Efficient N-heterocyclic carbene nickel pincer complexes catalyzed cross coupling of benzylic ammonium salts with boronic acids

Article abstract of DOI:10.1016/j.cclet.2016.09.006

Pyridine-bridged bis-benzimidazolylidene nickel complexes exhibited very high catalytic activity toward cross coupling of inactive (hetero)aryl benzylic ammonium salts with (hetero)aryl and alkenyl boronic acids under mild reaction conditions. Even at 2 mol% catalyst loading, a wide range of substrates for both coupling partners with different steric and electronic properties were well tolerated.

Full text of DOI:10.1016/j.cclet.2016.09.006

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CCLET 3823 1–4
Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
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Original article
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Efficient N-heterocyclic carbene nickel pincer complexes catalyzed
cross coupling of benzylic ammonium salts with boronic acids
Q1 Xi-Yu Liu a,1, Hai-Bo Zhu a,1, Ya-Jing Shen a, Jian Jiang a, Tao Tu a,b,
*
5
6
7
a Department of Chemistry, Fudan University, Shanghai 200433, China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Pyridine-bridged bis-benzimidazolylidene nickel complexes exhibited very high catalytic activity
toward cross coupling of inactive (hetero)aryl benzylic ammonium salts with (hetero)aryl and alkenyl
boronic acids under mild reaction conditions. Even at 2 mol% catalyst loading, a wide range of substrates
for both coupling partners with different steric and electronic properties were well tolerated.
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Received 25 July 2016
Received in revised form 29 August 2016
Accepted 9 September 2016
Available online xxx
Keywords:
N-heterocylic carbene
Nickel
Cross coupling
Benzylic ammonium salts
Boronic acid
8
9
1. Introduction
catalyzed carbon–carbon bond formation reactions using benzylic 29
electrophiles [16–21]. In recent, Watson and coworkers firstly 30
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22
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27
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As essential building blocks and intermediates for bioactive
organic molecules [1,2] and pharmaceuticals [3–5], diaryl-
methanes have drawn considerable attentions. Tremendous efforts
have been devoted to develop efficient, straightforward and
practical protocols to access these valuable targets. The transition
metal catalyzed cross coupling of benzylic electrophiles with
Grignard or organozinc reagents represents one of the most
powerful approaches for this purpose, however, the electrophiles
usually restricted to benzylic halides, acetates, carbonates and
phosphates (Scheme 1a) [6–13]. Recently, benzylic ethers and
alcohols were also found as good coupling partners, which may be
regarded as one of significant advances in this research field
[14,15], whereas, the air-sensitive Grignard reagents were still
required and gravely hampered their feasibility. Therefore, it is
highly desirable to explore an efficient catalytic system to produce
realized the nickel-catalyzed cross coupling of benzylic ammonium 31
salts, readily prepared from amine precursors, with aryl boronic 32
acids, and good yields were observed with
a number of 33
diarylmethanes by using air sensitive Ni(cod)2 [22,23] and luxury 34
phosphine ligands [23]. Besides high catalyst loading (10 mol%) was 35
required to achieve satisfactory results, active benzylic ammonium 36
triflates have to be involved, whereas, the ammonium salts with 37
other inactive counter-ions (I, Br or Cl) are not well tolerated. 38
Although, N-heterocyclic carbene ligands have been regarded as 39
suitable replacements for the traditional air-sensitive phosphine 40
ligands and exhibited high activity in a number of transformations 41
[24], the catalyst derived from IMesÁHCl and Ni(cod)2 only resulted 42
in 28% yield under the identical reaction conditions [23].
43
Recently, we successfully developed pyridine-bridged bis-NHC 44
nickel complexes and unveiled excellent outcomes in the Suzuki– 45
Miyaura couplings of inactive ArCl under mild reaction conditions 46
with broad substrate scopes. Remarkably, inert and inexpensive 47
ArOTs, ArOMs and ArOPiv could be successfully coupled via 48
selective C–O activation [25–27]. Following our recent research 49
interests in exploring their applicability of pincer metal complexes 50
in catalysis and material sciences [28–34], herein, we sought to 51
extend their applicability of pyridine-bridged bis-NHC nickel 52
complexes in the cross-coupling of steric hindered (hetero)ar- 53
ylbenzylic ammonium salts with (hetero)aryl boronic acids via 54
C–N activation of benzylic substrates without Grignard reagents. 55
a
variety of diarylmethanes by cross couplings of benzylic
substrates without Grignard reagents.
In contrast, robust organoboron agents have been gradually
employed as crucial coupling partners in various transition metal
*
Corresponding author at: Department of Chemistry, Fudan University, Shanghai
200433, China.
Q2
E-mail address: taotu@fudan.edu.cn (T. Tu).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.cclet.2016.09.006
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: X.-Y. Liu, et al., Efficient N-heterocyclic carbene nickel pincer complexes catalyzed cross coupling of
benzylic ammonium salts with boronic acids, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.09.006

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CCLET 3823 1–4
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X.-Y. Liu et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
transformation (94 and 92%, respectively, Table 1, entries 11 and
77
12), highlighting the catalytic efficiency of pincer complex 1. In
contrast, its imidazolium analog 2 only afforded a moderate yield
(83%, Table 1, entry 13), which further confirmed our previous
78
79
80
observation that the NHC ligands with
act as stronger -donors and weaker
p
-extended aromatic rings
81
82
s
p
-acceptors and increase the
electron density of the catalytic center leading to higher catalytic
activity [25–34]. When the reaction was performed in the absence
of PPh3, only 37% yield was found, which demonstrated the
significance of PPh3 as reductant in this transformation (Table 1,
entry 14) [26]. As expected, no reaction took place in a blank test
under the identical reaction conditions (Table 1, entry 15).
With the optimized reaction conditions in hand, the substrate
scope with a series of aryl benzylic ammonium iodide was
investigated (Scheme 2). Both electron-rich and electron-deficient
substituents on the phenyl ring of ammonium salts were well
tolerated (6–11). Especially, the substrates with meta-, para-, and
ortho-substituents resulted in similar good yields for the corre-
sponding diarylmethanes 6a–c. Other para-substitutes even with
strong electron-donating and electron-withdrawing only unveiled
slightly impacts on the coupling process, up to 91% yields
were afforded (7–10). Unexpectedly, steric hindered substrate,
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85
86
87
88
89
90
91
92
93
94
95
96
97
98
Scheme 1. Transition-metal catalyzed cross-coupling reactions for accessing
diarylmethanes.
56
2. Results and discussion
a
-naphthylbenzylic ammonium salt even gave out an excellent
99
result (96% for 12). However, when the substrate with two ortho-
positions substituted (2,4,6-trimethylbenzylic salt), the yield was
dramatically dropped to 69% (11). Remarkably, heterocyclic salts
were also well tolerated. Excellent results were observed with the
couplings of 2-thienyl and N-methyl-3-indolylbenzylic ammo-
niums as substrates (97% and 95% for products 13 and 14,
respectively), which clearly indicated the protocol efficiency.
In consideration that steric hindered substrates are challenging
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108
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112
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114
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58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
Initially, the coupling of benzylic ammonium iodide 3a with
para-methoxyphenylboronic acid 4 was selected as model reaction
to investigate the catalytic activity of the selected pincer
complexes. In the presence of 5 mol% pincer complex 1, 20 mol%
PPh3 and 4.5 equiv. K2CO3, only trace amount of diarylmethane 5
was detected in dioxane at 80 8C for 24 h (Table 1, entry 1). Similar
outcome was obtained when strong base t-BuOK was applied
(Table 1, entry 2). Delightedly, when CsF was used instead, the
product 5 was isolated in a moderate yield (63%, Table 1, entry 3).
The yield could be further enhanced to 92%, when K3PO4 was
employed (Table 1, entry 4). Unexpectedly, further decreasing the
catalyst loading to 2 mol% and 1 mol% did not hamper the coupling
process, whereas, the yields was further increased to 97% and 94%,
respectively (Table 1, entries 5 and 6, respectively). The alternation
of solvents shows less impact on the yields of product 5, slightly
inferior outcomes were observed when other solvents were
involved (87–91%, Table 1, entries 7–10). Additional information
for the optimization of solvents and temperature was depicted in
the Supporting information. Interestingly, other the ammonium
salts containing bromide and chlorides are also active in this
and less studied in the Suzuki–Miayara coupling reactions,
a-
naphthylbenzylic ammonium iodide was selected to investigate
the scope of aryl boronic acids (Scheme 3). With 2 mol% pincer
nickel complex 1, all selected aryl boronic acids were good
coupling partners, even in the case of heterocylic ones (15–25). The
position and electronic properties of the substitutes of aryl bronic
acids hardly hampered the coupling process; excellent yields were
produced in all tested cases (93–97%, 16–20). When 4-bromo-
phenylboronic acid and 4-chlorophenylboronic acid were in-
volved, 60% and 44% yields were still obtained for 21a and 21b,
respectively, which provided a possibility for the further transfor-
mation of these products, although small amount of inevitable
homo-coupling products were also produced. Again, steric
Table 1
Optimization of the reaction conditions.a
Entry
[Cat]: (mol%)
Salt
Solvent
Base
Yield (%)b
1
1: (5)
1: (5)
1: (5)
1: (5)
1: (2)
1: (1)
1: (2)
1: (2)
1: (2)
1: (2)
1: (2)
1: (2)
2: (2)
1: (2)
–
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3a
3a
3a
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
THF
K2CO3
t-BuOK
CsF
Trace
Trace
63
2
3
4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
96
5
97
6
94
7
91
8
DME
90
9
DMF
91
10
11
12
13
14
15
t-BuOH
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
87
94
92
82
37c
NRd
a
Carried out with 0.5 mmol scale with 1 equiv. 3, 4 equiv. 4, 4.5 equiv. base, 1–
5 mol% pincer complex and 20 mol% PPh3 at 80 8C for 24 h.
Scheme 2. Scope of various (hetero)aryl ammonium iodide. (Reaction was carried
out with 0.5 mmol scale with 1 equiv. aryl benzylic ammonium iodide, 4 equiv. 4,
4.5 equiv. base, 2 mol% nickel pincer complex 1 and 20 mol% PPh3 at 80 8C for 24 h
N2. Isolate yield based on aryl benzylic ammonium iodide.)
b
Isolate yield based on compound 3.
c
Without PPh3.
d
Without catalyst.
Please cite this article in press as: X.-Y. Liu, et al., Efficient N-heterocyclic carbene nickel pincer complexes catalyzed cross coupling of
benzylic ammonium salts with boronic acids, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.09.006

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CCLET 3823 1–4
X.-Y. Liu et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
3
Beijing Synthware Glass. CDCl3 was purchased from Cambridge 155
Isotope Laboratories. 1H NMR, 13C NMR and 19F NMR spectra were 156
recorded on Jeol ECA-400 and Bruker 400 DRX spectrometers. 13
C
157
NMR spectra were referenced to the carbon signal of CDCl3 158
(77.0 ppm). GC–MS spectra were recorded on Agilent Technologies 159
1890A GC system and 5975C inert MSD with Triple-Axis Detector. 160
General procedure for diarylmethanes from benzylic ammoni- 161
um iodide and arylboronic acid: To a 50 mL Schlenk tube 162
containing sodium benzylic ammonium iodide (0.5 mmol), aryl- 163
boronic acid (2.0 mmol), K3PO4 (2.25 mmol), castalyst (5 mol%) 164
and PPh3 (20 mol%) were added and the tube was purged with N2 165
for
3 times. Then 1,4-dioxaxe (2.0 mL), subsequently, was 166
introduced to the tube. The resulted mixture was allowed to stir 167
for 24 h at 80 8C under atmosphere of N2. After the completion of 168
the reaction, the resulting mixture was filtered through a Celite 169
pad and concentrated under the vacuum and directly purified by 170
flash chromatography to give the desired product.
171
Acknowledgments
172
Financial support from the National Natural Science Foundation Q3173
of China (Nos. 21572036, 21172045 and 91127041), the Chang- 174
jiang Scholars and Innovative Research Team in University (IRT- 175
15R12), the Shanghai International Cooperation Program (No. 176
14230710600), the Doctoral Fund of Ministry of Education of China 177
(No. 20130071110032) and Department of Chemistry Fudan 178
Scheme 3. Reaction scope with (hetero)aryl boronic acids. (Reaction was carried out
with 0.5 mmol scale: 3 (1 equiv.), arylboronic acid (4 equiv.), 1 (2 mol%), PPh3
(20 mol%) and K3PO4 (4.5 equiv.) were dissolved in 2.0 mL solvent and stirred at
80 8C in 24 h under atmosphere of N2. Isolate yield based on 3.)
University is gratefully acknowledged.
179
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123
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127
128
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encumbered 1-napthalylboronic acid produced the corresponding
product 22 in excellent isolated yield (99%), whereas, less bulky 2-
substituted isomer delivery 98% yield for product 23. In the
heterocyclic cases, 2-methoxypyridine-3-boronic acid and quino-
line-6-boronic acid all resulted in excellent yields for products 24
Appendix A. Supplementary data
180
Supplementary data associated with this article can be found, in 181
the online version, at http://dx.doi.org/10.1016/j.cclet.2016.09.006. 182
and 25 (96% and 93%, respectively). Intriguingly, when
a
challenging alkenyl substrate (1-cyclohexenylboronic acid) was
applied, the corresponding product 26 was produced even in a
quantitative yield.
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In summary, robust pincer nickel NHC complexes unveiled high
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benzylic ammonium salts with boronic acids, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.09.006

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Please cite this article in press as: X.-Y. Liu, et al., Efficient N-heterocyclic carbene nickel pincer complexes catalyzed cross coupling of
benzylic ammonium salts with boronic acids, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.09.006