Synthesis of a class of binaphthyl monophosphine ligands with a naphthofuran skeleton and their applications in Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1039/c8nj00412a

A series of new monophosphine ligands containing a naphthofuran skeleton have been prepared, characterized and evaluated in palladium-catalyzed Suzuki-Miyaura coupling reactions. Using Pd₂(dba)₃-L6 as the catalyst the reactions were performed with high catalytic activity under mild reaction conditions and the desired coupling products were obtained in good to excellent yields. The new catalyst system also showed broad substrate adaptability and practicality for gram-scale production.

Full text of DOI:10.1039/c8nj00412a

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Synthesis of a class of binaphthyl monophosphine ligands with a
naphthofuran skeleton and their applications in Suzuki-Miyaura
coupling reactions
Zihong Zhou, Hao Liang, Wang Xia, Huixuan Chen, Yaqi Zhang, Xuefeng He, Sifan
a
Yu, Rihui Cao, and Liqin Qiu*
a
School of Chemistry, the Key Laboratory of Low-carbon Chemistry & Energy Conservation of
Guangdong Province, South China Sea Bio-Resource Exploitation and Utilization Collaborative
Innovation Center, Sun Yat-sen University, No. 135 Xingangxi Road, Guangzhou 510275,
People’s Republic of China.
Abstract: A series of new monophosphine ligands containing a naphthofuran skeleton have been
prepared, characterized and evaluated in the palladium-catalyzed Suzuki-Miyaura coupling
2 3
reactions. Using Pd (dba) -L6 as the catalyst the reactions were performed with high catalytic
activity under mild reaction conditions and the desired coupling products were obtained in good to
excellent yields. The new catalyst system also showed broad substrate adaptability and practicality
for gram-scale production.
Introduction
In 1979, Akira Suzuki and co-workers firstly reported Pd-catalyzed cross-coupling between an
1
,2
organoboron compound and an organic halide.
Nowadays, this kind of reaction named after
words for Suzuki-Miyaura coupling reaction has become one of the most efficient and powerful
methods for the construction of organic biaryl frameworks under mild reaction conditions because
of broad functional group tolerance of this transformation. Besides, organoboron reagents and aryl
halides can be easily obtained via various ways and they all exhibit low toxicities and high
3
-12
stabilities towards air and moisture.
Ligand structure is a key factor in this reaction, this is
based on two effects: 1) the ligands can increase catalyst stability by preventing cyclometalation
and promoting the formation of Pd(0); 2) the lone pairs of the hetero atom such as N or O might
coordinate with [Pd] or increase electron density to the ligand, and this will help to stabilize
1
0, 13, 14, 15
intermediate [Pd] complexes.
In 1998, Buchwald and co-workers reported an amino

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1
6
phosphine ligands for Suzuki coupling of aryl chlorides at room-temperature. Since than, a series
of bulkily substituted diphenyl monophosphine ligands haves been prepared and used for Suzuki
1
7-29
coupling.
17-44
Although many of efficient phosphine ligands had been developed over the past 35 years,
the
influence of their steric structure and electronic property on the reaction performance need to be
further explored for the diversity of substrate and reaction. Most recently, we developed a series of
45, 46
new biphenyl monophosine ligands with a chiral-bridged chain.
The lone pair electrons of the
alkoxy oxygen and amino nitrogen atoms might interact with the Pd center and increase the
electron density, which could be beneficial to stabilize the intermediate complexes of ligand and
1
3, 14, 15, 18, 21, 47-52
Pd.
Based on this viewpoint, continuing efforts in developing new biaryl
phosphine ligands proceed in our laboratory. Herein, we report the progress in preparation of a
class of binaphthyl monophosphine ligands containing a naphthofuran skeleton and their
applications in Suzuki-Miyaura coupling reactions.
Scheme 1. Synthesis of ligands L1-L6
Results and discussion
The synthetic protocol was shown in Scheme 1. Compound 8 was synthesized from BINOL as

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showed in detail in the supporting information. Then the target ligands were readily prepared in
high yields from compound 8 in 3 steps. Treatment of 8 with trifluoromethanesulfonic anhydride
in the presence of DIPEA afforded 9 quantitatively. Catalytic introduction of R
2
P(O)- moisty to
compound 9 was carried out by using R P(O)H in dppb, 5 mol% of Pd (dba) and DIPEA in an
2
2
3
anhydrous DMSO system. The corresponding product 10 were provided in 51~75 % yields. Other
Pd source or similar diphosphine ligands such as Pd(OAc) or dppp or dppf only gave 10 in low
2
yields. The desired binaphthyl monophosphine ligands with a naphthofuran skeleton (L1-L6) were
finally afforded in 79~89% yields by reduction of 10 in the presence of DIPEA and HSiCl in
3
toluene.
After acquiring ligands L1-L6 successfully, they were applied to palladium-catalyzed
Suzuki-Miyaura coupling reaction. As shown in table 1, cross-coupling of
1
-bromo-2-methoxynaphthalene and naphthalen-1-ylboronic acid was chosen as the model
reaction. With Pd (dba) -L6 as the catalyst, K PO as the base, various solvents including toluene,
2
3
3
4
o
THF, dioxane and DMF were examined at 80 C under N for 0.5 h (table 1, entries 1-4).
2
Interestingly, the solvent polarity was found to have great impact on the yield of the reaction, and
only 82 % yield was obtained using DMF as the solvent (table 1, entry 4). In contrast, toluene
showed the best performance with a 99% yield given (table 1, entry 1). Encouraged by the results,
the base optimization was then carried out. K PO was found to be the best one. Changing K PO
3
4
3
4
to Cs CO , KF, KHF or K CO , the yields decreased significantly (table 1, entries 5-8). Using
2
3
2
2
3
Pd(OAc) or PdCl instead of Pd (dba) , the desired product was only obtained in 89 % and 81 %
2
2
2
3
yield, respectively (table 1,entries 9-10). This displays the superiority of Pd (dba) as the
2
3
palladium source. On the basis of these results, different ligands (L1-L5) were further screened
table 1, entries 11-15), and L6 was the best one among all of the ligands. In order to further
investigate the effectiveness of L6-Pd (dba) , the amount of palladium was reduced to 0.5 mol%
(
2
3
and substrate A was increased to 0.4 mmol. There was a light yield decrease after a reaction time
o
o
of 6 hours (table 1, entry 16). In addition, catalytic activity of L6 in low temperature (40 C, 60 C)
and [Pd]/L ratio (1:0.5) were also tested. Product AB was given in 54 %, 71 % and 79 % yields
o
respectively, along with a lot of starting material retained. This implies that 80 C and the ratio of
1
:1 are the appropriate choice for the reaction temperature and [Pd]/L ratio in our catalyst system.
Given the above results, the optimal reaction conditions were the combination of Pd (dba) (1.0
2
3
o
mol%), L6 (2.0 mol%), K PO , toluene, 80 C and nitrogen atmosphere. As control, only a
3
4

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moderate yield was obtained for the reaction when Cy-MOP was employed as the ligand (81 % vs
9% yield, table 1, entry 17 vs entry 1). This clearly demonstrates the efficacy of introduction of
9
the naphthofuran moiety.
Table 1: Pd-catalyzed Suzuki-Miyaura coupling of 1-bromo-2-methoxynaphthalene and
a
naphthalen-1-ylboronic acid .
b
Entry
[Pd]
Base
L
Solvent
toluene
THF
Yield (%)
99
1
2
3
4
5
6
7
8
9
Pd
Pd
2
(dba)
(dba)
3
K
K
3
PO
PO
4
4
L6
2
3
3
L6
95
Pd (dba)3
K PO4
3
L6
dioxane
DMF
93
2
Pd
Pd
Pd
Pd
2
(dba)
2
(dba)
2
(dba)
2
(dba)
3
3
3
3
K
3
PO
Cs CO
KF
KHF
K CO3
4
L6
82
2
3
L6
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
90
L6
81
2
L6
83
Pd (dba)3
L6
79
2
2
Pd(OAc)
PdCl
2
K
3
K
3
K
3
K
3
PO
PO
PO
PO
4
4
4
4
L6
89
10
2
L6
81
11
2
Pd (dba)
3
3
L1
94
1
1
1
1
1
2
3
4
5
6
7
2
Pd (dba)
L2
95
Pd (dba)3
K PO4
3
L3
97
2
Pd
Pd
Pd
Pd
2
(dba)
2
(dba)
2
(dba)
2
(dba)
3
3
3
3
K
3
K
3
K
3
K
3
PO
PO
PO
PO
4
4
4
4
L4
96
L5
96
c
L6
92
1
Cy-MOP
81
a
Reaction conditions: 0.2 mmol of A, 0.4 mmol of B, 0.6 mmol of base, [Pd] (2 mol%), L (2
o
b
c
mol%), solvent (4.0 mL), 0.5 h, 80 C; Determined by isolated yield; 0.4 mmol of A, [Pd] (0.5
mol%), L (0.5 mol%), 6 h.
With the optimized reaction conditions in hand, we next turned our attention to the scope and
limitations of this reaction using a variety of boronic acids with 1-bromo-2-methoxynaphthalene
(
table 2). It was found that aryl boronic acids with ortho-substituted groups such as -OMe, -OEt,
Ph, -F, or -Cl reacted with A quickly to give the corresponding desired products in excellent
-
yields (table 2, entries 1-5). It is worth noting that the yields are also very high for aryl boronic

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acids possessing strong electron-withdrawing groups such as 4-OCF , 4-CF and 3-COOMe
3
3
substituents (table 2, entries 11, 12 and 14). Generally, alkyl-substituted aromatic derivatives are
important substructure in organic synthesis. Especially for arylcyclo propanes, they play an
5
3-57
important role in pharmaceutically active molecules.
Gratifyingly, reaction with alkylboronic
acids bearing cyclopropyl or n-butyl group still gave the desired products in good yields (table 2,
entries 15 and 16, AB15 and AB ), and the obtained results are better than those using phosphine
16
5
7
adducts of cyclopalladated ferrocenylimine as the catalyst. Further reducing the [Pd]/A ratio
from 1.0 mol% to 0.1 mol%, the conversion decreased obviously, but moderate yield was still
obtained (table 2, entry 2). When pyridylboronic acids were used as the substrate under the
above-mentioned general conditions, the reactions just only acquired low yields. Employing
i-PrOH instead of toluene as the solvent, however, 82% and 91% yields for the target products
(
AB and AB ) were obtained satisfactorily (table 2, entries 17 and 18).
17 18
a
Table 2: Suzuki-Miyauracoupling reactions using different boronic acids .
b
Entry
Boronic acid
[Pd] (mol%)
Time (h)
n
Yield of AB (%)
1
1
0.5
99
c
2
3
4
1
1
1
0.5
0.5
0.5
99(79 )
99
96
5
6
1
1
0.5
0.5
98
96

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7
8
9
1
1
1
0.5
0.5
0.5
99
99
98
10
1
0.5
97
1
1
1
1
1
0.5
0.5
0.5
97
98
97
1
2
3
1
14
1
0.5
96
1
5
6
1
1
1
1
89
93
1
d
1
7
8
2
2
6
6
82
d
1
91
a
Reaction conditions: 0.2 mmol of A, 0.4 mmol of B , 0.6mmol of K PO , L6 (1 equiv. of [Pd]),
n
3
4
o
b
c
toluene (4.0 mL), 80 C; Determined by isolated yield; gram scale, 4.3 mmol of A, 0.1 mol% of
d
[Pd], 0.1 mol% of L6, 36 h. i-PrOH (4.0 mL) as the solvent.
The substrate scope of the reaction was further extended to various aryl halides, and the results are
summarized in table 3. A variety of functional groups such as -CHO, -CH CN, -NH , -NO and
2
2
2
heterocyclic groups were tolerated under the reaction conditions. To our delight, the reactions
proceeded fluently in high yields even for bulky substrates with substituents at 2- or 6-position of
the aryl halides (table 3, entries 2 vs 3 and 5 vs 6). In addition, substrates with
electron-withdrawing substituents on the aromatic ring (table 3, entries 4 and 11) and hetero aryl
halides (table 3, entries 13 and 14) were also tested in this reaction. The desired products were
given in good to excellent yields, but 2 mol% of the Pd catalyst and 3 hours of reaction time were

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needed for the reaction of bromobenzene with an ortho-acetonitrile substituent (table 3, entry 4).
Even for the substrate with a bulky benzyloxy substituent and a strong electron-withdrawing
aldehyde group at the ortho-position, the reaction activity and yield were still good enough (table
3
, entry12). Particularly noteworthy is the coupling of aryl chloride, which is generally difficult
1
3, 18, 58, 59, 60
due to the low reactivity.
Delightedly, our catalyst system successfully provided the
desired products in good yields. For bulky 2,6-dimethylchlorobenzene the coupling yield reached
80% (table 3, entries 1, 7, 15 and 17). Coupling of 4-chloronitrobenzene gave product A15B up to
95% yield, this result is better than that using (2-mesitylindenyl)dicyclohexylphosphine/Pd(OAc)2
61
as the catalyst (table 3, entry 15, 95% vs 85%). Moreover, our catalyst system can also be used
for the synthesis of some complicated compounds. For example, cross-coupling of an indole
derivative and 1-naphthylboronic acid successfully provided the desired product A16B in 94%
yield (table 3, entry 16).
a
Table 3. Suzuki-Miyaura coupling reactions using different aryl halide
b
Entry
Aryl halide
[Pd] (mol%) Time (h)
Yield of A
n
B (%)
0
.5
99
1
1
1
c
4
89
2
3
0.5
0.5
99
1
99
4
5
2
1
3
87
92
0.5

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6
1
0.5
94
7
8
9
1
1
1
8
78
99
99
0.5
0.5
1
0
1
1
0.5
0.5
99
95
11
1
2
3
1
1
2
1
85
93
1
14
15
16
1
1
1
1
4
1
95
95
94

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17
1
12
80
a
Reaction conditions: 0.2 mmol of A
n
, 0.4 mmol of B, 0.6 mmol of K
3
PO
4
, L6 (1 equiv. of [Pd]),
o
b
c
o
toluene (4.0 mL), 80 C; Determined by isolated yield; A
n
is chlorobenzene, 100 C.
Conclusions
A series of novel binaphthyl monophosphine ligands with a naphthofuran skeleton L1-L6 were
successfully developed and applied to palladium-catalyzed Suzuki-Miyaura coupling reactions
with high catalytic activity and good to excellent yields. Extensive substrate structure adapt ability
in both aryl halides and boronic acids and high functional group tolerance were also displayed.
The introduction of naphthofuran skeleton with more rigidity and higher electron density gave the
ligands better performance than the parent MOP in the catalytic reactions.
Acknowledgments
We would like to thank the National Natural Science Foundation of China (NSFC) (21272283),
the Special Financial Fund of Innovative Development of Marine Economic Demonstration
Project (GD2012-D01-001), and the Dayawan District Science and Technology Program
(201202002) for financial support.
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New Journal of Chemistry
Page 12 of 12
View Article Online
DOI: 10.1039/C8NJ00412A
2 3
The L6-Pd (dba) showed excellent activity in the Suzuki-Miyaura reaction of sterically hindered
and electron-rich aryl halide with aryl boronic acids.
L6-Pd (dba) (0.05 ~ 0.25 mol%)
2
3
K PO
O
Ar
X
+ R B(OH)2
3
4
Ar R
PCy2
toluene
X = Br or Cl;
Ar = Aryl or Heterocyclic aryl;
R = Aryl or Heterocyclic aryl or alkyl
3
6 examples
up to 99% yield
L6