Cross-coupling of diarylborinic acids and anhydrides with arylhalides catalyzed by a phosphite/N-heterocyclic carbene co-supported palladium catalyst system

Article abstract of DOI:10.1021/jo301335x

A highly efficient cross-coupling of diarylborinic acids and anhydrides with aryl chlorides and bromides has been effected by using a palladium catalyst system co-supported by a strong σ-donor N-heterocyclic carbene (NHC), N,N′-bis(2,6-diisopropylphenyl) imidazol-2-ylidene, and a strong π-acceptor phosphite, triphenylphosphite, in tert-BuOH in the present of K₃PO₄·3H₂O. Unsymmetrical biaryls with a variety of functional groups could be obtained in good to excellent yields using as low as 0.01, 0.2-0.5, and 1 mol % palladium loadings for aryl bromides and activated and deactivated aryl chlorides, respectively, under mild conditions. A ligand synergy between the σ-donor NHC and the π-acceptor phosphite in the Pd/NHC/P(OPh)₃ catalytic system has been proposed to be responsible for the high efficacy to arylchlorides in the cross-coupling. A scalable and economical process has therefore been developed for synthesis of Sartan biphenyl from the Pd/NHC/P(OPh)₃ catalyzed cross-coupling of di(4-methylphenyl)borinic acid with 2-chlorobenzonitrile.

Full text of DOI:10.1021/jo301335x

Article
pubs.acs.org/joc
Cross-Coupling of Diarylborinic Acids and Anhydrides with
Arylhalides Catalyzed by a Phosphite/N-Heterocyclic Carbene Co-
supported Palladium Catalyst System
Xiaofeng Chen, Haihua Ke, Yao Chen, Changwei Guan, and Gang Zou*
Department of Fine Chemicals, East China University of Science & Technology, 130 Meilong Rd., Shanghai, China
S
* Supporting Information
ABSTRACT: A highly efficient cross-coupling of diarylborinic acids
and anhydrides with aryl chlorides and bromides has been effected by
using a palladium catalyst system co-supported by a strong σ-donor N-
heterocyclic carbene (NHC), N,N′-bis(2,6-diisopropylphenyl) imida-
zol-2-ylidene, and a strong π-acceptor phosphite, triphenylphosphite,
in tert-BuOH in the present of K3PO4·3H2O. Unsymmetrical biaryls
with a variety of functional groups could be obtained in good to
excellent yields using as low as 0.01, 0.2−0.5, and 1 mol % palladium loadings for aryl bromides and activated and deactivated aryl
chlorides, respectively, under mild conditions. A ligand synergy between the σ-donor NHC and the π-acceptor phosphite in the
Pd/NHC/P(OPh)3 catalytic system has been proposed to be responsible for the high efficacy to arylchlorides in the cross-
coupling. A scalable and economical process has therefore been developed for synthesis of Sartan biphenyl from the Pd/NHC/
P(OPh)3 catalyzed cross-coupling of di(4-methylphenyl)borinic acid with 2-chlorobenzonitrile.
Suzuki coupling make them less competitive with arylboronic
acids. Diarylborinic acids, except for slow crystallization,10 not
only have higher atom economy but also share most of the
friendly features of arylboronic acids. Therefore, it is surprising
that diarylborinic acids and derivatives have been rarely applied
in transition-metal-catalyzed cross-coupling reactions.11 In fact,
to the best of our knowledge, except for our pending patent,12
Winkle et al. described the only example of a Suzuki coupling
reaction of diarylborinic acids, in situ generated bis(3,5-
dimethylphenyl)borinic acid with a highly reactive vinyl triflate,
in developing a process for synthesis of a potent endothelin A
antagonist PD 0182783.11c More recently, Knochel et al.
reported the use of a mixture of magnesium mono-, di-, and
triorganoboronates generated in situ from arylbromides,
boronates, magnesium, and lithium chloride, although a
formula of Ar2B(OBu)2MgX was used to reflect the
stoichiometry, in Suzuki coupling with active arylhalides and
pseudohalides.13 Herein, we report a highly efficient cross-
coupling of diarylborinic acids and anhydrides with arylhalides
catalyzed by a σ-donor NHC and π-acceptor phosphite co-
supported palladium catalyst system.
INTRODUCTION
■
The advantages of Suzuki coupling over the other transition-
metal-catalyzed cross-coupling reactions are mainly attributed
to the friendly properties of organoboronic acids, e.g.,
nontoxicity, air/moisture stability, and tolerance of a variety
of functional groups.1 Innumerable improvements on the
original protocol have been reported with respect to catalyst/
ligand systems, solvents, additives, and experimental con-
ditions,1,2 among which a couple of breakthroughs include the
conquest of inert arylchlorides by developing highly active
catalyst systems of palladium supported by bulky electron-rich
phosphines3 or sterically demanding N-heterocyclic carbene
(NHC) ligands4 and use of organotrifluoroborates to improve
reaction stoichiometry as well as compatibility to reaction
conditions.5 Comparatively, few advances have been made on
the utility of high-order arylborons,6 e.g., diarylborinic acids,
triarylboranes, and tetraarylborates, in Suzuki coupling. High-
order arylborons not only have higher atom economy but also
could be more readily prepared by reaction of aryl magnesium
halides with boronates, the most traditional and economical
way for preparation of arylboronic acids albeit less functional
group compatible compared with the transition-metal-catalyzed
protocols.7 In fact, synthesis of arylboronic acids by reaction of
aryl magnesium halides with alkylboronates would always
generate diarylborinic acids and triarylboranes as major
byproducts unless harsh cryogenic conditions or extra additives
were used,7,8 while high-order arylborons could be prepared
from almost equivalent arylmagnesium halides and boronates
under noncryogenic conditions, even by one-pot procedures
from aryl halides, boronates, and magnesium.9
RESULTS AND DISCUSSION
■
Diarylborinic acids could be readily prepared by reaction of aryl
magnesium halides with an equivalent amount of alkylboro-
nates at room temperature or by a one-pot procedure from
arylbromides, tributylboronate, and magnesium in THF at 30−
40 °C.9c However, purification of diarylborinic acids proved
The air sensitivity of triarylboranes and poor solubility of
tetraarylborates in organic solvents as well as their reluctance in
Received: June 29, 2012
Published: August 6, 2012
© 2012 American Chemical Society
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Article
tedious, especially for those that have low melting points.
Fortunately, treatment of the crude products with 2-amino-
ethanol afforded crystalline solid materials of 2-aminoethoxy
diarylborates, which could be readily purified by recrystalliza-
tion in ethanol and converted back into diarylborinic acids in
almost quantitative yields by simple acidification with aqueous
HCl (10%) (Scheme 1). Diphenylborinic acid was further
converted to anhydride for convenience in reaction stoichiom-
etry due to its ease to dehydration.
Table 1. Cross-Coupling of Diphenylborinic Anhydride with
4-(Benzyloxy)phenyl Chloride
a
yield
b
entry
L
Add. (mol %)
Sol.
Base
(%)
c
1
PCy3
P(tBu)3
IPr
DMF
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
NaOH
32
70
45
68
78
18
trace
55
43
55
40
53
10
17
95
95
93
92
76
trace
95
90
30
98
65
74
81
18
86
71
c
2
DMF
c
Scheme 1. Scalable Process for Preparation of Diarylborinic
Acids
3
DMF
4
IPr
tBuOH
tBuOH
iPrOH
EtOH
THF
5
IPr
6
IPr
NaOH
7
IPr
NaOH
8
IPr
NaOH
9
IPr
dioxane
THF
NaOH
10
11
12
13
14
15
16
17
18
19
20
IPr
tBuOK
IPr
Et3N (40)
tBuOH
tBuOH
NaOH
IPr
pyridine (40)
NaOH
IPr
P(OMe)3 (40) tBuOH
P(OiPr)3 (40) tBuOH
P(OPh)3 (40) tBuOH
P(OPh)3 (20) tBuOH
NaOH
IPr
NaOH
IPr
NaOH
IPr
NaOH
IPr
P(OPh)3 (8)
P(OPh)3 (4)
P(OPh)3 (2)
P(OPh)3 (2)
P(OPh)3 (2)
P(OPh)3 (1)
P(OPh)3 (2)
P(OPh)3 (2)
P(OPh)3 (2)
P(OPh)3 (2)
P(OPh)3 (1)
P(OPh)3 (2)
P(OPh)3 (2)
P(OPh)3 (2)
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
NaOH
IPr
NaOH
IPr
NaOH
NaOH
Electron-rich arylchlorides belong to difficult electrophile
counterparts in Suzuki coupling, especially with a low catalyst
loading. Therefore, reaction of diphenylborinic anhydride 1a
with 4-(benzyloxy)phenyl chloride 2a was used as the model
reaction to screen catalyst systems. Almost no reaction was
observed for both diphenylborinic anhydride and Ar2B-
(OBu)2MgBr generated in situ with the catalyst system of
[Pd(dppf)Cl2]/Cs2CO3 in DMF that was reported to work for
active arylhalides.13 Using the well-established catalyst systems
for arylchlorides in Suzuki coupling, such as [Pd/P(Cy)3)],
[Pd/P(tBu)3)]3 and [Pd/(IPr)] (IPr = N,N′-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene,4 the desired cross-cou-
pling product 3aa was obtained in 32−78% yields depending
on the structure of supporting ligands for palladium and
strongly on reaction conditions (Table 1, entries 1−10).
The Pd(OAc)2/IPr·HCl combination was selected for
further optimization of the reaction parameters considering
that Pd/NHC systems have proven not only more practical
than the air-sensitive Pd/P(tBu)3 but also more tunable in
catalytic activity as pioneered by Nolan4 and Organ14 groups.
For example, NHC palladium complexes used along with a
secondary weakly coordinating ligand, such as pyridines,4d,14
phosphites,15 NEt3,16 etc., have been reported to show
significantly higher catalytic activity in cross-coupling through
either speeding formation of active species or trapping and
conserving the active species in solution.14b Unfortunately, the
yields of 3aa decreased when pyridine, P(OMe)3, P(OiPr)3, or
NEt3 was added to the catalyst system (Table 1, entries 11−
14). However, when P(OPh)3 was used in the range of 2−20
equiv to palladium, the yields of 3aa remarkably increased to
92−95% from 68−78%, although 1 equiv of P(OPh)3 showed
little effect (Table 1, entries 15−19), possibly benefiting from a
ligand synergy between the σ-donor NHC and the strong π-
acceptor triphenylphosphite.17 A control experiment in the
absence of NHC showed that almost no reaction occurred
under the otherwise identical conditions (Table 1, entry 20),
d
21
IPr
NaOH
e
22
IPr
NaOH
d
23
IPr
K2CO3
d
24
IPr
K3PO4·3H2O
Ba(OH)2·8H2O
Cs2CO3
d
25
IPr
d
26
IPr
e
27
IPr
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
d
28
IMes
SIPr
IPr
d
29
f
30
a
Reaction conditions: (Ph2B)2O (0.55 mmol), 4-(benzyloxy)phenyl
chloride (2.0 mmol), base (4.0 mmol), solvent (5 mL), N2, 80 °C, 12
h; P(tBu)3 and IPr used in form of P(tBu)3·HBF4 and IPr·HCl,
b
c
d
respectively; Isolated yields. At 110 °C. 1 mol % Pd(OAc)2 used.
e
f
0.5 mol % Pd(OAc)2 used. 1 mol % Pd(OAc)2 and 2 mol % IPr·HCl
used.
excluding the possibility of a catalysis of Pd species supported
by P(OPh)3 alone. This result and the fact that the catalyst
system has worked well in the presence of large excess (20
equiv) of P(OPh)3 support the ligand synergy, namely, at least
one P(OPh)3 molecule remains coordinated to the catalytic
active NHC palladium(0) species,15 e.g., [(NHC)Pd{P-
(OPh)3}n], during the catalytic cycle.
The model reaction still proceeded smoothly to offer the
cross-coupling product 3aa in 90−95% yields even when the
palladium catalyst loading was decreased to 0.5−1 mol % from
2 mol % (Table 1, entries 18, 21, and 22). Considering the
possible problems of functional group compatibility of NaOH,
a couple of weak bases such as K2CO3, K3PO4·3H2O,
Ba(OH)2·8H2O, and Cs2CO3 were investigated, and
K3PO4·3H2O was found to work even slightly better than
NaOH with 1 mol % palladium catalyst loading (Table 1,
entries 23−26). The structure of NHCs also affected the
activity of the corresponding NHC/phosphite palladium
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catalyst system. The saturated analogue of IPr, N,N′-bis(2,6-
diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr), also
worked in the system, while the less sterically demanding NHC,
N,N′-dimestylimidazol-2-ylidene (IMes), provided product 3aa
in only 18% yield (Table 1, entries 28 and 29). The catalysis
efficiency decreased remarkably when the mole ratio of NHC
to Pd was increased to 2:1 from 1:1 (Table 1, entry 30),
indicating that bis(NHC) palladium complexes [Pd(NHC)2]
would be less catalytically active than the species co-supported
by a NHC and phosphites, [Pd(NHC){P(OPh)3}n]. No
reaction occurred when 2-aminoethoxy diphenylborate was
used in the model reaction under the otherwise identical
conditions.
Scope of the cross-coupling of diarylborinic acids with
arylhalides was explored briefly (Table 2). A wide range of
functional groups such as aldehyde, ester, nitrile, and acetyl
survived the reaction (Table 2, entries 1−5). Electron-deficient
arylchlorides, such as 4-CH3COC6H4Cl, 4-CNC6H4Cl, and 2-
CNC6H4Cl, reacted faster than electron-neutral or -rich
analogues, e.g., 4-CH3C6H4Cl and 4-CH3OC6H4Cl, even with
a lower catalyst loading (0.5 mol % Pd(OAc)2).
It has been observed that the presence of pyridine or
triethylamine in the system led to a yield decrease in the model
reaction (Table 1, entries 11 and 12). Therefore, it is
interesting to investigate if the ligand synergy between NHC
and P(OPh)3 in [(NHC)Pd{P(OPh)3}n] could possibly be
disturbed by competitive coordination to Pd(NHC) of N
groups in substrates, such as 4-Me2NC6H4Cl and 3-
chloropyridine (Table 2, entries 8−12). When 4-Me2NC6H4Cl
or 3-chloropyridine was subjected to the standard conditions,
the desired reaction did take place but in obviously slower rates
(49% and 86%, respectively, 10 h). To confirm the depression
of the competitive coordination from N groups of substrates to
Pd(NHC) catalytic species on the reaction rates, control
experiments without use of P(OPh)3 and with a high loading of
P(OPh)3 (20 equiv to Pd) were conducted (Table 2, entries 9,
10, and 12). For the deactivated chloride, 4-Me2NC6H4Cl, no
reaction was detected after 10 h in the absence of P(OPh)3,
although the cross-coupling product 3aj was isolated in 24%
yield for 3-chloropyridine. In contrast, 63% and 98% yields of
3ai and 3aj were obtained for the reactions of 4-Me2NC6H4Cl
and 3-chloropyridine, respectively, with 20 equiv P(OPh)3 to
palladium. These results are obviously consistent with the
existence of competitive coordination of N-containing groups
in substrates to catalytic palladium species in the NHC and
phosphite co-supported catalyst system.
An electronic effect similar to that observed in the reaction of
aryl chlorides was also found for bromides. However, only 0.01
mol % palladium loading proved to be enough for the reaction
of arylbromides to complete within reasonable time (4−10 h)
regardless of the substitutents on benzene ring (Table 2, entries
13−18). It is noteworthy to point out that the competitive
depression by coordination of the Me2N- group in the reaction
of 4-dimethylaminophenyl chloride (49% yield, 1 mol %
Pd(OAc)2) was not observed for 4-dimethylaminophenyl
bromide (94% yield, 0.01 mol % Pd(OAc)2) due to the high
activity of the NHC/phosphite palladium catalyst system to
arylbromides (Table 2, entries 8 and 14). Electronic properties
of diarylborinic acids showed a negligible influence on the
coupling reaction (Table 2, entries 19−26). All of the
investigated examples gave the desired products in excellent
yields, although bis(4-methoxyphenyl)borinic acid 1e, (4-
Table 2. Scope of the Cross-Coupling of Diarylborinic Acids
with Arylhalides
a
Pd
T
yield
b
entry
1
R1
R2 or ArX
X
(mol %) (h) product
(%)
H (1a) 4-CHO
(2b)
Cl
0.5
0.5
4
3
3ab
3ac
95
2
H (1a) 4-CO2Me
(2c)
Cl
94
3
4
5
H (1a) 2-CN (2d) Cl
0.5
0.5
0.5
4
4
6
3ad
3ae
3af
91
97
90
H (1a) 4-CN (2e)
Cl
Cl
H (1a) 4-COMe
(2f)
c
6
H (1a) 4-Me (2g)
Cl
Cl
0.5
1.0
10
10
3ag
3ah
91
92
c
7
H (1a) 4-OMe
(2h)
8
H (1a) 4-NMe2
(2i)
Cl
Cl
Cl
Cl
Cl
Br
Br
1.0
10
10
10
10
6
3ai
3ai
3aj
3aj
3aj
3ah
3ai
49
63
24
86
98
96
94
d
e
9
H (1a) 4-NMe2
(2i)
1.0
10
11
12
H (1a) 3-Cl-Py
(2j)
0.5
H (1a) 3-Cl-Py
(2j)
0.5
d
H (1a) 3-Cl-Py
(2j)
0.5
c
13
H (1a) 4-OMe
(2k)
0.01
0.01
10
10
14
H (1a) 4-NMe2
(2l)
c
15
H (1a) 4-Me (2m) Br
0.01
0.01
10
10
3ag
3an
90
90
c
16
H (1a) 2,5-
Br
dimethyl
(2n)
17
H (1a) 4-COMe
(2o)
Br
0.01
6
3af
93
18
19
H (1a) 4-CN (2p) Br
0.01
1.0
4
3ae
3ba
94
96
4-Me
(1b)
4-OBn
(2a)
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
10
20
21
22
23
24
25
26
2-Me
(1c)
4-OBn
(2a)
1.0
1.0
1.0
0.5
0.5
0.5
0.5
10
10
10
6
3ca
3da
3ea
3bf
3cf
3df
3ef
88
98
92
95
90
93
4-F
(1d)
4-OBn
(2a)
4-OMe 4-OBn
(1e)
(2a)
4-Me
(1b)
4-COMe
(2f)
2-Me
(1c)
4-COMe
(2f)
6
4-F
(1d)
4-COMe
(2f)
6
f
4-OMe 4-COMe
(1e) (2f)
6
82 (98)
a
Reaction conditions: 1a (0.55 mmol) as anhydride or diarylborinic
acids 1b−e (1.1 mmol), arylhalides (2.0 mmol), K3PO4·3H2O (4.0
b
c
mmol), tBuOH (5 mL), N2, 80 °C. Isolated yields. 3 equiv of
arylhalide according to diarylborinic acids due to its low bp. 20 equiv
d
e
of P(OPh)3 with respect to Pd(OAc)2 used. No P(OPh)3 added.
f
0.65 equiv of di(4-methoxyphenyl)borinic acid 1e used.
MeOC6H4)2BOH, suffered from the common side reaction in
Suzuki coupling, proton deboronation.
To demonstrate potential of the cross-coupling of diary-
lborinic acids with arylchlorides in practical applications, we
performed a synthesis of 4′-methyl-2-cyanobiphenyl, the so-
called Sartan biphenyl, as an example. Sartan biphenyl is a key
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intermediate for the synthesis of a family of angiotensin II
receptor antagonists, Sartans, for treatment of hypertension18
and represents one of the most important fine chemicals that
have been produced by using transition-metal-catalyzed cross-
coupling reactions in industry. Currently, Sartan biphenyl has
to be produced by Kumada or Negishi coupling of 2-
chlorobenzonitrile with 4-methylphenyl magnesium or zinc
halide instead of Suzuki coupling due to the high cost of 4-
methylphenylboronic acid.19 Although there are a couple of
transition-metal-catalyzed borylations of aromatics with cat-
echolborane or bis(pinacolato)diboron for synthesis of 4-
methylphenylboronic acid,7 because of the high cost of these
boron reagents, none has advantages over the reaction of 4-
methylphenyl magnesium chloride with an excess amount of
trimethylboronate even under cryogenic conditions. Since di(4-
methylphenyl)borinic acid could be more economically
prepared by reaction of 4-methylphenyl magnesium chloride
with almost equivalent boronates at room temperature, we
anticipated that a more practical process for the synthesis of
Sartan biphenyl would be possible by using the cross-coupling
of diarylborinic acids with arylhalides (Scheme 2).
diarylborinic acids promise a potential of this technique as a
cost-effective version of Suzuki coupling. Using this technique,
a scalable and economical process has been developed for
synthesis of Sartan biphenyl, a key intermediate for production
of a family of angiotensin II receptor antagonists. These results
also imply that utility of diarylborinic acids in other transition-
metal-catalyzed coupling reactions of organoboronic acids
would be possible.
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out under a
nitrogen atmosphere unless otherwise stated. Commercially available
chemicals were used as received. IPr·HCl,20 IMes·HCl,20 and
SIPr·HBr21 were prepared according to previously reported
1
procedures. H and 13C NMR spectra were recorded in CDCl3 at
ambient temperature. Chemical shifts in NMR were reported in ppm
(δ), relative to the internal standard of tetramethylsilane (TMS). The
signals observed were described as s (singlet), d (doublet), t (triplet),
dd(double doublet), m (multiplets), b (broad). The number of
protons (n) for a given resonance was indicated as nH. Coupling
constants were reported as J in Hz. The high resolution mass spectra
(HRMS) were performed on an electron ionization mass spectrometer
with a quadrupole analyzer.
Typical Procedure for Synthesis of Diarylborinic Acids.
Under a N2 atmosphere, a mixture of B(OBu)3 (11.51 g, 50 mmol)
and bromobenzene (15.70 g, 100 mmol) in 50 mL of THF was added
dropwise to a stirred mixture of magnesium turnings (2.64 g, 110
mmol) and a small crystal of I2 in THF (50 mL) at 40 °C over a
period of 30 min. The reaction was maintained at 40 °C for an
additional 2 h and then hydrolyzed by the addition of 100 mL of 5%
HCl (aq) after being cooled to room temperature. The mixture was
extracted with EtOAc and then concentrated to 20 mL before 2-
ethanolamine (4.58 g, 75 mmol) was added. The resulting solution
was stirred at room temperature for 2 h and then washed with water.
The organic layer was concentrated under vacuum to obtain the crude
product of 2-aminoethoxydiphenyl borate, which was recrystallized in
ethanol and then acidified with 50 mL of 10% HCl (aq). The mixture
was extracted with EtOAc, washed with brine, dried over Na2SO4,
filtered, and concentrated under vacuum to afford diphenylborinic
acid, which was further converted into anhydride by heating at 80 °C
for 2 h under vacuum.
Scheme 2. Scalable Synthesis of Sartan Biphenyl by Cross-
Coupling of Di(4-methylphenyl)borinic Acid with 2-
Chlorobenzonitrile
In fact, di(4-methylphenyl)borinic acid was readily prepared
from 4-methylphenyl chloride, Mg, and B(OBu)3 in 78%
overall yield in mole scale. Tributylboronate B(OBu)3 was used
instead of atom-economical B(OMe)3 considering that the
former is more readily prepared by just refluxing B(OH)3 in
BuOH with Dean−Stark and that the byproduct BuOH could
be more easily recovered and reused than MeOH. It was found
that 0.2 mol % palladium loading and 1 equiv of K3PO4·3H2O
with respect to 2-chlorobenzonitrile was enough for the
coupling reaction to complete within 16 h and provide 4′-
methyl-2-cyanobiphenyl 3bd in excellent yields (95−97%) in
repeated experiments.
Diphenylborinic Anhydride 1a.22 White solid (6.75 g, 78%); mp
1
120−122 °C; H NMR (400 MHz, CDCl3) δ 7.89−7.61 (m, 8H),
7.53−7.36 (m, 12H); 13C NMR (100 MHz, CDCl3) δ 136.0, 131.4,
128.1. HRMS (EI) m/z (M+) calcd for C24H20B2O: 346.1700, found
346.1704.
Bis(4-methylphenyl)borinic Acid 1b.23 White solid (8.18 g,
78%); mp 42−44 °C; 1H NMR (400 MHz, CDCl3) δ 7.71 (d, J = 8.0
Hz, 4H), 7.26 (d, J = 8.4 Hz, 4H), 5.75 (bs, 1H), 2.41 (s, 6H); 13C
NMR (100 MHz, CDCl3) δ 141.3, 136.2, 135.0, 128.9, 21.8.
Bis(2-methylphenyl)borinic Acid 1c. White solid (7.67 g, 73%);
1
mp 66−67 °C; H NMR (400 MHz, CDCl3) δ 7.42 (d, J = 7.2 Hz,
2H), 7.35−7.30 (m, 2H), 7.20−7.16 (m, 4H), 5.96 (s, 1H), 2.39 (s,
6H); 13C NMR (100 MHz, CDCl3) δ 142.2, 134.5, 130.2, 130.0,
125.2, 22.7. HRMS (EI) m/z (M+) calcd for C14H15BO 210.1216,
found 210.1218.
CONCLUSION
■
In summary, a highly efficient cross-coupling of diarylborinic
acids and anhydrides with aryl chlorides and bromides has been
effected by using a palladium catalyst system co-supported by a
strong σ-donor NHC, N,N′-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene, and a strong π-acceptor phosphite,
triphenyl phosphite. The Pd/NHC/phosphite catalyst system
worked well for both active and inert aryl halides, providing the
desired unsymmetrical biphenyl products in good to excellent
yields under mild conditions and within reasonable reaction
time. The high efficacy of the catalyst system for deactivated
arylchlorides has been attributed to a ligand synergy between
NHC and phosphite in Pd/NHC/phosphite catalytic species
generated in situ. The higher process and atom economies of
Bis(4-fluorophenyl)borinic Acid 1d. White solid (8.72 g, 80%);
1
mp 78−80 °C; H NMR (400 MHz, CDCl3) δ 7.79−7.75 (m, 4H),
7.15−7.10 (m, 4H), 5.83 (s, 1H); 13C NMR (100 MHz, CDCl3) δ
165.2 (dd, J = 22.0, 251.0 Hz,), 137.6 (dd, J = 8.0, 109.0 Hz), 115.3
(dd, J = 15.0, 20.0 Hz). HRMS (EI) m/z (M+) calcd for C12H9BOF2
218.0715, found 218.0710.
Bis(4-methoxyphenyl)borinic Acid 1e.24 White solid (9.18 g,
76%); mp 96−98 °C; 1H NMR (400 MHz, CDCl3) δ 7.76 (d, J = 8.8
Hz, 4H), 6.96 (d, J = 8.8 Hz, 4H), 5.75 (s, 1H), 3.85 (s, 6H); 13C
NMR (100 MHz, CDCl3) δ 161.9, 137.9, 136.8, 113.5, 55.2.
Typical Procedure for Cross-Coupling Reaction. Under a N2
atmosphere, to a 10 mL flask were added 4-benzyloxychlorobenzene
(0.44 g, 2.0 mmol), diphenylboronic anhydride (0.19 g, 0.55 mmol),
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Pd(OAc)2 (4.50 mg, 0.02 mmol), IPr·HCl (8.50 mg, 0.02 mmol),
P(OPh)3 (12.40 mg, 0.04 mmol), K3PO4·3H2O (1.06 g, 4.0 mmol),
and tBuOH (5 mL). The mixture was stirred at 80 °C and monitored
by TLC until 4-benzyloxychlorobenzene was completely consumed.
The reaction mixture was diluted with CH2Cl2 (15 mL), followed by
washing with H2O (2 × 10 mL). The organic layer was dried over
Na2SO4, filtered, and evaporated under reduced pressure to give crude
product. The pure product was obtained by column chromatography
on silica gel with EtOAc/petroleum ether.
Hz, 2H), 7.40 (t, J = 7.2 Hz, 2H), 7.35−7.31 (m, 1H), 7.26−7.21 (m,
6H), 7.02 (d, J = 8.8 Hz, 2H), 5.09 (s, 2H), 2.28 (s, 3H); 13C NMR
(100 MHz, CDCl3) δ 156.6, 140.4, 135.9, 134.3, 133.5, 129.2, 129.1,
128.8, 127.5, 126.8, 126.4, 125.9, 124.7, 113.3, 68.8, 19.5; HRMS (EI)
m/z (M+) calcd for C20H18O 274.1358, found 274.1360.
4-Benzyloxy-4′-fluorobiphenyl 3da. White solid (0.545 g,
1
98%); mp 139−140 °C; H NMR (400 MHz, CDCl3) δ 7.41−7.36
(m, 6H), 7.31 (t, J = 7.4 Hz, 2H), 7.25 (t, J = 7.2 Hz, 1H), 7.00 (t, J =
8.4 Hz, 2H), 6.95 (d, J = 8.8 Hz, 2H), 5.02 (s, 2H); 13C NMR (100
MHz, CDCl3) δ 161.1(d, J = 250 Hz), 157.3, 135.9, 132.0, 127.6,
127.2, 127.1, 127.0, 126.4, 114.5 (d, J = 21 Hz), 114.1, 69.0; HRMS
(EI) m/z (M+) calcd for C19H15OF 278.1107, found 278.1108.
4-Benzyloxy-4′-methoxybiphenyl 3ea.34 White solid; mp
4-Benzyloxybiphenyl 3aa.25 White solid (0.510 g, 98%); mp
1
131−133 °C; H NMR (400 MHz, CDCl3) δ 7.55−7.51 (m, 4H),
7.45−7.37 (m, 6H), 7.34−7.27 (m, 2H), 7.04 (d, J = 8.4 Hz, 2H), 5.07
(s, 2H); 13C NMR (100 MHz, CDCl3) δ 157.3, 139.7, 135.9, 133.0,
127.7, 127.6, 127.1, 126.9, 126.4, 125.7, 125.6, 114.1, 69.0.
1
172−174 °C; H NMR (400 MHz, CDCl3) δ 7.41−7.37 (m, 6H),
Biphenyl-4-carbaldehyde 3ab.26 Yellow solid; mp 57−59 °C;
1H NMR (400 MHz, CDCl3) δ 10.05 (s, 1H), 7.95 (d, J = 8.4 Hz,
2H), 7.75 (d, J = 8.0 Hz, 2H), 7.63 (d, J = 7.6 Hz, 2H), 7.48 (t, J = 7.4
Hz, 2H), 7.41 (t, J = 7.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ
192.0, 147.2, 139.7, 135.2, 130.3, 129.1, 128.5, 127.7, 127.4.
7.32 (t, J = 7.2 Hz, 2H), 7.25 (t, J = 7.2 Hz, 1H), 6.95 (d, J = 8.4 Hz,
2H), 6.88 (d, J = 8.4 Hz, 2H), 5.02 (s, 2H), 3.76 (s, 3H); 13C NMR
(100 MHz, CDCl3) δ 157.7, 156.9, 136.0, 132.7, 132.4, 128.3, 127.6,
126.9, 126.7, 126.5, 114.1, 113.1, 69.1, 54.3.
1-(4′-Methylbiphenyl-4-yl)ethanone 3bf.35 White solid; mp
118−119 °C; 1H NMR (400 MHz, CDCl3) δ 8.02 (dd, J = 2.0, 8.4 Hz,
2H), 7.66 (dd, J = 2.0, 8.4 Hz, 2H), 7.53 (dd, J = 2.0, 8.0 Hz, 2H), 7.28
(d, J = 6.4 Hz, 2H), 2.63 (s, 3H), 2.41 (s, 3H); 13C NMR (100 MHz,
CDCl3) δ 197.8, 145.7, 138.3, 136.9, 135.6, 129.7, 129.0, 127.1, 126.9,
26.7, 21.2.
Methyl Biphenyl-4-carboxylate 3ac.27 White solid; mp 114−
1
115 °C; H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 8.4 Hz, 2H),
7.59−7.54 (m, 4H), 7.39 (t, J = 7.4 Hz, 2H), 7.32 (t, J = 7.4 Hz, 1H),
3.86 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 165.9, 144.5, 138.9,
129.0, 127.9, 127.8, 127.1, 126.2, 126.0, 51.0.
1-(2′-Methylbiphenyl-4-yl)ethanone 3cf.36 Colorless oil; 1H
NMR (400 MHz, CDCl3) δ 8.01 (d, J = 8.4 Hz, 2H), 7.43 (d, J = 8.4
Hz, 2H), 7.31−7.21 (m, 4H), 2.65 (s, 3H), 2.27 (s, 3H); 13C NMR
(100 MHz, CDCl3) δ 197.9, 147.0, 140.8, 135.6, 135.2, 130.6, 129.6,
129.5, 128.3, 128.0, 126.0, 26.7, 20.5.
2-Cyanobiphenyl 3ad.28 White solid; mp 35−37 °C;1H NMR
(400 MHz, CDCl3) δ 7.76 (d, J = 7.6 Hz, 1H), 7.63 (t, J = 7.6 Hz,
1H), 7.65−7.41 (m, 7H); 13C NMR (100 MHz, CDCl3) δ 144.2,
137.0, 132.6, 131.8, 128.9, 127.6, 126.5, 117.6, 110.1.
4-Cyanobiphenyl 3ae.28 White solid; mp 86−89 °C; H NMR
1
1-(4′-Fluorobiphenyl-4-yl)ethanone 3df.37 White solid; mp
108−109 °C; 1H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 8.4 Hz, 2H),
7.65−7.57 (m, 4H), 7.16 (t, J = 8.8 Hz, 2H), 2.64 (s, 3H); 13C NMR
(100 MHz, CDCl3) δ 197.7, 162.9 (d, J = 247.0 Hz), 144.7, 135.9 (d, J
= 3.0 Hz), 135.8, 129.0, 128.9 (d, J = 8.0 Hz), 127.0, 115.9 (d, J = 22.0
Hz), 26.6.
(400 MHz, CDCl3) δ 7.65−7.58 (m, 4H), 7.52−7.49 (m, 2H), 7.42−
7.32 (m, 3H); 13C NMR (100 MHz, CDCl3) δ 144.5, 138.0, 131.5,
128.0, 127.6, 126.6, 126.1, 117.9, 109.7.
4-Acetylbiphenyl 3af.28 White solid; mp 120−122 °C; 1H NMR
(400 MHz, CDCl3) δ 8.03 (d, J = 8.4 Hz, 2H), 7.68 (d, J = 8.4 Hz,
2H), 7.62 (d, J = 6.8 Hz, 2H), 7.49−7.45 (m, 2H), 7.42−7.38 (m,
1H), 2.63 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 197.8, 145.7,
139.8, 135.8, 129.0, 128.9, 128.3, 127.3, 127.2, 26.7.
1-(4′-Methoxybiphenyl-4-yl)ethanone 3ef.38 White solid; mp
156−158 °C; 1H NMR (400 MHz, CDCl3) δ 8.00 (d, J = 8.4 Hz, 2H),
7.63 (d, J = 8.4 Hz, 2H), 7.57 (d, J = 8.8 Hz, 2H), 6.99 (d, J = 8.8 Hz,
2H), 3.85 (s, 3H), 2.62 (s, 3H); 13C NMR (100 MHz, CDCl3) δ
197.8, 159.9, 145.3, 135.2, 132.2, 129.0, 128.4, 126.6, 114.4, 55.4, 26.7.
Synthesis of Sartan Biphenyl by Cross-Coupling of Di(4-
methylphenyl)borinic Acid with 2-Chlorobenzonitrile. Under a
N2 atmosphere, to a 250 mL flask were added 2-chlorobenzonitrile
(19.2 g, 140 mmol), di(4-methylphenyl)borinic acid (16.4 g, 78
mmol), Pd(OAc)2 (0.063 g, 0.28 mmol), IPr·HCl (0.119 g, 0.28
mmol), P(OPh)3 (0.174 g, 0.56 mmol), K3PO4·3H2O (37.1 g, 140
mmol), and tBuOH (100 mL). The mixture was stirred at 80 °C for 16
h. After removal of tBuOH by distillation, the residue was washed with
H2O and extracted with EtOAc. The organic layer was dried over
Na2SO4, filtered, and concentrated to afford the crude product, which
was purified by distillation under vacuum (154−156 °C, 1 mmHg) to
give 4′-methyl-2-cyanobiphenyl 3bd (26.1 g, 97%), which solidifies at
room temperature.
4-Methylbiphenyl 3ag.29 White solid; mp 45−48 °C; H NMR
1
(400 MHz, CDCl3) δ 7.57 (d, J = 7.6 Hz, 2H), 7.48 (d, J = 7.6 Hz,
2H), 7.41 (t, J = 7.6 Hz, 2H), 7.31 (t, J = 7.4 Hz, 1H), 7.24 (d, J = 7.6
Hz, 2H), 2.38 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 141.3, 138.5,
137.1, 129.6, 128.8, 127.1, 21.2.
4-Methoxybiphenyl 3ah.28 White solid; mp 88−89 °C; 1H NMR
(400 MHz, CDCl3) δ 7.53 (t, J = 8.6 Hz, 4H), 7.40 (t, J = 8.0 Hz, 2H),
7.29 (t, J = 7.4 Hz, 1H), 6.97 (d, J = 8.4 Hz, 2H), 3.83 (s, 3H); 13C
NMR (100 MHz, CDCl3) δ 159.2, 140.9, 133.8, 128.8, 128.2, 126.8,
126.7, 114.3, 55.4.
4-N,N-Dimethylaminobiphenyl 3ai.30 White solid; mp 121−
123 °C; 1H NMR (400 MHz, CDCl3) δ 7.48 (d, J = 8.4 Hz, 2H), 7.43
(d, J = 8.8 Hz, 2H), 7.31 (t, J = 7.8 Hz, 2H), 7.17 (t, J = 7.4 Hz, 1H),
6.72 (d, J = 8.8 Hz, 2H), 2.90 (s, 6H); 13C NMR (100 MHz, CDCl3) δ
148.9, 140.1, 128.1, 127.6, 126.6, 125.2, 124.9, 111.7, 39.4.
3-Phenylpyridine 3aj.31 Colorless oil; 1H NMR (400 MHz,
CDCl3) δ 8.85 (s, 1H), 8.59 (d, J = 4.4 Hz, 1H), 7.86 (d, J = 8.0 Hz,
1H), 7.57 (d, J = 8.0 Hz, 2H), 7.47 (t, J = 7.4 Hz, 2H), 7.41−7.33 (m,
2H); 13C NMR (100 MHz, CDCl3) δ 148.3, 148.1, 137.6, 136.4,
134.1, 129.0, 128.0, 127.0, 123.4.
2-Cyano-4′-methylbiphenyl 3bd.39 White solid; mp 49−50 °C;
1H NMR (400 MHz, CDCl3) δ 7.74 (d, J = 7.6 Hz, 1H), 7.62 (t, J =
7.2 Hz, 1H), 7.51−7.39 (m, 4H), 7.30 (d, J = 8.0 Hz, 2H), 2.42 (s,
3H); 13C NMR (100 MHz, CDCl3) δ 145.5, 138.7, 135.3, 133.8,
132.9, 130.1, 129.5, 128.7, 127.4, 119.0, 111.1, 21.3.
2,5-Dimethylbiphenyl 3an.32 White solid; mp 107−109 °C; H
1
NMR (400 MHz, CDCl3) δ 7.41−7.37 (m, 2H), 7.32−7.30 (m, 3H),
7.15 (d, J = 7.6 Hz, 1H), 7.07 (d, J = 7.6 Hz, 2H), 2.34 (s, 3H), 2.23
(s, 3H); 13C NMR (100 MHz, CDCl3) δ 142.3, 141.9, 135.3, 132.3,
130.7, 130.4, 129.3, 128.2, 128.1, 126.8, 21.1, 20.1.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra for all compounds reported. This
material is available free of charge via the Internet at http://
pubs.acs.org.
4-Benzyloxy-4′-methylbipheny 3ba.33 White solid; mp 140−
142 °C; 1H NMR (400 MHz, CDCl3) δ 7.44−7.36 (m, 6H), 7.32 (t, J
= 7.6 Hz, 2H), 7.25 (t, J = 7.0 Hz, 1H), 7.14 (d, J = 8.0 Hz, 2H), 6.95
(d, J = 8.4 Hz, 2H), 5.01 (s, 2H), 2.30 (s, 3H); 13C NMR (100 MHz,
CDCl3) δ 158.2, 138.0, 137.1, 136.4, 134.0, 129.5, 128.7, 128.0, 127.6,
126.7, 115.2, 70.1, 21.2.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zougang@ecust.edu.cn.
4-Benzyloxy-2′-methylbiphenyl 3ca. White solid (0.482 g,
88%); mp 62−63 °C; 1H NMR (400 MHz, CDCl3) δ 7.46 (d, J = 7.2
7576
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undergo oxidation upon long-term storage could be one of factors
discouraging their use in the cross-coupling reactions.
(12) Zou, G.; Ke, H.; Li, X.; Lu, Q. A process for preparation of 4-
cyanobiphenyl, CN201110407815.8, 2011.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(13) Haag, B. A.; Samann, C.; Jana, A.; Knochel, P. Angew. Chem., Int.
̈
We are grateful for financial support provided by the National
Science Foundation of China (no. 20972049).
Ed. 2011, 50, 7290−7294.
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