Nickel(ii) N-heterocyclic carbene complexes as efficient catalysts for the Suzuki—Miyaura reaction

Article abstract of DOI:10.1007/s11172-020-2818-3

Catalytic activity of nickel(ii) and palladium(ii) N-heterocyclic carbene (NHC) complexes derived from imidazole, benzimidazole, and 1,2,4-triazole was comparatively evaluated in the cross-coupling reactions of aryl halides with arylboronic acids. Readily available nickel bis-NHC complexes (NHC)₂NiX₂ (X = Cl, Br, or I) exhibited the activity comparable to that of the structurally related palladium complexes and, consequently, can be applied as efficient catalysts for the Suzuki—Miyaura reaction.

Full text of DOI:10.1007/s11172-020-2818-3

Russian Chemical Bulletin, International Edition, Vol. 69, No. 4, pp. 683—690, April, 2020
683
Nickel(II) N-heterocyclic carbene complexes as efficient catalysts
for the Suzuki—Miyaura reaction*
S. B. Soliev, A. V. Astakhov, D. V. Pasyukov, and V. M. Chernyshev
M. I. Platov South-Russian State Polytechnic University (NPI),
32 ul. Prosveshcheniya, 346428 Novocherkassk, Russian Federation.
E-mail: chern13@yandex.ru
1
Catalytic activity of nickel(II) and palladium(II) N-heterocyclic carbene (NHC) complexes
derived from imidazole, benzimidazole, and 1,2,4-triazole was comparatively evaluated in the
cross-coupling reactions of aryl halides with arylboronic acids. Readily available nickel bis-NHC
complexes (NHC) NiX (X = Cl, Br, or I) exhibited the activity comparable to that of the
2
2
structurally related palladium complexes and, consequently, can be applied as efficient catalysts
for the Suzuki—Miyaura reaction.
Key words: nickel, palladium, N-heterocyclic carbenes, catalysis, Suzuki—Miyaura reaction.
After forty years since the publication of first results,1,2
the Suzuki—Miyaura reaction has become not only an
important and powerful tool for the academic research
used to form the carbon—carbon bonds3 but also found
application in industry for the manufacturing of drugs,
pesticides, and functional materials.7 The most studied
—6
,8
and widely used catalysts for this reaction are palladium
and its complexes with various ligands;3
—6,9—11
however,
over recent years a considerable attention was paid to more
available nickel compounds.3
,12—15
Nickel N-heterocyclic
carbene (Ni/NHC) complexes are considered as promis-
ing catalysts.1
6—20
However, the catalytic activity of nickel
mono-NHC complexes, viz. compounds 1 (Fig. 1), and
Ni pincer complexes3
,12—20
in the Suzuki—Miyaura reac-
tions have been mainly explored. Although some nickel
pincer complexes show a relatively high catalytic activity
2
1—27
even at the catalyst loadings of ~0.1—1 mol.%.
The
synthesis of such complexes is often a multi-step process
and the corresponding ligands are hardly available.
Nickel bis-NHC complexes 2 of the general formula
(
NHC) NiX (X = Cl, Br, or I) (Fig. 1) is of special inter-
2 2
est as the catalysts. Complexes 2 can be synthesized in one
step by heating nickel salts with readily available azolium
salts (the NHC proligands).2
8—30
However, despite the
availability of nickel bis-NHC complexes 2, their catalytic
activity in the Suzuki—Miyaura reactions is still insuffi-
ciently explored. To the best of our knowledge, there are
only two publications on the cross-couplings of aryl halides
Compounds 2 and 3b,c
2a,d,g
2b,f
R
Me
Bn
Mes
Bu
X
I
Br
Cl
Br
Cl
with arylboronic acids catalyzed by (NHC) NiX com-
2
2
3
1,32
31
plexes.
Louie and co-workers have shown only the
2
2
2
c, 3b
e,h, 3c
i
Bn
*
Based on the materials of the International Conference "Catalysis
and Organic Synthesis" (ICCOS-2019) (September 15—20, 2019,
Moscow, Russia).
Fig. 1. Ni/NHC and Pd/NHC complexes used in the present
work as the catalysts for the Suzuki—Miyaura reaction.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 0683—0690, April, 2020.
066-5285/20/6904-0683 © 2020 Springer Science+Business Media LLC
1

6
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Soliev et al.
principal possibility to catalyze the cross-coupling reac-
tions using nickel bis-NHC complexes with a relatively
high loading (10 mol.%). Bernhammer and Huynh3 have
described the complexes with the alkylthio groups at the
NHC nitrogen atoms. These alkylthio groups are capable
of chelating nickel and thus can significantly affect the
catalytic activity of the complex.32 In addition, the cross-
coupling reactions were studied only in the presence of
triphenylphosphine as a promoter.
In the present work, we studied the possibility to
employ nickel bis-NHC complexes 2a—i as the catalysts
for the Suzuki—Miyaura reaction and compared the
catalytic activities of compounds 2a—i, nickel(II) mono-
NHC complexes 1a—c containing labile coligands, and
widely used palladium(II) compounds 3a—c.
Table 1. Catalytic activity of Ni and Pd compounds in the cross-
coupling reaction of 4-bromoanisole (4a) with phenylboronic
a
2
acid (5a)
b
Sc (%)
Run
Catalyst
C (%)
Yield (%)
6a
6b
7a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1a
1b
1c
2a
2b
2c
2d
2e
2f
2g
2h
2i
10
32
18
45
44
32
52
58
34
37
35
23
45
1
—
63
61
76
80
88
90
93
91
76
77
78
76
—
—d —d —d
20
11
34
35
28
47
54
31
28
27
18
34
13
10
19
19
16
17
17
16
17
19
17
19
12
17
11
19
14
15
14
13
19
17
15
11
11
11
13
18
15
0
1
2
3
4
5
Structures of the studied Ni/NHC (1a—c and 2a—i)
and Pd/NHC (3a—c) complexes is shown in Fig. 1.
Complexes 1a—c, 2a,d—i, and 3a—c were synthesized
2a
Ni(OAc)2
3a
following the earlier described procedures.3
0,33—38
Com-
d
d
—
—
plex 2b was synthesized by heating 1,3-dibenzyl-1H-imid-
100
100
100
160
89
97
92
91
89
97
92
55
18
15
17
19
azolium bromide with anhydrous Ni(OAc) in a melt of
16
17
18
3b
3c
2
tetrabutylammonium bromide, and compound 2c was
obtained by heating 1,3-bis(2,4,6-trimethylphenyl)-1H-
imidazolium chloride with the nickel pyridine complex
Pd(OAc)2
a
Reaction conditions: aryl bromide 4a (0.25 mmol), PhB(OH)2
(5a) (0.35 mmol), KOH (0.35 mmol), catalyst (2.5 mol, 1 mol.%),
Ni(Py )Cl in pyridine in the presence of NaH.
2
2
Cross-coupling of 4-bromoanisole 4a with phenyl-
boronic acid 5a was chosen as a model reaction for the
comparative evaluation of the catalytic activity of nickel
and palladium complexes (Scheme 1 and Table 1).
aqueous dioxane (2 mL, 1% of H O, v/v), 80 C, 3 h.
2
b
Conversion of compound 4a.
c
Selectivity for the formation of compound 6a.
d
Trace amounts.
Scheme 1
nickel acetate was almost inactive under the reaction
conditions used, while palladium acetate was very active.
These data may indicate significant differences in the
mechanisms of catalysis by Ni/NHC and Pd/NHC com-
plexes in the Suzuki—Miyaura reaction. According to the
published data, not only the molecular Pd complexes with
various ligands but also the NHC-free palladium species,
viz., the products of decomposition of the Pd complexes
(
nanoparticles, clusters, etc.), can efficiently catalyze the
4
,39
Suzuki—Miyaura reaction.
In the Pd/NHC catalytic
systems, decomposition of the complex via the Pd—NHC
bond cleavage can lead to the generation of the ligandless
palladium species, whose activity is comparable to that of
1
0,37,40—46
the molecular Pd/NHC complexes.
The cata-
lytic inertness of Ni(OAc)2 along with the significant
activity of Pd(OAc) under the identical reaction condi-
2
According to the data listed in Table 1, complex 2e is
the most active and selective catalyst among the studied
nickel complexes. A comparison of the activity of nickel
and palladium catalysts revealed several significant differ-
ences in the structure—catalytic activity relationship. First,
activity of bis-nickel NHC complexes 2a—i is higher than
that of mono-NHC complexes 1a—c, while palladium
bis-NHC (3a) and mono-NHC (3b,c) complexes show
similar activity under the reaction conditions used. Second,
tions (see Table 1) allows us to suggest that the molecular
catalysis by the Ni/NHC complexes plays the key role in
the Ni-catalyzed Suzuki—Miyaura reaction.
However, it should be noted that both the Pd/NHC
and Ni/NHC complexes decompose during the catalytic
reaction. Electrospray ionization high-resolution mass
spectrometry (ESI-MS) of the reaction mixtures obtained
by cross-couplings of 4a with 5a catalyzed by complexes
2e and 3a revealed the same decomposition products for

(
NHC) NiX catalysts for the Suzuki reaction
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
685
2
2
Apparently, main decomposition routes for the Ni/NHC
and Pd/NHC complexes during the catalytic reactions are
similar and include reactions (1)—(4) (Scheme 2). Cation
+
[
8a] could be a result of decomposition of complexes via
3
7,42,44
30,37,45,47
the H—NHC coupling
Scheme 2, reactions (1) and (2), respectively). The pres-
and protolysis
(see
+
+
ence of the intense picks of ions [9a] and [9b] indicates
a significant contribution of the R—NHC coupling to
decomposition of both the Ni/NHC and Pd/NHC com-
3
1,37,41,43,44,48,49
plexes (see Scheme 2, reaction (3)).
+
Detection of the signals of cation [10 + H] under condi-
tions ensuring the absence of any contact of the reaction
mixture with air indicates that complexes decompose via
the O—NHC coupling reaction (see Scheme 2, reac-
4
5
tion (4)). It has been recently demonstrated that azolium
+
+
+
salts with the cations of the [8a] , [9a] , and [9b] types
that are resulted from the decomposition of Pd/NHC
complexes can stabilize the active forms of palladium and
3
7,44
+
+
+
+
promote the catalysis by the ligandless Pd species.
The
Fig. 2. Structures of cations [8a] , [9a] , [9b] , and [10 + H]
experimental data given in Table 1 do not allow us to
completely exclude the possibility of catalysis by the li-
gandless Ni species stabilized by heterocyclic products of
the Ni/NHC complexes decomposition.
To verify the assumption about the key role of the
molecular Ni/NHC catalysis in the Suzuki—Miyaura
and the m/z values found by electrospray ionization high resolu-
tion mass spectrometry.
+
+
+
both catalysts, namely, cations [8a] , [9a] , [9b] , and
+
[
10 + H] (Fig. 2).
Scheme 2
M = Pd, Ni

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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
Soliev et al.
reaction, the activities of catalytic systems based on com-
plexes 2e and 3a without (method A, Scheme 3) and after
thermal destruction at 80 C in the presence of KOH in
Scheme 3
1
% aqueous dioxane (method B, Scheme 3) were com-
pared. According to HPLC, LC/MS, and GC/MS data,
upon heating with KOH complexes 2e and 3a decompose
via reactions (2) and (4) (see Scheme 2) with the decom-
position degree of ca. 100% for complex 2e and 60±5%
for 3a. To estimate the possible effects of the products of
transformations of the NHC ligands on the activity of the
generated ligandless metals species, we studied the effect
of additives of azolium salts 8a and 9b and benzimidazol-
one 10 on the activity of Ni(OAc) and Pd(OAc) (Table 2).
2
2
For the comparison, the effect of common stabilizer
of the nanoparticles, tetrabutylammonium bromide
+
—
(
[Bu N] Br ) was also evaluated. The catalytic activity
4
of the obtained systems was investigated using the cross-
coupling of 4-chloroacetophenone (4b) with phenyl-
boronic acid (5a) as a model reaction (see Table 2). In the
Ni-catalyzed reaction, only the catalytic system based on
the intact complex 2e (without preliminary heating with
KOH; method A) was significantly active. Complex 2e
completely lost the activity after heating with KOH
Reagents and conditions: aryl chloride 4b (0.25 mmol), 5a
(
0.35 mmol), KOH (0.35 mmol), aqueous dioxane (2 mL, 1% of
H O (v/v)), catalyst (2.5 mol, 1 mol.%), 3 h, 80 C. Method A:
2
all reagents and catalyst were added simultaneously. Method B:
catalyst and KOH were heated at 80 C for 30 min, then com-
pounds 4b and 5a were added.
(
method B). The systems based on Ni(OAc) including
systems. A slight increase in the activity of Pd(OAc) was
2
2
those with additives of compounds 8a, 9b, 10, and
also observed in the presence of azolium salts 8a and 9b.
The similar effect observed previously in the Mizoroki—
Heck reaction can apparently be explained in terms of the
stabilization of the active ligandless palladium species with
+
–
[
Bu N] Br were found to be inactive both intact and
4
after thermal destruction in the presence of KOH. A com-
pletely different pattern was observed for the palladium
catalysts (see Table 2). The activity of the system based
on complex 3a increased by 10% after its heat treatment
in the presence of alkali. Palladium acetate without addi-
tives was only moderately active but the addition of
3
7,44
tetraalkylammonium and azolium salts.
Therefore, the experimental data shown in Scheme 3
and Table 2 confirm the assumption about the key role of
the molecular Ni/NHC catalysis in the Suzuki—Miyaura
reaction and the significant difference between the Ni/
NHC and Pd/NHC catalytic systems. The metal—NHC
+
–
[
Bu N] Br led to a significant increase in the catalytic
4
activity of both intact and thermally treated catalytic
Table 2. Effect of products of the NHC ligand-transformation on the activity of ligandless Ni
a
and Pd species in the cross-coupling reaction of compounds 4b and 5a
Run
Catalyst
Conversion of 4b (%)
Yield of 6b (%)
A
B
A
B
1
2
3
4
5
6
7
8
9
1
1
1
1
2e
92
~1
~1
~3
~1
~1
79
26
65
75
36
26
38
~1
~1
~1
~2
~1
~1
89
24
68
75
38
28
40
90
—b
b
b
Ni(OAc)2
—
—
—
b
b
Ni(OAc) + TBAB (2 equiv.)
—
2
b
Ni(OAc) + 8a (2 equiv.)
~1
—
2
b
b
Ni(OAc) + 9b (2 equiv.)
—
—
—
—
2
b
b
Ni(OAc) + 10 (2 equiv.)
2
3a
77
25
63
73
33
23
35
87
23
64
73
35
25
38
Pd(OAc)2
Pd(OAc) + TBAB (2 equiv.)
2
0
1
2
3
Pd(OAc) + TBAB (100 equiv.)
2
Pd(OAc) + 8a (2 equiv.)
2
Pd(OAc) + 9b (2 equiv.)
2
Pd(OAc) + 10 (2 equiv.)
2
a
b
Conditions are shown in Scheme 3.
Trace amounts.

(
NHC) NiX catalysts for the Suzuki reaction
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
687
2
2
bond cleavage in the systems based on Pd/NHC complexes
The cross-coupling reaction between 4-bromoanisole
(4a) and phenylboronic acid (5a) in toluene was very slow
(see Table 3, runs 1—3); in propan-2-ol, the selectivity
significantly decreased due to the predominant formation
of compound 7a, the product of the reductive dehalogena-
tion of aryl bromide 4a (runs 4—7). The reactions in
0
activated the catalysis by the ligandless Pd species, activ-
0
ity of which depended on the stabilization of the Pd
nanoparticles; while in the case of the Ni/NHC systems,
cleavage of the metal—NHC bond deactivated the catalyst
even in the presence of the stabilizers of the nanoparticles.
The enhanced activity of nickel bis-NHC complexes 2 is
due apparently to their higher stability than that of mono-
NHC complexes 1. It is also interesting to note that in the
presence of the strong base and water and in the absence
of the cross-coupling reagents, complexes 2 apparently
rapidly decompose, while in the presence of aryl halide
and arylboronic acid, the system remains active for several
hours. Probably, catalytic intermediates formed during
the Suzuki—Miyaura reaction are more stable towards
hydrolysis and the O—NHC coupling than the starting
complexes 2.
aqueous dioxane (1 vol.% of H O) in the presence of KOH
2
as a base were relatively fast and gave the target product
6a in the high yields (experiments 12 and 14—18). An
increase in the water content up to 5% (v/v) decelerated
the reaction (run 13). The highest yield of compound 6a
was achieved by carrying out the reaction at 120 C for 3 h
with the catalyst loading of 1 mol.% (run 16). Neither the
increase in the catalyst loading up to 3 mol.% nor the
elongation of the reaction time up to 6 h significantly
increased the yield of product 5a. Therefore, the conditions
of run 16 were assumed as the optimal.
The conditions of cross-coupling reaction between
compounds 4a and 5a catalyzed by complex 2e were op-
timized in order to design an efficient catalytic system for
the synthesis of biaryls (Table 3). The effects of various
solvents, bases, content of water in the solvent, cata-
lyst loadings, temperature, and the reaction time were
evaluated.
Under the optimized conditions, the cross-coupling
of aryl bromides and chlorides 4a—p with arylboronic
acids 5a—c gave the corresponding biaryls 6a—r in the
yields of 62—97% (Scheme 4 and Table 4). It is worthy to
note that both aryl chlorides and aryl bromides gave nearly
the same yields of biaryls (see Table 4). Earlier, the simi-
lar phenomenon was observed for the Ni/NHC-catalyzed
Table 3. Effect of the reaction conditions on the yield of compounds 6a and 7a in the cross-coupling reaction of
a
4-bromoanisole (4a) with phenylboronic acid (5a) catalyzed by complex 2e
Run
Solvent
Base
T/C
C b (%)
Yield (%))
6
a
7a
t
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
Toluene
Toluene
Toluene
Bu ONa
120
120
120
80
26
11
7
25
10
7
1
с
KOH
—
—
3
с
K CO
2
3
i
Pr OH
K CO
8
5
2
3
i
Pr OH
KOH
80
68
100
100
18
27
5
55
58
38
83
80
80
48
16
28
40
40
17
25
3
48
54
35
79
77
77
45
13
40
59
58
1
i
t
Pr OH
Bu OK
80
i
t
Pr OH
Bu ONa
80
t
Dioxane
Dioxane
Bu ONa
80
KOH
80
2
0
1
2
3
4
5
6
Dioxane + 1% H O
K CO
80
2
2
2
3
Dioxane + 1% H O
NaOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
80
7
2
Dioxane + 1% H O
80
4
2
Dioxane + 5% H O
80
3
2
d
e
Dioxane + 1% H O
120
120
120
120
120
5
2
Dioxane + 1% H O
3
2
Dioxane + 1% H O
2
2
f
7
8
Dioxane + 1% H O
3
2
g
Dioxane + 1% H O
2
2
a
Reaction conditions: 2e (1 mol.%), aryl bromide 4a (0.25 mmol), PhB(OH) (5a) (0.35 mmol), base (0.35 mmol),
2
solvent (2 mL), 3 h.
b
Conversion of compound 4a.
c
Trace amounts.
d
3
mol.% of 2e were used.
e
f
Reaction time was 6 h.
.5 mol.% of 2e were used.
0
0
g
.1 mol.% of 2e were used.

6
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
Soliev et al.
Scheme 4*
4-alkyl-substituted bromobenzenes and 2-chlorotoluene,
2
1—26
were used (see Table 4). It should be noted that earlier
the efficient Ni/NHC catalysis of the cross-coupling reactions
of deactivated aryl bromides and chlorides was mainly
ensured by the pincer-type complexes or complexes bearing
chelating ligands with the nickel loadings up to 1 mol.%
often in the presence of additives of phosphines or tetraalkyl-
ammonium salts. Catalyst 2e proposed herein can be
synthesized more easily and shows comparable efficacy in
the absence of phosphines or any other stabilizers. Note that
in some reactions of more active 4-bromoacetophenone (3c)
with phenylboronic acid (5a) with the loading of catalyst 2e
reduced to 0.1 mol.% the yield of product 6b reached 55—57%.
In conclusion, nickel bis-NHC complexes (NHC) NiX2
2
(
X = Cl, Br, or I) synthesized from readily available N,N´-di-
4
: X = Br; R = 4-OMe (a), 4-Ac (c), H (e), 4-Me (f), 4-Et (g), 4-F (h),
alkylazolium salts showed high catalytic activity in the
cross-coupling reaction of aryl halides with arylboronic
acids (the Suzuki—Miyaura reaction). This cross-coupling
proceeds by the mechanism of the molecular Ni/NHC-
catalysis. In contrast to the Pd/NHC systems, decompo-
4
-Me N (i), 4-PhO (g), 4-CN (k), 4-Cl (l), 4-Br (m), 3-Me (n); X = Cl,
2
R = 4-Ac (b), 4-OMe (d), 2-Me (o), H (p)
5
: R´ = H (a), OMe (b), CF3 (c)
*
Substituents R and R´ for products 6a—r are listed in Table 4.
0
Reagents and conditions: 2e (1 mol.%), KOH, dioxane, 120 C, 3 h.
sition of which gave NHC-free Pd species that can parti-
cipate in the catalysis, in the case of the Ni/NHC-catalyzed
Suzuki—Miyaura reaction decomposition of the complexes
via the Ni—NHC bond cleavage led to the loss of catalytic
activity. A new efficient catalytic system consisting of
cross-couplings of aryl halides with arylboronic acids.25,50
The suggested catalytic system with the catalyst loading
of only 1 mol.% provided sufficiently high yields of biaryls
(
60—77%) even when deactivated aryl halides, e.g.,
complex (NHC) NiX (NHC = N,N´-dibutylbenzimidazol-
2 2
Table 4. Yields of biaryls 6a—r in the reaction of aryl halides 4a—p with arylboronic acids 5a—c catalyzed by complex 2ea
b
Run
Aryl halide 4
Arylboronic acid 5
C (%)
Product
R
R´
Yield of 6 (%)
1
2
3
4
5
6
7
8
9
4a
4d
4b
4c
4e
4f
4g
4h
4i
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
5c
5c
5c
5c
5c
5c
80
76
90
100
82
77
80
81
70
83
91
87
91
75
75
85
68
100
100
65
63
78
6a
6a
6b
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
6l
6m
6n
6o
6p
6p
6o
6q
6r
4-OMe
4-OMe
4-Ac
H
H
77
73
89
95
78
77
70
81
62
75
90
71
87
75
72
78
62
96
97
56
60
75
H
4-Ac
H
H
H
4-Me
4-Et
H
H
4-F
H
4-N(Me)2
4-PhO
4-CN
4-Cl
H
1
1
1
0
1
2
4j
4k
4l
H
H
с
H
1
3d
4m
4n
4o
4c
4a
4b
4c
4d
4o
4p
Ph
H
1
1
1
1
1
1
2
2
2
4
5
6
7
8
9
0
1
2
3-Me
2-Me
4-Ac
H
H
OMe
CF3
CF3
CF3
CF3
CF3
CF3
4-OMe
4-Ac
4-Ac
4-OMe
2-Me
H
a
Reaction conditions: aryl halide 4a—p (0.25 mmol), arylboronic acid 5a—c (0.35 mmol), KOH (0.35 mmol), aqueous dioxane
(
2 mL, 1% of H O, v/v), complex 2e (2.5 mol, 1 mol.%), 120 C, 3 h.
2
b
c
Conversion of aryl halide 4a—p.
1
,4-Diphenylbenzene (6k) was formed in the reaction mixture in the yield of 12—19%.
d
PhB(OH) (5a) (0.7 mmol, 2.8 equiv.) and KOH (0.7 mmol) were used.
2

(
NHC) NiX catalysts for the Suzuki reaction
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
689
2
2
2
-ylidene and X = Br) and KOH in an aqueous 1,4-dioxane
CH3); 2.50 (s, 12 H, CH3); 6.75 (s, 4 H, CH of imidazole); 6.94
1
13
(
s, 8 H, Ar). H and C NMR spectra (CDCl ) of the obtained
(
1%, v/v) was suggested. This catalytic system enables the
3
5
3
product were identical to those reported previously.
Suzuki-Miyaura cross-coupling reactions of aryl bromides
and chlorides containing both electron-withdrawing and
electron-releasing substituents. The activity of the pro-
posed catalytic system is comparable to that of the systems
based on nickel complexes bearing the NHC pincer
ligands. The obtained results form the basis for further
studies of the catalytic activity of the readily available
nickel bis-NHC complexes in the reactions, wherein acti-
vation of the carbon—halogen bond is required.
Evaluation of the catalytic activity of nickel and palladium
complexes in the Suzuki—Miyaura reaction (general procedure).
The reaction was carried out under an argon atmosphere in
a screw top glass vial (V = 7 mL) equipped with a magnetic stir
bar. To a solution of the appropriate base (0.35 mmol), aryl
halide 4a—p (0.25 mmol), and arylboronic acid 5a—c (0.35 mmol)
in the appropriate solvent (0.5 mL), a solution of the appropriate
catalyst (0.0625—7.5 mol, 0.025—3 mol.%) in the same solvent
(
1.5 mL) was added (see Tables 1, 3, and 4). The vial was herme-
tically sealed and heated under stirring (see Tables 1, 3, and 4).
The mixture was cooled to room temperature and an internal
standard (a solution of naphthalene (16 mg, 0.125 mmol) in
acetonitrile (2 mL)) was added. An aliquot (2 μL) of the result-
ing mixture was taken, dissolved in acetonitrile (1 mL), and
analyzed by GC/MS (see Tables 1, 3, and 4).
Experimental
1H and 13C NMR spectra were recorded on a Bruker DRX-500
1
13
spectrometer (500 ( H) and 125 ( C) MHz) using SiMe as an
4
internalstandard. Gaschromatography-massspectrometry(GC/MS)
was performed on an Agilent 7890A chromatograph equipped
with an Agilent 5975C mass-selective detector (EI, 70 eV) and an
HP-5MS capillary column (30 m0.25 mm0.25 m). High per-
formance liquid chromatography (HPLC) was carried out using an
Agilent 1260 Infinity LC chromatograph equipped with a diode
array detector (detection at 260 nm) and a Zorbax SB-C18 reversed-
phase column (504.6 mm) thermostated at 30 C. Electrospray ioni-
zation high-resolution mass spectra (HRMS) were recorded on
a Bruker maXis Q-TOF spectrometer. Yields of the cross-coupling
products were determined by GC/MS. The cross-coupling prod-
ucts were identified by comparing their mass spectra with those
of the reference samples and the spectra from the NIST database.
Evaluation of the effect of products of the decomposition of
nickel and palladium complexes on their catalytic activity in the
Suzuki—Miyaura reaction. Method A (The reaction was carried
out with intact catalyst (without preliminary heating of the
catalyst with KOH)). The reaction was carried out under argon
in a screw top glass vial (V = 7 mL) equipped with a magnetic
stir bar. To a solution of KOH (20 mg, 0.35 mmol), 4-chloro-
acetophenone (4b) (38.6 mg, 0.25 mmol), and phenylboronic
acid (5a) (42.7 mg, 0.35 mmol) in an aqueous dioxane (0.5 mL,
1
1
% of H O, v/v), a solution of the appropriate catalyst (2.5 μmol,
2
mol.%) in an aqueous dioxane (1.5 mL, 1% of H O, v/v) was
2
added (see Table 2). The vial was sealed and heated at 80 C for
3
h under stirring. The yield of compound 6b (see Table 2) was
Complexes 1a—c,3
3,34,51,52
2a,d—i,
28—30
and 3a—c
36—38
determined by GC/MS as described above.
were synthesized by known procedures. The other reagents are
commercially available. Aqueous dioxane was prepared by add-
ing the calculated amount of water to the anhydrous solvent.
Bis(1,3-dibenzyl-1,3-dihydro-2H-imidazol-2-ylidene)(di-
bromo)nickel (2b). A mixture of 1,3-dibenzyl-1H-imidazolium
Method B (The reaction was carried out with the catalyst
preliminary heated of with KOH). The reaction was carried out
under argon in a screw cap septum glass vial (V = 7 mL) equipped
with a magnetic stir bar. A solution of the appropriate catalyst
(
2.5 μmol, 1 mol.%) (see Table 2) and KOH (20 mg, 0.35 mmol)
bromide (8b) (0.658 g, 2 mmol) and anhydrous Ni(OAc) (0.177 g,
2
in an aqueous dioxane (1.5 mL, 1% of H O, v/v) was heated at
2
1
mmol) was heated in a melt of tetrabutylammonium bromide
8
0 C for 3 h under stirring. Then a solution of 4-chloroaceto-
(
2 g) at 130 C under stirring in vacuo (1 Torr) for 5 h. After
phenone (4b) (38.6 mg, 0.25 mmol) and phenylboronic acid (5a)
cooling to 20 C, CH Cl (10 mL) and water (10 mL) were
2
2
(
42.7 mg, 0.35 mmol) in an aqueous dioxane (0.5 mL, 1% of
2
added and the mixture was stirred until completed dissolution.
H O, v/v) was added through the septum cap via a syringe. The
The organic layer was separated, washed with water (210 mL),
resulting mixture was heated at 80 C for 3 h under stirring. The yield
and dried with anhydrous Na SO . The solvent was removed
2
4
of compound 6b was determined as described above (see Table 2).
in vacuo and the residue was purified by silica gel column chroma-
tography (210 cm column, elution with CHCl ). Yield 0.342 g
3
The authors are grateful to Academician of the Russian
Academy of Sciences V. P. Ananikov for the fruitful dis-
cussion of the results and valuable comments. The authors
are also grateful to the Shared Research Center "Nano-
technologies" of the M. I. Platov South-Russian State
Polytechnic University (NPI) and the Department of
Structural Studies of the N. D. Zelinsky Institute of
Organic Chemistry of the Russian Academy of Sciences
for carrying out the analytical experiments.
1
13
(
48%), red prismatic crystals. H and C NMR spectra of the
2
9
obtained product were identical to those reported previously.
Bis(1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-
-ylidene)(dichloro)nickel (2c). To a vigorously stirred suspension
2
of NaH (25.2 mg, 1.05 mmol) in anhydrous pyridine (3 mL),
a suspension of 1,3-bis(2,4,6-trimethylphenyl)-1H-imidazolium
chloride (8c) (0.341 g, 1 mmol) and dichloro[bis(pyridine)]nickel
(
NiCl Py ) (0.143 g, 0.5 mmol) in pyridine (27 mL) was added
2 2
under an argon stream. The resulting mixture was stirred at 20 C
for 12 h and pyridine was removed in vacuo. The residue was
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 16-29-10786).
dissolved in CH Cl (30 mL) and the resulting solution was
2
2
washed with water (230 mL), dried with anhydrous Na SO ,
2
4
and passed through a short Celite layer. The solution was con-
centrated to ~2 mL and the residue was treated dropwise with
hexane (~5 mL). The precipitate formed was collected by filtra-
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Received October 9, 2019;
in revised form January 20, 2020;
accepted January 29, 2020
3