Efficient phosphine ligands for the one-pot palladium-catalyzed borylation/Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1039/c4ob02436b

We report the synthesis of 2-(anthracen-9-yl)-1H-inden-3-yl dicyclohexylphosphine and its use in palladium-catalyzed borylation/Suzuki-Miyaura cross-coupling reaction to prepare a variety of symmetrical and unsymmetrical biaryl compounds in excellent yield. This journal is

Full text of DOI:10.1039/c4ob02436b

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Efficient phosphine ligands for the one-pot
palladium-catalyzed borylation/Suzuki–Miyaura
cross-coupling reaction†
Cite this: DOI: 10.1039/c4ob02436b
You Chen,a Hui Peng,b Yun-Xiao Pi,c Tong Meng,a Ze-Yu Lian,a Meng-Qi Yan,a
Yan Liu,a Sheng-Hua Liua and Guang-Ao Yu*a
Received 20th November 2014,
Accepted 20th January 2015
We report the synthesis of 2-(anthracen-9-yl)-1H-inden-3-yl dicyclohexylphosphine and its use in palla-
dium-catalyzed borylation/Suzuki–Miyaura cross-coupling reaction to prepare a variety of symmetrical
and unsymmetrical biaryl compounds in excellent yield.
DOI: 10.1039/c4ob02436b
www.rsc.org/obc
In addition, when boronic acid is not commercially available,
its synthesis is required, adding additional, often lengthy,
Introduction
The Suzuki–Miyaura cross-coupling of boronic acids with steps to the synthetic process.6
organic halides is one of the most widely applied methods in
Over the last two decades, progress has been made to cir-
modern synthetic organic chemistry.1 Since the first report of cumvent some of the limitations of the Suzuki–Miyaura cross-
the palladium-catalyzed cross-coupling reaction between an coupling reaction with the advent of the one-pot borylation/
aryl halide and an arylboronic acid by Suzuki and Miyaura in Suzuki–Miyaura cross-coupling reaction. The first system was
1981,2 it has emerged as a synthetic method that tolerates a reported by Miyaura in 1997,6d which involved converting an
wide range of functional groups providing reliable and aryl triflate in situ into a boronate ester followed by the
efficient access to structurally diverse biaryl motifs.3 It is for addition of a second aryl triflate along with the palladium
these reasons that it remains one of the most important catalyst and a base. Since then, the development of one-pot,
methods of choice for C–C bond formation in both industrial catalytic C–H or C–X borylation/Suzuki–Miyaura coupling pro-
and academic groups. Fueled by the commercial availability of cesses have allowed the preparation of unsymmetrical biaryl
numerous organic halides and boronic acids, and the constant compounds.7,8 This protocol has been applied to the syn-
development of improved catalyst systems,4 intense research theses of pharmaceuticals.9 However, a high catalyst loading
efforts continue in this area.
(5–10 mol%) and the addition of a second portion of the palla-
With all of the advances, the Suzuki–Miyaura cross-coup- dium catalyst are necessary. In 2007, Buchwald reported that
ling reaction still suffers a major limitation in that it relies Pd2(dba)3 and XPhos actively combine to efficiently catalyze
upon the direct use of boronic acids. Although many boronic the one-pot borylation/Suzuki–Miyaura coupling reaction
acids are commercially available, they can be very expensive without the need to add a second portion of the catalyst prior
and decompose upon storage over time, often resulting in the to conducting the Suzuki–Miyaura coupling reaction.10 In this
need to use at least 1.2 equiv. (with regard to the organic process, an excess amount of the phosphine ligand (P/Pd, 4 : 1)
halide) in a typical Suzuki–Miyaura cross-coupling reaction.5 was required. Recently, Buchwald demonstrated an efficient
synthesis of biaryl compounds from aryl halides using a lithia-
tion/borylation/Suzuki–Miyaura coupling sequence under con-
tinuous-flow conditions.11 In addition, the groups of Molander,12
aKey Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of
Kwong,13 Zhang,14 Wu,15 and others have reported their pro-
Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of
gress. Despite these advances, many limitations remain. These
China. E-mail: yuguang@mail.ccnu.edu.cn
bDepartment of Chemistry, State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, People’s Republic of China
cState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of
include: (1) high catalyst loading is employed or a second
loading of the catalyst is often required to facilitate the
Suzuki–Miyaura coupling reaction in the second step, and (2)
heteroaryl halides, which are important for medicinal chem-
istry applications, react slowly with a narrow scope and often
require a higher catalyst loading when compared with aryl
halides.
China
†Electronic supplementary information (ESI) available. CCDC 1041773. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4ob02436b
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Table 1 Optimization of the reaction conditionsa
Entry
Pd precursor
Ligand
Base
Yieldb (%)
1
2
3
4
5
6
7
8
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(OAc)2
PdCl2
—
1
—
1
1
1
1
1
1
—
—
KOAc
KOAc
0
0
0
12
40
44
Cs2CO3
CsOH·H2O
tBuONa
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
Scheme 1 Ligands used in this study.
36
81 (91)c
47
9
10
11
1
2
44
58
Pd(dba)2
We have developed a series of 2-aryl indenyl phosphine
ligands and have demonstrated their high reactivity in the
Suzuki–Miyaura coupling reaction, Buchwald–Hartwig amin-
ation reaction, dehydrogenation reactions and hydration reac-
tions.16 Herein we report the application of 2-aryl indenyl
phosphine ligands 1 and 216c (Scheme 1) in the one-pot, palla-
dium-catalyzed borylation/Suzuki–Miyaura cross-coupling reac-
tion. Excellent reactivity has been achieved on a series of aryl
halide and heteroaryl halide substrates.
a Reaction conditions: 2.4 mmol 4-chlorotoluene, 1.0 mmol bis-
(pinacolato)diboron, 0.02 mmol Pd source, 0.04 mmol ligand,
3.0 mmol base, 2.0 mL DMAc, 100 °C, 20 h. b Isolated yield. c GC yield.
metrical biaryl product was observed (Table 1, entry 4). The
use of the base was found to be crucial for a high yield. Strong
bases such as Cs2CO3, CsOH·H2O and tBuONa provided mod-
erate yields (Table 1, entries 5–7), while the weak base
K3PO4·3H2O proved to be the most efficient and gave an 81%
yield (GC yield is 91%) (Table 1, entry 8). A scanning of
commercially available palladium precursors revealed that
Pd(dba)2 was the best choice (Table 1, entries 8–10). In addition
to ligand 1, the use of ligand 2 was examined for comparison
with moderate conversions being observed in this reaction
(Table 1, entry 11).
Under the optimized reaction conditions (Table 1, entry
8), a wide range of aryl chlorides reacted smoothly to provide
symmetrical biaryl products (Table 2). For example, 3-chloro-
toluene was directly converted into the symmetrical biaryl
product in high yield (Table 2, entry 1). Borylation/Suzuki–
Miyaura cross-coupling reactions of substrates bearing elec-
tron-donating substituents such as –OCH3 and –OH occurred
smoothly, providing high yields of the expected products
(Table 2, entries 2 and 3). The reaction of the relatively hin-
dered 2-chlorotoluene gave the desired product in moderate
yield (Table 2, entry 4). Aryl chlorides bearing electron-with-
drawing groups such as –F, –CF3, –NO2 and –COCH3 at the
para position also permit the borylation/Suzuki–Miyaura
cross-coupling reaction, giving the required products in good
yields (Table 2, entries 5–8). Moreover, the base-sensitive –CN
group was tolerated using our catalytic system and the corres-
ponding symmetrical biaryl product was obtained in a near-
quantitative yield (Table 2, entry 9). In addition, 2-chloro-
naphthalene and 3-chloronaphthalene underwent the
desired reaction, giving the symmetrical binaphthalenes in
moderate yield (Table 2, entries 10 and 11). To further extend
the scope of the reaction using our catalytic system, we inves-
tigated the reaction of 3-chloropyridine; fortunately, the
desired 3,3′-bipyridine was obtained in 76% yield (Table 2,
entry 12).
Results and discussion
The synthesis of 1 was accomplished in three steps from
indenyl boronic ester 3, which could be prepared on a kilo-
gram scale from 2-bromo-1H-indene. The palladium-catalyzed
Suzuki–Miyaura coupling of 3 with 9-bromoanthracene pro-
vided 4 in 87% yield.17 Then, straightforward deprotonation of
4 using nBuLi and trapping of the lithiated intermediate with
dicyclohexyl phosphine chloride (Cy2PCl) afforded the mono-
phosphine ligand 1 in 68% yield (Scheme 2). Ligand 1 was
found to be stable under air.
To test the effectiveness of this new ligand 1, we initially
investigated the one-pot reaction of 4-chlorotoluene with bis-
(pinacolato)diboron to establish the feasibility of our strategy
and to optimize the reaction conditions (Table 1). First, the
reaction was conducted without any base and/or phosphine
ligand and no product was obtained (Table 1, entries 1–3).
Then 2.0 mol% Pd(dba)2, 4.0 mol% phosphine ligand 1 and
3.0 equivalent KOAc were used and a 12% yield of the sym-
Scheme 2 Synthesis of ligand 1.
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jected to the Pd-catalyzed borylation conditions with the
addition of the second aryl chloride and K3PO4·3H2O. No
workup was performed or catalyst added prior to conducting
the second reaction in the sequence (Table 3). For example,
bromobenzene was subject to the Suzuki–Miyaura cross-
coupling reaction after the borylation reaction; the resulting
boronic ester could smoothly couple with 3-chloroanisole and
give the desired product in 96% yield (Table 3, entry 1). Aryl
bromides bearing electron-donating and electron-withdrawing
groups such as –OH, –F, and –NO2 were also tolerated in the
borylation and subsequent cross-coupling reaction with aryl
chloride and afforded the corresponding products in high
yields (Table 3, entries 2–4). It is noteworthy that this protocol
avoids the α-arylation reaction of ketones with aryl boronate
esters, and no reduction of ketones as previously observed in
the aryl halide borylation of ketones was observed when they
were employed in the second step (Table 3, entry 5).18 In
addition, aryl bromide bearing the base-sensitive –CN group
was tolerated in our catalytic system (Table 3, entries 6–10).
Moreover, aryl chlorides with both electron-donating and elec-
tron-withdrawing groups could be employed in the second
step while maintaining good to excellent yields of the unsym-
metrical biaryl products (Table 3, entries 6–10).
Heterocyclic compounds are of particular interest in the
pharmaceutical industry.19 The application of heterocyclic
compounds in cross-coupling reactions still remains a syn-
thetic challenge,20 because the ligating ability of the hetero-
atoms present can lead to catalyst deactivation. In addition, the
electronic properties at certain positions in the heterocycle can
be unfavorable for the elementary reactions required for these
catalytic processes.21 We next turned our attention to exploring
the scope of heteroaryl halides. As few heteroaryls performed
exceptionally well under the general borylation conditions, we
focused on their use as coupling partners in the second step.
As outlined in Table 4, all heteroaryl chlorides undergo
efficient cross-coupling reactions. For example, 3-chloro-
pyridine and 2-chloropyridine provided good to excellent
yields (Table 4, entries 1–3). 2-Chlorothiophene coupled
smoothly and provided a moderate yield over two steps
(Table 4, entry 4). 4-Chlorobenzonitrile was also tolerated in
the borylation reaction and subsequent cross-coupling reac-
tion with heteroaryl chlorides such as 2-chloropyridine,
3-chloropyridine, 2-chloropyrazine and 2-chloroquinoxaline, to
afford the desired heteroaryl products in good yield (Table 4,
entries 5–8). In addition, 3-chloropyridine was tolerated in the
borylation reaction and subsequent reaction with 2-chloro-
quinoxaline to give the corresponding heteroaryl product in
51% yield (Table 4, entry 9).
Table 2 Palladium-catalyzed one-pot preparation of symmetrical biaryl
compoundsa
Yieldb
(%)
Entry
1
Substrate
Product
90
2
95
3
4
90
72
5
6
7
8
82
84
88
81
9
97
70
10
11
12
72
76
a Reaction conditions: 2.4 mmol aryl chlorides or heteroaryl chlorides,
1.0 mmol bis(pinacolato)diboron, 2.0 mmol% Pd(dba)2, 4.0 mmol%
ligand 1, 3.0 mmol K3PO4·3H2O, 2.0 mL DMAc, 100 °C, 20 h. b Isolated
yield.
To carry out an economical large-scale synthesis of fine
chemicals, a low loading catalyst is needed. To further study
the efficiency of our catalytic system, we then studied the TON
(turnover number) of our catalytic system at low loading. For
example, carrying out the one-pot reaction of 3-chloropyridine
with bis(pinacolato)diboron at a palladium loading of 0.2 mol%
led to an yield of 27% after 20 h (Table 5, entry 1). This corres-
ponds to a TON of 135.
To further extend the scope of our catalytic system, we
carried out reactions towards the direct synthesis of unsymme-
trical biaryl compounds. In this endeavor, a catalytic system
based upon Pd(dba)2 and ligand 1 proved to be effective for
borylation as well as the subsequent Suzuki–Miyaura cross-
coupling reaction. In this process, aryl bromides were sub-
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Table 3 Palladium-catalyzed one-pot two-step preparation of unsymmetrical biaryl compoundsa
Entry
1
Ar–X
Ar′Cl
Product
Yieldb (%)
96
2
88
82
81
93
95
88
82
88
89
3
4
5
6
7
8
9
10
a Reaction conditions: 1.2 mmol the first aryl bromides, 1.0 mmol the second aryl chlorides, 1.2 mmol bis(pinacolato)diboron, 2.0 mmol%
Pd(dba)2, 4.0 mmol% ligand 1, 3.0 mmol KOAc, 3.0 mmol K3PO4·3H2O, 2.0 mL DMAc, 100 °C. b Isolated yield.
into symmetrical biaryl compounds. A direct synthesis of
Conclusion
unsymmetrical biaryls from aryl chlorides or aryl bromides
In summary, a novel, efficient and air-stable anthracenyl sub- using a one-pot, two-step procedure can also be successfully
stituted indenyl phosphine ligand 1 was synthesized in high accomplished using this catalyst system without an excess
yield and used to generate a very active and broadly useful Pd amount of ligand and the addition of a catalyst in the second
catalyst system for the borylation/Suzuki–Miyaura cross-coup- step. Such methodology tolerated not only a number of func-
ling reaction. With the Pd/1 catalyst system, a range of aryl tional groups, but also was applied successfully to a range of
chlorides bearing various functional groups can be converted heteroaryl chlorides.
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Table 4 Use of heteroaryl chlorides as electrophiles in the palladium-catalyzed one-pot two-step preparation of unsymmetrical biaryl compoundsa
Entry
1
Ar–X
Ar′Cl
Product
Yieldb (%)
87
2
3
4
92
94
65
5
6
7
81
89
88
8
9
91
51
a Reaction conditions: 1.2 mmol of aryl halides, 1.0 mmol of heteroaryl chlorides, 1.2 mmol of bis(pinacolato)diboron, 2 mmol% Pd(dba)2,
4 mmol% L1, 3.0 mmol KOAc, 3.0 mmol of K3PO4·3H2O, 2.0 mL DMAc, 100 °C. b Isolated yield.
Acknowledgements
Table 5 Palladium-catalyzed one-pot preparation of symmetrical
heteroaryl compounds using ultra-low loading of the catalysta
The authors would like to thank the National Natural Science
Foundation of China (no. 21472060, 21072071 and 21272088)
and the Program for Academic Leader in Wuhan Municipality
(no. 201271130441) for financial support.
Entry
Mol% Pd
Yieldb (%)
TON
1
2
0.2
0.02
27
<5
135
—
Notes and references
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a Reaction conditions: 2.4 mmol 3-chloropyridine, 1.0 mmol bis-
(pinacolato)diboron, Pd(dba)2/ligand 1 = 1 : 2, 3.0 mmol K3PO4·3H2O,
2.0 mL DMAc, 100 °C, 20 h. b Isolated yield.
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