Abnormal-NHC-Supported Nickel Catalysts for Hydroheteroarylation of Vinylarenes

Article abstract of DOI:10.1021/acs.organomet.7b00628

Herein we report the hydroheteroarylation of vinylarenes with benzoxazole in the presence of a free abnormal N-heterocyclic carbene and Ni(COD)₂, resulting in 1,1-diarylethane products exclusively. In an attempt to understand the mechanism of this catalytic reaction, two abnormal-NHC (aNHC)-coordinated Ni(II) cyclooctenyl complexes were isolated and their solid-state structures were determined by X-ray crystallographic studies. These Ni(II) cyclooctenyl complexes act as active catalyst precursors to generate in situ aNHC-Ni(0) species, which undergo oxidative addition with heteroarene to form Ni(II) hydride intermediates.

Full text of DOI:10.1021/acs.organomet.7b00628

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Abnormal-NHC-Supported Nickel Catalysts for Hydroheteroarylation
of Vinylarenes
Gonela Vijaykumar, Anex Jose, Pavan K. Vardhanapu, Sreejyothi P, and Swadhin K. Mandal*
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
S
* Supporting Information
ABSTRACT: Herein we report the hydroheteroarylation of
vinylarenes with benzoxazole in the presence of a free
abnormal N-heterocyclic carbene and Ni(COD)2, resulting
in 1,1-diarylethane products exclusively. In an attempt to
understand the mechanism of this catalytic reaction, two
abnormal-NHC (aNHC)-coordinated Ni(II) cyclooctenyl
complexes were isolated and their solid-state structures were
determined by X-ray crystallographic studies. These Ni(II) cyclooctenyl complexes act as active catalyst precursors to generate in
situ aNHC-Ni(0) species, which undergo oxidative addition with heteroarene to form Ni(II) hydride intermediates.
ransition-metal-catalyzed hydroheteroarylation of olefins
through C−H bond activation provides a highly atom
lysts14 since early reports by Crabtree, Nolan, and co-workers.15
The chemistry of aNHC gained momentum since the seminal
effort by Bertrand and co-workers to isolate the first aNHC in
2009.16 Recently, we have reported the same isolated aNHC,
derived from 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimida-
zolium salt, as an impressive ligand for convenient metal-free
catalysts17 and organometallic catalysts18 for a variety of organic
conversions.
As a part of our ongoing interest to develop catalysts based
on an aNHC ligand,17,18 herein we report that the Ni(COD)2/
aNHC combination can promote hydroheteroarylation of
vinylarenes with benzoxazole to afford selectively 1,1-diary-
lalkane derivatives. Further, in an effort to isolate the in situ
generated catalyst, the present study shows that a Ni(II)
cyclooctenyl complex is formed in solution. We have been able
to isolate two such aNHC-coordinated Ni(II) cyclooctenyl
complexes and characterized them by single-crystal X-ray
crystallography. On the basis of this observation, as well as
spectroscopic evidence from the use of stoichiometric reactions,
we propose a mechanistic pathway which involves activation of
the C−H bond of the heteroarene to form a Ni(II)-H complex,
assisted by flexible coordination adjustment by an abnormal
NHC ligand.
T
economical approach for the synthesis of 1,1- or 1,2-
diarylalkanes1 that are found in many pharmaceuticals and
biologically active molecules.2 Over the past decade, a variety of
transition-metal catalysts has shown significant activity in the
selective hydroheteroarylation of alkenes.3−8 There have been
intense efforts in recent years to develop hydroheteroarylation
catalysts with cost-efficient and cheaper metals. For example, in
2011, Yoshikai and co-workers have developed catalysts based
on Co-PCy3 as well as Co-NHC (NHC = 1,3-dimesitylimida-
zol-2-ylidene (IMes)) for hydroheteroarylation of vinylarenes
where the use of ligands can control the regioselectivity of the
reaction.9 Among the base-metal-catalyzed reactions, Ni-
(COD)2 (COD = 1,5-cyclooctadiene) and N-heterocyclic
carbene (NHC) based combinations have been the most well
studied for hydroheteroarylation, which was first introduced by
Nakao and Hiyama after their initial report on hydro-
heteroarylation of vinylarene in the presence of Ni(COD)2
and an NHC ligand (1,3-dimesitylimidazol-2-ylidene (IMes))
that exclusively afforded a 1,1-diarylethane derivative (branched
product).10 Ong and co-workers later reported Ni(COD)2 and
amino NHC based ligand combination in the presence as well
as in the absence of a Lewis acid (AlMe3) to understand the
influence of a Lewis acid on regioselectivity.11 The same group
in 2015 reported p-C−H alkylation of pyridine with
allylbenzene using a Ni(COD)2/NHC system.12 Recently,
Hartwig and co-workers investigated the anti-Markovnikov
hydroheteroarylation of aliphatic alkenes with a Ni(COD)2/
NHC based catalyst.13 All of these reports involving Ni-
(COD)2/NHC based catalysts postulate involvement of a
Ni(0)-NHC complex as the active catalyst generated in situ,
which undergoes oxidative addition with heteroarene. However,
a similar report on hydroheteroarylation catalysis with Ni-
(COD)2 is not known using an abnormal NHC (aNHC). It
may be noted that aNHCs are considered to be excellent
building blocks in designing numerous organometallic cata-
RESULTS AND DISCUSSION
■
Optimization of the catalytic study began by screening the
reaction of benzoxazole with styrene in the presence of
[Ni(COD)2] (5 mol %) and the aNHC ligand 1,3-bis(2,6-
diisopropylphenyl)-2,4-diphenylimidazol-5-ylidene (1) or 1-
(2,6-diisopropylphenyl)-2,3,5-triphenylimidazol-4-ylidene (2)
(Table 1) under different conditions. The product formation
1
was monitored by H NMR spectroscopy. We observed full
conversion using aNHC ligand 1 in hexane at 80 °C, to afford
Received: August 18, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00628
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Table 1. Evaluation of Ligands, Solvents, and Temperatures
for the Hydroheteroarylation of Styrene with Benzoxazole
Table 2. Scope of the Ni(COD)2/aNHC-Catalyzed
Hydroheteroarylation of Vinylarenes with Benzoxazole
a
a
b
entry
aNHC
solvent
temp (°C)
yield (%)
c
1
2
3
4
5
6
1
2
1
1
1
1
−
hexane
hexane
benzene
toluene
THF
80
80
98 (95)
65
90
90
85
110
85
<5
dioxane
hexane
90
<5
d
7
80
NR
a
Reaction conditions: ligand (5 mol %), Ni(COD)2 (5 mol %), 0.5
b
mmol of benzoxazole, 0.75 mmol of styrene, 5 mL of solvent, 1H
NMR spectroscopic yield. Isolated yield after column chromatog-
raphy. No ligand. NR signifies no reaction.
c
d
2-(1-phenylethyl)benzoxazole as the desired product in 95%
yield (Table 1, entry 1, and Table 2, entry 1). The formation of
regioisomeric 1,2-diarylethane was not observed. The use of the
somewhat sterically less hindered aNHC ligand 2 in hexane
gave an inferior result with 65% conversion (Table 1, entry 2;
see DFT results below on the relative reactivities of 1 and 2).
When the reaction was performed in benzene and toluene using
ligand 1, lesser conversions were obtained (90% and 85%;
Table 1, entries 3 and 4), and in THF as well as in dioxane, it
afforded very low conversion (<5%) (Table 1, entries 5 and 6).
The reaction did not proceed without any aNHC ligand when
the other conditions were maintained (Table 1, entry 7), which
confirms that the presence of the aNHC ligand is vital for this
catalysis.
Next, a range of vinylarenes was explored to check the scope
of this catalytic reaction under the optimized reaction
conditions, and the results are summarized in Table 2. A
variety of substrates including mono-, di-, and trisubstituted
styrene derivatives participated in the hydroheteroarylation
reaction successfully, leading to high isolated yields of 1,1-
diarylethane derivatives. Styrene with electron-donating groups
(Me, Et, OMe, Ph) attached at the para or ortho position
furnished the 1,1-diarylethane products (Table 2, entries 2−5
and 9) in high yields. In addition, styrenes containing electron-
withdrawing substituents (F, Cl, CF3) at the para position
provided the corresponding 1,1-diarylethane products in very
good yields (Table 2, entries 6−8). Notably, an aromatic C−F
bond was tolerated under our protocol, although activation of
the Ar−F bond has been reported with bis[1,3-bis(2′,6′-
diisopropylphenyl)imidazolin-2-ylidene]nickel(0).19 A styrene
derivative containing an electron-donating methyl group at the
meta position on the aromatic ring also resulted in a good yield
of 78% (Table 2, entry 10). A styrene derivative bearing an
electron-withdrawing trifluoromethyl substituent at the meta
position on the aromatic ring afforded the desired branched
product in very good yield (82%) with exclusive regioselectivity
(Table 2, entry 11). The reaction with 1-vinylnaphthalene
produced the corresponding branched product 2-(1-(naph-
thalen-1-yl)ethyl)benzoxazole in 94% yield (Table 2, entry 12).
Furthermore, the presence of electron-donating methoxy
substituents at meta positions was tolerated under our
conditions to afford the corresponding product in 72% yield
a
Reaction conditions: 5 mol % of ligand 1, 5 mol % of Ni(COD)2,
benzoxazole (0.5 mmol), vinylarene (0.75 mmol), and 5 mL of dry
hexane. Isolated yield after column chromatography. Reaction with
isolated complex 3.
b
c
(Table 2, entry 13). In addition, 3,4,5-trimethoxystyrene was
also able to undergo the hydroheteroarylation reaction,
resulting in the 1,1-diarylethane derivative in 65% yield
(Table 2, entry 14). The exclusive formation of branched
product involving styrene substrates has also been observed in
earlier studies,10a,11 which has been explained previously by Shi
and co-workers in their computational study on Ni-catalyzed
hydroheteroarylation reactions.10b
The reaction was also successfully applied to various
substituted benzoxazole derivatives to yield the corresponding
1,1-diarylethane products in excellent yields. Benzoxazole
having a methyl substituent on the benzene ring at the C5 or
C6 position gave the corresponding 1,1-diarylethane product
exclusively in 94% or 92% yield (Table 3). Benzoxazole bearing
a chloro, methoxy, or phenyl group at the C5 position
underwent the hydroheteroarylation process to produce the
branched alkylation product 5-chloro-2-(1-phenylethyl)-
benzoxazole in 85% yield, 5-methoxy-2-(1-phenylethyl)-
benzoxazole in 93% yield, and 5-phenyl-2-(1-phenylethyl)-
benzoxazole in 87% yield, respectively (Table 3).
In an effort to characterize the expected aNHC-Ni(0)
complex13 as the catalytically active species as well as to
B
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Table 3. Scope of the Ni(COD)2/aNHC-Catalyzed
Hydroheteroarylation of Styrene with Substituted
a
Benzoxazoles
Figure 1. Molecular structures of 3 and 4. Thermal ellipsoids represent
50% probability; hydrogen atoms are omitted for the sake of clarity.
Selected bond distances (Å) and angles (deg) are as follows: for 3,
Ni1−C1 1.912(4), Ni1−C5 1.947(4), Ni1−C40 2.081(4), Ni1−C41
1.963(4), Ni1−C42 2.045(4), C1−Ni1−C5 84.09(17), C1−Ni1−C42
177.32(18), C5−Ni1−C40 168.44(18), C5−Ni1−C42 96.74(19),
C1−Ni1−C40 105.98(19); for 4, Ni1−C1 1.897(3), Ni1−C5
1.927(3), Ni1−C34 2.067(3), Ni1−C35 1.965(3), Ni1−C36
2.049(3), C1−Ni1−C5 85.24(11), C1−Ni1−C36 170.41(11), C5−
Ni1−C34 167.93(11), C5−Ni1−C36 100.57(11), C1−Ni1−C34
98.89(11).
a
Reaction conditions: 5 mol % of ligand 1, 5 mol % of Ni(COD)2,
substituted benzoxazole (0.5 mmol), styrene (0.75 mmol), and 5 mL
of dry hexane. Isolated yields were obtained by column chromatog-
raphy.
understand the mechanistic cycle, we have performed
stoichiometric reactions between Ni(COD)2 and aNHC
ligands 1 and 2, respectively. To our surprise, we have
observed formation of the aryl C−H activated nickel(II)
cyclooctenyl complex 3 or 4 (Scheme 1) instead of the
nickel cyclooctenyl complex,20 where nickel-bound carbene
carbon appeared at δ 184.4 ppm and cyclocotenyl carbons
appeared at δ 106.8, 73.9, and 59.1 ppm in the 13C NMR
spectrum. Formation of a nickel(II) cyclooctenyl complex (3 or
4) may be rationalized by considering isomerization of 1,5-
COD to 1,3-COD followed by loss of a 1,3-COD ligand.
Subsequent C−H bond activation (ortho metalation) of one of
the adjacent aryl groups leading to generation of a Ni−H
species followed by nickel hydride migration to the 1,3-COD
ligand (Scheme 2) can generate the η3-allyl mode (cyclo-
octenyl) of binding to Ni(II). Such a reaction pathway has been
proposed following a previous report by Caddick, Cloke, and
co-workers.20
Scheme 1. Synthesis of Complexes 3 and 4
Scheme 2. Proposed Mechanism for the Formation of
Nickelacycle 3
anticipated aNHC-Ni(0) complex. Complexes 3 and 4 were
characterized by elemental analysis, NMR spectroscopy, and
finally X-ray crystallographic structures were determined. The
X-ray structures of 3 and 4 revealed that these complexes are
cyclooctenyl Ni(II) complexes in which one of the C−H bonds
of a phenyl group present in the aNHC ligand has undergone
ortho metalation (Figure 1). This observation was also fully
supported by NMR studies.
The molecular structures of 3 and 4 were determined by X-
ray crystallographic studies, which confirmed the abnormal
mode of binding of the NHC. The X-ray structures also
confirmed that one of the ortho C−H bonds of a phenyl group
has undergone activation. The solid-state structures (Figure 1)
showed that C4-aryl ortho C−H bond activation has taken
place during the reaction with aNHC ligand 1 while N-aryl
ortho C−H bond activation has taken place in the case of
reaction with aNHC ligand 2. The geometry around the nickel
ion is distorted square planar where nickel(II) binds to a
carbene carbon, a C−H activated ortho aryl carbon, and a
cyclooctenyl ligand in an η3 fashion. The Ni−C bond lengths
revealed that the Ni−C(carbene) bond distances (1.912(4) Å
in 3 and 1.897(3) Å in 4) are shorter than the Ni−C(aryl)
bond distances (1.947(4) Å in 3 and 1.927(3) Å in 4). These
Ni−C(carbene) bond distances are comparable to those
The 1H NMR spectra of 3 and 4 reveal that the coordinated
cyclooctenyl protons resonate at δ 5.17, 4.03, and 3.66 ppm in
3 and at δ 5.43, 4.21, and 3.91 ppm in 4. The abnormal carbene
carbon and the ortho-metalated carbon resonances coordinated
to Ni ion appeared in a highly downfield region (at δ 171.6 and
171.3 ppm in 3; at δ 166.3 and 163.7 ppm in 4). In addition to
this observation, nickel-coordinated cyclocotenyl carbon
resonances appeared at δ 108.9, 69.1, and 64.5 ppm in 3 and
at δ 109.2, 69.4, and 69.3 ppm in 4. These NMR chemical shift
values are comparable to the values reported for the NHC
C
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observed in earlier studies.20 To check whether these
complexes can act as catalysts for the hydroheteroarylation
reaction, we have carried out a catalytic reaction with the
isolated aNHC-Ni(II) complex 3. The reaction of benzoxazole
and styrene in the presence of a catalytic amount of complex 3
(5 mol %) in hexane at 80 °C afforded the corresponding 1,1-
diarylethane product 2-(1-phenylethyl)benzoxazole exclusively
in 96% yield (a similar yield (95%) was obtained with
Ni(COD)2/aNHC combinations; see Table 2, entry 1). This
result indicates that complex 3 is generated in situ by a reaction
between the free aNHC and Ni(COD)2 during the catalytic
reaction.
It is interesting to note that the use of the sterically less
hindered aNHC ligand 2 afforded low conversion (65%)
during catalysis in comparison to that for the sterically hindered
aNHC ligand 1 (98%) (vide supra; Table 1, entries 1 and 2).
To understand the role of the ligand in determining the
catalytic efficiency, we resorted further to study the electronic
structure of complexes 3 (with ligand 1) and 4 (with ligand 2),
which are considered to be active catalysts for this trans-
formation. Theoretical calculations were performed at the
DFT-B3LYP/LANL2DZ (for Ni) and 6-31+g(d) levels (for
other atoms). On comparison to the HOMO energy levels of
these complexes (see Figure 2) it may be concluded that
observed the presence of free 1,3-COD in the 1H NMR
spectrum of the reaction mixture (see Figure S46 in the
Supporting Information). The 13C NMR spectrum of the
reaction mixture from this stoichiometric reaction reveals that
one of the downfield 13C resonance signals (out of two signals
at δ ∼170 ppm) vanishes, indicating that the C−H activated
ortho aryl carbon−Ni bond cleaves during this step (see Figure
S47 in the Supporting Information). Additionally, we also
found the absence of an aryl proton, which appeared at a highly
shielded region in the 1H NMR spectrum of compound 3 (at δ
6.15 ppm). This particular signal at δ 6.15 ppm has been
assigned for a phenyl C−H proton at the ortho position of the
Ni−C(aryl) carbon, which has undergone ortho metalation.
Such an upfield shift on ortho metalation of the aNHC phenyl
proton has been previously noted in case of C−H activated
aNHC-Pd dimers.21 On the basis of these observations, it may
be proposed that the Ni(II) cyclooctenyl complex 3 acts as an
active catalyst precursor, generating the aNHC-Ni(0) inter-
mediate 3A (Scheme 3), which can undergo oxidative addition
Scheme 3. Plausible Mechanistic Cycle
Figure 2. HOMO energy levels of complexes 3 and 4 (isovalue 0.04,
density 0.0004 e−/au3).
complex 3 has a higher energy HOMO in comparison to that in
complex 4. It is interesting to note that the major contribution
to the HOMO is different in these complexes. For complex 3,
the HOMO is mainly located on the Ni atom as well as on the
aNHC backbone, whereas in complex 4, the HOMO lies
mainly on the Ni atom and the anionic cyclooctenyl ligand.
Looking at the chemical structure of these two complexes, we
note that the major difference lies in the presence of isopropyl
substituents on the N-phenyl rings of the aNHC backbone for
complex 3 whereas they are absent in complex 4. However,
from this calculation, it appears that the influence of this
electron-donating substituents is evident from a charge
population analysis, which clearly indicated that the imidazole
ring in complex 3 has a higher charge population in comparison
to that in complex 4 (see the Supporting Information). The
presence of these electron-donating substituents in the aNHC
backbone in 3 (as evident from the presence of relatively higher
energy HOMO, see Figure 2) would make oxidative addition
more facile than that in the other complex (4).
with heteroarene to produce the Ni(II)-hydride intermediate
3B. We have carried out a variable-temperature (VT) NMR
experiment on compound 3 in C6D6 by gradually increasing the
temperature up to 80 °C (see the Supporting Information for
1
the VT spectra). The H NMR spectrum of 3 recorded at 60
°C in C6D6 clearly showed the appearance of free 1,3-
cyclooctadiene (δ 5.85 and 5.55 ppm). The 1H NMR spectrum
recorded at 80 °C showed nearly complete disappearance of the
phenyl C−H proton at the ortho position of the Ni−C(aryl)
carbon and cyclooctenyl coordinated protons (at room
temperature these signals appeared at δ 6.15 ppm, assigned
Next, the stoichiometric reaction between 3 and benzoxazole
in a 1:1 ratio at 80 °C in benzene-d6 revealed a sharp change in
the 1H NMR spectrum. A highly upfield signal at δ −11.33 ppm
1
appeared in the H NMR spectrum, which supports formation
of a Ni−H compound. During this stoichiometric reaction, we
D
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outside the glovebox, and heated at 80 °C for 6 h. The resulting
mixture was filtered through a silica gel pad and then concentrated
under vacuum. The residue was purified by flash chromatography over
silica gel to afford the corresponding products in yields given in Tables
2 and 3.
to the phenyl C−H proton at ortho position of the Ni−C(aryl)
carbon, and at δ 5.17, 4.04, and 3.66 ppm, assigned to
cyclooctenyl coordinated protons in complex 3; see Figure S48
in the Supporting Information). However, the 1H NMR
spectrum on cooling to room temperature from 80 °C did not
change further. Such an 1H NMR spectroscopic change of 3 on
heating may be attributed to the intermediacy of 3A. On the
basis of the above experimental observations and previous
literature reports,13 we propose a mechanistic pathway for the
hydroheteroarylation reaction (Scheme 3). The aNHC-Ni(0)
species 3A can undergo oxidative addition with heteroarene to
form the Ni(II)-H intermediate 3B, which has been detected by
1H NMR spectroscopy (see above). In addition, the formation
of such an NHC-nickel hydride complex has been reported
earlier.11 In the next step, vinylarene coordinates to nickel ion
in an η2 fashion to form compound 3C. Subsequently, the Ni−
H bond can undergo regioselective insertion to the olefin,
which forms the compound 3D. Compound 3D can undergo
reductive elimination to release the product, and recoordination
of the free 1,3-COD regenerates 3A.
General Procedure for the Preparation of Complex 3. Under
a nitrogen atmosphere, benzene (10 mL) was added to a mixture of
1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimidazol-5-ylidene (1; 540
mg, 1 mmol) and Ni(COD)2 (275 mg, 1 mmol) at room temperature,
and this mixture was stirred for 30 min to give the dark red compound
3. Analytically pure dark red crystals of the title compound were
obtained by recrystallization from a benzene/hexane solvent mixture at
1
room temperature. Yield: 593 mg (84%); H NMR (400 MHz, C6D6,
25 °C, TMS): δ 8.36 (d, J = 7.2 Hz, 1H), 7.28 (t, J = 7.7 Hz, 1H), 7.20
(ddd, J = 7.7, 3.4, 1.6 Hz, 3H), 7.13 (dd, J = 7.6, 1.2 Hz, 1H), 7.04 (dd,
J = 7.6, 1.2 Hz, 1H), 7.01−6.93 (m, 3H), 6.83 (t, J = 7.8 Hz, 1H), 6.57
(dd, J = 6.9, 2.8 Hz, 3H), 6.14 (d, J = 7.5 Hz, 1H), 5.17 (t, J = 8.3 Hz,
1H), 4.04 (q, J = 8.7 Hz, 1H), 3.66 (dt, J = 13.7, 6.8 Hz, 1H), 3.29−
3.16 (m, 2H), 3.04 (dt, J = 13.7, 6.8 Hz, 1H), 2.75 (dt, J = 13.7, 6.8
Hz, 1H), 2.64−2.51 (m, 1H), 2.35−2.22 (m, 1H), 2.14 (s, 1H), 1.98
(dt, J = 17.5, 9.0 Hz, 1H), 1.68−1.51 (m, 2H), 1.44 (dd, J = 12.3, 5.5
Hz, 6H), 1.33 (d, J = 6.8 Hz, 3H), 1.30−1.22 (m, 1H), 1.11 (d, J = 6.9
Hz, 3H), 1.06 (dd, J = 9.4, 6.8 Hz, 6H), 0.91 (d, J = 6.8 Hz, 3H), 0.63
(d, J = 6.8 Hz, 3H), 0.59 (d, J = 6.9 Hz, 3H) ppm. 13C NMR (125
MHz, C6D6, 25 °C, TMS): δ 171.6, 171.3, 151.9, 146.3, 145.4, 145.3,
144.6, 144.0, 143.3, 139.6, 138.2, 133.0, 131.0, 130.0, 128.5, 128.3,
128.1, 127.9, 126.9,125.4, 125.3, 125.1, 124.7, 124.7, 122.7, 118.6,
108.9, 69.1, 64.5, 33.2, 31.8, 30.0, 29.2, 29.1, 29.0, 28.9, 25.0, 24.3,
24.2, 24.2, 23.4, 23.3, 23.3, 23.0, 22.5 ppm. Anal. Calcd for
C47H56N2Ni: C, 79.77; H, 7.98; N, 3.96. Found: C, 79.72; H, 7.92;
N, 3.92.
General Procedure for the Preparation of Complex 4. Under
a nitrogen atmosphere, benzene (10 mL) was added to a mixture of 1-
(2,6-diisopropylphenyl)-2,3,5-triphenylimidazol-4-ylidene (2; 456 mg,
1 mmol) and Ni(COD)2 (275 mg, 1 mmol) at room temperature, and
this mixture was stirred for 30 min to give the dark red compound 4.
Analytically pure dark red crystals of the title compound were obtained
by recrystallization from benzene/hexane solvents mixture at room
temperature. Yield: 499 mg (82%); 1H NMR (400 MHz, C6D6, 25 °C,
TMS): δ 8.59 (d, J = 7.0 Hz, 1H), 7.54 (d, J = 8.3 Hz, 2H), 7.27 (t, J =
7.3 Hz, 1H), 7.06−6.94 (m, 5H), 6.86 (d, J = 7.3 Hz, 4H), 6.83−6.74
(m, 2H), 6.70 (t, J = 7.2 Hz, 2H), 5.43 (t, J = 8.4 Hz, 1H), 4.22 (q, J =
8.2 Hz, 1H), 3.96−3.81 (m, 1H), 2.71−2.55 (m, 2H), 2.46 (dd, J =
13.3, 6.6 Hz, 2H), 2.36−2.24 (m, 1H), 2.18−2.09 (m, 1H), 1.92 (ddd,
J = 14.4, 8.7, 4.9 Hz, 1H), 1.74−1.62 (m, 1H), 1.51−1.42 (m, 2H),
1.33−1.24 (m, 2H), 1.19 (d, J = 6.8 Hz, 3H), 0.78 (d, J = 6.6 Hz, 3H),
0.47 (d, J = 6.8 Hz, 3H), 0.37 (d, J = 6.6 Hz, 3H) ppm. 13C NMR (125
MHz, C6D6, 25 °C, TMS): δ 166.4, 163.8, 152.1, 146.7, 146.2, 144.9,
141.0, 139.4, 132.9, 131.8, 131.7, 131.2, 130.6, 130.5, 130.0, 128.7,
128.6, 128.3, 127.7, 127.5, 126.8, 126.1, 124.9, 124.6, 122.7, 115.0,
109.3, 69.4, 69.3, 32.7, 32.0, 30.4, 30.0, 28.4, 28.3, 24.7, 24.5, 23.8, 23.5
ppm. Anal. Calcd for C41H44N2Ni: C, 78.98; H, 7.11; N, 4.49. Found:
C, 78.90; H, 7.16; N, 4.52.
CONCLUSIONS
■
In conclusion, we have developed for the first time abnormal
NHC ligand based Ni catalysts for regioselective hydro-
heteroarylation of vinylarenes with benzoxazoles, leading to a
wide range of 1,1-diarylethane products exclusively. A
mechanistic investigation by performing stoichiometric reac-
tions shows that a Ni(II)-aNHC complex acts as a precatalyst
for this reaction. Two such Ni(II)-aNHC based complexes
were isolated and fully characterized by s single-crystal X-ray
study. These Ni(II) cycloocetenyl complexes generate in situ
aNHC-Ni(0) species, which undergo oxidative addition with
heteroarene to produce Ni(II)-hydride species.
EXPERIMENTAL SECTION
■
General Methods and Instrumentation. All manipulations were
performed under a dry and oxygen-free atmosphere (nitrogen) using
standard Schlenk techniques or inside a glovebox maintained below
0.1 ppm of O2 and H2O. All glasswares were oven-dried (130 °C) and
evacuated while hot prior to use. All solvents were distilled from Na/
benzophenone prior to use. All other chemicals were purchased from
Sigma−Aldrich and used as received. Elemental analyses were carried
out using a PerkinElmer 2400 CHN analyzer, and samples were
prepared by keeping them under reduced pressure (10−2 mbar)
overnight. The HRMS data were obtained using a Finnigan MAT 8230
instrument. Analytical TLC was performed on a Merck 60 F254 silica
1
gel plate (0.25 mm thickness). H, 13C, and 19F NMR spectra were
recorded on a JEOL ECS400 MHz spectrometer and on a Bruker
Avance III 500 MHz spectrometer. 1,3-Bis(2,6-diisopropylphenyl)-2,4-
diphenylimidazol-5-ylidene (1) and 1-(2,6-diisopropylphenyl)-2,3,5-
triphenylimidazol-4-ylidene (2) were prepared according to the
literature procedure.16
X-ray Crystallographic Details. A single crystal of complex 3 or
4 was mounted on a glass tip. Intensity data were collected for 3 on a
Bruker APEX-II CCD diffractometer and for 4 on a Super Nova, Dual,
Mo at zero, Eos diffractometer. Atomic coordinates, isotropic and
anisotropic displacement parameters of all non-hydrogen atoms were
refined using Olex2,22 and the structure was solved with the
Superflip23 structure solution program using charge flipping and
refined with the ShelXL24 refinement package using least-squares
minimization.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00628.
Spectroscopic data, scanned spectra, X-ray crystallo-
graphic data for 3 and 4, and details on DFT calculations
(PDF)
Cartesian coordinates of calculated structures (XYZ)
General Procedure for the Hydroheteroarylation of Benzox-
azole and Vinylarenes. Inside a glovebox, benzoxazole (0.5 mmol)
was added to a solution of Ni(COD)2 (5 mol %) and ligand (5 mol %)
in hexane with subsequent addition of vinylarene (0.75 mmol) in a 10
mL sealed tube. The tube was sealed with a Teflon screw cap, taken
Accession Codes
CCDC 1565776−1565777 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
E
DOI: 10.1021/acs.organomet.7b00628
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(d) Hota, P. K.; Vijaykumar, G.; Pariyar, A.; Sau, S. C.; Sen, T. K.;
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Mandal, S. K. Adv. Synth. Catal. 2015, 357, 3162−3170.
(19) Bohm, V. P. W.; Gstottmayr, C. W. K.; Weskamp, T.;
̈
̈
Herrmann, W. A. Angew. Chem., Int. Ed. 2001, 40, 3387−3389.
(20) Caddick, S.; Cloke, F. G. N.; Hitchcock, P. B.; de K. Lewis, A. K.
Angew. Chem., Int. Ed. 2004, 43, 5824−5827.
AUTHOR INFORMATION
■
Corresponding Author
(21) Sau, S. C.; Santra, S.; Sen, T. K.; Mandal, S. K.; Koley, D. Chem.
Commun. 2012, 48, 555−557.
*E-mail for S.K.M.: swadhin.mandal@iiserkol.ac.in.
(22) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.;
Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339−341.
(23) (a) Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40, 786−
790. (b) Palatinus, L.; van der Lee, A. J. Appl. Crystallogr. 2008, 41,
975−984. (c) Palatinus, L.; Prathapa, S. J.; van Smaalen, S. J. Appl.
Crystallogr. 2012, 45, 575−580.
ORCID
Swadhin K. Mandal: 0000-0003-3471-7053
Notes
The authors declare no competing financial interest.
(24) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71,
3−8.
ACKNOWLEDGMENTS
■
We thank the SERB (DST) of India (Grant No. SR/S1/IC-25/
2012) for financial support. G.V. and P.K.V. thank the UGC,
New Delhi, India, for research fellowships. A.J. thanks the DST
for an Inspire fellowship. S.P. thanks the IISER-Kolkata for a
fellowship.
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DOI: 10.1021/acs.organomet.7b00628
Organometallics XXXX, XXX, XXX−XXX