(N-heterocyclic carbene)PdCl(N-heterocyclic carboxylate) complexes: Synthesis and catalytic activities towards arylation of benzoxazoles with aryl halides

Article abstract of DOI:10.1002/aoc.4394

A series of N-heterocyclic carboxylate-stabilized N-heterocyclic carbene palladium complexes have been synthesized and fully characterized. The solid-state structures indicate that each of the palladium centers is coordinated by an N-heterocyclic carbene, a chloride and a bidentate N,O-donor N-heterocyclic carboxylate ligand. The catalytic performance of the complexes was screened and the results revealed that the complexes exhibit moderate to high catalytic activities for the direct C─H bond arylation of benzoxazoles with aryl bromides.

Full text of DOI:10.1002/aoc.4394

Received: 12 February 2018
DOI: 10.1002/aoc.4394
Revised: 13 March 2018
Accepted: 23 March 2018
F U L L P A P E R
(N‐heterocyclic carbene)PdCl(N‐heterocyclic carboxylate)
complexes: Synthesis and catalytic activities towards
arylation of benzoxazoles with aryl halides
Jin Yang
Department of Chemistry, Huaibei
A series of N‐heterocyclic carboxylate‐stabilized N‐heterocyclic carbene
Normal University, Huaibei, Anhui
235000, People's Republic of China
palladium complexes have been synthesized and fully characterized. The solid‐
state structures indicate that each of the palladium centers is coordinated by an
Correspondence
N‐heterocyclic carbene, a chloride and a bidentate N,O‐donor N‐heterocyclic
Jin Yang, Department of Chemistry,
Huaibei Normal University, Huaibei,
carboxylate ligand. The catalytic performance of the complexes was screened
Anhui 235000, People's Republic of China.
and the results revealed that the complexes exhibit moderate to high catalytic
Email: yangjinlz@163.com
activities for the direct C─H bond arylation of benzoxazoles with aryl bromides.
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 21301061
KEYWORDS
benzoxazoles, direct arylation, N‐heterocyclic carbene palladium, N‐heterocyclic carboxylic acid
1 | INTRODUCTION
reported arylation of benzoxazoles catalyzed by a well‐
defined (IPr)PdCl2(1‐methylimidazole) complex.[6b] In
Over the past few decades, direct arylation of benzoxazoles
has received considerable attention because 2‐aryl‐
substituted benzoxazoles are important building blocks
for biologically active compounds and functional
materials.[1] Remarkable progress has been made in the
synthesis of these compounds and their further
functionalization.[2] Since Ohta et al. first reported the
arylation of heteroarenes with aryl halides using
[Pd(PPh3)4] catalyst,[3] the Pd‐catalyzed arylation of
benzoxazoles has been developed as a convenient alterna-
tive for the straightforward synthesis of such compounds.
However, in most cases, excessive phosphine ligands with
Pd salts were used as efficient catalysts for the reaction.[4]
It is important to note that phosphine‐free ligands, espe-
cially N‐heterocyclic carbenes (NHCs), have generated
interest due to their favorable characteristics, such as
being cost‐effective and easily available as well as
exhibiting relatively high stability.[5] However, so far, only
a few examples of NHC–Pd complex‐catalyzed arylation of
benzoxazoles using aryl halides have been described.[6]
Arslan et al. first reported a mixed‐halide NHC–Pd
complex and established its efficient catalytic activity for
arylation of benzoxazoles.[6a] Shao and co‐workers
addition, Yang et al. reported chelating NHC–Pd com-
plexes as active catalysts for arylation of benzoxazoles.[6c]
In these well‐defined NHC–Pd complexes, not only the
NHC moieties, but also the ancillary ligands around the
Pd center affect the reactivity profile. Our group has
focused on the synthesis of well‐defined NHC–Pd com-
plexes and explored their catalytic activities.[7] By simply
changing the ancillary ligands, easily available and diverse
heteroleptic NHC–Pd complexes were developed.
Herein, we report a series of heteroleptic NHC–Pd com-
plexes using N‐heterocyclic carboxylic acids as the N,O‐
chelate ancillary ligands (Scheme 1). Furthermore, as a
preliminary application, the obtained complexes were used
in the direct arylation of benzoxazoles with aryl halides.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis and Characterization of
Complexes
The reactions of commercially available imidazolium
salts, PdCl2 and N‐heterocyclic carboxylic acids in the
Appl Organometal Chem. 2018;e4394.
wileyonlinelibrary.com/journal/aoc
Copyright © 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4394

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SCHEME 1 Synthetic route of [(NHC)PdCl(N‐heterocyclic carboxylate)] complexes
presence of K2CO3 in tetrahydrofuran (THF) afforded the
corresponding (NHC)PdCl(N‐heterocyclic carboxylate)
complexes in moderate yields. The compounds were fully
characterized using NMR and Fourier transform infrared
(FT‐IR) spectroscopies, high‐resolution mass spectrome-
try (HR‐MS) and elemental analysis. The 1H NMR spectra
of complexes 1a, 1b, 2a, 2b, 3a and 3b in CDCl3 show the
complete disappearance of the proton signals of carbox-
ylic acids and the distinctive stoichiometric proton signals
of NHCs and N‐heterocyclic carboxylates. In addition, the
13C NMR spectra exhibited singlet resonances at 153.1,
185.7, 158.1, 188.3, 155.5 and 182.5 ppm, ascribed to the
characteristic peaks of the carbene carbons. All of the
complexes were further characterized using HR‐MS.
Moreover, crystals of 1a, 1b and 3a were grown in a
mixture of dichloromethane and n‐hexane, and their
structures were determined using single‐crystal X‐ray
diffraction. The solid‐state molecular structures of the
complexes are depicted in Figures 1–3 and selected bond
lengths and angles are listed in the captions. As
commonly observed for square planar Pd complexes, the
obtained heteroleptic NHC–Pd complexes contain an
essentially square planar Pd center, which is coordinated
by an NHC, a chloride and a bidentate N,O‐donor N‐het-
erocyclic carboxylate. The neutral σ‐donating quinoline
nitrogen atom is located trans to NHC, while the carbox-
ylate oxygen is situated in cis position and in a direction
opposite to the chloride. The PdCNOCl coordination
planes are oriented approximately perpendicular to the
FIGURE
1
Crystal structure of 1a⋅0.5(C6H14)⋅CH2Cl2 with
thermal ellipsoids drawn at the 30% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles
(°): Pd(1)–C(1) 1.9590(14), Pd(1)–N(3) 2.1473(12), Pd(1)–O(1)
1.9959(13), Pd(1)–Cl(1) 2.2632(6); C(1)–Pd(1)–O(1) 89.96(6), O(1)–
Pd(1)–N(3) 81.04(5), C(1)–Pd(1)–Cl(1) 87.12(5), N(3)–Pd(1)–Cl(1)
101.91(4)
carbene rings with dihedral angles ranging from 70.25°
to 81.34° and are twisted from the quinoline ring by
11.11° to 33.90°. Due to the torsion tension of chelating

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NHC‐bearing complexes, the Pd─C bond lengths
(1.9590(14)–1.973(3) Å), Pd─N bond lengths (2.090(3)–
2.165(2) Å) and Pd─O bond lengths (1.9959(13)–2.011(2)
Å) are similar to those found in (IPr)PdCl(pyridine‐2‐
carboxylate).[8]
2.2 | Direct Arylation of Benzoxazoles
with Aryl Halides
With the isolation and characterization of this series of
(NHC)PdCl(N‐heterocyclic carboxylate) complexes, we
were able to determine and compare their catalytic activ-
ities in the arylation of benzoxazoles with aryl halides. In
order to screen the experimental conditions, complex 1a
was selected as the catalyst in the reaction of 4‐
bromoanisole with benzoxazole as the model partners at
130 °C for 12 h. The catalyst was associated with various
bases, solvents and reaction temperatures in order to
determine the most efficient catalytic systems (Table 1).
Initially, the influence of the solvent was explored and a
significant solvent effect was observed. When toluene
was used as solvent, the product was obtained in a yield
of only 45%. Further investigation indicated that polar
solvents, such as dimethylformamide (DMF) and
dimethylacetamide (DMAc), could efficiently promote
the reaction, and the desired product was generated in
moderate yields. However, inferior results were obtained
when 1,4‐dioxide or CH3CN was employed as reaction
medium at 100 °C. Among the tested bases, NaOtBu gave
better conversions. Two alternative bases, LiOtBu and
KOtBu, were demonstrated to be less effective than
NaOtBu. In the presence of other bases such as K3PO4,
Cs2CO3, NaOH and KOH, low yields of the product were
obtained. The loading of the catalyst was also examined.
The use of 0.5 mol% Pd catalyst gave the product in a
yield of only 65%. On increasing the amount of catalyst
loading up to 1.0 mol%, a high yield was obtained. How-
ever, the yields were not improved significantly upon
increasing the amount of catalyst loading up to
2.0 mol%. Furthermore, the reaction temperature was
also examined; decreasing the reaction temperature from
130 to 100 °C had a detrimental effect on the yield.
Finally, in view of the above analysis, we obtained the
optimized catalytic conditions for the reaction: DMF as
solvent, NaOtBu as base, 1.0 mol% of complex 1a, at
130 °C for 12 h.
FIGURE
2
Crystal structure of 1b⋅2CH2Cl2 with thermal
ellipsoids drawn at the 30% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°): Pd(1)–
C(1) 1.965(3), Pd(1)–N(3) 2.165(2), Pd(1)–O(1) 2.011(2), Pd(1)–Cl(1)
22.2923(8); C(1)–Pd(1)–O(1) 91.82(10), O(1)–Pd(1)–N(3) 80.16(9),
C(1)–Pd(1)–Cl(1) 83.88(8), N(3)–Pd(1)–Cl(1) 104.15(7)
FIGURE
3
Crystal structure of 3a⋅2CH2Cl2 with thermal
ellipsoids drawn at the 30% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°): Pd(1)–
C(1) 1.973(3), Pd(1)–N(3) 2.090(3), Pd(1)–O(1) 1.999(3), Pd(1)–Cl(1)
2.2769(10); C(1)–Pd(1)–O(1) 91.23(13), O(1)–Pd(1)–N(3) 89.55(12),
C(1)–Pd(1)–Cl(1) 86.01(10), N(3)–Pd(1)–Cl(1) 93.22(9)
Under the optimum reaction conditions, arylation of
benzoxazoles with a variety of aryl bromides catalyzed
by complexes 1a, 1b, 2a, 2b, 3a and 3b was carried out
and the results are presented in Table 2. Initially, we
investigated the reactions of benzoxazole with 4‐
bromoanisole, bromobenzene and 4‐bromotoluene. The
arylation products were obtained in good yields for all
ring, the dihedral angle between the PdCNOCl coordina-
tion plane and the quinoline ring in the six‐membered
compound 3a is significantly larger than observed in
five‐membered compounds 1a and 1b. As usual in

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TABLE 1 Influence of reaction conditions for complex 1a‐catalyzed arylation of benzoxazole with 4‐bromoanisolea
Entry
[Pd] (%)
Solvent
Base
Yield (%)b
1
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
2.0
1.0
Toluene
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
LiOtBu
NaOtBu
KOtBu
K3PO4
Cs2CO3
NaOH
KOH
45
65
60
35
20
88
94
90
74
70
78
80
65
95
45
2
3
DMAc
1,4‐Dioxane
CH3CN
DMF
4
5
6
7
DMF
8
DMF
9
DMF
10
11
12
13
14
15c
DMF
DMF
DMF
DMF
NaOtBu
NaOtBu
NaOtBu
DMF
DMF
aReaction conditions: 4‐bromoanisole (0.50 mmol), benzoxazole (0.75 mmol), NHC–Pd 1a (1.0 mol%), base (1.0 mmol), solvent (2.0 ml) at 130 °C for 12 h.
bIsolated yield.
cReaction performed at 100 °C.
TABLE 2 Arylation of benzoxazole with aryl bromides catalyzed by (NHC)PdCl(N‐heterocyclic carboxylate) complexesa
[NHC–Pd catalyst]/yield (%)b
Entry
Benzoxazole
Aryl bromide
1a
1b
2a
2b
3a
3b
1
R1 = H
R2 = 4‐OMe
R2 = H
87
72
82
84
82
80
85
55
50
85
78
84
86
86
82
88
65
56
83
75
80
82
83
80
84
58
46
86
80
85
87
85
84
88
60
52
85
80
80
82
83
82
86
60
53
88
82
83
88
87
86
88
65
55
2
R1 = H
3
R1 = H
R2 = 4‐Me
4
R1 = H
R1 = H
R2 = 3,5‐Me2
R2 = 3,4‐Me2
R2 = 4‐Me
5
6
R1 = 5‐Me
R1 = 5‐OMe
R1 = H
7
R2 = 4‐Me
8c
9c
4‐Chloroanisole
4‐Chlorotoluene
R1 = H
aReaction conditions: aryl bromide (0.50 mmol), benzoxazole (0.75 mmol), NaOtBu (1.0 mmol), NHC–Pd complex (1 mol%) in DMF (2.0 ml) at 130 °C for 12 h.
bIsolated yield.
cReaction performed at 150 °C for 24 h.
six catalysts. Aryl bromides with an electron‐donating
group such as methoxy or methyl on the aromatic ring
gave the products in good yields. Moreover, aryl bromides
without an electron‐donating group were also arylated in
moderate yields, as demonstrated in the arylation
reactions using bromobenzene. We also found that
increasing the steric hindrance of the aryl bromide did
not reduce the yields of the products. 1‐Bromo‐3,5‐
dimethylbenzene and 1‐bromo‐3,4‐dimethylbenzene
could also react with benzoxazole and gave products in

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TABLE 3 Comparison of activity of well‐defined NHC–Pd complexes in arylation of benzoxazole with 4‐bromoanisolea
Entry
IPr–Pd complex
Yield (%)b
1
2
3
4
(IPr)PdCl(quinoline‐2‐carboxylate) (1a)
(IPr)PdCl(isoquinoline‐1‐carboxylate) (2a)
(IPr)PdCl2(quinoline)
87
83
92
86
(IPr)PdCl2(isoquinoline)
aReaction conditions: 4‐bromoanisole (0.50 mmol), benzoxazole (0.75 mmol), NaOtBu (1.0 mmol), NHC–Pd complex (1 mol%) in DMF (2.0 ml) at 130 °C for
12 h.
bIsolated yield.
good yields. Since the complexes were effective catalyst
precursors for the reactions of benzoxazole with aryl
bromides, they were then subjected to substituted
benzoxazoles, in order to examine their versatility. 5‐
Methylbenzoxazole and 5‐methoxybenzoxazole were then
used as substrates to react with 4‐bromotoluene and all the
catalysts displayed good outcomes in coupling reaction.
The reaction showed a good tolerance of different groups
on the aromatic ring. Nevertheless, when aryl chlorides,
such as 4‐chloroanisole and 4‐chlorotoluene, were selected
as substrates, the desired products were afforded in low
yields under the optimum reaction conditions, which indi-
cated the higher bond energies of the C─Cl bonds. How-
ever, when the reaction time was increased to 24 h and
the temperature was increased to 150 °C, moderate yields
were achieved which indicated that the catalysts were still
active for C─Cl bond atn high temperature.
carboxylate group. The well‐defined (IPr)PdCl2(iso)quin-
oline complexes have been synthesized and showed high
catalytic activity for Buchwald–Hartwig arylamination of
aryl chlorides.[10] As evident from Table 3, (IPr)PdCl2(iso)
quinoline complexes show slightly higher activities in
comparison with complexes 1a and 2a. As ancillary
ligands, the (iso)quinoline ligands could alter the σ‐dona-
tion property of the NHC and easily dissociate from the
Pd center to form the [(NHC)Pd(0)] active species, while
the N‐heterocyclic carboxylates, as stronger bidentate
chelate ligands, might with difficultly dissociate from
the Pd center or remain coordinated to Pd during the
catalytic cycle. The relatively good stability of the
obtained complexes may be disadvantageous to the for-
mation of active species. The real role of the N,O‐donors
in the catalytic process needs to be further investigated.
Although the obtained complexes exhibited good
catalytic activities towards arylation of benzoxazoles with
aryl bromides, there is no advantage in reaction activity
3 | CONCLUSIONS
A series of heteroleptic palladium complexes containing
both NHCs and N‐heterocyclic carboxylates were
synthesized and fully characterized. The catalytic
behavior of the complexes was investigated in direct
arylation of benzoxazoles with aryl halides. The (NHC)
PdCl(N‐heterocyclic carboxylate) complexes were found
to be suitable catalysts for arylation of benzoxazoles with
aryl bromides. With the complexes, even non‐activated
aryl chlorides could be used in arylation of benzoxazoles
with moderate yields. Further modification of the NHC–
Pd complexes and their application are in progress in
our laboratory.
compared
to
the
well‐defined
(IPr)PdCl2(1‐
methylimidazole) reported by Shao and co‐workers.[6b]
A key factor for the effective catalysis of well‐defined
NHC–Pd complexes is to transform the stable Pd(II) com-
plex into the active Pd(0) species in the transition state of
the catalytic cycle. In this process, the Pd(II) complex
must lose some of the ligands attached to the metal
center. The ‘throw‐away’ ligand(s) should incorporate
the active metal center to generate an active Pd(0) species
during the catalytic process.[9] Actually, the well‐defined
(IPr)PdCl2(1‐methylimidazole) could easily lose the 1‐
methylimidazole during the activation step. However, in
our systems, the removal of the bidentate chelate ligand,
which is a sluggish activation step, restricted the rapid
formation of the active intermediate. In order to gain
more information about the influence of the ancillary
ligand on the catalytic activity, a comparison of the cata-
lytic activities between the obtained complexes and (IPr)
PdCl2(iso)quinoline complexes for the same reactions
was conducted to explain the functionalization of the
4 | EXPERIMENTAL
4.1 | General Considerations
All reactions and manipulations were carried out under
air atmosphere. The chemicals were purchased from
commercial suppliers and were used without purification.

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NMR spectra were performed in CDCl3 with
tetramethylsilane as an internal standard and recorded
ppm): 9.43 (d, J = 9.0 Hz, 1H, CH‐quinoline), 8.18 (d,
J = 8.5 Hz, 1H, CH‐quinoline), 7.97 (d, J = 8.5 Hz, 1H,
CH‐quinoline), 7.65 (d, J = 8.0 Hz, 1H, CH‐quinoline),
7.59 (t, J = 8.0 Hz, 1H, CH‐quinoline), 7.45 (t,
J = 7.5 Hz, 1H, CH‐quinoline), 7.37 (t, J = 7.5 Hz, 2H,
p‐CH‐aniline), 7.28 (d, J = 8.0 Hz, 4H, m‐CH‐aniline),
4.16 (s, 2H, NCH2CH2N), 4.14 (s, 2H, NCH2CH2N), 3.58
(sept, J = 6.5 Hz, 2H, CH(CH3)2), 3.52 (sept, J = 6.5 Hz,
2H, CH(CH3)2), 1.56 (d, J = 6.5 Hz, 6H, CH(CH3)2), 1.53
(d, J = 6.5 Hz, 6H, CH(CH3)2), 1.32–1.28 (m, 12H,
CH(CH3)2). 13C NMR (CDCl3, 125 MHz, δ, ppm): 182.5
(Ccarbene),173.0 (Ccarboxylate), 153.0, 147.8, 147.6, 146.8,
139.8, 134.5, 130.7, 130.3, 129.5, 128.8, 128.3, 127.3,
124.5, 122.3, 53.6 (NCH2CH2N), 28.7 (CH(CH3)2), 26.7
(CH(CH3)2), 26.6 (CH(CH3)2), 24.2 (CH(CH3)2), 23.7
(CH(CH3)2). FT‐IR (KBr, cm−1): 3072, 2962, 1659, 1618,
1597, 1562, 1489, 1460, 1425, 1329, 1303, 1272, 1174,
1151, 1107, 1049, 898, 803. HR‐MS (ESI): calcd for
C37H45ClN3O2Pd [M + H+]+ 704.2235; found 704.2241.
Anal. Calcd for [(SIPr)PdCl(quinoline‐2‐carboxylate)]
(C37H44ClN3O2Pd) (%): C, 63.07; H, 6.29; N, 5.96. Found
(%): C, 63.35; H, 6.47; N, 6.22.
1
at 400 MHz (for H NMR) and 100 MHz (for 13C NMR)
with a Bruker Avance 400 NMR spectrometer. High‐reso-
lution mass spectra were recorded with an Agilent 6550
iFunnel Q‐TOF MS system. FT‐IR spectra were recorded
with a Bruker IFS 120HR spectrometer using KBr discs.
The C, H and N analyses were performed with a Vario
El III Elementar. Column chromatography was carried
out using 230–400 mesh silica gel.
4.2 | General Procedure for Preparation of
(NHC)PdCl(N‐heterocyclic carboxylate)
Complexes
A sealable reaction tube equipped with a magnetic stir
bar was charged with imidazolium salt (0.6 mmol), PdCl2
(0.5 mmol), K2CO3 (1.5 mmol), (iso)quinoline carboxylic
acid (0.6 mmol) and anhydrous THF (5.0 ml). The mix-
ture was stirred under reflux for 12 h and then cooled
to room temperature and condensed under vacuum. The
residue was purified by flash chromatography on short
silica gel and recrystallized from n‐hexane–CH2Cl2 to
afford the desired products.
4.2.3 | [(IPr)PdCl(isoquinoline‐1‐carboxyl-
ate)] (2a)
4.2.1 | [(IPr)PdCl(quinoline‐2‐carboxyl-
ate)] (1a)
The procedure yielded 240 mg (68%) of the pure product
2a as a yellow powder. H NMR (CDCl3, 500 MHz, δ,
1
The procedure yielded 260 mg (74%) of the pure product
1a as a yellow powder. H NMR (CDCl3, 500 MHz, δ,
ppm): 9.83 (d, J = 8.5 Hz, 1H, CH‐isoquinoline), 8.59 (d,
J = 6.5 Hz, 1H, CH‐isoquinoline), 7.72–7.66 (m, 2H),
7.63–7.60 (m, 2H), 7.47 (t, J = 8.0 Hz, 2H, p‐CH‐aniline),
7.34 (d, J = 8.0 Hz, 4H, m‐CH‐aniline), 7.20 (s, 2H,
NCH═CHN), 3.01 (sept, J = 6.5 Hz, 4H, CH(CH3)2),
1.47 (d, J = 6.5 Hz, 12H, CH(CH3)2), 1.16 (d, J = 6.5 Hz,
12H, CH(CH3)2). 13C NMR (CDCl3, 125 MHz, δ, ppm):
173.3 (Ccarboxylate), 158.1 (Ccarbene), 148.9, 146.7, 138.3,
137.7, 134.5, 131.8, 130.3, 129.1, 127.9, 127.4, 126.5,
125.0, 124.7, 124.0, 28.6 (CH(CH3)2), 26.2 (CH(CH3)2),
22.9 (CH(CH3)2). FT‐IR (KBr, cm−1): 2963, 2927, 2868,
1657, 1621, 1591, 1501, 1467, 1414, 1383, 1353, 1332,
1291, 1210, 1153, 802. HR‐MS (ESI): calcd for
C37H43ClN3O2Pd [M + H+]+ 702.2079; found 702.2087.
Anal. Calcd for [(IPr)PdCl(isoquinoline‐1‐carboxylate)]
(C37H42ClN3O2Pd) (%): C, 63.25; H, 6.03; N, 5.98. Found
(%): C, 63.55; H, 6.31; N, 6.25.
1
ppm): 9.43 (d, J = 9.0 Hz, 1H, CH‐quinoline), 8.21 (d,
J = 8.0 Hz, 1H, CH‐quinoline), 7.99 (d, J = 8.0 Hz, 1H,
CH‐quinoline), 7.67 (d, J = 8.0 Hz, 1H, CH‐quinoline),
7.60 (t, J = 8.0 Hz, 1H, CH‐quinoline), 7.49–7.45 (m,
3H), 7.34 (d, J = 8.0 Hz, 4H, m‐CH‐aniline), 7.19 (s, 2H,
NCH═CHN), 3.07 (sept, J = 6.5 Hz, 4H, CH(CH3)2),
1.47 (d, J = 6.5 Hz, 12H, CH(CH3)2), 1.16 (d, J = 5.0 Hz,
12H, CH(CH3)2). 13C NMR (CDCl3, 125 MHz, δ, ppm):
173.1 (Ccarboxylate), 153.1 (Ccarbene), 151.4, 146.8, 139.8,
134.4, 130.7, 130.4, 128.9, 128.3, 127.4, 125.1, 124.1,
122.4, 28.7 (CH(CH3)2), 26.3 (CH(CH3)2). FT‐IR (KBr,
cm−1): 2965, 1659, 1618, 1562, 1514, 1462, 1413, 1383,
1365, 1349, 1269, 1209, 1174, 1152, 1107, 802. HR‐MS
(ESI): calcd for C37H43ClN3O2Pd [M + H+]+ 702.2079;
found 702.2083. Anal. Calcd for [(IPr)PdCl(quinoline‐2‐
carboxylate)] (C37H42ClN3O2Pd) (%): C, 63.25; H, 6.03;
N, 5.98. Found (%): C, 63.54; H, 6.15; N, 6.27.
4.2.4 | [(SIPr)PdCl(isoquinoline‐1‐carbox-
ylate)] (2b)
4.2.2 | [(SIPr)PdCl(quinoline‐2‐carboxyl-
ate)] (1b)
The procedure yielded 255 mg (72%) of the pure product
2b as a yellow powder. H NMR (CDCl3, 500 MHz, δ,
1
The procedure yielded 270 mg (77%) of the pure product
1b as a yellow powder. H NMR (CDCl3, 500 MHz, δ,
ppm): 9.81 (d, J = 8.5 Hz, 1H, CH‐isoquinoline), 8.56 (d,
J = 6.5 Hz, 1H, CH‐isoquinoline), 7.70–7.64 (m, 2H,
1

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CH‐isoquinoline), 7.60 (t, J = 7.5 Hz, 2H, p‐CH‐aniline),
7.38 (t, J = 7.5 Hz, 1H, CH‐isoquinoline), 7.29 (d,
J = 7.5 Hz, 4H, m‐CH‐aniline), 4.13 (s, 4H, NCH2CH2N),
3.49 (sept, J = 6.5 Hz, 4H, CH(CH3)2), 1.54 (d, J = 6.5 Hz,
12H, CH(CH3)2), 1.30 (d, J = 6.5 Hz, CH(CH3)2). 13C
NMR (CDCl3, 125 MHz, δ, ppm): 188.3 (Ccarbene), 173.1
(Ccarboxylate), 148.8, 147.6, 138.2, 137.6, 134.7, 131.8,
129.5, 129.0, 127.8, 127.4, 126.5, 124.7, 124.5, 53.8
(NCH2CH2N), 28.7 (CH(CH3)2), 26.6 (CH(CH3)2). FT‐IR
(KBr, cm−1): 3072, 2962, 2868, 1660, 1622, 1590, 1483,
1455, 1426, 1383, 1368, 1353, 1327, 1293, 1180, 1152,
1102, 1051, 1018, 880, 840, 803. HR‐MS (ESI): calcd for
C37H45ClN3O2Pd [M + H+]+ 704.2235; found 704.2240.
Anal. Calcd for [(SIPr)PdCl(isoquinoline‐1‐carboxylate)]
(C37H44ClN3O2Pd) (%): C, 63.07; H, 6.29; N, 5.96. Found
(%): C, 63.45; H, 6.58; N, 6.25.
4.13 (s, 2H, NCH2CH2N), 4.11 (s, 2H, NCH2CH2N), 3.68
(sept, J = 6.5 Hz, 2H, CH(CH3)2), 3.50 (sept, J = 6.5 Hz,
4H, CH(CH3)2), 1.51 (d, J = 6.5 Hz, 12H, CH(CH3)2),
1.30–1.28 (m, 12H, CH(CH3)2). 13C NMR (CDCl3,
125 MHz, δ, ppm): 185.7 (Ccarbene), 166.8 (Ccarboxylate),
153.9, 148.1, 147.4, 143.4, 139.7, 137.6, 135.0, 131.4,
130.3, 129.4, 128.5, 126.7, 124.5, 124.3, 120.4, 107.9, 53.6
(NCH2CH2N), 26.7 (CH(CH3)2), 26.6 (CH(CH3)2), 24.0
(CH(CH3)2), 23.9 (CH(CH3)2). FT‐IR (KBr, cm−1): 3069,
2964, 2926, 2867, 1659, 1638, 1620, 1591, 1465, 1410,
1382, 1352, 1330, 1291, 1253, 1152, 1065, 801. HR‐MS
(ESI): calcd for C37H45ClN3O2Pd [M + H+]+ 704.2235;
found 704.2242. Anal. Calcd for [(SIPr)PdCl(quinoline‐2‐
carboxylate)] (C37H44ClN3O2Pd) (%): C, 63.07; H, 6.29;
N, 5.96. Found (%): C, 62.88; H, 6.56; N, 6.24.
4.2.5 | [(IPr)PdCl(quinoline‐8‐carboxyl-
ate)] (3a)
4.3 | General Procedure for Arylation of
Benzoxazoles with Aryl Bromides
The procedure yielded 225 mg (64%) of the pure product
3a as a yellow powder. H NMR (CDCl3, 500 MHz, δ,
Direct arylation reactions were carried out under aerobic
conditions. In the parallel reaction, aryl bromide
(0.5 mmol), benzoxazole (0.75 mmol), NaOtBu
(1.0 mmol), the (NHC)PdCl(N‐heterocyclic carboxylate)
complex (1 mol%) and DMF (2.0 ml) were introduced in
a reaction tube, equipped with a magnetic stirring bar.
The reaction mixture was stirred for 12 h at 130 °C. After
completion of the reaction, the reaction mixture was
cooled to room temperature and then evaporated under
vacuum. The residue was subjected to purification via
column chromatography with petroleum ether–EtOAc
(20:1) as eluent to afford the products.
1
ppm): 9.15 (d, J = 5.0 Hz, 1H, CH‐quinoline), 8.75 (d,
J = 7.5 Hz, 1H, CH‐quinoline), 7.96 (d, J = 8.0 Hz, 1H,
CH‐quinoline), 7.63 (d, J = 8.0 Hz, 1H, CH‐quinoline),
7.48–7.44 (m, 3H), 7.33 (t, J = 7.5 Hz, 4H, m‐CH‐aniline),
7.16 (s, 2H, NCH═CHN), 7.06 (t, J = 6.0 Hz, 1H, CH‐
quinoline), 3.32 (br, 2H, CH(CH3)2), 2.97 (br, 2H,
CH(CH3)2), 1.46 (d, J = 6.5 Hz, 12H, CH(CH3)2), 1.15
(d, J = 7.0 Hz, 12H, CH(CH3)2). 13C NMR (CDCl3,
125 MHz, δ, ppm): 166.3 (Ccarboxylate), 155.5 (Ccarbene),
154.0, 147.0, 146.9, 146.9, 146.8, 146.8, 143.5, 139.7,
137.6, 134.8, 131.5, 130.2, 128.5, 126.7, 124.8, 124.0,
120.4, 28.6 (CH(CH3)2), 26.3 (CH(CH3)2), 23.1
(CH(CH3)2). FT‐IR (KBr, cm−1): 2965, 2928, 2870, 1659,
1629, 1508, 1466, 1414, 1342, 1307, 1208, 1084, 1059,
892, 802. HR‐MS (ESI): calcd for C37H43ClN3O2Pd
[M + H+]+ 702.2079; found 702.2085. Anal. Calcd for
[(IPr)PdCl(quinoline‐8‐carboxylate)] (C37H42ClN3O2Pd)
(%): C, 63.25; H, 6.03; N, 5.98. Found (%): C, 63.01; H,
6.29; N, 6.27.
4.4 | X‐ray Crystallography
Single crystals for X‐ray diffraction analysis were
obtained by slow diffusion of n‐hexane into their CH2Cl2
solutions. Data collection was performed with a Bruker‐
AXS SMART CMOS area detector diffractometer at
296 K using ω rotation scans with a scan width of 0.5°
and Mo Kα radiation (λ = 0.71073 Å). Multi‐scan correc-
tions were applied using SADABS.[11] Structure solutions
and refinements were performed with the SHELX‐97
package.[12] All non‐hydrogen atoms were refined aniso-
tropically by full‐matrix least‐squares on F2. The hydro-
gen atoms bound to carbon were included in idealized
geometric positions with thermal parameters equivalent
to 1.2 times those of carbon atoms. CCDC 1819838,
1819839 and 1819841 contain the supplementary crystal-
lographic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre (https://www.ccdc.cam.ac.uk/structures/).
4.2.6 | [(SIPr)PdCl(quinoline‐8‐carboxyl-
ate)] (3b)
The procedure yielded 220 mg (62%) of the pure product
1
3b as a yellow powder. H NMR (CDCl3, 500 MHz, δ,
ppm): 9.16 (d, J = 5.0 Hz, 1H, CH‐quinoline), 8.74 (d,
J = 8.0 Hz, 1H, CH‐quinoline), 7.99 (d, J = 8.0 Hz, 1H,
CH‐quinoline), 7.65 (d, J = 8.0 Hz, 1H, CH‐quinoline),
7.47 (t, J = 7.5 Hz, 1H, CH‐quinoline), 7.37 (J = 8.0 Hz,
2H, p‐CH‐aniline), 7.29 (d, J = 10.0 Hz, 4H, m‐CH‐ani-
line), 7.08 (dd, J = 7.5 and 2.0 Hz, 1H, CH‐quinoline),

8 of 8
YANG
2017, 31, e3734; d) H. Wang, J. Yang, Appl. Organometal. Chem.
2017, 31, e3543; e) J. Yang, J. Coord. Chem. 2017, 70, 441; f) J.
Yang, New J. Chem. 2016, 40, 9739; g) J. Yang, P. Li, Y. Zhang,
L. Wang, J. Organometal. Chem. 2014, 766, 73; h) J. Yang, P.
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P. Li, Y. Zhang, L. Wang, Dalton. Trans. 2014, 43, 7166; j) J.
Yang, L. Wang, Dalton. Trans. 2012, 41, 12031.
ACKNOWLEDGMENT
Financial support from the National Natural Science
Foundation of China (no. 21301061) is gratefully
acknowledged.
ORCID
[8] a) Y.‐J. Li, J.‐L. Zhang, X.‐J. Li, Y. Geng, X.‐H. Xu, Z. Jin,
J. Organometal. Chem. 2013, 737, 12; b) Z.‐M. Zhang, Y.‐J.
Gao, J.‐M. Lu, Tetrahedron 2017, 73, 7308.
Jin Yang http://orcid.org/0000-0003-0757-7155
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