Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

Article abstract of DOI:10.1016/j.jorganchem.2009.09.031

The unsymmetrical, achiral and chiral NCN pincer ligand precursors (3a-3d) with oxazoline and pyrazole as N donors as well as (3e) which has oxazolinyl and amino group have been synthesized in a facile manner in four steps starting from commercially available isophthalaldehyde. Direct C2 metallation of the precursors (3a-3e) with Pd(OAc)₂ in refluxing HOAc, followed by treatment with LiCl at room temperature provided convenient access to the corresponding pincer palladium(II) complexes (4a-4e). The molecular structure of complex 4e has been determined by X-ray single-crystal diffraction. The obtained Pd complexes exhibited good activities in the Suzuki reactions of aryl bromides and activated aryl chlorides with phenylboronic acid.

Full text of DOI:10.1016/j.jorganchem.2009.09.031

Journal of Organometallic Chemistry 695 (2010) 82–89
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand
precursors and their complexes with palladium(II)
*
*
Xin-Qi Hao, Ya-Nan Wang, Jun-Rui Liu, Kun-Lun Wang, Jun-Fang Gong , Mao-Ping Song
Department of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Zhengzhou University, Zhengzhou 450052, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The unsymmetrical, achiral and chiral NCN pincer ligand precursors (3a-3d) with oxazoline and pyrazole
as N donors as well as (3e) which has oxazolinyl and amino group have been synthesized in a facile man-
ner in four steps starting from commercially available isophthalaldehyde. Direct C2 metallation of the
precursors (3a–3e) with Pd(OAc)2 in refluxing HOAc, followed by treatment with LiCl at room tempera-
ture provided convenient access to the corresponding pincer palladium(II) complexes (4a–4e). The
molecular structure of complex 4e has been determined by X-ray single-crystal diffraction. The obtained
Pd complexes exhibited good activities in the Suzuki reactions of aryl bromides and activated aryl chlo-
rides with phenylboronic acid.
Received 11 June 2009
Received in revised form 18 September
2009
Accepted 25 September 2009
Available online 3 October 2009
Keywords:
Unsymmetrical
NCN pincer palladium complexes
Oxazolinyl
Ó 2009 Elsevier B.V. All rights reserved.
Chiral
Suzuki reaction
1. Introduction
a better tuning of the catalytic properties or give unique reactivity
of the corresponding metal pincer complexes. In fact, some
Palladium complexes with NCN pincer ligands have been exten-
sively studied in recent years due to their high stability, feasible
structural modifications, and remarkable catalytic activities in
organometallic catalysis [1–4]. Much of the research has focused
on symmetrical NCN pincer palladium complexes, which are sym-
metrical with two identical N donors such as amines [5–7], imines
[8,9], pyridines [10–12], oxazolines [13–20] or other N-containing
heterocycles [21,22] and 2 equiv. five-membered palladacycles
(Scheme 1). Some of the complexes have been successfully applied
as catalysts for stannylation of allyl [5] or propargylic substrates
[6], Diels–Alder reaction [15], Heck [8,10,21,22], Suzuki and Sono-
gashira [22] coupling reactions. And the chiral complexes proved
unsymmetrical pincer Pd complexes have been found to be much
more active than the related symmetrical ones under certain
circumstances [26,27]. Herein, we would like to report a simple
protocol for the synthesis of the unsymmetrical, oxazolinyl-con-
taining achiral and chiral NCN pincer ligand precursors (3a–3e)
and their corresponding Pd (II) derivatives (4a–4e) (Scheme 2).
To the best of our knowledge, there are no reports concerning
the preparation and use of unsymmetrical NCN pincer Pd(II)
complexes. The Pd complexes (4a–4d) were unusual in that they
not only had different N-heterocyclic donors, but also contained
both five- and six-membered metallacycles in the molecules. The
obtained Pd complexes were applied to the Suzuki reactions of aryl
halides with phenylboronic acid.
to be effective in asymmetric Michael reaction between
a-cyano-
carboxylates and methyl vinyl ketone (up to 83% ee) [8,17]. Follow-
ing our interest in the metal pincer complexes and their
applications, we recently reported the symmetrical chiral NCN pin-
cer Pt(II) and Pd(II) complexes with 1,3-bis(20-imidazolinyl)ben-
zenes [23,24] and particularly unsymmetrical PCN pincer Pd(II)
complexes containing phosphinito group by one-pot phosphoryla-
tion/palladation reaction [25,26]. We reasoned that different do-
nors such as ‘‘hard” N and ‘‘soft” P in PCN pincers might provide
2. Results and discussion
2.1. Synthesis and characterization
The unsymmetrical NCN pincer ligand precursors 3a–3e were
prepared from commercially available isophthalaldehyde in four
steps as shown in Scheme 2. First, selective reduction of one alde-
hyde group in isophthalaldehyde by NaBH4 at 0 °C in MeOH readily
afforded 3-(hydroxymethyl) benzaldehyde, which after bromina-
tion with PBr3 led to 3-(bromomethyl)benzaldehyde (1). Then
nucleophilic substitution of 1 with 3,5-dimethylpyrazole, pyrazole
or diethylamine in the presence of K2CO3 in DMF or dioxane gave
* Corresponding authors. Tel.: +86 371 67763869; fax: +86 371 67766667
(J.-F. Gong).
E-mail addresses: gongjf@zzu.edu.cn (J.-F. Gong), mpsong9350@zzu.edu.cn
(M.-P. Song).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.031

X.-Q. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 82–89
83
ular note is that the significant upfield shifts of 5-aryl protons
d ꢀ ꢁ0.6 ppm) and large downfield shifts of the 3-pyrazolyl pro-
tons (
gands 3. In fact, the downfield shift was also observed for certain
protons of substituent in the oxazoline or pyrazole ring. For exam-
O
R'
R'
(D
NMe2
Pd
NMe2
N
Pd
N
R
N
Pd
N
D
d ꢀ 1.0 ppm) in complexes 4 compared to those in the li-
R
R
N
L
L
Pd
N
L
L
ple, the CH proton of i-Pr in the oxazoline ring (
CH3 protons in the 3-position of pyrazole ring (
D
D
d ꢀ 1.0 ppm) and
R
d ꢀ 0.5 ppm). In
O
the 13C NMR spectra of the ligands 3, the resonances due to the car-
bon atom of C@N appeared around 164 ppm for oxazoline ring, and
148 or 140 ppm for pyrazole ring. The resonances due to C-2 atom
of central aryl appeared around 130 ppm. For the pincer Pd com-
plexes 4, the corresponding resonances obviously shifted down-
field. They were observed around 175 ppm, 153 or 145 ppm and
150 ppm, respectively, again indicative of N–Pd coordinations
and C–Pd bond formation in the complexes. Finally, it should be
pointed out that in the 13C NMR spectra of 4b and 4d, the signal
for one of the quaternary carbons on the central aryl ring over-
lapped with the signal at d 132.7 ppm.
Scheme 1. Structure of the most common types of NCN pincer palladium
complexes.
pyrazolyl- or amino-containing m-benzaldehyde derivatives 2a–
2c. Finally, the aldehyde group in 2 was directly converted to 2-
oxazoline to yield achiral and chiral NCN ligand precursors 3a–3e
by the reaction of aldehyde 2 with 2-aminoethanol or (S)-valinol
using molecular iodine and potassium carbonate in tert-butyl alco-
hol according to the published procedure [28]. With the expected
ligand precursors in hand, the synthesis of the corresponding pin-
cer Pd(II) complexes was attempted. The direct cyclopalladation
reaction was carried out with 3a–3e and 1.2 equiv. of Pd(OAc)2 in
refluxing acetic acid for 48 h, followed by treatment with LiCl at
room temperature in acetone/water for another 48 h. The com-
plexes 4a–4e were successfully isolated as air- and moisture-stable
yellow solids in 16–70% yields after purification.
2.2. Crystal structure
The molecular structure of 4e was confirmed by a single-crystal
X-ray analysis. The molecular is shown in Fig. 1. Crystallographic
and data collection parameters are summarized in Table 2. Se-
lected bond lengths (Å) and angles (°) are given in Table 3. Fig. 1
shows clearly that the ligand is coordinated to the Pd(II) centre
via oxazolinyl-N, diethylamino-N and one aryl-C in a tridentate
manner and a tetracyclic system is thus formed by the central aryl
ring, the oxazoline ring and the two five-membered matallacycles.
The palladium atom adopts a typical distorted-square-planar con-
figuration defined by C (central aryl), N, N and Cl atoms. The Pd–C
bond distance is 1.915(7) Å (or 1.910(8) Å) and the bond distances
between Pd(II) and the two nitrogen atoms are 2.058(7) (or
2.072(7) Å) and 2.119(6) Å (or 2.126(6) Å) for N(1) and N(2),
respectively. The angle of C–Pd–Cl (179.0° or 177.6°) is almost lin-
ear, while the N–Pd–N angle (around 161°) is small, which are in
accordance with a pincer consisting two five-membered-ring
All the new compounds were well characterized by HRMS, 1H
NMR, 13C NMR, IR and ESI-MS. Comparison of the 1H NMR spectra
of the NCHN ligands 3 with those of their corresponding Pd pincer
complexes 4, the formation of 4 was clearly indicated by the disap-
pearance of the singlet at d 7.76–7.90 ppm corresponding to the
central aryl proton located ortho to both N donors in 3. Addition-
ally, it was found that the signals of central aryl protons in 4 were
shifted upfield. While those of the N-heterocycles including oxaz-
oline and pyrazole rings as well as the CH2 protons in NEt2 were
shifted downfield due to the N–Pd coordinations and C–Pd bond
formation in the Pd complexes (selected 1H NMR data of ligands
3 and Pd complexes 4 are given in Table 1). Among them, of partic-
R
R
1) NaBH4, MeOH, 0oC
OHC
K2CO3, DMF, 120 oC
Br
HN
N
R
N
N
2) PBr3, CHCl3, rt
OHC
OHC
CHO
OH
2a R = Me
1
R
2b R = H
R1
4
5
2
5
6
3
2
6
4
3
1) Pd(OAc)2, HOAc
reflux, 48 h
H2N
1
O
O
I2, K2CO3, t-BuOH, 70oC
1
R
R
2) LiCl, acetone,
H2O, rt, 48 h
N
N
N
Pd
N
N
N
R1
R1
Cl
R
R
4a R = CH3, R1 = H
3a R = CH3, R1 = H
4b R = R1 = H
3b R = R1 = H
4c R = CH3, R1 = i-Pr
4d R = H, R1 = i-Pr
3c R = CH3, R1 = i-Pr
3d R = H, R1 = i-Pr
4
5
6
5
6
3
2
1) Pd(OAc)2, HOAc
reflux, 48 h
4
3
HNEt2, K2CO3
NH2CH2CH2OH
O
dioxane, 110 oC
OHC
1
O
2) LiCl, acetone,
I2, K2CO3,
t-BuOH, 70oC
OHC
1
H2O, rt, 48 h
N
2
Pd
N
N
Br
N
N
Et
Et
Et
Et
Et
Cl
Et
1
3e
4e
2c
Scheme 2.

84
X.-Q. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 82–89
Table 1
Selected 1H NMR data of NCHN ligands 3 and Pd complexes 4 (in CDCl3, d: ppm).
Ligand or
complex
Ar2 (1H) or
Ar1
Ar6 or Ar5 (1H)
Ar5 or Ar4 (1H)
Ar4 or Ar3
(1H)
PzH or N(CH2CH3)2
OxH
3a
4a
3b
7.76 (s)
–
7.84 (s)
7.83 (d)
7.21 (dd)
7.87 (d)
7.34 (t)
7.11 (d)
7.29 (d)
5.85 (s, 1H)
5.92 (s, 1H)
7.55 (d, 1H), 7.40 (d, 1H),
6.28 (t, 1H)
8.57 (d, 1H), 7.66 (d, 1H),
6.38 (t, 1H)
4.42 (t, 2H), 4.05 (t, 2H)
4.76 (t, 2H), 4.18 (t, 2H)
4.42 (t, 2H), 4.04 (t, 2H)
7.10–7.05 (m)
7.38 (t)
7.32 (t)
7.37 (t)
4b
3c
4c
3d
4d
–
7.26–7.25 (m)
7.84 (d)
7.14–7.07 (m)
7.09–7.05 (m)
7.07–7.01 (m)
4.78 (t, 2H), 4.15 (t, 2H)
7.78 (s)
7.08 (d)
7.28 (d)
5.85 (s, 1H)
4.40–4.36 (m, 1H), 4.14–4.07 (m,
2H)
4.63 (dd, 1H), 4.57 (t, 1H), 4.46–
4.41 (m, 1H)
4.42–4.38 (m, 1H), 4.15–4.07 (m,
2H)
4.57–4.49 (m, 2H), 4.34–4.29 (m,
1H)
–
7.19 (dd)
5.92 (s, 1H)
7.87 (s)
–
7.88 (d)
7.55 (d, 1H), 7.39 (d, 1H),
6.28 (t, 1H)
8.55 (d, 1H), 7.60 (d, 1H),
6.30 (t, 1H)
7.20–7.17 (m)
3e
4e
7.90 (s)
–
7.82 (d)
7.07 (d)
7.36 (t)
7.01 (t)
7.49 (d)
6.94 (d)
2.52 (q, 4H)
3.44–3.37 (m, 2H) 2.79–
2.72 (m, 2H)
4.43 (t, 2H), 4.06 (t, 2H)
4.71 (t, 2H), 4.01 (t, 2H)
Table 3
Selected bond lengths (Å) and angles (°) (corresponding values for the unshown
second structure are given in brackets) for complex 4e.
Pd(1)–C(4) [Pd(10)–C(40)]
Pd(1)–N(1) [Pd(10)–N(10)]
Pd(1)–N(2) [Pd(10)–N(20)]
Pd(1)–Cl(1) [Pd(10)–Cl(10)]
N(1)–C(2) [N(10)–C(20)]
N(1)–C(3) [N(10)–C(30)]
N(2)–C(10) [N(20)–C(100)]
N(2)–C(11) [N(20)–C(110)]
N(2)–C(13) [N(20)–C(130)]
O(1)–C(3) [O(10)–C(30)]
O(1)–C(1) [O(10)–C(10)]
C(1)–C(2) [C(10)–C(20)]
C(3)–C(5) [C(30)–C(50)]
C(4)–C(5) [C(40)–C(50)]
C(4)–C(9) [C(40)–C(90)]
C(9)–C(10) [C(90)–C(100)]
1.915(7) [1.910(8)]
2.058(7) [2.072(7)]
2.119(6) [2.126(6)]
2.4162(19) [2.426(2)]
1.461(10) [1.475(10)]
1.296(11) [1.277(12)]
1.530(11) [1.518(10)]
1.488(13) [1.504(11)]
1.489(13) [1.518(11)]
1.326(10) [1.360(10)]
1.468(13) [1.441(13)]
1.532(14) [1.549(15)]
1.470(12) [1.440(13)]
1.386(13) [1.400(13)]
1.373(12) [1.408(11)]
1.485(13) [1.476(12)]
Fig. 1. Molecular structure of complex 4e (representation of one of the two
independent crystal structures). Hydrogen atoms are omitted for clarity.
C(4)–Pd(1)–N(1) [C(40)–Pd(10)–N(10)]
C(4)–Pd(1)–N(2) [C(40)–Pd(10)–N(20)]
N(1)–Pd(1)–N(2) [N(10)–Pd(10)–N(20)]
C(4)–Pd(1)–Cl(1) [C(40)–Pd(10)–Cl(10)]
N(1)–Pd(1)–Cl(1) [N(10)–Pd(10)–Cl(10)]
N(2)–Pd(1)–Cl(1) [N(20)–Pd(10)–Cl(10)]
79.9(3) [79.6(3)]
82.1(3) [82.0(3)]
161.7(3) [160.7(3)]
179.0(3) [177.6(3)]
99.51(19) [101.1(2)]
98.48(19) [97.50(18)]
Table 2
Crystallographic and data collection parameters for complex 4e.
Formula
C14H19ClN2OPd
Mr
373.16
0.20 ꢃ 0.18 ꢃ 0.18
20.064(4)
7.1728(14)
20.597(4)
90
Crystal size (mm)
a (Å)
b (Å)
c (Å)
angles around Pd(II) centre are similar to those observed in the re-
lated bis(oxazoline) pincer Pd(II) complexes [20,29]. In the crystal
of complex 4e, chlorine atom forms hydrogen bond with the adja-
cent CH2 group of oxazoline ring [Cl(1)ꢂ ꢂ ꢂH(10B) 2.834 Å,
Cl(1)ꢂ ꢂ ꢂC(10) 3.549 Å, C(10)–H(10B)ꢂ ꢂ ꢂCl(1) 132° and Cl(10)ꢂ ꢂ ꢂH(1A)
2.843 Å, Cl(10)ꢂ ꢂ ꢂC(1) 3.607 Å, C(1)–H(1A)ꢂ ꢂ ꢂCl(10) 136°]. There also
exist intermolecular hydrogen bonds between Pd atom and the
adjacent CH2 group of oxazoline ring [Pd(1)ꢂ ꢂ ꢂH(10A) 2.977 Å,
Pd(1)ꢂ ꢂ ꢂC(10) 3.907 Å, C(10)–H(10A)ꢂ ꢂ ꢂPd(1) 161°]. These hydrogen
bonds are attributed to construct the 1D chain structure of com-
plex 4e (Fig. 2).
a
(°)
b (°)
90
90
c
(°)
V (Å3)
2964.2(10)
8
Pca2(1)
1.672
1.426
1.98–26.00
9947
Z
Space group
DCalcd (g cmꢁ3
)
l
(mmꢁ1
)
h range (°)
Number of data collected
Number of unique data
Observed data [I ꢄ 2
R (all data)
Rw (all data)
R (I > 2
Rw (I > 2
5658
5658
r(I)]
0.0562
0.1443
0.0508
0.1336
1504
2.3. Suzuki reaction
r
(I))
(I))
r
All the palladium complexes 4 including the chiral ones were
tested as precatalysts for the Suzuki coupling reactions. The com-
plexes are thermally stable and inert toward air and moisture both
in the solid state and in solution. These properties allowed for the
catalytic experiments open to air in the analytical grade solvents
which were used without further purification. Initial catalytic
studies were performed on the coupling of 3-bromotoluene with
F(0 0 0)
Peak/hole (e Åꢁ3
)
1.256/ꢁ1.317
palladacycles [8,19,20,24,26] and reflect a relative steric strain of
the almost planar tetracyclic system. All the bond distances and

X.-Q. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 82–89
85
Fig. 2. 1-D chain structure of complex 4e formed by hydrogen bonds. Non-hydrogen bonding H atoms are omitted for clarity.
phenylboronic acid as a model reaction in the presence of 0.5 mol%
of complex 4b. As shown in Table 4, when the reaction was con-
ducted in DMF at 140 °C for 12 h, KFꢂ2H2O, K2CO3 and Na2CO3 as
a base gave excellent yields (entries 1–6). When toluene was used
as solvent, excellent yields were also obtained at 110 °C after 12 h
with KFꢂ2H2O, K2CO3, K3PO4ꢂ3H2O and NaOH as a base (entries 7–
12). Several other solvents using K2CO3 as the base were also
tested. Among them, THF was not effective (entry 13) and water
produced a moderate yield (entry 14). Dioxane gave the best result
after 7 h (>99% yield, entry 15). Finally, the influence of the cata-
lytic loading on the reaction was investigated (entries 16–19).
When the reaction was carried out in the presence of 0.1 mol% of
complex 4b with K2CO3 as the base in dioxane at 110 °C, the cou-
pled product could be isolated in >99% yield after 7 h (entry 19).
Under these optimized reaction conditions, several electron-
neutral aryl bromides such as boromobenzene, 3-bromotoluene
and even the sterically hindered 2-bromo-m-xylene could be cou-
pled efficiently with phenylboronic acid, giving the corresponding
biaryls in excellent yields. And it was found that complex 4a with
3,5-dimethyl pyrazolyl group and 4b with pyrazolyl group dis-
played comparable activity in these reactions (Table 5, entries
1–8). While in the case of electron-rich aryl bromides such as 4-
bromoanisole, complex 4a exhibited higher activity than complex
4b (entries 9–10). Similarly, 4c was more active than 4d (entries
11–12). Among the five Pd complexes, 4c with 4-i-Pr-oxazolinyl
and 3,5-dimethyl pyrazolyl group was the most active, providing
the 4-methoxybiphenyl in an excellent yield (entries 9–13). While
for heteroaryl bromides such as 2-bromothiophene and 2-bromo-
pyridine, complex 4a and 4c were found to be inferior to 4b and
excellent yields could also be achieved in the presence of 4b (en-
tries 14–19).
Table 4
Optimization of reaction conditions for the coupling of 3-bromotoluene with
phenylboronic acid.a
In the following experiments, the catalytic activities of unsym-
metrical NCN Pd complex 4c and the symmetrical NCN complex 5
[24] as well as the related unsymmetrical PCN complexes 6–7
[25,26] were compared under two different reaction conditions.
It was found that the symmetrical Pd complex 5 exhibited an obvi-
ously lower activity than the other unsymmetrical ones, especially
at reduced temperature giving only trace amount of the coupled
product (Table 6). The three unsymmetrical Pd complexes showed
comparable activities at 110 °C, giving the product in almost quan-
titative yield (entries 1, 3 and 4). While at 50 °C, complexes 4c and
6a were inferior to 6b and 7, indicating that the activity was influ-
enced by the substituent on the N- or P-donors (entries 5, 7–9).
Since palladium black formation was observed under the investi-
gated reaction conditions, the Suzuki reaction catalyzed by these
systems was believed to proceed via a Pd(0)–Pd(II) catalytic cycle.
Consequently, the lower activity of complex 5 was possibly due to
its higher stability that would lead to a slower release of the true
catalytic active Pd(0) species. In contrast, the relatively lower sta-
bility of the unsymmetrical complexes resulted from the hemila-
bile coordination of the ligands made it easier for these
complexes to generate active Pd(0).
H3C
H3C
Complex 4b
+
B(OH)2
Br
Entry Solvent
Base
Temperature (°C) Time (h) Yieldb (%)
1
2
3
4
5
6
7
8
DMF
DMF
DMF
DMF
DMF
DMF
K3PO4ꢂ3H2O
NaOAcꢂ3H2O
140
140
12
12
12
12
12
12
12
12
12
12
12
12
12
12
7
47.6
14.3
45.2
94.0
>99
>99
45.2
19.0
>99
96.4
>99
>99
8.3
78.6
>99
>99
>99
39.3
>99
K2HPO4ꢂ3H2O 140
KFꢂ2H2O
140
140
140
K2CO3
Na2CO3
Toluene K2HPO4ꢂ3H2O 110
Toluene Na2CO3
Toluene KFꢂ2H2O
Toluene K2CO3
110
110
110
110
110
65
100
110
140
110
110
110
9
10
11
12
13
14
15
16c
17c
18d
19d
Toluene K3PO4ꢂ3H2O
Toluene NaOH
THF
H2O
K2CO3
K2CO3
Dioxane K2CO3
DMF K2CO3
Dioxane K2CO3
DMF K2CO3
Dioxane K2CO3
12
12
7
The Suzuki reaction of aryl chlorides with phenylboronic acid
catalyzed by complex 4 was also investigated since aryl chlorides
are more readily available, less expensive but less reactive than
aryl bromides (Scheme 3). It turned out that for an activated
chloride such as 4-chloroacetophenone, high yields could be ob-
tained in DMF with K3PO4ꢂ7H2O as the base at 120 °C after 12 h
in the presence of 1 mol% of complex 4a (>99%) or 4b (86.7%).
7
a
Reaction conditions: complex 4b (0.5 mol%), 3-bromotoluene (0.5 mmol),
PhB(OH)2 (0.75 mmol), base (1.0 mmol), solvent (3 mL), under air.
b
Isolated yield.
0.25 mol% of complex 4b.
0.1 mol% of complex 4b.
c
d

86
X.-Q. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 82–89
Table 5
Suzuki reactions of aryl bromides with phenylboronic acid catalyzed by complex 4.a
Entry
Aryl bromide
Cat. (mol%)
Product
Yield b(%)
1
2
4a (0.1)
4b (0.1)
>99
>99
Br
3
4
4a (0.1)
4b (0.1)
>99
>99
H3C
H3C
Br
5
6
4a (0.1)
4b (0.1)
>99
91.7
CH3
Br
CH3
Br
CH3
7
8
4a (0.1)
4b (0.1)
>99
>99
CH3
CH3
CH3
9
4a (0.1)
4b (0.1)
4c (0.1)
4d (0.1)
4e (0.1)
4a (0.1)
4b (0.1)
4c (0.1)
4a (0.1)
4b (0.1)
4c (0.1)
85.9
67.3
>99
Br
H3CO
H3CO
10
11
12
13
14
15
16
17
18
19
83.4
70.0
88.8
97.5
89.8
65.8
92.9
39.5
Br
Br
S
S
N
N
a
Reaction conditions: aryl bromide (0.5 mmol), PhB(OH)2 (0.75 mmol), K2CO3 (1.0 mmol), dioxane (3 mL), 110 °C, 7 h, under air.
Isolated yield.
b
Table 6
Relative catalytic activities of unsymmetrical and symmetrical pincer Pd(II) complexes 4–7 in the Suzuki coupling of 4-bromoanisole with phenylboronic acid.a
Cat.
+
B(OH)2
H3CO
Br
H3CO
Cl
N
O
P
N
N
N
N
Pd
Cl
Pd Cl
PPh2
R
(R')2
N
N
Pd
O
R
Cl
6a R = CH3; R' = Cy
6b R = H; R' = Ph
7
5
Entry
Cat. (mol%)
Solvent
Base
Temperature (°C)
Time (h)
Yieldb (%)
1
2
3
4
4c (0.1)
5 (0.1)
6a (0.1)
7 (0.1)
4c (0.3)
5 (0.3)
6a (0.3)
6b (0.3)
7 (0.3)
Dioxane
Dioxane
Dioxane
Dioxane
EtOH
EtOH
EtOH
EtOH
EtOH
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4ꢂ7H2O
K3PO4ꢂ7H2O
K3PO4ꢂ7H2O
K3PO4ꢂ7H2O
K3PO4ꢂ7H2O
110
110
110
110
50
50
50
50
50
7
7
7
7
10
10
10
10
10
>99
11.9
>99
>99
5c
6c
7c
8c
9c,d
82.0
trace
80.9
95.0
91.0
a
b
c
Reaction conditions: 4-bromoanisole (0.5 mmol), PhB(OH)2 (0.75 mmol), base (1.0 mmol), solvent (3 mL), under air.
Isolated yield.
PhB(OH)2 (0.6 mmol).
From Ref. [26].
d
O
O
Complex 4a or 4b
K3PO4·7H2O, DMF
120 oC, 12 h
+
B(OH)2
Cl
H3C
H3C
Scheme 3.

X.-Q. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 82–89
87
However, for methyl-substituted chlorobenzenes only poor yields
(<40%) were obtained under similar reaction conditions.
In conclusion, we have synthesized a series of unsymmetrical,
oxazolinyl-containing achiral and chiral NCN pincer ligand precur-
sors starting from easily available isophthalaldehyde. The corre-
sponding pincer Pd complexes were prepared via direct C2
palladation of the ligand precursors and used as efficient catalysts
for Suzuki reactions of aryl bromides and activated aryl chlorides
with phenylboronic acid. Further studies will focus on the synthe-
sis of chiral pincer Pt complexes with the ligands and their applica-
tions in asymmetric catalysis.
Method B: A mixture of 3-(bromomethyl)benzaldehyde (1)
(396.0 mg, 2.0 mmol), 3,5-dimethylpyrazole, pyrazole (2.5 mmol)
or diethylamine (2.0 mmol), and potassium carbonate (552.0 mg,
4.0 mmol) was heated at 120 °C in DMF (5 mL) for 24 h (or
110 °C in 30 mL of dioxane for 5 h for the reaction of 1 with HNEt2).
Then solvent was removed in vacuo and water (15 mL) was added.
The aqueous layer was extracted with chloroform. The organic lay-
ers were dried over MgSO4, filtered, and evaporated. The crude was
purified by preparative TLC on silica gel plates eluting with ace-
tone/petroleum ether (1/2) for 2a and 2b or ethyl acetate for 2c.
Compound 2c is a known compound.
2a: 270.5 mg. Yield: 63.2%. Colorless oil. 1H NMR (400 MHz,
CDCl3): d 9.96 (d, J = 1.0 Hz, 1H, CHO), 7.77 (d, J = 7.6 Hz, 1H,
ArH), 7.59 (s, 1H, ArH), 7.48 (t, J = 7.6 Hz, 1H, ArH), 7.33 (d,
J = 7.6 Hz, 1H, ArH), 5.88 (s, 1H, PzH), 5.28 (s, 2H, CH2), 2.25 (s,
3H, CH3), 2.16 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): d 192.0,
148.0, 139.2, 138.7, 136.7, 132.6, 129.5, 128.9, 127.6, 105.8, 51.9,
3. Experimental
3.1. General
13.5, 11.1. IR (KBr, cmꢁ1):
m 2923, 2821, 1699, 1589, 1554, 1458,
Compounds 1 [30,31], 3,5-dimethylpyrazole [32], 2 [25,33] and
(S)-valinol [34] were prepared according to the literature methods.
All the other reagents were used as commercial sources. Melting
points were measured using a WC-1 microscopic apparatus and
were uncorrected. IR spectra were collected on a Bruker VECTOR22
spectrophotometer in KBr pellets. 1H and 13C NMR spectra were re-
corded on a Bruker DPX-400 spectrometer. Mass spectra were per-
formed on the Agilent LC/MSD Trap XCT instrument. HRMS were
measured on a Micromass Q-TOF mass spectrometry (Waters,
Manchester, UK) with an ESI source. Elemental analyses were mea-
sured on a Thermo Flash EA 1112 elemental analyzer. Optical rota-
tions of chiral compounds were recorded on a Perkin–Elmer 341
polarimeter.
1424, 1285, 1349, 1308, 1280, 1240, 1142, 1084, 1030, 973, 783,
708. MS (m/z, ESI+): 215 (M+H). HRMS (m/z, ESI+): found for
M+H = 215.1185, C13H15N2O requires 215.1184.
2b: 178.9 mg. Yield: 48.1%. Colorless oil. 1H NMR (400 MHz,
CDCl3): d 9.97 (s, 1H, CHO), 7.80 (d, J = 7.6 Hz, 1H, ArH), 7.69 (s,
1H, ArH), 7.57 (d, J = 1.6 Hz, 1H, PzH), 7.50 (t, J = 7.6 Hz, 1H, ArH),
7.46 (d, J = 2.0 Hz, 2H, PzH), 7.46–7.44 (m, 1H, ArH), 6.31 (t,
J = 2.0 Hz, 1H, PzH), 5.40 (s, 2H, CH2). 13C NMR (100 MHz, CDCl3):
d 191.8, 139.8, 137.9, 136.6, 133.2, 129.4, 129.3, 129.2, 128.2,
106.2, 55.0. IR (KBr, cmꢁ1):
m 2935, 2840, 2734, 1699, 1592,
1513, 1446, 1394, 1353, 1285, 1243, 1145, 1089, 1050, 967, 919,
754. MS (m/z, ESI+): 187 (M+H). HRMS (m/z, ESI+): found for
M+H = 187.0877, C11H11N2O requires 187.0871.
3.2. Synthesis of 3-(bromomethyl)benzaldehyde (1)
3.4. Synthesis of the unsymmetrical NCN ligand precursors 3a–3e by
direct oxidative conversion of aldehyde in 2a-2c to 2-oxazoline
To a solution of isophthalaldehyde (4.75 g, 35.5 mmol) in etha-
nol (105 mL) was added sodium borohydride (0.33 g, 8.85 mmol)
at 0 °C and the mixture was stirred overnight at 0 °C. After the sol-
vent was removed, CH2Cl2 were added to the residue. The com-
bined extracts were washed with brine, dried, and then
concentrated in vacuo. The crude product was purified by SiO2 col-
umn chromatography (eluent: 1/1 n-hexane/ethyl acetate) to fur-
nish the desired 3-(hydroxymethyl)benzaldehyde (3.94 g, 81.7%)
as a colorless oil. To a solution of 3-(hydroxymethyl)benzaldehyde
(1.36 g, 10.0 mmol) in CHCl3 (20 mL), PBr3 (1.12 mL, 12.0 mmol)
was added at room temperature and stirred for 3 h. Then a spot
of water was added to end the reaction, the organic phase was sep-
arated and the aqueous layer was extracted twice with CH2Cl2
(2 ꢃ 20 mL). The organic phase was dried over MgSO4, filtered,
and evaporated. The residue was purified by SiO2 column chroma-
tography (eluent: CH2Cl2) to afford the desired 3-(bromo-
methyl)benzaldehyde (1) (1.42 g, 71.6%) as a colorless oil which
solidified at 0 °C.
To a solution of 2a (214.0 mg, 1 mmol) [or 2b (186.0 mg,
1 mmol) or 2c (191.0 mg, 1 mmol)] in t-BuOH (10 mL) was added
2-aminoethanol (67.2 mg, 1.1 mmol) [or (S)-valinol (113.3 mg,
1.1 mmol)]. The mixture was stirred at room temperature under
N2 atmosphere for 30 min, and K2CO3 (414.6 mg, 3 mmol) and I2
(507.6 mg, 2 mmol) were added to the mixture and stirred at
70 °C. After 18 h, the mixture was quenched with satd. aq. Na2SO3
until the color of iodine almost disappeared and was extracted
with Et2O. The organic layer was washed with brine and dried over
Na2SO4, filtered, and evaporated. The crude was purified by pre-
parative TLC on silica gel plates eluting with AcOEt/petroleum
ether (1/1) to afford 3a–3e.
3a: 103.5 mg. Yield: 40.6%. Colorless solid. M.p. 65–66 °C. 1H
NMR (400 MHz, CDCl3): d 7.83 (d, J = 7.7 Hz, 1H, ArH), 7.76 (s,
1H, ArH), 7.34 (t, J = 7.7 Hz, 1H, ArH), 7.11 (d, J = 7.7 Hz, 1H, ArH),
5.85 (s, 1H, PzH), 5.24 (s, 2H, CH2), 4.42 (t, J = 9.5 Hz, 2H, OxH),
4.05 (t, J = 9.5 Hz, 2H, OxH), 2.25 (s, 3H, CH3), 2.14 (s, 3H, CH3).
13C NMR (100 MHz, CDCl3): d 164.3, 147.7, 139.1, 137.7, 129.4,
128.8, 128.0, 127.2, 126.3, 105.6, 67.6, 54.8, 52.2, 13.5, 11.1. IR
3.3. Synthesis of pyrazolyl- or amino-containing m-benzaldehyde
derivatives 2
(KBr, cmꢁ1):
m 3275, 2925, 2874, 1721, 1646, 1583, 1551, 1482,
1456, 1425, 1367, 1327, 1256, 1177, 1086, 1029, 980, 956, 914,
800, 777, 709. MS (m/z, ESI+): 256 (M+H). HRMS (m/z, ESI+): found
for M+H = 256.1451 and M+Na = 278.1259, C15H18N3O and
C15H17N3NaO require 256.1450 and 278.1269, respectively.
Method A: Under nitrogen atmosphere, a mixture of 3-(bromo-
methyl)benzaldehyde (1) (396.0 mg, 2.0 mmol), 3,5-dimethyl-
pyrazole (2.0 mmol), and NaH (144.0 mg, 6.0 mmol) in 30 mL
of dioxane was refluxed with stirring for 3 days. After being
cooled, the reaction was quenched with water. The aqueous
layer was then extracted with dichloromethane, and the organic
layers were dried over MgSO4, filtered, and evaporated. The
crude was purified by preparative TLC on silica gel plates eluting
with acetone/petroleum ether (1:2) to afford 2a (101.8 mg,
23.8%) as a colorless oil.
3b: 81.5 mg. Yield: 35.9%. Colorless oil. 1H NMR (400 MHz,
CDCl3): d 7.87 (d, J = 7.6 Hz, 1H, ArH), 7.84 (s, 1H, ArH), 7.55 (d,
J = 1.3 Hz, 1H, PzH), 7.40 (d, J = 2.0 Hz, 1H, PzH), 7.38 (t,
J = 7.6 Hz, 1H, ArH), 7.29 (d, J = 8.9 Hz, 1H, ArH), 6.28 (t,
J = 2.0 Hz, 1H, PzH), 5.34 (s, 2H, CH2), 4.42 (t, J = 9.6 Hz, 2H, OxH),
4.04 (t, J = 9.6 Hz, 2H, OxH). 13C NMR (100 MHz, CDCl3): d 164.2,
139.6, 136.9, 130.4, 129.2, 128.9, 128.1, 127.7, 127.3, 106.1, 67.6,

88
X.-Q. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 82–89
55.4, 54.8. IR (KBr, cmꢁ1):
m
3106, 2935, 2877, 1719, 1650, 1606,
791, 729. MS (m/z, ESI+): 360 (MꢁCl). HRMS (m/z, ESI+): found for
MꢁCl = 360.0314, C15H16N3OPd requires 360.0328. Anal. Calc. for
C15H16ClN3OPd: C, 45.47; H, 4.07; N, 10.61. Found: C, 46.09; H,
4.45; N 10.04%.
1586, 1513, 1483, 1450, 1434, 1395, 1362, 1269, 1189, 1088,
1067, 980, 954, 916, 805, 756, 709. MS (m/z, ESI+): 228 (M+H).
HRMS
(m/z,
ESI+):
found
for
M+H = 228.1132
and
M+Na = 250.0975, C13H14N3O and C13H13N3NaO require 228.1137
and 250.0956, respectively.
4b: Yield: 43.5%. Yellow solid. M.p. >250 °C. 1H NMR (400 MHz,
CDCl3): d 8.57 (d, J = 1.6 Hz, 1H, PzH), 7.66 (d, J = 1.8 Hz, 1H, PzH),
7.26–7.25 (m, 1H, ArH), 7.14–7.07 (m, 2H, ArH), 6.38 (t,
J = 2.2 Hz, 1H, PzH), 5.28 (s, 2H, CH2), 4.78 (t, J = 9.5 Hz, 2H, OxH),
4.15 (t, J = 9.5 Hz, 2H, OxH). 13C NMR (100 MHz, CDCl3): d 175.4,
149.1, 145.0, 132.7, 131.9, 128.8, 125.4, 124.6, 106.6, 71.0, 56.3,
3c: 150.9 mg. Yield: 50.8%. Colorless oil. ½a D20
ꢅ
¼ þ1ꢆ (c 0.15,
CH2Cl2). 1H NMR (400 MHz, CDCl3): d 7.84 (d, J = 7.6 Hz, 1H, ArH),
7.78 (s, 1H, ArH), 7.32 (t, J = 7.8 Hz, 1H, ArH), 7.08 (d, J = 8.0 Hz,
1H, ArH), 5.85 (s, 1H, PzH), 5.25 (s, 2H, CH2), 4.40–4.36 (m, 1H,
OxH), 4.14–4.07 (m, 2H, OxH), 2.24 (s, 3H, CH3), 2.13 (s, 3H, CH3),
1.89–1.82 (m, 1H, CH(CH3)2), 1.02 (d, J = 6.8 Hz, 3H, CH(CH3)2),
0.92 (d, J = 6.8 Hz, 3H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): d
163.1, 147.7, 139.2, 137.6, 129.4, 128.8, 128.2, 127.4, 126.4,
105.7, 72.5, 70.1, 52.2, 32.8, 18.9, 18.0, 13.5, 11.1. IR (KBr, cmꢁ1):
50.8. IR (KBr, cmꢁ1):
m 3117, 2923, 2853, 1739, 1639, 1573, 1515,
1476, 1436, 1406, 1370, 1268, 1173, 1148, 1103, 1078, 1057,
997, 959, 919, 862, 779, 725. MS (m/z, ESI+): 332 (MꢁCl). HRMS
(m/z, ESI+): found for MꢁCl = 332.0016, C13H12N3OPd requires
332.0015. Anal. Calc. for C13H12ClN3OPd: C, 42.41; H, 3.29; N,
11.41. Found: C, 42.65; H, 3.39; N 11.21%.
m
3198, 2960, 2921, 2872, 1717, 1649, 1605, 1588, 1555, 1461,
1430, 1383, 1356, 1307, 1279, 1188, 1110, 1079, 1030, 975, 923,
784, 710. MS (m/z, ESI+): 298 (M+H). HRMS (m/z, ESI+): found for
M+H = 298.1912, C18H24N3O requires 298.1919.
4c: Yield: 37.8%. Yellow solid. M.p. >250 °C. ½a D20
ꢅ
¼ þ58ꢆ (c 0.06,
CH2Cl2). 1H NMR (400 MHz, CDCl3): d 7.19 (dd, J = 2.3, 6.5 Hz, 1H,
ArH), 7.09–7.05 (m, 2H, ArH), 5.92 (s, 1H, PzH), 5.07 (d,
J = 14.9 Hz, 1H, CHH), 5.02 (d, J = 14.9 Hz, 1H, CHH), 4.63 (dd,
J = 5.1, 8.8 Hz, 1H, OxH), 4.57 (t, J = 9.1 Hz, 1H, OxH), 4.46–4.41
(m, 1H, OxH), 2.97–2.93 (m, 1H, CH(CH3)2), 2.72 (s, 3H, CH3),
2.37 (s, 3H, CH3), 0.94 (d, J = 7.0 Hz, 3H, CH(CH3)2), 0.84 (d,
J = 7.0 Hz, 3H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): d 173.9,
153.3, 151.5, 140.6, 134.5, 131.6, 128.4, 125.0, 124.5, 107.8, 70.6,
3d: 97.5 mg. Yield: 36.1%. Colorless oil. ½a D20
ꢅ
¼ ꢁ25ꢆ (c 0.05,
CH2Cl2). 1H NMR (400 MHz, CDCl3): d 7.88 (d, J = 8.8 Hz, 1H, ArH),
7.87 (s, 1H, ArH), 7.55 (d, J = 1.5 Hz, 1H, PzH), 7.39 (d, J = 2.4 Hz,
1H, PzH), 7.37 (t, J = 7.6 Hz, 1H, ArH), 7.28 (d, J = 7.7 Hz, 1H, ArH),
6.28 (t, J = 2.0 Hz, 1H, PzH), 5.34 (s, 2H, CH2), 4.42–4.38 (m, 1H,
OxH), 4.15–4.07 (m, 2H, OxH), 1.88–1.83 (m, 1H, CH(CH3)2), 1.02
(d, J = 6.8 Hz, 3H, CH(CH3)2), 0.92 (d, J = 6.8 Hz, 3H, CH(CH3)2). 13C
NMR (100 MHz, CDCl3): d 163.0, 139.7, 136.9, 130.5, 129.3, 128.9,
128.4, 127.9, 127.5, 106.1, 72.6, 70.2, 55.6, 32.8, 19.0, 18.1. IR
66.7, 52.0, 29.1, 18.9, 16.4, 14.0, 12.2. IR (KBr, cmꢁ1):
m 2953,
2925, 2864, 1618, 1558, 1484, 1463, 1433, 1395, 1347, 1276,
1181, 1149, 1054, 997, 972, 936, 815. MS (m/z, ESI+): 402 (MꢁCl).
HRMS (m/z, ESI+): found for MꢁCl = 402.0798, C18H22N3OPd re-
quires 402.0798. Anal. Calc. for C18H22ClN3OPd: C, 49.33; H, 5.06;
N, 9.59. Found: C, 49.17; H, 5.12; N 9.41%.
(KBr, cmꢁ1):
m 3101, 2961, 2929, 2872, 1721, 1651, 1606, 1585,
1515, 1451, 1393, 1359, 1284, 1189, 1088, 1049, 975, 921, 803,
753, 711. MS (m/z, ESI+): 270 (M+H). HRMS (m/z, ESI+): found for
M+H = 270.1606
and
M+Na = 292.1433,
C16H20N3O
and
4d: Yield: 69.5%. Yellow solid. M.p. >250 °C. ½a D20
ꢅ
¼ þ137ꢆ (c
C16H19N3NaO require 270.1606 and 292.1426, respectively.
0.13, CH2Cl2). 1H NMR (400 MHz, CDCl3): d 8.55 (d, J = 2.0 Hz, 1H,
PzH), 7.60 (d, J = 2.2 Hz, 1H, PzH), 7.20–7.17 (m, 1H, ArH), 7.07–
7.01 (m, 2H, ArH), 6.30 (t, J = 2.3 Hz, 1H, PzH), 5.26 (d, J = 15.3 Hz,
1H, CHH), 5.17 (d, J = 15.3 Hz, 1H, CHH), 4.57–4.49 (m, 2H, OxH),
4.34–4.29 (m, 1H, OxH), 2.80–2.76 (m, 1H, CH(CH3)2), 0.87 (d,
J = 7.0 Hz, 3H, CH(CH3)2), 0.75 (d, J = 7.0 Hz, 3H, CH(CH3)2). 13C
NMR (100 MHz, CDCl3): d 174.1, 149.0, 145.0, 132.7, 131.8, 128.7,
125.5, 124.6, 106.5, 70.6, 66.9, 56.3, 29.1, 18.8, 14.2. IR (KBr,
3e: 31.3 mg. Yield: 13.5%. Colorless oil. 1H NMR (400 MHz,
CDCl3): d 7.90 (s, 1H, ArH), 7.82 (d, J = 7.7 Hz, 1H, ArH), 7.49 (d,
J = 7.7 Hz, 1H, ArH), 7.36 (t, J = 7.7 Hz, 1H, ArH), 4.43 (t, J = 9.5 Hz,
2H, OxH), 4.06 (t, J = 9.5 Hz, 2H, OxH), 3.59 (s, 2H, CH2), 2.52 (q,
J = 7.2 Hz, 4H, CH2CH3), 1.04 (t, J = 7.2 Hz, 6H, CH2CH3). 13C NMR
(100 MHz, CDCl3): d 165.2, 140.6, 132.4, 129.1, 128.7, 127.9,
127.1, 68.0, 57.6, 55.3, 47.1, 12.1. IR (KBr, cmꢁ1):
m 3421, 3128,
2969, 2925, 2804, 1720, 1650, 1595, 1451, 1393, 1361, 1270,
1193, 1071, 986, 952, 914, 803, 709. MS (m/z, ESI+): 233 (M+H).
HRMS (m/z, ESI+): found for M+H = 233.1652, C14H21N2O requires
233.1654.
cmꢁ1):
m 3123, 2958, 2926, 2869, 1712, 1674, 1626, 1575, 1516,
1483, 1438, 1402, 1371, 1277, 1182, 1152, 1105, 1075, 995, 935,
866, 755, 727. MS (m/z, ESI+): 374 (MꢁCl). HRMS (m/z, ESI+): found
for MꢁCl = 374.0485, C16H18N3OPd requires 374.0485. Anal. Calc.
for C16H18ClN3OPd: C, 46.85; H, 4.42; N, 10.24. Found: C, 47.12;
H, 4.84; N 10.04%.
3.5. Synthesis of pincer palladium complexes 4a–4e
4e: Yield: 16.3%. Yellow solid. M.p. 196 °C. 1H NMR (400 MHz,
CDCl3): d 7.07 (d, J = 7.3 Hz, 1H, ArH), 7.01 (t, J = 7.6 Hz, 1H, ArH),
6.94 (d, J = 7.6 Hz, 1H, ArH), 4.71 (t, J = 9.5 Hz, 2H, OxH), 4.07 (s,
2H, CH2), 4.01 (t, J = 9.5 Hz, 2H, OxH), 3.44–3.37 (m, 2H, CH2CH3),
2.79–2.72 (m, 2H, CH2CH3), 1.57 (t, J = 7.0 Hz, 6H, CH2CH3). 13C
NMR (100 MHz, CDCl3): d 175.1, 160.4, 147.9, 128.8, 124.3, 123.9,
A mixture of unsymmetrical NCN pincer ligand precursors 3a–
3e (0.2 mmol) and Pd(OAc)2 (54.0 mg, 0.24 mmol) in dry HOAc
(60 mL) were refluxed for 48 h under nitrogen atmosphere. The
solvent was removed under reduced pressure and a solution of
lithium chloride (102.0 mg, 2.4 mmol) in acetone/water (3/2,
35 mL) was added. The resulting solution was stirred at room tem-
perature for 48 h. The solution was extracted with dichlorometh-
ane and the organic layer was washed with brine, dried over
MgSO4 and evaporated. The crude was purified by preparative
TLC on silica gel plates eluting with AcOEt/CH2Cl2 (1:10–1:20) to
afford the corresponding pincer palladium complexes 4a–4e.
4a: Yield: 20.3%. Yellow solid. M.p. >250 °C. 1H NMR (400 MHz,
CDCl3): d 7.21 (dd, J = 1.7, 7.0 Hz, 1H, ArH), 7.10–7.05 (m, 2H, ArH),
5.92 (s, 1H, PzH), 5.03 (s, 2H, CH2), 4.76 (t, J = 9.5 Hz, 2H, OxH), 4.18
(t, J = 9.5 Hz, 2H, OxH), 2.72 (s, 3H, CH3), 2.37 (s, 3H, CH3). 13C NMR
(100 MHz, CDCl3): d 175.0, 153.4, 151.6, 140.7, 134.4, 131.7, 128.3,
123.3, 70.7, 66.7, 58.0, 50.5, 14.1. IR (KBr, cmꢁ1):
m 3432, 3131,
2968, 2925, 2868, 1609, 1565, 1473, 1433, 1396, 1269, 1183,
1143, 1091, 1045, 976, 923, 802, 729. MS (m/z, ESI+): 337 (MꢁCl).
HRMS (m/z, ESI+): found for MꢁCl = 337.0529, C14H19N2OPd re-
quires 337.0532. Anal. Calc. for C14H19ClN2OPd: C, 45.06; H, 5.13;
N, 7.51. Found: C, 45.37; H, 5.24; N 7.48%.
3.6. X-ray crystallography
The crystals of 4e were obtained by recrystallization from
CH2Cl2/petroleum ether at room temperature. Crystallographic data
for 4e were measured on a Rigaku-Raxis-IV X-ray diffractometer
124.9, 124.5, 107.8, 70.9, 52.0, 50.5, 16.2, 12.1. IR (KBr, cmꢁ1):
m
3134, 2921, 2885, 2853, 1729, 1633, 1575, 1554, 1468, 1440,
using graphite-monochromated Mo Ka radiation (k = 0.71073 Å)
1398, 1358, 1277, 1179, 1152, 1123, 1049, 989, 945, 913, 861,
at 291(2) K. The hydrogen atoms were included but not refined.

X.-Q. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 82–89
89
The full-matrix least-squares calculations on F2 were applied on the
final refinement. The structure was solved by direct methods. All
non-hydrogen atoms were described anisotropically. Its raw data
were corrected and the structure was solved using the SHELXL-97
program.
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3.7. General procedure for the Suzuki reaction
The palladium complex 4 was dissolved in dioxane or DMF
(3 mL). To this solution was added aryl halide (0.5 mmol), phenyl-
boronic acid (0.75 mmol) and base (1.0 mmol). The reaction mix-
ture was then placed in an oil bath with stirring and heated for a
certain time. After the reaction mixture was cooled to ambient
temperature, 15 mL of diethyl ether and 15 mL of water were
added. The organic layer was separated, and the aqueous phase
was extracted with 10 mL of diethyl ether. The combined organic
phase was dried over MgSO4, filtered and evaporated. The products
were isolated by flash chromatography on silica gel (the purified
products were identified by comparison of melting points with
the literature values or by 1H NMR spectra).
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4. Supplementary material
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Moreno, R. SanMartin, B. Inés, M.T. Herrero, E. Domínguez, Curr. Org. Chem. 13
(2009) 878.
CCDC 748266 contains the supplementary crystallographic data
for 4e. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
ta_request/cif.
Acknowledgements
We are grateful to the National Science Foundation of China
(Nos. 20872133, 20572102) and the Innovation Fund for Outstand-
ing Scholar of Henan Province (074200510005) for financial sup-
port of this work.
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