A bench-stable Pd catalyst for the hydroarylation of fullerene with boronic acids

Article abstract of DOI:10.1021/ol801936j

(Chemical Equation Presented) A Pd(II) catalyst for the hydroarylation of fullerene with boronic acids is presented. Treatment of C₆₀ with an arylboronic acid in the presence of a catalytic amount of Pd(2-PyCH=NPh) (OCOC₆F₅

Full text of DOI:10.1021/ol801936j

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4609-4612
A Bench-Stable Pd Catalyst for the
Hydroarylation of Fullerene with Boronic
Acids
Susumu Mori, Masakazu Nambo, Liang-Chen Chi, Jean Bouffard, and
Kenichiro Itami*
Department of Chemistry, Graduate School of Science, Nagoya UniVersity,
Nagoya 464-8602, Japan, and PRESTO, Japan Science and Technology Agency,
Nagoya 464-8602, Japan
itami@chem.nagoya-u.ac.jp
Received August 19, 2008
ABSTRACT
A Pd(II) catalyst for the hydroarylation of fullerene with boronic acids is presented. Treatment of C60 with an arylboronic acid in the presence
of a catalytic amount of Pd(2-PyCHdNPh)(OCOC6F5)2 in H2O/1,2-Cl2C6H4 at room temperature furnishes the hydroarylation product (Ar-C60-H)
in good yield with high selectivity. This complex possesses high catalytic activity paired with bench stability in the solid state.
The chemical modification of fullerenes provides an important
opportunity for tailoring the properties of nanocarbon-based
materials.1 Although a number of privileged reactions have
emerged,1 the synthetic toolbox of highly versatile reactions
for fullerene functionalization remains comparatively limited.
Thus, the development of new general synthetic methods is
critical for the synthesis of new nanocarbon-based materials.
Recently, we initiated a program aimed at the develop-
ment of new functionalization chemistry of nanocarbons
using transition metal catalysts.2,3 As our initial foray into
the area of nanocarbon chemistry, we developed a Rh-
catalyzed hydroarylation of C60 with organoboron reagents
(4) For the synthesis of hydroarylation products from aromatic com-
pounds with AlCl3 as a promoter, see: (a) Olah, G. A.; Bucsi, I.; Lambert,
C.; Aniszfeld, R.; Trivedi, N. J.; Sensharma, D. K.; Prakash, G. K. J. Am.
Chem. Soc. 1991, 113, 9387. (b) Iwashita, A.; Matsuo, Y.; Nakamura, E.
Angew. Chem., Int. Ed. 2007, 46, 3513. (c) Kokubo, K.; Tochika, S.; Kato,
M.; Sol, Y.; Oshima, T. Org. Lett. 2008, 10, 3335.
* Address correspondence to this author at Nagoya University.
(1) (a) Hirsch, A.; Brettreich, M. Fullerenes: Chemistry and Reactions;
Wliey-VCH: Weinheim, Germany, 2005. (b) Kadish, K. M.; Ruoff, R. S.,
Eds. Fullerenes: Chemistry, Physics, and Technology; Wiley: New York,
2000. (c) Martin, N. Chem. Commun. 2006, 2093.
(5) Pioneering works: (a) Sakai, M.; Hayashi, H.; Miyaura, N. Orga-
nometallics 1997, 16, 4229. (b) Takaya, Y.; Ogasawara, M.; Hayashi, T.;
Sakai, M.; Miyaura, N. J. Am. Chem. Soc. 1998, 120, 5579. Reviews: (c)
Fagnou, K.; Lautens, M. Chem. ReV. 200, 103, 169. (d) Hayashi, T.;
Yamasaki, K. Chem. ReV. 2003, 103, 2829. Selected recent examples in
this area: (e) Tseng, N.-W.; Mancuso, J.; Lautens, M. J. Am. Chem. Soc.
2006, 128, 5338. (f) Shintani, R.; Duan, W.-L.; Hayashi, T. J. Am. Chem.
Soc. 2006, 128, 5628. (g) Tonogaki, K.; Itami, K.; Yoshida, J. Org. Lett.
2006, 8, 1419. (h) Miura, T.; Murakami, M. Chem. Commun. 2007, 217.
(i) Walter, C.; Auer, G.; Oestreich, M. Angew. Chem., Int. Ed. 2006, 45,
5675. (j) Kurahashi, T.; Shinokubo, H.; Osuka, A. Angew. Chem., Int. Ed.
2006, 45, 6336. (k) Uehara, K.; Miyamura, S.; Satoh, T.; Miura, M. J.
Organomet. Chem. 2006, 691, 2821. (l) Ukai, K.; Aoki, M.; Takaya, J.;
Iwasawa, N. J. Am. Chem. Soc. 2006, 128, 8706.
(2) Nambo, M.; Noyori, R.; Itami, K. J. Am. Chem. Soc. 2007, 129,
8080.
(3) Chemical transformations of fullerenes by means of metal catalysis:
(a) Becker, L.; Evans, T. P.; Bada, J. L. J. Org. Chem. 1993, 58, 7630. (b)
Shiu, L.-L.; Lin, T.-I.; Peng, S.-M.; Her, G.-R.; Ju, D. D.; Lin, S.-K.; Hwang,
J.-H.; Mou, C. Y.; Luh, T.-Y. J. Chem. Soc., Chem. Commun. 1994, 647.
(c) Shen, C. K. F.; Chien, K.-M.; Liu, T.-Y.; Lin, T.-I.; Her, G.-R.; Luh,
T.-Y. Tetrahedron Lett. 1995, 36, 5383. (d) Gan, L.; Huang, S.; Zhang, X.;
Zhang, A.; Cheng, B.; Cheng, H.; Li, X.; Shang, G. J. Am. Chem. Soc.
2002, 124, 13384. (e) Matsuo, Y.; Iwashita, A.; Nakamura, E. Chem. Lett.
2006, 35, 858.
10.1021/ol801936j CCC: $40.75
Published on Web 09/19/2008
2008 American Chemical Society

(Scheme 1).2,4 Under the influence of catalytic amounts of
[Rh(cod)(CH3CN)2]BF4 (1), arylboronic acids react with C60
to investigate the use of Pd(II) complexes as potential second-
generation catalysts for the functionalization of C60. Herein
we report a highly active yet bench-stable Pd(II) catalyst (2)
for the hydroarylation of C60 with boronic acids (Scheme
1).
Scheme 1. Hydroarylation of Fullerene with Boronic Acids
In early experiments, we found that a cationic Pd(II)
complex such as [Pd(dppe)(PhCN)2]SbF6 (Miyaura catalyst)9
can catalyze the reaction between C60 and PhB(OH)2 (3a)
in H2O/1,2-Cl2C6H4, furnishing the corresponding hydro-
phenylation product 4a (Table 1, entries 1 and 2). As in the
Catalyzed by Rh Complex 1 and Pd Complex 2
Table 1. Discovery of Palladium Catalysis for the
Hydroarylation of C60 with Boronic Acidsa
additive
(amount)
yield
(%)b
entry
Pd catalyst
PdCl2(dppe)d
1c
2e
3
4
5
AgSbF6 (20%)
16
40
3
17
40
in the presence of water to give a formal hydroarylation product
(Ar-C60-H).2 This reaction enables the introduction of various
aryl groups and a hydrogen atom on the fullerene surface. This
catalytic reaction has been developed on the basis of the ability
of Rh(I) complexes to catalyze the hydroarylation of electron-
deficient alkenes and alkynes with arylboronic acids,5 and of
the general behavior of C60 as an electron acceptor.1,6
[Pd(dppe)(PhCN)2](SbF6)2
Pd(OAc)2
Pd(OAc)2/bpyf
Pd(OAc)2/bpyf
CF3CO2H (20%)
a Reaction conditions: C60 (60 µmol), 3a (90 µmol), Pd catalyst (10
mol %), 1,2-Cl2C6H4 (7.2 mL), H2O (0.6 mL), 60 °C, 6 h. b Determined
by HPLC analysis with C70 as an internal standard. c Reaction was
conducted for 12 h. d dppe: 1,2-bis(diphenylphosphino)ethane. e Reaction
was conducted at 23 °C. f bpy: 2,2′-bipyridyl. 20 mol % of bpy was
employed.
Inspired by the parallel progress of Pd(II) catalysis in
organoboron-based hydroarylation chemistry,7-13 we decided
(6) The addition of organometallic reagent to C60: (a) Wudl, F.; Hirsch,
A.; Khemani, K. C.; Suzuki, T.; Allemand, P. M.; Kosch, A.; Eckert, H.;
Srdanov, G.; Webb, H. M. ACS Symposium Series 481; American Chemical
Society: Washington, DC, 1992; p 161. (b) Hirsch, A.; Soi, A.; Karfunkel,
H. R. Angew. Chem., Int. Ed. Engl. 1992, 31, 766. (c) Fagan, P. J.; Krusic,
P. J.; Evans, D. H.; Lerke, S. A.; Johnston, E. J. Am. Chem. Soc. 1992,
114, 9697. (d) Hirsch, A.; Gro¨sser, T.; Skiebe, A.; Soi, A. Chem. Ber. 1993,
126, 1061. (e) Nagashima, H.; Terasaki, H.; Kimura, E.; Nakajima, K.;
Itoh, K. J. Org. Chem. 1994, 59, 1246. (f) Komatsu, K.; Murata, Y.;
Takimoto, N.; Mori, S.; Sugita, N.; Wan, T. S. M. J. Org. Chem. 1994, 59,
6101. (g) Sawamura, M.; Iikura, H.; Nakamura, E. J. Am. Chem. Soc. 1996,
118, 12850. (h) Matsuo, Y.; Tahara, K.; Morita, K.; Matsuo, K.; Nakamura,
E. Angew. Chem., Int. Ed. 2007, 46, 2844.
rhodium-catalyzed reaction,2 the addition takes place across
the CdC bond between the two six-membered rings on C60
(1,2-bond). Although Pd(OAc)2 alone is nearly inactive (entry
3), the addition of 2,2′-bipyridyl (bpy)10 as a ligand (Lu
catalyst) affords 4a in 17% yield (entry 4). Catalytic activity
is further increased with CF3CO2H (20 mol %) as additive,
producing 4a in 40% yield (entry 5).
On the basis of these encouraging results, further
optimization of catalyst was conducted. Following the
Pd(OAc)2/bpy/CF3CO2H catalyst lead (Table 1, entry 5),
(7) First demonstration of Pd catalysis: Cho, C. S.; Motofusa, S.; Ohe,
K.; Uemura, S.; Shim, S. C. J. Org. Chem. 1995, 60, 883.
(8) Pd0/CHCl3 systems: (a) Yamamoto, T.; Ohta, T.; Ito, Y. Org. Lett.
2005, 7, 4153. (b) Yamamoto, T.; Iizuka, M.; Ohta, T.; Ito, Y. Chem. Lett.
2006, 35, 198.
(11) PdII/phosphite systems: Horiguchi, H.; Tsurugi, H.; Satoh, T.; Miura,
(9) Cationic PdII systems: (a) Nishikata, T.; Yamamoto, Y.; Miyaura,
N. Angew. Chem., Int. Ed. 2003, 42, 2768. (b) Nishikata, T.; Yamamoto,
Y.; Miyaura, N. Organometallics 2004, 23, 4317. (c) Nishikata, T.;
Yamamoto, Y.; Miyaura, N. Chem. Lett. 2005, 34, 720. (d) Gini, F.; Hessen,
B.; Minnaard, A. J. Org. Lett. 2005, 7, 5309. (e) Nishikata, T.; Yamamoto,
Y.; Gridnev, I. D.; Miyaura, N. Organometallics 2005, 24, 5025. (f)
Nishikata, T.; Yamamoto, Y.; Miyaura, N. Tetrahedron Lett. 2007, 48, 4007.
(g) Nishikata, T.; Yamamoto, Y.; Miyaura, N. AdV. Synth. Catal. 2007,
349, 1759. (h) Nishikata, T.; Kobayashi, Y.; Kobayshi, K.; Yamamoto, Y.;
Miyaura, N. Synlett 2007, 3055. (i) Nishikata, T.; Yamamoto, Y.; Miyaura,
N. Chem. Lett. 2007, 36, 1442. (j) Yamamoto, Y.; Nishikata, T.; Miyaura,
N. Pure Appl. Chem. 2008, 80, 807.
M. J. Org. Chem. 2008, 73, 1590
(12) PdII/carbene systems: Zhang, T.; Shi, M. Chem. Eur. J. 2008, 14,
3759.
.
(13) Palladacycle systems: (a) He, P.; Lu, Y.; Dong, C.-G.; Hu, Q.-S.
Org. Lett. 2007, 9, 343. (b) Suzuma, Y.; Yamamoto, T.; Ohta, T.; Ito, Y.
Chem. Lett. 2007, 36, 470. (c) He, P.; Lu, Y.; Hu, Q.-S. Tetrahedron Lett.
2007, 48, 5283. (d) Bedford, R. B.; Betham, M.; Charmant, J. P. H.;
Haddow, M. F.; Orpen, A. G.; Pilarski, L. T.; Coles, S. J.; Hursthouse,
M. B. Organometallics 2007, 26, 6346.
(14) In view of cost, simplicity, and stability, the catalyst modification
of [Pd(dppe)(PhCN)2]SbF6 was not conducted.
(15) Throughout this paper, “selectivity” stands for a ratio of the quantity
of desired product (4) over the quantity of substrate (C60) converted (Orchin,
M.; Macomber, R. S.; Pinhas, A. R.; Wilson, R. M. The Vocabulary and
Concepts of Organic Chemistry, ed.; Wiley-Interscience: Hoboken, NJ,
2005). The most common side products for reactions where the selectivity
is low are multiple hydroarylation products.
(10) PdII/bpy systems: (a) Lu, X.; Lin, S. J. Org. Chem. 2005, 70, 9651.
(b) Lin, S.; Lu, X. Tetrahedron Lett. 2006, 47, 7167. (c) Zhao, B.; Lu, X.
Org. Lett. 2006, 8, 5987. (d) Zhao, B.; Lu, X. Tetrahedron Lett. 2006, 47,
6765. (e) Lin, S.; Lu, X. J. Org. Chem. 2007, 72, 9757. (f) Dai, H.; Yang,
M.; Lu, X. AdV. Synth. Catal. 2008, 350, 249
.
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Org. Lett., Vol. 10, No. 20, 2008

various nitrogen-based ligands and acids were screened
(Table 2, molar ratio, C60/3a/Pd(OAc)2/ligand/acid ) 1.0:
1.5:0.1:0.2:0.2).14 The yield and selectivity15 were deter-
Table 2. Optimization of Palladium Catalyst for the
Hydroarylation of C60 with Boronic Acidsa,b
Figure 1. Gram-scale synthesis and X-ray crystal structure of 2.
Table 3. Hydroarylation of C60 with Boronic Acids Catalyzed
by Palladium Complex 2a
a Reaction conditions: C60 (60 µmol), 3a (90 µmol), Pd(OAc)2 (10 mol
%), ligand (20 mol %), acid (20 mol %), 1,2-Cl2C6H4 (7.2 mL), H2O (0.9
mL), 60 °C, 12 h, under argon. b Determined by HPLC analysis with C70
as an internal standard. c Selectivity: [yield of 4a]/[conversion of C60]. d 40
mol % of pyridine was employed. e Not determined.
mined by HPLC analysis of the crude reaction mixture
with C70 as an internal standard.
First, the effect of acid was examined in combination with
Pd(OAc)2/bpy (entries 1-4). By increacing the acidity of
the carboxylic acid additive (CH3CO2H f CCl3CO2H f
CF3CO2H), an increase in product yield and selectivity was
observed. Interestingly, the use of C6F5CO2H resulted in
highest selectivity for 4a (>95%) among those acids
screened, albeit with moderate product yield (entry 4). Next
we examined various nitrogen-based ligands in combination
with Pd(OAc)2/C6F5CO2H (entries 5-9). Since the use of
pyridine in place of bpy shut down the reaction (entry 5),
the use of a nitrogen-based bidentate ligand seems to be
important for activity and/or stability of the Pd catalyst. After
extensive screening of such ligands, 2-PyCHdNPh (5a) was
identified as an optimal ligand, producing 4a in 61% yield
with very high selectivity (>95%) (entry 6). Although the
reason is not clear yet, 5a is a far superior ligand among
other related imine-based ligands such as 2-PyC(CH3)dNPh
(5b), 2-PyCHdN(cyclo-C6H11) (5c), and PhNdCHsCHdNPh
(5d) (entries 7-9).
a Reaction conditions: C60 (60 µmol), ArB(OH)2 (3: 90 µmol), 2 (6.0
µmol), 1,2-Cl2C6H4 (7.2 mL), H2O (0.6 mL), 60 °C, 12 h, under argon.
b Yield of 4 determined by HPLC analysis with C70 as an internal standard.
The number in parentheses is the isolated yield.
Org. Lett., Vol. 10, No. 20, 2008
4611

With this interesting new catalytic system in hand, we then
attempted to isolate the Pd species generated in situ from
Pd(OAc)2/5a/C6F5CO2H (Figure 1). By treating Pd(OAc)2
with 5a and C6F5CO2H in sequence, the complex Pd(2-
PyCHdNPh)(OCOC6F5)2 (2) can be isolated as an air-stable
yellow solid.16 The routine gram-scale preparation of 2 can
be performed by a similar procedure (4.1 g, 82%). The X-ray
crystal structure reveals a typical square-planar coordination
around the Pd(II) center (Figure 1). A notable feature of 2
is its shelf stability in the solid state. Virtually no decom-
position of 2 has been detected by NMR after prolonged
exposure (>6 months) to air and moisture. Since the
corresponding palladium diacetate complex easily decom-
poses under air/moisture, the pentafluorobenzoate moiety
clearly enhances not only the activity but also the stability
of palladium catalyst.
The thus-isolated Pd complex 2 can catalyze the reaction of
C60 with various electronically diverse arylboronic acids (3a-j)
in H2O/1,2-Cl2C6H4 (Table 3). Interestingly, unlike the previous
Rh-catalyzed reaction,2 electron-deficient aryl groups can be
introduced onto C60 with reasonable efficiency. This permits
the incorporation of functionality, such as acetyl group, nitro
group, and halogens, onto C60. In particular, tolerance to
carbon-halogen bonds is attractive for further synthetic ma-
nipulations. In all cases examined thus far, the monoadditon
product (4) was obtained with high selectivities (>90%), as
judged by the HPLC analysis of crude reaction mixture.
More interestingly, room temperature hydroarylation is
possible with Pd catalyst 2 (Table 4). In all cases examined,
the hydroarylation products (4) were produced with reason-
able efficiency. It should be noted that under otherwise
a
Table 4. Room-Temperature Hydroarylation of C60
yield of 4b (%)
entry
3
Rh catalyst 1
Pd catalyst 2
1
2
3
4
5
6
7
3a
3a
3b
3c
3d
3e
3f
<1
<1
<1
<1
5
49
49c
47
41
42
38
51
<1
<1
a Reaction conditions: C60 (60 µmol), ArB(OH)2 (3: 90 µmol), catalyst
(6.0 µmol), 1,2-Cl2C6H4 (7.2 mL), H2O (0.6 mL), 23 °C, 12 h, under argon.
b Yield of 4 determined by HPLC analysis with C70 as an internal standard.
c Reaction was conducted under air.
identical conditions, our first-generation Rh catalyst 1 is
virtually inactive for room temperature hydroarylation (Table
4). More interestingly, the reaction proceeds under air with
similar efficiency (entry 2).
In summary, a new Pd(II) catalyst for the hydroarylation
of fullerene with boronic acids has been developed. The
complex Pd(2-PyCHdNPh)(OCOC6F5)2 (2) possesses good
catalytic activity paired with bench stability in the solid state.
In addition to significantly broadening the scope of fullerene
functionalization chemistry, the Pd(II) catalyst 2 may
demonstrate utility in small-molecule reactions.17
(16) Similar Pd(II) complexes have been reported for the bis-alkoxy-
carbonylation of styrene: Bianchini, C.; Lee, H. M.; Mantovani, G.; Meli,
A.; Oberhauser, W. New J. Chem. 2002, 26, 387.
Acknowledgment. This work was supported by the
PRESTO program of the Japan Science and Technology
Agency (JST) and a Grant-in-Aid for Scientific Research
from MEXT and JSPS. M.N. is a recipient of a JSPS
Predoctoral Fellowship for Young Scientists. J.B. is a
recipient of a JSPS Postdoctoral Fellowship. We thank Mr.
Shunsuke Ohishi (Nagoya University) for assistance in X-ray
crystal structure analysis.
(17) For example, we have identified that the Pd catalyst 2 is active in
the hydroarylation (conjugate addition) of cyclic and acyclic enones with
boronic acids. The reactions proceed at room temperature under air (see
the Supporting Information for details):
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
.
OL801936J
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Org. Lett., Vol. 10, No. 20, 2008