N-heterocyclic carbene-palladium(II) complexes with benzoxazole or benzothiazole ligands: Synthesis, characterization, and application to Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1016/j.jorganchem.2015.12.039

A series of novel N-heterocyclic carbene-palladium(II) complexes 3 were conveniently prepared from commercially available imidazolium salts, palladium chloride, and benzoxazole or benzothiazole in one step. All of the new complexes have been fully characterized by elemental analysis, ¹H, ¹³C NMR, and IR spectra. In addition, the molecular structures of complexes 3a-d have also been determined by X-ray single-crystal diffraction. The obtained palladium(II) catalysts showed efficient catalytic activity toward the Suzuki-Miyaura coupling of aryl as well as benzyl chlorides with arylboronic acids. Under the optimal reaction conditions, the expected biaryl products were obtained in moderate to high yields.

Full text of DOI:10.1016/j.jorganchem.2015.12.039

Accepted Manuscript
N-heterocyclic carbene-palladium(II) complexes with benzoxazole or benzothiazole
ligands: synthesis, characterization, and application to Suzuki-Miyaura cross-coupling
reaction
Tao Wang, Huanping Xie, Lantao Liu, Wen-Xian Zhao
PII:
S0022-328X(15)30253-9
10.1016/j.jorganchem.2015.12.039
JOM 19345
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 2 September 2015
Revised Date: 19 December 2015
Accepted Date: 27 December 2015
Please cite this article as: T. Wang, H. Xie, L. Liu, W.-X. Zhao, N-heterocyclic carbene-palladium(II)
complexes with benzoxazole or benzothiazole ligands: synthesis, characterization, and application to
Suzuki-Miyaura cross-coupling reaction, Journal of Organometallic Chemistry (2016), doi: 10.1016/
j.jorganchem.2015.12.039.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
N-heterocyclic carbene-palladium(II) complexes with benzoxazole
or benzothiazole ligands: synthesis, characterization, and
application to Suzuki-Miyaura cross-coupling reaction
Tao Wanga, Huanping Xiea,b, Lantao Liua*, Wen-Xian Zhaoa,b*
aSchool of Chemistry and Chemical Engineering, Shangqiu Normal University,
Shangqiu, Henan, 476000, People’s Republic of China
bCollege of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou, Henan, 450052, People’s Republic of China
ABSTRACT: A series of novel N-heterocyclic carbene-palladium(II) complexes 3 were
conveniently prepared from commercially available imidazolium salts, palladium chloride,
and benzoxazole or benzothiazole in one step. All of the new complexes have been fully
1
characterized by elemental analysis, H, 13C NMR, and IR spectra. In addition, the
molecular structures of complexes 3a-d have also been determined by X-ray single-crystal
diffraction. The obtained palladium(II) catalysts showed efficient catalytic activity toward
the Suzuki-Miyaura coupling of aryl as well as benzyl chlorides with arylboronic acids.
Under the optimal reaction conditions, the expected biaryl products were obtained in
moderate to high yields.
Keywords: N-heterocyclic carbene; Palladium; Suzuki-Miyaura cross-coupling; Catalysis
1. Introduction
Palladium-catalyzed reactions have become one of the most important and frequently
applied methods for the synthesis of useful organic compounds in agrochemistry,
pharmaceutical chemistry, materials and synthetic chemistry.[1] N-heterocyclic carbene
(NHC)-palladium complexes, as an important and fascinating subclass of palladium
catalysts, have been developed and applied into numerous catalytic reactions such as
*Tel./Fax: +86 0370-2595126; e-mail: liult05@iccas.ac.cn or zhwx2595126@163.com.
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amination reactions, direct C-H bond arylation reactions, and cross-coupling reactions
including Heck, Suzuki-Miyaura, Negishi, and Kumada-Tamao-Corriu(KTC) reactions.[2]
In previous reports, it was found that the catalytic activity of N-heterocyclic carbene
(NHC)-palladium complexes is dependent largely on the electronic properties and steric
encumbrance of the NHC.[2a, 2d, 2f] Consequently, N-heterocyclic carbenes (NHCs) such as
IPr, IXy, IMes, SIPr, SIXy, SIMes and other ligand architecture were successively
designed. NHCs in Pd-catalyzed organic reactions have led to a number of exciting
discoveries.[3] In addition to the NHC moieties, the ancillary ligands which are attached to
the metal center also played key roles in the catalytic transformations of the NHC-Pd
complexes. For example, Navarro and co-workers[4] explored the applications of the
NHC-Pd(II)-Et3N complex in Mizoroki-Heck couplings, providing the desired products in
good yields without the need of an inert atmosphere and in relatively short reaction times.
The groups of Organ[3c,3j], Tu[5], Thiel[3d] and Brenner[3h] demonstrated that the related
NHC-Pd(II)-pyridine complexes were highly effective catalysts in several catalytic
reactions. While the NHC-Pd(II)-2-phenylimidazole complexes[6] readily catalyzed the
Suzuki-Miyaura cross-coupling reaction of aryl as well as benzyl chlorides with different
boronic acids in excellent yields using very low catalyst concentrations of 0.05 mol%.
Recently, Shao and Lu have successfully developed a well-defined N-heterocyclic
carbene-Pd(II)-1-methylimidazole [NHC-Pd(II)-Im][7] complex, and found it to be an
efficient catalyst for the Suzuki-Miyaura coupling,[3b,7a] arylation of symmetric dialkyl
ketones,[7d] direct C-H bond arylation reactions[7b,7c]. Additionally, N-heterocyclic
carbene-palladium(II)-4,5-dihydrooxazole complexes [NHC-Pd(II)-Ox] have been
successfully applied to the amination of aryl chlorides.[8] Despite the impressive progress
made so far in the N-heterocyclic carbene-palladium(II) complexes, however, we were
interested in the development of easily prepared and highly reactive, and stable NHC-Pd(II)
complexes, which can be readily tuned via variation of N-heterocyclic carbenes and
efficient ancillary ligands. In our previous work, we reported the achiral CNN pincer Pd(II)
complexes with N-substituted-2-aminomethyl-6-phenylpyridine ligands, which were used
as effective catalysts for allylation of aldehydes as well as three-component allylation
reactions of aldehydes, arylamines and allyltributyltin.[9] Very recently, some novel chiral
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CNN pincer Pd(II) complexes based on 2-aryl-6-(oxazolinyl)pyridine ligands which were
conveniently synthesized and well characterized by us, have proved to be excellent chiral
catalysts for the asymmetric allylation of isatins with allyltributyltin (up to 86% ee) as well
as asymmetric Suzuki-Miyaura coupling reaction (up to 68% ee).[10] Encouraged by the
results mentioned above and also in continuation of our efforts to develop palladium
complexes for organic synthesis, herein we would like to present facile synthesis and
structural characterization of the N-heterocyclic carbene palladium(II) complexes with
benzoxazole or benzothiazole as ancillary ligands, which can be prepared straightforward
within only one step (Scheme 1). The application of the resulting bench-stable complexes
in the Suzuki-Miyaura coupling of aryl and benzyl chlorides with arylboronic acids are
also presented below.
Scheme 1. Synthesis of N-heterocyclic carbene-palladium(II) complexes.
2. Results and discussion
2.1. Synthesis and characterization of the palladium(II) complexes
According to the literature method reported,[3j,8] the synthesis of the required
N-heterocyclic carbene-palladium(II) complexes 3 were easily done in a one-step sequence
from commercially available imidazolium salts, palladium chloride, and benzoxazole or
benzothiazole as shown in Scheme 1. The expected palladium(II) complexes 3 were
isolated in good yields (65-76%) after purification and fully characterized by elemental
analysis, 1H NMR, 13C NMR, and IR spectra. Additionally, the NHC-Pd complexes exhibit
generally high stabilities towards heat, air and moisture.
The molecular structures of Pd complexes 3a-d were unambiguously determined by
X-ray single crystal analysis. The molecules are illustrated in Figures 1 and 2, respectively.
Selected bond lengths and bond angles are listed in Table 1. Complexes 3a-d showed
slightly distorted-square-planar configuration for the central palladium atom. The two
chloride anions perpendicular to the plane of the NHC ligands and the benzoxazole or
benzothiazole is trans to it. All of the bond lengths and angles around the Pd(II) center in
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the four complexes are similar. The values of bond lengths and angles also compare well
to those of the related NHC-Pd(II) complexes with N-containing compounds.[8,11] The
Pd-N bond lengths (around 2.090 Å) in complexes 3a-d are slightly longer than that of
Pd-Ccarbene (around 1.960 Å). The angles of Ccarbene-Pd-N and Cl(1)-Pd-Cl(2) are almost
close to 180o, while the Ccarbene-Pd-Cl(1) angles, Ccarbene-Pd-Cl(2) angles, N-Pd-Cl(1)
angles and N-Pd-Cl(2) angles are almost close to 90 o.
Figure 1 Molecular structures of the Pd(II) complexes 3a (left) and 3b (right). Hydrogen
atoms are omitted for clarity.
Figure 2 Molecular structures of the Pd(II) complexes 3c (left) and 3d (right). Hydrogen
atoms and solvent molecules are omitted for clarity.
Table 1 Selected bond lengths (Å) and angles (deg) for complexes 3a-d
3d.CH2Cl2
1.974(4)
3a
3b
3c
Pd(1)-C(3)/Pd(1)-C(8)
Pd(1)-N(3)/Pd(1)-N(1)
Pd(1)-Cl(1)
1.953(3)
2.100(3)
2.2754(9)
2.3011(9)
173.59(12)
1.967(2)
2.097(2)
2.3015(8)
2.2948(8)
174.44(10)
91.38(8)
88.22(8)
91.13(8)
89.26(8)
179.58(3)
1.967(2)
2.088(2)
2.2948(7)
2.3045(8)
175.43(10)
91.27(7)
91.34(7)
88.00(7)
89.62(7)
176.11(3)
2.097(3)
2.3013(10)
2.3020(11)
173.36(14)
92.10(11)
90.43(11)
89.86(9)
Pd(1)-Cl(2)
C(3)-Pd(1)-N(3)/C(8)-Pd(1)-N(1)
C(3)-Pd(1)-Cl(1)/C(8)-Pd(1)-Cl(1) 89.31(9)
C(3)-Pd(1)-Cl(2)/C(8)-Pd(1)-Cl(2) 91.62(9)
N(3)-Pd(1)-Cl(1)/N(1)-Pd(1)-Cl(1) 90.21(9)
N(3)-Pd(1)-Cl(2)/N(1)-Pd(1)-Cl(2) 89.08(8)
87.88(9)
Cl(1)-Pd(1)-Cl(2)
177.84(4)
176.60(4)
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2.2. Suzuki-Miyaura coupling of aryl as well as benzyl chlorides with arylboronic acids
The Suzuki-Miyaura coupling reaction stands out as one of the most powerful,
convenient, and versatile methods to create carbon-carbon bonds and thus has found
widespread applications in natural products, advanced materials and organic synthesis.[12]
Although most of the successful examples concern the use of aryl iodides and bromides in
the reaction, the application of aryl chlorides has recently attracted much attention mainly
due to their good stability, low cost and wide availability. However, Suzuki-Miyaura
coupling of aryl chlorides is still regarded as a challenging task because of high C-Cl bond
strength. As a result, the development of known or new catalysts with excellent catalytic
activities is in great demand to improve this transformation. Thus, we wonder whether the
present NHC-Pd(II) complexes can behave better in the reaction. The coupling of
1-chloro-4-methoxybenzene with phenylboronic acid was chosen as a model to optimize
the reaction conditions (Table 2). It was found that in the presence of 2.0 mol% of
complex 3a with KOtBu, Cs2CO3 or NaOtBu as the base in EtOH at 80 °C the
corresponding biaryl product 6a was isolated in high yield after 15 h (entries 1, 3, 5).
When K2CO3 or K3PO4 was used as the base, the yield drastically decreased (entries 2, 6).
With Na2CO3 as the base afforded trace amounts of product (entry 4).
The drastically different yields between Na, K and Cs carbonate bases possible are due to t
he different basic character of the EtOH solutions. [13] In a word, base plays crucial role for
the yield of the reaction. To our delight, EtOH mixed with different proportions of water
still be able to provide good yields (entries 7-9). Several other solvents including THF,
i
toluene, 1,4-dioxane and PrOH mixed with an equal volume of water were tested, and
iPrOH was found to be the most appropriate, giving the biaryl product in a >99% yield
(entries 10-13). And a reduction in the catalyst loading from 2.0 mol% to 1.0 mol% led to
decreased yield (entry 14). By reducing the reaction time to 12 h the product 6a was
obtained in a 85% yield (entry 15). In addition, complex 3b showed an activity
comparable to that of 3a (entry 16), while 3c and 3d were less efficient under the same
reaction conditions (entries 17 and 18). The nitrogen atoms of benzoxazole ligands
coordinate more weakly to palladium than that of benzothiazole ligands. So the
disassociation of benzoxazole ligands is relatively easier to occur during the reaction
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process which might lead to the higher reactivity of corresponding complexes.
Table 2 Optimization of reaction conditions for Suzuki-Miyaura reaction of
1-chloro-4-methoxybenzene with phenylboronic acid catalyzed by the NHC-Pd(II)
complexes 3a
Entry
1
Cat.
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
Base
KOtBu
K2CO3
Cs2CO3
Na2CO3
NaOtBu
K3PO4
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
Solvent
EtOH
Yield (%)b
> 99
53
2
EtOH
3
EtOH
> 99
trace
> 99
80
4
EtOH
5
EtOH
6
EtOH
7
EtOH/H2O (2/1)
EtOH/H2O (1/1)
EtOH/H2O (1/2)
THF/H2O (1/1)
Toluene/H2O (1/1)
1,4-dioxane/H2O (1/1)
iPrOH/H2O (1/1)
iPrOH/H2O (1/1)
iPrOH/H2O (1/1)
iPrOH/H2O (1/1)
iPrOH/H2O (1/1)
iPrOH/H2O (1/1)
> 99
84
8
9
60
10
11
12
13
14c
15d
16
17
18
67
19
88
> 99
72
85
91
72
25
aAll reactions were carried out using 4a (0.50 mmol), 5a (0.75 mmol), base (2.0 equiv), 3
o
b
c
(2.0 mol%) in solvent (2.0 mL) at 80 C for 15 h. Isolated yields. Cat. (1.0 mol%).
dReaction time was 12 h.
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With optimized conditions in hand, we next set out to explore the substrate scope of the
cross coupling of aryl chlorides and aryl boronic acids. As shown in Table 3, a series of
i
aryl chlorides 4 and aryl boronic acids 5 were investigated in PrOH-H2O in the presence
o
of 2.0 mol% complex 3a and 2.0 equiv KOtBu at 80 C for 15 h (Table 3). Gratifyingly,
most of the coupling reaction proceeded rapidly and efficiently to provide the
corresponding biaryl products in excellent yields. Both electron-donating and
electron-withdrawing groups in the phenyl ring of aryl chlorides were tolerated and good
results were obtained (entries 1-10). Roughly, the electron-donating group in the phenyl
ring of aryl chlorides showed some beneficial effect on the yields of the catalysis products.
The position of the group on the aryl ring did not seem to affect the product yields
apparently. For example, the reaction was quite feasible with ortho-substituted aryl
chlorides when the phenylboronic acid was used (entries 3 and 6). Particularly, when
heteroaromatic aryl chlorides such as 2-chloropyridine and 3-chloropyridine were used as
the substrates, high yield of the corresponding products was always observed (entries
9-10). Subsequently, the scope of complex 3c catalytic system was further investigated
with respect to arylboronic acids (entries 11-17). In most cases, the reaction worked well
and approached to corresponding products in very high yield. However, electron-deficient
and sterically hindered boronic acids resulted in a significant decrease in yields (entries 15
and 17). In addition, in the case of 3-pyridinylboronic acid or 4-pyridinylboronic acid
afforded trace amounts of product under the present reaction conditions (data not shown in
Table 3). It was believed that possible reason for this was these substrates bearing ligating
atoms which may deactivate the catalyst. Overview, the results indicated that complex 3a
was still efficient in the catalytic process.
Inspired by these results and our attention was next turned to the cross coupling reaction
of benzyl chlorides and aryl boronic acids. As shown in Table 4, the reactions proceed
smoothly to afford diarylmethanes in excellent yields. Particularly, when electron-deficient
boronic acids such as 4-trifluoromethyl-phenylboronic acid were used as the substrates,
high yield of the corresponding product was always observed (98% yield, entry 5). The
above results confirm that the NHC-Pd complexes are really effective catalysts for the
Suzuki-Miyaura reaction. It is worth pointing out that the present NHC-Pd(II) catalysts
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invariably provided much higher yields than the related catalysts[3h, 14] under similar
reaction conditions.
Table 3 Substrate scope for the catalytic Suzuki-Miyaura reaction of aryl chlorides using
the NHC-Pd(II) complex 3a as the catalysta
Entry
1
4 (Ar1)
5 (Ar2)
5a (Ph)
Product
6a
Yield (%)b
> 99
99
4a (4-MeO-C6H4)
4b (3-MeO-C6H4)
4c (2-MeO-C6H4)
4d (4-Me-C6H4)
4e (3-Me-C6H4)
4f (2-Me-C6H4)
4g (4-CH3CO-C6H4)
4h (4-NO2-C6H4)
4i (2-Pyridyl)
2
5a (Ph)
6b
6c
3
5a (Ph)
99
4
5a (Ph)
> 99
99
6d
6e
5
5a (Ph)
6
5a (Ph)
92
6f
7
5a (Ph)
86
6g
8
5a (Ph)
93
6h
6i
9
5a (Ph)
97
10
11
12
13
14
15
16
17
4j (3-Pyridyl)
5a (Ph)
96
6j
4a (4-MeO-C6H4)
4a (4-MeO-C6H4)
4a (4-MeO-C6H4)
4a (4-MeO-C6H4)
4a (4-MeO-C6H4)
4a (4-MeO-C6H4)
4a (4-MeO-C6H4)
5b (4-Me-C6H4)
5c (3-Me-C6H4)
5d (2-Me-C6H4)
5e (4-F-C6H4)
5f (4-CF3-C6H4)
5g (1-Naphthyl)
5h (2-Naphthyl)
98
6k
6l
97
92
6m
6n
6o
98
68
> 99
50
6p
6q
aAll reactions were carried out using 4 (0.50 mmol), 5 (0.75 mmol), KOtBu (2.0 equiv), 3a
(2.0 mol%) in iPrOH/H2O (2.0 mL, V:V = 1:1) at 80 oC for 15 h. bIsolated yields.
Table 4 Substrate scope for the catalytic Suzuki-Miyaura reaction of benzyl chlorides
using the NHC-Pd(II) complex 3a as the catalysta
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Entry
5 (Ar2)
Product
8a
Yield (%)b
1
2
3
4
5
6
5b (4-Me-C6H4)
5c (3-Me-C6H4)
5d (2-Me-C6H4)
5e (4-F-C6H4)
5f (4-CF3-C6H4)
5g (1-Naphthyl)
> 99
98
8b
98
8c
99
8d
98
8e
> 99
8f
aAll reactions were carried out using 7 (0.50 mmol), 5 (0.75 mmol), KOtBu (2.0 equiv), 3a
(2.0 mol%) in iPrOH/H2O (2.0 mL, V:V = 1:1) at 80 oC for 15 h. bIsolated yields.
3. Conclusion
In summary, four new N-heterocyclic carbene-palladium(II) complexes based on
benzoxazole or benzothiazole ligands have been conveniently synthesized and well
characterized. These complexes have been utilized as the catalysts for the Suzuki-Miyaura
coupling of aryl chlorides and benzyl chlorides with arylboronic acids, producing the
corresponding biaryl products in high yields (up to 99%). Also, the NHC-Pd
complexes include benzoxazole as ligand was found to exhibit much higher activity than
the related NHC-Pd complexes with benzothiazole in the Suzuki-Miyaura coupling.
Further modification of the N-heterocyclic carbene-palladium(II) complexes and their
applications in other reactions are in progress.
4. Experimental
4.1. General
Reactions for the preparation of N-heterocyclic carbene-palladium(II) complexes as
well as all the catalytic reactions were carried out under nitrogen atmosphere. Solvents
were dried with standard methods and freshly distilled prior to use if needed. All other
chemicals were used as purchased. Melting points were measured on a XT4A melting
point apparatus and were uncorrected. IR spectra were collected on a Bruker VECTOR22
1
spectrophotometer in KBr pellets. H and 13C NMR spectra were recorded on a Bruker
DPX 400 instrument using TMS as an internal standard. Elemental analyses were
measured on a Thermo Flash EA 1112 elemental analyzer.
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4.2. General procedure for the synthesis of N-heterocyclic carbene-palladium(II)
complexes
Under an N2 atmosphere, the mixture of imidazolium salts 1 (1.1 mmol), benzoxazole or
benzothiazole (2.0 mmol), PdCl2 (1.0 mmol) and K2CO3 (1.1 mmol) was stirred in
anhydrous THF (10 mL) under reflux for 16 h. After cooling, filtration and evaporation,
the residue was puried by preparative TLC on silica gel plates eluting with CH2Cl2 to
afford the corresponding N-heterocyclic carbene-palladium(II) complexes 3a-d.
Complex (3a): 458.4 mg, 67% yield, orange solids. mp: 139-142 oC. 1H NMR (400 MHz,
CDCl3): δ 8.30 (s, 1H, OCH=N), 8.02-8.00 (m, 1H, ArH), 7.51 (t, J = 7.8 Hz, 2H, ArH),
7.40-7.36 (m, 5H, ArH), 7.30-7.23 (m, 2H, ArH), 7.16 (s, 2H, NCH=), 3.25-3.15 (m, 4H,
CH(CH3)2), 1.49 (d, J = 6.6 Hz, 12H, CH3CHCH3), 1.13 (d, J = 6.9 Hz, 12H, CH3CHCH3).
13C NMR (100 MHz, CDCl3): δ 156.5(Pd-Ccarbene), 153.5, 148.4, 145.9, 134.1, 129.3,
125.3, 125.1, 124.0, 123.7, 123.0, 120.5, 27.8, 25.4, 22.1. IR (KBr, cm-1): υ 3122, 3091,
2965, 2927, 2866, 1593, 1529, 1454, 1414, 1382, 1348, 1137, 800, 756, 708. Anal. Calcd.
for C34H41Cl2N3OPd: C, 59.61; H, 6.03; N, 6.13. Found: C, 59.58; H, 6.08; N, 6.09%.
Complex (3b): 532.1 mg, 76% yield, orange solids. mp: >250 oC. 1H NMR (400 MHz,
CDCl3): δ 8.91 (s, 1H, SCH=N), 8.26 (d, J = 7.2 Hz, 1H, ArH), 7.69 (d, J = 6.8 Hz, 1H,
ArH), 7.58-7.54 (m, 2H, ArH), 7.42-7.40 (m, 4H, ArH), 7.37-7.31 (m, 2H, ArH), 7.18 (s,
2H, NCH=), 3.26-3.16 (m, 4H, CH(CH3)2), 1.47 (d, J = 6.6 Hz, 12H, CH3CHCH3), 1.13 (d,
J = 6.9 Hz, 12H, CH3CHCH3). 13C NMR (100 MHz, CDCl3): δ 156.5(Pd-Ccarbene), 153.5,
148.4, 145.9, 134.1, 129.3, 125.3, 125.1, 124.0, 123.7, 123.0, 120.5, 27.8, 25.4, 22.1. IR
(KBr, cm-1): υ 3171, 3136, 3105, 2921, 2851, 1603, 1530, 1491, 1459, 1412, 1383, 1345,
1237, 1021, 928, 864, 826, 762, 730, 709, 426. Anal. Calcd. for C34H41Cl2N3SPd: C, 58.25;
H, 5.89; N, 5.99. Found: C, 58.15; H, 5.96; N, 5.85%.
Complex (3c): 390.1 mg, 65% yield, orange solids. mp: >250 oC. 1H NMR (400 MHz,
CDCl3): δ 8.23 (s, 1H, OCH=N), 7.80 (d, J = 8.0 Hz, 1H, ArH), 7.40 (d, J = 8.0 Hz, 1H,
ArH), 7.32-7.23 (m, 2H, ArH), 7.10 (s, 2H, NCH=), 7.08 (s, 4H, ArH), 2.40 (s, 6H, CH3),
2.38 (s, 12H, CH3). 13C NMR (100 MHz, CDCl3): δ 154.7(Pd-Ccarbene), 139.3, 136.5,
135.1, 129.3, 126.3, 125.1, 124.2, 121.7, 111.0, 21.3, 19.2. IR (KBr, cm-1): υ 3122, 3089,
2963, 2929, 2869, 1605, 1529, 1466, 1457, 1411, 1382, 1362, 1346, 1332, 1267, 1205,
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944, 923, 800, 758, 731, 699, 425. Anal. Calcd. for C28H29Cl2N3OPd: C, 55.97; H, 4.86; N,
6.99. Found: C, 55.63; H, 5.06; N, 6.72%.
Complex (3d): 462.0 mg, 75% yield, orange solids. mp: >250 oC. 1H NMR (400 MHz,
CDCl3): δ 8.85 (s, 1H, SCH=N), 8.09 (d, J = 7.4 Hz, 1H, ArH), 7.71 (d, J = 7.1 Hz, 1H,
ArH), 7.37-7.31 (m, 2H, ArH), 7.12 (s, 6H, ArH and NCH=), 2.45 (s, 6H, CH3), 2.38 (s,
12H, CH3). 13C NMR (100 MHz, CDCl3): δ 157.4(Pd-Ccarbene), 139.3, 136.7, 135.1,
129.3, 126.4, 126.2, 124.6, 124.2, 121.6, 21.3, 19.2. IR (KBr, cm-1): υ 3162, 3133, 3122,
2954, 2918, 2855, 1607, 1529, 1487, 1455, 1408, 1378, 1370, 1341, 1302, 1280, 1247,
1234, 1160, 1132, 1099, 930, 870, 863, 848, 745, 734, 703. Anal. Calcd. for
C28H29Cl2N3SPd: C, 54.51; H, 4.74; N, 6.81. Found: C, 54.31; H, 4.96; N, 6.63%.
4.3. General procedure for the catalytic Suzuki-Miyaura coupling reaction
A Schlenk flask was charged with aryl chlorides (0.20 mmol), arylboronic acids (0.30
mmol), N-heterocyclic carbene-palladium(II) complex 3 (2 mol %), KOtBu (2.0 equiv),
o
iPrOH (1 mL) and H2O (1 mL). The mixture was stirred at 80 C for 15 h under N2. After
cooling, the reaction mixture was evaporated and the product was isolated by preparative
1
TLC on silica gel plates. The purified products were identified by H NMR spectra and
their analytical data are given in Supporting Information.
4.4. Crystal structure determination and data collection
Crystals of 3a-d were obtained by recrystallization from CH2Cl2/n-hexane at ambient
temperature. Their data were collected on an Oxford Diffraction Gemini E diffractometer
with graphite-monochromated Cu Kα (λ = 1.5418 Å, for 3a-b) or Mo Kα radiation (λ =
0.7107 Å, for 3c-d). The structures were solved by direct methods using the SHELXS-97
program, and all non-hydrogen atoms were refined anisotropically on F2 by the full-matrix
least-squares technique, which used the SHELXL-97 crystallographic software package.[15]
The hydrogen atoms were included but not refined. Details of the crystal structure
determination of the Pd(II) complexes are summarized in Table 5. CCDCs 1413911,
1413912, 1413913 and 1413782 contain the crystallographic data for complexes 3a, 3b, 3c
and 3d, respectively. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
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Table 5 Crystal data and structure refinement parameters for complexes 3a-d
3d.CH2Cl2
C29H31Cl4N3SPd
701.83
3a
3b
3c
Empirical formula
Mr
C34H41Cl2N3OPd
685.00
C34H41Cl2N3SPd
701.06
C28H29Cl2N3OPd
600.84
temperature (K)
wavelength (Å)
crystal system
cryst size (mm)
a (Å)
291.15
291.15
291.15
291.15
1.54184
Monoclinic
1.54184
Triclinic
0.71073
0.71073
Orthorhombic
Monoclinic
0.20 x 0.20 x 0.18 0.18 x 0.16 x 0.13 0.25 x 0.20 x 0.17 0.20 x 0.18 x 0.16
14.33347(13)
33.7182(3)
15.71676(14)
90
10.7554(5)
12.4577(6)
12.8064(6)
84.337(4)
88.720(4)
83.079(4)
1694.99(14)
2
10.10616(14)
32.1576(6)
16.5544(3)
90
40.9687(13)
11.9097(2)
14.9999(4)
90
b (Å)
c (Å)
α (o)
β (o)
100.4678(9)
90
90
119.779(4)
90
γ (o)
90
V (Å3)
7469.47(12)
8
5380.03(15)
8
6352.4(4)
8
Z
space group
Dcalcd (g cm-3)
ꢀ (mm-1)
θ range (o)
F(000)
P2(1)/n
1.218
P-1
Pccn
C(2)/c
1.374
1.484
1.468
5.525
6.638
0.914
1.009
3.15-72.39
2832.0
3.468-67.08
724.0
3.031-26.37
2448.0
19649
3.021-26.37
2848.0
25263
no.
of
data 28773
12167
collected
no. of unique data
R(int)
14512
6052
5500
6488
0.0243
0.0291
0.0276
0.0318
final R indices
(I > 2σ(I))
R1 = 0.0411
wR2 = 0.1093
R1 = 0.0323
wR2 = 0.0773
R1 = 0.0391
wR2 = 0.0814
R1 = 0.0333
wR2 = 0.0719
R1 = 0.0427
wR2 = 0.0768
R1 = 0.0439
wR2 = 0.1089
R1 = 0.0571
wR2 = 0.1173
R indices (all data) R1 = 0.0502
wR2 = 0.1149
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Largest diff peak
and hole (e·Å-3)
1.42 and -0.86
0.32 and -0.40
0.49 and -0.43
0.67 and -0.76
Acknowledgments
We are grateful to the National Natural Sciences Foundation of China (no. U1404205,
21202095 and 21172139) and the Program for Science & Technology Innovation Talents
in Universities of Henan Province (14HASTIT016) for financial support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/xxxxxx.
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Highlights
.
Four new N-heterocyclic carbene-palladium(II) complexes based on benzoxazole
or benzothiazole ligands have been conveniently synthesized.
.
.
All of the new complexes have been well characterized.
The catalytic activity of Pd(II) complexes in the Suzuki-Miyaura coupling of aryl
as well as benzyl chlorides with arylboronic acids are investigated.