Monovalent nickel complex bearing a bulky N-heterocyclic carbene catalyzes buchwaldhartwig amination of aryl halides under mild conditions

Article abstract of DOI:10.1246/cl.2011.1036

Monovalent nickel complex bearing a bulky N-heterocyclic carbene (NHC) and triphenylphosphine was synthesized and demonstrated to catalyze the BuchwaldHartwig amination of aryl halides with diarylamines to form triarylamines under mild conditions.

Full text of DOI:10.1246/cl.2011.1036

doi:10.1246/cl.2011.1036
1036
Published on the web September 5, 2011
Monovalent Nickel Complex Bearing a Bulky N-Heterocyclic Carbene Catalyzes
Buchwald-Hartwig Amination of Aryl Halides under Mild Conditions
Shinya Nagao,1 Taisuke Matsumoto,2 Yuji Koga,1 and Kouki Matsubara*1
1Department of Chemistry, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180
2Analytical Center, Institute for Materials Chemistry and Engineering, Kyushu University,
6-1 Kasuga, Fukuoka 816-8580
(Received June 22, 2011; CL-110522; E-mail: kmatsuba@fukuoka-u.ac.jp)
Ar
N
Monovalent nickel complex bearing a bulky N-heterocyclic
carbene (NHC) and triphenylphosphine was synthesized and
demonstrated to catalyze the Buchwald-Hartwig amination of
aryl halides with diarylamines to form triarylamines under mild
conditions.
N
N
Ar
Ar
N
Ar
N
N
N
Ar
N
Ar
N
Ar
Ar
Ar
PPh3
Ni Cl
P
Cl
Cl
1
Ni
Ni
Ni Cl
N
Ar
N
Ar
Ar
N
(Ar: 2,6-diisopropylphenyl)
2
Catalytic N-arylation of amine is an attractive and useful
synthetic process to produce arylamines efficiently under rather
mild conditions.1 In order to make triarylamines from poorly
nucleophilic diarylamines particularly, several studies using Pd,2
Ni,3 and Cu4 catalysts have been reported recently. Such
chemicals are known to have electronic, photoelectronic, and
magnetic properties, and benefit as a wide spectrum of new
materials, such as organic light-emitting diodes, photovoltaic
cells, nonlinear optics, and organic photoconductors.5,6 Most
reports for the nickel- or palladium-catalyzed reactions have
shown that zero- and divalent species are the key intermediates
in the catalytic cycles. However, in several cross-coupling
reactions, such as the Kumada-Tamao-Corriu cross coupling,7
and Negishi coupling reactions,8 radical catalysis has been
proposed via the formation of monovalent species as the
intermediates which make the reactions faster than those with
zero- and divalent species. As for the Buchwald-Hartwig
amination, Gao et al. proposed similar catalytic cycles with
magnesium amide and nickel complexes,3 however the evidence
is still unsatisfying.
Previously we showed that a monovalent nickel NHC
complex was isolated and used as a catalyst for the Kumada-
Tamao-Corriu cross coupling for the first time.9 Louie et al. also
reported nickel(I) NHC complexes, which were active for
Suzuki and Kumada-Tamao-Corriu cross-coupling reactions.10
However, the activities of the catalysts were not very high,
probably because the steric hindrance derived from two bulky
carbene ligands restricts the access of substrates. Now, we
prepared a new NHC/phosphine mixed complex of monovalent
nickel, which catalyzed the Buchwald-Hartwig amination of
aryl halides under ambient conditions.
3
Scheme 1.
Figure 1.
»molT vs. T plot for 3, obtained from SQUID measure-
ment.
again formed only 3. As the compound 2 reacts with chloroform
to form diamagnetic nickel dichloride, [NiCl2(IPr)2] (IPr: 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene), the compound
3 also reacts smoothly at ambient temperature to form the
analogous nickel(II) dichloride, [NiCl2(PPh3)(IPr)].13 The easy
radical abstraction of chlorine suggested the radical nature of 3.
The compound 3 is paramagnetic, demonstrated by a
magnetic susceptibility measurement; the value of »molT is
¹1
0.37 cm3 K mol at 5 K (Figure 1). This indicates that the spin
quantum number, S, is 1/2. The structure was finally determined
by single-crystal X-ray diffraction14 as shown in Figure 2 in
combination with elemental analysis. Notably the nickel atom
takes a planar Y-shape three-coordinate conformation, where the
sum of the three angles around Ni(1) is 359.6(1)°, in contrast to
2 taking T-shape coordination. The T- or Y-shape of the three-
coordinate nickel species has sometimes been discussed in
reports,15 probably due to not steric reasons but to some
electronic influence arising from the ligands.16 In our case, we
discussed that difference of the electronic properties in NHC and
PPh3 ligands may also lead to such shape differences. Each bond
distance between Ni(1) and Cl(1), P(1), or C(1) is in the range
of normal single bond length, 2.179(9), 2.20(1), or 1.930(3),
respectively. Similar results have been shown in the structure of
Our previous report also showed the existence of an
equilibrium (Scheme 1), indicating the labile nature of one
of the NHC ligands in 2. This prompted us to introduce a
phosphine ligand to dinickel(I) dichloride 1 instead of NHC.11
The monovalent nickel complex 3 bearing NHC/phosphine
ligand formed quantitatively in the presence of 2 equiv of PPh3
(Scheme 1), as expected. The compound 3 could be isolated in
66% yield by recrystallization from THF/hexane solution at
¹30 °C.12 Dissolving the crystals in benzene-d6 generated the
mixture of 1 and 3 again, suggesting that a similar equilibrium to
2 exists in solution. Addition of 2 equiv of PPh3 to the mixture
Chem. Lett. 2011, 40, 1036-1038
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1037
without ligand (80%) < 1 with PPh3 (12.5 mol %) (99%),
suggesting that the in situ formation of 3 is the important
process to stabilize and/or to activate the catalyst system. It is of
interest that the reaction was not affected by the presence of
excess phosphine ligand, suggesting that coordination of the
substrates to coordinatively unsaturated nickel species might not
be the rate-determining step in the catalytic cycle.
The catalyst activity of the system using 3 and PPh3 is
comparable to that using 1 and PPh3, but sometimes better than
using 1. Thus, in the presence of 3, PPh3 (25 mol %, 5 equiv to
nickel), and base, we added several aryl halides to diphenyl-
amine (eq 3).18 At first, the reaction of activated 4-bromoben-
zophenone resulted in the formation of the coupling product
efficiently at 40 °C as a surprise to us (Table 1, Entry 1). Even
at room temperature, the reactions of 4-bromobenzophenone,
4-bromotoluene, 4-iodoanisole, and p-bromonitrobenzene pro-
ceeded to form the triarylamine derivatives in 40-70% yields
(Entries 2, 7, 10, and 11). The reactions of bromides and iodides
at 70 °C gave the amination products in good yields (Entries 5,
6, 8, 9, and 12). Using chlorobenzene or 4-bromostyrene as a
reactant provided the desired product in moderate yield, 41 or
43% (Entry 4 or 13). The yield from 4-bromostyrene did not
improve even in longer reaction time. We assumed that the
catalyst could be decomposed or deactivated with the olefinic
substrate, because the yield only increased when we added
10 mol % of the nickel catalyst.
Figure 2. ORTEP drawing of [NiICl(IPr)(PPh3)] (3) (50% proba-
bility of thermal ellipsoids). All hydrogen atoms are omitted for
clarity. Representative bond distances and angles are as follows:
Ni(1)-Cl(1) = 2.179(9),
Ni(1)-P(1) = 2.20(1),
Ni(1)-C(1) =
1.930(3), C(2)-C(3) = 1.346(4) ¡; Cl(1)-Ni(1)-P(1) = 113.31(4),
Cl(1)-Ni(1)-C(1) = 134.2(1), P(1)-Ni(1)-C(1) = 112.1(1)°.
2 previously, suggesting that most of the unpaired electron does
not exist in the ligands but in the independent d orbital of
nickel.9
cat. 3 (5 mol%)
PPh3 (25 mol%)
At first we examined Kumada-Tamao-Corriu cross-cou-
pling reactions of aryl halides in the presence of the monovalent
nickel complex. Because the solution of 3 forms a mixture of 1,
3, and free phosphine, we added 5 equiv of PPh3 to the solution
of 1 before the reactions. As expected, the complex showed
good performance toward the coupling reaction as shown in
eq 1. For example, only 1.0 mol % of 3 (1 (0.5 mol %) + 5 equiv
PPh3) provided 4-methoxybiphenyl and p-terphenyl in 98 and
92% yields in 3 h, whereas 1.0 mol % of 2 (1 (0.5 mol %) + 5
equiv IPr) gave these compounds in 93 and 89% yields in longer
time, 18 h.
+
X
Ph2NH
ð3Þ
Ph2N
NaOt-Bu (1.5 equiv)
toluene
R
R
Table 1. Buchwald-Hartwig amination of aryl halides mediated
by 3a
Entry
Aryl halides
Temp/°C
Time/h
Yield/%b
O
1
40
24
97
Br
Br
Cl
O
O
2
25
72
68
1 (0.5 mol %)
PPh3 (5 equiv to 1)
ð1Þ
3
4
70
70
72
24
96
41
+
R
X
ClMg
R'
R
R'
THF, r.t
3 h
R, R', X
yield / %
Cl
98
92
86
69
OMe, H, Br
Ph, H, Br
H, Me, Br
H, Me, Cl
5
6
70
70
24
24
74
77
Br
I
O
cat. 1 (2.5 mol%)
O
ligand (12.5 mol%)
Ph2N
ð2Þ
+
Ph2NH
7
8
25
70
70
25
25
72
24
48
72
72
43
60
66
52
49
NaOt-Bu (1.5 equiv)
toluene, 80°C, 2 h
Br
Br
ligand yield / %
Br
OMe
IPr
-
PPh3
61
80
99
9
Br
I
OMe
NO2
10
11
We also examined the Buchwald-Hartwig amination of aryl
halides with diphenylamine, since triarylamine was generated
efficiently in the reaction of diphenylamine with bromobenzene
in the presence of a similar nickel complex, [NiIICl2(IPr)(PPh3)],
previously.17 At first, we examined amination reactions at 80 °C
for 2 h using some catalyst systems (eq 2). Significantly, the
reaction yields increased in the following order: 1 with IPr
(12.5 mol %, 2.5 equiv to nickel) (61% isolated yield) < 1
Br
Br
Br
12
13
70
70
72
72
75
43
aThe reaction was carried out with diphenylamine (1.2 equiv) and
NaOt-Bu. bIsolated yields were determined after silica gel column
chromatography.
Chem. Lett. 2011, 40, 1036-1038
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1038
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Ni(I)Ln
M
R
X
Transmet.
R
R'
Red.
Elim.
M
X
(Pd)
3
4
X
Ni(I)Ln
R' Ni(III)Ln
(Pd)
R
R
R'
X
Oxid. Add.
Figure 3. Proposed mechanism of the cross-coupling reactions.
Several reports describe the mechanism of the “radical”
catalysis in nickel and palladium chemistries: transmetallation
occurs to exchange halogen ligand of Ni(I) (or Pd(I)) halide and
subsequent oxidative addition of aryl halide forms Ni(III) (or
Pd(III)) species, easily giving a coupling product (Figure 3).3,7,8
We also considered that the formation of nickel(I) phenyl or
amide species could be the initial reaction in the Kumada-
Tamao-Corriu coupling or Hartwig-Buchwald amination, re-
spectively, that was inferred from preliminary experimental
results.19 Detailed mechanistic study for the reactions are now in
progress.
In summary, we successfully isolated and structurally
determined the phosphorus-substituted NHC complex of mono-
valent nickel from dinickel(I) halide. The complex has a unique
three-coordinate Y-shape structure in the solid state and a
suitable nature for catalysis in solution, making the unsaturated
site easily with liberation of phosphine ligand, that may be
attributed to the radical nature of the nickel(I) center. As
expected, the complex has high catalyst activities toward the
Kumada-Tamao-Corriu cross coupling and Buchwald-Hartwig
amination of aryl halides. Strikingly, amination of aryl halide
with diphenylamine proceeded to form triphenylamine deriva-
tives even at low temperatures, that has never been reported
previously to the best of our knowledge.
5
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12 Procedure for 3: In a glovebox, [Ni(®-Cl)(IPr)]2 (1) (0.03 mmol,
29.0 mg), PPh3 (0.06 mmol, 15.7 mg), and THF (0.5 mL) were
added to a 5 mL screw-capped tube. After the compounds were
dissolved, hexane (1.5 mL) was slowly added to the solution and
cooled to ¹30 °C. Yellow prismatic crystals of 3 were obtained,
after removal of the liquid and washing with small amount of
hexane (28.1 mg, 66% yield). Anal. Calcd for C45H51N2PClNi:
C, 72.55; H, 6.90; N, 3.76%. Found: 72.47; H, 6.95; N, 3.93%.
Details are shown in the Supporting Information, which is
available electronically on the CSJ-Journal Web site, http://
www.csj.jp/journals/chem-lett/index.html.
We thank Dr. Shinji Kanegawa and Dr. Yusuke Sunada in
the Institute for Materials Chemistry and Engineering, Kyushu
University, for measuring the magnetic susceptibility. This work
is financially supported by the Japan Society for the Promotion
of Science (Grant-in-Aid for Scientific Research No. 22550104).
13 K. Matsubara, K. Ueno, Y. Shibata, Organometallics 2006, 25,
3422.
14 CCDC 828967 contains the supplementary crystallographic data
for this paper. Copy of the data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi.
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18 We added 25 mol % of triphenylphosphine in these reactions.
However, using about 10 mol % also provided the similar
results.
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
References and Notes
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2
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script in preparation.
Chem. Lett. 2011, 40, 1036-1038
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/