Highly efficient Cu(OAc)2-catalyzed dimerization of monofunctionalized hydrofullerenes leading to single-bonded [60]fullerene dimers

Article abstract of DOI:10.1002/anie.201107505

Dimers are a girl's best friend: The title reaction allows the formation of single-bonded fullerene dimers with extremely high chemical yield and high compatibility with various functional groups, which are highly soluble in organic solvents. The use of Cu(OAc)₂ catalyst with dimethylformamide or acetonitrile as additives under air is the critical factor in achieving the highly efficient catalytic dimerization. Copyright

Full text of DOI:10.1002/anie.201107505

.
Angewandte
Communications
DOI: 10.1002/anie.201107505
Fullerenes
Highly Efficient Cu(OAc) -Catalyzed Dimerization of
2
Monofunctionalized Hydrofullerenes Leading to Single-Bonded
60]Fullerene Dimers**
[
Shirong Lu, Tienan Jin,* Eunsang Kwon, Ming Bao, and Yoshinori Yamamoto*
[
6]
The single-bonded fullerene dimer RC ÀC R, which has a
conditions. However, examples of catalytic dimerization
for the synthesis of single-bonded fullerene dimers have never
6
0
60
direct covalent bond between two C60 cages, is an interesting
and unusual structure that is expected to display interesting
optical and electronic properties through the interaction of
[
6f]
been reported.
Recently, we developed the cobalt-catalyzed hydroalky-
lation of C60 with active alkyl bromides at ambient temper-
ature to give monoalkylated hydrofullerenes in good to high
yields. Our interest in catalytic CÀH bond activation led us
to consider the possibility of catalytic CÀH bond functional-
ization of the monosubstituted hydrofullerenes toward the
construction of fullerene dimers. A study by Komatsu and
co-workers revealed that RC C could be formed from full-
erenyl radical cation (RHC C ), which in turn was generated
from monofunctionalized hydrofullerene by one-electron
[1]
two adjacent fullerene cages. Since the pioneering work by
[
2]
Krusic and co-workers, studies on ESR and X-ray crystal-
structure analysis revealed that singly bonded fullerene
dimers consist of racemic and meso isomers, which are in
[
7]
[3]
equilibrium with the monomer radical (RC C) in solution.
6
0
[
8]
Some experimental and theoretical studies on these single-
bonded fullerene dimers demonstrated that they were usually
formed by dimerization of RC C radicals, which were
6
0
+
6
0
60
[
4]
generated by various radical reactions; for example, the
[
9]
photoirradiation of C with perfluoroalkyl iodides in the
oxidation in sulfuric and sulfonic acids. In addition, Yu
6
3a]
0
[
II
presence of (R Sn) , Mn(OAc) -mediated radical reaction
and co-workers reported the Cu -catalyzed aryl CÀH bond
3
2
3
[
3c]
[5a]
[10]
of C60 with phosphonate esters or dialkyl malonates, and
one-electron oxidation of the monoanion RC
functionalization by using O ; the reaction was proposed to
2
À
+
by oxi-
proceed through the formation of an aryl radical cation (ArC )
species by one electron transfer from the aryl ring to the Cu
6
0
[3b,5b–d]
II
dants.
However, their application to the elaborated
fullerene dimers is generally limited by low functional-group
tolerance, low yield, and the use of large excess amounts of
reagents. Functionalization of fullerene by transition-metal
catalysis has recently been considered to be a promising and
innovative strategy in fullerene chemistry, which offers
advantages for high chemical yield, selectivity control, and
high functional-group compatibility under mild reaction
catalyst followed by formation of an aryl radical (ArC). Taking
these results into consideration, we turned our attention to
metal oxidants as catalysts. Herein, we report a novel and
efficient Cu(OAc) -catalyzed homo-dimerization of various
2
monofunctionalized hydrofullerenes that affords the single-
bonded fullerene dimers 2 as a mixture of racemic and meso
isomers in excellent yields in the presence of a small amount
of dimethylformamide (DMF) at room temperature under air
[
Eq. (1); ODCB = 1,2-dichlorobenzene]. Notably, the cross-
[
*] Prof. Dr. T. Jin, Prof. Dr. Y. Yamamoto
WPI Advanced Institute for Materials Research (WPI-AIMR)
Tohoku University, Sendai 980-8577 (Japan)
E-mail: tjin@m.tohoku.ac.jp
dimers 3 (see below) were prepared for the first time through
cross-dimerization of two different hydrofullerenes.
yoshi@m.tohoku.ac.jp
Homepage: http://www.wpi-aimr.tohoku.ac.jp/~yamamoto/
S. Lu
Department of Chemistry, Graduate School of Science
Tohoku University (Japan)
and
State Key Laboratory of Fine Chemicals
Dalian University of Technology (China)
Dr. E. Kwon
Research and Analytical Center for Giant Molecules
Graduate School of Science, Tohoku University (Japan)
Prof. Dr. M. Bao
State Key Laboratory of Fine Chemicals
Dalian University of Technology (China)
[
**] T.J. acknowledges a Grant-in-Aid for Young Scientists (B). S.L.
acknowledges the support of the JSPS Predoctoral Fellowships for
Young Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107505.
802
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Angew. Chem. Int. Ed. 2012, 51, 802 –806

Angewandte
Chemie
We initiated this unprecedented but highly promising
catalytic dimerization by examining solvents and catalysts at
room temperature under air. The yields in Table 1 were
94%, respectively, within 4 h (Table 1, entries 4 and 5). A
decrease of the amount of DMF resulted in slightly increased
yield (Table 1, entry 6).
It should be noted that, in the absence of air, the present
reaction produced the corresponding dimer 2a in only 14%
Table 1: Screening of catalysts and solvents for the homo-dimerization of
monofunctionalized [60]hydrofullerenes.
[
a]
yield under a catalytic amount of Cu(OAc) (Table 1, entry 7).
2
This result indicates that air has an important role, which is
indispensable for achieving high catalytic activity. Moreover,
recent studies on the formation of the single-bonded fullerene
1
dimers show that the fullerene radical (R C C) is almost
6
0
certainly a key intermediate, which can be formed from the
1
À
0
corresponding fullerene anion (R C ) by O2 or I2 oxi-
dant.
6
[
3b,5b–d]
If this is the case, the fact that a stoichiometric
amount of Cu(OAc) affords 2a in 93% yield in the absence
2
1
of air means that Cu(OAc) can generate R C C under the
2
60
present solvent systems (Table 1, entry 8). To gain further
insight into the role of the metal oxidant, we examined
various metal catalysts. Other coinage-metal catalysts with
lower acidity than Cu(OAc) , such as Cu(OMe) and AgOAc,
2
2
[
b]
[b]
Entry Catalyst
Solvent (1:3)
ODCB
1,4-dioxane/ODCB
THF/ODCB
t [h] 2a [%]
1a [%]
are also active, but the yields are lower than that with
Cu(OAc) and longer reaction times are needed (Table 1,
entries 9 and 10). Lewis acidic metal catalysts, such as CuCl2
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
24
24
24
4
trace
trace
20
97
96
78
4
4
2
and Cu(OTf) , are totally inactive, although Au(OAc)
2
3
CH CN/ODCB
92
showed good catalytic activity notwithstanding its higher
acidity (Table 1, entries 11–13). In contrast, several other
transition-metal acetates, such as Pd(OAc) , Ni(OAc) , Zn-
3
DMF/ODCB
DMF/ODCB (1:10)
DMF/ODCB
4
94
Cu(OAc)2
4
4
97 (92)
14
2
2
[
[
c]
Cu(OAc)2
Cu(OAc)2
Cu(OCH3)2 CH CN/ODCB
AgOAc
CuCl2
84
2
(
OAc) , and Mn(OAc) , are less active or completely
c,d]
2
3
DMF/ODCB
2
93
ineffective (Table 1, entries 14–17). It is clear that the reaction
did not proceed without metal catalysts (Table 1, entry 18). In
addition, under optimized conditions (10 mol% of Cu(OAc)2
in a 1:10 mixture of DMF and ODCB at room temperature
under air for 4 h), the reaction proceeded with similar
efficiency on a one-gram scale, giving 2a in 90% yield.
The structure for the meso isomer of 2a was unambigu-
ously confirmed by single-crystal X-ray diffraction analysis
48
48
48
48
48
48
48
24
24
24
66
81
trace
0
86
12
20
0
14
25
13
96
94
11
85
78
98
82
99
3
1
1
1
1
1
1
1
1
1
CH CN/ODCB
3
CH CN/ODCB
3
Cu(OTf)2
Au(OAc)3
CH CN/ODCB
3
CH CN/ODCB
3
Pd(OAc)2
CH CN/ODCB
3
Ni(OAc)2
Zn(OAc)2
Mn(OAc)3
none
CH CN/ODCB
3
CH CN/ODCB
3
CH CN/ODCB
3
[
12]
(Figure 1). A crystal of the meso isomer was grown as the
CH CN/ODCB
0
3
predominant component in CHCl /CS2 at À158C from a
3
[
a] Conditions: The catalyst (10 mol%) was added to a solution (10 mL)
mixture of meso and racemic products. The fullerene–full-
erene CÀC bond (1.53(2) ꢀ) is slightly shorter than the
of 1a (0.1 mmol) and additives in ODCB under air. The resulting mixture
was stirred at room temperature for the times shown. [b] Yields
determined by HPLC by using C60 as an internal standard. Yield of
product isolated by silica-gel chromatography is shown in parentheses.
[
3b,c]
reported one.
The UV/Vis spectrum of 2a in chloroform
showed a broad absorption band at 445 nm, which is red-
shifted relative to that of the corresponding hydrofullerene 1a
[c] Reaction was conducted under argon atmosphere. [d] 1.0 equivalent
of Cu(OAc) was used.
2
(433 nm). The cyclic voltammogram (CV) of 2a in ODCB
determined by HPLC analysis using C60 as an internal
standard. Homo-dimerization of the monofunctionalized
[
7]
hydrofullerene 1a
in the presence of Cu(OAc)2
(
10 mol%) by using ODCB as a sole solvent afforded only
trace amounts of the corresponding dimer 2a, and 1a was
recovered almost quantitatively (Table 1, entry 1). The re-
markable enhancement of efficiency on addition of organo-
metal reagents to C60 by adding pyridine or DMF reported by
[
11]
the Nakamura group
led us to examine polar solvents
mixed with ODCB. The addition of 1,4-dioxane did not affect
the yield of 2a, whereas the use of tetrahydrofuran afforded
2
a in 20% yield (Table 1, entries 2 and 3). We further
examined more-polar additives. To our delight, the addition
Figure 1. ORTEP of the meso isomer of 2a and CVs of 1a and 2a in
of CH CN or DMF improved the yields of 2a to 92% and
ODCB.
3
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803

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Angewandte
Communications
exhibited three reversible waves on reduction, which would
correspond to the formation of monoanion, dianon, and
noteworthy that hydrofullerene 1j with a propargyl ester
group on the benzyl moiety produced the desired dimer 2j in
91% yield without the formation of any conjugated diynes
[
3b]
trianion of its monomer (Figure 1).
The different values
[
13]
between dimer 2a and monomer 1a on half-wave potentials
generated from the Glaser reaction (Table 2, entry 9).
red
[7]
(
E1/2 ) clearly indicate the electronic interaction between the
Alkyl-substituted hydrofullerenes with an ester group (1d)
as well as benzyl- and THP-protected alcohols (1e and 1 f) are
also active substrates, producing the corresponding dimers
2d–f in high yields, whereas hydrofullerene 1g with an
unprotected hydroxy group showed a lower activity, giving 2g
in 70% yield in the presence of 30 mol% of Cu(OAc)2
catalyst under longer reaction times (24 h) (Table 2,
entries 3–6). The reaction also proceeded well with hydro-
fullerenes with a benzene ring substituted directly to the C60
moiety, giving 2h and 2i in 81% and 84% yield, respectively
(Table 2, entries 7 and 8). The present copper catalysis was
applied to the construction of a fullerene-bound macro-
molecule, which may be useful for versatile electronic
materials. For example, under the standard conditions, the
fullerene-bound dendrimer 1k afforded the corresponding
two C60 cages of 2a.
The present Cu(OAc) -catalyzed dimerization of mono-
2
substituted hydrofullerenes 1 showed high compatibility with
a variety of functional groups, producing the corresponding
single-bonded dimers 2 in very high yields (Table 2; THP =
tetrahydropyranyl). All of the reactions were monitored by
TLC and HPLC analysis, and the corresponding dimers 2
were isolated by silica gel column chromatography as a
mixture of meso and racemic isomers. In each case, a very
small amount of recovered 1 and unidentified products was
observed, but these impurities could be removed by silica gel
column chromatography. An n-Butyl (1b) or a methoxy
group (1c) on the benzene ring of monobenzyl-substituted
hydrofullerenes was tolerated, affording the corresponding
[
7]
[7]
dimers 2b,c in excellent yields (Table 2, entries 1 and 2). It is
dendritic dimer 2k in 79% yield (Table 2, entry 10). It
should be mentioned that the prepared dimers, except for 2d
and 2h, showed good solubility in toluene and ODCB. In
particular, the dendritic dimer 2k exhibited dramatically
improved solubility in various organic solvents.
Table 2: Cu(OAc) -catalyzed homo-dimerization of monosubstituted
hydrofullerenes.
2
[
a]
This method also provides an interesting opportunity for
the synthesis of novel cross-dimers containing two different
functionalized C60 units (Table 3). Under the standard con-
ditions, cross-dimerization of two different hydrofullerenes
1
a and 1 f proceeded smoothly to produce the desired cross-
dimer 3a in 31% yield together with the two homodimers 2a
and 2 f in 26% and 30% yield, respectively (Table 3, entry 1).
Cross-dimer 3a was speculated to consist of four isomers
according to the four possible kinds of monoradicals (Fig-
[
3b,c]
ure 2),
although the individual isomers cannot be com-
1
pletely identified from H NMR spectra and HPLC analysis.
Similarly, cross-dimerizations of 1a and 1e, as well as 1e and
1
f, afforded the cross-dimers 3b and 3c, respectively, in
similar yields together with the corresponding homodimers
Table 3, entries 2 and 3). All products were isolated by silica
(
gel chromatography. Interestingly, the cross-dimers 3 are in
equilibrium with homodimers 2 in solution. For example,
1
monitoring by H NMR spectroscopy showed that the pure
[
a]
Table 3: Cu(OAc) -catalyzed cross-dimerization.
2
[
b]
[b]
Entry
1
t [h]
2
Yield 2 [%]
Yield 1 [%]
1
2
3
4
5
6
7
8
9
0
1b
1c
1d
1e
1 f
1g
1h
1i
4
4
4
4
4
24
5
5.5
4.5
5
2b
2c
2d
2e
2 f
2g
2h
2i
90
96
88
90
94
70
81
84
91
79
2
trace
2
trace
3
1
2
[b]
[b]
Entry
1 (R )
1 (R )
Homo-dimer 2 [%]
Cross-dimer 3 [%]
[
c]
3
trace
2
trace
trace
1
2
3
1a
1a
1e
1 f
1e
1 f
26 (2a); 30 (2 f)
25 (2a); 31 (2e)
29 (2e); 31 (2 f)
31 (3a)
32 (3b)
33 (3c)
1j
1k
2j
2k
1
[
a] Conditions: To a solution of two different hydrofullerenes 1
[
a] Conditions: Cu(OAc) (10 mol%) was added to a solution of 1
(0.1 mmol) and DMF (1 mL) in 1,2-dichlorobenzene (ODCB, 10 mL)
2
(
0.1 mmol) and DMF (1 mL) in ODCB (10 mL) under air. The resulting
under an air atmosphere was added Cu(OAc) (10 mol%). The resulting
2
mixture was stirred at room temperature for the times shown. [b] Yields
mixture was stirred at room temperature for 5 h. [b] Dimers 2 and 3 were
of isolated product. [c] 30 mol% of Cu(OAc) was used.
isolated by silica-gel chromatography.
2
804
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Chemie
moiety to Cu(OAc) , leading to radical cation B. We assume
2
that coordination of Cu(OAc) to the C moiety to form
2
60
intermediate A may accelerate the one-electron oxidation,
wherein polar solvents such as DMF and CH CN stabilize A.
3
Elimination of a proton on the radical cation B by the acetate
anion forms the monoradical C, which produces the corre-
sponding single-bonded dimers. Dimerization of two mono-
radicals with R and S configuration gives the meso isomer (R–
S), whereas dimerization of two monoradicals with the same
configuration affords a racemic mixture of R–R and S–S
[
3c]
isomers. We also speculate that polar solvents stabilize the
radical cation B to assist the subsequent release of the proton.
Finally, the Cu(OAc) catalyst is regenerated by oxidation of
2
CuOAc under an air atmosphere and in the presence of acetic
acid.
In conclusion, we have demonstrated for the first time that
single-bonded fullerene dimers can be synthesized in excel-
lent yields through the catalytic dimerization of monosub-
stituted hydrofullerenes. A wide range of functional groups
are tolerated. In contrast to the previously reported methods,
the present catalytic reaction can be carried out under very
mild conditions with remarkably higher yields. Fullerene-
bound dendritic homodimers and various cross-dimers, which
are otherwise difficult to obtain, were synthesized in good to
high yields for the first time. The present catalytic dimeriza-
tion of monosubstituted hydrofullerenes seems to proceed
Figure 2. Four plausible isomers of cross-dimers 3 speculated from
their four different radical monomers.
dimer 3a was gradually converted into 2a and 2 f in CDCl3/
CS at ambient temperature and reached equilibrium after
2
1
2 h, although this interconversion rate between the cross-
through Cu(OAc) -catalyzed generation of a fullerenyl rad-
ical cation species followed by formation of a fullerenyl
2
1
dimer and homodimers was slow enough to identify its H and
1
3
C NMR spectra. Furthermore, 3a was obtained by mixing of
the two homodimers 2a and 2 f in CDCl /CS at ambient
radical. The Cu(OAc) catalyst combined with a small amount
2
of DMF under air is crucial for the efficient formation of the
corresponding single-bonded fullerene dimers. Investigation
on further catalytic functionalization of fullerenes and
application to materials science are in progress.
3
2
temperature through similar interconversion. In sharp con-
trast, 3a is very stable in the solid state; it can be stored for
several months at ambient temperature without any structural
changes. The single-bonded dimers dissociate to the stable
monoradicals in solution, followed by recombination of the
resulting monoradicals to form various dimers, which was in
good agreement with the previous studies reported by the Experimental Section
[
3b,c]
groups of Komatsu and Wang.
This interconversion
2a: To a solution of 1a (87 mg, 0.1 mmol) in DMF and ODCB (1:10,
11 mL) was added Cu(OAc) (1.8 mg, 0.01 mmol) at room temper-
provides an alternative approach for the construction of
cross-dimers from various homodimers in solution.
2
ature under air. The reaction mixture was stirred at room temperature
for 4 h. The reaction was monitored by HPLC analysis (elution with
A plausible mechanism for the present catalytic dimeri-
zation based on our experimental observations, as well as
works on the generation of radical cation species by the
À1
toluene at 0.6 mLmin flow rate, detection at 320 nm). The mixture
was subjected directly with silica gel chromatography (toluene/
hexane = 1:1). A solution of 2a in toluene and hexane was evaporated
at below 608C, and the residue was washed with acetone to afford 2a
in 92% yield (80 mg).
[
9]
[10]
groups of Komatsu and Yu,
is shown in Scheme 1.
Initially, the one-electron oxidation of hydrofullerene 1
takes place through the single-electron transfer from the C60
3
a: To a solution of 1a (44 mg, 0.05 mmol) and 1 f (46 mg,
.05 mmol) in DMF and ODCB (1:10, 11 mL) was added Cu(OAc)2
1.8 mg, 0.01 mmol) at room temperature under air. The reaction
0
(
mixture was stirred at room temperature for 5 h. The reaction was
À1
monitored by HPLC analysis (elution with toluene at 0.6 mLmin
flow rate, detection at 320 nm). The mixture was subjected directly
with silica gel chromatography using toluene as an eluent. Product 2a
was isolated as the first fragment, followed by 3a and 2 f. A solution of
3
a in toluene was evaporated at below 608C, and the residue was
washed with acetone to remove the remaining toluene, affording 3a in
1% yield (28 mg). Similarly, 2a and 2 f were obtained in 26%
23 mg) and 30% (26 mg) yield, respectively, after evaporation of
3
(
toluene and washing with acetone.
Scheme 1. A plausible mechanism for the Cu(OAc) -catalyzed dimeri-
Received: October 25, 2011
2
zation of monosubstituted hydrofullerenes.
Published online: December 2, 2011
Angew. Chem. Int. Ed. 2012, 51, 802 –806
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
805

.
Angewandte
Communications
Keywords: copper · dimerization · fullerenes ·
homogeneous catalysis · radical reactions
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