Nickel-catalyzed reductive cross-coupling of aryl halides

Article abstract of DOI:10.1055/s-0032-1318237

This work highlights unsymmetrical biaryl compounds via direct nickel-catalyzed reductive coupling of two aryl halides. By tuning the ligand structures, the reaction of two electron-enriched aryl halides also provided the coupling product in good yields, with an excess of 1.4 equivalents of one of the halides. The mild reaction conditions display excellent functional-group tolerance and generally gave the coupling products in moderate to good yields. In addition to aryl bromides, activated aryl chlorides are effective.

Full text of DOI:10.1055/s-0032-1318237

LETTER
▌619
lNettierckel-Catalyzed Reductive Cross-Coupling of Aryl Halides
Reductive Cross-Coupling of Aryl Halides
Qun Qian,*1 Zhenhua Zang,1 Shulin Wang,1 Yang Chen, Kunhua Lin,* Hegui Gong
Department of Chemistry, Shanghai University, 99 Shang-Da Road, Shanghai 200444, P. R. of China
Fax +86(21)66134594; E-mail: qianqun@shu.edu.cn
Received: 27.12.2012; Accepted after revision: 28.01.2013
equimolar electron-poor 2-chloropyrimidine or 2-chloro-
pyrazine with functionalized aryl halides. The high cou-
pling efficiency appears to be a result of taking advantage
of the gap of electronic properties between pyridine/pyr-
Abstract: This work highlights unsymmetrical biaryl compounds
via direct nickel-catalyzed reductive coupling of two aryl halides.
By tuning the ligand structures, the reaction of two electron-en-
riched aryl halides also provided the coupling product in good
yields, with an excess of 1.4 equivalents of one of the halides. The azine and benzene-derived aromatics. It should also be
mild reaction conditions display excellent functional-group toler-
ance and generally gave the coupling products in moderate to good
yields. In addition to aryl bromides, activated aryl chlorides are ef-
balt and nickel methods suffer from limitations in
fective.
noted that bipyridine was the only ligand being investigat-
ed in the Ni-catalyzed protocols. In addition, both the co-
substrate scope. For example, the coupling of two elec-
tron-enriched aryl halides has not been reported, and the
Key words: nickel, cross-coupling, biaryl, aryl halides
nickel method is limited to N-heteroaryl halides with aryl
iodides. It is therefore important to further optimize the
nickel-catalyzed strategies, in particular the examination
of other ligands so as to achieve higher compatibility for
broader scope of substrates.
Aryl–aryl compounds are important organic units that are
often found as part of skeletons in natural products2 and
have been intensively applied to several fields, including
ligand design3 and materials science.4 As such, numerous
methods have been developed in the synthesis of aryl–aryl
bonds.2,3,5 In particular, unsymmetrical biaryl compounds
are generally obtained via conventional cross-coupling
protocols involving organometallic reagents and aryl
electrophiles.2,3,5 In addition to direct coupling of aryl C–
H bonds,6 reductive coupling of two aryl electrophiles, in
particular the Ni-catalyzed dimerization and oligomeriza-
tion of aryl electrophiles, has also become a straightfor-
ward approach to aryl–aryl compounds in which the
preparation of organometallic reagents can be avoided.5
With our recent success in the nickel-catalyzed reductive
coupling of alkyl halides with other electrophiles,9–11 we
reasoned that under nickel-catalyzed conditions, the effi-
ciency of reductive coupling of two different aryl halides
may be improved by tuning the reaction parameters such
as ligand structures, solvent, and temperature, which may
provide a complementary solution to the current methods.
In this paper, we highlight the Ni-catalyzed reductive cou-
pling of two different benzene-derived aromatic halides.
The reactions were carried out at ambient temperature
with a wide range of functional groups being tolerated
(Scheme 1).
However, direct reductive coupling of two unsymmetrical
aryl electrophiles, for example, halides, is generally less
effective due to intrinsic poor chemoselectivities between
two structurally similar aryl coupling partners.7 Although
a variety of transition metals including Co, Ni, and Pd
have been successfully employed, no general solutions
have been achieved to overcome the poor selectivity is-
sue.5,7,8 Gosmini recently developed a relatively efficient
cobalt-catalyzed method for the coupling of electron-defi-
cient with electron-enriched aryl and heteroaryl halides.7a
However, the reactions require the use of two equivalents
of the second aryl halides, and generally moderate to good
yields are obtained.
Gosmini's work:
X
R2
NiBr2bpy (cat.)
R2
R1
N
Mn, H+
DMF
R1
Y
N
(1 equiv)
this work:
(1.3 equiv)
NiI2 (10 mol%)
2a (10 mol%)
+
X
Y
R1
(1 equiv)
R2
R1
Zn (200 mol%)
additvies
DMA
(1.4 equiv)
R1, R2 = eletron-donating and
On the other hand, Gosmini further demonstrated that the
Ni-catalyzed electrochemical and chemical coupling of
aryl with pyridyl/pyrazinyl halides offered a facile syn-
thetic route to unsymmetrical aryl–pyridyl or aryl–pyr-
azinyl compounds (Scheme 1).7b The use of Zn and Mn as
the terminal reductant allowed the effective coupling of
X, Y = Br, Cl, I
electron-withdrawing groups
Scheme 1 Nickel-catalyzed chemically reductive synthesis of un-
symmetrical biaryl compounds
After extensive investigation of the reaction conditions
for the coupling of bromobenzene (100 mol%) and elec-
tron-enriched aryl bromide 1 (140 mol%), we identified
that NiI2 (10 mol%) as the precatalyst, 4,4′-dimethylbi-
SYNLETT 2013, 24, 0619–0624
Advanced online publication: 20.02.2013
DOI: 10.1055/s-0032-1318237; Art ID: ST-2012-W1102-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

620
Q. Qian et al.
LETTER
X
N
DMA provided the coupling product in a best yield of
63% (Table 1, entry 1). The addition of Bu4NI seems to be
important as the yield decreased to 53% without it, possi-
bly due to the removal of the salts on the zinc surface (Ta-
ble 1, entry 2). In the initial search for the optimal
conditions with no addition of Bu4NI, other ligands such
as tridentate pybox ligands 3a and 3b, bidentate ligands
2b–c, 4a–b, 5a–c (Figure 1), other nickel sources such as
NiBr2 and Ni(cod)2, and other solvent such as DMF were
less effective (Table 1, entries 3–15). The use of other ad-
ditives is also important. Without pyridine and Bu4NI no
product was observed (Table 1, entry 15). We reasoned
that the role of MgCl2 and Bu4NI is probably to remove
the salts on the zinc surface. The major side reaction arose
from the homocoupling of aryl halides. On the other hand,
the use of two equivalents of 1 decreased the yield (Table
1, entry 16).
R
R
O
O
N
N
N
N
R
R
2a, R = Me
2b, R = t-Bu
2c, R = H
3a, X = H, R = i-Pr
3b, X = Cl, R = H
X
R
R
O
N
N
N
N
R
4a, R = OMe
4b, R = Me
5a, X = Cl, R = H
5b, X = Cl, R = i-Pr
5c, X = OMe, R = i-Pr
Figure 1 Structures of ligands
With the optimized conditions in hand, the limitation and
scope of the aryl halides were examined (Table 2). In con-
trast to 1, reaction of bromobenzene with electron-en-
riched 1-bromo-3,4-dimethoxybenzene and 1-bromo-4-
methoxybenzene provided the coupling products in 56%
pyridine (10 mol%) as the ligand, zinc powder (200
mol%) as the reductant, in the presence of MgCl2 (100
mol%), pyridine (100 mol%), and Bu4NI (100 mol%) in
Table 1 Optimization of the Reaction Conditions
OMe
OMe
OMe
OMe
'standard conditions'
NiI2 (10 mol%), 2a (10 mol%)
Br
+
OMe
Ph Br
Ph
Zn (200 mol%)
MgCl2 (100 mol%)
pyridine (100 mol%)
Bu4NI (100 mol%)
DMA (1 mL)
OMe
1 (1.4 equiv)
Entry
Deviation from the standard conditions
Yield (%)a
1
2
none
63
53
48
48
without Bu4NI
3
2b instead of 2a, without Bu4NI
2c instead of 2a, without Bu4NI
4
5
i-Pr-Pybox 3a instead of 2a, without Bu4NI
Cl-Pybox 3b instead of 2a, without Bu4NI
4a instead of 2a, without Bu4NI
trace
22
6
7
trace
36
8
4b instead of 2a, without Bu4NI
9
5a instead of 2a, without Bu4NI
<10
trace
trace
35
10
11
12
13
14
15
16
5b instead of 2a, without Bu4NI
5c instead of 2a, without Bu4NI
NiBr2 and 2b instead of NiI2 and 2a, without Bu4NI
Ni(cod)2 and 2b instead of NiI2 and 2a, without Bu4NI
DMF instead of DMA, without Bu4NI
2b instead of 2a, 100 mol% MgCl2, without pyridine, without Bu4NI
PhBr/1 = 1:2
trace
39
n.d.
22
a Isolated yields.
Synlett 2013, 24, 619–624
© Georg Thieme Verlag Stuttgart · New York

LETTER
Reductive Cross-Coupling of Aryl Halides
621
and 28% yield (Table 2, entries 1 and 2), suggesting a rides and bromides were employed as the limiting
more electron-rich aromatic ring with respect to benzene reagents, coupling with 1, bromobenzene, and other elec-
promote the coupling efficiency. However, the coupling tron-deficient aryl chlorides or bromides usually resulted
efficiency of bromobenzene with 1-bromo-4-methoxy- in low yields (Table 2, entries 10–14), although a 52%
benzene can be boosted when 4,4′-di-tert-butylbipyridine yield was observed for the coupling of methyl 4-bromo-
(2b) was used as the ligand. Interestingly, coupling of 1- benzoate and bromobenzene (Table 2, entry 11). Finally,
bromo-4-methoxybenzene with 1 generated the desired coupling of electron-enriched 5-iodo-1,2,3-trimethoxy-
product in 60% yield (Table 2, entry 3). The use of 1-chlo- benzene and 5-bromo-1,2,3-trimethoxybenzene with
ro-4-methoxybenzene was comparable to the bromo ana- methyl 4-iodobenzoate and 1-(4-bromophenyl)ethanone,
logue (Table 2, entry 4). The use of benzyl 4- provided the products in 55% and 57% yield, respectively
bromophenylcarbamate as the limiting reagent, coupling (Table 2, entries 15 and 16). The electron-enriched 1-
with 1-bromo-4-methoxybenzene offered the biaryl prod- iodo-4-methoxybenzene and its bromo analogue, on the
uct in 54% yield, although a relatively acidic NH was other hand, only delivered 32% and trace yields of the
present (Table 2, entry 5). In addition, 1-bromo-3-meth- coupling product with methyl 4-bromobenzoate and 1-(4-
oxybenzene appeared to be as efficient as 1-bromo-4-me- bromophenyl)ethanone, respectively (Table 2, entries 17
thoxybenzene (Table 2, entry 6). Coupling of bromoben- and 18), suggesting that certain difference of electron
zene with electron-deficient aryl bromides and chlorides properties between the coupling partners is important for
also provided the coupling products higher than 60% (Ta- the high coupling efficiency.
ble 2, entries 7–9). When the electron-deficient aryl chlo-
Table 2 Examples of the Synthesis of Unsymmetrical Biaryl Compounds
Entry
1
Ar1X (1 equiv)
Ar2Y (1.4 equiv)
Yield of Ar1Ar2 (%)a
OMe
56
Br
Br
Br
Br
OMe
2
3
28 (55)b
OMe
OMe
OMe
MeO
MeO
Br
Cl
Br
60
OMe
OMe
Br
Br
Br
Br
OMe
OMe
OMe
OMe
OMe
OMe
COOMe
4
5
6
58
54
61
CbzHN
MeO
Br
Br
Br
Br
Br
7
8
9
65
64
60
O
Br
Cl
CN
OMe
OMe
OMe
NC
Cl
Br
10
43
© Georg Thieme Verlag Stuttgart · New York
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Q. Qian et al.
LETTER
Table 2 Examples of the Synthesis of Unsymmetrical Biaryl Compounds (continued)
Entry
11
Ar1X (1 equiv)
Ar2Y (1.4 equiv)
Yield of Ar1Ar2 (%)a
52
MeO2C
Br
Br
Br
MeO2C
MeO2C
Cl
12
13
14
40
48
30
Cl
Cl
Cl
CF3
NC
Cl
CF3
MeO
MeO
X
15
16
Y
R
MeO
55
57
Y = I, R = CO2Me
Y = Br, R = C(O)Me
X = I, Br
Br
R
17
18
MeO
X
32
trace
R = CO2Me
R = C(O)Me
X = I, Br
a Isolated yields.
b 4,4′-Di-tert-butylbipyridine (2b) was used.
To further test the compatibility of this unsymmetrical bi- To test whether the reaction proceeds through an in situ
aryl formation method, we used heteroaromatic halides as Negishi process, we first conducted the cross-coupling of
substrates for the coupling with aryl bromides (Table 3). the halides as in entries 3 and 11 in Table 2, using the op-
To our surprise, 8-bromoquinoline gave the coupling timized conditions but in the absence of ligand. Only re-
product in excellent yield, implying that coordination of covered aryl bromides were isolated, suggesting that in
1-nitrogen to the nickel center played an important role the absence of ligand under the Ni/Zn conditions neither
(Table 3, entry 1). The coupling of pyridine derivatives electron-rich nor electron-poor aryl bromides were con-
such as 2-bromo- and 2-chloropyridine with methyl 4- verted into organozinc reagents. In addition, coupling of
bromobenzoate and 1-bromo-4-methoxybenzene deliv- dimethyl 4-bromophthalate-derived organozinc·LiCl with
ered the coupling products in 68% and 52% yield, respec- 1 using the optimized conditions, without zinc powder,
tively (Table 3, entries 2 and 3). Reaction of 2- did not generate the cross-coupling product but recovered
bromothiophene with bromobenzene did not generate the aryl bromide 1. Therefore, the in situ Negishi process is
desired product.
excluded.
Gosmini’s mechanistic proposal for the Co-catalyzed bi-
aryl formation, a CoI to CoIII process was discussed with-
out in situ formation of organo-Mn reagents followed by
Negishi-type reaction.7a Also in line with the studies on
the catalytic and stiochiometric NiI-catalyzed dimeriza-
tion of aryl halides leading to symmetric biaryl
compounds5,12 we proposed a similar NiI/NiIII catalytic
process in the current method. NiIIX2 was first reduced to
NiIX by single-electron reduction with Zn, which then ox-
idatively adds to ArX to form an Ar1NiIIIX2 complex. Sub-
sequent reduction of the Ar1NiIIIX2 complex with Zn
generates an Ar1NiI intermediate, which undergoes oxida-
tive addition to Ar2Y, forming an Ar1NiIIIAr2Y species
prior to the product formation (Scheme 2). Alternatively,
Ni0 could be generated by two-electron reduction of
NiIIX2 with Zn. Oxidative addition of aryl halide to Ni0
generates an Ar1NiIIX intermediate, which then undergoes
a one-electron reduction to give an Ar1NiI species (path B,
Scheme 2).12c,13 Similar to pathway A, an oxidative addi-
Table 3 Coupling of Hetereoaromatic Halides
Entry HetAr1X
Ar2Y
Yield of
HetAr1Ar2 (%)a
1
86
68
Br
CO2Me
N
Br
N
Br
Br
CO2Me
OMe
2
Br
3
4
52
N
S
Cl
Br
n.d.b
Br
a Isolated yields.
b n.d. = not detected.
Synlett 2013, 24, 619–624
© Georg Thieme Verlag Stuttgart · New York

LETTER
Reductive Cross-Coupling of Aryl Halides
623
NiX2
Zn
NiX2
NiIX
Zn
ZnY
Ar1Ar2
ZnX2
Ar1Ar2
ArX
ZnX
Zn
NiI
ArX
Ni0
Ar1NiIIIX2
Zn
Ar1NiIIX
Zn
Ar1NiIIIAr2Y
Ar1NiIIIAr2Y
Ar1NiI
Ar1NiI
Ar2Y
Ar2Y
ZnX2
ZnX
path A
path B
Scheme 2 Proposed catalytic cycle
tion of Ar2Y to Ar1NiI is followed by reductive elimina-
tion to give a second NiI, which is reduced to Ni0.
3,4,3′,5-Tetramethoxybiphenyl (Table 2, Entry 6)
According to the general procedure, this compound was obtained as
a white solid; mp 89–92 °C. IR (KBr): νmax = 2997 (OCH3, νCH1 ),
2938 (OCH3, νCH), 2834 (OCH3, νCH), 1258 (=COCH3, νCO). H
NMR (500 MHz, CDCl3): δ = 3.87 (3 H, s), 3.89 (3 H, s), 3.92 (6 H,
s), 6.77 (2 H, s), 6.89 (1 H, dd, J = 8.2, 2.5 Hz), 7.09 (1 H, t, J = 2.3
Hz), 7.14 (1 H, m), 7.35 (1 H, t, J = 7.9 Hz). 13C NMR (125 MHz,
CDCl3): δ = 55.4, 56.2 (2 C), 60.9, 104.5 (2 C), 112.3, 113.2, 119.6,
129.8, 137.1, 137.7, 142.9, 153.4 (2 C), 159.9. HRMS (EI): m/z
calcd for C16H18O3: 258.1256; found: 274.1204 [M+ + 1].
In conclusion, we have optimized the Ni-catalyzed reduc-
tive coupling of two aryl halides by tuning the reaction pa-
rameters. With the present reaction conditions, reasonably
good coupling results could be achieved for the electron-
enriched aryl halides, wherein one of the coupling aryl ha-
lides requires only 1.4 equivalents excess. The coupling of
electron-deficient pyridyl and quinoline bromides with
benzene-based aryl halides also offered the coupling
products in good to excellent yields. The mild reaction
Acknowledgment
conditions also display excellent functional-group toler- Financial support was provided by the Chinese NSF (Nos. 2097209
and 21172140), the general research grant (No. 10YZ04), and the
Program for Professor of Special Appointment (Eastern Scholar) at
Shanghai Institutions of Higher Learning Shanghai Education
ance.
All experiments were carried out under dry nitrogen atmosphere.
DMA (anhyd and 99.5% ultra pure, Acros), NiI2 (anhyd, Alfa
Aesar), anhyd MgCl2 (Alfa Aesar), other chemicals were purchased
from Aldrich Chemical company and were used without purifica-
tion.
Committee. Dr. Hongmei Deng (Shanghai University) is thanked
for help in the NMR facility.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
General Experimental Procedure
To a flame-dried Schlenk tube equipped with a stir bar was loaded
ligand (10 mol%), zinc powder (200 mol%), and Bu4NI (100
mol%). The tube was moved to a dry glove box, at which point NiI2
(10 mol%) and MgCl2 (100 mol%) were added. The tube was
capped with a rubber septum, and it was moved out of the glove
box. DMA (1 mL), aryl Ar1X (0.15 mmol), Ar2Y (0.21 mmol), and
pyridine (100 mol%) were then added via syringe. After the reaction
mixture was allowed to stir for 12 h under N2 atmosphere at 25 °C,
it was directly loaded onto a silica column without workup. The res-
idue in the reaction vessel was rinsed with small amount of CH2Cl2.
Flash column chromatography provided the product as a solid or oil.
References
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a white solid; mp 184–187 °C. IR (KBr): νmax = 3327 (O=CNH,
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Synlett 2013, 24, 619–624

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LETTER
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Synlett 2013, 24, 619–624
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