Highly efficient palladium-catalyzed cross-coupling of diarylborinic acids with arenediazoniums for practical diaryl synthesis

Article abstract of DOI:10.1016/j.tetlet.2019.151491

A highly efficient cross-coupling of cost-effective diarylborinic acids with both isolatable and latent arenediazoniums, i.e. tetrafluoroborates and aryltriazenes, respectively, has been developed with a practical palladium catalyst system under base-free conditions in open flask at room temperature. A variety of electronically and sterically various biaryls, in particular, those bearing a coordinative ortho-substituent, could be obtained in good to excellent yields by using 0.3 mol% palladium acetate as catalyst. Features of the protocol including cost-effectiveness of diarylborinic acids, efficacy to heteroatom ortho-substituted substrates and high chemoselectivity to aryl chlorides have been clearly demonstrated in practical synthesis of fungicide Boscalid.

Full text of DOI:10.1016/j.tetlet.2019.151491

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Highly efficient palladium-catalyzed cross-coupling of diarylborinic acids with
arenediazoniums for practical diaryl synthesis
Fengze Wang, Chen Wang, Guoping Sun, Gang Zou
PII:
S0040-4039(19)31290-0
DOI:
https://doi.org/10.1016/j.tetlet.2019.151491
TETL 151491
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Tetrahedron Letters
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1 December 2019
4 December 2019
Please cite this article as: Wang, F., Wang, C., Sun, G., Zou, G., Highly efficient palladium-catalyzed cross-coupling
of diarylborinic acids with arenediazoniums for practical diaryl synthesis, Tetrahedron Letters (2019), doi: https://
doi.org/10.1016/j.tetlet.2019.151491
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Graphical Abstract
Highly efficient palladium-catalyzed cross-
coupling of diarylborinic acids with
arenediazoniums for practical diaryl
synthesis
Leave this area blank for abstract info.
Fengze Wang,a Chen Wang,a Guoping Sunb and Gang Zou*a
aSchool of Chemistry & Molecular Engineering, East China University of Science & Technology, 130 Meilong Rd,
Shanghai, China. bJiangsu Danxia New Material Co. Ltd., Jingsi Road, Ecological Chemical Technology Industrial Park,
Suqian, Jiangsu Province, China
R1
R1
Ar
N2L
Ar
Ar
0.3mol% Pd(OAc)2
+
B OH
sol, r.t., 12h, open flask
up to 99%
R
R
L = BF4, NMe2

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Highly efficient palladium-catalyzed cross-coupling of diarylborinic acids with
arenediazoniums for practical diaryl synthesis
Fengze Wang,a Chen Wang,a Guoping Sunb and Gang Zou*a
aSchool of Chemistry & Molecular Engineering, East China University of Science & Technology, 130 Meilong Rd, Shanghai, China.
bJiangsu Danxia New Material Co. Ltd., Jingsi Road, Ecological Chemical Technology Industrial Park, Suqian, Jiangsu Province, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A highly efficient cross-coupling of cost-effective diarylborinic acids with both isolatable and
latent arenediazoniums, i.e. tetrafluoroborates and aryltriazenes, respectively, has been
developed with a practical palladium catalyst system under base-free conditions in open flask at
room temperature. A variety of electronically and sterically various biaryls, in particular, those
bearing a coordinative ortho-substituent, could be obtained in good to excellent yields by using
0.3mol% palladium acetate as catalyst. Features of the protocol including cost-effectiveness of
diarylborinic acids, efficacy to heteroatom ortho-substituted substrates and high
chemoselectivity to aryl chlorides have been clearly demonstrated in practical synthesis of
fungicide Boscalid.
Available online
Keywords:
Suzuki coupling
diarylborinic acid
arenediazonium
aryltriazene
2009 Elsevier Ltd. All rights reserved.
1. Introduction
Suzuki coupling of arenediazonium salts has increasingly
attracted interests1 as a complement to the conventional aryl
halide and phenol derived electrophiles since the seminal
publications by Genêt2 and Sengupta groups.3 independently.
Several advantages of using arenediazoniums as aryl sources in
Suzuki-coupling have come to be recognized.1f Firstly, the
superior reactivity of arenediazoniums allows high
chemoselectivity. Secondly, the reaction could even work
efficiently in open flask under mild conditions without basic
additives. Thirdly, a variety of arenediazoniums could be
readily prepared from cheap commercial anilines, promising a
high potential in large-scale production. Moreover, some doubly
or multiply substituted aryl halides, the conventional
electrophiles in Suzuki coupling, are in fact prepared from
anilines via Sandmeyer reaction, a diazotization-halogenation
procedure, taking advantage of the strong electronic effects of
amino or its precursor nitro group. To further improve the
protocol, many efforts have been recently devoted towards
stabilized diazoniums, requirement of 2equiv. basic additives
and loss of half amount of aryl groups in boronates almost
nullified the benefits of the high reactivity of arenediazoniums
and atom economy of tetraarylboronates. Similar to
tetraarylboronates, diarylborinic acids could be economically
prepared via Barbier-type one-pot procedure from aryl bromides,
magnesium and trialkylboronates under non-cryogenic
conditions.7 We have shown that diarylborinic acids could be
efficiently used as a cost-effective alternative to arylboronic
acids in palladium and nickel catalyzed cross-coupling for biaryl
synthesis.8 In particular, the aerobic copper-catalyzed Chan-Lam
coupling, which appeared to be incompatible with reducing
ability of high-order arylborons, could also well accommodate
diarylborinic acids as aryl source with proper catalyst systems.9
We anticipate that Suzuki coupling of arenediazoniums with
diarylborinic acids should be also feasible. Herein, we report a
highly efficient cross-coupling of diarylborinic acids with
isolatable or latent arenediazoniums catalyzed by simple
ligandless palladium catalyst under base-free conditions in open
flask at room temperature, in which both aryl groups of
diarylborinic acids could be utilized at low catalyst loading.
development of sustainable procedures by in situ generation of
4
arenediazoniums1d,
and/or use of heterogeneous palladium
catalysts.1b, 1e, 5 However, the high cost of arylboronic acids that
generally have to be used as the nucleophile counterpart still
remains to be addressed, in particular, in potential application of
the protocol in industrial production of fine chemicals.
2. Results and Discussion
Cross-coupling of 2-nitrobezenediazonium tetrafluoroborate
(1a) with bis(4-chlorophenyl)borinic acid (2a) was taken as the
model for condition optimization considering the desired product
4'-chloro-2-nitro-biphenyl (3aa) could be used for synthesis of
fungicide Boscalid (Table 1).
Zarei and co-workers have reported Suzuki coupling of
sodium tetraarylboronates with arenediazoniums grafted on SiO2
under basic aqueous conditions.6 However, only two aryl groups
in tetraarylboronates could be utilized. The use of support-

2
Tetrahedron Letters
Table 1. Optimization of palladium-catalyzed cross-coupling of diarylborinic acids with arenediazoniums a
NO2
NO2
Cat.
Cl
B(OH)
Cl
+
L
base. sol. r.t.
2
1a
1a
3aa
2a
L = N2+BF4
(
), N2NMe2
(
-
1a'
)
Entry
1
N /B (mol ratio)
1a/2a (1.0)
1a/2a (1.2)
1a/2a (0.8)
1a/2a (1.2)
1a/2a (1.2)c
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
1a/2a (1.2)
2a/1a’ (0.65)
2a/1a’ (0.65)
2a/1a’ (0.65)
2a/1a’ (0.65)
2a/1a’ (0.65)
2a/1a’ (0.65)
Cat. (mol%)
PdCl2 (1)
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
H2O
Additive (equiv)
Time (h)
Yield (%)b
49
/
12
12
12
12
12
12
12
12
12
12
12
6
2
PdCl2 (1)
/
59
3
PdCl2 (1)
/
37
4
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Pd(OAc)2 (0.5)
Pd(OAc)2 (0.3)
Pd(OAc)2 (0.1)
5%Pd/C (1)
/
78
5
/
trace
10
6
/
7
THF
/
86
8
Dioxane
CH2Cl2
DMF
/
92
9
/
trace
92
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
/
EtOH
/
90
i-PrOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
MeOH
MeOH
EtOH
/
96
/
6
99
/
6
97
/
12
12
6
99
/
88
/
9
5%Pd/C (1)
/
12
6
83
5%Pd/C (1)
/
82d
75
5%Pd/C (1)
/
12
12
12
12
6
5%Pd/C (1)
THF
/
20
5%Pd/C (1)
i-PrOH
Dioxane
H2O
/
48
5%Pd/C (1)
/
14
5%Pd/C (1)
/
trace
14
5%Pd/C (1)
MeOH
MeOH
MeOH
t-BuOH
MeOH
Dioxane
MeOH
Dioxane
Dioxane
K2CO3 (1.0)
K3PO4·3H2O (1.0)
NaF (1.0)
12
6
5%Pd/C (1)
trace
43
5%Pd/C (1)
12
12
12
12
12
12
12
Pd(OAc)2 (0.3)
Pd(OAc)2 (0.3)
Pd(OAc)2 (0.3)
Pd(OAc)2 (0.3)
Pd(OAc)2 (0.3)
Pd(OAc)2 (0.3)
BF3·Et2O (1.5）
BF3·Et2O (1.5）
BF3·Et2O (1.5）
BF3·2MeOH (1.5）
BF3·dioxane (1.5）
BF3·dioxane (1.2）
67
75
65
84
96
77
aReaction conditions: reaction was carried out at 0.5mmol scale with respect to 2a for arenediazoniums or 1.0mmol scale with respect to 1a’, solvent (4 mL),
open flask, at room temperature. bIsolated yields. c0.3mmol tetraarylboroate used. dAt 50oC.
The product 3aa was obtained in 37-59% yields with 0.8-
1.2equiv. 1a using 1mol% PdCl2 as catalyst in methanol at room
temperature (Table 1, entries 1-3). The yield could be increased
to 78% by using 1mol% Pd(OAc)2 (Table 1, entry 4). When
sodium tetra(4-chlorophenyl) boronate was used only trace
amount of 3aa was detected along with nitrobenzene as the
1a and 2a, e.g. THF (86%, 12h), dioxane (92%, 12h), DMF
(90%, 12h), EtOH (90%, 12h), i-PrOH (96%, 6h) and t-BuOH
(98%, 6h), gave 3aa in excellent yields while poor yields were
obtained in CH2Cl2 and H2O (Table 1, entries 6-13). In fact, the
catalyst loading could be reduced to 0.3mol% without significant
decrease in 3aa yields (Table 1, entries 13-15). Use of
heterogeneous palladium catalyst allows readily isolation, reuse
or recovery of the noble metal component, therefore having
attracted extensive interests.10 Several reports have been
published on use of solid supported palladium catalysts in the
major
product
formed
via
protodediazotization
of
arenediazonium 1a (Table 1, entry 5). Solvent screening implied
that homogenous catalysis should be crucial for the cross-
coupling. Therefore, solvents that could dissolve both substrates

3
1e, 5, 10c
Suzuki coupling of arenediazoniums.1b,
In particular,
competitive reaction of CN group with diazoniums and fast
Felpin and co-workers reported that palladium nanoparticles of
low oxidation degree and uniformly dispersed on the charcoal
showed very high activity.5c Given the widespread use and
readily availability of simple commercial Pd/C as well as to test
the homogeneous catalysis speculation (vide supra) we
investigated the performance of 5%Pd/C catalyst in the model
reaction. Very low yields were obtained in the solvents that
worked well for Pd(OAc)2 although a comparable yield could be
obtained in MeOH (82-83% vs 78%) (Table 1, entries 17-24).
The low efficiency of Pd/C catalyst appeared to indirectly
support the speculation of homogenous catalysis. Unlike the
conventional Suzuki coupling, the presence of a common base,
e.g. K2CO3, K3PO4·3H2O and NaF etc., proved to be deleterious,
even completely block the coupling reaction (Table 1, entries 25-
27). Therefore, the optimal conditions were set as 1.2equiv.
arenediazonium tetrafluoroborates catalyzed by 0.3mol%
Pd(OAc)2 in t-BuOH at room temperature.
dediazonation due to strong electron-donating ability of MeO
group, respectively. However, coordination of an ortho-
substituent, e.g. o-NO2 (3ab-3ah) and o-OMe (3ma), at
arenediazonium tetrafluoroborates appeared to surpass its
electronic and steric effects, remarkably increasing the product
yields12 although sterically demanding o-tert-butylbenzene
diazonium tetrafluoroborate gave a trace amount of product. In
the case of aryltriazenes, lower functional group compatibility
was observed than that of the corresponding arenediazonium
tetrafluoroborates, possibly due to the presence of strong Lewis
acid BF3 in the system. For example, the poorer yields were
obtained for aryltriazenes bearing p-CN (1f’, 26%) and p-SO2Me
(1g’, trace). An ortho-substituent, e.g. o-OMe (1m’, 75%) and o-
Me (1q’, 79%), also appeared to slightly decrease the reaction
yields although a NO2 group could be tolerated in both cases.
The low dependence of the palladium catalyzed cross-coupling
of arenediazonium tetrafluoroborates on the electronic
activation/deactivation of aryl substituents in both coupling
counterparts indicated that neither the oxidative addition (OA) of
arenediazoniums nor the transmetalation from diarylborinic acids
but the reductive elimination (RE) should be the rate-
determining step in the widely accepted mechanism for Suzuki
coupling (Scheme 1). The higher yields obtained from
heteroatom ortho-substituted arenediazonium tetrafluoroborates
are consistent with the RE rate-determining mechanism although
the chelate stabilization should also contribute via preventing
protodediazotization of arenediazoniums.12 The lower functional
group compatibility and the larger steric effects of aryltriazenes
could reasonably be attributed to the use of excess BF3 to in-situ
generate arenediazoniums prior to the catalysis.
Aryltriazenes which release labile arenediazoniums when
treated with Brönsted or Lewis acids could serve as protected
diazoniums to obtain control of chemoselectivity or improve
safety, handling and storing. Aryltriazenes have been shown to
be effective thermally stable surrogates of arenediazonium
tetrafluoroborates in the presence of BF3 in Suzuki coupling by
Tamao et al.11 After establishment of the optimal conditions for
cross-coupling of arenediazonium tetrafluoroborates with
diarylborinic acids, we further investigated the reactivity of
aryltriazene 1a’ in the model reaction. However, unsatisfactory
yields were obtained when the catalyst system for 1a was
directly adopted with the aid of BF3·Et2O, due to a strongly
exothermic ligand exchange between BF3 complexes (Table 1,
entries 28-30). To our delight, cross-coupling of 1a’ with a slight
excess (1.3equiv.) 2a gave 3aa in 96% yield with 0.3mol%
Pd(OAc)2 by using 1.5equiv. pre-formed BF3·dioxane complex
in dioxane at room temperature (Table 1, entry 32), remarkably
more practical than the coupling of aryltriazenes with excess
(2equiv.) arylboronic acids reported in literature, for example,
Pd0(sol)n
Ar N2+L-
Ar Ar'
L = BF4, BF3(NMe2)
RE
OA
rate-determining
Ar Pd
Ar Pd
Ar'
N2+L-
11c
5mol% Pd(OAc)2 in dioxane or complicated catalyst systems,
e.g. 2mol% Pd2(dba)3 / 8mol%P(t-Bu)3) in DME 11a and 2mol%
polystyrene-supported N-heterocyclic carbene palladium catalyst
in dioxane.11b
B(OH)(L)2
Ar Pd+L-
Ar'B(OH)L
Ar'2B(OH)
N2
With the optimal conditions in hands, scope of the cross-
coupling of arenediazonium tetrafluoroborates and aryltriazenes
with diarylborinic acids was explored (Table 2). Reaction of
bis(o-tolyl)borinic acid (2b) with 1a (94%) or 1a’ (94%) also
gave 3ab in comparable yields to its para-analog 2a, indicating
small steric hindrance could be smoothly overcome. Electronic
effects of diarylborinic acids proved to be negligible since
excellent yields (93-96%) could be obtained in the reaction with
1a or 1a’ regardless of an electron-donating (Me, or OMe) or
withdrawing group (F) on the phenyl rings of borinic acids
(Table 2, 3ac-3ah). However, the influence of substituents of
arenediazonium tetrafluoroborates or aryltriazenes on cross-
coupling is more than simple electronic or steric effects.
Generally, arenediazonium tetrafluoroborates bearing weakly
electron-withdrawing (Cl) or -donating (Me) groups cross-
coupled with 2a affording 3ka, 3la, or 3qa-3ta in excellent
yields while those bearing strong electron-withdrawing (m-NO2
1b, p-NO2 1c, p-CO2Et 1d, p-Ac 1e, p-CN 1f, p-MeSO2 1g) or –
donating (p-OMe 1n, m-MeS 1o, m-AcNH 1p) para- or meta-
substituents gave slightly lower yields (54-86%) for cross-
coupling products (3ba-3ga, 3na-3pa). The poor yields for 3fa
(55%, p-CN) and 3na (54%, p-OMe) may be attributed to the
Scheme 1 A mechanistic explanation
To investigate the chemoselectivity of diazonium vs bromide,
reactions of bromo (1i, 1i’, 1j and 1j’) and electronic analogs, p-
chloro (1k and 1k’) diazoniums, with 2a were carried out. In the
case of tetrafluoroborates, an excellent yield (96%) was obtained
for 3ka while the bromo containing products 3ia (72%) and 3ja
(75%) were isolated in significantly lower yields due to the
competitive reaction of Caryl-Br bond. These results indicated
only modest selectivity of diazoniums vs bromide could be
achieved in Suzuki coupling with diarylboronic acids albeit the
former was described as super-electrophiles.13 The yields from
reaction of the in situ generated arenediazoniums 1i’ (53%) and
1j’ (55%) further decreased because the aryltriazenes have to be
converted into diazoniums before the desired cross-coupling
could occur while no influence was observed for the chloro
analog (1k’, 96%).
Table 2. Scope of the Pd-catalyzed cross-coupling of diarylborinic acids with arenediazoniums

4
Tetrahedron
0.3 mol% Pd(OAc)2
Conditions A or B, rt., air, 12h
+
N2L
1a-t
B(OH)
2
R2
R2
R1
R1
3ab-ta
2a-h
1a t
-
A: t-BuOH for
L=BF4,
1a' t'
1a'-t'
B: 1.5equiv. BF3, dioxane for
-
L=NMe2,
OMe
Me
NO2
NO2
NO2
NO2
NO2
NO2
Me
OMe
OMe
Me
3ae
3af
3ag
3ad
3ac
3ab
A: 95%, B: 95%
A: 94%, B: 94%
A: 96%, B: 97%
A: 93%, B: 95%
A: 96%, B: 96%
A: 94%, B: 96%
Cl
Cl
Cl
F
Cl
Cl
NO2
O2N
O
3ca
A: 85%, B: 91%
3da
3ea
3ba
O2N
3ah
EtO2C
A: 86%, B: 82%
A: 83%, B: 85%
A: 76%, B: 80%
A: 95%, B: 92%
Cl
Cl
Cl
NO2
Cl
Br
O
NC
S
O
Br
3ia
3ja
3ga
3ha
3fa
Me
Cl
A: 72%, B: 53%
A: 75%, B: 55%
A: 63%, B: trace
A: 91%, B: 85%
A: 55%, B: 26%
Cl
Cl
Cl
Cl
Cl
Cl
OMe
MeS
Cl
Cl
MeO
3na
3la
3ma
A: 90%, B: 75%
3ka
3oa
A: 54%, B: 71%
A: 95%, B: 90%
A: 98%, B: 96%
A: 75%, B: 78%
Cl
Me
Cl
Cl
Cl
Cl
Me
AcHN
Me
Me
Me
3ra
3qa
3sa
3ta
3pa
A: 87%, B: 80%
A: 89%, B: 79%
A: 92%, B: 87%
A: 88%, B: 76%
A: 79%, 78%
aReaction conditions: 1 (1.2 mmol), 2 (0.5 mmol), or 1’ (1 mmol), 2 (0.65 mmol), 4 mL solvent at rt. in open flask for 12h; isolated yields for products obtained
from conditions A and B, respectively.
NO2
Cl
N2BF4
Application of the palladium catalyzed cross-coupling of
arenediazoniums with diarylboronic acids was demonstrated in
gram-scale synthesis of Boscalid (Scheme 2), a fungicide
developed by BASF company.14 With the above optimal catalyst
system, the key intermediate 3aa could be obtained in excellent
yields (98-99%), which was chemoselectively converted into
aniline intermediate 4 in almost quantitative yield by charcoal-
assisted, FeCl3 catalyzed nitro reduction with hydrazine although
selective (NO2) reduction via palladium catalyzed hydrogenation
was hampered by the concurrence of dehalogenation. Boscalid
was finally obtained in 93% yield from the condensation of
aniline 4 with nicotinoyl chloride under routine amidation
conditions. With this three-step process, Boscalid could be
obtained in 90% overall yield from readily available and cost-
effective starting materials under practical conditions, promising
a potential in a large scale production.
0.6equiv.
1a
Cl
NO2
0.3 mol% Pd(OAc)2
tBuOH, r.t. 12h, 98%
B
OH
2a
3aa
Cl
Cl
O
Cl
N
Cl
5mol% FeCl3
C*, N2H4.H2O
NH2
MeOH, H2O, 85 o
2h, 99%
C
Et3N, THF 25 o
1 h, 93%
C
4
Cl
N
O
Cl
NH
Boscalid
5
( )
Scheme 2. Synthesis of Boscalid in gram-scale
3. Conclusions
In summary, a highly efficient cross-coupling of diarylborinic
acids with arenediazonium tetrafluoroborates or aryltriazenes

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variety of electronically and sterically various biaryls in good to
excellent yields. Both aryl groups of diarylborinic acids could be
utilized effectively, making the protocol
a cost-effective
6.
7.
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alternative to the conventional version of arylboronic acids.
Common alcohol and ether solvents that could efficiently
dissolve the substrates worked well, among which t-BuOH
performed best for arenediazonium tetrafluoroborates while
dioxane appeared to be the choice of solvent in the presence of
1.5equiv, BF3 as activator for aryltriazenes. Features of the
protocol, i.e. the cost-effectiveness of diarylborinic acids, high
efficacy to heteroatom ortho-substituted substrates, low loading
of the simple and ligandless palladium catalyst, good
chemoselectivity to halides (Cl) and mild reaction conditions,
have been clearly demonstrated in a highly efficient and practical
synthesis of fungicide Boscalid, promising a good potential in
application in large-scale production of fine chemicals.
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We are grateful for financial support provided by the National
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Supplementary material available: experimental procedures,
compound characterization and copies of NMR spectra for
aryltriazenes and biaryls.
Graphical Abstract
5.
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R1
R1
Ar
N2L
Ar
Ar
0.3mol% Pd(OAc)2
+
B OH
sol, r.t., 12h, open flask
up to 99%
R
R
L = BF4, NMe2
High efficacy to heteroatom ortho-substituted
diazoniums
Highlights
Cost-effective arylboron reagent
Good functional group compatibility
Low loading of simple ligandless catalyst
Mild, open-flask and base-free conditions

6
Tetrahedron Letters
Declaration of interests
☒ The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to influence
the work reported in this paper.
☐The authors declare the following financial
interests/personal relationships which may be
considered as potential competing interests: