Sterically Encumbered Tetraarylimidazolium Carbene Pd-PEPPSI Complexes: Highly Efficient Direct Arylation of Imidazoles with Aryl Bromides under Aerobic Conditions

Article abstract of DOI:10.1021/acs.organomet.6b00391

A series of sterically encumbered tetraarylimidazolium carbene Pd-PEPPSI complexes were conveniently prepared and fully characterized. These sterically encumbered Pd-PEPPSI complexes act as active precatalysts for the direct arylation of imidazoles with aryl bromides under aerobic conditions. The catalytic performance of Pd-PEPPSI complexes in cross-coupling processes is investigated. Under the optimal protocols, the cross-coupling reactions regioselectively produced C5-arylation products in moderate to excellent yields, which could tolerate a wide range of functional aryl bromides.

Full text of DOI:10.1021/acs.organomet.6b00391

Article
pubs.acs.org/Organometallics
Sterically Encumbered Tetraarylimidazolium Carbene Pd-PEPPSI
Complexes: Highly Efficient Direct Arylation of Imidazoles with Aryl
Bromides under Aerobic Conditions
Xu-Xian He,† Yinwu Li,‡ Bei-Bei Ma,† Zhuofeng Ke,*,‡ and Feng-Shou Liu*,†
†School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan, Guangdong 528458, People’s
Republic of China
‡MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Materials Science and Engineering, Sun Yat-sen
University, Guangzhou 510275, People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of sterically encumbered tetraarylimi-
dazolium carbene Pd-PEPPSI complexes were conveniently
prepared and fully characterized. These sterically encumbered
Pd-PEPPSI complexes act as active precatalysts for the direct
arylation of imidazoles with aryl bromides under aerobic
conditions. The catalytic performance of Pd-PEPPSI com-
plexes in cross-coupling processes is investigated. Under the
optimal protocols, the cross-coupling reactions regioselectively produced C5-arylation products in moderate to excellent yields,
which could tolerate a wide range of functional aryl bromides.
Despite the significant progress that has been achieved in
traditional palladium-catalyzed cross-coupling reactions,15 the
application of NHC palladium complexes to the direct C−H
arylation of imidazoles with aryl halides is relatively rare and
some shortcomings still remain. In most cases, the substrate
scope is generally limited, and a high palladium loading of 2.5−
5 mol % is required in these catalytic processes. Moreover, even
though NHC palladium precursors are generally stable to heat
and moisture, their catalytic species are often sensitive to the
presence of oxygen, forming the unreactive peroxo species
LPd(O2) rapidly, which demands an anhydrous and oxygen-
free environment during their catalytic reactions.16 Conse-
quently, it is challenging to achieve direct C−H bond arylation
under aerobic conditions with the NHC palladium complexes.
The catalytic activities of NHC palladium complexes are
directly related to the unique properties of the supporting
NHCs: their σ-donor character and their steric environment.
Both properties have strong effects on the oxidative addition
and reductive elimination processes in palladium-catalyzed
cross-coupling reactions. Therefore, the development of a new
NHC catalytic system to overcome these difficulties is highly
desired. To date, although the design of NHC palladium
complexes is mainly based on tuning steric hindrance in N-
phenyl rings, it is noteworthy that the introduction of bulky
ligands into the skeleton of NHC palladium complexes has
INTRODUCTION
■
Over the past decade, the palladium-catalyzed direct arylation
reactions of heteroarenes with aryl halides have become some
of the most reliable approaches to assemble skeletons for
pharmaceuticals and functional materials.1,2 These reactions
exploit the direct activation of the C−H bonds of heteroarenes,
which are atom economical and environmentally friendly and
avoid the need for prefunctionalization of organometallic
derivatives in traditional palladium-catalyzed cross-couplings.3
However, it is also quite challenging for the arylation of
imidazoles, because a kinetically significant aryl C−H bond
activation step is involved.4 By far, Pd(OAc)2 species associated
with various phosphines or nitrogen-based ligands are the most
common catalytic systems for direct arylation of imidazoles.
Ligands such as PPh3,5 AsPh3,6 Pcy3,7 P(2-furyl)3,8 P(n-
Bu)Ad2,9 dppb,10 X-phos,11 and Xantphos12 as well as 1,10-
phenanthroline13 have been commonly employed. In parallel,
the recent development of N-heterocyclic carbenes (NHCs) as
prospective ligands has been regarded as promising, since their
strong σ donation and easily tunable steric properties have
important advantages over their phosphine counterparts.14 For
instance, Lee and co-workers have shown that Pd(L)(PCy3)-
(OAc)2 (L = 1,3-dibenzylimidazol-2-ylidene) successfully
catalyzed direct C5-arylation reactions between imidazoles
and aryl halides.14c Recently, Huynh found that the
heterotetrakis(carbene) complexes [PdX2(iPr2-bimy)]2(μ-ditz)
emerged as a powerful strategy.17 For example, Ces
́
ar and
i
Lavigne found that the adjunction of one or two N,N-dimethyl
groups into the backbone framework of IPr, which greatly
(X = Br, CH3COO, CF3COO; Pr2-bimy = 1,3-diisopropyl-
benzimidazolin-2-ylidene; μ-ditz = diNHC ligand 1,2,4-
trimethyltriazolidine-3,5-diylidene) can catalyze the direct C−
H arylation of imidazoles with aryl iodides and aryl bromides in
moderate yields.14d
Received: May 13, 2016
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Tetraarylimidazolium Palladium Complexes
increased the electron-donating ability of the carbene ligand,
result in highly efficient catalytic performance in the
Buchwald−Hartwig amination reaction.17d However, the
synthetic procedures for these complexes involve multiple
steps with harsh reaction conditions. Moreover, it is notable
that NHCs with diaryl backbones, namely tetraarylimidazolium
carbenes, also show great electron-donor ability.18 Inspired by
the promisingly strong σ-donation ability of the tetraarylimi-
dazolium carbenes, we envision that strong binding of these
kinds of electron-rich and bulky carbene ligands will help in the
validation of the Pd−Ccarbene bond and enhance the stability of
palladium complexes. Therefore, an expected excellent
reactivity throughout the course of direct arylation could be
achieved under mild reaction conditions. Herein, we describe a
series of generally applicable Pd-PEPPSI complexes with a
“larger steric hindrance backbone” (Scheme 1) for efficient and
direct C-5 arylation of imidazoles with aryl bromides.
oil of complicated composition. To access this compound, a
variety of methods, such as using ZnCl2/paraformaldehyde and
AgOTf/chloromethyl pivalate, were explored; however, this
resulted in no benefit.21
With these imidazolium salts in hand, we then started
metalation studies using the one-pot method developed by
Organ and co-workers for the synthesis of Pd-PEPPSI
complexes.22a Thus, using K2CO3 as base and pyridine or 3-
chloropyridine as solvent, the treatment of ligand precursors
L1−L3 with PdCl2 smoothly afforded C1−C6 in moderate
yields of 51−69%. To our delight, all of these Pd-PEPPSI
complexes proved to be both air- and moisture-stable and could
be stored on the shelf. They were readily soluble in
dichloromethane and chloroform but insoluble in diethyl
ether and hydrocarbon solvents. The structures of C1−C6
were fully characterized by NMR, ESI-MS, and elemental
analysis and further confirmed by X-ray single-crystal
1
diffraction. In the H NMR spectra of C1−C6, the resonances
RESULTS AND DISCUSSION
for the imidazolium C−H protons (NCHN) had disappeared,
and the chemical shifts of other protons were similar to those of
the corresponding precursors. The chemical shift of the α-
hydrogen on pyridines appeared at δ 8.58−8.50 ppm, which
was observed to be shifted upfield in comparison with their
corresponding free pyridines (δ 8.62 ppm).22b In the 13C NMR
spectra, the signals of the Pd−Ccarbene in C1−C6 appear at
153.7−150.1 ppm, which were similar to those of the reported
Pd-PEPPSI complexes.23 Moreover, unlike the behavior of α-
diimine (1) and imidazolium salt (L1), C1 and C4 exhibited
two isomers in solution, indicating that the molecules could
adopt syn and anti configurations. Reasonably, the CAr−N bond
rotation which would freely occur in the case of 1 and L1 would
be retarded when the coordinated palladium was used.
■
Synthesis and Characterization of the Tetraarylimi-
dazolium Carbene Ligands and the Pd-PEPPSI Com-
plexes. The procedures for the synthesis of sterically
encumbered tetraarylimidazolium carbene ligands and the
corresponding Pd-PEPPSI complexes C1−C6 are shown in
Scheme 1. Previously, the key intermediates of α-diimine
compounds were prepared by intermolecular cyanide-catalyzed
aldimine coupling or simply acid-catalyzed condensation.19
However, the preparation of α-diimines bearing steric bulk in
the N-phenyl rings with a diphenyl backbone failed using
traditional methods. To overcome this synthetic challenge, we
undertook a strategy to condense anilines onto benzil via
trimethylaluminum-promoted routes, and it afforded the
desired products 1−3 in high yields.20 The respective
imidazolium salts L1−L3 were subsequently obtained by the
cyclization of the α-diimines with chloromethyl ethyl ether,
giving moderate to high yields (69−87%). The formation of a
NCHN bond in the imidazolium salts was confirmed by the
characteristic signals in 1H NMR spectra, in which the
resonance of imidazolium C−H protons appeared at δ
10.06−11.61 ppm. We considered that unsymmetrical 2-methyl
substitution on the N-phenyl moiety, of both the α-diimine (1)
and imidazolium salt (L1), would exhibit isomers. However,
there no splitting of peaks was observed in the NMR spectrum,
which indicates the rapid rotation of the CAr−N bond in these
cases in solution. Nevertheless, attempts to synthesize the
imidazolium salt with a 2,6-diisopropyl group on the N-phenyl
rings did not lead to the desired product, instead giving a black
Single crystals of C1, C2, and C4 suitable for X-ray
diffraction studies were obtained by layering their dichloro-
methane solutions with hexane at ambient temperature
(Figures 1−3). Because of the unsymmetrical substitution of
the methyl in C1 and C4, the solid molecules could adopt a syn
or anti configuration. As can be seen in Figures 1 and 3, these
two single crystals exhibited an anti configuration with the
methyl fragments pointing toward each other above and below
the NHC ring. The palladium atoms in these structures adopt a
slightly distorted square planar coordination geometry with the
NHC and pyridine ligands trans to each other. The N-phenyl
rings were found to be approximately perpendicular to the
NHC ring with dihedral angles of 71.10 (67.70), 81.83 (81.79),
and 69.19° (67.87°), for complexes C1, C2, and C4,
respectively. These data suggest that greater steric repulsion
B
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Figure 1. Molecular structure of C1 depicted with 30% thermal
ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): Pd(1)−C(29) 1.957(3), Pd(1)−
N(3) 2.109(2), Pd(1)−Cl(1) 2.3083(9), Pd(1)−Cl(2) 2.3053(8);
N(3)−Pd(1)−C(29) 177.69(11), N(3)−Pd(1)−Cl(1) 91.81(8),
C(29)−Pd(1)−Cl(1) 89.29(9), N(3)−Pd(1)−Cl(2) 90.34(8),
C(29)−Pd(1)−Cl(2) 88.63(9), Cl(1)−Pd(1)−Cl(2) 177.13(3).
Figure 3. Molecular structure of C4 depicted with 30% thermal
ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): Pd(1)−C(29) 1.944(5), Pd(1)−
N(3) 2.099(5), Pd(1)−Cl(1) 2.3044(16), Pd(1)−Cl(2) 2.3124(14),;
N(3)−Pd(1)−C(29) 176.40(18), N(3)−Pd(1)−Cl(1) 92.32(13),
C(29)−Pd(1)−Cl(1) 89.54(14), N(3)−Pd(1)−Cl(2) 91.14(13),
C(29)−Pd(1)−Cl(2) 87.05(14), Cl(1)−Pd(1)−Cl(2) 176.45(5).
The Pd-PEPPSI complex structures were fully optimized with
the BP86 functional25 as implemented in the Gaussian09
program.26 The triple-ζ SBK ECP basis set was used for
palladium and the 6-31G* set was used for main-group
atoms.27 It was shown that the structural differences between
optimized structures and solid-state structures are very small
(Table S1 in the Supporting Information). As shown in Chart
1, the range of buried volumes lies between 30.9 and 33.2% for
the NHC ligands in Pd-PEPPSI complexes. Unsurprisingly,
with a buried volume of 31.0−33.2%, the tetraaryl NHC ligands
were found to be more sterically encumbered than the IMe (%
VBur = 30.9%) and IEt (%VBur = 31.1%) ligands.
Direct C−H Bond Arylation Catalyzed by Pd-PEPPSI
Complexes. To verify the robust catalytic efficiency of the
synthesized bulky Pd-PEPPSI complexes in hand, direct
arylation reactions were carried out without the need for
anhydrous conditions or an inert atmosphere. Initially, the
reaction of 1-methyl-1H-imidazole (7a) with 1-bromo-4-
chlorobenzene (8a) was examined as a model reaction in the
presence of pivalic acid (PivOH) and K2CO3 in N,N-
dimethylacetamide (DMAc) at 130 °C. In the preliminary
studies, regioselective arylation on the C5 position was
observed because the hydrogen on C5 is more reactive than
that on the C4 position.28 Moreover, in the course of this
reaction, no cleavage of the carbon−chlorine bond of the
product was detected. Then, a variety of palladium precatalysts
were evaluated. As illustrated in entry 1 of Table 1, a yield of
49% was obtained when 5 mol % of Pd(OAc)2 was applied,
which is compatible with the results reported by Bellina.29
However, the poor efficiency in the ligand-free catalytic system
suggested that the catalytic species tend to be deactivated in the
absence of ligand protection under aerobic conditions. In
contrast, Pd-PEPPSI-IEt, with 2,6-diethyl substituents on each
N-phenyl ring, was then evaluated. This resulted in a moderate
efficiency of 52% yield when a palladium loading of 0.5 mol %
was used (entry 2, Table 1). Moreover, the sterically bulky Pd-
PEPPSI-IPr bearing 2,6-diisopropyl groups on the N-phenyl
moiety afforded a very efficient yield of 93% (entry 3, Table 1).
With the tetraarylimidazolium type of Pd-PEPPSI complexes in
hand, we found that C3, bearing 2,6-diethyl substituents on N-
Figure 2. Molecular structure of C2 depicted with 30% thermal
ellipsoids. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): Pd(1)−C(16) 1.979(2), Pd(1)−
N(2) 2.125(2), Pd(1)−Cl(1) 2.3086(5), Pd(1)−Cl(1A) 2.3086(5);
N(2)−Pd(1)−C(16) 180.0, N(2)−Pd(1)−Cl(1) 89.735(11), C(16)−
Pd(1)−Cl(1) 90.265(11), N(2)−Pd(1)−Cl(1A) 89.735(11), C(16)−
Pd(1)−Cl(1A) 90.265(11), Cl(1)−Pd(1)−Cl(1A) 179.47(2).
of the bulky substituents on the N-phenyl ring would cause
more shielding on the apical positions of the palladium center.
Moreover, dihedral angles between the diphenyl framework and
NHC ring were evaluated, which turned out to be 47.02
(43.74°), 49.33 (49.31) and 47.55° (46.13°) for complexes C1,
C2, and C4, respectively. Obviously, the backbones of the
diphenyl also exhibit a certain steric effect on retarding the
rotation of the N-phenyl rings, which would provide further
protection for the metal center. The Pd−NPy and Pd−Cl bond
lengths are quite similar to those for the reported Pd-PEPPSI
complexes.23 On the other hand, the Pd−Ccarbene bond length
in C2 (1.979(2) Å) is much longer than that of the
[Pd(IMes)(3-ClPy)Cl2] analogue (1.962(4) Å). This is
presumably also due to the bulky steric hindrance brought
about by the diphenyl framework. In order to further
understand the spatial situation of the palladium center, the
percent buried volumes (%VBur) of the investigated NHC
ligands were obtained using the web application SambVca.24
C
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a
Chart 1. Comparison of the Buried Volume (%VBur) of NHC Ligands in Pd-PEPPSI Complexes
a
The buried volume of NHC ligands in the Pd-PEPPSI complexes were obtained via SambVca with DFT optimized [Pd(NHC)(3Cl-Py)Cl2]
structures. Parameters: sphere radius, 3.5; distance from the center of the sphere, 2.10; bond radii scaled by 1.17.
a
in good agreement with the trends in the buried volumes of
tetraaryl NHC ligands, implying that sterically bulky species
would lead to fast reductive elimination, which suppresses
undesired side reactions and catalyst decomposition in aerobic
conditions. In addition to the dominant NHC ligands, the effect
of ancillary ligands was also evaluated and it was found that 3-
chloropyridine showed activities slightly higher than those of
the pyridine analogues. Probably, the more electron-deficient
ancillary ligands would favor their release from the Pd(0)
during the reaction process, therefore liberating the catalytic
species.23 In order to promote the cross-coupling reaction
further, a high palladium loading of 1 mol % was performed. To
our delight, we found that C3 was the optimal precatalyst,
providing the desired coupling product in 88% yield (entry 12,
Table 1).
This preliminary result encouraged us to optimize the
reaction conditions with C3 as precatalyst. As shown in Table
2, the bases were crucial to the efficiency of this reaction.
Among a variety of bases screened, K2CO3 displayed the
highest reactivity. K3PO4, KOAc, and Cs2CO3 were inferior,
while NaOtBu shut down the reaction completely (entries 1−5,
Table 2). The influence of the nature of the acid additives was
subsequently examined. It is noteworthy that, in the absence of
PivOH, the reaction delivered the product in poor to low yields
(entries 6−14, Table 2). Obviously, the excellent performance
of the acid additive of PivOH is consistent with a concerted
metalation−deprotonation (CMD) pathway in the catalytic
process.30 Further screening of solvents revealed that DMAc
was the best solvent for the present system. Other solvents,
such as DMF, NMP, toluene, dioxane, and xylene, were less
effective, and DMSO was ineffective. Thus, the optimal result
was obtained by using 1 mol % of C3, K2CO3 as base, and
PivOH as acid additive in DMAc at 130 °C for 12 h. Under
these reaction conditions, minor side reactions, such as
homocoupling of the aryl bromides and C-4 arylation of the
imidazole, were observed during the course of this reaction.
With the optimized protocol in hand, we next set out to
explore the substrate scope of the aryl bromides for the direct
arylation of 1-methyl-1H-imidazole (Table 3). Gratifyingly, a
wide range of functional groups on the aryl bromides, such as
chloro, cyano, ester, nitro, aldehyde, acetyl, trifluoromethyl, and
fluoro, were well tolerated, producing the corresponding
products 9aa−ah) in high to excellent yields (70−96%). It is
noteworthy that the utilization of sterically hindered 1-naphthyl
bromide afforded the desired coupling product 9aj in excellent
yield (94%). Moreover, we found that a heteroaryl bromide
such as 4-bromoisoquinoline was compatible and a satisfactory
Table 1. Screening of the Precatalysts on Direct Arylation
b
d
entry
cat.
Pd (mol %)
yield (%)
selectivity (%)
1
Pd(OAc)2
5
49
52
93
46
59
74
43
57
64
74
83
99
98
98
97
96
97
98
97
96
97
97
98
98
97
97
2
Pd-PEPPSI-IEt
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
3
Pd-PEPPSI-IPr
4
C1
C2
C3
C4
C5
C6
C1
C2
C3
C4
C5
C6
5
6
7
8
9
10
11
12
13
14
15
1
c
1
88 (84)
1
71
66
86
1
1
a
Reaction conditions: 1-methyl-1H-imidazole (2 mmol), 1-bromo-4-
chlorobenzene (1 mmol), palladium source (0.5−5 mol %), PivOH
(0.3 mmol), K2CO3 (2 mmol), DMAc (3 mL), 130 °C, 12 h, in an
aerobic atmosphere. GC yield using (trifluoromethyl)benzene as an
internal standard. Isolated yields in parentheses. Percent yield of the
C5-arylated product (there is a trace amount of C4-arylated product).
b
c
d
phenyl ring, gave a satisfactory yield of 74% (entry 6, Table 1).
Although C3 exhibited lower productivity to some extent in
comparison to commercial Pd-PEPPSI-IPr with a bulkier N-
phenyl moiety, its catalytic efficiency was also promising. This
study indicates that the shielding of the palladium center with
bulky backbones could enhance the catalytic activity, proving
the superiority of the tetraarylimidazolium Pd-PEPPSI
complexes.
Moreover, the substituents on the N-phenyl ring on the
NHC (C1−C6) were also found to play a key role in governing
the efficiencies of catalysts. As can be seen in Table 1, the
sterically demanding NHC ligands afforded higher conversion
to the desired product. For example, C1 (%VBur = 31.0%, Chart
1), with a 2-methyl substituent on the N-phenyl ring, showed
moderate efficiency and provided a GC yield of 46% (entry 4,
Table 1). In comparison, when the size of the substituent was
increased through the use of 2,6-dimethyl and 2,6-diethyl
substituents (C2 and C3), higher yields of 59−74% were
obtained (entries 5 and 6, Table 1). These observed results are
D
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Table 2. Optimization of Conditions in Direct Palladium-
Catalyzed Cross-Coupling Reactions
Table 3. Palladium-Catalyzed Direct Arylation of Imidazole
with Aryl Bromides
a
a
yield
b
selectivity
c
entry solvent
base
additives
PivOH
(%)
(%)
1
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMF
K2CO3
K3PO4
KOAc
88
74
30
27
NR
5
98
98
94
95
2
PivOH
PivOH
PivOH
PivOH
3
4
Cs2CO3
NaOtBu
K2CO3
K3PO4
KOAc
5
6
96
91
7
19
NR
32
NR
45
10
NR
73
63
77
NR
17
23
9
8
9
Cs2CO3
NaOtBu
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
96
10
11
12
13
14
15
16
17
18
19
20
HOAc
97
93
PhCOOH
CF3COOH
PivOH+TBAB
PivOH
97
97
98
NMP
PivOH
DMSO
toluene
dioxane
xylene
PivOH
PivOH
97
97
92
PivOH
PivOH
a
Conditions: 1-methyl-1H-imidazole (2.0 mmol), 1-bromo-4-chlor-
obenzene (1.0 mmol), C3 (1 mol %), additives (0.3 mmol), base (2
b
mmol), solvent (3 mL), 130 °C, 12 h, in an aerobic atmosphere. GC
c
yield using (trifluoromethyl)benzene as an internal standard. Percent
yield of the C5-arylated product (there is a trace amount of C4-
arylated product).
yield of 70% (9al) was obtained. However, introducing
electron-donating substituents on the aryl bromide lowered
the activity. For example, in the case of 3-bromoanisole, the
reaction produced 9ak in a much lower yield of 33%.
Additionally, we investigated the direct arylation between
other imidazoles and aryl bromides, such as 1,2-dimethyl-1H-
imidazole (7b). As shown in Table 3, we were pleased to find
that the cross-coupling products 9ba−bl were obtained in good
yields. A variety of aryl bromides bearing electron-withdrawing
and electron-donating substituents, as well as heteroaryl
bromides, were compatible. Moreover, more challenging aryl
chlorides such as 4-chlorobenzonitrile and 4-chlorobenzalde-
hyde were used. However, this revealed that the efficiency
decreased dramatically, affording the desired products 9ad,ae in
22% and 29% yields, respectively.
a
Conditions: imidazoles (2.0 mmol), aryl bromide (1.0 mmol), C3 (1
mol %), PivOH (0.3 mmol), K2CO3 (2 mmol), DMAc (3 mL), 130
°C, 12 h, in an aerobic atmosphere. Isolated yields are given . Isolated
yield in parentheses when aryl chloride was used as the substrate.
b
EXPERIMENTAL SECTION
■
Physical Measurements and Materials. 2-Aminotoluene, 2,6-
dimethylaniline, and 2,6-diethylaniline were purchased from Aldrich
Chemical and were distilled under reduced pressure before being used.
Palladium chloride and trimethylaluminum (2 M, hexane) was
purchased from Aldrich Chemical. Benzil, chloromethyl ethyl ether,
pyridine, 3-chloropyridine, inorganic bases, and the solvents were
purchased from Guangzhou Chemical Reagent Factory and were used
as received. 1-Methyl-1H-imidazole, 1,2-dimethyl-1H-imidazole, and
the aryl bromides were purchased from Darui Chemical Reagent
Factory. The α-diimine compound [2,6-(CH3)2C6H3)N*C(Ph)C-
(Ph)N(2,6-(CH3)2-C6H3)] (2) was prepared according to our
previous report.20 Pd-PEPPSI-IEt was synthesized according to
literature methods.23
CONCLUSION
■
In summary, a series of sterically encumbered tetraarylimida-
zolium carbene Pd-PEPPSI complexes have been conveniently
synthesized and fully characterized. These complexes have been
utilized for the direct arylation of imidazoles with aryl bromides,
which demonstrated a powerful strategy in Pd-PEPPSI catalyst
design on backbones. Remarkably, these protocols are atom
economical and general for various coupling partners in
moderate to excellent yields in aerobic conditions at a low
palladium loading.
E
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The NMR data of compounds were obtained on a Varian Mercury-
Plus 400 MHz spectrometer at ambient temperature with the
decoupled nucleus, using CDCl3 as solvent, and referenced versus
TMS as a standard. Elemental analyses were determined with a Vario
EL Series Elemental Analyzer from Elementar. The X-ray diffraction
data of single crystals were obtained with the ω−2θ scan mode on a
Bruker SMART 1000 CCD diffractometer with graphite-monochro-
mated Mo Kα radiation (λ = 0.71073 Å) at 173 K for C1, C2, and C4.
The structure was solved using direct methods, and further refinement
with full-matrix least squares on F2 was obtained with the SHELXTL
program package.31 All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were introduced in calculated positions with the
displacement factors of the host carbon atoms.
General Procedures for the Synthesis of α-Diimine
Compounds. Aniline (24 mmol) was diluted in toluene under a
nitrogen atmosphere, trimethylaluminum (12 mL, 2 M) was added
through a syringe at ambient temperature, and then the reaction
mixture was heated to 90 °C for 2 h. When the determined time was
reached, the solution was cooled and the reaction mixture was carefully
treated with benzil (2.10 g, 10 mmol). The reaction mixture was
refluxed for another 6 h. After completion of the reaction, the reaction
mixture was cooled to room temperature, hydrolyzed with 5% aqueous
NaOH solution, and then stirred for another 1 h. After stirring was
stopped for a moment, the mixture was layered, the aqueous layer was
extracted three times with ethyl acetate, and then the organic layers
were combined, dried over anhydrous magnesium sulfate, filtered, and
concentrated under vacuum. The resulting crude oil was purified by
column chromatography as bright yellow to yellow crystals.
Ar-H, 4H), 2.67-2.58 (m, CH2, 4H), 2.43-2.36 (m, CH2, 4H), 1.25 (t, J
= 7.5 Hz, CH3, 12H). 13C NMR (100 MHz, CDCl3): δ 140.1, 132.0,
131.3, 130.1, 130.0, 129.6, 128.7, 127.2, 126.7, 124.6, 24.1, 13.8. ESI-
+
MS: m/z 485.3, [L3 − Cl]+ (C35H37N2 , calcd 485.68).
General Procedures for the Synthesis of Pd-PEPPSI
Compounds. A mixture of tetraarylimidazolium salt (1.1 mmol),
palladium dichloride (0.177 g, 1 mmol), and K2CO3 (0.690 g, 10
mmol) in 3-chloropyridine or pyridine (4 mL) was stirred at 90 °C for
24 h. When the determined time was reached, the solution was cooled
to room temperature and 20 mL of dichloromethane was added, and
then the reaction mixture was placed on a short silica gel column and
washed with substantial dichloromethane. Evaporation of the filtrate
provided a yellow-brown solid. The solid was then washed and stirred
with hexane (15 mL) for 1 h. The precipitate was isolated by filtration.
The yellow solid was slowly treated with dichloromethane until it was
dissolved completely, and then a large amount of Et2O was added,
causing the formation of a white precipitate. The suspension was
filtered through a sintered funnel. Drying the solid in vacuo produced
the desired palladium complexes as white powders.
[(2-(CH3)C6H4)NC(Ph)]2CHPdCl2(3-Cl-Py) (C1). This compound was
1
obtained as a white powder (0.352 g, 51%). H NMR (400 MHz,
CDCl3): δ (ppm) 8.58 (m, Ar-H, 1H), 8.50-8.48 (m, Ar-H, 1H), 8.26
(m, Ar-H, 1H), 7.60 (m, Ar-H, 1H), 7.59 (m, Ar-H, 1H), 7.52 (m, Ar-
H, 1H), 7.40-7.34 (m, Ar-H, 3H), 7.22-6.92 (m, Ar-H, 13H), 2.28 (s,
CH3, 2 H), 1.95 (s, CH3, 4 H). 13C NMR (100 MHz, CDCl3): δ
(ppm) 150.1, 149.1, 137.6, 136.7, 136.5, 136.3, 133.2, 132.2, 131.0,
130.9, 130.8, 130.2, 130.0, 129.6, 128.6, 128.5, 128.2, 128.1, 127.6,
127.4, 126.4, 126.2, 124.3, 18.5. Anal. Calcd for C34H28Cl3N3Pd: C,
59.06; H, 4.08; N, 6.08. Found: C, 59,09; H, 4.18; N, 5.98. ESI-MS:
m/z 657.2, [L1PdCl(3-ClPy)]+ (C34H28Cl2N3Pd+, calcd 655.93);
[(2-CH3C6H4)NC(Ph)C(Ph)N(2-CH3C6H4)] (1). This compound
1
was obtained as bright yellow crystals in 86% yield. H NMR (400
+
401.0, [L1 − Cl]+ (C29H25N2 , calcd 401.52).
MHz, CDCl3): δ 7.94 (d, J = 7.6 Hz, Ar-H, 4H), 7.47-7.39 (m, Ar-H,
6H), 6.93-6.87 (m, Ar-H, 4H), 6.77 (t, J = 7.2 Hz, Ar-H, 2H), 6.49 (d,
J = 7.8 Hz, Ar-H, 2H), 1.28 (s, CH3, 6H). 13C NMR (100 MHz,
CDCl3): δ 162.5, 147.9, 138.3, 132.3, 130.9, 129.9, 128.7, 128.4, 125.5,
125.0, 116.7, 16.6.
[(2,6-(CH3)2C6H3)NC(Ph)]2CHPdCl2(3-Cl-Py) (C2). This compound
was obtained as a white powder (0.500 g, 69%). 1H NMR (400 MHz,
CDCl3): δ (ppm) 8.53-8.43 (m, Ar-H, 2H), 7.58-7.55 (m, Ar-H, 1H),
7.29-7.25 (m, Ar-H, 2H), 7.20-7.05 (m, Ar-H, 12H), 6.93-6.91 (m, Ar-
H, 3H), 2.39 (s, CH3, 12H). 13C NMR (100 MHz, CDCl3): δ (ppm)
151.4, 150.4, 149.4, 137.4, 137.1, 136.0, 133.6, 131.9, 129.7, 129.4,
128.5, 128.3, 128.2, 127.6, 124.2, 19.9. Anal. Calcd for C36H32Cl3N3Pd:
C, 60.10; H, 4.48; N, 5.84. Found: C, 60.26; H, 4.38; N, 5.71. ESI-MS:
m/z 721.2, [L2PdCl2(3-ClPy)]+ (C36H32Cl3N3Pd+, calcd 719.44);
610.0, [L2PdCl + K]+ (C31H28ClKN2Pd+, calcd 609.54); 534.9,
[(2,6-(C2H5)2C6H3)NC(Ph)C(Ph)N(2,6-(C2H5)2C6H3)] (3). This
1
compound was obtained as bright yellow crystals in 51% yield. H
NMR (400 MHz, CDCl3) δ: 8.21 (s, Ar-H, 2H), 7.53 (s, Ar-H, 2H),
7.25 (t, Ar-H, 4H), 6.94 (d, Ar-H, 7H), 6.73 (s, Ar-H, 1H), 2.63-1.78
(m, CH2, 8H), 1.07 (t, CH3, 12H). 13C NMR (100 MHz, CDCl3): δ
(ppm) 165.3, 164.5, 146.7, 146.1, 138.1, 134.8, 131.8, 131.7, 131.0,
130.8, 129.6, 128.6, 127.8, 127.6, 125.8, 125.3, 124.9, 124.1, 24.7, 24.1,
13.6, 13.0.
+
[L2Pd]+ (C31H28N2Pd+, calcd 534.99); 429.1, [L2 − Cl]+ (C31H29N2 ,
calcd 429.57).
[(2,6-(C2H5)2C6H3)NC(Ph)]2CHPdCl2(3-Cl-Py) (C3). This compound
was obtained as a white powder (0.505 g, 65%). 1H NMR (400 MHz,
CDCl3): δ (ppm) 8.50 (d, J = 2.3 Hz, Ar-H, 1H), 8.42 (m, Ar-H, 1H),
7.54 (m, Ar-H, 1H), 7.42 (t, J = 7.7 Hz, Ar-H, 2H), 7.25 (m, Ar-H,
4H), 7.19−7.12 (m, Ar-H, 2H), 7.10−7.02 (m, Ar-H, 5H), 6.94-6.76
(m, Ar-H, 4H), 3.17 (m, CH2, 4H), 2.34 (m, CH2, 4H), 1.15 (t, J = 7.5
Hz, CH3, 12H). 13C NMR (100 MHz, CDCl3): δ (ppm) 152.1, 150.3,
149.4, 141.8, 137.3, 135.0 , 133.7, 131.9, 129.9, 129.6, 128.4, 128.1,
127.7, 125.2, 124.2, 24.3, 13.1. Anal. Calcd for C40H40Cl3N3Pd: C,
61.95; H, 5.20; N, 5.42. Found: C, 62.07; H, 5.14; N, 5.28. ESI-MS:
m/z 816.8, [L3PdCl2(3-ClPy) + K]+ (C40H40Cl3KN3Pd+, calcd
814.64); 739.9, [L3PdCl(3-ClPy)]+ (C40H40Cl2N3Pd+, calcd 740.09);
700.1, [L3PdCl2 + K]+ (C35H36Cl2N2PdK+, calcd 700.10); 665.1,
[L3PdCl + K]+ (C35H36ClN2PdK+, calcd 665.64); 591.2, [L3Pd]+
General Procedures for the Synthesis of Tetraarylimidazo-
lium Salts. α-Diimine compounds and chloromethyl ethyl ether were
combined under a nitrogen atmosphere at room temperature, and then
the reaction mixture was heated to 100 °C overnight. When the
determined time was reached, the solution was cooled to room
temperature, and the reaction mixture was treated with anhydrous
Et2O and stirred for 1 h, causing the formation of a great deal of
precipitate. The solid was isolated by filtration and washed three times
with anhydrous Et2O. The resulting crude product was generally
obtained in excellent purity, and no further purification was required.
+
[(2-CH3C6H4)NC(Ph)]2CH2 Cl− (L1). This compound was obtained
as a yellowish powder (0.760 g, 87%). 1H NMR (400 MHz, CDCl3): δ
10.06 (s, C2-H, 1H), 7.98 (s, Ar-H, 2H), 7.39-7.18 (m, Ar-H, 15H),
6.60 (s, Ar-H, 1H), 2.19 (s, CH3, 6H). 13C NMR (100 MHz, CDCl3):
δ 141.0, 137.2, 133.9, 132.4, 131.0, 130.4, 129.9, 128.9 128.6, 124.8,
+
(C35H36N2Pd+, calcd 591.09); 485.3, [L3 − Cl]+ (C35H37N2 , calcd
+
17.7. ESI-MS: m/z 401.0, [L1 − Cl]+ (C29H25N2 , calcd 401.52).
485.68).
+
[(2,6-(CH3)2C6H3)NC(Ph)]2CH2 Cl− (L2). This compound was
[(2-(CH3)C6H4)NC(Ph)]2CHPdCl2(Py) (C4). This compound was
obtained as a yellowish powder (0.684 g, 73%). 1H NMR (400
MHz, CDCl3): δ 11.44 (s, C2-H, 1H), 7.33 (m, Ar-H, 4H), 7.22 (s,
Ar-H, 8H), 7.01 (m, Ar-H, 4H), 2.24 (s, CH3, 12H) 13C NMR (100
MHz, CDCl3): δ 140.1, 135.0, 131.9, 131.6, 131.0, 130.2, 129.6, 129.2,
obtained as a white powder (0.340 g, 61%). H NMR (400 MHz,
1
CDCl3): δ (ppm) 8.50 (m, Ar-H, 2H), 8.28-8.26 (m, Ar-H, 1H), 7.92
(m, Ar-H, 1H), 7.59-7.34 (m, Ar-H, 5H), 7.21-6.93 (m, Ar-H, 14H),
2.29 (s, CH3, 2H), 1.95 (s, CH3, 4H). 13C NMR (100 MHz, CDCl3):
δ (ppm) 151.2, 151.1, 137.5, 136.7, 136.5, 136.3, 133.0, 131.1, 131.0,
130.9, 130.8, 130.2, 130.0, 129.5, 128.5, 128.4, 128.2, 128.1, 127.6,
126.3, 126.2, 18.7, 18.4. Anal. Calcd for C34H29Cl2N3Pd: C, 62.16; H,
4.45; N, 6.40. Found: C, 62.01; H, 4.48; N, 6.37. ESI-MS: m/z 586.6,
[L1Pd(Py)]2+ (C34H29N3Pd+, calcd 586.03); 401.0, [L1 − Cl]+
+
128.9, 124.7, 18.3. ESI-MS: m/z 429.1, [L2 − Cl]+ (C31H29N2 , calcd
429.57).
+
[(2,6-(C2H5)2C6H3)NC(Ph)]2CH2 Cl− (L3). This compound was
obtained as a yellowish powder (0.723 g, 69%). 1H NMR (400
MHz, CDCl3): δ 11.61 (s, C2-H, 1H), 7.48 (t, J = 7.7 Hz, Ar-H, 2H),
7.34- 7.29 (m, Ar-H, 4H), 7.24-7.19 (q, Ar-H, 6H), 7.01 (d, J = 8.0 Hz,
+
(C29H25N2 , calcd 401.52).
F
DOI: 10.1021/acs.organomet.6b00391
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
[(2,6-(CH3)2C6H3)NC(Ph)]2CHPdCl2(Py) (C5). This compound was
obtained as a white powder (0.456 g, 67%). H NMR (400 MHz,
3.73 (s, CH3, 3H), 2.63 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3):
δ (ppm) 197.4, 140.1, 136.1, 134.4, 132.4, 129.4, 128.8, 127.9, 32.9
26.6.
1
CDCl3): δ (ppm) 8.46 (m, Ar-H, 2H), 7.55 (m, Ar-H, 1H), 7.30-7.24
(m, Ar-H, 2H), 7.21-7.06 (m, Ar-H, 12H), 6.92 (m, Ar-H, 4H), 2.40
(s, CH3, 12H). 13C NMR (100 MHz, CDCl3): δ (ppm) 151.4, 137.3,
137.2, 137.0, 136.1, 133.5, 129.7, 129.3, 128.4, 128.3, 128.2, 127.7,
123.9, 19.9. Anal. Calcd for C36H33Cl2N3Pd: C, 63.12; H, 4.86; N,
6.13. Found: C, 63.27; H, 4.88; N, 6.02. ESI-MS: m/z 645.5,
[L2PdCl2+K]+ (C31H28Cl2KN2Pd+, calcd 644.99); 612.8, [L2Pd-
(Py)]2+(C36H33N3Pd2+, calcd 612.09).
1-Methyl-5-(4-(trifluoromethyl)phenyl)-1H-imidazole (9ag).13b
1H NMR (400 MHz, CDCl3): δ (ppm) 7.67 (d, J = 8.1 Hz, Ar-H,
2H), 7.56−7.46 (m, Ar-H, 3H), 7.15 (s, Ar-H, 1H), 3.68 (s, CH3, 3H).
13C NMR (101 MHz, CDCl3): δ (ppm) 139.8, 133.3, 132.0, 129.9 (q,
J = 32.0 Hz), 129.0, 128.3, 125.7 (q, J = 4.0 Hz), 125.3 (q, J = 270 Hz),
32.61 (s).
5-(4-Fluorophenyl)-1-methyl-1H-imidazole (9ah).29 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.52 (s, Ar-H, 1H), 7.41−7.33 (m,
Ar-H, 2H), 7.15 (t, J = 8.7 Hz, Ar-H, 2H), 7.08 (s, Ar-H, 1H), 3.65 (s,
CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 163.8 (d, J = 255.0
Hz), 139.0, 132.4, 130.3 (d, J = 8.0 Hz), 128.1, 125.9 (d, J = 4.0 Hz),
115.8 (d, J = 26.0 Hz), 32.5.
[(2,6-(C2H5)2C6H3)NC(Ph)]2CHPdCl2(Py) (C6). This compound was
1
obtained as a white powder (0.428 g, 58%). H NMR (400 MHz,
CDCl3): δ (ppm) 8.52-8.31 (m, Ar-H, 2H), 7.59-7.49 (m, Ar-H, 1H),
7.42 (t, J = 7.7 Hz, Ar-H, 2H), 7.24 (d, J = 7.7 Hz, Ar-H, 4H), 7.16 (t,
J = 7.4 Hz, Ar-H, 2H), 7.08 (m, Ar-H, 6H), 6.89 (d, J = 7.5 Hz, Ar-H,
4H), 3.19 (m, CH2, 4H), 2.35 (m, CH2, 4H), 1.15 (t, J = 7.5 Hz, CH3,
12H). 13C NMR (100 MHz, CDCl3): δ (ppm) 153.7, 151.4, 141.9,
137.2, 135.1, 133.6, 129.9, 129.6, 128.3, 128.1, 127. 9, 125.1, 123.9,
24.3, 13.1. Anal. Calcd for C40H41Cl2N3Pd: C, 64.83; H, 5.58; N, 5.67.
Found: C, 64.98; H, 5.61; N, 5.63. ESI-MS: m/z 706.7, [L3PdCl-
(Py)]+ (C40H41ClN3Pd+, calcd 705.65); 485.3, [L3 − Cl]+
1-Methyl-5-phenyl-1H-imidazole (9ai).14d 1H NMR (400 MHz,
CDCl3): δ (ppm) 7.52 (s, Ar-H, 1H), 7.47−7.34 (m, Ar-H, 5H), 7.11
(s, Ar-H, 1H), 3.66 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ
(ppm) 138.9, 133.4, 129.7, 128.7, 128.4, 127.9, 127.4, 32.5.
1-Methyl-5-(naphthalen-1-yl)-1H-imidazole (9aj).33 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.97−7.89 (m, Ar-H, 2H), 7.65 (d, J
= 8.1 Hz, Ar-H, 2H), 7.57−7.42 (m, Ar-H, 4H), 7.16 (s, Ar-H, 1H),
3.42 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 138.5,
133.7, 132.9, 131.1, 129.4, 129.2, 129.0, 128.4, 127.2, 126.7, 126.2,
125.5, 125.2, 32.0.
+
(C35H37N2 , calcd 485.68).
General Procedure for Direct Arylation Promoted by
Palladium Complexes. Unless otherwise noted, the direct C−H
activation arylation reactions were carried out under aerobic
conditions. The reaction temperatures are reported as the temperature
of the heating vessel unless otherwise stated. All solvents were used as
received, and no further purification was needed. A parallel reactor
containing a stirring bar was charged with Pd-PEPPSI complexes (0.01
mmol), 1-methyl-1H-imidazole or 1,2-dimethyl-1H-imidazole (2.0
mmol), aryl bromide (1.0 mmol), base (2 mmol), acid additive (0.3
mmol), and 3 mL of solvent. The reaction was carried out at 130 °C
for 12 h. After completion of the reaction, the reaction mixture was
cooled to ambient temperature and 20 mL of water was added. The
mixture was diluted with dichloromethane (5 mL), followed by
extraction three times (3 × 5 mL) with dichloromethane. The organic
layer was dried with anhydrous magnesium sulfate, filtered, and
evaporated under reduced pressure. The crude cross-coupling
products were purified by silica gel column chromatography using
petroleum ether/dichloromethane (20/1) as eluent. The isolated
cross-coupling products were characterized by 1H NMR and 13C
NMR, and the spectra can be found in the Supporting Information.
5-(4-Chlorophenyl)-1-methyl-1H-imidazole (9aa).8a 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.52 (s, Ar-H, 1H), 7.41 (d, J = 8.6
Hz, Ar-H, 2H), 7.32 (d, J = 8.6 Hz, Ar-H, 2H), 7.10 (s, Ar-H, 1H),
3.65 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm)139.4,
133.9, 132.3, 129.6, 129.0, 128.5, 128.3, 32.5.
5-(3-Methoxyphenyl)-1-methyl-1H-imidazole (9ak).34 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.49 (s, Ar-H, 1H), 7.33 (t, J = 8.8 Hz,
Ar-H, 1H), 7.09 (s, Ar-H, 1H), 6.99−6.87 (m, Ar-H, 3H), 3.82 (s,
OCH3, 3H), 3.66 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ
(ppm) 159.6, 138.9, 130.9, 129.7, 127.8, 120.7, 114.2, 113.2, 55.2, 32.5.
5-(1-Methyl-1H-imidazol-5-yl)isoquinoline (9al).34 1H NMR (400
MHz, CDCl3): δ (ppm) 9.29 (s, Ar-H, 1H), 8.47 (s, Ar-H, 1H), 8.05
(d, J = 8.0 Hz, Ar-H, 1H), 7.74−7.62 (m, Ar-H, 4H), 7.20 (d, J = 0.9
Hz, Ar-H, 1H), 3.46 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ
(ppm) 153.3, 144.4, 139.2, 135.3, 131.3, 130.4, 128.0, 127.7, 124.4,
121.0, 32.1.
5-(4-Chlorophenyl)-1,2-dimethyl-1H-imidazole (9ba).14c 1H
NMR (400 MHz, CDCl3): δ (ppm) 7.38 (d, J = 8.7 Hz, Ar-H, 2H),
7.27 (d, J = 6.3 Hz, Ar-H, 2H), 6.94 (s, Ar-H, 1H), 3.50 (s, CH3, 3H),
2.43 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 146.3,
133.6, 132.3, 129.7, 129.0, 128.9, 126.2, 31.3, 13.7.
Methyl 4-(1,2-Dimethyl-1H-imidazol-5-yl)benzoate (9bb).34 1H
NMR (400 MHz, CDCl3): δ (ppm) 8.17−8.02 (m, Ar-H, 2H), 7.50−
7.38 (m, Ar-H, 2H), 7.04 (s, Ar-H, 1H), 3.93 (s, CH3, 3H), 3.56 (s,
CH3, 3H), 2.45 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm)
166.7, 147.1, 135.0, 132.6, 130.0, 128.9, 127.9, 127.1, 52.2, 31.6, 13.7.
1,2-Dimethyl-5-(4-nitrophenyl)-1H-imidazole (9bc).32 1H NMR
(400 MHz, CDCl3): δ (ppm) 8.29 (d, J = 7.9 Hz, Ar-H, 2H), 7.53 (d, J
= 8.5 Hz, Ar-H, 2H), 7.13 (s, Ar-H, 1H), 3.61 (s, CH3, 3H), 2.48 (s,
CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 148.0, 146.6,
136.5, 131.5, 128.4, 124.2, 124.1, 31.6, 14.0.
Methyl 4-(1-Methyl-1H-imidazol-5-yl)benzoate (9ab).5b 1H NMR
(400 MHz, CDCl3): δ (ppm) 8.08 (d, J = 6.8 Hz, Ar-H, 2H), 7.56 (s,
Ar-H, 1H), 7.46 (d, J = 6.9 Hz, Ar-H, 2H), 7.19 (s, Ar-H, 1H), 3.92 (s,
CH3, 3H), 3.70 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ
(ppm)166.6 140.0, 134.2, 132.5, 130.0, 129.2, 129.1, 127.8, 52.2, 32.8.
1-Methyl-5-(4-nitrophenyl)-1H-imidazole (9ac).32 1H NMR (400
MHz, CDCl3): δ (ppm) 8.27 (d, J = 8.8 Hz, Ar-H, 2H), 7.56 (d, J =
8.8 Hz, Ar-H, 3H), 7.24 (s, Ar-H, 1H), 3.74 (s, CH3, 3H). 13C NMR
(101 MHz, CDCl3): δ (ppm) 146.7, 140.8, 136.1, 131.5, 130.5, 128.7,
124.3, 33.3.
4-(1,2-Dimethyl-1H-imidazol-5-yl)benzonitrile (9bd).14c 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.71 (d, J = 8.6 Hz, Ar-H, 2H), 7.47 (d, J
= 8.6 Hz, Ar-H, 2H), 7.07 (s, Ar-H, 1H), 3.58 (s, CH3, 3H), 2.47 (s,
CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 147.8, 135.1,
132.5, 131.9, 128.3, 127.9, 118.7, 110.9, 31.8, 13.6.
4-(1,2-Dimethyl-1H-imidazol-5-yl)benzaldehyde (9be).14c 1H
NMR (400 MHz, CDCl3): δ (ppm) 10.01 (s, CHO, 1H), 7.97−
7.86 (m, Ar-H, 2H), 7.58−7.47 (m, Ar-H, 2H), 7.07 (d, J = 2.4 Hz, Ar-
H, 1H), 3.58 (s, CH3, 3H), 2.45 (s, CH3, 3H). 13C NMR (101 MHz,
CDCl3): δ (ppm) 191.5, 147.5, 136.4, 135.0, 132.4, 130.1, 128.2,
127.6, 31.7, 13.7.
4-(1-Methyl-1H-imidazol-5-yl)benzonitrile (9ad).14c 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.72 (d, J = 9.4 Hz, Ar-H, 2H), 7.57
(s, Ar-H, 1H), 7.52 (d, J = 8.1 Hz, Ar-H, 2H), 7.21 (s, Ar-H, 1H), 3.72
(s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 140.6, 134.4
132.6, 129.8, 128.3, 126.2, 118.5, 111.3, 32.9.
4-(1-Methyl-1H-imidazol-5-yl)benzaldehyde (9ae).14d 1H NMR
(400 MHz, CDCl3): δ (ppm) 10.02 (s, Ar-H, 1H), 7.92 (d, J = 10.0
Hz, Ar-H, 2H), 7.56 (d, J = 8.3 Hz, Ar-H, 3H), 7.22 (s, Ar-H, 1H),
3.73 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm) 191.8,
140.4, 135.8, 135.3, 132.3, 130.2, 129.8, 128.2, 33.1.
1-(4-(1,2-Dimethyl-1H-imidazol-5-yl)phenyl)ethanone (9bf).14c
1H NMR (400 MHz, CDCl3): δ (ppm) 7.98 (d, J = 8.6 Hz, Ar-H,
2H), 7.43 (d, J = 8.5 Hz, Ar-H, 2H), 7.02 (s, Ar-H, 1H), 3.55 (s, CH3,
3H), 2.59 (s, CH3, 3H), 2.43 (s, CH3, 3H). 13C NMR (101 MHz,
CDCl3): δ (ppm) 197.5, 147.2, 135.8, 135.2, 132.6, 128.8, 128.0,
127.2, 31.7, 26.6, 13.7.
1-(4-(1-Methyl-1H-imidazol-5-yl)phenyl)ethanone (9af).14d 1H
NMR (400 MHz, CDCl3): δ (ppm) 8.02 (d, J = 7.2 Hz, Ar-H, 2H),
7.56 (s, Ar-H, 1H), 7.51 (d, J = 8.5 Hz, Ar-H, 2H), 7.21 (s, Ar-H, 1H),
1,2-Dimethyl-5-(4-(trifluoromethyl)phenyl)-1H-imidazole
(9bg).34 1H NMR (400 MHz, CDCl3): δ (ppm) 7.67 (d, J = 8.2 Hz,
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Ar-H, 2H), 7.47 (d, J = 8.1 Hz, Ar-H, 2H), 7.02 (s, Ar-H, 1H), 3.55 (s,
CH3, 3H), 2.46 (s, CH3, 3H). 13C NMR (101 MHz, CDCl3): δ (ppm)
147.0, 134.1, 132.2, 129.9 (q, J = 33.0 Hz), 128.4, 128.1 (q, J = 271.0
Hz), 127.1, 125.7 (q, J = 4.0 Hz), 31.5, 13.7.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00391.
■
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Details of NMR spectra of all α-diimines, N-hetero-
carbene salts, palladium complexes, and cross-coupling
products (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for Z.K.: kezhf3@mail.sysu.edu.cn.
*E-mail for F.-S.L.: fengshou2004@126.com.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Foundation for Distinguished
Young Talents in Higher Education of Guangdong (no.
Yq2013101), the NSFC (Grants 21203256 and 21473261),
the Guangdong Natural Science Funds for Distinguished
Young Scholar (no. 2015A030306027), and the Project of
innovation for enhancing Guangdong Pharmaceutical Univer-
sity, provincial experimental teaching demonstration center of
chemistry & chemical engineering. Computing facilities were
supported in part by the Guangdong Province Key Laboratory
of Computational Science, the Joint Funds of NSFC-
Guangdong for Supercomputing Applications, and the National
Supercomputing Center in Guangzhou. We also acknowledge
Dr. Hui Huang’s helpful discussion.
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