Efficient C-N and C-S bond formation using the highly active [Ni(allyl)Cl(IPr*OMe)] precatalyst

Article abstract of DOI:10.1002/ejoc.201402022

Two new [Ni(allyl)Cl(NHC)] complexes with the bulky yet flexible N-heterocyclic carbene (NHC) ligands IPr* {N,N'-bis[2,6- bis(diphenylmethyl)-4-methylphenyl]imidazole-2-ylidene} and IPr* ^OMe {N,N'-bis[2,6-bis(diphenylmethyl)-4-methoxyphenyl]imidazol-2- ylidene} are reported. These complexes were employed in the amination and sulfination of aryl halide species and were shown to perform well in these reactions, which typically required less than half as much catalyst as previous state-of-the-art nickel complexes. Copyright

Full text of DOI:10.1002/ejoc.201402022

FULL PAPER
DOI: 10.1002/ejoc.201402022
Efficient C–N and C–S Bond Formation Using the Highly Active
OMe
[Ni(allyl)Cl(IPr*
)] Precatalyst
Anthony R. Martin,[a][‡] David J. Nelson, Sébastien Meiries, Alexandra M. Z. Slawin,
[a]
[a]
[a]
and Steven P. Nolan*[
a]
Keywords: Nickel / Carbene ligands / Cross-coupling / Arenes / Amination / Sulfination
Two new [Ni(allyl)Cl(NHC)] complexes with the bulky yet
complexes were employed in the amination and sulfination
flexible N-heterocyclic carbene (NHC) ligands IPr* {N,NЈ-
of aryl halide species and were shown to perform well in
these reactions, which typically required less than half as
much catalyst as previous state-of-the-art nickel complexes.
bis[2,6-bis(diphenylmethyl)-4-methylphenyl]imidazole-2-
OMe
ylidene} and IPr*
{N,NЈ-bis[2,6-bis(diphenylmethyl)-4-
methoxyphenyl]imidazol-2-ylidene} are reported. These
Introduction
In the field of organotransition-metal-catalyzed trans-
formations, the role of ancillary ligands is of paramount
importance. In the search for efficient catalytic systems,
chemists have designed and prepared a wide variety of li-
gand families. These include, among others, N-heterocyclic
[
1]
[2]
[3]
carbenes (NHCs),
pincers,
phosphines,
phosphor-
amidites, and phenanthrolines. The incredible diversity Figure 1. IPr* and IPr*OMe NHC ligands on Pd centers in Suzuki
[
4]
[5]
and Buchwald–Hartwig coupling reactions and the [Ni(allyl)-
with regard to these ligands reflects the central role they
Cl(NHC)] structures described herein.
hold in transition-metal catalysis.
Since the isolation of the first stable NHC by Arduengo
and co-workers in the early 1990s, this class of compounds
has attracted considerable attention.[ More recently,
Markò reported the synthesis of an NHC ligand with flexi-
ble steric bulk, IPr* {IPr* = N,NЈ-bis[2,6-bis(diphen-
ylmethyl)-4-methylphenyl]imidazole-2-ylidene}.[ Our labo-
ratory has explored the use of this ligand coordinated to
In spite of its higher natural abundance in comparison
to palladium and, therefore, lower cost, there has been less
research on the topic of nickel in cross-coupling reactions.
Most recently, we focused our attention on the synthesis of
6]
well-defined [Ni-NHC] catalysts that have flexible steric
7]
OMe
bulk in the form of the IPr* and IPr*
ligands. These
new nickel-based complexes were subsequently assessed in
[8]
OMe
various transition metals. This ligand and its IPr*
[9g]
C–N bond-forming reactions. Although interesting cata-
OMe
{IPr*
= N,NЈ-bis[2,6-bis(diphenylmethyl)-4-methoxy-
lytic activity was observed for the first time with [NiCpCl-
phenyl]imidazol-2-ylidene} congener have led to highly
active well-defined palladium-NHC complexes that have
proven quite efficient when used in the Buchwald–Hartwig
and challenging Suzuki–Miyaura cross-coupling reactions
OMe
(NHC)] (NHC = IPr*
, Cp = cyclopentadienyl) com-
plexes in a Buchwald–Hartwig amination, the difficult acti-
vation of these precatalysts hampered access to outstanding
levels of activity. Hence, more easily activated Ni-based
[
9]
(see Figure 1).
OMe
complexes that have IPr* or IPr*
NHC ligands might
lead to highly active catalytic systems.
With this hypothesis in mind, we surveyed the reactivity
[
a] EaStCHEM School of Chemistry, University of St Andrews,
St Andrews, KY16 9ST, UK
OMe
of [Ni(allyl)Cl(IPr*)] and [Ni(allyl)Cl(IPr*
(see Figure 1), as the analogous palladium-based systems
PdCl(LX)(NHC)] (LX = allyl or cinnamyl) proved easy to
)] precatalysts
E-mail: snolan@st-andrews.ac.uk
http://chemistry.st-and.ac.uk/staff/spn/group/SP_Nolan/
Home.html
[
[
10]
[‡] Current address: Institut de Chimie de Nice, UMR 7272,
Université de Nice Sophia Antipolis, CNRS, Parc Valrose,
initiate. Therefore, we report herein the syntheses and ap-
plications of [Ni(allyl)Cl(NHC)] complexes with the IPr*
0
6108 Nice cedex 2, France
OMe
and IPr*
ligands in both Buchwald–Hartwig and thio-
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402022.
phenol arylation reactions.
Eur. J. Org. Chem. 2014, 3127–3131
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. P. Nolan et al.
FULL PAPER
Results and Discussion
The first step of our study was to synthesize the [Ni-
OMe
(allyl)Cl(NHC)] complexes that have IPr* and IPr*
as
the NHC ligands. For this purpose, we used the method
described by Dible and Sigman.[ This two-step protocol
11]
consists of the in situ formation of the [Ni(allyl)(μ-Cl)] di-
2
mer followed by the addition of a previously isolated free
carbene (see Scheme 1). Complexes 1 and 2 were isolated in
78 and 85% yield, respectively.
Scheme 1. Synthesis of [Ni(allyl)Cl(IPr*)] (1) and [Ni(allyl)Cl-
OMe
(IPr*
)] (2).
After characterization by proton and carbon NMR spec-
troscopy, the structures of 1 and 2 were unambiguously
confirmed by single-crystal X-ray diffraction analysis, and
their purity was confirmed by elemental analysis. Notably,
1
and 2 were found to be bench stable in the solid state,
although they do decompose rapidly in nondegassed sol-
[
11]
vents. The square-planar geometry of 1 and 2 was found
to be similar to that of previously reported complexes of
[
11]
the general formula [Ni(allyl)Cl(NHC)] (see Figure 2).
Calculations of the percent buried volume (%Vbur) using
Figure 2. Molecular structures of 1 (top) and 2 (bottom) with the
thermal ellipsoids displayed at 50% probability.
the SambVca web application were carried out for the two
newly synthesized complexes.[
12]
As expected, the volume
occupied in the metal coordination sphere by the IPr* and
OMe
IPr*
ligands (42.4 and 42.7%, respectively) was greater
Taking into consideration that the precatalyst that was
than that of frequently used ligands such as IPr (36.9%, used by Nicasio is of the same general formula [Ni(allyl)-
calculated by using the crystal structure reported by Sigman Cl(NHC)] as 1 and 2, we began our study by probing the
[
11]
et al.).
activity of our complexes under the conditions that were
Although [Pd-NHC] based catalytic systems have been reported by Nicasio and selected the coupling of 4-chloro-
extensively surveyed in the Buchwald–Hartwig reaction, anisole and morpholine as the benchmark reaction (see
much less attention has been devoted to nickel systems. Sig- Table 1).
nificant advances in the field of the Ni/NHC catalyzed am-
Although 1 (1 mol-%) was unable to catalyze this reac-
[
13]
ination of aryl halides have been described by Fort
and tion (see Table 1, Entry 1), to our delight, 2 performed
[
14]
Yang,
in which they employed in situ generated [Ni- much better and eventually led to the full conversion of 4-
NHC] complexes. However, the use of well-defined [Ni- chloroanisole with only 2 mol-% of the catalyst (see Table 1,
NHC] precatalysts remains scarce in the literature. Matsub- Entries 2 and 3). Further studies showed that both the na-
ara reported the use of a mixed [Ni(NHC)(phosphine)] cat- ture of the base and the counterions are important, with
alyst, however, this system required a high temperature the optimal base being sodium tert-butoxide (see Table 1,
(100 °C), high catalyst loading (10 mol-%), and long reac- Entries 3–7). In addition, various solvents were screened,
tion time (24 h) and was only efficient for activated aryl but THF remained the best choice (see Table 1, Entries 8–
[
15]
bromides. As mentioned above, our group also reported 11).
OMe
the use of 5 mol-% of the well-defined [NiClCp(IPr*
)]
After determining the optimized reaction conditions (see
complex for C–N bond formation.[ However, to the best Table 1, Entry 3), which were similar to those reported for
9g]
[
16]
of our knowledge, the most efficient catalytic system was [Ni(allyl)Cl(IPr)], we focused our attention on the scope
reported by Nicasio et al. They employed [Ni(allyl)Cl(IPr)] and limitation of the present systems (see Table 2). With
as a precatalyst (5 mol-%) to arylate morpholine with aryl regard to the electronic effects of the chlorides, both neutral
chlorides at 60 °C in tetrahydrofuran (THF). In the simple and activated substrates could be coupled to morpholine
cases of neutral or activated aryl chlorides, the coupling using only 1 mol-% of 2 to give the product in nearly quan-
could be performed at room temperature.[
16]
titative yields in short reaction times (see Table 2, Entries 1–
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Eur. J. Org. Chem. 2014, 3127–3131

Efficient C–N and C–S Bond Formation
Table 1. Optimization of the [Ni(NHC)] catalyzed arylation of
morpholine.
Table 2. Scope of the amination using [Ni(allyl)Cl(IPr*OMe)].[a]
[
a]
Entry
Precatalyst
Solvent
Base
% Conversion[b]
1
2
3
4
5
6
7
8
9
1 (1 mol-%)
2 (1 mol-%)
2 (2 mol-%)
2 (1 mol-%)
2 (1 mol-%)
2 (1 mol-%)
2 (1 mol-%)
2 (1 mol-%)
2 (1 mol-%)
2 (1 mol-%)
2 (1 mol-%)
THF
THF
THF
THF
THF
THF
THF
toluene
NaOtBu
NaOtBu
NaOtBu
KOtBu
0
52
Ͼ99
16
18
0
KOtAm[
NaOH
c]
K
2
CO
3
0
NaOtBu
NaOtBu
NaOtBu
33
15
18
33
[c]
DME
DCM[
c]
10
1
1
2-Me-THF NaOtBu
[
a] Reagents and conditions: 4-chloroanisole (0.5 mmol), morph-
oline (0.75 mmol), base (0.6 mmol), [Ni] (1–2 mol-%), 60 °C, 24 h.
b] Conversion into coupling product, as determined by GC analy-
[
sis. [c] tAm = tert-amyl, DME = 1,2-dimethoxyethane, DCM =
dichloromethane.
3). The more challenging 4-chloroanisole could also afford
the coupled product in high yield within only 3 h, however,
the catalyst loading had to be increased to 2 mol-% (see
Table 2, Entry 4). Although the electronic effects of the sub-
stituted aryl chloride do not play a major role in the out-
come of the reaction, the steric effects had more of an influ-
ence on the results. In fact, changing the substrate from 4-
to 3- and 2-chlorotoluene resulted in decreased reactivity
(see Table 2, Entries 1, 5, and 6). 3-Chlorotoluene was ef-
ficiently coupled to morpholine under the standard condi-
tions, whereas 2-chlorotoluene had to be treated with
2
mol-% of 2 for 16 h to avoid a drop in the yield. After
surveying the steric and electronic effects of the aryl chlor-
ides, we focused our attention on the amine partner. As
described above, morpholine, which is a cyclic dialkylamine,
can satisfactorily undergo a reaction with various electro-
philes. We found that the less nucleophilic N-methylaniline
proved to be a good coupling partner (see Table 2, En-
tries 7–9). The use of even less nucleophilic anilines was
more problematic (see Table 2, Entries 10 and 11). However,
the lower reactivity of these substrates could be circum-
vented by using 1,4-dioxane as the solvent along with
higher temperatures and 16 h of reaction time. Under these
conditions, the corresponding cross-coupling products
[
(
a] Reagents and conditions: aryl chloride (0.5 mmol), amine
0.75 mmol), NaOtBu (0.6 mmol), [Ni] (1–2 mol-%), 60 °C. [b] Iso-
lated yield (average of two runs). [c] 1,4-Dioxane as solvent, 100 °C.
d] Competing α-arylation occurred.
[
could be isolated in very high yields. Finally, we investigated the best of our knowledge, the use of only 1 mol-% of 2 for
the use of 3-chloropyridine as the electrophile (see Table 2, the reaction with both neutral and activated halides (see
Entry 12). Surprisingly, in this case, both a longer reaction Table 2, Entries 1–3) represents the lowest catalyst loading
time and higher temperature were required to access the reported to date. Moreover, our system exhibits outstanding
target compound in high yield. Acyclic dialkylamines did efficiency when compared with the previous state-of-the-
[
16]
not undergo arylation (Table 2, Entry 13), which illustrates art
catalyst for use with deactivated substrates (see
Table 2, Entry 4), by virtue of the lower catalyst loadings
a limitation of this system.
In comparison with previous reports in the field of well- (2 vs. 5 mol-%) and the shorter reaction times (3 vs. 24 h).
defined [Ni-NHC] catalyzed aminations of aryl chlorides, Although palladium catalysts are still typically more ef-
the system reported herein performed more efficiently. To ficient (with catalyst loadings as low as approximately
Eur. J. Org. Chem. 2014, 3127–3131
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3129

S. P. Nolan et al.
FULL PAPER
0
.01 mol-%), this work represents a significant improve- Table 3. Scope of the arylation of thiophenols.[a]
ment in the performance of nickel complexes and reduces
the performance gap between the palladium and nickel sys-
tems.
In a related area, the use of nickel catalysts to couple
thiols with aryl halides is an area of research that remains
largely underexplored. A few well-defined and in situ
formed Ni/phosphine complexes have been developed for
this purpose, although they generally suffer from serious
R1
R2
% Yield[b]
Entry
X
2 [mol-%]
1
2
3
4
5
6
7
8
9
H
H
H
H
OMe
CF
H
H
H
I
I
I
I
I
I
Br
Br
Cl
Cl
Br
4-Cl
0.5
1
1
1
1
1
1
2
2
98
97
86
86
90
–
83
43
–
4-Me
4-OMe
2-Me
4-Me
4-Me
H
4-OMe
4-Me
4-CF
4-CN
[
17]
[17a]
drawbacks. In fact, very harsh conditions,
high cata-
represent
[
17d]
[17b,17c]
lyst loadings,
and the use of additives
3
the main limitations of these systems. More recently, Ni/
[
16,18]
NHC based complexes have also been reported.
and Zhang
Ying
[
18a]
disclosed the first NHC-based Ni catalyst
10
1
H
H
3
1
1
74
83
that was formed in situ using [Ni(COD) ] (COD = cyclooc-
2
1
tadiene) and NHC·HX in the presence of a base. The active
0
species was proposed to be a bis(NHC)-Ni complex, which [a] Reagents and conditions: aryl halide (0.5 mmol), thiol
(
0.6 mmol), NaOtBu (0.6 mmol), [Ni] (0.5–2 mol-%), 100 °C, 24 h,
DMF (1 mL). [b] Isolated yield (average of two runs).
efficiently catalyzed the coupling of aryl iodides and brom-
ides with aryl and alkyl thiols. Subsequently, Jun and
Lee[
18c]
as well as Shi
[18b]
reported further advances in this
field with a solid supported [Ni-NHC] system and the inex- Entries 8 and 9), whereas activated 4-trifluoromethylchloro-
pensive [Ni(OAc) ] precursor, respectively. However, the benzene was well tolerated (see Table 3, Entry 10).
2
catalyst loadings were rather high (10 mol-%). In 2010,
Nicasio reported the only well-defined [Ni-NHC] precata-
lyst for this type of transformation. Efficient couplings Conclusions
could be performed by using 1–5 mol-% of [Ni(allyl)-
In summary, we have reported the synthesis of two new
Cl(IPr)], with the amount dependent on the nature of the
[
Ni(allyl)Cl(NHC)] precatalysts that have the bulky yet flex-
[
16]
aryl halide.
OMe
ible IPr* (1) and IPr*
(2) ligands. These complexes fur-
We surveyed the activity of complexes 1 and 2 with thio-
ther improved the catalytic performance of the nickel com-
plexes in cross-coupling reactions, which is a step towards
the development of nickel species that are competitive with
palladium complexes. Complex 2 proved to be highly active
in Buchwald–Hartwig arylamination reactions with catalyst
loadings as low as 1 mol-%. The coupling reactions proved
to be tolerant of the electronic effects of the halide partner,
whereas its steric properties had more of an influence on
the results. Precatalyst 2 also exhibited outstanding catalytic
performance in C–S bond-forming reactions with a catalyst
loading as low as 0.5 mol-%. Moreover, the coupling of aryl
bromides and chlorides were possible, depending on the ac-
tivation of the arene. Further work is currently ongoing in
phenol and 4-iodotoluene as model substrates and em-
ployed previously reported conditions [KOtBu, N,N-di-
methylformamide (DMF), 100 °C].[
18a]
Using a low catalyst
loading (0.2 mol-%) to compare 1 and 2 with [Ni(allyl)-
Cl(IPr)] resulted in 45, 52, and 33% conversion, respec-
tively, according to GC analysis. Hence, 2, which was the
best precatalyst for the amination reaction, also proved to
be the most efficient for the C–S bond formation. Next, the
scope and limitations of 2 were assessed (see Table 3).
Activated aryl iodides underwent the coupling reaction
efficiently with only 0.5 mol-% of 2 (see Table 3, Entry 1).
This is, to the best of our knowledge, the lowest catalyst
loading for the [Ni-NHC] catalyzed arylation of thiophenol.
Both neutral and deactivated (electron-rich) aryl iodides
also underwent facile coupling to afford the bis(aryl) mer-
captans in high yields (see Table 3, Entries 2–4). The present
system tolerated moderate steric hindrance as shown in
Table 3, Entry 4, and 2-iodotoluene was coupled to thio-
phenol in 86% yield. Electron-rich 4-methoxythio-
our laboratories to understand the difference in activity be-
OMe
tween IPr* and IPr*
as well as to develop new and more
active nickel complexes for cross-coupling applications.
Experimental Section
phenol appeared to be a good substrate for this reaction, General Procedure for the Buchwald–Hartwig Amination Reaction:
In a glovebox, a vial containing a stir bar was charged with Na-
whereas the electron-poor 4-trifluoromethylthiophenol did
OMe
OtBu (57 mg, 0.60 mmol, 1.2 equiv.) and [Ni(allyl)Cl(IPr*
)] (2,
not undergo reaction (see Table 3, Entries 5 and 6). The in-
fluence of the aryl halide proved to be highly important.
Bromobenzene was successfully used as the electrophile in
the reaction that employed only 1 mol-% of 2 (see Table 3,
Entry 7). This constitutes a fivefold decrease in the catalyst
loading in comparison with the most closely related precat-
5.4–10.8 mg, 1–2 mol-%), and the system was sealed with a screw
cap fitted with a septum. If solid, the amine (0.75 mmol, 1.5 equiv.)
and/or the aryl chloride (0.50 mmol, 1.0 equiv.) were added. Out-
side of the glovebox, THF (1 mL) was added, and then if liquid,
the amine and/or the aryl chloride were added. Finally, the vial was
heated at 60 °C for the time reported in Table 2. The solution was
alyst, [Ni(allyl)Cl(IPr)]. Electron-rich bromides and neutral then cooled to room temperature and adsorbed onto silica gel prior
chlorides did not undergo efficient coupling (see Table 3, to purification by flash column chromatography.
3130
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 3127–3131

Efficient C–N and C–S Bond Formation
General Procedure for the Arylation of Thiols: In a glovebox, a vial
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containing a stir bar was charged with NaOtBu (57 mg, 0.60 mmol,
OMe
1.2 equiv.) and [Ni(allyl)Cl(IPr*
)] (2, 0.5–2 mol-%), and the sys-
tem was sealed with a screw cap fitted with a septum. If solid,
the aryl halide (0.50 mmol, 1.0 equiv.) was added. Outside of the
glovebox, DMF (1 mL) was added, and then if liquid, the thiol
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(0.60 mmol, 1.2 equiv.) and the aryl halide were added. Finally, the
vial was heated at 100 °C for 24 h. The solution was then cooled
to room temperature and diluted with ethyl acetate (30 mL). The
resulting solution was subsequently washed with NaOH (1 m, 3ϫ)
and brine. The organic extract was dried with MgSO , filtered, and
4
concentrated in vacuo. The residue was purified by chromatog-
raphy on a silica gel column.
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CCDC-970312 (for 1) and -970313 (for 2) contain the supplemen-
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tained free of charge from The Cambridge Crystallographic Data
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and NMR spectra of all compounds.
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