Nickel-Catalyzed C(sp2)-H Borylation of Arenes

Article abstract of DOI:10.1021/acs.organomet.9b00364

In this study, C(sp²)-H borylation of arenes was accomplished by a nickel catalyst, resulting in good yield. Alkyl and alkoxy arenes were successfully functionalized, affording C(sp²)-H borylated compounds. It was unraveled that the well-defined abnormal N-heterocyclic carbene based Ni(II) complex breaks into Ni nanoparticles (Ni-NPs), which act as catalytically active species. A series of controlled reactions under stoichiometric conditions along with spectroscopic studies and single-crystal X-ray crystallographic study helped us to understand the formation of Ni-NPs along with formation of a boron(III) compound during this reaction.

Full text of DOI:10.1021/acs.organomet.9b00364

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Cite This: Organometallics XXXX, XXX, XXX−XXX
2
Nickel-Catalyzed C(sp )−H Borylation of Arenes
Arpan Das, Pradip Kumar Hota, and Swadhin K. Mandal*
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
S Supporting Information
*
2
ABSTRACT: In this study, C(sp )−H borylation of arenes
was accomplished by a nickel catalyst, resulting in good yield.
Alkyl and alkoxy arenes were successfully functionalized,
2
affording C(sp )−H borylated compounds. It was unraveled
that the well-defined abnormal N-heterocyclic carbene based
Ni(II) complex breaks into Ni nanoparticles (Ni-NPs), which
act as catalytically active species. A series of controlled
reactions under stoichiometric conditions along with spectro-
scopic studies and single-crystal X-ray crystallographic study
helped us to understand the formation of Ni-NPs along with
formation of a boron(III) compound during this reaction.
2
rganoboron compounds are considered as extremely
useful reagents in organic synthesis, since they are
Scheme 1. Overview of Transition-Metal-Catalyzed C(sp )−
O
H Borylation of Arenes
important synthons to prepare various core moieties of
1
2
pharmaceutically important molecules, natural products,
3
and organic materials. Apart from the transition-metal-
catalyzed coupling of aryl halides or pseudohalides with
boronyl esters, the direct functionalization of C−H bonds
using a borane source is one of the most effective methods to
4
,5
synthesize such compounds.
The direct C−H bond
functionalization has several benefits in comparison to
conventional synthetic routes for the preparation of aryl
boronates. In this method, prefunctionalized arenes are not
needed, making this process more atom economical and
2
cheaper. In this context, transition-metal-catalyzed C(sp )−H
borylation of arenes has come to the limelight in modern
catalysis, as it helps to introduce a synthetically important
boron moiety directly into an aromatic ring in absence of any
directing group. Earlier, [Ir(COD)(OMe)] (COD = 1,5-
2
cyclooctadiene) and [Ir(COE)(OMe)] (COE = cyclooctene)
2
in association with an organic ligand such as bipyridine or
phenanthroline have been extensively used for such trans-
formations due to their high activity and good functional group
tal discovery, a plethora of base-metal catalysts has come into
the picture toward catalytic C−H borylation. In 2010, Kuang,
6
tolerance. Later on, the versatility of the iridium catalysts
Wang, and co-workers used γ-Fe O nanoparticles toward
2 3
encouraged many others to explore the activity of iridium-,
rhodium-, and platinum-based catalysts under homogeneous
conditions as well as heterogeneous conditions toward the
arene borylation in presence of more than an equivalent
13
amount of base and oxidant. Mankad et al. relied upon
metal−metal cooperativity to accomplish this task by
introducing various heterobimetallic systems under photo-
7
−10
catalytic C−H borylation reactions (Scheme 1).
Despite
14
chemical conditions. Darcel, Etienne, Sortais, Bontemps, and
co-workers have introduced Fe(Me) (dmpe) (dmpe = 1,2-
their very high catalytic activity, high cost, and lower
abundance of these rare metals in earth’s crust raises concern
over their use as catalysts, replacement by nonprecious, earth-
abundant, nontoxic metal catalysts is thus considered as an
2
2
bis(dimethylphosphino)ethane) for borylation of arenes with
15
the help of UV irradiation. All of these methods involving
11
base-metal catalysts still suffered from major drawbacks such as
versatility in substrate scope, use of excess base, oxidant, H2
alternative to such precious metals. In 1995, Hartwig and co-
workers demonstrated that the complex (Cp)Fe(CO) (Bcat)
2
(
Cp = cyclopentadienyl, cat = catecholate) can stoichiometri-
cally transfer its catechol−borane part to borylate benzene
Received: May 31, 2019
12
under photochemical conditions. Soon after this experimen-
©
XXXX American Chemical Society
A
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acceptor, and photochemical setup involving UV or Hg lamp.
Chirik and co-workers further attempted to address such
drawbacks by activating arenes under thermal conditions. In
Table 1. Optimization of Nickel-Catalyzed C−H
a
Borylation
2
014, Chirik et al. first demonstrated a series of highly active
cobalt complexes bearing pincer ligands which can borylate
arenes with only 1 mol % loading of catalyst under thermal
conditions. Later on, the underlying mechanism was
1
6
1
7
explained in detail to demonstrate the site-selective
borylation of fluoroarenes in presence of fluorine as an
18
ortho-directing group.
However, borylation of more challenging long branched
alkyl chain substituted arenes was not addressed in such
studies. In parallel efforts, Chatani et al. and Itami et al. have
2
b
demonstrated nickel-catalyzed C(sp )−H borylation of arenes
entry
catalyst (mol %)
time (h)
temp (°C)
yield (%)
19,20
and heteroarenes.
Chatani and co-workers introduced an
1
2
3
4
5
6
7
8
Ni(COD) /aNHC (1)
16
16
12
16
16
16
16
16
16
16
16
16
16
80
80
80
80
<5
60
74
85
0
2
N-heterocyclic carbene (NHC) salt with a catalytic amount of
Ni(COD)2 (COD = 1,5-cyclooctadiene) and base to
accomplish the borylation of arenes under thermal conditions
Ni(COD) /aNHC (2.5)
2
Ni(COD) /aNHC (5)
2
Ni(COD) /aNHC (5)
2
19
using pinacolborane (HBPin). However, such a methodology
has a very limited scope of arenes (only four examples), as it
Ni(COD) /aNHC (5)
room temp
2
Ni(COD) (5)
80
80
80
80
80
80
80
80
0
0
2
2
suffers from the selectivity of C(sp )−H borylation over
aNHC (5)
3
C(sp )−H for substituted arenes (Scheme 1). On the other
Ni(COD) /IPr (5)
14
17
88
44
55
<5
2
hand, Itami and co-workers used a phosphine-based ligand in
9
Ni(COD) /SIPr (5)
2
combination with Ni(COD) in presence of cesium fluoride
10
11
12
13
1 (5)
2 (5)
1 (2.5)
1 (1)
2
and excess of arene (>50 equiv) to obtain the desired product
20
at very high temperature (140 °C). Once again, this report
documents only three arenes, raising concern over the general
applicability of this method.
a
All reactions were conducted with HBPin (0.5 mmol) and benzene
7.5 mmol) under neat conditions. Isolated yield based on HBPin.
b
Despite all these advances, the major limitation using base-
metal catalysts without photochemical stimulation lies in the
fact that they cannot borylate a wide range of arene substrates
using HBPin. Furthermore, this field has not yet fully explored
the scope of the nickel catalyst. It may be worth mentioning
that Chatani et al. as well as Itami et al. in two independent
studies earlier suggested that the nickel-catalyzed borylation
reaction does not follow the conventional pathway reported
(
increment in yield (74%) was found when a 5 mol % of
Ni(COD) and aNHC catalyst combination was used (Table
, entry 3). Delightfully, the yield was further improved to 85%
on elongating the time period to 16 h (Table 1, entry 4). The
reaction did not proceed at room temperature (Table 1, entry
2
1
5
). Reaction with either Ni(COD) or aNHC did not result in
2
2
1,22
17
15
earlier for iridium-,
cobalt-, and iron-catalyzed
any product formation, authenticating the indispensable role of
an active catalyst in this reaction (Table 1, entries 6 and 7).
Next, we examined two other normal N-heterocyclic carbenes
processes; rather, it might follow a heterogeneous pathway.
The reports both suggested formation of black particles during
the catalysis and concluded that more detailed study is needed
to get insights into the nature of these black particles. In this
study, we report the first well-defined abnormal N-heterocyclic
carbene, a 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimida-
zol-5-ylidene (aNHC)-based nickel catalyst, for borylation of
(
IPr and SIPr) in combination with Ni(COD) . In both cases,
2
the desired product was formed in very low yield, 14−17%
Table 1, entries 8 and 9). It may be concluded from the above
observations that the abnormal N-heterocyclic carbene
aNHC) has a better catalytic activity than the normal N-
heterocyclic carbene (NHC) in association with Ni(COD)2.
The theoretically calculated energies for HOMO and HOMO-
(
(
2
C(sp )−H bonds for a wide variety of arenes. During the
catalytic reaction, we also observed formation of black
particles, which we have characterized as nickel(0) nano-
particles (Ni-NPs).
1
orbitals for aNHC (−4.403 and −4.879 eV, respectively) are
much higher than that of the normal isomeric NHC (−5.000
and −5.279 eV, respectively), which makes aNHC more
RESULTS AND DISCUSSION
The present study was initiated with optimization of reaction
conditions for the direct C−H borylation of benzene using
,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBPin), Ni(COD) ,
23
■
nucleophilic than its normal analogue.
Next, we prepared the two nickel catalysts 1 and 2 (Figure
1a) through modification of the reported procedure by using
1,3-bis(2,6-diisopropylphenyl)-2,5-diphenyl-1H-imidazol-3-
ium chloride hydrochloride (L1) or 1,3-bis(2,6-diisopropyl-
phenyl)-5-(4-methoxyphenyl)-2-phenyl-1H-imidazol-3-ium
chloride hydrochloride (L2) instead of the corresponding free
4
2
and abnormal N-heterocyclic carbene (aNHC). Various
reaction conditions were examined by altering time, solvent,
catalyst loading, and temperature, and in each case, formation
of the desired product was observed but with varied efficiency
24
carbene. The syntheses of 1 and 2 were accomplished by
(
Table 1). The borylated product was obtained in negligible
reacting Ni(COD) with L1 and L2 in the presence of a base,
potassium bis(trimethylsilyl)amide (KHMDS), in a 1:1:2.1
stoichiometric ratio.
2
yield (<5%) with 1 mol % loading of Ni(COD) and aNHC
catalyst combination in 16 h at 80 °C (Table 1, entry 1). When
the Ni(COD) and aNHC loadings were increased to 2.5 mol
2
The addition of dry THF to this mixture at −78 °C under an
2
%
and all other parameters kept unchanged, the yield was
N atmosphere resulted in a sharp color change from pale
2
significantly increased to 60% (Table 1, entry 2), and a further
yellow to dark red with the formation of complex 1 or 2
B
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8
8% yield (Table 1, entry 10). A further decrease in mole
percent of 1 did not give any satisfactory yield (Table 1, entries
2 and 13). However, introduction of 2 resulted in low yield
44%, Table 1, entry 11). This observation may be explained
1
(
25
by considering a previous report which documents that the
carbene generated from L2 is less stable than that generated
from L1. It may be possible that, during the metathesis step, it
is quenched before formation of the analogue of compound 3
(
Figure 2a) and ends up with a smaller yield. These
observations encouraged us to take forward this borylation
reaction with catalyst 1 to explore further the scope of the
reaction. To increase the scope of boranes, we have also
introduced B Pin /9-BBN as a borane source but no product
2
2
formation was observed under our standard protocol for this
borylation reaction. A 69:31 selectivity of meta- and para-
substituted products was obtained in the case of toluene. It is
worth mentioning that borylation of toluene, ethylbenzene, or
2
any alkyl benzene took place selectively at the C(sp )−H
Figure 1. (a) Syntheses of aNHC-based nickel complexes. (b)
Molecular structure of 2. Thermal ellipsoids represent 50%
probability; residual solvent molecules and disordered atoms are
omitted for the sake of clarity. Selected bond distances (Å) and angles
19
position instead of the benzylic position (Scheme 2, 4b−
m,p,q), resulting in moderate to excellent yield (57−83%).
Furthermore, a meta-borylated product was obtained as the
major species, which is consistent with the previously reported
(
deg) are as follows: Ni1−C3 1.907(2), Ni1−C5 1.954(2), Ni1−C9
26,27
iridium catalysts.
Next, xylene derivatives were borylated in
2
8
.022(2), Ni1−C10 1.965(2), Ni1−C11 2.101(2); C3−Ni1−C5
4.66(9), C3−Ni1−C9 172.54(9), C9−Ni1−C11 73.76(9), and
very good yield (68−70%, Scheme 2, 4k,l). The most difficult
substrate among xylenes, p-xylene resulted in moderate yield
C10−Ni1−C9 41.42(9).
(
54%, Scheme 2, 4m). In case of m-xylene, a very good
selectivity was noted, as only a single borylated product at the
C5-position of the phenyl ring was obtained. The borylation of
a solid substrate was carried out: for example, naphthalene is
considered to be one of the most challenging substrates to
borylate through a direct C−H bond. Even iridium-catalyzed
borylation of naphthalene resulted in diborylated products in
major amount. Delightfully, the present reaction conditions
resulted selectively in monoborylated naphthalene in 61%
isolated yield with high β/α selectivity (86:14, Scheme 2, 4o).
Another solid substrate, biphenyl, can also be borylated in
(
Figure 1a). Analytically pure crystals were obtained from a
toluene/hexane mixture at 15 °C after 3 days. Complex 1 was
characterized by H and C NMR spectroscopy, and these
data matched with the literature data. Complex 2 was
characterized by an array of spectroscopic tools ( H and
1
13
24
1
13
C
NMR spectroscopy) as well as elemental analysis and single-
crystal X-ray diffraction studies. The molecular structure of 2 is
shown in Figure 1b.
Complexes 1 and 2 were tested for their catalytic activity.
The reaction proceeded smoothly with catalyst 1, affording
28
Figure 2. (a) Synthesis of compound 3. (b) Molecular structure of 3. Thermal ellipsoids represent 50% probability; hydrogen atoms and residual
solvent molecules are omitted for the sake of clarity. Selected bond distances (Å) and angles (deg) are as follows: C3−B1 1.6920(17), C5−B1
1
.6738(18), O1−B1 1.4717(16), O2−B1 1.4569(16), O1−B1−O2 106.24(10), O1−B1−C5 114.04(10), O2−B1−C3 120.71(10), and C5−B1−
C3 93.80(9).
C
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a
Scheme 2. Reaction Scope for Nickel-Catalyzed C−H Borylation of Arenes
a
All reactions were conducted with HBPin (0.5 mmol) and arene (7.5 mmol). The isolated yield based on HBPin is given in parentheses. Product
ratios were determined via NMR spectroscopy analysis. Conditions: 4c,e−o, 100 °C for 24 h; 4p,q, 100 °C for 24 h, 7.5 mol % catalyst was used.
b
Conditions: heteroarene (0.5 mmol), HBPin (0.75 mmol), catalyst (0.025 mmol), 80 °C, 16 h, methylcyclohexane as solvent. For 4r−t, isolated
yields are based on heteroarene.
good yield under our reaction conditions (66%, Scheme 2, 4n).
Furthermore, it was noted that even an arene bearing sterically
bulky groups such as isopropyl, tert-butyl, and cyclohexyl
resulted in such borylation successfully with impressive yield
we examined whether heteroarenes such as pyrrole and indole
can be borylated using our protocol. N-Methylpyrrole was
borylated successfully at the C-2 position with excellent yield
(
92%, Scheme 2, 4r). N-Methylindole and N-benzylindole also
(
58−67%, Scheme 2, 4g−i). A similar finding was also
resulted in the formation of C-2 borylated products in good
yield (77−79%, Scheme 2, 4s,t), which may be compared with
a previous report leading to single isomeric borylated products
observed for the borylation of n-propyl-, n-butyl-, and n-
hexylbenzene (57−74%, Scheme 2, 4e,f,j). The m:p
selectivities are also consistent with the previously reported
iridium(0) nanoparticle catalyzed borylation of arenes. This
result encouraged us to check whether more sterically hindered
19
10
for heteroarenes. To the best of our knowledge, earlier
2
studies never addressed direct C(sp )−H borylation of n-
2
hexylbenzene, n-butylbenzene, n-propylbenzene, phenylcyclo-
arenes can undergo C(sp )−H borylation under the optimized
hexane, naphthalene, biphenyl, 1,3-diisopropylbenzene, and
conditions. Borylation of 1,3-diisopropylbenzene and 1,3-
dimethoxybenzene led to the formation of single isomer
selectively (Scheme 2, 4p,q), however, with relatively low
yields of 43% and 46%, respectively. This result is consistent
with earlier results reporting borylation at the most sterically
1
,3-dimethoxybenzene using first-row transition-metal cata-
lysts.
Having the optimized reaction conditions in hand, we
continued to explore the borylation of different arenes under
neat conditions. Borylation of benzene and toluene proceeded
7
accessible C−H bonds (C5-position of the phenyl ring). Next,
D
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Figure 3. (a) TEM image of the Ni-NPs obtained from a 1:1 reaction mixture of 1 and HBPin. (b) STEM image of Ni-NPs. (c) Histogram of the
Ni-NPs size distribution (average diameter 3.4(±0.5) nm), constructed from the measurement of 200 NPs. (d) XPS spectrum of Ni-NPs.
smoothly, affording 88% and 83% yields, respectively (Scheme
spectroscopy (δ 2.21 (m, 8H), 5.57 (m, 4H) ppm) from the
30
2
, 4a,b).
reaction mixture in C D . Furthermore, we observed the
6 6
While performing the catalytic reactions, we noticed that
formation of black particles during the course of the reaction,
which have been characterized as Ni-NPs by transmission
electron microscopy (TEM) and X-ray photoelectron spectro-
scopic (XPS) measurements (Figure 3). The formation of Ni-
NPs is similar to that reported for reduction of Ni(II) salts or
there was deposition of black particles on the wall of the
reaction vessel. This is reminiscent of the earlier observation
reported by Chatani and Itami et al. in two independent
2
studies during nickel-catalyzed C(sp )−H borylation reac-
1
9,20
31,32
tions.
However, these studies did not provide any further
by decomposition of Ni(COD)2 in ionic liquids.
details on such black particle formation. Next, we performed
the catalytic borylation reaction in the presence of mercury (10
Furthermore, transmission electron microscopy (TEM) of
the black particles displayed spherical metallic nanoparticles
distributed uniformly (Figure 3a). The scanning transmission
electron microscopic (STEM) images displayed a clear picture
of the shape and size of Ni-NPs (Figure 3b). The TEM images
unveiled a highly monodisperse nature of the nickel nano-
particles of average diameter 3.4(±0.5) nm (Figure 3a,c). In
addition, an X-ray photoelectron spectroscopic (XPS)
measurement was performed to gain more insight on the
oxidation state of nickel which revealed Ni 2p3/2,1/2 peaks at
851.9 and 869.2 eV, confirming the formation of metallic
19
equiv with respect to 1). The addition of mercury completely
arrested the reaction, as one might expect in the case of a
heterogeneous reaction. A kinetic isotope experiment was
carried out with equimolar amounts of benzene and benzene-
d , and the k /k value was measured as 1.76 (see Figure S70
6
H
D
in the Supporting Information), which is comparable to that
reported (1.99) earlier, indicating that breaking of a C−H
19,20
bond may be involved in the rate-determining step.
To
gather further information, a series of stoichiometric reactions
was performed. First, a stoichiometric mixture of HBPin and 1
in C D was heated at 80 °C for 2 h over a hot plate under
33
nickel (Figure 3d). The formation of Ni-NPs along with
compound 3 in the course of C−H borylation of arenes was
6
6
1
closed conditions (Figure 2a). The mixture was cooled down
to room temperature and kept for crystallization after adding
hexane. Within 10 h, appearance of yellowish crystals was
observed along with a black precipitate. One of these crystals
was analyzed by a single-crystal X-ray diffractometer.
The molecular structure of 3 (Figure 2b) revealed the
formation of an unprecedented compound in which the Ni(II)
ion was replaced by the B(III) ion to form the tetrahedrally
coordinated boron compound 3 (Figure 2a). The crystals of 3
were found to be highly air and moisture sensitive, and it
immediately decomposes on exposure to air. Additionally, to
support the findings, we noted that, on addition of HBPin to 1,
well-characterized using several spectroscopic techniques ( H,
1
3
11
C, B NMR spectroscopy, TEM, XPS) and an X-ray
crystallographic study.
A series of control experiments was carried out to check the
catalytic activity of Ni-NPs. At first, we prepared nickel
nanoparticles (Ni-NPs) from another Ni precursor such as
Ni(COD) in the presence of a stoichiometric amount of
2
HBPin (black mass deposition) on heating at 80 °C for 2 h,
but failed to accomplish the catalytic borylation reaction of
benzene. Next, Ni-NPs were isolated from the stoichiometric
reaction of 1 and HBPin. However, a trace amount of product
was realized using only Ni-NPs (Table 2, entry 1).
Alternatively, the catalytic activity was tested using compound
3, which did not result in the formation of the desired product
(Table 2, entry 2). However, an excellent yield was achieved
when a mixture of Ni-NPs and 3 was introduced (Table 2,
entry 3). When all these results are combined, it may be
concluded that the presence of a heterogeneous mixture of Ni-
NPs and 3 is needed to proceed with the catalysis.
gas evolution was observed, which was characterized as H by
2
1
29
H NMR spectroscopy (δ 4.57 in CD CN). Attempts to get
3
more information about the intermediates in the course of
1
1
formation of 3 were unsuccessful. We performed an B NMR
spectroscopy study by adding a stoichiometric amount of
HBPin to a C D solution of 1. However, within 15 min of
6
6
addition of HBPin to the solution of 1, the formation of 3 was
detected through 11B NMR spectroscopic experiment (see
Figure S68 in the Supporting Information). After 2 h of
heating at 80 °C, HBPin was fully consumed and the peak due
to 3 became prominent (see Figure S69 in the Supporting
This reaction follows a heterogeneous pathway where Ni-
NPs in the presence of compound 3 form the catalytically
active species. The mechanism of the reaction may be
proposed on the basis of the previously reported iridium
10,34,35
Information). The formation of free 1,5-cyclooctadiene was
nanoparticle catalyzed borylation of arenes
following
1
also authenticated as a side product along with H by H NMR
activation of either an arene C−H bond to produce reactive
2
E
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a
was then passed through a pad of Celite. Solvent was evaporated
under reduced pressure using a rotary evaporator. Borylated products
were isolated through a silica column (100−200 mesh) using an ethyl
acetate and hexane mixture.
Table 2. Controlled Experiments
General Procedure for Borylation of Heteroarenes. In a
nitrogen-filled glovebox, 1 (0.025 mmol), HBpin (0.75 mmol), the
appropriate heteroarene (0.5 mmol), and dry methylcyclohexane were
added together in a 15 mL sealed tube equipped with a Teflon-sealed
screw cap. Next, the mixture was heated at 80 °C for 16 h. After
completion of the reaction, the reaction mixture was diluted with 50
mL of an ethyl acetate and hexane (1/5) mixture. It was then passed
through a pad of Celite. Solvent was evaporated under reduced
pressure using a rotary evaporator. Borylated products were isolated
through a silica (100−200 mesh) column using an ethyl acetate and
hexane mixture.
b
entry
catalyst
yield
1
2
3
Ni-NPs
3
Ni-NPs and 3
<5%
0
83%
a
b
Benzene (3.75 mmol), HBPin (0.25 mmol). Isolated yield based on
HBPin.
1
9
Hg Drop Test. Inside a N -filled glovebox, 1 (17.7 mg, 0.025
2
mmol), HBpin (72 μL, 0.5 mmol), and Hg (55 mg, 0.275 mmol)
were placed in a 15 mL sealed tube containing 1 mL of benzene. The
sealed tube was tightly sealed with a screw cap and heated to 80 °C
for 16 h with stirring over a hot plate. The resulting mixture was
diluted with 20 mL of an ethyl acetate and hexane (1/5) mixture to
pass it through a bed of silica, and solvent was fully removed under
vacuum. The formation of the desired borylated product was not
observed by proton NMR spectroscopy.
Ni−Ar or Ni−H species or reaction with HBPin to produce
Ni−H and Ni-BPin species. This reductive elimination may
result in the formation of the product and generates the active
catalyst with the simultaneous liberation of H gas.
In conclusion, this report documents catalytic C(sp )−H
2
2
borylation of a wide variety of arenes using a nickel catalyst.
For the first time, we could perform borylation of n-
hexylbenzene, n-butylbenzene, n-propylbenzene, phenylcyclo-
hexane, naphthalene, biphenyl, 1,3-diisopropylbenzene, and
Preparation of 1 and 2. A 50 mL Schlenk flask was charged with
1,3-bis(2,6-diisopropylphenyl)-2,5-diphenyl-1H-imidazol-3-ium chlor-
ide hydrochloride and L1 (613 mg, 1.0 mmol) and kept under
vacuum for 2 h at 80 °C. Next, it was transferred to a nitrogen-filled
1
,3-dimethoxybenzene using a first row transition metal
2
glovebox to add Ni(COD) (275 mg, 1.0 mmol) and potassium
catalyzed direct C(sp )−H borylation reaction. Mono- and
disubstituted alkyl and alkoxy arenes as well as solid substrates
can also be borylated under these reaction conditions. Active
reaction intermediates were also characterized by an array of
spectroscopic and microscopic studies as well as X-ray
crystallographic studies.
2
bis(trimethylsilyl)amide (419 mg, 2.1 mmol). It was taken out of the
glovebox and 15 mL of dry THF was added through a cannula under
a nitrogen atmosphere at −78 °C. The mixture was stirred for 2 h at
room temperature. In this process, the color changed from pale yellow
to dark red. The reaction mixture was fully dried under vacuum, and
the complex was extracted in dry benzene. Analytically pure dark
yellow crystals of 1 were obtained by recrystallization from benzene/
1
13
EXPERIMENTAL SECTION
hexane (1/7 v/v) mixture at 25 °C after 3 days. H and C NMR
■
24
General Considerations. All air- and moisture-sensitive manip-
ulations were carried out using standard high-vacuum-line, Schlenk, or
cannula techniques or in an MBraun drybox containing an
atmosphere of purified nitrogen. Glasswares were dried overnight at
spectra were recorded and matched with the literature data: 568.4
mg, yield 80%.
The preparation of 2 was accomplished using 1,3-bis(2,6-
diisopropylphenyl)-5-(4-methoxyphenyl)-2-phenyl-1H-imidazol-3-
1
30 °C before use. All solvents and arenes were dried over a Na/
ium chloride hydrochloride, L2 (322 mg, 0.5 mmol), Ni(COD)
2
benzophenone mixture and distilled under inert conditions before use.
(137.5 mg, 0.5 mmol), and potassium bis(trimethylsilyl)amide (220
1
13
The H and C NMR spectra were recorded on 400 and 500 MHz
NMR spectrometers with residual undeuterated solvent as an internal
standard. 11B NMR spectra were obtained by using a Bruker Avance
mg, 1.1 mmol) following a procedure similar to that described for 1. 2
1
was characterized by single-crystal X-ray crystallography and H NMR
13
and C NMR spectroscopic measurements: 280.5 mg, yield 76%.
1
1
5
00 MHz NMR spectrometer. Chemical shifts for B NMR spectra
Anal. Calcd for C54
H
64
N
2
NiO: C, 79.50; H, 7.91; N, 3.43. Found: C,
Complex 2. H NMR (400 MHz, C D , 298 K): δ (ppm) 8.10 (d, J
6 6
were referenced using Et O·BF as an external standard. Chemical
79.34; H, 8.10; N, 3.73.
2
3
1
shifts (δ) are given in ppm, and J values are given in Hz. For
transmission electron microscopy (TEM) imaging, Ni-NPs were
dispersed in hexane by ultrasonication to obtain a stable dispersion of
the particles. One drop of this solution was drop-cast onto the carbon-
coated Cu-based grid (Ted Pella, Inc., 300 mesh Cu), and TEM
images were recorded with JEOL apparatus, Model JEM-2100F. TEM
images in STEM (HAADF) mode were taken at the UHR-FEG-TEM,
DST-FIST facility of IISER Kolkata. For X-ray photoelectron
spectroscopy (XPS) experiments, Ni-NPs were dispersed in hexane
by sonication and 10−15 drops of solution were drop-cast onto a glass
= 7.2 Hz, 1H), 7.31−7.20 (m, 3H), 7.17 (s, 1H), 7.05 (d, J = 7.6 Hz,
1H), 6.98 (dd, J = 6.8 Hz, 3.2 Hz, 3H), 6.59 (m, 3H), 6.30 (dd, J =
8.4 Hz, 2.4 Hz, 1H), 6.03 (d, J = 8.4 Hz, 1H), 5.16 (t, J = 8.0 Hz,
1H), 4.01 (q, J = 8.4 Hz, 1H), 3.65 (quin, J = 7.2 Hz, 1H), 3.54 (s,
3H), 3.26 (q, J = 6.8 Hz, 1H), 3.19 (q, J = 6.8 Hz, 1H), 3.05 (quin, J
= 6.8 Hz, 1H), 2.77 (quin, J = 6.8 Hz, 1H), 2.64−2.57 (m, 1H),
2.33−2.24 (m, 1H), 2.14−2.10 (m, 1H), 2.00−1.92 (m, 1H), 1.61−
1.54 (m, 2H), 1.41 (d, J = 6.8 Hz, 6H), 1.33 (d, J = 6.8 Hz, 3H),
1.27−1.23 (m, 1H), 1.13 (d, J = 6.8 Hz, 3H), 1.09 (t, J = 6.4 Hz, 6H),
0.94 (d, J = 6.8 Hz, 3H), 0.65 (J = 6.8 Hz, 3H), 0.62 (d, J = 6.8 Hz,
2
plate (area <1 cm ). Solvent was fully dried under vacuum, and the
1
3
experiment was conducted on an Omicron Nano Technology (Model
3H). C NMR (100 MHz, C D , 298 K): δ (ppm) 172.8 (Ni−CAr),
6 6
0571) XPS spectrometer using an aluminum anode (Al Kα, 1486.6
168.8 (Ni−CCarbene), 157.7, 151.0, 145.3, 144.4, 144.3, 143.0, 137.8,
137.2, 136.5, 132.0, 129.9, 128.9, 128.4, 127.5, 127.1, 126.9, 124.5,
124.3, 124.1, 123.7, 123.6, 117.8, 107.9, 106.0, 68.3, 63.3, 53.6, 32.1,
31.0, 29.0, 28.2, 28.1, 28.0, 27.9, 24.0, 23.2, 23.1, 22.4, 22.3, 22.2,
22.0, 21.5.
eV) run at 15 kV and 10 mA as an X-ray source.
General Procedure for Borylation of Arenes. In a nitrogen-
filled glovebox, a 15 mL sealed tube equipped with a Teflon-sealed
screw cap was charged with a magnetic bar, nickel complex 1 (17.7
mg, 0.025 mmol), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBPin, 72
μL, 0.5 mmol), and the corresponding arene (7.5 mmol).
Subsequently, the mixture was heated to 80 or 100 °C for 16−24
h. After completion of the reaction, the reaction mixture was diluted
with 50 mL of an ethyl acetate and hexane (1/5) solvent mixture. It
Preparation of 3. Inside a N -filled glovebox, a 15 mL J. Young
2
pressure tube equipped with a PTFE screw cap was charged with 50
mg of complex 1 (0.07 mmol) and it was dissolved in a minimum
amount of dry benzene-d (1.5 mL), resulting in a bright red solution.
6
Next, 10.3 μL of 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBPin,
F
DOI: 10.1021/acs.organomet.9b00364
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
0
.07 mmol) was placed in the vial. The mixture slowly turned into
Notes
dark red from bright red. The tube was capped tightly and taken out
of the glovebox to stir the mixture at 80 °C for 2 h under closed
conditions. The mixture was cooled to room temperature and taken
inside a glovebox. The black particles had already settled down at the
bottom of the tube. The supernatant liquid was gently decanted to
one 5 mL vial. Dry hexane (2.5 mL) was added carefully to make
separate layers (1:1.7 v/v). These layers were kept for crystallization
at room temperature after adding hexane (1.5 mL). After 10 h, good-
quality yellowish block-shaped crystals were found to grow inside the
vial: 20.8 mg, yield 44%. Due to high air and moisture sensitivity, the
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the SERB (DST) of India (Grant No. EMR/2017/
00772). A.D. and P.K.H. thank the IISER Kolkata for
■
0
research fellowships. A.D. thanks Sourav Ghosh, IISER-
Kolkata, for his help in TEM and XPS studies. The authors
also thank the NMR, X-ray, and TEM facilities of IISER-
Kolkata.
1
elemental analysis experiment of 3 was unsuccessful. H NMR (400
MHz, C D , 298 K): δ (ppm) 8.15 (d, J = 6.8 Hz, 1H), 7.25−7.19 (m,
6
6
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,5-Cyclooctadiene Liberation Experiment. Inside a N -filled
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AUTHOR INFORMATION
Corresponding Author
E-mail for S.K.M.: swadhin.mandal@iiserkol.ac.in.
■
*
4
(
ORCID
Swadhin K. Mandal: 0000-0003-3471-7053
G
DOI: 10.1021/acs.organomet.9b00364
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DOI: 10.1021/acs.organomet.9b00364
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