Triarylphosphanes with dendritically arranged tetraethylene glycol moieties at the periphery: An efficient ligand for the palladium-catalyzed Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1002/anie.200802683

Peripheral vision: A second generation dendritic phosphane, with tetraethylene glycol (TEG) moieties at the periphery, has been designed and prepared (see structure). It is particularly effective as a ligand for the title transformation when using aryl chloride substrates. To realize high catalytic activity, the TEG groups must be connected to the phosphane framework. (Figure Presented)

Full text of DOI:10.1002/anie.200802683

Communications
DOI: 10.1002/anie.200802683
Dendritic Catalysts
Triarylphosphanes with Dendritically Arranged Tetraethylene Glycol
Moieties at the Periphery: An Efficient Ligand for the Palladium-
Catalyzed Suzuki–Miyaura Coupling Reaction**
Tetsuaki Fujihara, Shohei Yoshida, Hidetoshi Ohta, and Yasushi Tsuji*
Development of a highly efficient catalyst systems is one of
the most important tasks in organic chemistry. For homoge-
neous transition-metal catalysts,[1] modification within close
proximity of the metal center (within a few angstroms) is a
the TEG functionality of 1a is folded in a particular manner
around the catalyst center to enhance the catalytic activity.
However, the catalytic activity of these NHC ligands was not
good enough to activate the less reactive aryl chlorides during
coupling reactions. Recently, highly active catalyst systems
have been developed for the Suzuki–Miyaura coupling
reaction of aryl chlorides. These systems mainly employ a
basic, bulky phosphane derivative as a ligand.[8] Herein, we
design and prepare a series of new triarylphosphanes in which
the TEG or n-C12 moieties are arranged dendritically[9] at the
periphery of the phosphanes. Among them, the second
generation dendritic derivative, with the TEG chains, is
distinctly effective as a ligand and provides a highly active Pd
catalyst system for the Suzuki–Miyaura coupling reaction.
The novel phosphanes with TEG (2a1 and 2a2) or n-C12
(2b1 and 2b2) moieties are shown in Scheme 2. These long
chains are arranged dendritically at the periphery of the
phosphanes: 2a2 and 2b2 are higher dendritic analogues of 2a1
and 2b1, respectively. These phosphanes were prepared in a
high yielding and straightforward manner by the reaction of
tri(4-hydroxyphenyl)phosphane oxide with the corresponding
conventional way in which to develop efficient catalysts for
[2]
À
various reactions, including C C bond-formation. Mean-
while, the catalytic surroundings can be expanded to the nano
scale, and this unique environment has offered new prospects
for homogeneous catalysis.[3] For constructing such an dis-
tinctive catalytic environment, phosphanes[1d,e] and N-hetero-
cyclic carbene (NHC)[4] ligands are the most suitable compo-
nents; this is because a wide variety of these ligands have been
successfully utilized to realize high catalytic activity and
selectivity.
Recently, our research group has designed novel NHC
ligands (Scheme 1),[5] where 1a bears tetraethylene glycol
(TEG)[6] functionality and 1b has n-C12 alkyl chains. With
Scheme 1. N-Heterocyclic carbene ligands bearing tetraethylene glycol
or n-C12 moieties.
these ligands, the TEG moieties of 1a considerably enhanced
the catalytic activity of the palladium-catalyzed Suzuki–
Miyaura coupling reaction[7] of aryl bromides, whereas the
n-C12 alkyl chains of 1b has no effect on the catalysis. Here,
[*] Prof. Dr. T. Fujihara, S. Yoshida, Dr. H. Ohta, Prof. Dr. Y. Tsuji
Department of Energy and Hydrocarbon Chemistry
Graduate School of Engineering, Kyoto University
Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2514
E-mail: ytsuji@scl.kyoto-u.ac.jp
Homepage: http://twww.ehcc.kyoto-u.ac.jp/
[**] This work was supported by a Grant-in-Aid for Scientific Research on
Priority Areas (“Synergy of Elements” and “Chemistry of Concerto
Catalysis”) from the Ministry of Education, Culture, Sports, Science,
and Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802683.
Scheme 2. Novel phosphane ligands bearing tetraethylene glycol or n-
C12 moieties.
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benzyl chlorides (3a1,[10a] 3a2, 3b1,[10b] and 3b2[10c]) and
subsequent reduction of the phosphane oxides (4)[11] with
PhSiH3 (Scheme 3). These new phosphanes have been fully
characterized (see the Supporting Information), and the
MALDI-TOF mass spectra show the molecular ion peaks at
m/z 2438 (2a1), 4930 (2a2), 2240 (2b1), and 4535 (2b2),
respectively.
Table 1: Effect of a phosphane and related ligand on the coupling
reaction.[a]
Entry
Ligand
Yield [%][b]
1
2
3
4
5
6
7
8
2a1
2a2
2b1
2b2
PPh3
none
1a
36
93 (92)[c]
2
4
0
0
9
1b
<1
[a] 4-Chlorotoluene (2.0 mmol), phenylboronic acid (4.0 mmol), [PdCl2-
(PhCN)2] (0.0020 mmol, 0.10 mol%), phosphane (0.0040 mmol,
0.20 mol%), and K2CO3 (4.0 mmol) in THF (1.0 mL) at 608C for 20 h.
[b] Yield based on the GC internal standard technique. [c] Yield of
isolated product.
In THF, the catalyst system with 2a2 afforded the product in
high yield (Table 2, entry 1). However, 1,4-dioxane and
toluene, very common solvents for the Suzuki–Miyaura
coupling reaction, were not suitable for the present system
(Table 2, entries 2 and 3). In the solvents DME, DMF, 2-
Table 2: Effect of solvent and base on the coupling reaction.[a]
Entry
Solvent
Base
Yield [%][b]
1
2
3
4
5
6
7
8
THF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KF
K3PO4
Rb2CO3
Na2CO3
Cs2CO3
Li2CO3
93
15
7
33
10
43
9
78
43
75
33
25
3
1,4-dioxane
toluene
DME
DMF
2-propanol
H2O
THF
THF
THF
THF
Scheme 3. Preparation of phosphanes 2a1, 2a2, 2b1, and 2b2.
The phosphanes 2an and 2bn (n = 1 and 2) were employed
as ligands in the Suzuki–Miyaura coupling of 4-chlorotoluene
with phenylboronic acid in the presence of a catalytic amount
of [PdCl2(PhCN)2] with K2CO3 in THFat 608C (Table 1). As a
ligand, the addition of 2a1 bearing the TEG moieties afforded
4-phenyltoluene in 36% yield (Table 1, entry 1). Remarkably,
the higher dendritic analogue 2a2 is a much more efficient
ligand and afforded the product in 93% yield, even with a low
catalyst loading (0.1 mol%; Table 1, entry 2). In contrast, 2b1
with n-C12 alkyl chains gave the product in only 2% yield
under the same reaction conditions (Table 1, entry 3). Unlike
2a2, 2b2 (the higher dendritic analogue of 2b1) did not show a
positive dendrimer effect[12] and afforded the product in only
4% yield (Table 1, entry 4). Moreover, even with phosphane
ligand PPh3 (the common core of 2a1, 2a2, 2b1, and 2b2) or
without it, no conversion of 4-chlorotoluene was observed
(Table 1, entries 5 and 6). The use of NHC ligands 1a and 1b
in the reaction was not successful under the same reaction
conditions (Table 1, entries 7 and 8). Overall, the higher
dendritic phosphane 2a2 had particular efficiency with aryl
chloride in the Suzuki–Miyaura coupling reaction.
9
10
11
12
13
THF
THF
[a] 4-Chlorotoluene (2.0 mmol), phenylboronic acid (4.0 mmol), [PdCl2-
(PhCN)2] (0.0020 mmol, 0.10 mol%), 2a2 (0.0040 mmol, 0.20 mol%),
base (4.0 mmol), and solvent (1.0 mL) at 608C for 20 h. [b] Yield based
on the GC internal standard technique. DME=1,2-dimethoxyethane,
DMF=N,N-dimethylformamide.
propanol, and water,[13] the products were obtained in only
low to moderate yields (Table 2, entries 4–7). Thus, THF
seems to play an important role in the catalysis. Actually,
upon addition of a small amount of THF to the reaction
described by entry 3 (with 0.1 mL of THF and 0.9 mL of
toluene as the solvent), the yield of the product increased
noticeably to 24%. It is well known that the Suzuki–Miyaura
coupling reaction is affected by the type of base used.
Potassium salts have been frequently employed in the
The efficiency of 2a2 as the ligand is effected considerably
by the type of solvent and base used in the reaction (Table 2).
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reaction. In fact, K2CO3 afforded the best result (93% yield;
obtained in moderate yield (Table 3, entry 6). Unfortunately,
2,2’,6,6’-biphenyl derivatives could not be obtained from the
present coupling reaction. The substrates 1-chloronathphar-
ene or 1-naphthylboronic acid gave the corresponding
products in high yields (Table 3, entries 7 and 8). More
reactive aryl bromides were also smoothly coupled to give the
corresponding products as exemplified by entry 9 of Table 3.
To clarify the unique role of the TEG moieties of 2a2 in
the catalysis (vide supra), control experiments were carried
out with several ligand systems in the presence of [PdCl2-
(PhCN)2] (Table 4). The ligand system, consisting of a mixture
of PPh3 and tetraethylene glycol dimethyl ether (5) in place of
2a2 (molar ratio of 5/PPh3 18:1), did not afford any product
(Table 4, entry 1). Two other ligand systems were examined:
one was a combination of PPh3 and the ester 6a2 having two
Ra1 groups (Table 4, entry 2), and the other was a mixture of
PPh3 and 4a2 (the oxide of 2a2; Table 4, entry 3). Both ligand
Table 2, entry 1), whereas KFand K3PO4 afforded the product
in 78% and 43% yields, respectively (Table 2, entries 8 and
9). Rb2CO3 may also be a suitable base (Table 2, entry 10),
however, only low yields were obtained with Na2CO3,
Cs2CO3, and Li2CO3 (Table 2, entries 11–13).
The efficacy of 2a as the ligand was further confirmed by
using various aryl chloride and arylboronic acid substrates
under the optimum reaction conditions in THF with K2CO3
(Table 3). The electron-poor aryl chloride was smoothly
converted into the corresponding biphenyl compound in
95% yield (Table 3, entry 1). In the case of an electron-rich
substrate, the desired product was obtained in 92% yield by
using a higher catalyst loading (0.50 mol%; Table 3, entry 2).
4-Fluorophenylboronic acid afforded the product in 93%
yield (Table 3, entry 3). In the reaction of 2-chlorotoluene
with phenylboronic acid, the product was obtained in high
yield (Table 3, entry 4). The coupling reaction of 2-chloro-1,3-
dimethylbenzene with phenylboronic acid also provided the
product in 93% yield (Table 3, entry 5). In the more sterically
demanding coupling reaction of 2-chloro-1,3-dimethylben-
zene with 2-methylphenylboronic acid, the product was
Table 4: Effect of several ligand systems of relevance on the effective-
ness of 2a2.[a]
Table 3: The reaction of various substrates under the optimum reaction
Entry
Ligand system
Yield [%][b]
conditions.[a]
1
PPh3 (0.20 mol%) + 5 (3.6 mol%)
PPh3 (0.2 mol%) + 6a2 (0.60 mol%)
PPh3 (0.2 mol%) + 4a2 (0.20 mol%)
2a2 (0.2 mol%) + Hg (25 mol%)
0
0
0
2[c]
3
4
55
[a] 4-Chlorotoluene (2.0 mmol), phenylboronic acid (4.0 mmol), [PdCl2-
(PhCN)2] (0.0020 mmol, 0.10 mol%), and K2CO3 (4.0 mmol) in THF
(1.0 mL) at 608C for 20 h. [b] Yield based on the GC internal standard
technique. [c] For Ra1, see Scheme 3.
Entry Aryl chloride
1
Arylboronic acid Product
Yield [%][b]
95
2[c]
3
92
93
4
5
83 (99)
93
systems did not afford the product. These results clearly
indicate that the TEG moieties linked to the phosphane
framework are indispensable in causing the high catalytic
activity. On the other hand, dendrimers are known to stabilize
nanoparticles.[14] Therefore, to examine a possibility of nano-
particles acting as active catalyst species, the mercury test[15]
was performed. Since the product was obtained in significant
yield (55%), even in the presence of 25 mol% mercury
(Table 4, entry 4), it is unlikely that nanoparticles acted as the
active catalyst species.
6
7
(55)
87 (96)
The basicity of these new phosphanes was evaluated by
1
looking at the JPÀSe coupling constants of the corresponding
phosphane selenides Se PR3. It is well known that small JP-Se
8
9
86 (93)
93
1
=
values for phosphane selenides correspond to a higher
basicity of the parent phosphanes.[16] The coupling constants
(hence basicity) of 2a1 (J = 714 Hz), 2a2 (J = 715 Hz), 2b1 (J =
707 Hz), and 2b2 (J = 715 Hz) were all very similar and
comparable to that of PPh3 (J = 730 Hz), but much larger
(therefore a weaker base) than those of basic phosphanes
such as P(tBu)3 (J = 686 Hz) and tricyclohexylphosphane (J =
674 Hz). The 31P{1H} NMR spectra of 2an and 2bn (n = 1 or 2)
[a] Aryl chloride (2.0 mmol), arylboronic acid (4.0 mmol), [PdCl2(PhCN)2]
(0.0020 mmol, 0.10 mol%), 2a2 (0.0040 mmol, 0.20 mol%), K2CO3
(4.0 mmol), and THF (1.0 mL) at 608C for 20 h. [b] Yield of isolated
product. The data in parentheses show yields based on the GC internal
standard technique. [c] [PdCl2(PhCN)2] (0.010 mmol, 0.50 mol%) and 2a2
(0.020 mmol, 1.0 mol%) were used.
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show that 31P resonances are very similar when recorded in
[D8]THF and CDCl3.[17] Thus, the environment close of the
phosphorus atoms of 2an and 2bn (n = 1 or 2) seem to
resemble each other. This observation may be a result of the
intrinsic dendritic structures. The TEG and n-C12 moieties are
at the periphery of the phosphanes, which have the common
triarylphosphane cores.
The reasons for the distinct efficacy of 2a2 as a ligand
(vide supra) are not obvious. Figure 1 shows an optimized
structure of 2a2 calculated by ONIOM[18] (B3LYP/LANL2D-
Z:UFF)-CONFLEX[19] methods. The TEG moieties are very
flexible, and are folded in a particular manner around the
triarylphosphane core in the optimized structure (the triphen-
ylphosphane core is represented by the space-filling model
shown in Figure 1). The density of the TEG moieties of 2a2
(in the case where K2CO3 is the base) and/or the arylboronic
acids during the coupling reaction. Therefore, it is conceivable
that both the transmetalation and the oxidative addition steps
would be accelerated cooperatively because of the strong
interaction of the TEG moieties surrounding the active
catalyst center.
Received: June 7, 2008
Published online: September 26, 2008
Keywords: dendrimers · palladium · phosphanes ·
.
Suzuki–Miyaura coupling
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À
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