Phosphine-functionalized NHC Ni(II) and Ni(0) complexes: Synthesis, characterization and catalytic properties

Article abstract of DOI:10.1039/c7dt01805c

Two families of nickel complexes bearing chelating diphenylphosphine-functionalized NHC ligands [Ni^II(ArNHCPPh₂)(allyl)]Cl 1a (Ar = Mes); 1b, (Ar = 2,6-iPr₂-C₆H₃) and [Ni⁰(ArNHCPPh₂)(alkene)] 2a (Ar = 2,6-iPr₂-C₆H₃, alkene = styrene); 2b (Ar = 2,6-iPr₂-C₆H₃, alkene = diethyl fumarate) have been prepared and fully characterized. VT-NMR experiments in solution reveal that the allyl derivatives 1a-b are stereochemically nonrigid. The solid-state structure of the Ni⁰ derivative 2b is also reported. These complexes display interesting catalytic properties in various cross-coupling reactions. The precatalyst [Ni⁰(ArNHCPPh₂)(styrene)] 2a was found to be the most active system. The bulkiness of the N-substituent on the imidazole ring and the low oxidation state of the metal center in 2a accounted for its enhanced catalytic performance. This system catalyzed effectively the coupling of (hetero)aryl chlorides with a range of nucleophiles including Grignard reagents, boronic acids, secondary amines and indoles.

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Phosphine-functionalized NHC Ni(II) and Ni(0) complexes:
†
synthesis, characterization and catalytic properties
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
,a
,a
,b
S. G. Rull, R. J. Rama, E. Álvarez, M. R. Fructos,* T. R. Belderrain,* M. C. Nicasio*
II
DOI: 10.1039/x0xx00000x
Two families of nickel complexes bearing chelating diphenylphosphine-functionalized NHC ligands [Ni (ArNHCPPh
2
)(allyl)]Cl
0
1
a (Ar = Mes); -1b, (Ar = 2,6-iPr
2
-C
6
H
3
) and [Ni (ArNHCPPh
2
)(alkene)] 2a (Ar = 2,6-iPr
2
-C
6
H
3
, alkene = styrene); -2b (Ar = 2,6-
www.rsc.org/
iPr -C
2
H
6 3
, alkene = diethyl fumarate) have been prepared and fully characterized. VT-NMR experiments in solution reveal
0
that the allyl derivatives 1a-b are stereochemically nonrigid. The solid-state structure of the Ni derivative 2b is also
reported. These complexes display interesting catalytic properties in various cross-coupling reactions. The precatalyst
0
[
Ni (ArNHCPPh
2
)(styrene)] 2a was found to be the most active system. The bulkiness of the N-substituent on the imidazole
ring and the low oxidation state of the metal center in 2a accounted for its enhanced catalytic performance. This system
catalyzed effectively the coupling of (hetero)aryl chlorides with a range of nucleophiles including Grignard reagents,
boronic acids, seconday amines and indoles.
ascribed to the synergistic effect between the two ligands, i.e., the
dissociation of the more labile phosphine ligand provides a vacant
coordination site for the oxidative addition, whereas the tightly
Introduction
The field of organic chemistry, largely dominated by palladium
catalysis, is gradually giving way to more sustainable transition
bound NHC ligand facilitates the oxidative addition and the
reductive elimination, the latter being accelerated by phosphine
recoordination. This kind of dynamic behavior has been also
attributed to bidentate phosphine-functionalized N-heterocyclic
1
metal catalysts, such as those based on earth-abundant 3d metals.
Nickel represents a very good example of this tendency, as
evidenced by the significant progress achieved in Ni-mediated C-C
1
0,11
carbenes (NHCP).
and C-heteroatom bond formation and other coupling reactions
2
over the last decade. Not only is nickel a cheaper alternative to Palladium-based catalysts bearing bidentate NHCP ligands have not
1
2,13
palladium, but it also offers diverse reactivity and novel catalytic been extensively applied in cross-coupling reactions.
Regarding
2
,3
transformations.
to nickel, we are aware of only two reports on the applications of
NHCP/Ni complexes as catalysts for C-C couplings. Poli and co-
workers have described the use of zwitterionic Ni(II) complexes
with phosphine-functionalized imidazolium ligands as catalysts in
Within the area of cross-coupling chemistry, nickel catalysis has
enabled the efficient coupling of reluctant electrophiles under
4
palladium catalysis, such as phenol derivatives. Most of these
1
4
the Kumada-Tamao-Corriu coupling reaction. Lee and co-workers
have shown that mono- and bis-NHCP Ni(II) chelates are effective
compelling results have been achieved with the use of N-
5
heterocyclic carbenes (NHC) as supporting ligands. The remarkable
1
5
catalysts for Suzuki coupling of aryl chlorides. So far, the
application of this type of ligand in Ni-catalyzed C-heteroatom
cross-coupling reactions remains largely unexplored.
σ-donor character of the NHCs ligands and the efficient steric
protection provided by the substituents on the N atoms enhance
the stability of the catalyst and its activity, when compared to
6
-7
phosphines. In the last years, several groups have reported the Part of our research interest is focused on the development of
beneficial effect of using mixed NHC/PPh complexes in cross- molecularly defined Ni catalysts for cross-coupling reactions. We
3
8
,9
coupling reactions. The enhanced activity of such systems is have described the efficiency of two monodentate NHC/Ni catalyst
II
0
precursors, [Ni (IPr)(allyl)Cl] and [Ni (IPr)(styrene)
2
] (IPr = N,N’-bis-
1
6
(
2,6-diisopropylphenyl)imidazole-2-ylidene), in C-C
and C-
1
7
heteroatom couplings reactions. We became interested in
phosphine-functionalized NHC ligands since Ni-based catalyst
systems containing these ligands have been underexplored in cross-
coupling chemistry. For this purpose, we decided to prepare
monochelate NHCP-Ni compounds analogues to those of
monodentate IPr complexes above mentioned. In this work, we
account the synthesis and structural characterization of two
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ARTICLE
Journal Name
families
of
Ni(II)
and
Ni(0)
complexes
containing
diphenylphosphine-functionalized NHC ligands. These complexes
are active catalysts in the coupling of Grignard reagents and boronic
acids with aryl chlorides. Moreover, the performance of NHCP/Ni
complexes in the arylation of N-nucleophiles has been evaluated for
the first time.
Results and Discussion
II
Synthesis and Characterization of Cationic ArNHCPPh
2
-Ni (allyl)
Complexes. In this study, we chose two chelating NHCP ligands
containing a diphenylphosphine arm. The ligand bulkiness was then
controlled by the substituents on the N atom of the imidazolyl
II
Scheme 1 Synthesis of cationic Ni -allyl complexes 1a-b
.
2
.65 and 2.53 ppm). In addition, the rotation of the mesityl
II
moiety. The target ionic Ni complexes 1a and 1b were synthesized
substituent of the NHC moiety around the C-N bond was hindered,
as inferred from the observation of a singlet for each type of methyl
protons of the mesityl ring.
following a two-step procedure (Scheme 1). First, the phosphine-
8
c,18
imidazolium salt, prepared as reported,
deprotonated at -30ºC in THF using 1 equiv of K[N(SiMe
Subsequently, a solution of the Ni(II) precursor [NiCl(allyl)] in THF
was conveniently
3 2
)
].
The chemical shifts of the allyl group in 1a deserve further
comments. The resonances of the allyl terminal protons of the
carbon atom trans to the NHC moiety at low temperature (δ 3.53
and 1.88 for Hsyn and Hanti, respectively) were slighted shifted to
higher frequencies than those corresponding to the carbon atom
2
was added to the previous mixture leading to the isolation of the
complexes 1a-b as yellow-orange solids in 87-90% yield.
Complexes 1a-b are quite robust in solid-state and in solution. Their
elemental analyses were in agreement with the expected
trans to the phosphine arm (δ 3.45 and 1.82 for Hsyn and Hanti
respectively). Since the difference in chemical shift between the
,
1
31
1
13
1
compositions. H, P{ H} and C{ H} NMR spectra of both
complexes showed the presence of only one configurational isomer
two syn (or the two anti) protons depends on the σ-donor
3
1
1
9
2 2
in solution. The P resonances for 1a-b, in CD Cl , were observed as
properties of the ligands attached to the metal center, the small
variance observed was a consequence of minor differences in the σ-
donation capabilities of both NHC and phosphine arms of this
ligand. The similarity in the electronic character of both donors was
also reflected in the small difference in chemical shift between the
singlets at 20.6 and 22.9 ppm, respectively. Upon coordination,
these resonances were shifted at higher frequency with respect to
the corresponding for the free imidazolium salts (-23.2 and -24.0,
respectively). In addition to this, the carbene carbon resonance
1
3
appeared as a doublet (δ 171.0, ,JCP = 20 Hz, for 1a and 172.5, JCP
=
two terminal allyl C resonances (4 ppm), appearing the CH
2
trans
2
4 Hz, for 1b) as a result of the bidentate coordination of the
to P at slightly higher frequency (δ 67.1, JCP = 29.1 Hz) than that of
1
2
0
ligands. At room temperature, the H NMR spectra of these
compounds showed three distinct resonances for the five allyl
protons: a multiplet for the central proton (at 4.97 ppm for 1a and
the CH group trans to C (δ 63.1).
2
For complex 1b the temperature had to be lowered to -50 ºC to
stop the syn-syn, anti-anti exchange (see ESI). The presence of
isopropyl groups on the NHC bound phenyl ring did not produce
significant changes in the chemical shifts of the allyl moiety with
regard to those of derivative 1a.
4
.96 ppm for 1b), a broad singlet for the syn protons (ca. 3.48 and
.46 ppm for 1a and 1b respectively) and a doublet for the anti
3
protons (ca. 1.99 and 1.95 ppm for 1a and 1b, respectively).
Furthermore, resonances due to the CH -CH - bridge of the
ArNHCPPh ligands were also very broad. From these data, it was
2
2
It has been proposed that the syn-syn and anti-anti exchange
proceeds through apparent allyl rotation, which can involve the
2
clear that a fluxional process involving the exchange between syn-
syn and anti-anti protons takes place in solution. VT NMR studies
were then carried out (see ESI). On cooling the CD Cl
2 2
solution of solvent or other donor molecules. To test which pathway was
2
1
dissociation of one arm of the chelate or the coordination of
2
2
complex 1a to -30 ºC five distinct resonances for the allyl protons responsible for the observed fluxional behavior of 1a-b at room
were clearly observed, indicating that the syn-syn and anti-anti temperature, complex 1b was treated with stoichiometric amounts
exchange was inhibited. At this temperature, it was also possible to of silver hexafluoroantimonate, affording the isolation of 1b-SF6.
observe independent broad resonances for each of the protons of Unlike in the case with the chloride derivative, the room-
1
ethylene bridge of the bidentate ligand (ca. 5.11, 4.47,
temperature H NMR of 1b-SF6 showed five nonequivalent allyl
protons (Figure 1). This result suggests that the apparent rotation
proceed through an associative pathway involving the coordination
2
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Alternatively, the styrene-derivative 2a could also be prepared by
the reduction of the cationic complex 1b with di-n-butylmagnesium
in THF, in the presence of 2 equiv of styrene (see ESI for
experimental details). Complexes 2a-b are stable in solid state
under an inert atmosphere, but they are degraded in solution when
exposing to air. Both complexes are very soluble in common polar
solvents, like THF or diethyl ether and sparingly soluble in non-polar
solvents, such as hexane or toluene. Compounds 2a-b represent the
first examples of
R1
K[N(SiMe
R2 (3 equiv)
] (1 equiv)
Ni(cod)
+
2
N
Ph
Ph
3
)
2
P
N
THF, 90 min, rt
Ni
1
N
Ar
N
PPh
2
Ar
Fig. 1 Room temperature H NMR (CD
bottom).
2
Cl
2
) spectra of 1b-SF
6
(top) and 1b
R1
R2
(
Cl
8
5-90%
2 6 3
Ar = 2,6-iPr C H
8
of chloride anion. When heating a toluene-d solution of 1b, a
2
a, alkene = styrene
2b, alkene = diethyl fumarate
second fluxional process was detected. At 80 ºC, all terminal allyl
protons exchanged between them, simultaneously. This syn-anti
0
3
Scheme 2
Synthesis of Ni -alkene complexes 2a-b.
exchange, occurring at both sides of the allyl ligand, is linked to η -
1
3
20b,22b,23
η -η allyl rearrangement.
zerovalent NHCPPh
2
-Ni derivatives stabilized by coordination to
alkenes. It is worth mentioning that attempts to prepare the
The coalescence temperatures for the syn-syn and anti-anti
1
13
1
corresponding ethylene derivatives failed. H and C{ H} of 2a in
interconversion processes were determined from the variable
1
C D displayed four distinct signals for the four methyl groups of the
6
6
temperature H NMR studies carried out with 1a
-b
. Thus, the
isopropyl substituents and two resonances for the two methine
groups, indicating that both isopropyl substituents were in different
chemical environment. In addition, the methylene protons of the
coalescence of the signals of the syn protons occurred at 288 and
83 K for 1a and 1b, respectively, whereas that of the anti protons
2
was observed at 258 K for complex 1b. It was not possible to
determine the coalescence of the anti proton resonances for 1a due
to strong overlap with the signal of the methyl groups. With the
coalescence temperatures and the separation of the allylic
resonances (see ESI for details) the estimated free energies of
2 2
fragment N-CH -CH -P were also inequivalent and appeared as four
multiplets. The olefinic protons of the styrene appeared as three
1
multiplets (ca. 3.71, 2.15 and 1.89 ppm) in the H NMR spectrum,
31
and a singlet (δ 52.2) and a doublet due to the coupling with the
P
nucleus (δ 33.4, JCP = 24 Hz) were observed for the olefinic carbons
-1
activation for the interconversion processes (kJ mol ) were as
follows: 1a, syn-syn, 58.9; 1b, syn-syn, 57.5; 1b, anti-anti, 53.3.
These values are similar to those reported for apparent allyl
13
1
in the C{ H} NMR. The resonances attributed to the alkene, both
1
13
1
in the H and C{ H} NMR, appeared at lower frequencies than
2
5
those of the free styrene, as expected for a strong metal-alkene π
22c-d,23c
13
1
rotation processes.
Synthesis and Characterization of neutral ArNHCPPh
Complexes. Due to their enhanced stability towards oxidation, Ni
back-bonding. Finally, in the C{ H} NMR spectrum the carbenic
0
-Ni (alkene) carbon signal was observed at δ 195.5 and appeared as a doublet
2
II
31
1
(JCP = 7.5 Hz) due to the coupling with the phosphorus. The P{ H}
complexes are commonly employed as catalyst precursors in most NMR displayed a singlet at 15.2 ppm, indicative of the coordination
Ni-mediated cross-coupling processes. To avoid the in situ of the phosphine arm to the nickel center (see above).
0
reduction step associated with the catalyst activation, the use of Ni
complexes can be advantageous. However, examples of catalyst
31
The P resonance for the diethyl fumarate complex 2b appeared at
1
13
2
3.7 ppm in C
those already discussed for 2a and deserve no further comments
see ESI).
6 6
D . Its H and C NMR features were very similar to
0
systems based on molecularly defined Ni complexes are
1
7b,24
scarce.
ArNHCPPh
Ni(ArNHCPPh
Having this in mind, we tackled the synthesis of
(
0
2
-Ni derivatives. The preparation of the title compounds
2
The structure of compound 2b was confirmed by X-ray diffraction
studies. Figure 2 shows the molecular structure of 2b together with
the most relevant bond distances and angles. To the best of our
knowledge, this is the first reported structure of a phosphine-
[
2
)(alkene)] (Ar = 2,6-iPr -C
6
H
3
, alkene = styrene, 2a
;
alkene = diethyl fumarate, 2b) was accomplished following a
modified procedure to that we reported for the complex
1
7b
[
(IPr)Ni(styrene)
products was higher when carrying out the reactions in a one-pot
fashion. Thus, an equimolar mixture of [Ni(cod) ], the ligand salt
2
].
We observed that the purity of the nickel
0
functionalized Ni -alkene complex. The nickel center is bonded to
the phosphorus and carbenic carbon atoms of the bidentate
2
2
ArNHCPPh ligand and to one molecule of diethylfumarate, in a
and the base were reacted in THF in the presence of an excess of
the alkene (3 equiv), at room temperature (Scheme 2). After the
workup procedure, complexes 2a and 2b were obtained in high
yields as yellow-orange solids.
distorted trigonal planar environment. The C=C double bond
distance of the coordinated alkene (1.434(4) Å) is longer than that
of free alkene (1.318(2) Å) as a result of the strong π back-
donation. Interestingly, both Ni−C (alkene) bond distances are very
2
6
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[
Ni] (0.1-5 mol%)
similar (1.976(3) and 1.965(3) Å), indicating that the phosphorus
and the carbenic carbon donors have comparable electron-donating
properties. A similar situation is encountered in a series of alkene-
Ar
Cl
+
MgBr
Ar
THF/t.a.
0
entry
precatalyst
Ar
time
yield
Pd complexes reported recently with structurally related
b
1
2g
(mol %)
1a (5)
1b (5)
2a (5)
2b (5)
1a (5)
1b (5)
2a (5)
2b (5)
2a (5)
2a (1)
2a (1)
2a (0.1)
2a (1)
2a (1)
(h)
18
18
18
18
18
18
18
18
18
18
1
(%)
phosphine-functionalized NHC ligands. The Ni−carbene distance
1.898 (3) Å) is within the range of values reported for other
1
2
3
4
5
6
7
8
9
p-Me-C
p-Me-C
p-Me-C
p-Me-C
6
6
6
6
H
4
H
4
H
4
H
4
80
92
98
95
82
88
92
84
97
91
94
76
81
41
(
0
27
NHC−Ni complexes. The alkene double bond is almost coplanar
with the nickel, the phosphorus and the carbenic carbon atoms
(
with a slight deviation of 15.64(0.12)° between the two planes
p-Et-C
p-Et-C
p-Et-C
p-Et-C
6
6
6
6
H
4
Ni1−C31−C3 and P1−Ni1−C1).
H
4
Catalytic Results. We began our catalytic survey by examining the
performances of complexes 1a-2b in the room temperature
Kumada-Tamao-Corriu (KTC) coupling of 4-chlorotoluene with
phenylmagnesium bromide. As depicted in Table 1, all precatalysts
were active in this reaction, with complex 2a affording the highest
yield (entry 3). Full selectivity towards the coupling product was
observed in all cases. A similar trend in the catalytic behavior of 1a-
H
4
H
4
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
1
1
1
1
1
0
1
2
3
4
1
2-quinolyl
1
2-quinoxalyl
1
2
b was obtained when 4-ethyl-chlorobenzene was used as the
a
coupling partner (entries 5-8). Next, we gauged the catalytic activity
Reaction conditions: ArCl (0.5 mmol), PhMgBr (0.75 mmol), precatalyst (0.1-5
b
1
mol%), THF (1 mL). Yields determined by H NMR using an internal standard.
of the most active catalyst system 2a in the challenging coupling of
2
-chloropyridine with phenylmagnesium bromide. Full conversion
Table 2. Cross-Coupling of Aromatic Chlorides with PhB(OH)
2
Catalyzed by Ni
a
Complexes.
[
Ni] (1-3 mol%)
PO
R
Cl
+
B(OH)
2
R
K
3
4
Toluene / 80 ºC / 18 h
b
entry
precatalyst
mol%)
R
yield (%)
(
1
2
3
4
5
6
7
8
9
1a (3)
1b (3)
2a (3)
2b (3)
2a (1)
1a (3)
1b (3)
2a (3)
2b (3)
Me
Me
68
70
98
68
72
90
92
95
79
Me
Me
Me
COMe
COMe
COMe
COMe
Fig. 2
Selected bond distances (Å) and angles (º) for 2b: Ni(1)-C(1), 1.898(3); Ni(1)-P(1),
.1527(7); Ni(1)-C(31), 1.977(2); Ni(1)-C(32), 1.965(2); C(31)-C(32), 1.435(4);
Molecular structure of 2b. Hydrogen atoms are omitted for clarity.
2
a
2 3 4
Reaction conditions: ArCl (0.5 mmol), PhB(OH) (0.65 mmol), K PO (1.3 mmol),
C(32)-Ni(1)-C(31), 42.69(11); C(1)-Ni(1)-P(1), 96.78(7); C(32)-Ni(1)-P(1), 107.35(8);
C(1)-Ni(1)-C(31), 113.39(11); C(1)-Ni(1)-C(32), 155.78(11).
b
1
precatalyst (1-3 mol%), toluene (2 mL), 18 h at 80 ºC. Yields determined by
NMR using an internal standard.
H
was attained using 5 mol % of the Ni precatalyst (entry 9). The
catalyst loading was subsequently decreased to 1 mol% and the
reaction was complete within 1 hour (entry 11). However, a further Next, we explored the activities of the NHCP-Ni complexes in the
reduction of the catalyst loading to 0.1 mol% produced lower yield Suzuki-Miyaura coupling of p-chlorotoluene with phenylboronic
(
76%) of the couple product (entry 12). Under the optimized acid (Table 2). These complexes showed moderate to high activities
catalytic conditions, other heteroaryl chlorides such as 2- (69-98% conversions) under somewhat standard conditions, e.g. 3
chloroquinoline and 2-chloroquinoxaline afforded 81% and 41%
yields of product, respectively, in the reaction with
phenylmagnesium bromide (entries 13-14).
3 4
mol% of the Ni precatalyst, K PO and toluene at 80 ºC (entries 1-4).
As in the previous case, the styrene derivative 2a afforded the
highest yield of the biaryl product. When the catalyst loading was
reduced to 1 mol %, complex 2a exhibited only moderate activity.
Using more reactive 4-chloroacetophenone as the coupling partner
led to excellent yields of product with 1a-b and 2a and good yield
Table 1 Cross-Coupling of (Hetero)aromatic Chlorides with PhMgBr Catalyzed by Ni
a
Complexes.
(
79%) with 2b at 3 mol% loading of catalysts (entries 6-9)
4
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Having demonstrated the good activities of compounds 1a
ARTICLE
-
2b, and 11). Under these conditions, 2a also catalyzed competently the
0
in particular, that of the styrene-Ni derivative 2a in the C-C cross- coupling of other heteroaryl chlorides such as, 2-chloroquinoline
coupling reactions examined, we focused on the Buchwald-Hartwig and 2-chloroquinoxaline affording the products in excellent yields
amination reaction. At stated above, the application of phosphine- (entries 12-13).
functionalized NHCs ligands in the amination of aryl halides have
The outcomes of the catalytic studies presented in Tables 1-3
received scant attention. We have found only two examples of use
demonstrate that the NHCP-Ni complexes 1a-2b, prepared in this
of phosphine-functionalized imidazolium ligands with palladium in
work, are all active in the cross-coupling processes examined. The
13
Buchwald-Hartwig aminations. However, no report on the
performance of this type of ligand in nickel-catalyzed Buchwald-
Hartwig aminations has yet been described. Hence, we evaluated
0
best catalyst among them is the styrene-Ni complex 2a. Its superior
performance not only can be ascribed to the larger steric bulkiness
2 6 3
of the substituent 2,6-iPr -C H on the imidazolyl moiety, but also to
the performance of NHCP-Ni complexes 1a
-2b in the amination of
a faster rate of activation since the metal precursor is in its zero
2
-chloropyridine with morpholine following catalytic conditions
oxidation state. The good catalytic activity showed by 2a in the C-C
1
7b
reported by us previously, i.e., NaOtBu as base, 5 mol% catalyst
loading, THF (1 mL), at 110 ºC. Complexes 1a-2b displayed poor to
moderate yields in reactions carried out for 16 hours (Table 3,
entries 1-4). A more reactive substrate, 2-chloroquinoline, was then
employed as the coupling partner and, as expected, the activities of
these NHCP-Ni systems were significantly enhanced (entries 5-8).
Once again, the styrene-derivative 2a displayed the best catalytic
behavior, leading to complete conversion of the substrates. Good
conversion was also obtained for 2-chloroquinoxaline as substrate
16
couplings compares well to those reported by Poli at al. and Lee et
17
II
al. for their Ni systems bearing bidentate NHCP ligands.
Furthermore, this catalyst system is also applicable for the coupling
of challenging heteroaryl chlorides with secondary amines
(
morpholine) and indole.
Conclusions
II
Two sets of nickel complexes [Ni (ArNHCPPh
2
)(allyl)]Cl,
(entry 9).
0
[
Ni (ArNHCPPh )(alkene)] have been easily prepared from the
2
Table 3. Cross-Coupling of Heteroaryl Chorides and N-Nucleophiles Catalyzed by Ni corresponding Ni precursor and phosphine-functionalized
a
II
Complexes.
imidazolium salt. The fluxional behavior observed for the allyl-Ni
derivatives at room temperature can be attributed to an apparent
allyl rotation facilitated by the chloride anion. At high temperature,
3
1
3
a
η -η -η allyl isomerization is also taking place. The
0
diethylfumarate-Ni complex, 2b, has been characterized by X-ray
1
2
precatalyst
(mol%)
1a (5)
(Het)Ar
HNR R
Yield
diffraction. This represents the first reported structure of
b
0
entry
(%)
ArNHCPPh -Ni -alkene complex. These complexes have been tested
2
1
2
3
4
5
6
7
8
9
2-pyridyl
2-pyridyl
morpholine
morpholine
morpholine
morpholine
morpholine
morpholine
morpholine
morpholine
morpholine
indole
24
35
46
40
80
83
96
82
82
for catalytic activity in cross-coupling processes. The complex
0
1b (5)
[
Ni (ArNHCPPh
2
)(styrene)] (Ar = 2,6-iPr
2
-C
6
H
3
), 2a, has shown its
2a (5)
2-pyridyl
potential as a versatile catalyst for the coupling of (hetero)aryl
chlorides with a range of nucleophiles including phenyl Grignard,
phenylboronic acid, morpholine and indole. The bulkiness of the N-
substituent on the imidazole ring and the low oxidation state of the
metal center in complex 2a have found to be beneficial for its
enhanced catalytic activity. This is the first example of NHCP/Ni
catalyst system applied in Buchwald-Hartwig aminations.
2b (5)
2-pyridyl
1a (5)
2-quinolyl
2-quinolyl
2-quinolyl
2-quinolyl
2-quinoxalyl
2-pyridyl
1b (5)
2a (5)
2b (5)
2a (5)
c
1
1
1
1
0
1
2
3
2a (5)
60
c
2a (10)
2a (10)
2a (10)
2-pyridyl
indole
94
c
2-quinolyl
2-quinoxalyl
indole
94
c
indole
95
Experimental Section
a
Reaction conditions: Heteroaryl chloride (0.5 mmol), N-nucleophile (0.6 mmol), General Procedures.
b
NaO
tBu (0.6 mmol), catalyst (5 or 10 mol%), dioxane, 16 h at 110 ºC. Yields
All reactions and manipulations were carried out under a nitrogen
atmosphere by using standard Schlenk techniques or under
nitrogen atmosphere in an Mbraun glovebox. All substrates were
purchased from Aldrich and used without further purification.
1
c
determined by H NMR using an internal standard. Reaction using LiO
t
Bu (0.6
mmol) as the base.
2
8
Solvents were distilled and degassed before use. [Ni(cod)
2
],
1
7c
29
Then, we surveyed the activity of 2a in the N-arylation of indoles,
[
Ni(allyl)Cl]
2
and the phosphine-functionalized NHCPPh
2
8
c,18
a challenging substrate due to its poor nucleophilicity, high acidity
and strong coordination ability of the NH group. 2-Chloropyridine
and indole were the substrates selected for the catalytic tests.
Carrying out the reaction with 5 mol% Ni at 110 ºC for 16 h,
provided moderate yield of the N-coupled product (Table 3, entry
ligands
were prepared according to literature methods. NMR
spectra were recorded on Agilent 400 MR or Agilent 500 DD2. FTIR
1
spectra were recorded on a Nicolet IR200 FTIR spectrometer. H
1
3
and C NMR shifts were measured relative to deuterated solvents
peaks but are reported relative to tetramethylsilane. Elemental
analyses were performed on a PerkinElmer Series II CHNS/O
Analyzer 2400.
1
0). Increasing the catalyst loading up to 10 mol% resulted in the
complete conversion of the substrate into the couple product (entry
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Dalton Transactions
Page 6 of 8
View Article Online
DOI: 10.1039/C7DT01805C
ARTICLE
Synthesis
Journal Name
Hz, C ), 131.9 (d, J = 3 Hz, C ), 131.5 (d, J = 11 Hz, C ), 131.3 (d,
Ar
CP
Ar
CP
Ar
i
[
Ni(ArNHCPPh
imidazolium salt (1.5 mmol) and potassium bis(trimethylsilyl)amide Hz, CAr), 124.9 (CAr), 124.3 (d, JCP = 6 Hz, CAr), 123.6 (CAr), 115.3
0.3 g, 1.5 mmol) were stirred in THF (10 mL) at -30 ºC for 2 hours. A (CHallyl), 67.6 (d, JCP = 18 Hz, CH2allyl), 63.2 (d, JCP = 5 Hz, CH2allyl), 47.1
solution of [Ni(allyl)Cl]
(0.2 g, 0.75 mmol) in THF (5 mL) cooled at - (d, JCP = 4 Hz, NCH ), 28.5 (d, JCP = 18 Hz, PCH ), 27.0 (CH-iPr), 26.8
0 ºC was added to the former suspension, and the mixture was (CH-iPr), 25.4 (CH -iPr), 24.9 (CH -iPr), 22.9 (CH -iPr), 22.6 (CH -iPr).
P{ H} NMR (202 MHz, CD Cl 21.3. Anal. Calcd for
PNiSbꢀ0.4CH Cl : C, 48.04; H, 4.83; N, 3.46. Found: C,
2
)(allyl)]Cl (Ar = Mes (1a), (2,6- Pr-C
6
H
3
(1b)). The
JCP = 3 Hz, CAr), 130.6 (CAr), 129.6, (d, JCP = 2 Hz, CAr), 129.5 (d, JCP = 2
(
2
2
2
3
3
3
3
3
3
1
1
allowed to reach room temperature. The solvent was removed
under vacuum and the residue was dissolved in dichloromethane
2
2
) δ
C
32
H
38
F
6
N
2
2
2
and filtered through a Celite pad. The solution was taken to dryness 47.83; H, 4.83; N, 3.46.
and the solid washed with diethyl ether and dried under vacuum.
[
2 6 3
Ni(ArNHCPPh )(alkene)] (Ar = (2,6-iPr-C H ; alkene = styrene (2a),
Recrystallization from toluene afforded the complexes as dark
diethyl fumarate, (2b)). The imidazolium salt (0.48g, 1 mmol),
potassium bis(trimethylsilyl)amide (0.2 g, 1 mmol), Ni(cod) (0.27 g,
mmol) and 3 equivalents of the corresponding alkene were
orange solids. Yields: 0.72 g, 90 % for 1a; 0.81 g, 87 % for 1b. Data
2
1
for 1a: H NMR (500 MHz, CD
2
2
Cl , -30 ºC) δ 8.13 (s, 1H, CHimid), 7.56-
1
7
.41 (m, 10 H, CHAr), 7.03 (s, 1 H, CHimid), 7.01 (s, 1 H, CHAr), 6.95 (s,
H, CHAr), 5.11 (m, 1 H, NCH ), 4.99 (m, 1 H, Hmeso), 4.48 (m, 1 H,
), 3.53 (m, 1 H, Hsyn), 3.45 (m, 1 H, Hsyn), 2.65 (m, 1 H, PCH ),
.53 (m, 1 H, PCH ), 2.32 (s, 3 H, CH ), 1.94 (s, 3 H, CH ), 1.92 (s, 3H,
CH ), 1.88-1.80 (m, 2 H, Hanti). C{ H} NMR (125 MHz, CD Cl , -30
ºC) δ 171.0 (d, JCP = 20 Hz, NCN), 139.6 (CAr), 136.3 (CAr), 133.4 (d, JCP
dissolved in THF (5 mL). The mixture was stirred for 90 minutes at
room temperature and then, it was filtered through a pad of Celite.
The volatiles were removed under reduced pressure. The yellow-
orange solid was washed with hexane to give the desired product.
1
2
NCH
2
2
2
2
3
1
3
1
3
1
3
2
2
Yield: 0.54 g, 90 % for 2a; 0.57 g, 85 % for 2b. Data for 2a: H NMR
(
500 MHz, C
CHAr), 7.10-6.85 (m, 13 H, CHAr), 6.41 (s, 1 H, CHimid), 6.13 (s, 1 H,
CHimid), 3.78-3.68 (m, 1 H, NCH and CHolefin), 3.39 (m, 1 H, NCH ),
.95 (hept, 1 H, JHH = 7 Hz, CH-iPr), 2.66 (hept, 1 H, JHH = 7 Hz, CH-
iPr), 2.15 (m, 1H, CHolefin), 1.94-1.85 (m, 2 H, PCH and CHolefin), 1.57
6 6
D ) δ 7.60 (t, 2 H, JHH = 8.5 Hz, CHAr), 7.27-7.14 (m, 3 H,
=
13 Hz, CAr), 131.8 (CAr), 131.6 (CAr), 131.1 (CAr), 129.5 (CAr), 129.5
Ar), 129.4 (CAr), 129.2 (d, JCP = 3 Hz, CAr), 125.0 (CAr), 122.6 (CAr),
15.5 (CHallyl), 67.1 (d, JCP = 29 Hz, CH2allyl), 63.1 (CH2allyl), 46.6 (d, JCP
(C
2
2
1
=
1
3
3
2
3 Hz, NCH
2
), 26.7 (d, JCP = 26 Hz, PCH
2
), 21.2 (CH3Ar), 18.4 (CH3Ar),
Cl ) δ 20.6. Anal. Calcd for
PNiCl: C, 65.26; H, 6.04; N, 5.25. Found: C, 64.74; H, 5.96;
2
31
1
8.2 (CH3Ar). P{ H} NMR (202 MHz, CD
2
2
3
3
(
m, 1 H, PCH
Hz, CH -iPr), 1.05 (d, 3 H, JHH = 7 Hz, CH
Hz, CH -iPr). C{ H} NMR (125 MHz, C
NCN), 150.3 (CAr), 146.3 (CAr), 145.8 (CAr), 139.6 (d, JCP = 23 Hz, CAr),
2 3
), 1.30 (d, 3 H, JHH = 7 Hz, CH -iPr), 1.11 (d, 3 H, JHH =
C
29
H
32
N
2
3
3
7
3
3
-iPr), 0.89 (d, 3 H, JHH = 7
6 6
D ) δ 195.5 (d, JCP = 8 Hz,
1
N, 5.21. Data for 1b: H NMR (500 MHz, CD
H, CHimid), 7.59-7.41 (m, 11 H, CHAr), 7.31-7.14 (m, 2 H, CHAr), 7.10
s, 1 H, CHimid), 5.16 (m, 1 H, NCH ), 4.99 (m, 1 H, Hmeso), 4.38 (m, 1
H, NCH ), 3.53 (m, 1 H, Hsyn), 3.41 (m, 1 H, Hsyn), 2.66 (m, 1 H, PCH ),
.53-2.41 (m, 3 H, CH-iPr and PCH
2
2
Cl , -50 ºC) δ 8.31 (s, 1
13
1
3
(
2
1
1
38.1 (CAr), 135.9 (d, JCP = 25 Hz, CAr), 133.3 (d, JCP = 15 Hz, CAr),
31.7 (d, JCP = 13 Hz, CAr), 129.1 (CAr), 128.3 (CAr), 128.0 (CAr), 127.6
Ar), 123.6 (CAr), 123.3 (CAr), 123.2 (CAr), 121.0 (CAr), 120.1 (CAr),
2
2
2
2
), 1.76 (m, 1 H, Hanti), 1.69 (m, 1
(
C
3
3
H, Hanti), 1.19 (d, 3 H, JHH = 6.5 Hz, CH
3
-iPr), 1.05 (d, 3 H, JHH = 6.9
1
19.8 (CAr), 52.2 (CH2olefin), 47.0 (d, JCP = 8 Hz, NCH
2
), 33.4 (d, JCP = 24
),
3
3
Hz, CH
.9 Hz, CH
CP = 24 Hz, NCN), 145.1 (CAr), 144.6 (CAr), 137.7 (CAr), 135.5 (CAr),
3
-iPr), 1.01 (d, 3 H, JHH = 6.5 Hz, CH
3
-iPr), 0.82 (d, 3 H, JHH
=
Hz, CHolefin), 28.4 (CH-iPr), 28.3 (CH-iPr), 27.7 (d, JCP = 23 Hz, PCH
2
13
1
6
3
-iPr). C{ H} NMR (125 MHz, CD
2
Cl
2
, -70 ºC) δ 172.5, (d,
31
1
2
5.4 (CH
NMR (202 MHz, C
or the molecular mass by HRMS have failed with this compound.
3
3 3 3
-iPr), 24.6 (CH -iPr), 23.7 (CH -iPr), 22.6 (CH -iPr). P{ H}
J
6 6
D
) δ 15.2. Attempts to obtain elemental analysis
133.1 (d, JCP = 13 Hz, CAr), 131.5 (CAr), 131.0 (CAr), 130.9 (CAr), 130.6
(
CAr), 129.9 (CAr), 129.1-128.9 (m, CAr), 128.7 (CAr), 127.9 (CAr), 124.9
1
3
Data for 2b: H NMR (500 MHz, C
6
D
6
): δ 8.07 (t, 2 H, JHH = 8.7 Hz,
(
C
Ar), 124.5 (CAr), 123.8 (CAr), 123.6 (d, JCP = 16 Hz, CAr), 114.7 (CHallyl),
6.4 (d, JCP = 22 Hz, CH2allyl), 62.8 (CH2allyl), 45.8 (NCH ), 28.0 (CH-iPr),
7.9 (CH-iPr), 26.2 (d, JCP = 27 Hz, PCH ), 25.6 (CH -iPr), 22.9 (CH
-iPr). P{ H} NMR (202 MHz, CD Cl ) δ
PNiClꢀ0.5C : C, 68.57; H, 6.81; N,
.50. Found: C, 68.94; H, 6.79; N, 4.25.
)(allyl)]SbF (Ar 2,6-
equivalent of AgSbF (0.123 g, 0.35 mmol) was added to a solution
3
CHAr), 7.49 (t, 2 H, JHH = 8.7 Hz, CHAr), 7.24-7.12 (m, 7 H, CHAr), 7.08-
6
2
7
.01 (m, 2 H, CHAr ), 6.46 (s, 1H, CHimid), 6.09 (br. s, 1 H, CHimid), 4.05
2
2
3
3
-
2
3
2
3
1
1
(dq, 1 H, JHH = 11 Hz, JHH = 7 Hz, COOCH
2
CH
3
), 3.97 (dq, 1 H, JHH
CH and
), 3.54 (m,
), 3.09 (sept, 1 H,
HH = 6.5 Hz, CH-iPr), 3.08 (sept, 1 H, JHH = 6.5 Hz, CH-iPr), 1.72 (m,
=
iPr), 22.2 (CH
3
-iPr), 21.1 (CH
3
2
2
3
1
1 Hz, JHH = 7 Hz, COOCH
2
CH
), 3.71 (dq, 1 H, JHH = 11 Hz, JHH = 7 Hz, COOCH
H, CHolefin), 3.33-3.21 (m, 2 H, CHolefin and NCH
3
), 3.90-3.81 (m, 2 H, COOCH
2
3
2
2.9. Anal. Calcd for C32
H N
38 2
7 8
H
2
3
NCH
2
2 3
CH
4
1
2
[
Ni(ArNHCPPh
2
6
=
i
Pr-C
6
H
3
(1b-SbF
6
)). One
3
3
J
3
6
1
1
H, PCH
2 3 2
), 1.60 (d, 3 H, JHH = 6.5 Hz, CH -iPr), 1.39 (m, 1 H, PCH ),
of complex 1b (0.2 g, 0.35 mmol) in dichloromethane (5 mL). The
3
3
.12 (d, 3 H, JHH = 6.5 Hz, CH
3
3
-iPr), 0.99 (d, 3 H, JHH = 6.5 Hz, CH -
reaction stirred for 10 min and then filtered through a pad of Celite.
3
3
iPr), 0.97 (t, 3 H, JHH = 7 Hz, COOCH
2
CH
), 0.70 (d, 3H, JHH = 7 Hz, CH
6
D ): δ 192.0 (NCN), 173.8 (d, JCP = 5 Hz, C=O), 145.9 (CAr),
3
), 0.84 (t, 3 H, JHH = 7 Hz,
The solvent was removed under reduce pressure to afford a pale
3
13
1
COOCH
2
CH
3
3
-iPr). C{ H} NMR (125
1
yellow solid in quantitative yield. Data for 1b-SbF
MHz, CD Cl
.16 (d, 1 H, JHH = 1.8 Hz, CHimid), 5.00 (m, 1 H, Hmeso), 4.65 (m, 1 H,
NCH ), 4.35 (m, 1 H, NCH ), 3.59 (m, 1 H, Hsyn), 3.54 (m, 1 H, Hsyn),
.68 (m, 1 H, PCH ), 2.57-2.44 (m, 3 H, CH-iPr and PCH ), 1.90-1.85
m, 2 H, Hanti), 1.21 (d, 3 H, JHH = 6.8 Hz, CH
6
: H NMR (500
MHz, C
6
2
2
) δ 7.63-7.44 (m, 10 H, CHAr), 7.36-7.28 (m, 2 H, CHAr),
1
C
1
C
1
44.9 (CAr), 137.4 (CAr), 137.2 (CAr), 136.9 (CAr), 135.1 (d, JCP = 31 Hz,
3
7
Ar), 133.3 (d, JCP = 14 Hz, CAr), 132.2 (d, JCP = 13 Hz, CAr), 129.4 (CAr),
29.1 (CAr), 128.9 (CAr), 128.4 (d, JCP = 9 Hz, CAr), 128.1 (d, JCP = 9 Hz,
Ar), 128.0 (CAr), 127.2 (CAr), 123.8 (d, JCP = 5 Hz, CAr), 123.3 (CAr),
2
2
2
2
2
3
3
(
3
-iPr), 1.09 (d, 3 H, JHH =
19.8 (CAr), 57.9 (COOCH
), 41.2 (d, JCP = 20 Hz, CHolefin), 34.0 (COOCH
5 Hz, PCH ), 26.3 (CH-iPr), 24.5 (CH-iPr), 23.3 (CH
iPr), 22.3 (CH -iPr), 21.8 (CH -iPr), 14.4 (d, JCP = 20 Hz, COOCH
2
CH
3
), 57.8 (CHolefin), 46.2 (d, JCP = 7 Hz,
CH ), 27.4 (d, JCP
-iPr), 22.4 (CH
CH
3
3
6
=
1
.8 Hz, CH
6.8 Hz, CH
9 Hz, NCN), 145.6 (CAr), 145.1 (CAr), 136.0 (CAr), 132.9 (d, JCP = 14
3
-iPr), 1.07 (d, 3 H, JHH = 6.8 Hz, CH
3
-iPr), 0,95 (d, 3 H, JHH
NCH
2
2
3
=
1
3
1
3
-iPr). C{ H} NMR (125 MHz, CD
2
Cl ) δ 172.9, (d, JCP
2
=
2
2
3
3
-
3
3
2
3
),
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Page 7 of 8
Ple Da sae l tdo onn To rt aa nd sj ua sct t mi o an r sg ins
View Article Online
DOI: 10.1039/C7DT01805C
Journal Name
ARTICLE
3
1
1
1
3.9 (COOCH
ν(C-O) = 1668 cm (str). Anal. Calcd for C37
.76; N, 4.17. Found: C, 66.28; H, 6.49; N, 4.29.
2
CH
3
-
). P{ H} NMR (202 MHz, C
6
D
6
): δ 23.7. IR (KBr):
L. Sun and Z.-J. Shi, Chem. Eur. J., 2011, 17, 1728; d) T.
Mesganaw and N. K. Garg, Org. Process Res. Dev., 2013, 17,
1
45 2 4
H N O PNi: C, 66.19; H,
2
2
9; e) J. Cornella, C. Zarate and R. Martin, Chem. Soc. Rev.,
014, 43, 8081; f) M. Tobisu and N. Chatani, Acc. Chem. Res.;
6
General procedure for Kumada-Tamao-Corriu reactions.
2015, 48, 1717.
To a mixture of the catalyst (0.1 to 5 mol%) and the (hetero)aryl
chloride (0.5 mmol) in THF (1 mL), phenylmagnesium chloride (0.75
mmol, 1 M in THF) was added under a nitrogen atmosphere. The
reaction mixture was stirred at a room temperature for 16 h. A
5
6
(a) M. Henrion, V. Ritleng and M. J. Chetcuti, ACS Catal.,
2
ACS Catal., 2016,
015,
5
, 1283; (b) V. Ritleng, M. Henrion, and M. J. Chetcuti,
, 890; (c) A. P. Prakasham and P. Ghosh,
6
Inorg. Chim. Acta, 2015, 431, 61.
For general reviews on the electronic properties of NHCs
ligands, see: (a) G. Frenking, M. Solá and S. F. Vyboishchikov,
J. Organomet. Chem., 2009, 690, 6178; (b) H. Jacobsen, A.
Correa, A. Poater, C. Constabile and L. Carvallo, Coord. Chem.
Rev., 2009, 253, 687; (c) D. J. Nelson and S. P. Nolan, Chem.
Soc. Rev., 2013, 42, 6723; For selected reviews and books on
NHCs ligands, see: (d) S. Díez-González, N. Marion and S. P.
Nolan, Chem. Rev., 2009, 109, 3612; (e) M. N. Hopkinson, C.
Richter, M. Schedler and F. Glorius, Nature, 2014, 510, 485;
saturated solution of NH
extracted with diethyl ether (3 x 5 mL). The combined organic layers
were dried over anhydrous MgSO and the solven was evaporated
4
Cl was added and the mixture was
4
1
to dryness. The yield of product was determined by H NMR using
anisole as internal standard.
General procedure for Suzuki-Miyaura reactions.
The catalyst (1 or
3 4
3 mol%), the base K PO (1.3 mmol),
(f) N-Heterocyclic Carbenes: From Laboratory Curiosities to
Efficient Synthetic Tools, ed. S. Díez-González, RSC Publishing,
Cambridge, U. K. 2011.
phenylboronic acid (0.65 mmol) and toluene (2 mL) were added in
turn to a vial equipped with a J Young tap and containing a
magnetic bar. The aryl chloride (0.5 mmol) was added under a
nitrogen atmosphere. The reaction mixture was stirred for 18 h at
7
8
9
Outstanding bisphosphine/Ni complexes capable to catalyze
the monoarylation of challenging N-nucleopliles, e.g.
ammonia and primary amines, have recently appeared in the
literature. See for example (a) S, Ge, R. A. Green, J. F.
Hartwig, J. Am. Chem. Soc., 2014, 136, 1617; (b) A. Borkenzo,
N. L. Rotta-Loria, P. M. MacQueen, C. M. Lavoie, R.
80 ºC in an oil bath. The reaction mixture was allowed to cool to
room temperature, diluted with ethyl acetate (10 mL) and filtered
through celite. After evaporation of the solvent, the crude was
1
analyzed by H NMR using anisole as internal standard.
McDonald, M. Stradiotto, Angew. Chem. Int. Ed., 2015, 54
3
,
773; (c) R. A. Green, J. F. Hartwig, Angew. Chem. Int. Ed.,
General procedure for Buchwald-Hartwig amination reactions.
The catalyst (2.5-10 mol%), the base NaOtBu or LiOtBu (0.6 mmol)
and dioxane (1 mL) were added in turn to a vial equipped with a J
Young tap and containing a magnetic bar. The N-nucleophile (0.6
mmol) and the (hetero)aryl chloride (0.5 mmol) were added under a
nitrogen atmosphere. The reaction mixture was stirred at 110ºC for
2015, 54, 3768; (d) C. M. Lavoie, P. M. MacQueen, N. L.
Rotta-Loria, R. S. Sawatzky, A. Borzenko, A. J. Chisholm, B. K.
V. Hargreaves, R. McDonald, M. J. Ferguson, M. Stradiotto,
Nat. Commun. 2016,
7, 11073.
For mixed monodentate PR
3
/NHC Pd-based systems in cross-
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16 h in an oil bath. The reaction mixture was allowed to cool to
room temperature, diluted with ethyl acetate (10 mL) and filtered
through celite. The clean solution was evaporated to dryness, and
1
the residue was analyzed by H NMR using anisole as internal
standard.
Acknowledgements
We thank MINECO (Proyectos CTQ2011-24502 and CTQ2014-
3
For mixed monodentate PR /NHC Ni-based systems in cross-
52769-C3-3-R) for financial support. SGR thanks the Ministerio de
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