New Nickel-Based Catalytic System with Pincer Pyrrole-Functionalized N-Heterocyclic Carbene as Ligand for Suzuki-Miyaura Cross-Coupling Reactions

Article abstract of DOI:10.1016/j.jorganchem.2021.122068

A new catalytic system with Ni(NO₃)₂·6H₂O as the catalyst and a pincer pyrrole-functionalized N-heterocyclic carbene as the ligand was employed in the Suzuki-Miyaura cross-coupling reactions of aryl iodides with arylboronic acids. With 5 mol% catalyst, the catalytic reactions proceeded at 160 °C, giving coupling products in isolated yields of up to 94% in short reaction times (1-4 h). The system worked efficiently with aryl iodides bearing electron-donating or electron-withdrawing groups and arylboronic acids with electron-donating groups. Steric effects were observed for both aryl iodides and arylboronic acids. It is proposed that the reactions underwent a Ni(I)/Ni(III) catalytic cycle.

Full text of DOI:10.1016/j.jorganchem.2021.122068

Journal of Organometallic Chemistry 953 (2021) 122068
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
New Nickel-Based Catalytic System with Pincer Pyrrole-Functionalized
N-Heterocyclic Carbene as Ligand for Suzuki-Miyaura Cross-Coupling
Reactions
Zhifo Guo, Xiangyang Lei∗
Department of Chemistry and Biochemistry, Lamar University, Beaumont, TX, 77710, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A
new catalytic system with Ni(NO3)2·6H2O as the catalyst and a pincer pyrrole-functionalized N-
Received 1 August 2021
Revised 24 August 2021
Accepted 27 August 2021
Available online 1 September 2021
heterocyclic carbene as the ligand was employed in the Suzuki-Miyaura cross-coupling reactions of aryl
iodides with arylboronic acids. With 5 mol% catalyst, the catalytic reactions proceeded at 160 °C, giving
coupling products in isolated yields of up to 94% in short reaction times (1-4 h). The system worked effi-
ciently with aryl iodides bearing electron-donating or electron-withdrawing groups and arylboronic acids
with electron-donating groups. Steric effects were observed for both aryl iodides and arylboronic acids. It
is proposed that the reactions underwent a Ni(I)/Ni(III) catalytic cycle.
Keywords:
N-heterocyclic carbene
pincer ligand
© 2021 Elsevier B.V. All rights reserved.
nickel catalysis
Suzuki-Miyaura cross-coupling
Introduction
with a pincer pyrrole-functionalized NHC (L1 in Scheme 1), which
can be formed in situ from bis(imidazolium) (Pre-L1) in the pres-
Transition metal-catalyzed cross-coupling reactions have be-
come one of the most important methods for the formation
of carbon-carbon and carbon-heteroatom bonds. Recently, nickel
catalysis has been applied in a wide variety of cross-coupling re-
actions, including Suzuki-Miyaura, Kumada, Hiyama, Negishi, and
Stille couplings [1–4]. Among them, the Suzuki- Miyaura coupling
has become one of the most effective reactions for the construction
of carbon−carbon bonds [5–12]. Although the Ni-catalyzed Suzuki
cross-coupling reactions have been greatly promoted, there are
still limitations on the scope of substrates and relevant functional
groups [13]. Development of new ligands might provide solutions
to overcome these limitations. N-heterocyclic carbenes (NHCs),
which are electron-rich and behave like typical σ-donor ligands,
have found broad applications in coordination chemistry and ho-
mogeneous catalysis [14,15]. Particularly, the introduction of NHCs
in the second-generation Grubbs catalysts has boosted ruthenium-
catalyzed metathesis and related reactions [16–20]. Multiple ex-
amples have also been reported on their application as ligands in
nickel catalysis [21–24]. More recently, nickel-based catalytic sys-
tems with pincer ligands have attracted significant attention as ef-
ficient catalysts for cross-coupling reactions [25–28]. Herein, we
reported Ni(II)-catalyzed Suzuki-Miyaura cross-coupling reactions
ence of base, as the ligand. To our best knowledge, this is the first
employment of a CNC-type pincer ligand with two NHC donors
and one anionic N-donor bridge in nickel catalysis.
Experimental
Materials and methods
Pre-ligand Pre-L1 was prepared according to literature proce-
dures [29]. Bases were ground into powder and dried under vac-
uum at 125°C for 4 h prior to use. Toluene was refluxed over
Na/benzophenone and distilled under nitrogen. All other reagents
and solvents were purchased from commercial suppliers and used
as received. All glasswares were dried in an oven at 120°C for
8 h and cooled under nitrogen atmosphere in a desiccator be-
fore use. An Agilent 6890N Network Gas Chromatograph, equipped
with a capillary column (HP-5) and a TCD detector, was used to as-
say the reaction yields for the optimization of reaction conditions.
Column chromatography was performed with 300–400 mesh sil-
ica gel. NMR spectra were recorded on a Bruker Avance 400 MHz
spectrometer in CDCl3 and referenced to the residual solvent sig-
nals (1H: 7.26 ppm; 13C: 77.16 ppm). Peaks were characterized as
singlet (s), doublet (d), triplet (t), quartet (q), doublet of doublet
(dd), and multiplet (m). Chemical shifts were expressed in ppm
and J values were given in Hz.
∗
Corresponding author.
E-mail address: xlei@lamar.edu (X. Lei).
https://doi.org/10.1016/j.jorganchem.2021.122068
0022-328X/© 2021 Elsevier B.V. All rights reserved.

Z. Guo and X. Lei
Journal of Organometallic Chemistry 953 (2021) 122068
Table 1
Optimization of reaction conditionsa.
Entry
Ligand
Catalyst
Base
Time (h)
Yield (%)b
1
L1
L2
L3
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(Ph3P)2Cl2
Ni(OAc)2
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
Cs2CO3
CsF
4
4
4
4
4
4
4
4
4
1
1
1
1
2
2
99
5
2
3
4
4
42
57
98
92
94
65c
99
11
30
67
81d
3e
5
6
NiI2
7
NiBr2
8
NiCl2·6H2O
9
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
Ni(NO3)2·6H2O
10
11
12
13
14
15
K2CO3
K3PO4
K3PO4
a
Reaction conditions: 1a (1.0 mmol), 2a (2.0 mmol), Ni(II) (0.05 mmol), ligand (0.05 mmol; Pre-L1 was used to form L1 in situ), base (3.0 mmol), toluene (3 mL), 160 °C.
b
c
GC yield using hexadecane as the internal standard.
2a (1.2 mmol).
d
e
1-Bromo-4-methoxybenzene.
1-Chloro-4-methoxybenzene.
the same reaction conditions, ligand L1 is much more efficient
than L2 and L3 (Table 1, entries 1-3). Among the six nickel (II)
salts that were screened, Ni(Ph3P)2Cl2 and Ni(OAc)2 gave moder-
ate yields (entries 4 and 5), and all other four gave excellent yields
(>90%, entries 1 and 6-8). Salt Ni(NO3)2·6H2O was chosen for fur-
ther study due to its highest yield and lower cost. A decrease of
the amount of 2a from 2 eq. to 1.2 eq. resulted in much lower
yield (entries 1 and 9). Using TLC to monitor the reaction, it was
found that the same reaction was complete in 1 h, resulting in a
shorter reaction time (entry 10 vs. entry 1). The studies on the ef-
fect of base on the coupling reaction showed that K3PO4 was the
most efficient base (entry 10) while other bases such as Cs2CO3,
CsF, and K2CO3 gave poor to moderate yields (entries 11-13). The
efficiency of the base in the reaction is determined by multiple
factors including its basicity, its solubility in the solvent, and the
affinity of its metal cation for the iodide ion. The decrease in yields
using Cs2CO3, CsF, or K2CO3 as base is probably due to one or
more of these factors. Furthermore, the results showed that un-
der the same reaction conditions, the reactivity of aryl halides in
this coupling reaction follows the sequence of iodides > bromides
>> chlorides (entries 10, 14 and 15), which is consistent with the
recently reported results that were obtained with a PCP-type Ni(II)
pincer complex [25]. The higher reactivity of this catalytic system
towards aryl iodides than aryl chlorides is probably due to its dif-
ferent catalytic cycle from those we previously reported [31–33].
Finally, the optimal reaction conditions for the coupling reaction
were set as entry 10 in Table 1.
Scheme 1. In situ formation of pincer NHC ligand (L1) from bis(imidazolium) (Pre-
L1)
General procedure for the Suzuki-Miyaura cross-coupling reactions
Aryl iodide (1.0 mmol), arylboronic acid (2.0 mmol),
Ni(NO3)2·6H2O (0.05 mmol), pre-ligand Pre-L1 (0.05 mmol),
base (3.0 mmol), and toluene (3 mL) were added to a glass tube,
which was then sealed with a PTFE cap. After the reaction mixture
was stirred vigorously at room temperature for 5 min, the sealed
glass tube with the reaction mixture was placed in a Radleys
Carousel 12 Plus Reaction Station, which was preheated to the
described temperature. After the reaction was stirred for the
required time and then cooled down to room temperature, water
(10 mL) was added to the reaction mixture. The resulting mixture
was extracted with ethyl acetate (3 × 10 mL). The combined
organic layers were dried over anhydrous Na2SO4, filtered and
concentrated to dryness. The remaining residue was analyzed by
GC or purified by flash chromatography on silica gel with ethyl
acetate-hexanes (0-20% ethyl acetate in hexanes).
Results and Discussion
Scope of substrates
Optimization of reaction conditions
With the optimal reaction conditions, we investigated the scope
of aryl iodides by examining the coupling of phenylboronic acid
(2a) with a variety of aryl iodides (1). As shown in Table 2,
substrates with electron-donating groups such as methoxy and
methyl groups gave excellent yields (entries 1-3), while substrates
with electron-withdrawing groups such as chloro, fluoro, trifluo-
romethyl, ester, and acyl groups gave moderate to good yields (en-
tries 6-16). Steric effects were observed as ortho-substituted io-
dides generally gave lower yields than their para-substituted iso-
mers (entries 1 and 5, 2 and 4, respectively). This reaction also
As shown in Scheme 1, ligand L1 can be generated in situ
by deprotonation of the corresponding bis(imidazolium) (Pre-L1).
Pre-L1 was prepared according to literature procedures [29]. With
the coupling of 4-iodoanisole (1a) with phenylboronic acid (2a)
as the model reaction (Table 1), we first compared the efficiency
of L1 with 2,2’-bipyridine (L2) and 1,10-phenanthroline (L3), both
of which are commercially available and often used as ligand in
Ni-catalyzed coupling reactions [30]. The results show that under
2

Z. Guo and X. Lei
Journal of Organometallic Chemistry 953 (2021) 122068
Table 2
Scope of aryl iodidesa.
Entry
Ar-I
Product
Time (h)
Yield (%)b
1
p-MeO-Ph-I (1a)
p-Me-Ph-I (1b)
p-MeO-Ph-Ph (3a)
p-Me-Ph-Ph (3b)
1
2
2
2
2
2
2
2
2
2
2
2
4
2
4
4
2
2
2
1
2
4
2
94
90
90
30
48
84
70
84
78
32
88
50
76
68
88
66
71c
38c
54c
50
41d
22d
21c
2
3
m-Me-Ph-I (1c)
m-Me-Ph-Ph (3c)
4
o-Me-Ph-I (1d)
o-Me-Ph-Ph (3d)
5
o-MeO-Ph-I (1e)
p-Cl-Ph-I (1f)
o-MeO-Ph-Ph (3e)
p-Cl-Ph-Ph (3f)
6
7
p-F-Ph-I (1g)
p-F-Ph-Ph (3g)
8
p-CF3-Ph-I (1h)
p-CF3-Ph-Ph (3h)
9
m-CF3-Ph-I (1i)
m-CF3-Ph-Ph (3i)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
o-CF3-Ph-I (1j)
o-CF3-Ph-Ph (3j)
p-EtOOC-Ph-I (1k)
o-MeOOC-Ph-I (1l)
p-OHC-Ph-I (1m)
p-MeOC-Ph-I (1n)
p-PhOC-Ph-I (1o)
o-PhOC-Ph-I (1p)
4-Iodopyridine (1q)
3-Iodopyridine (1r)
2-Iodopyridine (1s)
p-Ph-Ph-I (1t)
p-EtOOC-Ph-Ph (3k)
o-MeOOC-Ph-Ph (3l)
p-OHC-Ph-Ph (3m)
p-MeOC-Ph-Ph (3n)
p-PhOC-Ph-Ph (3o)
o-PhOC-Ph-Ph (3p)
Pyridine-4-Ph-p-Me (3q)
Pyridine-3-Ph-p-Me (3r)
Pyridine-2-Ph-p-Me (3s)
p-Ph-Ph-Ph (3t)
m-I-Ph-I (1u)
m-Ph-Ph-Ph (3u)
2,6-Bis(I-CH2)pyridine (1v)
m-MeOOC-Ph-CH2I (1w)
2,6-Bis(Ph-CH2)pyridine (3v)
m-MeOOC-Ph-CH2-Ph-p-Me (3w)
a
Reaction conditions: 1 (1.0 mmol), 2a (2.0 mmol), Ni(NO3)2·6H2O (0.05 mmol), Pre-L1 (0.05 mmol),
K3PO4 (3.0 mmol), toluene (3 mL), 160 °C, Ph = C6H5 or C6H4 where appropriate.
b
Isolated yields.
c
p-MePhB(OH)2 (2.0 mmol).
d
2a (4.0 mmol), Ni(NO3)2·6H2O (1.0 mmol), Pre-L1 (1.0 mmol), K3PO4 (6.0 mmol), toluene (6 mL).
Table 3
Scope of arylboronic acidsa.
Entry
Ar’B(OH)2
Product
Time (h)
Yield (%)b
1
p-MeO-PhB(OH)2 (2b)
p-Me-PhB(OH)2 (2c)
p-MeO-Ph-Ph-p-OMe (3x)
p-Me-Ph-Ph-p-OMe (3y)
1
1
2
2
1
1
3
3
4
4
4
4
4
92
92
84
45
92
87
51
50
5
2
3
m-Me-PhB(OH)2 (2d)
o-Me-PhB(OH)2 (2e)
m-Me-Ph-Ph-p-OMe (3z)
4
o-Me-Ph-Ph-p-OMe (3a’)
5
1-B(OH)2-naphthalene (2f)
2-B(OH)2-naphthalene (2g)
p-Cl-PhB(OH)2 (2h)
1-(p-MeO-Ph)-naphthalene (3b’)
2-(p-MeO-Ph)-naphthalene (3c’)
p-Cl-Ph-Ph-p-OMe (3d’)
6
7
8
p-F-PhB(OH)2 (2i)
p-F-Ph-Ph-p-OMe (3e’)
9
p-MeOOC-PhB(OH)2 (2j)
m-MeOOC-PhB(OH)2 (2k)
2-B(OH)2-furan (2l)
p-MeOOC-Ph-Ph-p-OMe (3f’)
m-MeOOC-Ph-Ph-p-OMe (3g’)
2-(p-MeO-Ph)-furan (3h’)
2-(p-MeO-Ph)-benzofuran (3i’)
(E)-1-Methoxy-4-styrylbenzene (3j’)
10
11
12
13
8
21
23
17
2-B(OH)2-benzofuran (2m)
(E)-Styrylboronic acid (2n)
a
Reaction conditions: 1a (1.0 mmol), 2 (2.0 mmol), Ni(NO3)2·6H2O (0.05 mmol), Pre-L1 (0.05 mmol),
K3PO4 (3.0 mmol), toluene (3 mL), 160 °C, Ph = C6H5 or C6H4 where appropriate.
b
Isolated yields.
works for heteroaromatic iodides (entries 17-19), giving moderate
yields. The formation of p-terphenyl (3t) and m-terphenyl (3u) in-
dicates that this catalytic protocol is efficient for the preparation
of polyaryls (entries 20 and 21). Pleasingly, the coupling reaction
occurred on benzylic sp3-hybridized carbons as well, though the
yields were low (entries 22 and 23).
arylboronic acids and resulted in low to moderate yields (entries 7-
10). Steric effect was observed as the yields followed the sequence
of para > meta > ortho (entries 2-4). This protocol also works for
heteroarylboronic acids (entries 11 and 12) and a vinylboronic acid
(entry 13) with low yet acceptable yields.
Next, we examined the scope of arylboronic acids. As shown
in Table 3, phenylboronic acids bearing electron-donating groups
(entries 1-3) and naphthylboronic acids (entries 5 and 6) success-
fully coupled with 4-iodoanisole (1a) in good to excellent yields.
The presence of electron-withdrawing groups generally deactivates
Reaction mechanism
A plausible reaction mechanism is proposed in Fig. 1. Similar to
the Suzuki-Miyaura cross-coupling reactions catalyzed by a Ni(II)
PCP-type pincer complex [25] and a Ni(II) PNP-type pincer com-
3

Z. Guo and X. Lei
Journal of Organometallic Chemistry 953 (2021) 122068
Fig. 1. Proposed reaction mechanism
plex, [26] respectively, it is believed that the reaction system re-
ported here follows a Ni(I)/Ni(III) catalytic cycle. First, in the pres-
ence of base under heating, bis(imidazolium) Pre-L1 reacts via lig-
and L1 with Ni(NO3)2 to form a CNC-type bis(carbene)-containing
pincer complex (4), which displays a square-planar geometry about
nickel [34]. The conversion of complex 4 to the catalytically active
Ni(I) species (5) is achieved by a single-electron reduction with
arylboronic acid (2) as the reductant. Oxidative addition of aryl
iodide (1) to Ni(I) species 5 gives intermediate 6, which under-
goes transmetalation with arylboronic acid (2) in the presence of
base to produce intermediate 7. The reductive elimination of inter-
mediate 7 regenerates active Ni(I) species 5 so that the catalytic
cycle continues. It is proposed that both the oxidative addition
and transmetalation steps also undergo a single-electron pathway.
The fact that this reaction system gives best yields for aryl iodides
(Table 1, entries 10, 14 and 15) supports a Ni(I)/Ni(III) catalytic cy-
cle because generally, a Ni(0)/Ni(II) catalytic cycle favors aryl chlo-
rides [31–33].
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgement
We thank the Robert A. Welch Foundation for financial support
of this research.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2021.
122068.
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