
2.139(8)
2.113(3)
Pd1–Br1
(or Cl1)
2.4453(8)
Pd1–Br2
(or Cl2)
2.4236(8)
2.4219(8)
2.4065(14) 2.2938(8)
Angel (°)
C1-Pd1-N3
175.4(2)
175.86(16)
89.92(11)
177.7(4)
86.8(2)
175.16(12)
85.90(9)
C1–Pd1–Br1
(or Cl1)
90.90(16)
C1–Pd1–Br2
(or Cl2)
86.89(17)
91.26(15)
91.72(15)
88.78(11)
92.21(13)
88.64(12)
87.9(2)
91.6(2)
93.4(2)
91.64(9)
90.05(8)
92.69(8)
N3–Pd1–Br1
(or Cl1)
N3–Pd1–Br2
(or Cl2)
Fig. 3 ORTEP structure of complex 3c showing 30 % probability
ellipsoids; hydrogen atoms are omitted for clarity
Synthesis of (S)-4-benzyl-1-(2,6-diisopropylphenyl)-3-
(naphthalen-1-ylmethyl)-4,5-dihydro-1H-imidazol-3-
ium chloride (2d)
400 MHz): d 10.38 (s, 1H), 8.47 (d, J = 8.0 Hz, 1H),
7.92–7.95 (m, 2H), 7.65–7.71 (m, 2H), 7.50–7.59 (m, 2H),
7.29–7.34 (m, 4H), 7.13 (t, J = 8.0 Hz, 4H), 6.75
(d, J = 12.0 Hz, 1H), 5.18 (d, J = 12.0 Hz, 1H),
4.36–4.45 (br, 1H), 3.69–3.80 (m, 2H), 3.42 (d,
J = 12.0 Hz, 1H), 3.05–3.11 (m, 1H), 2.47–2.60 (m, 2H),
1.27 (d, J = 4.0 Hz, 3H), 1.21 (d, J = 4.0 Hz, 3H), 1.16
(d, J = 4.0 Hz, 3H), 1.06 (d, J = 4.0 Hz, 3H). 13C NMR
(CDCl3, 100 MHz): 158.7, 146.3, 146.0, 133.6, 133.3,
The synthesis of 2d was carried out in the same way as that
described for 2b, but 1-naphthylmethyl chloride (0.707 g,
4 mmol) was used instead of 2-napthylmethyl bromide.
Yield: 0.98 g (99 %). White solid, mp: 211–212 °C;
[a]2D5 = ?50.8° (c 1.0, DCM); 1H NMR (CDCl3,
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Transition Met Chem (2013) 38:367–375
371
1 mol% of 3 or 4,
2 eq. base
+
R
X
Table 3 Sonogashira coupling reactions catalyzed by palladium complexes
DMSO, 100 °C,
1 h
1 eq.
2 eq.
Entry
R
X
Base
Complex
Yield (%)a
1
NO2
NO2
NO2
NO2
NO2
NO2
NO2
H
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
3a
3b
3c
3d
3e
4d
4e
3a
3b
3c
3d
3e
4d
4e
3a
99
97
98
98
97
99
99
43
28
31
30
28
32
34
80
2
3
4
5
6
7
8
9
H
10
11
12
13
14
15
a
H
H
H
H
H
H
Determined by GC–MS with 1,2,3,4-tetrahydronaphthalene as an internal standard
Synthesis of Pd[(s)-3-C3H5-4-(C5H5CH2)-1-(2,6-
133.1, 131.0, 130.2, 129.7, 129.5, 129.4, 129.3, 128.6,
127.9, 127.8, 127.7, 126.8, 126.7, 125.6, 124.7, 60.3, 56.9,
50.0, 36.8, 28.6, 28.4, 25.3, 25.1, 23.9, 23.7. Anal. Calc. for
C33H37BrN2 (497.11 g/mol): C, 79.7; H, 7.5; N, 5.6.
Found: C, 79.4; H, 7.4; N, 5.9 %.
iPr2C6H3)–C3H3N2](C5H5N)Br2 (3a)
To an oven-dried 50 mL of r.b.f. containing 2a (0.133 g,
0.3 mmol), PdCl2 (0.053 g, 0.3 mmol), K2CO3 (0.415 g,
3 mmol) and NaBr (0.155 g, 1.5 mmol) with a septum,
pyridine (5 mL) was injected into under argon. The
reaction mixture was stirred at 80 °C for 12 h. After
cooling to room temperature, the mixture was filtered
through a plug of Celite and washed by DCM. After the
volatile was removed under vacuum, the solid was re-
crystallized by DCM/ether to give 3a as a yellow solid.
Synthesis of (S)-4-benzyl-1-(2,6-diisopropylphenyl)-3-
(2,4,6-trimethylbenzyl)-4,5-dihydro-1H-imidazol-3-
ium chloride (2e)
The synthesis of 2e was carried out in the same way as that
described for 2b, but 2,4,6-trimethylbenzyl chloride
(0.675 g, 4 mmol) was used instead of 2-napthylmethyl
bromide. Yield: 0.83 g (85 %). White solid, mp:
212–213 °C; [a]2D5 = -50.1° (c 1.0, DCM); 1H NMR
(CDCl3, 400 MHz): d 8.77 (s, 1H), 7.35 (t, J = 4.0 Hz, 3H),
7.30 (d, J = 8.0 Hz, 1H), 7.21 (d, J = 8.0 Hz, 2H), 7.16
(t, J = 8.0 Hz, 2H), 6.92 (s, 2H), 5.50 (d, J = 12.0 Hz, 1H),
5.27 (d, J = 12.0 Hz, 1H), 5.19 (s, br, 1H), 4.22
(t, J = 12.0 Hz, 1H), 3.78-3.82 (m, 1H), 3.44 (d, J =
12.0 Hz, 1H), 2.99-3.05 (m, 1H), 2.86-2.91 (m, 1H), 2.47 (s,
6H), 2.27 (s, 3H), 1.20 (t, J = 4.0 Hz, 6H), 1.15 (t, J =
8.0 Hz, 6H). 13CNMR(CDCl3, 100 MHz):157.5, 146.4, 146.3,
139.4, 137.9, 133.9, 131.1, 130.1, 129.7, 129.6, 129.4, 127.9,
125.6, 125.0, 124.8, 61.8, 57.6, 45.9, 37.8, 28.8, 28.6, 25.2, 25.0,
23.9, 21.0, 20.2. Anal. Calc. for C32H41ClN2 (489.13 g/mol): C,
78.6; H, 8.4; N, 5.7. Found: C, 78.2; H, 8.6; N, 6.0 %.
Yield: 0.20 g (95 %). [a]2D5 = ?34.8° (c 1.0, DCM); H
1
NMR (CDCl3, 400 MHz): d 8.72 (d, J = 8.0 Hz, 2H),
7.62 (t, J = 8.0 Hz, 1H), 7.38 (t, J = 4.0 Hz, 1H), 7.15-
7.30 (m, 9H), 6.27–6.37 (m, 1H), 5.95 (dd, J = 8.0 Hz,
J = 4.0 Hz, 1H), 5.43 (t, J = 12.0 Hz, 2H), 4.30–4.39
(m, 2H), 3.88 (t, J = 12.0 Hz, 1H), 3.57–3.64 (m, 2H),
3.30–3.39 (m, 2H), 2.77 (dd, J = 4.0 Hz, J = 12.0 Hz,
1H), 1.48 (d, J = 8.0 Hz, 3H), 1.43 (d, J = 8.0 Hz, 3H),
1.17 (d, J = 4.0 Hz, 3H), 1.07 (d, J = 8.0 Hz, 3H). 13C
NMR (CDCl3, 100 MHz): 182.9, 152.4, 148.3, 137.6,
136.0, 134.1, 133.6, 129.6, 129.3, 129.0, 127.2, 124.7,
124.6, 124.3, 119.8, 59.8, 58.7, 52.7, 38.0, 28.6, 28.4,
27.3, 27.2, 24.5. Anal. Calc. for C30H37Br2N3Pd
(705.86 g/mol): C, 51.1; H, 5.3; N, 5.9. Found: C, 51.4;
H, 5.6; N, 6.0 %.
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Synthesis of Pd[(s)-4-(C5H5CH2)-1-(2,6-iPr2C6H3)-3-
(C10H7-2-CH2)-C3H3N2](C5H5N)Br2 (3b)
1H), 7.33 (d, J = 8.0 Hz, 2H), 7.23–7.20 (m, 3H), 7.13
(d, J = 8.0 Hz, 3H), 6.93 (d, J = 12.0 Hz, 1H), 6.79
(d, J = 8.0 Hz, 2H), 5.37 (d, J = 12.0 Hz, 1H), 4.07–4.01
(m, 1H), 3.74 (t, J = 12.0 Hz, 1H), 3.59–3.53 (m, 1H),
3.42–3.37 (m, 2H), 3.20–3.13 (m, 1H), 2.81–2.75 (m, 1H),
1.58 (d J = 8.0 Hz, 3H), 1.55 (d, J = 8.0 Hz, 3H), 1.26
(d, J = 8.0 Hz, 3H), 0.99 (d, J = 8.0 Hz, 3H). 13C NMR
(CDCl3, 100 MHz): 182.8, 152.5, 148.3, 148.1, 137.6, 135.
9, 134.3, 134.0, 132.3, 130.8, 129.7, 129.5, 129.3, 128.8,
128.7, 128.6, 126.9, 126.8, 126.5, 125.5, 125.3, 124.6,
124.5, 124.4, 59.7, 58.1, 53.4, 38.2, 28.8, 28.3, 27.4, 26.9,
24.8, 24.3. Anal. Calc. for C38H41Br2N3Pd (805.98 g/mol):
C, 56.6; H, 5.1; N, 5.2. Found: C, 56.4; H, 5.2; N, 5.4 %.
The synthesis of 3b was carried out in the same way as that
described for 3a, but 2b (0.162 g, 0.3 mmol) was used
instead 2a. Yield: 0.22 g (89 %). Yellow solid, [a]D25
=
-88.6° (c 1.0, DCM); 1H NMR (CDCl3, 400 MHz): d 8.76
(d, J = 4.0 Hz, 2H), 7.95 (s, 3H), 7.89–7.90 (m, 2H), 7.62
(t, J = 4.0 Hz, 1H), 7.52–7.54 (m, 2H), 7.40 (t, J = 8.0 Hz,
1H), 7.31 (d, J = 8.0 Hz, 1H), 7.24 (s, 1H), 7.19 (br, 5H),
6.98–6.99 (m, 2H), 6.84 (d, J = 12.0 Hz, 1H), 4.95 (d,
J = 12.0 Hz, 1H), 4.00 (br, 1H), 3.81–3.90 (m, 2H),
3.52–3.56 (m, 1H), 3.40–3.44 (m, 1H), 3.29–3.34 (m, 1H),
3.81 (t, J = 8.0 Hz, 1H), 1.51–1.56 (m, 6H), 1.20 (d,
J = 8.0 Hz, 3H), 1.08 (d, J = 8.0 Hz, 3H). 13C NMR
(CDCl3, 100 MHz): 183.1, 154.3, 152.4, 148.3, 148.2, 137.5,
136.0, 134.2, 133.4, 133.3, 132.9, 129.7, 129.3, 128.9, 128.1,
128.0, 127.8, 127.1, 126.8, 126.3, 126.2, 125.0, 124.6, 124.3,
59.2, 58.8, 54.0, 38.4, 28.7, 28.4, 27.2, 27.1, 24.7, 24.4. Anal.
Calc. for C38H41Br2N3Pd (805.98 g/mol): C, 56.6; H, 5.1; N,
5.2. Found: C, 56.4; H, 5.0; N, 5.4 %.
Synthesis of Pd[(s)-4-(C5H5CH2)-1-(2,6-iPr2C6H3)-3-
(2,4,6-Me3C6H2CH2)-C3H3N2](C5H5N)Br2 (3e)
The synthesis of 3e was carried out in the same way as that
described for 3a, but2e (0.147 g, 0.3 mmol) was used instead
2a. Yield: 0.21 g (87 %). Yellow solid, [a]2D5 = ?11.3° (c
1.0, DCM); 1H NMR (CDCl3, 400 MHz): d 8.78 (d,
J = 8.0 Hz, 2H), 7.64 (t, J = 4.0 Hz, 1H), 7.39 (t, J =
8.0 Hz, 1H), 7.32 (d, J = 8.0 Hz, 1H), 7.21 (t, J = 8.0 Hz,
3H), 7.14 (d, J = 8.0 Hz, 3H), 6.98 (s, 2H), 6.67 (d, J =
4.0 Hz, 2H), 6.02 (d, J = 12.0 Hz, 1H), 5.56 (d, J =
12.0 Hz, 1H), 3.94-4.00 (m, 1H), 3.87 (t, J = 8.0 Hz, 1H),
3.63 (t, J = 8.0 Hz, 1H), 3.45 (dd, J = 12.0 Hz, J = 4.0 Hz,
1H), 3.17-3.25 (m, 1H), 2.95 (d, J = 12.0 Hz, 1H), 2.72
(t, J = 12.0 Hz, 1H), 2.55 (s, 6H), 2.36 (s, 3H),
1.55 (d, J = 8.0 Hz, 3H), 1.45 (d, J = 8.0 Hz, 3H), 1.27
(d, J = 8.0 Hz, 3H), 1.06 (d, J = 8.0 Hz, 3H). 13C NMR
(CDCl3, 100 MHz): 183.0, 152.5, 148.4, 148.2, 138.5, 138.0,
137.5, 136.0, 134.4, 129.7, 129.6, 129.2, 128.8, 128.1, 126.9,
124.6, 124.5, 124.4, 60.2, 57.9, 49.3, 38.4, 28.7, 28.3, 27.3,
27.1, 24.7, 24.3, 21.1, 21.0. Anal. Calc. for C37H45Br2N3Pd
(805.98 g/mol): C, 55.7; H, 5.7; N, 5.3. Found: C, 55.4; H,
5.4; N, 5.5 %.
Synthesis of Pd[(s)-4-(C5H5CH2)-3-((C6H5)2-4-CH2)-1-
(2,6-iPr2C6H3)-C3H3N2](C5H5N)Br2 (3c)
The synthesis of 3c was carried out in the same way as that
described for 3a, but 2c (0.170 g, 0.3 mmol) was used
instead 2a. Yield: 0.25 g (99 %). Yellow solid, [a]D25
=
-65.0° (c 1.0, DCM); 1H NMR (CDCl3, 400 MHz): d 8.75
(d, J = 8.0 Hz, 2H), 7.76–7.61 (m, 8H), 7.48 (t, J =
4.0 Hz, 2H), 7.42–7.36 (m, 2H), 7.31 (d, J = 8.0 Hz, 1H),
7.23–7.18 (m, 5H), 7.02 (d, J = 8.0 Hz, 2H), 6.76
(d, J = 12.0 Hz, 1H), 4.84 (d, J = 12.0 Hz, 2H),
4.10–4.02 (m, 1H), 3.89 (t, J = 8.0 Hz, 1H), 3.85–3.78 (m,
1H), 3.58–3.54 (m, 1H), 3.42–3.29 (m, 2H), 2.82–2.75 (m,
1H), 1.54 (d, J = 8.0 Hz, 3H), 1.49 (d, J = 8.0 Hz, 3H),
1.21 (d, J = 8.0 Hz, 3H), 1.08 (d, J = 8.0 Hz, 3H). 13C
NMR (CDCl3, 100 MHz): 183.0, 154.4, 152.4, 148.3,
148.2, 141.1, 140.7, 137.6, 135.9, 134.4, 134.1, 129.7,
129.6, 129.3, 128.9, 127.6, 127.5, 127.1, 124.7, 124.6,
124.3, 59.2, 58.7, 53.5, 38.1, 28.7, 28.4, 27.3, 27.2, 24.7,
24.4. Anal. Calc. for C40H43Br2N3Pd (832.02 g/mol): C,
57.7; H, 5.2; N, 5.0. Found: C, 57.5; H, 5.1; N, 5.3 %.
Synthesis of Pd[(s)-4-(C5H5CH2)-1-(2,6-iPr2C6H3)-3-
(C10H7-1-CH2)-C3H3N2](C5H5N)Cl2 (4d)
To an oven-dried 50 mL of round-bottomed flask con-
taining 2d (0.149 g, 0.3 mmol), PdCl2 (0.053 g,
0.3 mmol), and K2CO3 (0.415 g, 3 mmol) and equipped
with a septum, pyridine (5 mL) was injected under argon.
The mixture was stirred at 80 °C for 12 h. After cooling to
room temperature, the mixture was filtered through a plug
of Celite and washed by CH2Cl2. After the volatiles were
removed under vacuum, the solid was recrystallized from
CH2Cl2/ether to give 4d as a light yellow solid. Yield:
Synthesis of Pd[(s)-4-(C5H5CH2)-1-(2,6-iPr2C6H3)-3-
(C10H7-1-CH2)-C3H3N2](C5H5N)Br2 (3d)
The synthesis of 3d was carried out in the same way as that
described for 3a, but 2d (0.149 g, 0.3 mmol) was used
instead 2a. Yield: 0.20 g (83 %). Yellow solid, [a]D25
=
-20.5° (c 1.0, DCM); 1H NMR (CDCl3, 400 MHz): d 8.78
(d, J = 4.0 Hz, 2H), 7.94 (d, J = 8.0 Hz, 2H), 7.65
(t, J = 8.0 Hz, 3H), 7.59–7.52 (m, 2H), 7.40 (d, J = 8.0 Hz,
1
0.17 g (81 %). [a]2D5 = -15.8° (c 1.0, DCM); H NMR
(CDCl3, 400 MHz): d 8.80 (d, J = 8.0 Hz, 2H), 7.94
(d, J = 8.0 Hz, 2H), 7.79 (t, J = 8.0 Hz, 1H), 7.62–7.67
123

Transition Met Chem (2013) 38:367–375
373
(m, 3H), 7.51–7.59 (m, 2H), 7.31–7.41 (m, 4H), 7.20–7.23
(m, 2H), 7.11–7.15 (m, 3H), 6.83 (d, J = 8.0 Hz, 2H), 5.35
(d, J = 12.0 Hz, 1H), 3.84–3.91 (m, 1H), 3.71 (t, J =
8.0 Hz, 1H), 3.54–3.62 (m, 1H), 3.37–3.41 (m, 2H),
3.08–3.16 (m, 1H), 2.73 (t, J = 12.0 Hz, 1H), 1.59 (d, J =
8.0 Hz, 3H), 1.52 (d, J = 8.0 Hz, 3H), 1.25 (d, J =
8.0 Hz, 3H), 0.99 (d, J = 8.0 Hz, 3H). 13C NMR (CDCl3,
100 MHz): 182.9, 153.4, 151.3, 148.2, 148.1, 137.8, 135.8,
134.3, 134.1, 132.3, 130.7, 129.6, 129.3, 128.8, 128.7,
128.4, 127.0, 126.9, 126.5, 125.2, 125.0, 124.5, 124.3,
59.6, 58.1, 52.5, 38.5, 28.7, 28.2, 27.1, 26.8, 24.5, 24.0.
Anal. Calc. for C38H41Cl2N3Pd (717.08 g/mol): C, 63.6; H,
5.8; N, 5.9. Found: C, 63.3; H, 5.5; N, 6.0 %.
in the final stage of full-matrix least-squares refinement.
The structure was solved by directed methods using the
SHELXS-97 program, and absorption correction was per-
formed by SADABS program.
Results
Synthesis
The optically pure imidazoline 1 (Scheme 1) was prepared
effectively by a five-step sequence in an overall 27 % yield
starting from the relatively inexpensive amino acid L-
phenylalanine, according to the literature procedures [25,
26]. The direct substitution reaction of imidazoline 1 with
readily available alkyl halides gives the corresponding
imidazolium bromides or chlorides 2 in excellent yield.
The reaction for compound 2a was performed at 65 °C for
10 h due to the low boiling point of allyl bromide, whereas
the reactions for 2b–2e were carried out at 110 °C for 5 h.
The synthesis of NHC–PdBr2–Py complexes 3 was
achieved by refluxing the corresponding imidazolium salts
2a–2e with palladium chloride in the presence of K2CO3 in
pyridine, plus excess NaBr as a bromide provider, as shown
in Scheme 1. The same reaction with imidazolium chlo-
rides 2d–2e provided NHC–PdCl2–Py complexes 4 if no
NaBr was added.
Synthesis of Pd[(s)-4-(C5H5CH2)-1-(2,6-iPr2C6H3)-3-
(2,4,6-Me3C6H2CH2)-C3H3N2](C5H5N)Cl2 (4e)
The synthesis of 4e was carried out in the same way as that
described for 4d, but 2e (0.147 g, 0.3 mmol) was used
instead 2d. Yield: 0.20 g (92 %). Light yellow solid,
[a]2D5 = ?9.4° (c 1.0, DCM); 1H NMR (CDCl3, 400 MHz):
d 8.77 (d, J = 8.0 Hz, 2H), 7.65 (t, J = 4.0 Hz, 1H),
7.34–7.39 (m, 1H), 7.29–7.33 (m, 1H), 7.15–7.23 (m, 6H),
6.98 (s, 2H), 6.74 (d, J = 4.0 Hz, 2H), 6.19 (d, J = 8.0 Hz,
1H), 5.51 (d, J = 8.0 Hz, 1H), 3.78–3.86 (m, 2H),
3.64–3.70 (m, 1H), 3.44 (dd, J = 8.0 Hz, J = 4.0 Hz, 1H),
3.13–3.19 (m, 1H), 3.01 (d, J = 12.0 Hz, 1H), 2.67 (t, J =
12.0 Hz, 1H), 2.55 (s, 6H), 2.35 (s, 3H), 1.56 (d,
J = 8.0 Hz, 3H), 1.44 (d, J = 8.0 Hz, 3H), 1.27 (d,
J = 8.0 Hz, 3H), 1.07 (d, J = 8.0 Hz, 3H). 13C NMR
(CDCl3, 100 MHz): 183.1, 153.4, 151.2, 148.2, 148.1,
138.4, 137.9, 137.7, 135.9, 134.3, 129.8, 129.5, 129.2,
128.8, 128.0, 126.9, 125.0, 124.5, 124.4, 124.3, 60.0, 57.9,
48.4, 38.6, 28.6, 28.1, 27.0, 26.9, 24.4, 24.0, 21.1, 20.9.
Anal. Calc. for C37H45Cl2N3Pd (709.10 g/mol): C, 62.7; H,
6.4; N, 5.9. Found: C, 62.3; H, 6.2; N, 6.2 %.
All of these palladium complexes are air- and mois-
ture-stable and can be stored under air in the solid state
for more than 6 months without any noticeable decom-
position. Complexes 3 and 4 were fully characterized by
NMR spectroscopy and gave satisfactory elemental anal-
1
yses. There are a few features of their H and 13C NMR
spectra that confirm the formation of NHC–Pd–Py com-
1
plexes. Firstly, the H NMR spectra of complexes 3 and 4
in CCl3D show the complete absence of the acidic proton
of NCHN from the ligand precursor [2a (9.71 ppm), 2b
(10.12 ppm), 2c (10.01 ppm), 2d (10.38 ppm) and 2e
(8.77 ppm)], which is diagnostic for the formation of
metal carbene complexes. Furthermore, the 13C NMR
spectra for 3 and 4 provide direct evidence of the meta-
lation of the ligand, as seen by the distinctive signal of
the carbene carbon [3a (182.9 ppm), 3b (183.1 ppm), 3c
(183.0 ppm), 3d (182.8 ppm), 3e (183.0 ppm), 4d
(182.9 ppm) and 4e (183.1 ppm)]. This signal is signifi-
cantly shifted downfield relative to those of the imi-
dazolium salts [2a (158.5 ppm), 2b (159.2 ppm), 2c
(158.4 ppm), 2d (158.7 ppm) and 2e (157.5 ppm)].
X-ray crystallography
Intensity data were collected with a Rigaku Mercury CCD
area detector in x scan mode using Mo-Ka radiation
˚
(k = 0.71075 A). The diffraction intensities were cor-
rected for Lorentz polarization effects and empirical
absorption corrections. Details of the intensity data col-
lection and crystal data are given by full-matrix least-
squares procedures based on F2 [29]. All the non-hydrogen
atoms were refined anisotropically. The hydrogen atoms in
these complexes were all generated geometrically (C–H
1
Lastly, the signals of the pyridine ligands in the H NMR
˚
bond lengths fixed at 0.95 A), assigned appropriate iso-
spectra of the complexes are shifted compared to those of
free pyridine, confirming the coordination of pyridine to
palladium. Therefore, the NMR data indicate that the
NHC–Pd complexes were prepared successfully.
tropic thermal parameters and allowed to ride on their
parent carbon atoms. All the hydrogen atoms were held
stationary and included in the structure factor calculations
123

374
Transition Met Chem (2013) 38:367–375
Description of the structures
these complexes. Next, we tested their activities in the
reaction of bromobenzene with phenylacetylene under the
same conditions, with the results shown in Table 3 (entries
8–14). Only moderate yields were observed in these exper-
iments. Catalyst 3a with the less sterically hindered allyl
group provided the best yield of 43 %, whereas the other
catalysts (3b–3e and 4d–4e) with more sterically hindered
substituents gave lower yields (about 30 %). It has been
found preciously that sterically demanding ligands can per-
mit high catalytic activity [28]; however, this is not true in the
present case. It is possible that transient coordination of the
allyl group in complex 3a helps to stabilize the highly
reactive low-valent palladium intermediate and permit high
catalytic activity. The effect of the different aromatic sub-
stituents and halide ligands on the catalytic activity is not
very obvious. The yield of the reaction of bromobenzene
with phenyl acetylene can be improved to 80 % with 3a by
using K3PO4 as base instead of K2CO3 (Table 3, entry 15).
We also tested an asymmetric Suzuki reaction with palla-
dium complex 3a in the reaction of 1-bromonaphthalene
with naphthalen-1-ylboronic acid; unfortunately, only about
5 % ee with 80 % yield was detected. We are currently
optimizing the reaction conditions to further improve the
catalytic performance.
Crystal of complexes of 3a and 4e were obtained from
layered petroleum ether/CH2Cl2 solutions; 3b was obtained
from a layered acetonitrile/CH2Cl2 solution; and 3c was
obtained from a layered ethyl acetate/CH2Cl2 solution. The
molecular structures of 3a–3c and 4e were determined by
X-ray diffraction. Details of crystal data, data collections,
and structure refinement are summarized in Table 1. The
molecular diagrams of 3a–3c and 4e are shown in Figs. 1,
2, 3 and 4, respectively, and selected bond lengths and
angles are given in Table 2. All four complexes show
slightly distorted square-planar geometry around palla-
dium, which is surrounded by imidazolylidene, two halide
ligands in a trans configuration, and a pyridine ligand in
each case.
The two bromides in 3a–3c are in a slightly bent
geometry, with Br–Pd–Br angles from 169.49° to 172.28°,
while the Cl–Pd–Cl moiety in 4e is also in a slightly bent
geometry with an angle of 172.82°. The Ccarbene, Pd and
N
pyridine atoms are nearly colinear in these complexes, with
C–Pd–N angles ranging from 175.16° to 177.73°. How-
ever, the pyridyl and imidazolidene rings bisect each other
with dihedral angles of 45.7° for 3a, 51.7° for 3b, 63.3° for
3c and 65.2° for 4e. The Pd–Ccarbene distances range from
˚
1.939(9) to 1.970(4) A, and the Pd–Npyridine distances
˚
range from 2.102(5) to 2.139(8) A, which are similar to
Conclusions
those shown by similar palladium complexes [24, 27].
Other distances and angles lie in the expected ranges. It is
worth noting that various p–p stacking interactions exist in
these complexes. In the solid-state structure of 3a, there is
intermolecular face-to-face p–p stacking between adjacent
In conclusion, the synthesis of a series of novel chiral
Pd(NHC)PyBr2 (3a–3e) and Pd(NHC)PyCl2 (4d–4e)
complexes from a commercially available and inexpensive
amino acid, L-phenylalanine, was reported. The solid-state
structures of 3a–3c and 4d were determined by X-ray
diffraction. All these complexes show slightly distorted
square-planar geometries around palladium, which is
coordinated by imidazolylidene, two halide ligands in a
trans configuration and a pyridine. Complex 3a showed
good catalytic activity for Cu-free Sonogashira coupling
under aerobic conditions.
˚
phenyl rings; the shortest atom-to-atom distance is 3.6 A
[C(22)–C(23)]. In 3b, the pyridyl and phenyl rings from
one molecule have intermolecular p–p interactions with
adjacent phenyl and napthyl rings. The shortest atom-to-
˚
atom distance of pyridine to the phenyl ring is 3.4 A
[C(38)–C(31)], and that of the phenyl ring to the napthyl
˚
ring is 3.5 A [C(33)–C(20)]. However, there are only very
weak p–p interactions in 3c and 4e.
Catalytic studies
Supplementary material
The Pd catalyzed Cu-free Sonogashira coupling reaction
under aerobic conditions was chosen to evaluate the cata-
lytic activities of 3a–3d and 4d–4e. Firstly, we performed
the reaction of p-nitrobromobenzene (1 mmol) with phen-
ylacetylene (2 mmol) in DMSO (2 mL) in the presence of
K2CO3 (2 mmol) as base at 100 °C for 1 h with 1 mol %
Pd catalyst loading. As can be seen from the results shown
in Table 3 (entries 1–7), all the complexes showed good
catalytic activity in the tested reactions, and there was
almost no difference in the catalytic performance among
Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre, CCDC, 12 Union
Road, Cambridge CB21EZ, UK. Copies of the data can be
obtained free of charge on quoting the depository numbers
CCDC-899890 (3a), CCDC-899891 (3b), CCDC-899892
(3c) and CCDC-899893 (4e) (Fax: ?44-1223-336-003;
E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.
uk).
123

Transition Met Chem (2013) 38:367–375
375
Acknowledgments We are grateful to the foundation of the
National Natural Science Foundation of China (Grants 21071121,
21172188 and 21104064), Scientific Research Foundation (SRF) for
the Returned Overseas Chinese Scholars (ROCS), State Education
Ministry (SEM), the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD), 333 Project for the
Cultivation of High-level Talents (33GC10002) and Graduate Foun-
dation of Jiangsu Education Committee (CXLX11_0909 and
CXLX11_0910) for financial support of this work.
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