Bulky α-diimine palladium complexes: Highly efficient for direct C-H bond arylation of heteroarenes under aerobic conditions

Article abstract of DOI:10.1039/c6dt02544g

Through the strategy to enhance the bulkiness on both the backbone and the N-aryl moieties, we designed and synthesized a type of bulky α-diimine palladium complex (i.e., {[Ar-NC(R)-C(R)N-Ar]PdCl₂, (Ar = 2-benzhydryl-4,6-dimethylphenyl)}, C1, R = H; C2, R = An; C3, R = Ph). The structures of these palladium complexes were well characterized, while C1 and C3 were further characterized by X-ray diffraction. The catalytic performances of the precatalysts were screened for direct C-H bond arylation of heteroarenes. The bidentate N,N-palladium complex C3 with both a backbone and N-aryl bulkiness was found to be a highly efficient precatalyst under aerobic conditions. With a low palladium loading of 0.5-0.1 mol%, a variety of heteroarenes with challenging bulky steric aryl bromides as well as heteroaryl bromides are all applicable for this cross-coupling reaction.

Full text of DOI:10.1039/c6dt02544g

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Bulky α-diimine palladium complexes: highly
efficient for direct C–H bond arylation of
heteroarenes under aerobic conditions†
Cite this: DOI: 10.1039/c6dt02544g
Jia-Sheng Ouyang,a Yan-Fang Li,a Dong-Sheng Shen,a Zhuofeng Ke*b and
Feng-Shou Liu*a
Through the strategy to enhance the bulkiness on both the backbone and the N-aryl moieties, we
designed and synthesized a type of bulky α-diimine palladium complex (i.e., {[Ar–NvC(R)–C(R)vN–Ar]
PdCl2, (Ar = 2-benzhydryl-4,6-dimethylphenyl)}, C1, R = H; C2, R = An; C3, R = Ph). The structures of
these palladium complexes were well characterized, while C1 and C3 were further characterized by X-ray
diffraction. The catalytic performances of the precatalysts were screened for direct C–H bond arylation of
heteroarenes. The bidentate N,N-palladium complex C3 with both a backbone and N-aryl bulkiness was
found to be a highly efficient precatalyst under aerobic conditions. With a low palladium loading of
0.5–0.1 mol%, a variety of heteroarenes with challenging bulky steric aryl bromides as well as heteroaryl
bromides are all applicable for this cross-coupling reaction.
Received 25th June 2016,
Accepted 4th August 2016
DOI: 10.1039/c6dt02544g
www.rsc.org/dalton
tedious preparation procedures and toxic reagents. From the
viewpoint of economy and environmental friendliness, the
Introduction
Arylated heteroarenes are important structural motifs widely development of phosphine-free ligands is highly desired, as
found in many natural products, pharmaceuticals, and they would provide a more efficient and straightforward access
functional materials.1 Consequently, much effort has been to bi(hetero)arenes.6 In this context, nitrogen-based ligands,
made in the development of more efficient and versatile such as 1,10-phenanthroline,7 2,2′-bipyridyl8 and others,9 were
methods for the construction of arylated heteroarenes.2 Since found to catalyze the direct C–H arylation of heteroarenes with
the pioneering work of Ohta on direct arylation of indoles with aryl halides to yield regioselective products. Very recently,
chloropyrazines,3 the palladium-catalyzed direct C–H bond Wu’s group successfully established the N,O-based bis(alkoxo)
arylation of heteroarenes has become an alternative approach palladium-catalyzed direct arylation of thiophenes.10 Although
to traditional cross-coupling, as there is no need for the pre- remarkable advances have been achieved, some fundamental
paration of organometallic reagents avoiding the generation of challenges still remain. The cleavage of the C–H bond is
stoichiometric metal wastes.4 In the past few decades, great thermodynamically unfavorable due to its high bond-
efforts have been devoted to ligand design to improve the dissociation energy.11 Generally, the catalytic process was per-
palladium-catalyzed reaction efficiency.5,6
formed in the temperature range of 100–150 °C, while the pal-
Previously reported ligands for the direct arylation of ladium catalytic species tend to decompose under these
heteroarenes are mostly electron-rich and sterically bulky conditions. To ensure a satisfactory yield of the desired
phosphine compounds, which turned out to be very effective product, a loading of 5–10 mol% palladium as well as an air
toward aryl halides to install a variety of arylated hetero- and moisture-free environment were required.6 Obviously, the
arenes.5 However, these phosphine-based ligands suffer from high cost of these catalytic systems and inconvenient manipu-
lation greatly impeded their use in large-scale applications.
Furthermore, although such an approach is attractive, the
aSchool of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
University, Zhongshan, Guangdong, 528458, China. E-mail: fengshou2004@126.com
bSchool of Materials Science and Engineering, MOE Key Laboratory of Bioinorganic
and Synthetic Chemistry, Sun Yat-sen University, Guangzhou 510275, China.
E-mail: kezhf3@mail.sysu.edu.cn
cross-coupling reaction of sterically hindered aryl/heteroaryl
bromides is another challenge to overcome (Chart 1).
To achieve a highly efficient phosphine-free palladium cata-
lyst system under aerobic reaction conditions with low palla-
dium loading, we have recently reported a type of nitrogen-
based α-hydroxyimine palladium complex.12 We found that the
key to ensure excellent catalytic activities is to incorporate
†Electronic supplementary information (ESI) available. CCDC 1486526 and
1486527. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6dt02544g
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Chart 1 The design of the nitrogen-based palladium precatalyst.
ortho substituents on the N-aryl moiety which positions the
steric bulk at the axial sites lying above and below the coordi-
nation plane. Moreover, the backbone of the ligand also played
an important role, as its bulky steric hindrance derived from
the backbone could retard the CAr–N rotation further.13
Considering the nature of the oxygen in the hydroxyl group
that has been coordinated to the palladium in
α-hydroxyimines, one side of the coordination plane is less
protected. Hence, we envisioned that bulkier α-diimine
ligands bearing two N-aryl moieties in the structural motifs
would better shield the axial site than that of the N,O-based
palladium complexes.14 Moreover, the steric properties of
these ligands can be easily tuned with appropriate substituents.
Realizing that the α-diimine ligands can enable new
opportunities in cross-coupling15 and olefin poly-
merization,13,16 as part of our ongoing research interest in
direct arylation using phosphine-free ligands, we report herein
the use of bulky α-diimine compounds as ligands to evaluate
the reactivity in the cross-coupling of a variety of heteroarenes
with aryl bromides under aerobic conditions.
Scheme 1 Synthetic route for the α-diimine palladium complexes.
Results and discussion
Synthesis and characterization of α-diimine palladium
complexes
Fig. 1 Molecular structure of C1·DMF depicted with 30% thermal ellip-
soids. Hydrogen atoms and DMF solvent have been omitted for clarity.
Selected bond distances (Å) and angles (°): Pd(1)–N(1) 2.0198(17), Pd(1)–
N(2) 2.0278(16), Pd(1)–Cl(1) 2.2635(5), Pd(1)–Cl(2) 2.2778(5), N(1)–Pd(1)–
N(2) 79.70(7), N(1)–Pd(1)–Cl(1) 94.25(5), N(2)–Pd(1)–Cl(1) 173.55(5),
N(1)–Pd(1)–Cl(2) 174.11(5), N(2)–Pd(1)–Cl(2) 94.95(5), Cl(1)–Pd(1)–Cl(2)
91.18(2).
The α-diimine ligands in this study were synthesized using a
similar method reported previously,17 and the corresponding
α-diimine palladium complexes (Scheme 1) were readily pre-
pared by the reaction of the ligands with palladium dichloride
in methanol. These complexes are air and moisture stable, and
can be stored on the shelf in solid form for more than six
months without noticeable decomposition in NMR determi-
nation. These complexes are well soluble in CH2Cl2, CHCl3,
DMF and DMAc, while they are poorly soluble in hexane, exhibits a slightly distorted square-planar coordination geo-
MeOH and EtOH. Their chemical structures were characterized metry. The 2-benzhydryl groups on N-aryl moieties were located
by NMR spectroscopy and elemental analysis. Although the as the anti-form lying above and below the coordination plane.
potential existence of isomers was derived from the unsym- Bearing the bulky steric substituents, the N-aryl moieties are
metric nature of the N-aryl moieties, the NMR analysis demon- nearly perpendicular to the five-membered chelate ring with a
strated one sole isomer in solution at ambient temperature.
dihedral angle of 83.96 and 77.64°, respectively, which
To further reveal the structures of these complexes, a single suggests that the axial sites of the palladium center would be
crystal of complex C1, suitable for X-ray crystallographic blocked tightly. Nevertheless, the Pd–N and Pd–Cl bond dis-
analysis, was grown from methanol and DMF, while a single tances as well as N(1)–Pd–N(2) bond angles in C1 are in
crystal of C3 was grown from hexane and dichloromethane accordance with the values of other α-diimine palladium com-
solution. ORTEP diagrams are given in Fig. 1 and 2, respec- plexes.18 In contrast, with the diphenyl backbone framework,
tively, along with selected bond lengths and angles. In the the bond lengths of Pd–N(1) and Pd–N(2) in C3 are 2.033(2)
solid structure, complex C1 with the backbone of hydrogen and 2.044(2) Å, respectively, which are longer than those of C1
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Table 1 Screening of palladium complexes for the direct arylation
reaction of benzo[b]thiophene with 1-bromo-2-methylnaphthalenea
Fig. 2 Molecular structure of C3·2CH2Cl2 depicted with 30% thermal
ellipsoids. Hydrogen atoms and CH2Cl2 solvent have been omitted for
clarity. Selected bond distances (Å) and angles (°): Pd(1)–N(1) 2.033(2),
Pd(1)–N(2) 2.044(2), Pd(1)–Cl(1) 2.2682(7), Pd(1)–Cl(2) 2.2738(7), N(1)–Pd
(1)–N(2) 79.79(8), N(1)–Pd(1)–Cl(1) 95.07(6), N(2)–Pd(1)–Cl(1) 174.47(6),
N(1)–Pd(1)–Cl(2) 175.00(6), N(2)–Pd(1)–Cl(2) 97.13(6), Cl(1)–Pd(1)–Cl(2)
88.14(3).
a Reaction conditions: benzo[b]thiophene (2 mmol), 1-bromo-2-methyl-
naphthalene (1 mmol), palladium complex (0.005 mmol), PivOH
(0.3 mmol), K2CO3 (2 mmol) and DMAc (3 mL) in an aerobic environ-
ment. b Cross-coupling product determined by GC-FID using (trifluoro-
methyl)benzene as an internal standard. Cross-coupling was
performed in two parallel reactions and the yield is given in average.
c Isolated yields in parentheses.
(2.020(2) and 2.028(2) Å, respectively), indicating a more con-
strained structure. Moreover, the dihedral angles between the
N-aryl moieties and the chelate ring turned out to be 84.95
and 78.44°, respectively. Therefore, axial steric hindrance
around the palladium center could further hinder the rotation
of the CAr–N bond, which will play a significant role in the
catalytic performances.13,19
lytic properties was further evaluated and N,N-based palladium
complexes bearing 2,6-diisopropyls were further tested for
comparison. It was shown that IPr-Pd with two hydrogen
atoms and IPrAn-Pd with acenaphthyl as backbones gave 3a in
a much lower product yield of 42% and 57%, respectively.
Moreover, our previously reported bulky N,O-Pd afforded a
satisfactory yield of 88% under the present reaction conditions.12
Nevertheless, it was also inferior to that of C3. Obviously, the
bulky steric hindrance of 2-benzhydryl-4,6-dimethyl on N-aryl
The palladium-catalyzed direct C–H arylation reaction under
aerobic conditions
The direct C–H arylation between sterically hindered 1-bromo- moieties played a profound role in the excellent catalytic per-
2-methylnaphthalene and benzo[b]thiophene was studied as formance of C3, which is consistent with the reductive elimin-
the model reaction. Initially, all the palladium complexes ation process as the rate-determining step in the direct C–H
C1–C3 were screened under the same reaction conditions. The arylation.12,20
cross-coupling reactions were performed in DMAc under
Having identified the most effective precatalyst, the optim-
aerobic conditions at 130 °C for 12 h, in the presence of ization of the reaction conditions was investigated. First, the
0.5 mol% palladium loading with K2CO3 as the base and reaction was conducted in the absence of PivOH and the
PivOH (pivalic acid) as the acid additive. As shown in Table 1, product 3a was obtained in only 56% GC yield (Table 1, run 1),
the precatalysts with 2-benzhydryl-4,6-dimethyl on N-aryl moi- thus confirming the important role of the acid additive in the
eties showed superior activities. C1 with two hydrogen atoms direct C–H arylation reaction. We next performed a systematic
on the backbone gave the desired product 3a in 78% GC yield, screening of the bases and a significant acceleration was
while a higher yield of 84% was observed when acenaphthyl observed on introducing the carboxylate salts into the reaction.
was used as the backbone. To our delight, bearing a bulky K2CO3 was proved to be the most efficient and gave the desired
diphenyl backbone, C3 was identified as the most effective pre- product in 98% GC yield. It is noteworthy that only KOPiv
catalyst with almost quantitative GC yield (98%), and a satis- worked well and gave a satisfactory yield of 96%. Moreover,
factory isolated yield of 93% was obtained. As expected, our KOPiv associated with PivOH was also investigated, and it
designed N,N-based ligand with more steric hindrance on the afforded a yield of 97% (run 3, Table 2). Other carboxylate
backbone could retard the rotation of the CAr–N bonds, enhan- bases such as Na2CO3, NaHCO3, KOAc, and NaOAc were also
cing the steric hindrance of the ortho-substituents on N-aryl efficient and moderate to high yields were obtained, whereas
moieties to block the axial positions of the palladium center, other bases provided low yields for this transformation. The
and therefore suppressing the related catalytic decomposition indispensability of PivOH and the key role of the carboxylate
at high temperatures under aerobic reaction conditions. base suggest a concerted metalation–deprotonation (CMD)
Besides the backbone, the effect of the N-aryl moieties on cata- pathway involved in the direct C–H arylation.11,21 Moreover,
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Table 2 Screening of reaction conditions for the direct arylation reac-
tion of benzo[b]thiophene with 1-bromo-2-methylnaphthalenea
their meta- and para-substituted analogues in previous
studies,22 were successfully coupled with benzo[b]thiophene to
give the heterocyclic product 3a, 3b, and 3e in 90–96% yields
under the optimal reaction conditions. Moreover, it was found
that the cross-couplings with the aryl bromide partners
bearing electron-withdrawing, -neutral and -donating groups
and bulky groups could all take place efficiently to afford the
corresponding products in good to excellent yields (3c–3f,
Table 3). For example, substrates with an electron-withdrawing
group such as 4-bromobenzonitrile afforded 3f in a yield of
87%, while the substrate with an electron-donating group,
4-bromoanisole, afforded 3c in 71% yield. To further evaluate
the potential breadth of the substrate scope, a variety of five-
membered heteroarenes bearing C-2 electron-donating and
electron-withdrawing functional groups were also successfully
coupled with bulky aryl bromides in excellent yields (4a–13b,
Table 3). Heteroarenes, such as 2-methylthiophene,
2-ethylthiophene, 2-chlorothiophene, thiophene-2-carbonitrile,
Run
Pd loading
Solvent
Base
T (°C)
Yieldb (%)
1c
2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMF
K2CO3
K2CO3
PivOK
PivOK
KOAc
Na2CO3
NaHCO3
NaOAc
KF
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
100
110
120
130
56
98
97
96
67
76
82
65
36
49
39
31
35
33
96
79
33
11
NR
NR
38
55
83
61
3
4c
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
K3PO4
LiOH
KOH
NatOBu
KtOBu
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
thiophene-2-carbaldehyde,
imidazo[1,2-a]pyridine,
2,4-di-
methylthiazole, 2-methyl-4-phenylthiazole and 4-methyl-
thiazole gave the desired arylated products 4a–11b in 83–99%
yields under the optimized conditions. To our delight, the
NMP
Toluene
Xylene
Dioxane
DMSO
DMAc
DMAc
DMAc
DMAc
Table 3 The palladium-catalyzed direct arylation of heteroarenes with
aryl bromidesa
a Reaction conditions: benzo[b]thiophene (2 mmol), 1-bromo-2-methyl-
naphthalene (1 mmol), palladium complex C3 (0.005 or 0.001 mmol),
PivOH (0.3 mmol), K2CO3 (2 mmol) and DMAc (3 mL) in an aerobic
environment. b Cross-coupling product determined by GC-FID using
(trifluoromethyl)benzene as an internal standard. Cross-coupling was
performed in two parallel reactions and the yield is given in average.
c In the absence of PivOH.
a profound influence of the solvent has been found, with
toluene and xylene affording poor yields. Moreover, it was
observed that DMF and NMP gave 3a in moderate to excellent
yields, whereas DMAc afforded the best yield (Table 2). Other
reaction conditions were tested further. However, the decrease
in palladium loading to 0.1 mol% or lowering the reaction
temperature to 100 °C all impeded the efficiency. Thus, the
optimized reaction conditions were obtained by using a
mixture of benzo[b]thiophene (2.0 equiv.), 1-bromo-2-methyl-
naphthalene (1.0 equiv.), C3 as the precatalyst, a palladium
loading of 0.5 mol%, K2CO3 as a base and PivOH (30 mol%) as
an acid additive and DMAc as a solvent in an aerobic
environment.
Having established the optimal reaction conditions, the
coupling reactions between benzo[b]thiophene and a variety of
aryl bromides were screened. It is noteworthy that the bulky
hindrance of the electrophile substrates showed little effect on
the transformation. The ortho-substituted aryl bromides such
as 1-bromo-2-methylbenzene, 1-bromonaphthalene, and
1-bromo-2-methylnaphthalene, which are challenging coup-
ling partners and generally afforded much lower yields than
a Reaction conditions: heteroarene (2 mmol), aryl bromide (1 mmol),
palladium complex C3 (0.5 mol%), PivOH (0.3 mmol), K2CO3 (2 mmol)
and DMAc (3 mL), 130 °C for 12 h in an aerobic environment. Cross-
coupling was performed in two parallel reactions and the isolated
yields are given in average.
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much less reactive imidazoles23 such as 1-methyl-1H-imidazole bromides, such as p-cyano, methoxy, tert-butyl and
and 1,2-dimethyl-1H-imidazole also successfully coupled to ortho-methyl were well tolerated, leading to excellent yields.
provide 12b and 13b in nearly quantitative yield. It seems that
Biheteroaryls are well known as common substructures in
the electronic nature and steric hindrance of the aryl natural and biologically active compounds.24 The coupling of
heteroaryl bromides with heteroarenes remains quite an
important but challenging task due to the low reactivity of sub-
Table 4 The palladium-catalyzed direct arylation of heteroarenes with
heteroaryl bromidesa
strates and the poisoning of palladium species by hetero-
atoms.20 Encouraged by the above results using highly
sterically congested aryl bromides as the substrates, we then
expanded it to reactions between heteroarenes (such as thio-
phenes, thiazole, imidazole, and imidazo[1,2-a]pyridine) and
heteroaryl bromides under the optimal conditions. As can be
seen from Table 4, the reaction proceeded in excellent yields
in spite of the electronic nature of the heteroaryl bromides.
Electron-poor and electron-rich aromatic systems underwent
direct arylation in excellent yields irrespective of the substi-
tution pattern. The functional groups, such as pyridyl, pyrimi-
dine, isoquinoline and thiophene, were well tolerated. As a
result, a variety of desired biheteroarenes 16a–28c were
obtained. Inspired by the outstanding performance of the pre-
catalyst C3, we tested the possibility of a low palladium
loading of 0.1 mol% further. To our delight, it was found that
both electron-donating and -withdrawing substituents on the
five-membered heteroarenes were compatible in these reac-
tions to provide the respective products in high to excellent
yields (Table 4).
Recently, the conjugated poly(hetero)arenes have been
extensively investigated for the fabrication of functional
materials, such as optoelectronic and electrochromic
devices.25 The highly efficient palladium complexes prompt us
to explore the potential application in di-arylation reactions of
heteroarenes. As can be seen in Table 4, C3 was found to be
useful for carrying out the di-arylation reaction. For example,
3,4-ethylenedioxythiophene (EDOT) reacts with 3 equiv. of
3-bromopyridine and 4-bromoisoquinoline to give the 2,5-ary-
lated product of 29a and 29b in 98 and 97% yield, respectively.
Moreover, we attempted another strategy of the diheteroaryla-
tion of heteroarenes with dibromoarene. It is a pleasure that
3,5-dibromopyridine successfully coupled with 4-methyl-
thiazole and 1-methyl-1H-imidazole, affording the regio-
selective products 30 and 31 in excellent yields of 98 and 96%,
respectively. Significantly, this straightforward and selective
diarylation reaction provides a potential efficient method for
the direct arylation polymerization for the construction of
functional materials.26
Experimental
General procedures
a Reaction conditions: heteroarene (2 mmol), aryl bromide (1 mmol),
palladium complex C3 (0.5 mol%), PivOH (0.3 mmol), K2CO3 (2 mmol)
and DMAc (3 mL), 130 °C for 12 h in an aerobic environment. Cross-
2-Benzhydryl-4,6-dimethylaniline was prepared according to
coupling was performed in two parallel reactions and the isolated
previous reports.27 Heteroarenes and aryl bromides were pur-
chased from Darui Chemical Reagent Factory. Oxalaldehyde
aqueous (40%), acenaphthenequinone, benzil, and palladium
chloride were purchased from Beijing HWEK Chem. Co., Ltd.
Inorganic bases and solvents were purchased from Guangzhou
yields are given in average. b The reaction was performed at
a
palladium loading of 0.1 mol%. c Reaction conditions: 3,4-ethylene-
dioxythiophene (1 mmol), aryl bromide (3 mmol), other reaction
conditions were the same as those in footnote ‘a’. d Reaction con-
ditions: heteroarene (3 mmol), 3,5-dibromopyridine (1 mmol), other
reaction conditions were the same as those in footnote ‘a’.
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Chemical Reagent Factory and were used as received. IPr-Pd28a,b the crude material was purified by column chromatography
and IPrAn-Pd29 compounds were synthesized according to (hexane/EtOAc = 20/1) as the desired product.
literature methods.
Bis[N,N-(2-benzhyldryl-4,6-dimethyl)imino]acenaphthene
The NMR spectra were recorded on a Bruker DMX 400 MHz (L2) was obtained as orange crystals (3.0 g, 85% yield):
instrument at ambient temperature with the decoupled 1H NMR (400 MHz, CDCl3) δ 7.60 (d, J = 8.2 Hz, Ar–H, 2H),
nucleus, using TMS as an internal standard and CDCl3 as a 7.30–7.23 (m, Ar–H, 4H), 7.22–7.17 (m, Ar–H, 2H), 7.14–6.99
solvent. Elemental analysis was performed using a Flash (m, Ar–H, 7H), 6.88 (d, J = 7.3 Hz, Ar–H, 4H), 6.72 (s, Ar–H,
EA1112 microanalyzer. The X-ray diffraction data of single crys- 2H), 6.34 (dd, J = 16.1, 7.6 Hz, Ar–H, 6H), 6.11 (t, J = 7.4 Hz,
tals were collected with the ω–2θ scan mode on a Bruker Ar–H, 2H), 5.74 (s, Ar–H, 1H), 5.32 (s, Ar2C–H, 2H), 2.35 (s,
SMART 1000 CCD diffractometer with graphite-monochro- CH3, 6H), 2.28 (s, CH3, 6H); 13C NMR (101 MHz, CDCl3)
mated Mo Kα radiation (λ = 0.71073 Å) at 173 K for C1 and C3. δ 162.9, 146.4, 143.6, 141.7, 139.8, 133.0, 132.7, 129.8, 129.6,
Cell parameters were obtained by global refinement of the 129.5, 129.2, 128.9, 128.0, 128.0, 127.7, 127.1, 126.9, 125.9,
positions of all the collected reflections. The intensities were 125.0, 124.8, 122.4, 52.5, 21.2, 17.8. Anal. Calcd for: C54H44N2:
corrected for Lorentz and polarization effects and empirical C, 89.96; H, 6.15; N, 3.89. Found: C, 89.85; H, 6.21; N, 3.82.
absorption. The structures were solved by direct methods and
N,N-(1,2-Diphenylethane-1,2-diylidene)bis(2-benzhydryl-4,6-
refined by full-matrix least squares on F2. All hydrogen atoms dimthylaniline) (L3) was obtained as light yellow crystals
were placed in the calculated positions. Structure solution and (3.0 g, 82% yield): 1H NMR (400 MHz, CDCl3) δ 7.06 (t, J = 72.4
refinement were carried out by using the SHELXL-97 package. Hz, Ar–H, 22H), 6.78 (d, J = 30.6 Hz, Ar–H, 8H), 6.44 (d, J = 58.4
All non-hydrogen atoms were refined anisotropically. Hz, Ar–H, 2H), 5.55 (d, J = 19.2 Hz, Ar–H, 2H), 5.30 (s, Ar2C–H,
Hydrogen atoms were introduced in the calculated positions 2H), 2.14 (s, CH3, 6H), 1.90 (s, CH3, 6H); 13C NMR (101 MHz,
with the displacement factors of the host carbon atoms. CCDC CDCl3) δ 166.6, 144.9, 144.1, 143.1, 135.5, 135.2, 132.3, 131.6,
1486526 (C1) and 1486527 (C3) contain the supplementary 129.9, 129.5, 129.4, 128.9, 128.7, 128.0, 127.6, 126.0, 125.9,
crystallographic data for this paper.
125.0, 124.8, 51.4, 21.0, 18.9. Anal. Calcd for: C56H48N2:
C, 89.80; H, 6.46; N, 3.74. Found: C, 89.74; H, 6.35; N, 3.68.
The syntheses of N,N-bis-(2-benzhydryl-4,6-dimethyl)
ethanediimine (L1)
General procedures for the synthesis of α-diimine Pd
compounds
2-Benzhydryl-4,6-dimethylaniline (10 mmol) and CH3COOH
(4 mL) were added in ethanol (30 mL). To this solution, 40 wt%
glyoxal in H2O (5 mmol) was added slowly, while the yellow
suspension emerged immediately. The mixture was stirred for
8 h at 80 °C. When reaching the schedule time, the reaction
was stopped and the precipitation was filtered. The residue
was recrystallized in dichloromethane/ethanol (15 mL/20 mL)
to give a light yellow solid (2.4 g, 80% yield): 1H NMR
(400 MHz, CDCl3) δ 7.44 (s, Ar–H, 2H), 7.26 (t, J = 7.3 Hz, Ar–H,
8H), 7.20 (t, J = 6.9 Hz, Ar–H, 4H), 7.04 (d, J = 7.5 Hz, Ar–H,
8H), 6.97 (s, Ar–H, 2H), 6.58 (s, Ar–H, 2H), 5.47 (s, Ar2C–H,
2H), 2.24 (s, CH3, 6H), 2.03 (s, CH3, 6H); 13C NMR (101 MHz,
CDCl3) δ 164.1, 147.2, 143.5, 133.7, 133.4, 129.8, 129.5, 128.2,
128.0, 126.2, 125.0, 51.8, 21.0, 18.3. Anal. Calcd for: C44H40N2:
C, 88.55; H, 6.76; N, 4.69. Found: C, 88.34; H, 6.80; N, 4.61.
The α-diimine ligand (1.0 mol) and palladium dichloride
(0.177 g, 1.0 mmol) were mixed in 10 mL of methanol at room
temperature. After the reaction mixture was heated to reflux
overnight, the methanol was removed under reduced pressure.
The residue was dissolved in 5 mL of dichloromethane, and
20 mL hexane was added. The precipitate of the palladium
complex was collected by filtration and washed with hexane.
Drying in vacuum produced the desired palladium complex.
{N,N-Bis-(2-benzhydryl-4,6-dimethyl)ethanediimine}dichloro-
palladium C1 was obtained as an orange solid (0.69 g,
90% yield): 1H NMR (400 MHz, CDCl3) δ 7.28 (t, J = 5.0 Hz,
Ar–H, 6H), 7.23 (d, J = 7.3 Hz, Ar–H, 8H), 7.19 (t, J = 6.8 Hz,
Ar–H, 2H), 7.12 (d, J = 7.4 Hz, Ar–H, 4H), 7.02 (s, Ar–H, 2H),
6.55 (s, Ar–H, 2H), 6.41 (s, Ar–H, 2H), 6.20 (s, Ar2C–H, 2H), 2.34
(s, CH3, 6H), 2.23 (s, CH3, 6H); 13C NMR (101 MHz, CDCl3)
δ 166.9, 143.1, 142.8, 141.8, 138.5, 136.4, 130.0, 129.8, 129.7,
129.0, 128.7, 128.3, 128.2, 127.0, 126.7, 52.6, 21.4, 18.7. Anal.
Calcd for: C44H40Cl2N2Pd: C, 68.27; H, 5.21; N, 3.62. Found: C,
General procedures for the synthesis of α-diimine compounds
L2 and L3
2-Benzhydryl-4,6-dimethylaniline (12 mmol) was dissolved in 68.05; H, 5.14; N, 3.52.
30 mL of anhydrous toluene under a nitrogen atmosphere, tri-
{Bis[N,N-(2-benzhyldryl-4,6-dimethyl)imino]acenaphthene}
methylaluminum (5 mL, 1.0 M in hexane) was added slowly dichloropalladium C2 was obtained as an orange solid (0.79 g,
through a syringe at room temperature and the reaction 88% yield): 1H NMR (400 MHz, CDCl3) δ 7.84 (d, J = 8.3 Hz, Ar–H,
mixture was heated to reflux for 2 h. After cooling to room 2H), 7.33 (d, J = 7.5 Hz, Ar–H, 4H), 7.25 (dd, J = 16.6, 8.4 Hz,
temperature, the 1,2-diketone (5 mmol) was added, and then Ar–H, 8H), 7.11 (d, J = 8.3 Hz, Ar–H, 6H), 6.76 (s, Ar–H, 2H),
the mixture was stirred for 8 h at 110 °C under a nitrogen 6.63 (s, Ar–H, 2H), 6.38 (t, J = 7.4 Hz, Ar–H, 4H), 6.07 (t, J = 7.4 Hz,
atmosphere. When the determined time was reached, the reac- Ar–H, 2H), 5.32 (s, Ar2C–H, 2H), 2.54 (s, CH3, 6H), 2.34 (s,
tion mixture was carefully hydrolyzed with 5% iced aqueous CH3, 6H); 13C NMR (101 MHz, CDCl3) δ 176.9, 146.2, 142.2,
NaOH solution. The organic product was extracted with 140.9, 140.2, 138.2, 137.7, 131.4, 130.0, 129.9, 129.8, 129.6,
toluene and dried over MgSO4 and the solvent was evaporated, 129.3, 128.8, 128.0, 127.9, 127.7, 126.4, 125.3, 125.1, 123.7,
Dalton Trans.
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52.8, 21.6, 18.4. Anal. Calcd for: C54H44Cl2N2Pd: C, 72.20; H, were crucial to promote the efficiency of the cross-coupling
4.94; N, 3.12. Found: C, 71.98; H, 4.83; N, 3.17. reaction. Under the optimized conditions, an array of sterically
{N,N-(1,2-Diphenylethane-1,2-diylidene)bis(2-benzhydryl- hindered as well as heteroaryl bromides have been successfully
4,6-dimthylaniline)}dichloropalladium C3 was obtained as an coupled with various heteroarenes at a 0.5–0.1 mol% palla-
orange solid (0.80 g, 87% yield): 1H NMR (400 MHz, CDCl3) dium loading. The results highlight a new type of nitrogen-
δ 8.01 (dd, J = 13.9, 8.3 Hz, Ar–H, 3H), 7.85 (d, J = 7.3 Hz, Ar–H, based phosphine-free catalytic system to enhance catalytic
1H), 7.71 (d, J = 6.1 Hz, Ar–H, 5H), 7.48 (dd, J = 16.9, 9.0 Hz, activities under mild reaction conditions.
Ar–H, 3H), 7.28–7.19 (m, Ar–H, 11H), 7.16 (d, J = 8.1 Hz, Ar–H,
3H), 7.17–7.06 (m, Ar–H, 4H), 6.66 (d, J = 7.2 Hz, Ar–H, 3H),
6.56 (d, J = 7.2 Hz, Ar–H, 1H), 5.30 (s, Ar2C–H, 2H), 2.52 (s,
Acknowledgements
CH3, 6H), 2.41 (s, CH3, 6H); 13C NMR (101 MHz, CDCl3)
δ 175.0, 146.6, 139.6, 138.6, 137.9, 134.8, 131.3, 131.0, 130.0, This work was supported by the Foundation for Distinguished
129.7, 129.6, 129.1, 129.0, 128.5, 128.0, 127.3, 125.2, 124.7, Young Talents in Higher Education of Guangdong (no.
60.2, 21.3, 19.1. Anal. Calcd for: C56H48Cl2N2Pd: C, 72.61; Yq2013101), the NSFC (Grants 21203256 and 21473261), the
H, 5.22; N, 3.02. Found: C, 72.49; H, 5.28; N, 2.96.
Guangdong Natural Science Funds for Distinguished Young
Scholar (no. 2015A030306027) and the Project of innovation
for enhancing Guangdong Pharmaceutical University, provin-
cial experimental teaching demonstration center of chemistry
& chemical engineering. We also thank Dr Hui Huang for
helpful discussion.
General procedure for direct arylation promoted by palladium
complexes
Unless otherwise noted, the direct C–H arylation reactions
were carried out under aerobic conditions. The reaction temp-
eratures 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 contain-
Notes and references
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a
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standard stock solution of DMAc), heteroarene (2.0 mmol),
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solvent. The reaction was carried out at 130 °C for 12 h. After
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characterized by H NMR and 13C NMR, and the spectra can
be found in the ESI.†
Conclusions
We have developed a type of bulky α-diimine palladium
complex for direct C–H arylation under aerobic conditions.
The catalytic performances of the precatalysts were screened. It
was found that both the bulky backbone and the N-aryl moiety
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