NaOH-catalyzed dimerization of monofunctionalized hydrofullerenes: Transition-metal-free, general, and efficient synthesis of single-bonded [60]fullerene dimers

Article abstract of DOI:10.1021/ol301435m

Unprecedented, transition-metal-free NaOH-catalyzed homo- and cross-dimerizations of monofunctionalized hydrofullerenes are reported. Various single-bonded fullerene dimers were synthesized under mild reaction conditions with remarkably high yields. The u

Full text of DOI:10.1021/ol301435m

ORGANIC
LETTERS
2012
Vol. 14, No. 13
3466–3469
NaOH-Catalyzed Dimerization of
Monofunctionalized Hydrofullerenes:
Transition-Metal-Free, General, and
Efficient Synthesis of Single-Bonded
[60]Fullerene Dimers
Shirong Lu,†,‡ Tienan Jin,*,† Ming Bao,‡ and Yoshinori Yamamoto†,‡
WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University,
Sendai 980-8577, Japan, and State Key Laboratory of Fine Chemicals,
Dalian University of Technology, Dalian 116012, China
tjin@m.tohoku.ac.jp
Received May 24, 2012
ABSTRACT
Unprecedented, transition-metal-free NaOH-catalyzed homo- and cross-dimerizations of monofunctionalized hydrofullerenes are reported.
Various single-bonded fullerene dimers were synthesized under mild reaction conditions with remarkably high yields. The use of a NaOH catalyst
combined with tetrahydrofuran as a cosolvent under an air atmosphere is critical in achieving highly efficient catalytic dimerization.
The single-bonded fullerene dimer RC60ꢀC60R with a
direct covalent bond between two C60 cages has attracted
much attention due to not only its potential interesting
optical and electronic properties through the interaction of
two adjacent fullerene cages1 but also its useful functiona-
years, versatile synthetic methods for the construction of
the single-bonded fullerene dimers have been reported.2ꢀ8
For example, the photoirradiation radical addition of C60
with perfluoroalkyl iodides in the presence of (R3Sn)2,4a
Mn(OAc)3-mediated radical reaction of C60 with phos-
phonate esters or dialkyl malonates,4c,6 one-electron oxi-
dationofthe monoanion RC60ꢀ through anexcess amount
of oxidants,2b,4b,5d,7 and electroreduction of 1,2-(PhCH2)-
lization ability as a monomer radical (RC60 ) precursor.2
•
The pioneered experimental and theoretical studies on
these fullerene dimers revealed that they consist of
racemic and meso isomers which were usually formed by
8
HC60 have been reported to form the single-bonded
fullerene dimers. However, their wide application to the
the dimerization of RC60
.
• 3,4 Accordingly, in the past few
† Tohoku University.
‡ Dalian University of Technology.
(1) (a) Segura, J. S.; Martin, N. Chem. Soc. Rev. 2000, 29, 13. (b) Luo,
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Chem. Soc. 2004, 126, 8874. (b) Zhang, Y.; Matsuo, Y.; Li, C.-Z.;
Tanaka, H.; Nakamura, E. J. Am. Chem. Soc. 2011, 133, 8086.
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Murata, Y. Chem. Commun. 2011, 47, 6111.
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r
10.1021/ol301435m
Published on Web 06/18/2012
2012 American Chemical Society

elaborated fullerene dimers generally suffered from the
limited scope of functional groups, low chemical yields,
harsh reaction conditions, and use of large excess amounts
of toxic reagents or additives. Recent advances in the
catalytic functionalization of fullerene9 triggered us to
explore the highly efficient catalytic dimerization of hydro-
fullerenes which may offer advantages such as high che-
mical yields and selectivity as well as high functional group
compatibility under mild conditions.
Scheme 1. Cu(OAc)2- and NaOH-Catalyzed Dimerization of
Hydrofullerenes 1 for Synthesis of Single-Bonded [60]Fullerene
Dimers 2
Most recently, we found that the Cu(OAc)2 catalyst
combined with a small amount of DMF as a cosolvent
under air was crucial for dramatic enhancement of the
dimerization efficiency of monosubstituted hydrofuller-
enes (RHC60) (1),9m affording the corresponding fullerene
dimers 2 in high yields (Scheme 1).10 The reaction most
as-prepared NaOH (10 mol %) by using ODCB (ortho-
dichlorobenzene) as a sole solvent in air at rt did not
proceed, and 1a was recovered almost quantitatively
(Table 1, entry 1). The remarkable enhancement of effi-
ciency by adding DMF or CH3CN, which was reported by
our previous Cu(OAc)2-catalyzed dimerization,10 led us to
examine the notable cosolvent effect on the present dimer-
ization. To our delight, addition of the polar cosolvents,
such as DMF, THF, and CH3CN, improved the yields of
the corresponding dimer 2a drastically to 89%, 95%, and
90%, respectively (entries 2ꢀ4), although we reported
previously that the use of THF as a cosolvent combined
with the Cu(OAc)2 catalyst afforded only a 20% yield
of 2a.10 Addition of less-coordinative cosolvents, such as
toluene or chloroform, did not exert any influences on
the reaction efficiency (entries 5 and 6). Previously, we
also reported that the use of a catalytic or stoichiometric
amount of Cu(OAc)2 in the absence of air produced the
corresponding dimer 2a in 14% and 93% yields, res-
pectively.10 Interestingly, in sharp contrast to these results,
the present NaOH-mediated dimerization did not afford
the corresponding dimer 2a with either a catalytic or
stoichiometric amount of NaOH (entries 7 and 8). These
results not only indicate that air play as an important role
in the generation of monomer radical RC60• but also imply
that the present NaOH-catalyzed dimerization might form
probably proceeds through the Cu(OAc)2-catalyzed gen-
•þ
•
eration of a fullerenyl radical cation species (RHC60
followed by formation of a fullerenyl monoradical RC60
)
.
However, the drawback of toxic copper contamination of
the products and solvents may preclude its use in materials
science and practical synthetic application.11 Taking these
points into consideration, we envisioned that by using the
suitable, nontoxic base as a catalyst in the presence of air
and cosolvent, the catalytic dimerization of RHC60 (1)
should be accomplished through the generation of mono-
ꢀ
anion RC60 with base treatment followed by formation
of RC60• by one-electron oxidation.5d,12 Herein, we report
unprecedented, efficient, and nontoxic NaOH-catalyzed
homo- and cross-dimerizations of monofunctionalized
hydrofullerenes 1 that afford the single-bonded fullerene
dimers 2 as a mixture of racemic and meso isomers in
excellent yields in the presence of air and tetrahydrofuran
(THF) cosolvent under mild reaction conditions
(Scheme 1). To the best of our knowledge, this is the first
example for transition-metal-free catalytic functionalization
of fullerenes which provides an alternative and promising
method for catalytic dimerization of hydrofullerenes,
meeting the requirements for both green chemistry and
fullerene chemistry.
In the preliminary experiment, we found that the homo-
dimerization of monosubstituted 1a in the presence of an
•
RC60 in a completely different way from the Cu(OAc)2-
catalyzed reaction. To gain further insight into the role of
bases, we examined various base catalysts. Strong inor-
ganic bases, such as KOMe, KOtBu, CsOH, and KOH,
produced the corresponding fullerene dimer 2a in high
yields (entries 9ꢀ12), while other inorganic bases having a
relatively lower basicity, such as LiOH and Na2CO3, are
less active (entries 13 and 14). Strong organic base Me4-
NOH produced 2a in a short reaction time (2 h), but the
yield was lower than that of NaOH (entry 15). The weaker
organic base Et3N showed low catalytic activity regardless
of the prolonged reaction time (entry 16). It is clear that the
reaction did not proceed without base catalysts (entry 17).
With the optimized reaction conditions in hand,
various monosubstituted hydrofullerenes 1 were examined
as shown in Table 2. A variety of functional groups in hydro-
fullerenes 1 were tolerated under the present NaOH-
catalyzed dimerization conditions, affording the corre-
sponding single-bonded fullerene dimers 2 in very high
(9) (a) Martın, N.; Altable, M.; Filippone, S.; Martın-Domenech, A.;
´ ´
ꢀ
Poater, A.; Sola, M. Chem.;Eur. J. 2005, 11, 2716. (b) Matsuo, Y.;
Iwashita, A.; Nakamura, E. Chem. Lett. 2006, 35, 858. (c) Nambo, M.;
Noyori, R.; Itami, K. J. Am. Chem. Soc. 2007, 129, 8080. (d) Mori, S.;
Nambo, M.; Chi, L.-C.; Bouffard, J.; Itami, K. Org. Lett. 2008, 10, 4609.
(e) Nambo, M.; Wakamiya, A.; Yamaguchi, S.; Itami, K. J. Am. Chem.
Soc. 2009, 131, 15112. (f) Nambo, M.; Itami, K. Chem.;Eur. J. 2009, 15,
4760. (g) Zhu, B.; Wang, G.-W. J. Org. Chem. 2009, 74, 4426. (h) Zhu,
B.; Wang, G.-W. Org. Lett. 2009, 11, 4334. (i) Filippone, S.; Maroto,
E. E.; Martın-Domenech, A.; Suarez, M.; Martın, N. Nat. Chem. 2009,
´ ´
1, 578. (j) Xiao, Z.; Matsuo, Y.; Nakamura, E. J. Am. Chem. Soc. 2010,
ꢁ
132, 12234. (k) Maroto, E. E.; de Cozar, A.; Filippone, S.; Martın
o, F. P.; Martın, N. Angew. Chem., Int.
´
ꢁ
Domenech, A.; Suarez, M.; Cossı
´
´
Ed. 2011, 50, 6060. (l) Martın, N.; Altable, M.; Filippone, S.; Martın
´
´
ꢁ
Domenech, A.; Echegoyen, L.; Cardona, C. M. Angew. Chem., Int. Ed.
2006, 45, 110. (m) Lu, S.; Jin, T.; Bao, M.; Yamamoto, Y. J. Am. Chem.
Soc. 2011, 133, 12842.
(10) Lu, S.; Jin, T.; Kwon, E.; Bao, M.; Yamamoto, Y. Angew.
Chem., Int. Ed. 2012, 51, 802.
(11) Brewer, G. J. Clin. Neurophysiol. 2010, 121, 459.
(12) (a) Wallace, T. J.; Schriesheim, A.; Bartok, W. J. Org. Chem.
1963, 28, 1311. (b) Wallace, T. J.; Schriesheim, A. J. Org. Chem. 1962, 27,
1514.
Org. Lett., Vol. 14, No. 13, 2012
3467

Table 1. Screening of Catalysts and Solvents for Homodimer-
ization of Monofunctionalized [60]Hydrofullerenesa
Table 2. NaOH-Catalyzed Homodimerization of Monosubsti-
tuted Hydrofullerenesa
catalyst
solvent
(1:3)
time
(h)
2a
(%)b
1a
(%)b
entry
(1 M in H2O)
1
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
KOMe
KOtBue
CsOH
KOH
ODCB
12
12
12
12
20
20
12
12
13
13
13
13
20
20
2
trace
89
97
4
2
DMF/ODCB
THF/ODCB
CH3CN/ODCB
Toluene/ODCB
CHCl3/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
THF/ODCB
3
95 (91)
90
0
4
2
5
trace
0
99
99
98
97
0
6
7c
8c,d
9
trace
trace
93
time
(h)
yield 2
(%)b
entry
1
2
10
11
12
13
14
15
16
17
92
1
1
1b
1c
1d
1e
1f
13
12
13
12
20
20
20
20
15
14
2b
2c
2d
2e
2f
85
93
90
86
96
91
84
79
94
74
2c
3
92
4
91
0
LiOH
31
63
88
0
4
5c
6d
7c
8c
9
Na2CO3
Me4NOH
Et3N
9
88
1g
1h
1i
2g
2h
2i
26
12
26
70
99
none
0
1j
2j
a Conditions: To a 1,2-dichlorobenzene (ODCB) (3 mL) and cosolvent
(1 mL) of 1a (0.03 mmol) under air atmosphere was added base catalyst
(10 mol %, 1 M aqueous solution). The resulting mixture was stirred at rt
for the times shown. b HPLC yields determined using C60 as an internal
standard. Isolated yield by silica-gel chromatography is shown in par-
entheses. c Reaction wasconducted underargonatmosphere.d 1.0 equivof
NaOH catalyst was used. e A THF solution was used.
10c
1k
2k
a Conditions: To a 1,2-dichlorobenzene (ODCB, 3 mL) solution of
1 (0.03 mmol) and THF (1 mL) under an air atmosphere was added
NaOH (1 M in H2O, 10 mol %, 3 μL). The resulting mixture was stirred
at rt for the times shown. b Isolated yields. c 15 mol % NaOH was used.
d 20 mol % NaOH was used.
yields. All of the reactions were monitored by TLC and
HPLC analysis and the corresponding dimers 2 were
purified by silica gel column chromatography as a mixture
of meso and racemic isomers. 1,2-Benzyl(hydro)fullerenes
bearing an n-butyl (1b) or a methoxy group (1c) on the
benzene rings underwent the NaOH-catalyzed dimeriza-
tion smoothly, affording the corresponding dimers 2b and
2c in high yields (entries 1 and 2). Remarkably, hydro-
fullerenes 1d, 1e, and 1j having the base sensitive ester
substituents were also tolerated, giving the expected dimers
in high yields without any formation of the hydrolyzed
byproducts (entries 3, 4, and 9). Alkyl- or aryl-substituted
hydrofullerenes 1fꢀi having benzyl-, THP-, and n-octyl-
protected ethers are also active substrates, producing the
corresponding dimers 2fꢀi in high yields, whereas a large
amount of catalytic loading is required (entries 5ꢀ8). The
present NaOH-catalyzed dimerization was applied to the
construction of a fullerene-bound dendrimer, which is ex-
pected to show some special electronic property arising from
the fullerene cage and dendrimer moiety. Under the stan-
dard catalytic conditions, the fullerene-bound dendrimer 1k
afforded the corresponding dendritic dimer 2k in 74% yield,
which exhibited high solubility in various common organic
solvents, such as THF, CHCl3, and benzene (entry 10).
The present NaOH-catalyzed dimerization was also
successfully applied to the synthesis of cross-dimers con-
taining two different functionalized C60 units (Table 3).
Under the standard reaction conditions, cross-dimeriza-
tion of two different alkyl(hydro)fullerenes 1a and 1g
occurred smoothly to produce the desired cross-dimer 3a
in 30% yield together with the two homodimers 2a and 2g
in 24% and 29% yield, respectively (entry 1). It was noted
that these three dimers were purified by flash silica gel
chromatography. As we reported previously, cross-dimers
3 were speculated to consist of four isomers according
to the four possible kinds of monoradicals,10 although the
individual isomers cannot be completely identified from
1H NMR and HPLC analysis because they are difficult
to separate through silica gel chromatography and
HPLC. Similarly, the NaOH-catalyzed cross-dimerization
of alkyl(hydro)fullerenes 1a and 1f as well as 1f and 1g also
proceeded smoothly to afford the corresponding cross-
dimers 3b and 3c in 34% and 33% yield, respectively,
together with the corresponding homodimers (entries 2
and 3). The homodimers 2 are in equilibrium with cross-
dimers 3 in solvent at rt that provides an alternativemethod
for the synthesis of these promising cross-dimers 3 from
various homodimers 2 through their interconversion in
3468
Org. Lett., Vol. 14, No. 13, 2012

solution. It should be noted that both homo- and cross-
dimers are very stable in the solid state; they can be stored
for several months at ambient temperature without any
decomposition.
whereas dimerization of two monoradicals with the same
configuration gives a racemic mixture of R-R and S-S
isomers.4c
Scheme 2. A Plausible Mechanism for the NaOH-Catalyzed
Dimerization of Monosubstituted Hydrofullerenes
Table 3. NaOH-Catalyzed Cross-Dimerizationa
entry 1 (R1) 1 (R2) homodimer 2 (%)b cross-dimer 3 (%)b
1
2
3
1a
1a
1f
1g
1f
24 (2a); 29 (2g)
26 (2a); 28 (2f)
29 (2f); 29 (2g)
30 (3a)
34 (3b)
33 (3c)
1g
a Conditions: To a 1,2-dichlorobenzene (ODCB, 6 mL) solution of
two different hydrofullerenes 1 (0.03 mmol) and THF (2 mL) under an
air atmosphere was added NaOH (15 mol %, 1 M aqueous solution).
The resulting mixture was stirred at rt for 16 h. b Dimers 2 and 3 were
isolated by silica-gel chromatography.
A notable feature of an acidic CꢀH bond13 in hydro-
fullerenes 1 and our experimental observations, as well
as early works on base catalyzed dimerization of
mercaptans,12 led us to propose the monoanion oxidation
mechanism as shown in Scheme 2.14 Initially, deprotona-
tion of an acidic proton in hydrofullerenes 1 by the strong
base NaOH forms a fullerenyl monoanion sodium com-
plex A, wherein a polar solvent such as THF may stabilize
A. Indeed, when hydrofullerene 1a was treated with a
stoichiometric amount of NaOH in an argon atmosphere
without air, the color of the reaction mixture was changed
to dark green, indicating the generation of a monoanion
species (Table 1, entry 8).4b One-electron oxidation of the
monoanion sodium complex A by O2 in air forms the
monoradical B which produces the corresponding single-
bonded fullerene dimers 2.4b,5b,5d,12 Moreover, we assume
that further one-electron oxidation of monoanion A may
take place by an in situ generated •O2ꢀ oxidant, leading to
monoradical B.12a Finally, hydrolysis of sodium peroxide
regenerates the NaOH catalyst which enters into the next
catalytic circle. Dimerization of two monoradicals having
an R and S configuration produces the meso isomer (R-S),
In conclusion, we have developed unprecedented,
transition-metal-free NaOH-catalyzed homo- and cross-
dimerizations of monofunctionalized hydrofullerenes. The
present novel catalytic dimerization provided a general,
efficient, mild, and green method for the construction of
various single-bonded fullerene dimers with remarkably
high yields. Compared with our previous Cu(OAc)2-cata-
lyzed dimerization, the present NaOH catalysis avoided
use of the toxic and expensive copper salts while retaining
its high catalytic efficiency. The NaOH catalyst combined
with a small amount of THF as a cosolvent in air is crucial
for the efficient formation of the corresponding single-
bonded fullerene dimers. Investigation on their chemical
and electronic properties and application to materials
science are in progress.
Acknowledgment. This work was partially supported
by World Premier International Research Center Initiative
(WPI), MEXT (Japan). T.J. acknowledges a Grant-in-Aid
for Scientific Research on Innovative Areas “Organic
Synthesis Based on Reaction Integration. Development
of New Methods and Creation of New Substances” from
the MEXT (Japan). S.L. acknowledges the support of the
JSPS fellowships for Young Scientists.
(13) Matsuo, Y.; Iwashita, A.; Abe, Y.; Li, C.-Z.; Matsuo, K.;
Hashiguchi, M.; Nakamura, E. J. Am. Chem. Soc. 2008, 130, 15429.
(14) An alternative reaction pathway, such as the generation of a
fullerenol intermediate through hydroxylation of 1a, is unlikely because,
without any fullerenol, derivatives were observed from 1H NMR and
HPLC analysis under the standard reaction conditions and the reaction
using H2O2 instead of air; the reaction of 1a under aqueous basic
conditions in the presence of H2O2 (50 equiv) in an argon atmosphere
produced the corresponding dimer 2a in 23% yield (8 h) together with a
72% yield of the recovered 1a.
Supporting Information Available. Experimental pro-
cedures and characterization data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 13, 2012
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