Selective introduction of organic groups to C60 and C 70 using organoboron compounds and rhodium catalyst: A new synthetic approach to organo(hydro)fullerenes

Article abstract of DOI:10.1002/asia.201000583

A Rh-catalyzed reaction of C₆₀ and C₇₀ with organoboron compounds is described. This new catalytic method enables introduction of various organic groups onto C₆₀ and C₇₀. [Rh(cod)(MeCN)₂]BF₄

Full text of DOI:10.1002/asia.201000583

FULL PAPERS
DOI: 10.1002/asia.201000583
Selective Introduction of Organic Groups to C60 and C70 Using Organoboron
Compounds and Rhodium Catalyst: A New Synthetic Approach to
OrganoACTHNUGRTENUNG(hydro)fullerenes
Masakazu Nambo,[a] Yasutomo Segawa,[a] Atsushi Wakamiya,[b] and Kenichiro Itami*[a]
Dedicated to Professor Eiichi Nakamura on the occasion of his 60th birthday
Abstract: A Rh-catalyzed reaction of
C60 and C70 with organoboron com-
pounds is described. This new catalytic
method enables introduction of various
organic groups onto C60 and C70. [Rh-
tive manner. It was found that water is
an essential additive to promote the re-
action. By X-ray crystal structure anal-
ysis, we have confirmed the reaction
site of organometallic-based hydroary-
lation of C70 for the first time. Various
functional fullerenes, such as fullerene-
tagged amino acids and fullerene-
capped p systems, can be synthesized.
The X-ray crystal structure of biphen-
yl-attached C60 revealed an interesting
opportunity for the well-organized
alignment of bucky balls by taking ad-
vantage of CH–p interactions.
ACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(MeCN)2]BF4 proved to be the
most effective catalyst in terms of pro-
ductivity and selectivity. The reaction
generally proceeds with a high regiose-
lectivity and in a mono-addition selec-
Keywords: addition reactions · ful-
lerenes · homogeneous catalysis ·
organoboron compounds · rhodium
Introduction
Since the discovery of nanocarbons such as fullerenes and
carbon nanotubes, their structures, properties, and potential
applications have attracted attention. While various types of
nanocarbons have been discovered, chemical functionaliza-
tion with organic molecules is becoming one of new ways to
add the interesting properties directly, leading to the crea-
tion of functionalized nanocarbon materials.[1] Organic
chemists have developed a number of useful fullerene func-
tionalization reactions such as cyclopropanation with a-halo-
malonates,[2] Diels–Alder reaction with dienes,[3] and 1,3-
dipoar cycloaddition with azomethine ylides (Figure 1).[4]
Figure 1. Representative functionalized fullerenes.
Among these reactions, nucleophilic addition to fullerene
perhaps is the most simple and important reaction in fuller-
ene chemistry. In 1991, Hirsch et al. reported the first exam-
ple of nucleophilic addition to C60 where primary and secon-
dary amines add to give the multiaminated products.[5] In
1995, Wudl and co-workers reported the synthesis of cyano-
dihydrofullerene using cyanide anion as a nucleophile.[6] In
2005, Nakamura and Isobe reported the DMSO-promoted
hydrophosphorylation of C60.[7]
[a] M. Nambo, Dr. Y. Segawa, Prof. Dr. K. Itami
Department of Chemistry
Graduate School of Science
Nagoya University
Chikusa, Nagoya 464-8602 (Japan)
Fax : (+81)52-788-6098
E-mail: itami@chem.nagoya-u.ac.jp
[b] Prof. Dr. A. Wakamiya
Institute for Chemical Research
Kyoto University
Addition of Organometallic Compounds to C60
Uji, Kyoto 611-0011 (Japan)
Nucleophilic addition with organometallic compounds, such
as organolithium and organomagnesium compounds, is a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000583.
590
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Chem. Asian J. 2011, 6, 590 – 598

functional-group-compatible organometallic species in a cat-
alytic manner.
Addition of Organoboron Compounds to Electron-Deficient
Alkenes Catalyzed by Rhodium Complexes
Rhodium-catalyzed conjugate additions of organoboron
compounds to alkenes and alkynes have recently emerged
À
as an attractive C C bond-forming reaction. In 1997,
Miyaura discovered the Rh-catalyzed conjugate addition of
organoboron compounds to enones in aqueous solution.[15]
The advantages of this Rh-catalyzed reaction have been
demonstrated by the introduction of various aryl and alken-
Scheme 1. Addition of organometallic compounds to C60.
À
yl groups to C C double bonds as well as the achievement
of a high level of enantiocontrol by chiral ligands.[16] Fur-
thermore, the shelf stability, functional group compatibility,
widespread commercial availability, and ease of functionali-
zation of organoboron compounds confer them significant
practical advantages over lithium, magnesium, or copper or-
ganometallics.[17]
straightforward method to install an organic group onto the
fullerene
core
(Scheme 1).
Typically,
organo-
ACHTUNGTRENNUNG(hydro)fullerenes can be prepared by protonation of thus-
formed fullerenyl anions. In 1992, Hirsch reported the reac-
tions of alkyllithium or Grignard reagents with C60, produc-
ing alkylated fullerenes.[8] Fagan also reported the synthesis
of tBuC60H by the reaction between C60 and tert-butyllithi-
um.[9] Recently, Nakamura and Matsuo discovered that di-
methylformamide works as a promoter in the addition of
Grignard reagents to C60.[10] In 2003, Meier reported the Re-
formatsky-type reaction of C60 with various oraganozinc re-
agents prepared from Zn and activated alkyl bromides.[11]
Among these organometallic compounds, organocopper re-
agents represent very unique agents manifesting remarkable
addition patterns. In 1996, Nakamura and Sawamura report-
ed the regioselective fivefold addition of organocopper re-
agents to C60.[12] The group of Nakamura took full advantage
Inspired by the progress in Rh-catalyzed reaction of orga-
noboron compounds with various electron-deficient alkenes
and alkynes (Scheme 2), as well as by the general disposi-
tion of fullerenes as an electron acceptor, we successfully
discovered the Rh-catalyzed hydroarylation of C60 with aryl-
boron compounds in 2007.[18a] We also developed a unique
PdII catalyst for the hydroarylation of C60.[18b] In this paper,
we report the Rh-catalyzed selective introduction of organic
groups to C60 and C70 using organoboron compounds in
detail.
of this unique organocopper reaction to synthesize
a
number of interesting materials.[13]
Although these addition reactions have contributed signif-
icantly to the generation of interesting molecular entities
and fullerene-based functional materials, there remains con-
siderable room for further investigations, given that 1) high
loading of organometallic reagents is typically required,
2) selective mono-addition is often difficult, 3) functional
group compatibility is relatively low, and 4) the use of metal
catalysis, which offers various opportunities for selectivity
control, has not been forthcoming.[14] A key to addressing
these issues is to generate a reasonably nucleophilic but
Scheme 2. Rh-catalyzed addition of organoboron compounds to electron-
deficient alkenes and fullerenes.
Results and Discussion
Discovery of Organoboron-Based Hydroarylation of C60
with Rhodium Catalyst
Abstract in Japanese:
On the basis of representative Rh-catalyzed conjugate addi-
tion protocol, the reaction of C60 with arylboronic acids was
investigated. We found that the reaction of C60 (1.0 equiv)
with 4-methoxyphenylboronic acid (1.5 equiv) in the pres-
ence of [RhOHACTHUNRGTNEUNG(cod)]2 (10 mol%; cod=1,5-cyclooctadiene)
in H2O/toluene at 508C furnished 1-(4-methoxyphenyl)-1,9-
dihydrofullerene in 36% yield (Scheme 3). Unreacted C60
was recovered in 50% yield. The 13C NMR spectrum of the
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591

K. Itami et al.
FULL PAPERS
these studies, we judged the performance of catalysts not
only by the chemical yield of the desired product but also
by the selectivity (a ratio of the quantity of desired product
over the quantity of substrate conversion).[19] When selectiv-
ity is low, multiarylated products are formed in most instan-
ces.
When a mixture of the boronic acid and C60 was treated
with a catalytic amount of [RhClACTHUNRGTNEGN(U cod)]2 in H2O/1,2-Cl2C6H4
(1:9) at 608C for 3 h (molar ratio: C60/ArB(OH)2/Rh=
1:2:0.1), the corresponding hydroarylation product was ob-
tained in 27% yield with 90% selectivity (Table 1, entry 1).
The use of [RhOH
but this was achieved at the expense of low selectivity
(66%). When [Rh(cod)2]BF4 or [Rh(cod)2]SbF6 was used as
ACHTUNGTNER(NUNG cod)]2 gave rise to a higher yield (57%),
Scheme 3. Discovery of organoboron-based hydroarylation of C60 cata-
lyzed by rhodium complex.
N
ACHTUNGTRENNUNG
a catalyst, both a good yield (>61%) and excellent selectivi-
ty (>95%) were achieved (Table 1, entries 3 and 6). Howev-
er, other representative cationic complexes, such as [Rh-
product showed 34 signals for sp2-hybridized carbon atoms
(including p-anisyl group), indicating the Cs-symmetric
structure (1,2-isomer) of the product. Although a small
amount of multiarylated fullerenes was detected by LCMS
analysis, the present reaction proceeded in a mono-addition
selective manner. During our early-stage scouting experi-
ments, we also found that 2,6-dimethylphenylboronic acid
possesses higher reactivity, allowing the hydroarylation to
occur at room temperature (Scheme 3).
AHCTUNGTRENN(GNU cod)2]PF6, [RhACHTUNREGTG(NNNU cod)2]OTf, and [RhACHTNUGERTNNUG
(cod)2]BArF [ArF =3,5-
4
bis(trifluoromethyl)phenyl] were found to be totally inactive
(Table 1, entries 4, 5, and 7). These results were surprising
because it has been well-documented that these complexes
behave similarly in standard catalytic transformations.
Changing the diene ligand from cod to norbornadiene (nbd)
resulted in lower reactivity (Table 1, entry 8), indicating that
at least one of the dienes dissociates from rhodium before
becoming catalytically active. In line with this assumption,
the use of [RhACHTUNGTRENN(UG cod)AHCUTGNTREN(NUGN MeCN)2]BF4 resulted in a 67% yield
Effect of Catalyst Components
with 88% selectivity (Table 1, entry 9). We further found
that increasing the amount of water (H2O/1,2-Cl2C6H4 =1:4)
allowed an 84% yield (Table 1, entry 10). Under these con-
ditions, the amount of ArB(OH)2 and Rh complex could be
reduced to 1.2 and 0.05 equiv, respectively (Table 1, en-
tries 11 and 12).
Following the results of the initial experiments, the catalytic
activity of various RhI complexes was investigated using 2,6-
dimethylphenylboronic acid as a reagent (Table 1). During
Table 1. Effect of catalyst components.[a]
We also examined the reactivity of these rhodium com-
plexes with the corresponding pinacol ester (Table 1). Be-
cause the reactivity of pinacol ester is generally lower than
that of boronic acid, the reactions required higher tempera-
ture (1008C). The general trend was found to be similar to
that obtained with the reactions of boronic acid. The best
Entry
Rh Catalyst
Boronic acid
Pinacol ester
yield was obtained with use of [Rh
catalyst; a 60% yield of product was realized with excellent
selectivity (>95%). Thus, we identified [Rh(cod)-
(MeCN)2]BF4 as our first-choice standard catalyst for the re-
action of fullerenes.
ACHTUGTNRNENUG(cod)ACHTUNGTERN(NUGN MeCN)2]BF4 as
Yield[b] Selectivity[c] Yield[b] Selectivity[c]
[%]
[%]
90
66
>95
–
–
>95
–
>95
88
91
>95
>95
[%]
[%]
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
G
A
27
57
61
<1
<1
64
–
19
25
14
14
–
–
AHCTUNGTRENNUNG
N
E
68
78
88
88
–
R
N
N
Effect of Protic Additives
R
[g]
N
<1
17
–
–
–
–
4
A notable feature of the resultant organoACTHNUTRGNE(NUG hydro)fullerenes
A
À
is that they possess an acidic C H bond, reflecting the
highly conjugated fullerene backbone. For example, Fagan
and Evans have experimentally determined the pKa value of
tBuC60H to be 5.7 (in DMSO at 258C) by voltammetric
techniques.[5] Geerlings has also identified several highly
[Rh
N
G
60
–
>95
–
10[d] [Rh
11[d,e] [Rh
12[d,f] [Rh
N
T
–
–
–
–
G
[a] Molar ratio: C60/ArB(OR)2/Rh=1:2:0.1. Conditions: 608C, 3 h (boronic
acid); 1008C, 10 h (pinacol ester). [b] Conversion of C60 and yield of product
were determined by HPLC analysis using C70 as internal standard. [c] Deter-
mined by yield/conversion. [d] H2O/1,2-Cl2C6H4 =1:4. [e] C60/ArB(OR)2/Rh=
1:1.2:0.1. [f] C60/ArB(OR)2/Rh=1:1.2:0.05. [g] ArF =3,5-bis(trifluoromethyl)-
phenyl.
acidic organoACTHNUTRGNEUNG
(hydro)fullerenes by theoretical calculations.[20]
We envisioned that acidic (protic) additives might have sub-
stantial impact on the catalytic turnover. We therefore in-
vestigated several protic additives in the reaction (Table 2).
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Synthetic Approach to OrganoACHTUNTRGNEUNG(hydro)fullerenes
Table 2. Effect of protic additives.[a]
Entry
Additive/Solvent
Yield[b] [%]
pKa of Additive[c]
[d]
1
2
3
4
phenol/1,2-Cl2C6H4
<1
6
67
2
18.0
29.0
31.4
32.2
MeOH/1,2-Cl2C6H4 (1:9)
H2O/1,2-Cl2C6H4 (1:9)
tBuOH/1,2-Cl2C6H4 (1:9)
[a] Molar ratio: C60/ArB(OR)2/Rh=1:2:0.1. [b] Yield of product was de-
termined by HPLC analysis using C70 as internal standard. [c] Values in
DMSO (reference [21]). [d] Phenol (180 equiv) was added.
Figure 2. A possible mechanism of hydroarylation of C60.
The replacement of water with more acidic phenol resulted
in no conversion (Table 2, entry 1). Surprisingly, MeOH and
tBuOH, which have similar pKa values[21] to water, did not
promote the reaction efficiently (Table 2, entries 2 and 4). It
is clear from these experiments that the promoting effect of
water (our standard additive) cannot be explained solely by
its acidity.
Though the exact role of water remains unclear, this
addend clearly promotes the reaction. For example, when
the reaction of C60 and 2,6-dimethylphenylboronic acid was
conducted in the absence of water, the arylated product was
observed only in a trace amount (Scheme 4). However,
when water was added to the mixture (H2O/1,2-Cl2C6H4 =
1:4), the hydroarylation product was produced in 43% yield
after heating the mixture at 608C for 12 h.
agent in the protonation step. As shown in Figure 2, we pre-
sume HBF4 to be the protonating agent in the reaction.
Effect of Substitutents on Boron Atom
Recently, new organoboron compounds such as boronate
esters and potassium trifluoroborates have proved to be ef-
fective agents for transition-metal-catalyzed reactions.[23]
Moreover, various synthetic routes to organoboron com-
pounds have been developed, including Pd-catalyzed boryla-
tion of aryl halides[24] and Ir-catalyzed C H bond borylation
À
of arenes.[25] Thus, the effect of substituents on the boron
atom in the Rh-catalyzed hydrophenylation of C60 was in-
vestigated using various phenylboron compounds (Table 3).
Table 3. Effect of substituents on boron atom.[a]
Entry
1
Ph-B
Conv.[b] [%] Yield[c] [%] Selectivity[d] [%]
47
39
45
39
>95
>95
2
3
Scheme 4. Water is essential in Rh-catalyzed hydroarylation of C60.
<1
–
–
A Possible Mechanism
4
5
6
11
29
2
11
29
2
>95
>95
>95
Although the detailed mechanism of the Rh-catalyzed reac-
tion remains unknown, we assume a catalytic cycle shown as
in Figure 2;[15b,22] that is, 1) reaction of cationic Rh complex
and water producing Rh-OH species and HBF4, 2) transme-
talation of thus-formed Rh-OH with ArB(OH)2 giving Rh-
Ar species, 3) coordination of C60 to Rh, 4) arylrhodation
(insertion) across fullerenyl C=C bond, and 5) protonolysis
of thus-formed fullerenyl Rh species to liberates the hydro-
arylation product Ar-C60-H with the regeneration of the cat-
ionic Rh species. Because the hydrogen atom on the fuller-
ene core is highly acidic, we cannot assume water to be the
7
54
44
49
42
91
8[e]
Ph4BNa
>95
[a] Molar ratio: C60/Ph-B/Rh=1:1.5:0.1. [b] Conversion of C60 determined
by HPLC using C70 as an internal standard. [c] Yield of product deter-
mined by HPLC using C70 as an internal standard. [d] Selectivity was de-
termined by [yield of product]/[conversion of C60]. [e] 0.38 equivalents of
Ph4BNa were used.
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K. Itami et al.
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Table 4. Rh-catalyzed addition of organoboron compounds to C60.[a]
When the reaction of C60 with
phenylboronic acid was carried
out in H2O/1,2-Cl2C6H4 (1:4) at
608C, the hydrophenylated
product was obtained in 45%
with>95% selectivity. The re-
actions using 2-phenyl-1,3,2-di-
oxaborinane (Table 3, entry 2),
2-phenyl-1,3,2-dioxaborolane
(entry 4), and 5,5-dimethyl-2-
phenyl-1,3,2-dioxaborinane
Entry
1
2 (Yield)
2a (40%)
2b (40%)
Entry
11[d]
1
2 (Yield)
2j (42%)
1
2
N
ACHTUNGTRENNUNG
N
12[d]
A
2k (27%)
2l (42%)
(entry 5) gave the product in
39%, 11%, and 29% yield, re-
spectively. In contrast, when
3
N
2c (39%)
2d (46%)
13[d]
14
ACHTUNGTRENNUNG
4[b,c]
G
E
2m (48%)
2n (51%)
using
4,4,5,5-tetramethyl-2-
phenyl-1,3,2,-dioxaborolane as
a substrate, no reaction was ob-
served at 608C (Table 3,
entry 3). These differences in
reactivity should be partly due
to the steric bulkiness of the
boron center in these boronate
esters. When 2-phenyl-1,3,2-
benzodioxaborole was used,
product was hardly observed
(Table 3, entry 6). In the case of
5[c]
G
2e (69%)
15[b,c]
6
7
N
2 f (43%)
2g (44%)
16[b,c]
2o (49%)
2p (42%)
E
17[e]
8
9
R
2h (32%)
2i (35%)
2i (43%)
18[f]
19[d]
2q (34%)
2r (48%)
potassium
phenyltrifluorobo-
rate (Table 3, entry 7), the reac-
tion proceeded smoothly (49%
yield), albeit in somewhat lower
selectivity (91%). Sodium tet-
raphenylborate was found to be
comparable to phenylboronic
acid in terms of reactivity and
selectivity (Table 3, entry 8).
10
ACHTUNGTRENNUNG
[a] Molar ratio: C60/R-B/Rh=1:1.5:0.1. [b] Reaction was conducted at room temperature. [c] 1.2 equivalents of
arylboronic acid were used. [d] [RhOHACTHNUGTRENUNG(cod)]2 was used as a catalyst. [e] Reaction was conducted at 1008C.
[f] H2O/1,2-Cl2C6H4 =1:9.
Functional groups such as acetyl, formyl, and methoxycar-
bonyl groups, which usually cannot be applied in the typical
organolithium and organomagnesium methods, were tolerat-
ed under Rh-catalyzed conditions (Table 4, entries 11–13).
Extended p systems such as naphthyl and pyrenyl groups
can also be introduced (Table 4, entries 14–16). The use of
arylboronate ester having a ferrocenyl group (1p) furnished
Organic Groups Installable to C60
With optimized reaction conditions in hand, we next investi-
gated to identify organic groups that could be installed on
C60 under the present rhodium catalysis. Gratifyingly, the re-
action took place with various electronically and structurally
diverse arylboronic acids. Selected results are summarized in
Table 4. In all cases examined, the selectivity was good to
excellent. Interestingly, among ortho, meta, and para isomers
of tolylboronic acids, the most bulky o-tolylboronic acid
(1d) displayed the highest reactivity (Table 4, entries 2–4).
The reaction using 1d occurred smoothly even at room tem-
perature (Table 4, entry 4). The reactions using arylboronic
acids having a solubilizing group such as n-butyl or n-nonyl
group provided the hydroarylated products in good yields
(Table 4, entries 6 and 7). Boronic acids possessing an elec-
tron-donating group, such as methoxy group, tend to have
higher reactivity at the expense of lower selectivity (Table 4,
entry 9). In this case, we found that use of the corresponding
potassium trifluoroborate gave a better result (Table 4,
entry 10).
an interesting product having
a redox-active moiety
(Table 4, entry 17). When potassium (E)-styryltrifluorobo-
rate (1q) was used under optimized conditions, the corre-
sponding alkenylated product 2q was obtained (Table 4,
entry 18). Although some heteroarylboronic acids such as 3-
thienylboronic acid (1r) possessed low reactivity, the use of
highly reactive [RhOHACHTUNTRGNEUNG(cod)]2 was found to be a solution for
such sluggish reagents (Table 4, entry 19). Unfortunately,
alkyl- and alkynylboron compounds were virtually unreac-
tive under the present conditions.
This hydroarylation is applicable to the synthesis of func-
tionalized fullerene derivatives. For example, when protect-
ed phenylalanine equipped with a B(OH)2 group (1s) was
reacted with C60 under the standard conditions, a fullerene-
tagged amino acid 2s was obtained in 47% yield
(Scheme 5). This may be useful for a new peptide synthesis
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Chem. Asian J. 2011, 6, 590 – 598

Synthetic Approach to OrganoACHTUNTRGNEUNG(hydro)fullerenes
might be to assume that bulky
arylboronic acid undergoes
faster transmetalation. Such an
ortho-substitution
effect
in
boron-to-rhodium transmetala-
tion has been observed by
Hartwig and co-workers in the
stoichiometric reactions.[26]
X-Ray Crystal Structure of
Biphenyl-Attached C60 (2w)
Among synthesized products,
the structure of 2w was con-
firmed by single-crystal X-ray
diffraction analysis.
A single
Scheme 5. Synthesis of other functionalized fullerene derivatives.
crystal was obtained by diffu-
sion of dichloromethane into a
and various biological applications. Similarly, when 9,9-di-
hexylfluorene-2,7-diboronic acid (1t) was reacted with two
molar amounts of C60, a two-directional reaction occurred to
yield an interesting fullerene-capped p system 2t
(Scheme 5).
solution of 1,2-Cl2C6H4. In the structure of 2w, shown in Fig-
ure 3a,[27] the 2-biphenyl group and hydrogen atom are at-
Table 5. Effect of ortho-subsitutents on arylboronic acids.[a]
Effect of ortho-Subsitutents on Arylboronic Acids
A moderate electronic effect of the aryl group was observed
for the reaction efficiency, but we unexpectedly found that
ortho-substituted substrates, such as 1d, 1e, 1n, and 1o, re-
acted markedly faster than other arylboronic acids
(Table 4). To investigate this intriguing reactivity profile,
various ortho-substituted arylboronic acids were subjected
to the Rh-catalyzed reaction (Table 5). As already men-
tioned, the reaction using o-tolylboronic acid (1d) gave the
desired product 2d in 46% yield, whereas the reaction using
p-tolylboronic acid (1b) produced the product 2b only in
12% yield at room temperature (Table 5, entries 1 and 2).
Similarly, the reaction using o-ethylphenylboronic acid (1u)
proceeded well (Table 5, entry 3). However, o-isopropylphe-
nylboronic acid (1v) was found to have decreased reactivity
at room temperature (Table 5, entry 4). Interestingly, the re-
action using seemingly more bulky 2-biphenylboronic acid
(1w) furnished the desired product 2w in 64% yield at
608C (Table 5, entry 5). Although reaction of C60 with o-me-
thoxyphenylboronic acid (1x) did not proceed under stan-
dard conditions, the desired product 2x could be obtained in
Entry
R-B(OH)2 (1)
Product (2)
Yield [%]
1
2
3
1d
1b
1u
2d
2b
2u
46
12
55
4
1v
1w
1x
2v
2w
2x
27
64
45
5[a]
6[a,b]
[a] Reaction was conducted at 608C. [b] [RhOHACTHNUTRGENUG(N cod)]2 was used as a cat-
alyst.
45% yield when [RhOH
(Table 5, entry 6).
ACHTUNGTNER(NUGN cod)]2 was used as a catalyst
A similar promoting effect of the ortho substituent was
observed in the reactions using 3-thienylboronic acid deriva-
tives (Scheme 6). While 3-thienylboronic acid (1r) did not
react with C60 with our standard catalyst [Rh
ACHTUNGTRNE(NUNG cod)-
ACHTUNGTRENNUNG
and 1z) reacted smoothly to give the corresponding prod-
ucts in 37% and 50% yield, respectively.
Although the beneficial effect of ortho substituents on ar-
ylboronic acids is counterintuitive, one possible explanation
Scheme 6. Rh-catalyzed reactions using 3-thienylboronic acid derivatives.
Chem. Asian J. 2011, 6, 590 – 598
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FULL PAPERS
À
tached at the 6 6 bonds on the C60 core despite the bulki-
ness of the 2-biphenyl group. This result is consistent with
estimated to be 2.8 ꢁ. These intermolecular interactions
help the bucky ball to align in a well-organized manner.
the 13C NMR analysis, which indicated a Cs-symmetric struc-
Rh-Catalyzed Hydroarylation of C70
Encouraged by the reactions of C60, we then applied our
standard hydroarylation conditions to C70. When C70 was
treated with phenylboronic acid under the catalytic influ-
ence of [RhACHTUNGTRENN(GU cod)ACHTUNGTREN(NUNG MeCN)2]BF4 in H2O/1,2-Cl2C6H4, the ad-
dition took place smoothly to afford the hydrophenylation
product 3 in 44% yield (Scheme 7). Unreacted C70 was re-
covered in 36% yield. The LCMS analysis of the crude reac-
tion mixture indicated that the reaction occurred in a mono-
addition selective manner. However, unlike the hydroaryla-
tion of C60, regioisomers were detected in small amount.
Scheme 7. Rh-catalyzed hydroarylation of C70.
Hirsch and co-workers previously reported the synthesis
of Ph-C70-H by the reaction of C70 with phenylmagnesium
bromide followed by protonolysis. From the 13C NMR analy-
sis, they proposed their hydrophenylation product (Ph-C70-
H) to have Cs symmetry. However, they only proposed two
possible structures and the exact structural characterization
1
remained unresolved. Given the similarity in their H and
13C NMR spectra, it was indicated that our product 3 and
the Hirschꢂs product might
have the same structure.
Figure 3. a) Crystal structure of 2w·1CH2Cl2·C6H4Cl2 (50% thermal ellip-
soids, solvent molecules are omitted for clarity). b) Packing of 2w.
c) Mode of CH–p interaction.
2
In order to verify unambigu-
ously the molecular structure
(addition pattern) of 3, single-
crystal X-ray diffraction analy-
sis was performed (Figure 4).[27]
A single crystal was obtained
by the diffusion of toluene into
a solution of CS2. In the crystal
of 3, the phenyl group and hy-
drogen atom are located in 1,2-
fashion at the a bond of C70.
Moreover, it was found that the
phenyl group is attached at the
position close to the pole of the
C70 unit. As seen in the crystal
ture of 2w. In the solid-state structure of 2w, distortion of
the substituted carbon centers (C1 and C2) was found. The
decrease in the sum of the three C-C-C angles on fullerene
around the C1 and C2 atoms (ca. 3248 and 3338, respective-
ly) compared to that of C60 (3488) is in line with the change
of hybridization from sp2 to sp3. The fullerene moiety of 2w
was observed as a highly disordered structure, which may be
due to the low-barrier rotation around the C1–C60 axis in
the solid state. The crystal packing (Figure 3b) shows that
the biphenyl groups of neighboring 2w molecules vertically
face each other. As shown in Figure 3c, this interaction is
likely to be the CH–p interaction between the ortho hydro-
gen atom of the phenyl group of 2w and the benzene ring of
the neighboring 2w molecule; the shortest H–p distance is
Figure 4. Crystal structure of
structure of 2w, both C1 and
3·toluene (50% thermal ellip-
C2 in 3 have slightly distorted
soids, solvent molecules are
characters, where the sum of omitted for clarity).
596
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 590 – 598

Synthetic Approach to OrganoACHTUNTRGNEUNG(hydro)fullerenes
three C-C-C angles on C70 around C1 and C2 are about 3258
and 3348, respectively.
Acknowledgements
This work was supported in part by a Grant-in-Aid for Scientific Re-
search from MEXT and JSPS. M.N. is a recipient of a JSPS Predoctoral
Fellowship.
Nevertheless, this is the first example that concretely vali-
dates the reaction site of the organometallic mono-addition
to C70. Furthermore, it is notable that arylrhodium species,
generated under the present catalysis, selectively deliver an
aryl group to one of the five possible carbon atoms in C70.[28]
[1] a) A. Hirsch, M. Brettreich, Fullerenes: Chemistry and Reactions,
Wiley-VCH, Weinheim, 2005; b) Fullerenes: Chemistry, Physics, and
Technology (Eds.: K. M. Kadish, R. S. Ruoff), Wiley, New York,
2000; c) Fullerenes: Principles and Applications (Eds.: F. Langa, J.-F.
Nierengarten), RSC, Cambridge, 2007; d) N. Martꢃn, Chem.
Commun. 2006, 2093; e) N. Martꢃn, M. Altable, S. Filippone, A.
Martꢃn-Domenech, Synlett 2007, 3077.
Conclusions
In summary, we have developed a new rhodium-catalyzed
reaction of fullerenes with organoboron compounds. This
new catalytic method enables introduction of various organ-
ic groups onto C60 and C70. The reaction generally proceeds
with a high regioselectivity and in a mono-addition selective
manner. It was found that water is an essential additive to
promote the reaction. By X-ray crystal structure analysis, we
have confirmed the reaction site of organometallic-based hy-
droarylation of C70 for the first time. Various functional ful-
lerenes, such as fullerene-tagged amino acids and fullerene-
capped p systems, can be synthesized. Given the shelf stabil-
ity, functional group compatibility, widespread commercial
availability, and ease of functionalization of organoboron
compounds, the Rh-catalyzed reaction described herein
should find many uses in the preparation of organo-
À
[2] a) C. Bingel, Chem. Ber. 1993, 126, 1957; b) J.-F. Nierengarten, V.
Gramlich, F. Cardullo, F. Diederich, Angew. Chem. 1996, 108, 2242;
Angew. Chem. Int. Ed. Engl. 1996, 35, 2101; c) X. Camps, A. Hirsch,
J. Chem. Soc. Perkin Trans. 1 1997, 1595; d) J.-F. Nierengarten, D.
Felder, J.-F. Nicoud, Tetrahedron Lett. 1998, 39, 2747; e) F. Cheng,
X. Yang, H. Zhu, Y. Song, Tetrahedron Lett. 2000, 41, 3947.
[3] a) V. M. Rotello, J. B. Howard, T. Yadav, M. M. Conn, E. Viani,
L. M. Giovane, A. L. Lafleur, Tetrahedron Lett. 1993, 34, 1561;
b) M. Tsuda, T. Ishida, T. Nogami, S. Kurono, M. Ohashi, J. Chem.
Soc. Chem. Commun. 1993, 1296; c) K. Komatsu, Y. Murata, N.
Sugita, K. Takeuchi, T. S. M. Wan, Tetrahedron Lett. 1993, 34, 8473;
d) I. Lamparth, C. Maichle-Mçssmer, A. Hirsch, Angew. Chem.
1995, 107, 1755; Angew. Chem. Int. Ed. Engl. 1995, 34, 1607; e) Y.
Murata, N. Kato, K. Fujiwara, K. Komatsu, J. Org. Chem. 1999, 64,
3483; f) K. Mikami, S. Matsumoto, T. Tonoi, Y. Okubo, T. Suenobu,
S. Fukuzumi, Tetrahedron Lett. 1998, 39, 3733; g) J. L. Segura, N.
Martꢃn, Chem. Rev. 1999, 99, 3199; h) Y. Takaguchi, T. Tajima, K.
Ohta, J. Motoyoshiya, H. Aoyama, T. Wakahara, T. Akasaka, M. Fu-
jitsuka, O. Ito, Angew. Chem. 2002, 114, 845; Angew. Chem. Int. Ed.
2002, 41, 817.
ACHTUNGTRENNUNG(hydro)fullerenes. It should be noted that a number of C H
bond
transformations
of
the
resultant
organo-
ACHTUNGTRENNUNG(hydro)fullerenes would be possible with Pd catalysts as we
demonstrated previously.[14l] We believe that the described
chemistry not only broadens the scope of the functionaliza-
tion chemistry of fullerenes, thereby providing a new syn-
thetic avenue to a wide variety of previously unexplored
fullerene-based materials, but also represents a new direc-
tion for molecular catalysis in nanocarbon chemistry.
[4] a) M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 1993, 115,
9798; b) M. Prato, M. Maggini, Acc. Chem. Res. 1998, 31, 519; c) H.
Imahori, H. Norteda, H. Yamada, Y. Nishimura, I. Yamazaki, Y.
Sakata, S. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 100; d) G.-W.
Wang, T.-H. Zhang, E.-H. Hao, L.-J. Jiao, Y. Murata, K. Komatsu,
Tetrahedron 2003, 59, 55; e) G.-W. Wang, J.-X. Li, Y.-J. Li, Y.-C. Liu,
J. Org. Chem. 2006, 71, 680.
[5] A. Hirsch, Q. Li, F. Wudl, Angew. Chem. 1991, 103, 1339; Angew.
Chem. Int. Ed. Engl. 1991, 30, 1309.
[6] K. M. Keshavarz, B. Knight, G. Srdanov, F. Wudl, J. Am. Chem. Soc.
1995, 117, 11371.
Experimental Section
[7] H. Isobe, A.-J. Chen, N. Solin, E. Nakamura, Org. Lett. 2005, 7,
5633.
[8] a) A. Hirsch, A. Soi, H. R. Karfunkel, Angew. Chem. 1992, 104, 808;
Angew. Chem. Int. Ed. Engl. 1992, 31, 766; b) A. Hirsch, T. Grçsser,
A. Skiebe, A. Soi, Chem. Ber. 1993, 126, 1061.
[9] P. J. Fagan, P. J. Krusic, D. H. Evans, S. A. Lerke, E. Johnston, J.
Am. Chem. Soc. 1992, 114, 9697.
[10] Y. Matsuo, A. Iwashita, Y. Abe, C.-Z. Li, K. Matsuo, M. Hashiguchi,
E. Nakamura, J. Am. Chem. Soc. 2008, 130, 15429.
Typical procedure for rhodium-catalyzed hydroarylation of C60 using or-
ganoboron compounds (Table 4, entry 5): A 20 mL glass Schlenk flask
containing a magnetic stirring bar was flame-dried under vacuum and
filled with argon after cooling to room temperature. To this flask were
added dry o-dichlorobenzene (3.6 mL) and H2O (0.9 mL) under a stream
of argon. After performing three freeze-pump-thaw cycles, [Rh
ACHTUNGTRNE(NUNG cod)-
ACHTUNGTRENNUNG(MeCN)2]BF4 (1.1 mg, 2.9 mmol), C60 (21.6 mg, 30 mmol), and 2,6-dime-
thylphenylboronic acid (1e: 5.4 mg, 36 mmol) were added to the flask
under a stream of argon. After stirring the mixture at 608C for 6 h, the
mixture was cooled to room temperature. The yields of 1-(2,6-dimethyl-
phenyl)-1,9-dihydro[60]fullerene (2e) and C60 were determined to be
80% and 20%, respectively, by HPLC analysis (Buckyprep column; tolu-
ene as eluent; flow rate 0.5 mLminÀ1; UV detection at 326 nm) of this
crude mixture using C70 as an internal standard. The selectivity was deter-
mined to be greater than 95% from the [yield of 2e]/[conversion of C60]
value. Thereafter, the mixture was passed through a pad of celite with co-
pious washings with toluene (40 mL). The filtrate was concentrated (ca.
30 mL) and subjected to preparative recycling HPLC equipped with a
Buckyprep column (eluent: toluene) to afford 2e (17.2 mg, 69%) as dark
brown solid. C60 was recovered in 11% yield (2.4 mg).
[11] Z. Wang, M. S. Meier, J. Org. Chem. 2003, 68, 3043.
[12] M. Sawamura, H. Iikura, E. Nakamura, J. Am. Chem. Soc. 1996,
118, 12850.
[13] a) S. Zhou, C. Burger, B. Chu, M. Sawamura, N. Nagahama, M. To-
ganoh, U. E. Hackler, H. Isobe, E. Nakamura, Science 2001, 291,
1944; b) M. Sawamura, K. Kawai, Y. Matsuo, K. Kanie, T. Kato, E.
Nakamura, Nature 2002, 419, 702; c) Y. Matsuo, K. Tahara, M. Sa-
wamura, E. Nakamura, J. Am. Chem. Soc. 2004, 126, 8725; d) Y.-W.
Zhong, Y. Matsuo, E. Nakamura, J. Am. Chem. Soc. 2007, 129,
3052; e) Y. Matsuo, K. Tahara, K. Morita, K. Matsuo, E. Nakamura,
Angew. Chem. 2007, 119, 2902; Angew. Chem. Int. Ed. 2007, 46,
2844; f) T. Nakae, Y. Matsuo, E. Nakamura, Org. Lett. 2008, 10, 621;
g) Y. Matsuo, Y. Sato, T. Niinomi, I. Soga, H. Tanaka, E. Nakamura,
J. Am. Chem. Soc. 2009, 131, 16048.
Chem. Asian J. 2011, 6, 590 – 598
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
597

K. Itami et al.
FULL PAPERS
[14] Chemical transformations of fullerenes by means of metal catalysis:
a) L. Becker, T. P. Evans, J. L. Bada, J. Org. Chem. 1993, 58, 7630;
b) L.-L. Shiu, T.-I. Lin, S.-M. Peng, G.-R. Her, D. D. Ju, S.-K. Lin, J.-
H. Hwang, C. Y. Mou, T.-Y. Luh, J. Chem. Soc. Chem. Commun.
1994, 647; c) C. K. F. Shen, K.-M. Chien, T.-Y. Liu, T.-I. Lin, G.-R.
Her, T.-Y. Luh, Tetrahedron Lett. 1995, 36, 5383; d) L. Gan, S.
Huang, X. Zhang, A. Zhang, B. Cheng, H. Cheng, X. Li, G. Shang,
J. Am. Chem. Soc. 2002, 124, 13384; e) N. Martꢃn, M. Altable, S. Fili-
ppone, ꢄ. Martꢃn-Domenech, Chem. Commun. 2004, 1338; f) N.
Martꢃn, M. Altable, S. Filippone, ꢄ. Martꢃn-Domenech, A. Poater,
M. Solꢅ, Chem. Eur. J. 2005, 11, 2716; g) Y. Matsuo, A. Iwashita, E.
Nakamura, Chem. Lett. 2006, 35, 858; h) N. Martꢃn, M. Altable, S.
Filippone, A. Martꢃn-Domenech, L. Echegoyen, C. M. Cardona,
Angew. Chem. 2006, 118, 116; Angew. Chem. Int. Ed. 2006, 45, 110;
i) S. Filippone, M. I. Barroso, ꢄ. Martꢃn-Domenech, S. Osuna, M.
Solꢅ, N. Martꢃn, Chem. Eur. J. 2008, 14, 5198; j) B. Zhu, G.-W.
Wang, Org. Lett. 2009, 11, 4334; k) S. Filippone, E. E. Maroto, ꢄ.
Martꢃn-Domenech, M. Suarez, N. Martꢃn, Nat. Chem. 2009, 1, 578;
l) M. Nambo, K. Itami, Chem. Eur. J. 2009, 15, 4760.
[15] a) M. Sakai, H. Hayashi, N. Miyaura, Organometallics 1997, 16,
4229; Review: b) K. Fagnou, M. Lautens, Chem. Rev. 2003, 103, 169;
Selected examples: c) N. W. J. Mancuso, M. Lautens, J. Am. Chem.
Soc. 2006, 128, 5338; d) K. Tonogaki, K. Itami, J. Yoshida, Org. Lett.
2006, 8, 1419; e) T. Miura, M. Murakami, Chem. Commun. 2007,
217; f) T. Kurahashi, H. Shinokubo, A. Osuka, Angew. Chem. 2006,
118, 6484; Angew. Chem. Int. Ed. 2006, 45, 6336; g) K. Ueura, S.
Miyamura, T. Satoh, M. Miura, J. Organomet. Chem. 2006, 691,
2821; h) K. Ukai, M. Aoki, J. Takaya, N. Iwasawa, J. Am. Chem.
Soc. 2006, 128, 8706.
[16] a) Y. Takaya, M. Ogasawara, T. Hayashi, J. Am. Chem. Soc. 1998,
120, 5579. Review: b) T. Hayashi, K. Yamasaki, Chem. Rev. 2003,
103, 2829. Recent examples of Rh-catalyzed 1,4-addition of arylbor-
onic acids to a,b-unsaturated carbonyl compounds: c) J.-F. Paquin,
C. Defieber, C. R. J. Stephenson, E. M. Carreira, J. Am. Chem. Soc.
2005, 127, 10850; d) C. Walter, G. Auer, M. Oestreich, Angew.
Chem. 2006, 118, 5803; Angew. Chem. Int. Ed. 2006, 45, 5675; e) W.-
L. Duan, H. Iwamura, R. Shintani, T. Hayashi, J. Am. Chem. Soc.
2007, 129, 2130; f) R. Mariz, X. Luan, M. Gatti, A. Linden, R.
Dorta, J. Am. Chem. Soc. 2008, 130, 2172; g) T. Nishimura, J. Wang,
M. Nagaosa, K. Okamoto, R. Shintani, F.-Y. Kwong, W.-Y. Yu,
A. S. C. Chan, T. Hayashi, J. Am. Chem. Soc. 2010, 132, 464.
[17] Boronic acid derivatives are air-stable, storable, and easily accessible
by various methods. For general views for boronic acid derivatives,
see: Boronic Acids (Ed.: D. G. Hall), Wiley-VCH, Weinheim, 2005.
[18] a) M. Nambo, R. Noyori, K. Itami, J. Am. Chem. Soc. 2007, 129,
8080; b) S. Mori, M. Nambo, L.-C. Chi, J. Bouffard, K. Itami, Org.
Lett. 2008, 10, 4609.
[19] Throughout this paper, “selectivity” stands for a ratio of the quanti-
ty of desired product over the quantity of substrate converted (M.
Orchin, R. S. Macomber; A. R. Pinhas; R. M. Wilson, The Vocabu-
lary and Concepts of Organic Chemistry, 2nd ed., Wiley, New Jersey,
2005).
[20] a) M. E. Niyazymbetov, D. H. Evans, S. A. Lerke, P. A. Cahill, C. C.
Henderson, J. Phys. Chem. 1994, 98, 13093; b) G. V. Lier, B. Safi, P.
Gerrlings, J. Phys. Chem. Solids 1997, 58, 1719.
[21] a) W. N. Olmstead, Z. Margolin, F. G. Bordwell, J. Org. Chem. 1980,
45, 3295; b) F. G. Bordwell, R. J. McCallum, W. N. Olmstead, J. Org.
Chem. 1984, 49, 1424.
[22] a) S. Sakuma, N. Miyaura, J. Org. Chem. 2001, 66, 8944; b) T. Haya-
shi, M. Takahashi, Y. Takaya, M. Ogasawara, J. Am. Chem. Soc.
2002, 124, 5052.
[23] N. Miyaura, Bull. Chem. Soc. Jpn. 2008, 81, 1535.
[24] a) T. Ishiyama, M. Murata, N. Miyaura, J. Org. Chem. 1995, 60,
7508; b) T. Ishiyama, K. Ishida, N. Miyaura, Tetrahedron 2001, 57,
9813; c) C. Christophersen, M. Begtrup, S. Ebdrup, H. Petersen, P.
Vedsø, J. Org. Chem. 2003, 68, 9513; d) K. L. Billingsley, T. E.
Barder, S. L. Buchwald, Angew. Chem. 2007, 119, 5455; Angew.
Chem. Int. Ed. 2007, 46, 5359.
[25] a) J.-Y. Cho, M. K. Tse, D. Holmes, R. E. Maleczka Jr. , M. R.
Smith, III, Science 2002, 295, 305; b) T. Ishiyama, J. Takagi, K.
Ishida, N. Miyaura, N. R. Anastasi, J. F. Hartwig, J. Am. Chem. Soc.
2002, 124, 390; c) T. Ishiyama, J. Takagi, K. Ishida, J. F. Hartwig, N.
Miyaura, Angew. Chem. 2002, 114, 3182; Angew. Chem. Int. Ed.
2002, 41, 3056; d) G. A. Chotana, M. A. Rak, M. R. Smith, III, J.
Am. Chem. Soc. 2005, 127, 10539; e) S. Paul, G. A. Chotana, D.
Holmes, R. C. Reichle, R. E. Maleczka Jr. , M. R. Smith, III, J.
Am. Chem. Soc. 2006, 128, 15552; f) J. M. Murphy, C. C. Tzschucke,
J. F. Hartwig, Org. Lett. 2007, 9, 757.
[26] P. Zhao, C. D. Incarvito, J. F. Hartwig, J. Am. Chem. Soc. 2007, 129,
1876.
[27] CCDC 789387 (2w·1CH2Cl2·C6H4Cl2) and CCDC 789388 (3·toluene)
2
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
[28] The regioselective addition might be due to relatively high pyramid-
alization of pole carbon atoms of C70; a) M. Sawamura, M. Toganoh,
H. Iikura, Y. Matsuo, A. Hirai, E. Nakamura, J. Mater. Chem. 2002,
12, 2109; b) Z. Xiao, F. Wang, S. Huang, L. Gan, J. Zhou, G. Yuan,
M. Lu, J. Pan, J. Org. Chem. 2005, 70, 2060.
Received: August 18, 2010
Published online: October 20, 2010
598
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 590 – 598