Rh-catalyzed arylation and alkenylation of C60 using organoboron compounds

Article abstract of DOI:10.1021/ja073042x

A new organoboron-based arylation and alkenylation of C₆₀ catalyzed by a rhodium complex was developed. A treatment of C₆₀ with an organoboron compound in the presence of a catalytic amount of [Rh(cod)(CH₃CN)₂]B

Full text of DOI:10.1021/ja073042x

Published on Web 06/13/2007
Rh-Catalyzed Arylation and Alkenylation of C60 Using Organoboron
Compounds
Masakazu Nambo, Ryoji Noyori, and Kenichiro Itami*
Department of Chemistry and Research Center for Materials Science, Nagoya UniVersity, Nagoya 464-8602, Japan,
and PRESTO, Japan Science and Technology Agency
Received May 1, 2007; E-mail: itami@chem.nagoya-u.ac.jp
Table 1. Catalyst Screening and Reaction Optimizationa
Functionalized fullerenes have attracted significant attention as
promising structural/functional units in nanoscience.1 To this end,
a number of chemical reactions of fullerenes have been developed,
such as different types of cycloaddition and the addition of
nucleophiles and free radicals.1 The addition of organometallic
compounds, such as organolithiums or Grignard reagents, is one
such classical reaction in fullerene chemistry.2 Although these
addition reactions have contributed significantly to the generation
of interesting nanostructures,3 as exemplified by Nakamura’s
multifold addition of organocopper reagents, there remains con-
siderable room for further investigations, giving that (i) high loading
of organometallic reagents is typically required, (ii) selective
monoaddition is often difficult, (iii) functional group compatibility
is relatively low, and (iv) the use of metal catalysis, which offers
various opportunities for selectivity control, has not been forthcom-
ing. A key to addressing these issues is to generate a reasonably
nucleophilic but functional-group-compatible organometallic species
in a catalytic manner. Inspired by recent progress in rhodium-
catalyzed reaction of organoboron compounds with various elec-
trophiles, such as electron-deficient alkenes and alkynes,4 as well
as by the general disposition of C60 as an electron acceptor,1 we
envisioned the rhodium-catalyzed functionalization of C60 using
organoboron compounds.5
boronic acid
yieldb selectivityc
pinacol ester
entry
catalyst
yieldb
selectivityc
1
2
3
4
5
[RhCl(cod)]2
[RhOH(cod)]2
[Rh(cod)2]BF4
[Rh(cod)2]PF6
[Rh(cod)2]OTf
[Rh(nbd)2]BF4
[Rh(cod)(MeCN)2]BF4
[Rh(cod)(MeCN)2]BF4
[Rh(cod)(MeCN)2]BF4
[Rh(cod)(MeCN)2]BF4
27%
57%
61%
<1%
<1%
17%
67%
84%
80%
68%
90%
66%
>95%
-
-
-
19%
25%
14%
14%
-
60%
-
68%
78%
88%
88%
-
-
6
7
>95%
88%
91%
>95%
>95%
>95%
-
-
-
8d
9d,e
10d,f
-
-
a Molar ratio: C60/ArB(OR)2/Rh ) 1:2:0.1. Conditions: 60 °C, 3 h
(boronic acid); 100 °C, 10 h (pinacol ester). b Conversion of C60 and yield
of product were determined by HPLC analysis using C70 as an internal
standard. c Determined by [yield of product]/[conversion of C60]. d H2O/
e
f
1,2-Cl2C6H4 ) 1:4. C60/ArB(OR)2/Rh ) 1:1.2:0.1. C60/ArB(OR)2/Rh )
1:1.2:0.05.
[Rh(cod)2]OTf, were found to be totally inactive. These results were
surprising because it has been well-documented that these com-
plexes behave similarly in standard catalytic transformations.
However, any speculation regarding this dramatic counteranion
effect must await further mechanistic investigations. Changing the
diene ligand from cod to norbornadiene (nbd) resulted in lower
reactivity (entry 6), indicating that at least one of the dienes
dissociates from rhodium before becoming catalytically active. In
line with this assumption, the use of [Rh(cod)(MeCN)2]BF4 resulted
in a 67% yield with 88% selectivity (entry 7). We further found
that increasing the amount of water (H2O/1,2-Cl2C6H4 ) 1/4)
allowed an 84% yield (entry 8). Under these conditions, the amount
of ArB(OH)2 and Rh complex could be reduced to 1.2 and 0.05
equiv, respectively (entries 9 and 10).
We also examined the reactivity of these rhodium complexes
with the corresponding pinacol ester (Table 1, right column).
Because the reactivity of pinacol ester is generally lower than that
of boronic acid, the reactions were conducted at 100 °C. The general
trend was found to be similar to that obtained with the reactions of
boronic acid. Thus, we identified [Rh(cod)(MeCN)2]BF4 as our first-
choice standard catalyst precursor for the reaction of C60.
In early experiments, we found that 2,6-dimethylphenylboronic
acid possesses reasonable reactivity toward C60. For example, when
a mixture of the boronic acid and C60 was treated with a catalytic
amount of [RhCl(cod)]2 (cod ) 1,5-cyclooctadiene) in H2O/1,2-
Cl2C6H4 (1/9) at 60 °C for 3 h (molar ratio, C60/ArB(OH)2/Rh )
1:2:0.1), a hydroarylation adduct was obtained in 27% yield with
90% selectivity6 (Table 1, entry 1, left column). The hydroarylation
took place across the CdC bond between two six-membered rings
of C60.
On the basis of this preliminary result, we set out to investigate
the effect of rhodium(I) complexes with this model substrate (Table
1). All reactions were stopped after 3 h to ensure appropriate
comparison of catalytic activities. The use of [RhOH(cod)]2 gave
rise to higher yield (57%), but this was achieved at the expense of
low selectivity (66%); multiple addition products were detected by
LCMS analysis. When [Rh(cod)2]BF4 was used, both a good yield
(61%) and excellent selectivity (>95%) were achieved. However,
other representative cationic complexes, such as [Rh(cod)2]PF6 and
With a first-generation procedure in hand, we next examined
the reaction with several organoboron compounds in order to gain
a rough grasp of the scope of the present rhodium catalysis (Table
2). The reaction took place with various electronically and
9
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J. AM. CHEM. SOC. 2007, 129, 8080-8081
10.1021/ja073042x CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
molecular catalysis in nanocarbon chemistry.10 Work along this line,
as well as the development of a more efficient catalyst, is ongoing.
Table 2. Rh-Catalyzed Arylation and Alkenylation to C60 Using
Organoboron Compoundsa
Acknowledgment. This paper is dedicated to the memory of
Professor Yoshihiko Ito. Support has been provided in part by
PRESTO, Japan Science and Technology Agency (JST).
Supporting Information Available: Experimental procedures and
compound characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
entry
RB(OH)2 (1)
2
convb
yieldc
selectivityd
1
2
C6H5B(OH)2 (1a)
2a 47% 45% (40%) >95%
2b 50% 49% (40%) >95%
2c 64% 53% (46%) 83%
2d 80% 80% (69%) >95%
2e 38% 38% (32%) >95%
2f 52% 45% (35%) 87%
2f 62% 51% (43%) 82%
4-MeC6H4B(OH)2 (1b)
References
3e,f 2-MeC6H4B(OH)2 (1c)
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8g 4-MeC(O)C6H4B(OH)2 (1g) 2g 47% 47% (42%) >95%
9e,f 1-naphthylB(OH)2 (1h)
10e,f 1-pyrenylB(OH)2 (1i)
2h 68% 55% (51%) 81%
2i 69% 56% (49%) 81%
11h (E)-C6H5CHdCHBF3K (1j′) 2j 51% 36% (34%) 67%
12g 3-thienylB(OH)2 (1k)
2k 53% 53% (48%) >95%
a Molar ratio: C60/1/Rh ) 1:1.5:0.1. b Conversion of C60 determined by
HPLC using C70 as an internal standard. c Yield of 2 determined by HPLC
using C70 as an internal standard. The number in parenthesis is the isolated
yield. d Selectivities were determined by [yield of 2]/[conversion of C60].
e Reactions conducted at room temperature. C60/1/Rh ) 1:1.2:0.1. g [RhO-
f
H(cod)]2 was used. h H2O/1,2-Cl2C6H4 ) 1:9.
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(5) Boronic acid derivatives are air-stable, storable, and easily accessible by
various methods. For general views for boronic acid derivatives, see: Hall,
D. G., Ed. Boronic Acids; Wiley-VCH: Weinheim, Germany, 2005. Over
750 boronic acid derivatives are now commercially available from Sigma-
Aldrich (http://www.sigmaaldrich.com).
Figure 1. Other representative functionalized fullerenes.
structurally diverse boronic acids. In all cases examined, selectivity
was good to excellent. Functional groups, such as the acetyl group,
which usually cannot be applied in the existing organometallic
additions, have been found to be compatible (entry 8).7 Extended
π-systems such as naphthyl, pyrenyl, and styryl groups can also
be introduced (entries 9-11). Although some heteroarylboronic
acids, such as 3-thienylboronic acid, possessed low reactivity, the
use of highly reactive [RhOH(cod)]2 was found to be a solution
for such sluggish reagents (entry 12). The use of potassium trifluoro-
(organo)borates gave better results in some cases (entries 6 and
7).8 A moderate electronic effect of the aryl group was observed
for the reaction efficiency, but we unexpectedly found that ortho-
substituted 1c and 1d reacted markedly faster than the other
arylboronic acids.9
Other functionalized fullerenes were also created by the present
rhodium catalysis (Figure 1; see Supporting Information for details).
For example, when protected phenylalanine equipped with the
B(OH)2 group was reacted, a fullerene-tagged amino acid 3 was
obtained. This may be useful for various biological applications.1
When 9,9-dihexylfluorene-2,7-diboronic acid was reacted with 2
molar amounts of C60, two-directional reaction occurred, yielding
an interesting fullerene-capped π-system 4.
(6) Throughout this paper, “selectivity” stands for a ratio of the quantity of
desired product over the quantity of substrate converted (Orchin, M.;
Macomber, R. S.; Pinhas, A. R.; Wilson, R. M., The Vocabulary and
Concepts of Organic Chemistry, 2nd ed.; Wiley-Interscience: New Jersey,
2005).
(7) For an exception in this regard, see ref 3f.
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tions: (a) Navarre, L.; Pucheault, M.; Darses, S.; Genet, J.-P. Tetrahedron
Lett. 2005, 46, 4247. (b) Duursma, A.; Boiteau, J.-G.; Lefort, L.; Boogers,
J. A. F.; de Vries, A. H. M.; de Vries, J. G.; Minnaard, A. J.; Feringa, B.
L. J. Org. Chem. 2004, 69, 8045. (c) Moss, R. J.; Wadsworth, K. J.;
Chapman, C. J.; Frost, C. G. Chem. Commun. 2004, 1984.
(9) Recently, Hartwig also found a similar ortho-substitution effect in
transmetalation from boron to rhodium, which might be closely related
to the present reaction: (a) Zhao, P.; Incarvito, C. D.; Hartwig, J. F. J.
Am. Chem. Soc. 2007, 129, 1876. Kakiuchi also observed a seemingly
similar effect in ruthenium catalysis using arylboronates: (b) Kakiuchi,
F.; Matsuura, Y.; Kan, S.; Chatani, N. J. Am. Chem. Soc. 2005, 127, 5936.
(10) Catalytic modification of C60: (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.-
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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. For catalytic
transformation of functionalized fullerenes, see: (e) Matsuo, Y.; Iwashita,
A.; Nakamura, E. Chem. Lett. 2006, 35, 858.
In summary, we have established a new organoboron-based
arylation and alkenylation of C60 catalyzed by a rhodium complex.
The described chemistry not only broadens the scope of the
functionalization chemistry of fullerenes, thereby providing a new
synthetic avenue to a wide variety of previously unexplored
fullerene-based materials, but also represents a new direction for
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