A functionalized phosphine ligand with a pentaarylbenzene moiety in palladium-catalyzed Suzuki-Miyaura coupling of aryl chlorides

Article abstract of DOI:10.1016/j.tetlet.2008.10.071

A functionalized phosphine with a pentaarylbenzene moiety was found to facilitate the palladium-catalyzed Suzuki-Miyaura coupling of aryl chlorides. The new ligand achieved high turnover numbers (up to TON 5000).

Full text of DOI:10.1016/j.tetlet.2008.10.071

Tetrahedron Letters 49 (2008) 7430–7433
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A functionalized phosphine ligand with a pentaarylbenzene moiety
in palladium-catalyzed Suzuki–Miyaura coupling of aryl chlorides
*
*
Tetsuo Iwasawa , Toshinori Kamei, Satoshi Watanabe, Masaki Nishiuchi, Yasuhiko Kawamura
Institute of Technology and Science, Graduate School of The University of Tokushima, Minami-josanjima-cho, Tokushima 770-8506, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A functionalized phosphine with a pentaarylbenzene moiety was found to facilitate the palladium-cata-
lyzed Suzuki–Miyaura coupling of aryl chlorides. The new ligand achieved high turnover numbers (up to
TON 5000).
Received 8 September 2008
Revised 13 October 2008
Accepted 16 October 2008
Available online 21 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
The need for a functionalized pentaarylbenzene1 has been
growing in several application materials such as luminescent
materials,2 optical devices,3 supramolecular electronics,4 and cata-
lyst materials.5 The well-defined array of aromatic rings6 enhances
the properties inherent in functional groups and useful materials.7
Thus, these applications imply that it could be of interest to pre-
pare a novel pentaarylbenzene derivative endowed with functional
moieties.
On the other hand, we recently reported a simple method for
functionalizing a 1-iodo-2,3,4,5,6-pentaarylbenzene 1.8 The syn-
thetic approach employs iodide 1 and n-BuLi in toluene at ꢀ20 °C
to quickly conduct lithium–halogen exchange, followed by smooth
reactions with various electrophiles. The key to success was
increasing the solubility of 1 by installing four methyl groups onto
the phenyl rings. The reaction with 1,2-dibromobenzene as an
electrophile,9 for example, produced a bromide 2 in 91% yield
(Scheme 1).
molecule 3 was synthesized through the lithiation of 2, which is
outlined in Scheme 1. Then, the phosphine 3 was found to promote
the palladium-catalyzed Suzuki–Miyaura coupling of aryl chlo-
rides10 with high turnover numbers.11 Besides, a survey on the
structural features of 3 revealed that it is indispensable for the cat-
alyst activity to have the peripheral five aromatic rings in the pen-
taarylbenzene covalently bonded to a triphenylphosphine moiety.
The phosphine 3 was synthesized through the activation of 2,
which was derived from 1 according to our previous work (Scheme
1).8 The lithiation of 2 smoothly occurred in THF at ꢀ78 °C, and it
was followed by the clean reaction with chlorodiphenylphos-
phine.12 The crude was purified by silica gel column chromatogra-
phy to give 3 quantitatively. The stable solid material 3 was very
easy to handle, and well dissolved into CHCl3, CH2Cl2, and THF.
Note that any corresponding phosphine oxide was not observed
in 31P NMR after usual workup and purification with column chro-
matography. This indicates that phosphine 3 was basically robust:
the large pentaarylbenzene moiety would prevent the phospho-
rous lone-pair from being oxidized.13
In this Letter, we describe a novel pentaarylbenzene derivative
that works well as a catalyst material. First, the new phosphorous
Scheme 1. Synthesis of phosphine 3, and bromide 2 from the starting pentaarylbenzene 1.
* Corresponding authors. Tel.: +81 88 656 7405; fax: +81 655 7025 (T.I.).
E-mail addresses: iwasawa@chem.tokushima-u.ac.jp (T. Iwasawa), kawamura@chem.tokushima-u.ac.jp (Y. Kawamura).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.10.071

T. Iwasawa et al. / Tetrahedron Letters 49 (2008) 7430–7433
7431
Table 1
Effect of phosphines 3–6 on the coupling of 2-chloro-1,3-dimethylbenzene with
ortho-tolylboronic acida
S/Cb
Phosphines
Yieldc (%)
Figure 1. Phosphines 4, 5, and 6.
Entry
1
2
3
4
5
6
7
8
1000
2000
2000
2000
2000
2000
4000
4000
3
3
4
5
92
>99
81
Trace
Trace
0
>99
42
The readily availability of phosphine 3 led us to investigate the
performance as
Suzuki–Miyaura cross-coupling reaction of aryl chlorides.10,14 The
phosphines 4,5b 15 and 6, which are structurally similar to 3, were
a supporting ligand in palladium-catalyzed
6
None
3
4
5
also examined (Fig. 1).
In preliminary experiments, the reactions were carried out with
o-tolylboronic acid and 2-chloro-1,3-dimethylbenzene in the
presence of [Pd2(dba)3ꢁCHCl3] (dba = dibenzylideneacetone),
phosphine, KF, and refluxing THF (Table 1).16 The P/Pd ratio was
a
All reactions were performed in accordance with the representative procedure
in Ref. 16, unless otherwise noted.
b
Substrate to catalyst molar ratio.
Isolated yields.
c
Table 2
Evaluation of [Pd2(dba)3]/phosphine 3-catalyzed Suzuki–Miyaura coupling of aryl chloridesa
Entry
Aryl chlorides
Arylboronic acids
S/Cb
Products
Time (h)
14
Yieldc (%)
1
5000
92
2
5000
14
99
3
4
5000
50
23
18
84
0
5
6
7
500
500
13
2
99
>99
>99
18
1000
500
13
51
8
a
All reactions were performed in accordance with the representative procedure in Ref. 18, unless otherwise noted.
Substrate to catalyst molar ratio.
Isolated yields.
b
c

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T. Iwasawa et al. / Tetrahedron Letters 49 (2008) 7430–7433
Table 3
Effect of phosphines 3–6 on the Mizoroki–Heck reaction of 4-chlorotoluene with methyl acrylatea,b
Entry
Phosphines
Yieldc,d
1
2
3
4
5
3
4
5
6
50 (57)
1
Trace
Trace
0
None
a
All reactions were performed in accordance with the representative procedure in Ref. 21, unless otherwise noted.
Bath temperature was at 120 °C.
Isolated yields.
b
c
d
Isolated yield for the reaction time of 40 h is shown in parentheses.
maintained as 1.2,17 and efficiency of the supporting ligand 3 was
attached to 3 as compared with 4 contributes toward catalyzing
the reaction, in a similar vein of the cross-coupling in Table 1.
The increased rigidity in 3 would give emphasis to the stability
of catalyst, and it could endow the catalyst with the durable nature
to give high yields of products.
studied by raising the substrate to catalyst molar ratio (S/C). In en-
try 1, the reaction with S/C = 1000 yielded 2,20,6-trimethylbiphenyl
in 92%. In entry 2, the 3/Pd catalyst system was tolerable in high S/
C of 2000, and desired construction of the biaryl was achieved with
>99% yield. Then, the structurally related phosphines 4, 5, and 6
were also surveyed under the same conditions with S/C = 2000 (en-
tries 3–5). In entry 3, phosphine 4, which has a tetraphenylbenzene
covalently bonded to a triphenylphosphine moiety, gave the de-
sired biaryl in 81% yield. On the other hand, little amounts of cou-
pling adducts were observed with use of 5 and 6 (entries 4 and 5),
and no cross-coupling proceeded in the absence of phosphine (en-
try 6). Thus, S/C ratio was further increased to 4000 to compare the
catalytic activity between ligands 3 and 4 (entries 7 and 8). Sur-
prisingly, the reaction with 3 was smoothly catalyzed, providing
the target biaryl quantitatively (entry 7). However, in the case of
ligand 4, the yield was significantly reduced to 42% along with
the observation of palladium black formations. Hence, the com-
plete reaction with 3 was attributed to the peripheral one more
aryl group as compared with 4.
In summary, the pentaarylbenzene endowed with triphenyl-
phosphine was found to work well in palladium-catalyzed trans-
formation of aryl chlorides. One more aryl group of
3 as
compared with 4 was ensured to play a quintessential role in the
catalyst system, and accomplished the reactions with large turn-
over numbers, up to TON 5000. Work is now in progress to obtain
the single crystal of 3 and to amplify the property of pentaaryl-
benzene moiety as an attractive catalyst material.
Acknowledgments
We are pleased to thank Ms. Yasuko Yoshioka and Ms. Kazuyo
Yamashita for assistance with mass spectrometry.
Supplementary data
To examine the potential of 3 in other coupling partners, scope
and limitations of [Pd2(dba)3]/3-catalyzed Suzuki–Miyaura cou-
pling of aryl chlorides were attempted with P/Pd = 1.2. The results
are summarized in Table 2.18 In entries 1 and 2, the aryl chlorides
endowed with electron-deficient groups at ortho-position
smoothly reacted with p-tolylboronic acid under the condition of
S/C = 5000 to give 92% and 99% yields, respectively. In entry 3, 2-
chloroanisole bearing electron-rich group was also ensured to yield
the biaryl product in 84% with S/C = 5000.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.10.071.
References and notes
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16. Representative experimental procedure for Table 1, entry 7: KF (3.49 g, 60 mmol)
was dried in vacuo in a 50 mL flask with heating (heat gun), then o-tolylboronic
acid (3.27 g, 24 mmol), Pd2(dba)3ꢁCHCl3 (2.6 mg, 0.0025 mmol), and phosphine
3 (4.6 mg, 0.006 mmol) were added. The whole system was evacuated and
backfilled with argon three times, and the THF solution of 2-chloro-1,3-
dimethylbenzene (2.7 M, 10 mL) was added. The reaction mixture was stirred
at room temperature for 10 min, and then conducted in refluxing THF for 23 h.
After the reaction, the mixture was diluted with EtOAc, and was filtered.
Purification by silica gel column chromatography (hexane/CHCl3 = 4:1)
quantitatively afforded a desired biaryl5b (4.24 g) as a colorless oil. 1H NMR
(400 MHz, CDCl3) 7.41–7.33 (m, 3H), 7.30–7.21 (m, 3H), 7.16–7.13 (m, 1H),
2.10 (s, 3H), 2.08 (s, 6H). MS (EI) m/z: 196 (M+).
17. The P/Pd = 1.2, 2.0 and 3.0 in the reaction condition of Table 1 at entry 1 gave
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and phosphine 3 (4.6 mg, 0.006 mmol) were added. The whole system was
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chloro-3-methoxybenzaldehyde (0.8 M, 10 mL) was added. The reaction
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and was filtered. Purification by silica gel column chromatography (hexane/
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CHCl3 = 4:1) quantitatively afforded
a
desired biaryl23 (1.15 g) as white
needles. 1H NMR (400 MHz, CDCl3) d 9.62 (s, 1H), 7.63 (d, J = 7.8 Hz, 1H),
7.47 (dd, J = 7.8 Hz, 7.8 Hz, 1H), 7.35–7.24 (m, 3H), 7.21–7.19 (m, 1H), 7.14–
7.12 (m, 1H), 3.78 (s, 3H), 2.07 (s, 3H). 13C NMR (100 MHz, CDCl3) d 192.5,
157.2, 137.4, 135.3, 134.8, 133.3, 131.0, 130.0, 129.0, 128.4, 125.6, 119.1, 116.1,
56.1, 20.2. MS (EI) m/z: 226 (M+, 100%).
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21. Representative experimental procedure for Table 3, entry 1: Cs2CO3 (358 mg,
1.1 mmol) was dried in vacuo in a 20 mL Schlenk tube with heating (heat gun),
and to the flask were added Pd2(dba)3ꢁCHCl3 (15.5 mg, 0.015 mmol) and
phosphine 3 (27.9 mg, 0.036 mmol). The whole system was evacuated and
backfilled with argon three times, and the 1,4-dioxane (1 mL) and p-
chlorotoluene (0.118 mL, 1 mmol) were added. The mixture was conducted
at 75 °C (bath temperature) for 5 min, and methyl acrylate (0.18 mL, 2 mmol)
was added. The reaction was carried out at 120 °C (bath temperature) for 16 h.
After the reaction, the mixture was diluted with Et2O, and was filtered.
Purification by silica gel column chromatography (hexane/CHCl3 = 4:1)
afforded methyl (E)-p-methylcinnamate24 in 50% yield (88 mg) as white
12. Synthetic procedure for the phosphine 3: To a solution of 2 (668 mg, 1 mmol) in
THF (12 mL) at –78 °C was added n-BuLi (1.2 mmol, 1.6 M in hexane) dropwise
over 5 min, and the mixture was stirred for 2 h. ClPPh2 (0.22 mL, 1.2 mmol)
was slowly added, and the reaction mixture was allowed to warm to room
temperature. After stirring for 18 h, the solvent was evaporated, and the
residue was dissolved in CHCl3, washed with brine, and dried over Na2SO4.
Purification by silica gel column chromatography (hexane/CHCl3 = 4:1)
quantitatively afforded
3
(787 mg) as white solid materials. 1H NMR
(400 MHz, CDCl3) d 7.28–7.16 (m, 6H), 7.13–7.10 (m, 1H), 7.01–6.90 (m, 5H),
6.85–6.79 (m, 5H), 6.75–6.58 (m, 14H), 6.42 (dd, J = 7.8 Hz, 7.8 Hz, 1H), 6.33 (d,
J = 7.8 Hz, 1H), 6.24 (dd, J = 7.8 Hz, 15.1 Hz, 2H), 2.11 (s, 3H), 2.08 (s, 3H), 2.07
(br s, 3H ꢂ 2). 13C NMR (100 MHz, CDCl3) d 148.2, 147.9, 141.0, 140.9, 140.6,
140.44, 140.37, 139.23, 139.18, 138.8, 138.7, 138.6, 138.4, 138.23, 138.20,
138.1, 137.6, 136.1, 136.0, 135.0, 134.7, 134.3, 134.11, 134.09, 133.9, 133.7,
133.5, 132.5, 132.3, 132.2, 131.9, 131.7, 131.3, 131.1, 128.3, 128.2, 128.12,
128.06, 127.7, 127.6, 127.3, 127.2, 126.9, 126.5, 126.2, 126.1, 124.9, 21.23,
21,17. 31P NMR (162 MHz, CDCl3) d ꢀ12.6. MS (ESI) m/z: 775 ([M+H]+). Anal.
Calcd for C58H47P: C, 89.89; H, 6.11. Found: C, 89.85; H, 6.28.
needles. 1H NMR (400 MHz, CDCl3)
d 7.67 (d, J = 16 Hz, 1H), 7.42 (d,
J = 8.2 Hz, 2H), 7.19 (d, J = 8.2 Hz, 2H), 6.39 (d, J = 16 Hz, 1H), 3.80 (s, 3H),
2.38 (s, 3H). MS (EI) m/z: 176 (M+).
22. The reaction system in Table
3 was undertaken with styrene, methyl
methacrylate, 1-hexene, and methyl vinylketone. The styrene gave the
desired (E)-1-methyl-4-styrylbenzene in 41% yield. However, little amounts
of the target molecule were observed in use of methyl methacrylate (<4%
yield), and no cross-coupling with 1-hexene and methyl vinylketone
proceeded.
23. Meyers, A. I.; Himmelsbach, R. J. J. Am. Chem. Soc. 1985, 107, 682–685.
24. Ruan, J.; Li, X.; Saidi, O.; Xiao, J. J. Am. Chem. Soc. 2008, 130, 2424–2425.
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