An easily prepared tetraphosphine and its use in the palladium-catalyzed Suzuki-Miyaura coupling of aryl chlorides

Article abstract of DOI:10.1007/s10562-013-1028-0

An air-stable tetraphosphine N,N,N′,N′- tetra(diphenylphosphinomethyl)-benzene-1,3-diamine (L3) was easily prepared in two steps from triphenylphosphine, which in combination with [Pd(η³-C₃H₅)Cl]₂ affords an efficient catalyst for Suzuki-Miyaura coupling of activated chloroarenes. Even at high temperature of 130 C, this catalyst exhibits good stability and longevity, and could allow a high turnover number of 680,000 to be reached. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-013-1028-0

Catal Lett (2013) 143:1214–1219
DOI 10.1007/s10562-013-1028-0
An Easily Prepared Tetraphosphine and Its Use in the Palladium-
Catalyzed Suzuki–Miyaura Coupling of Aryl Chlorides
•
Kun Wang Wei Wang Heng Luo
•
•
•
•
•
Xueli Zheng Haiyan Fu Hua Chen
Ruixiang Li
Received: 21 April 2013 / Accepted: 22 May 2013 / Published online: 22 October 2013
Ó Springer Science+Business Media New York 2013
Abstract An air-stable tetraphosphine N,N,N0,N0-tetra
(diphenylphosphinomethyl)-benzene-1,3-diamine (L3) was
easily prepared in two steps from triphenylphosphine,
which in combination with [Pd(g3-C3H5)Cl]2 affords an
efficient catalyst for Suzuki–Miyaura coupling of activated
chloroarenes. Even at high temperature of 130 °C, this
catalyst exhibits good stability and longevity, and could
allow a high turnover number of 680,000 to be reached.
coupling conducted at low catalyst loadings was less
involved and is still an ongoing field [15–18].
NHC-based ligand could allow high turnover numbers
(TONs) to be reached [19, 20]; Buchwald’s monodentate
phosphines are indubitable in terms of reactivity and effi-
ciency [21–26]. Improved catalytic activities were detected
from systems that associate palladium to strong r-electron-
donating ligands. Bedford and coworkers proposed a con-
cept of catalyst longevity [27]. They attributed the spec-
tacular TONs they observed using palladacycle not to a
high activity of the catalyst, but more to an outstanding
catalyst longevity, consequently the stability of the cata-
lytic species. This study inspired chemists to develop
ligands which could stabilize palladium species efficiently.
A tetradentate phosphine Tedicyp [28–32] by Doucet and
Santelli displayed remarkable TONs, which was regarded
as a breakthrough in this area. This discovery prompted the
examination of various carbocyclic [33, 34], ferrocenyl-
based [35–37] and cavity-shaped [38, 39] multidentate
phosphines in low catalyst loading Suzuki couplings. A
cyclodextrin-tetraphosphine hybrid coined a-Cytep [39]
displayed exceptionally high TONs. The authors associated
this property with its ability to super-stabilize the catalytic
species over a very long time via multiple binding to the
metal. Multidentate phosphines shed light on the devel-
opment of ultra-low loading catalytic processes.
Keywords Suzuki–Miyaura coupling ꢀ Palladium ꢀ
Tetraphosphine ꢀ Aryl chlorides
1 Introduction
The palladium-catalyzed Suzuki–Miyaura cross-coupling
reaction, which is one of the most efficient methods for the
synthesis of biaryl compounds, is particularly popular in
the pharmaceutical and chemical industries [1–5]. For the
applications, the use of cheaper chloroaryls is important,
but their reluctance to perform the oxidative addition to
palladium makes the coupling procedure difficult [6, 7]. In
recent years, a great deal of attention has been focused on
the exploration of catalytic systems that could improve
efficiency of the reactions of aryl chlorides [8–14], but the
However, the long synthetic route of multidentate
phosphines results in the limitation on their applications
[28, 33–39]. In addition, due to the sensitivity to air, these
ligands were always protected and stored as their borane
complexes, and required deboronation before use in the
catalytic reaction [28, 39]. Although the Suzuki coupling
using multidentate phosphines has been much investigated,
the studies on the reaction of aryl chlorides were less
involved [29, 35, 39].
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-013-1028-0) contains supplementary
material, which is available to authorized users.
K. Wang ꢀ W. Wang ꢀ H. Luo ꢀ X. Zheng ꢀ H. Fu ꢀ H. Chen ꢀ
R. Li (&)
Key Laboratory of Green Chemistry and Technology, Ministry
of Education, Department of Chemistry, Sichuan University,
Chengdu 610064, Sichuan, People’s Republic of China
e-mail: liruixiang@scu.edu.cn
123

Palladium-Catalyzed Suzuki Reaction Using Tetraphosphine
1215
In this paper, we aim to synthesize an air-stable tetra-
phosphine, which could be easily prepared from commer-
cially available reactants, expecting to develop an effective
catalytic system for the low palladium-loading Suzuki–
Miyaura coupling of aryl chlorides.
by the cyclohexane moiety in L2 makes this tetraphosphine
air-sensitive. Consequently, it is unfavorable to use in
microscale catalytic experiments, and isolating oxygen for
storage is required.
Aiming to prepare a stable phosphine ligand which
could perform satisfactory results for the Suzuki coupling
of aryl chlorides, we replace the core structure of L1 with a
rigid moiety 1,3-phenyldiamine to obtain a new tetra-
phosphine N,N,N0,N0-tetra(diphenylphosphinomethyl)-ben-
zene-1,3-diamine (L3) (Fig. 1). Based upon the procedure
reported previously [42, 43], we modified the reaction
conditions to provide the target ligand in a simple protocol
with an overall yield of 73 % (Scheme 1). L3 is stable in
air and consequently, is very easy to store and handle.
We employed the coupling of 4-chloroacetophenone
with phenylboronic acid as the test case to gauge the per-
formance of our new ligand and to optimize the reaction
parameters (Table 1). Dimethylacetamide (DMAc) and
K3PO4 were adopted as solvent and base respectively for
good results. 98 % conversion of 4-chloroacetophenone
was observed using 0.05 mol% [Pd(g3-C3H5)Cl]2 and
0.1 mol% L3 at 130 °C for 20 h (Table 1, entry 9).
In the absence of ligand, only a trace amount (2 %) of
biaryl adduct was detected under the same reaction con-
ditions (Table 2, entry 1), and Pd black appeared rapidly. It
is suggested that the addition of L3 highly improved the
stability of the active palladium species, and the decom-
position of the catalyst could be avoided even at the rela-
tively high temperature of 130 °C.
2 Results and Discussion
We have reported the results in Suzuki–Miyaura coupling
based on tetraphosphine N,N,N0,N0-tetra(diphenylphosphi-
nomethyl)-1,2-ethylenediamine (L1) (Fig. 1) [40, 41],
which exhibits a good stabilization to the active palladium
species. Aryl and heteroaryl bromides coupled with phen-
ylboronic acid efficiently at low catalyst loadings, and a
high TON of 750,000 could be achieved. Unfortunately,
the catalyst system consisting of Pd/L1 is not active for
aryl chlorides.
Previous studies have demonstrated that in the catalytic
cycle the oxidative addition step is favored by electron-rich
ligands. On the other hand, the rate of reductive elimination
is usually increased by the coordination of bulky ligands to
the metal center. Therefore, those ligands combination of
both electronic and steric properties are of benefit for the
Suzuki reaction of non-activated substrates. Lacking of
sufficient steric hindrance in L1, which lies in the flexibility
of the 1,2-ethylenediamine backbone probably leads to the
unsatisfactory results for the couplings of chloroarenes.
To prove the hypothesis, we synthesized a new tetraphos-
phine N,N,N0,N0-tetra(diphenylphosphinomethyl)-cyclohex-
ane-1,2-diamine (L2) (Fig. 1) and tested its efficacy for the
palladium-catalyzed Suzuki reaction of aryl chlorides [42].
Under a relatively mild reaction condition (60 °C), aryl and
heteroaryl chlorides bearing electron-withdrawing substitu-
ents could be successfully transformed to the desired products
in excellent yields with 0.1 mol% Pd, and a yield of 80 % was
givenfor thecouplingof3-chloroanisole. Thisligandseemsto
possess a fine balance of electronic and steric properties.
However, the increased electron density at phosphorus caused
For the further evaluation of the importance of tetradentate
phosphine L3 on the complex, we compared the rate of the
reactions based on PPh3 and electronically similar diphos-
phines bearing diphenylphosphinomethyl group: 1,2-bis
(diphenylphosphino)ethane (dppe), 1,4-bis(diphenylphos-
phino)butane (dppb), 2,20-bis(diphenylphosphinomethyl)-1,10-
biphenyl (BISBI) and 1,2-bis(diphenylphosphinomethyl)ben-
zene (BDPX). This comparison was carried out with 4-chlo-
roacetophenone as the test substrate (Table 2, entries 5–21).
When 0.1 mol% Pd/PPh3 was used as catalyst, only 4 %
Fig. 1 Tetraphosphines we
have reported for the Suzuki
coupling
Scheme 1 Synthesis of N,N,N0,N0-tetra(diphenylphosphinomethyl)-benzene-1,3-diamine (L3)
123

1216
K. Wang et al.
Table 1 The effect of solvent and base on the Suzuki cross-coupling reaction
Entry
Solvent
Base
Yield (%)
1
2
3
4
5
6
7
8
9
2-Ethoxyethanol
Xylene
DMF
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
NaOH
KOH
41
39
35
62
71
83
76
87
98a
DMAc
DMAc
DMAc
DMAc
DMAc
K3PO4
K3PO4
DMAc
Reaction conditions: 4-chloroacetophenone 1 mmol, phenylboronic acid 1.5 mmol, base 2 mmol, solvent 2 mL, L3/[Pd(g3-C3H5)Cl]2 = 2/1,
[Pd(g3-C3H5)Cl]2 0.0005 mmol, L3 0.001 mmol, temperature 130 °C, 20 h, analyzed by GC, average of two runs
a
K3PO4 2.5 mmol
Table 2 The effect of phosphine ligands on the Suzuki cross-coupling reaction
Entry
Ligand
Pd loading (mol%)
Yield (%)
1
–
0.1
2
43
99
13
98
40
98a
4c
5a,b
31
34a
5
2
L1
L2
0.1
3
0.1
4
0.01
0.1
5
L3
6
0.01
0.01
0.1
7
8
PPh3
dppe
9
0.1
10
11
12
13
14
15
16
17
18
19
20
21
0.1
0.1
0.01
0.01
0.1
6a
dppb
36
40a
5
0.1
0.01
0.01
0.1
5a
BISBI
BDPX
18
21a
21
25a
0.1
0.1
0.1
Reaction conditions: 4-chloroacetophenone 1 mmol, phenylboronic acid 1.5 mmol, K3PO4 2.5 mmol, Ligand/[Pd(g3-C3H5)Cl]2 = 2/1, DMAc
2 mL, temperature 130 °C, 20 h, analyzed by GC, average of two runs
a
Reaction time 60 h
Ligand/[Pd(g3-C3H5)Cl]2 = 4/1
b
conversion was given (Table 2, entry 8). The utilization of
bidentate ligands led to higher yields (Table 2, entries 10, 14,
18, 20), and the backbone structure of phosphines seems to
affect the reaction rate. The diphosphines with alkyl backbones
(Table 2, entries 10, 14) provided better performance than
those with aryl backbones (Table 2, entries 18, 20). However,
123

Palladium-Catalyzed Suzuki Reaction Using Tetraphosphine
1217
at the end of the above reactions (Table 2, entries 8, 10, 14, 18,
20) Pd black was clearly observed, and the conversions were
not increased markedly even when the reaction time was
extended to 60 h (Table 2, entries 9, 11, 15, 19, 21). The result
using tetraphosphine L3 compared favorably with these mono-
or bidentate phosphines, and this advantage was shown more
obviously when the Pd loading was decreased to 0.01 mol%
(Table 2, entries 6, 7, 12, 13, 16, 17). The comparative inves-
tigation illustrates that the species formed by association of
tetraphosphine L3 and [Pd(g3-C3H5)Cl]2 appears to be more
stable than the systems formed with electronically similar
mono- or bidentate phosphines.
Table 3 Suzuki coupling of aryl chlorides with phenylboronic acid
based on Pd/L3
Entry
R1
R2
Ratio substrate/catalyst
Yield (%)
1
4-MeO
4-MeCO
4-MeCO
2-MeCO
2-MeCO
4-NO2
4-NO2
2-NO2
2-NO2
4-CF3
H
100
1,000
18
95
96a
2
H
3
H
10,000
1,000
4
H
44
93a
5
H
1,000
6
H
1,000
90
90a
7
H
10,000
1,000
In order to discuss the effect of structure of diamine
moiety in tetraphosphines on the reaction rate, the coupling
of 4-chloroacetophenone with phenylboronic acid was
investigated with L1, L2 and L3 under the same reaction
conditions, respectively (Table 2, entries 2–7). As expec-
ted, compared to the steric hindered ligand L2 and L3, L1
was unfavorable for the coupling. With 0.1 mol% Pd, just a
moderate yield of 43 % was given (Table 2, entry 2).
When the catalyst loading was reduced to 0.01 mol%, the
same reaction performed with L2 and L3 resulted in the
yield of 13 % (Table 2, entry 4) and 40 % (Table 2, entry
6), respectively. L2 associated to palladium showed a good
activity in our previous report, but this tetraphosphine
seems to compare unfavorably with L3 for the reactions at
low catalyst loading. L2 is air-sensitive and in microscale
experiments the probability of oxidation on phosphorus
would increase, which probably led to the inefficiency for
the generation of palladium complex and for the stabil-
ization to active palladium species. The aromatic backbone
in L3 could not provide a high electron density at phos-
phorus to accelerate the oxidative addition process as the
ligand with alkyl moieties. However, a steric factor from
the rigid diamine might be beneficial for the reductive
elimination step, and finally increase the reaction rate. As a
result, a higher yield could be achieved by use of L3.
Subsequently, we screened reactions of various aryl
chlorides with phenylboronic acid, and all the results are
illustrated in Table 3.
8
H
43
93a
9
H
1,000
10
11
12
13
14
15
16
17
18
19
20
21
22
23
H
1,000
98b
95a,b
64b
91b,c
34b
87a,b
61b
88b,c
90b
99b
87b
41a,b
60b,d
68b,e
4-CF3
H
10,000
1,000
4-CN
H
4-CN
H
1,000
2-CN
H
1,000
2-CN
H
1,000
4-CHO
4-CHO
2-CHO
2-NO2
2-NO2
2-NO2
2-NO2
2-NO2
H
1,000
H
1,000
H
1,000
4-CF3
4-CF3
4-CF3
4-CF3
4-CF3
10,000
100,000
1,000,000
1,000,000
1,000,000
Reaction conditions: aryl chloride 1 mmol, phenylboronic acid
1.5 mmol, K3PO4 2.5 mmol, L3/[Pd(g3-C3H5)Cl]2 = 2/1, DMAc
2 mL, temperature 130 °C, 20 h, isolated yield, average of two runs
a
Reaction time 60 h
b
K2CO3 as base (2.5 mmol)
c
Reaction time 40 h
d
Reaction time 120 h
e
Reaction time 200 h
comparison to the reported Pd/tetraphosphine catalyst sys-
tems [29, 35].
First, we investigated the Suzuki reaction of several para
substituted aryl chlorides. A strong substituent effect on the
reaction rate was unfolded. Electron-withdrawing groups in
the aryl chlorides support the process. In the presence of
0.01–0.1 mol% Pd/L3 catalyst, the couplings of 4-chloro-
acetophenone, 4-chloronitrobenzene, 4-chlorobenzotrifluo-
ride, 4-chlorobenzonitrile and 4-chlorobenzaldehyde with
phenylboronic acid proceeded to provide the corresponding
biaryls in good results (Table 3, entries 3, 7, 11, 13, 17). On
the other hand, electron-donating chloroaryls was unfa-
vorable. With 4-chloroanisole, a TON of 18 was obtained
(Table 3, entry 1), which exhibited a similar efficacy in
Some activated substrates bearing functional groups
were sensitive to the strong basicity of K3PO4, which
resulted in the formation of undesired products. Thus
K2CO3 was used instead (Table 3, entries 10–23).
Next, we studied the influence of an ortho substituent on
the chloroaryls, and an obvious steric effect was observed
on the reaction rate. 2-Chloroacetophenone, 2-chloroni-
trobenzene and 2-chlorobenzonitrile led to much lower
yields than the corresponding para substituted substrates
(Table 3, entries 4, 8, 14). However, 2-chlorobenzaldehyde
was an exception (Table 3, entry 18), which was more
123

1218
K. Wang et al.
beneficial than 4-chlorobenzaldehyde for the coupling
reaction under the same conditions.
of triethylamine (3 mL, 20 mmol) and [PPh2(CH2OH)2]Cl
(2.8 g, 10 mmol) in mixture solvent of 10 mL ethanol and
20 mL toluene. The solution was refluxed for 40 h. At the
end of reaction, the mixture was cooled to room tempera-
ture, and the product was washed with water (3 9 10 mL),
then dried over magnesium sulfate and filtered. Subse-
quently, the solvent was removed under vacuum and the
resulting crude product was refluxed in 30 mL ethanol for
2 h. The solution was slowly cooled to room temperature.
The pale yellow product was filtered, washed with ethanol
(3 9 5 mL) and dried under vacuum. Yield: 1.48 g (82 %).
We tried to determine the limitation of the catalyst
loading. The very activated substrate 2-chloro-5-(trifluo-
romethyl)nitrobenzene could be coupled with phenylbo-
ronic acid with a Pd loading as low as 0.0001 mol%. After
60 h, 41 % conversion was detected (Table 3, entry 21).
When the reaction time extended to 200 h, the desired
biaryls was given with a high TON of 680,000 (Table 3,
entry 23), and the yield did not increase any more even
when the reaction time was further prolonged. The results
indicated that the Pd/L3 catalyst system exhibits a good
longevity even at a high temperature, which is probably
attributed to the tetraphosphine L3 providing the active
palladium species with good stabilization. It is also dem-
onstrated that multidentate ligand had advantages for the
exploration of low loading catalyst systems.
1
31P NMR (162 MHz, CDCl3): d -26.88 ppm. H NMR
(400 MHz, DMSO): d 7.36–7.27 (m, 40H, PPH2), 6.89 (t,
J = 8.0 Hz, 1H, PH5), 6.29 (s, 1H, PH2), 6.21 (d, J =
2
8.0 Hz, 2H, PH4,6), 3.87 (d, JPH = 4.0 Hz, 8H, NCH2).
13C NMR (101 MHz, CDCl3): 148.17 (s, C61;3H4N2),
136.64 (d, 1JCP = -16.2 Hz, PC61H5), 132.23 (s, C65H4N2),
132.22 (d, JCP = 20.2 Hz, PC62;6H5), 128.20 (s, PC64H5),
2
127.36 (d, 3JCP = 22.2 Hz, PC63;5H5), 104.37 (s, C64;6H4N2),
3 Conclusion
1
101.67 (s, C62H4N2), 53.21 (d, JCP = -7.1 Hz, NCH2P).
HRMS (ESI) Calcd. for C58H52N2P4: 900.3081 [M]?, found:
In conclusion, we synthesized a simple and air-stable tet-
raphosphine N,N,N0,N0-tetra- (diphenylphosphinomethyl)-
benzene-1,3-diamine (L3) in two steps from PPh3, and a
catalyst system comprised of [Pd(g3-C3H5)Cl]2/L3 for the
Suzuki cross-coupling of aryl chlorides has been estab-
lished. Even at high temperature of 130 °C, this tetra-
phosphine could efficiently stabilize the active palladium
species over a long time. From the comparative investi-
gation, this catalyst is more stable than the complexes
formed with electronically similar mono- or diphosphines.
The couplings of electron-deficient chloroarenes can be
performed with 0.01–0.1 mol% catalyst, and a high TON
of 680,000 is obtained for the reaction of the very activated
substrate. Unfortunately, this Pd/tetraphosphine system is
unfavorable for electron-rich aryl chloride. Further work to
improve the structure of the ligand is in progress.
900.3076.
4.2 General Procedure of Suzuki Coupling Reactions
Tetraphosphine L3 (9.0 mg, 0.01 mmol) and [Pd(g3-C3H5)
Cl]2 (1.8 mg, 0.005 mmol) were added into a Schlenk tube
equipped with a magnetic bar, and then DMAc (1.0 mL)
was added. The mixture was stirred at room temperature
for 1.5 h. 4-Chloroacetophenone (130 lL, 1.0 mmol),
phenylboronic acid (183 mg, 1.5 mmol) and K3PO4
(531 mg, 2.5 mmol) were added into another Schlenk tube
with a magnetic bar. The dissolved mixture of L3/Pd
(100 lL, 0.001 mmol) was transferred to the Schlenk tube
of reactants by syringe. Then, DMAc (2 mL) was added.
The reaction mixture was heated at 130 °C for 20 h. At the
end of the reaction, the solution was cooled to room tem-
perature and water (5 mL) was added. The mixture solution
was extracted with ethyl acetate (3 9 5 mL) and the
organic layer was dried over magnesium sulfate. The dried
solution was filtered and reduced to ca 1–2 mL under
vacuum, then purified with silica gel chromatography to
give the corresponding product with an isolated yield.
4 Experimental
Reactions were carried out under a nitrogen atmosphere.
All chemicals were purchased from commercial suppliers.
Solvents were dried over appropriate drying agents and
distilled under nitrogen before use.
Acknowledgments This work was supported by the National Nat-
ural Science Foundation of China (No. 21202104).
4.1 Synthesis of N,N,N0,N0-
tetra(diphenylphosphinomethyl)-benzene-1,3-
diamine (L3)
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123