Cu-catalyzed C-H amination of Hydrofullerenes leading to 1,4-Difunctionalized Fullerenes

Article abstract of DOI:10.1021/ol403573r

A novel and highly efficient Cu-catalyzed C-H amination of the monofunctionalized hydrofullerenes for the synthesis of 1,4-difunctional fullerenes has been reported. A new series of 1,4-fullerene derivatives having various monoamine addends were synthesized in good to high yields under mild reaction conditions. The controlled experiments revealed that the reaction proceeds through the formation of a fullerene monoradical as a key intermediate followed by coupling with an amine radical.

Full text of DOI:10.1021/ol403573r

Letter
pubs.acs.org/OrgLett
Cu-Catalyzed C−H Amination of Hydrofullerenes Leading to
1,4‑Difunctionalized Fullerenes
Weili Si,† Shirong Lu,† Ming Bao,‡ Naoki Asao,† Yoshinori Yamamoto,†,‡ and Tienan Jin*,†
†WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
‡State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
S
* Supporting Information
ABSTRACT: A novel and highly efficient Cu-catalyzed C−H
amination of the monofunctionalized hydrofullerenes for the
synthesis of 1,4-difunctional fullerenes has been reported. A new
series of 1,4-fullerene derivatives having various monoamine addends
were synthesized in good to high yields under mild reaction
conditions. The controlled experiments revealed that the reaction
proceeds through the formation of a fullerene monoradical as a key
intermediate followed by coupling with an amine radical.
fullerene monoradical and an amine radical species under the
optimal reaction conditions.
ullerene derivatives with versatile functional groups on a
Ffullerene core not only increase their solubilities in various
organic solvents but also tune the energy levels and the packing
structures, while maintaining the electron affinity and the
electron transport ability of C60. These merits make functional
fullerenes attractive as excellent n-type materials in organic
electronics.1 The continuous creation of a new series of
functional fullerenes possessing a potential functionality in
materials science and medicinal chemistry is still highly
desirable.1,2 The transition-metal-catalyzed methodology has
been demonstrated as one of the most efficient methodologies
for functionalization of fullerenes, which offers much
opportunity for creating novel functional fullerenes.3 Recently,
we have reported that a fullerene monoradical can be readily
formed from the monofunctionalized hydrofullerenes catalyzed
by the Cu(OAc)2 oxidant or NaOH base under an air
atmosphere, affording the single-bonded fullerene dimers in
high yields.3k,4 The interesting catalytic generation and
reactivity of the fullerene monoradical species5 led us to
further explore its function as a useful intermediate for the
synthesis of new functional fullerenes. Indeed, the research
groups of Komatsu and Nakamura have reported that a
fullerene monoradical can be oxidized to a fullerene cation by
using excess amounts of strong acids or CuCl2, which reacted
with nucleophiles to produce the corresponding 1,4- or 1,2-
fullerenes, respectively.6 However, to the best of our knowl-
edge, direct and selective intermolecular radical coupling
between a fullerene monoradical and a certain radical species
generated in situ under a catalyst system has never been
reported.7 Herein, we report a novel Cu-catalyzed C−H
amination of the monosubstituted hydrofullerenes with various
amines to afford a new series of monoamine functionalized 1,4-
bisadducts in good to high yields. The controlled experiments
clearly revealed the involvement of a radical coupling between a
1,4-Difunctionalized fullerenes possess two alkyl or aryl
functional groups on the 1,4-position of the fullerene core and
exhibit a higher absorption extinction coefficient in the visible
region than that of methanofullerene, which have been used as
n-type semiconductors for high performance organic photo-
voltaics and thin film transistors.1,8 Those 1,4-fullerenes were
generally prepared by the reactions of fullerene anions with
alkyl halides and fullerene cations with aryl nucleophiles.6a,8−10
Only one example of the monoamine functionalized 1,4-
bisadduct has been reported by Wudl et al.,11 in which the
aniline substituted 1,4-bisadduct was prepared by the acid-
mediated reaction of 1,4-disubstituted fullerenol with aniline. It
was noted that, while the addition of amines to C60 resulted in
the formation of multiadducts,12 the present Cu-catalyzed C−H
amination afforded the corresponding 1,4-bisadducts in a high
monoselectivity. In this article, we provide not only a novel
transformation for selective functionalization of fullerenes but
also a new series of 1,4-fullerens with a wide range of
monoamine addends.13
On the basis of our previous Cu-catalyzed dimerization of the
monofunctionalized hydrofullerenes,3k we initiated our inves-
tigation by examining various copper catalysts in the reaction of
hydrofullerene 1a and dibenzylamine 2a in a 1:100 mixture of
DMF and ODCB (1,2-dichlorobenzene) at 70 °C under an
oxygen atmosphere (Table 1). The yields were determined by
HPLC analysis using C70 as an internal standard. The
Cu(OAc)2 catalyst which was the most sufficient catalyst for
dimerization of 1a exhibited low catalytic activity in producing
the amine-substituted 1,4-bisadduct 3a; instead, the single-
bonded fullerene dimer 4a was obtained in high yield (entry 1;
Received: December 10, 2013
Published: December 30, 2013
© 2013 American Chemical Society
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Letter
and DMF from 100:1 to 10:1 increased the yield of 3a to 71%
(entry 12). It was noted that the reaction produced the
corresponding amination product 3a in 43% yield under an air
atmosphere, indicating that the oxygen atmosphere is
indispensable in achieving the high chemical yield of 3a
(entry 13).
Table 1. Optimization Reaction Conditions for C−H
a
Amination of Hydrofullerene 1a
We noticed that, in the absence of a copper catalyst, the
present reaction produced the dimerization product 4a in over
90% yield without formation of 3a (entry 14), while it is not
surprising that we have demonstrated that the strong NaOH
base can be used as a catalyst for the efficient dimerization of
the hydrofullerene 1a at room temperature.4 However, the
similar reaction under an argon atmosphere afforded a trace
amount of 4a (entry 15). These results indicate that amine acts
as a base to promote the dimerization of 1a in the presence of
oxygen and implied that the present amination might proceed
through the dimerization of 1a to form 4a by the amine base
followed by amination of the dimer 4a by a copper catalyst to
form the dibenzylamine-substituted 1,4-bisadduct 3a.
The aforementioned results led us to examine the reaction
pathway from the single-bonded fullerene dimer to the amine-
substituted 1,4-bisadduct as shown in Table 2. When 4a was
treated with dibenzylamine 2a in the presence of a CuBr2
catalyst and oxygen, the corresponding 1,4-bisadduct 3a was
obtained in 77% yield, while the reaction produced 3a in a very
poor yield of 9% under an Ar atmosphere (entries 1 and 2). It
was noted that the reaction did not proceed in the absence of
the CuBr2 catalyst (entry 3). These results clearly indicate that
the amination of the single-bonded fullerene dimer is promoted
by the CuBr2 catalyst and oxygen is indispensable for the
present transformation. It was well demonstrated that the
single-bonded fullerene dimers dissociate to the stable
monoradicals in solution.3k,4,5 Consequently, the results in
both Tables 1 and 2 indicate that the present C−H amination
of hydrofullerene to the 1,4-bisadduct might proceed through
the formation of a fullerene radical intermediate followed by
coupling with an amine.
b
b
entry
catalyst
solvent (100:1)
3a (%)
4a (%)
1
2
3
4
5
6
7
8
9
10
Cu(OAc)2
Cu(OTf)2
CuCl2
CuBr2
Cu2O
CuCl
ODCB/DMF
5
85
38
54
20
54
60
34
70
39
39
13
0
ODCB/DMF
38
37
67
40
34
58
21
40
38
27
75 (71)
43
0
ODCB/DMF
ODCB/DMF
ODCB/DMF
ODCB/DMF
CuBr
ODCB/DMF
CuI
ODCB/DMF
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
none
ODCB/THF
ODCB/CH3CN
ODCB
c
11
d
12
ODCB/DMF (10:1)
ODCB/DMF (10:1)
ODCB/DMF (10:1)
ODCB/DMF (10:1)
e
13
0
14
90
trace
f
15
none
0
a
Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), Cu catalyst (10
mol %), ODCB/cosolvent (10 mL), oxygen balloon, 70 °C for 18 h.
Yields determined by HPLC by using C70 as an internal standard.
Yield of 3a isolated by silica gel chromatography is shown in
parentheses. 1a was recovered in 56% yield. C60 was obtained in 9%
yield. The reaction was conducted under an air atmosphere. 1a was
recovered in 23% yield. The reaction was conducted under an argon
b
c
d
e
f
atmosphere. 1a was recovered in 90% yield.
for the structure of 4a, see Table 2). Among the other Cu(II)
species such as Cu(OTf)2, CuCl2, and CuBr2 tested, the use of
Table 2. Control Experiments of Amination by Using the
a
Single-Bonded Fullerene Dimer 4a
As aforementioned, Matsuo and Nakamura et al. reported
that a fullerene cationic species can be generated from a
fullerene radical by using an excess amount of CuCl2 at high
temperature.6b,c Komatsu et al. demonstrated the fullerene
cationic species by the electrophilic addition to allylsilane or
benzene to form the corresponding 1,4-bisadducts.6a To
confirm whether our amination reaction conditions generate
the fullerene cation or not, the fullerene dimer 4a was treated
with anisole under our standard CuBr2-catalyzed reaction
conditions in the absence of amines (eq 1). However, the
b
b
entry
catalyst
oxidant
3a (%)
4a (%)
1
2
3
CuBr2
CuBr2
none
O2
77
9
trace
80
argon
O2
0
90
a
Reaction conditions: a mixture of meso and racemic dimer 4a (0.1
mmol), 2a (0.4 mmol), CuBr2 (10 mol %), ODCB/DMF (10:1, 10
b
mL), oxygen balloon, 70 °C, 18 h. Yields determined by HPLC using
C70 as an internal standard.
CuBr2 obviously increased the yield of 3a up to a 67% yield
together with a 20% yield of 4a (entries 2−5). The Cu(I)
species such as Cu2O, CuCl, CuBr, and CuI are also active, but
the yields of 3a are lower than that with CuBr2 (entries 6−8).
In contrast, several other metal catalysts, such as AuBr3, FeCl3,
CoBr2, PdCl2, and Mn(OAc)3, were totally inactive for the
formation of 3a and the dimer 4a was obtained in comparable
yields. The use of THF or CH3CN as a cosolvent instead of
DMF or ODCB as a single solvent did not result in increasing
the yield of 3a (entries 9−11). Changing the ratio of ODCB
reaction did not produce any anisole-substituted 1,2- or 1,4-
bisadduct and 4a was recovered in over 90% yield. This result
indicated that our reaction conditions are unable to generate
the fullerene cationic species from the fullerene radical.
On the basis of these results, the reaction mechanism is
proposed as shown in Scheme 1. Deprotonation of an acidic
proton in hydrofullerene 1 by an amine base forms a fullerenyl
monoanion, which converts to a monoradical A by an O2-
promoted one-electron oxidation.4 Meanwhile, coordination of
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Letter
reactions with cyclic amines, such as tetrahydroisoquinoline
(2f), thiomorpholine (2g), morpholine (2h), piperidine (2i),
and 1-phenylpiperazine (2j), produced the corresponding 1,4-
fullerenes 3f−j in high chemical yields; the yield of 3j was up to
96% by using 2j (entries 5−9). It should be mentioned that the
amine-substituted 1,4-fullerenes showed good solubility in
chloroform, toluene, and ODCB.
Scheme 1. Proposed Reaction Mechanism
The reaction was further examined by using various
monofunctionalized hydrofullerenes (1) and morpholine (2h)
under the standard conditions (Table 4). The reaction using
CuBr2 to amine 2 forms the Cu(II)-amine intermediate C along
with HBr. Intermediate C would be in equilibrium with amine
radical D and the Cu(I) species. Subsequent intermolecular
radical coupling between the fullerene monoradical A and the
amine radical D produces the corresponding 1,4-bisadduct 3.
The improved efficiency for formation of 3a by using DMF as
the cosolvent and the increased amount of DMF indicated that
the DMF polar solvent should stabilize both fullerene and
amine radicals, assisting the sufficient intermolecular radical
coupling.3k Finally, oxidation of CuBr under an oxygen
atmosphere in the presence of HBr regenerates the CuBr2
catalyst. The results of entries 4 and 7 in Table 1 suggest that
the bromide salts can improve the oxidation ability of the Cu(I)
species under the present reaction conditions.14
Table 4. CuBr2-Catalyzed Amination of Various
Hydrofullerenes 1 with Morpholine 2h
a
b
entry
1
time (h)
3
yield (%)
1
2
3
4
5
6
1b
1c
1d
1e
1f
13
13
24
13
13
42
3k
3l
58
74
41
72
3m
3n
3o
3p
c
Under the optimized conditions, the scope of the CuBr2-
catalyzed amination of the monofunctionalized hydrofullerene
1a with various amines was examined (Table 3). All of the
71
d
1g
73
a
Reaction conditions: 1 (0.1 mmol), 2h (0.2 mmol), CuBr2 (10 mol
b
%), ODCB/DMF (10:1, 10 mL), oxygen balloon, 70 °C. Yields of
Table 3. CuBr2-Catalyzed C−H Amination of
isolated products. A less than 2% yield of C60 was observed in every
a
c
d
Hydrofullerene 1a with Various Amines
example unless otherwise noted. C60 was obtained in 6% yield. C60
was obtained in 9% yield.
the monobenzyl hydrofullerenes 1b−d having a methoxy group
at the 2- or 3- or 4-position of the benzene ring afforded the
corresponding 1,4-fullerenes 3k−m in good yields (entries 1−
3). The monobenzyl hydrofullerenes 1e and 1f having an ester
or both ester and methoxy groups on the benzene ring were
also tolerated, producing the desired 1,4-fullerenes 3n and 3o in
72% and 71% yields, respectively (entries 4 and 5). The alky-
substituted hydrofullerene 1g with a benzyl-protected alcohol
gave the corresponding product 3p in 73% yield under longer
reaction times (entry 6). It was noted that, in every example, a
small amount of C60 was formed probably by the elimination of
the fullerene anions or the monoradicals shown in Scheme 1.
b
c
entry
1
2
time (h)
3
yield (%)
dimer 4a (%)
2b
2c
2d
2e
2f
16
20
16
29
36
55
12
13
17
3b
3c
3d
3e
3f
64
54
64
61
73
70
86
85
96
7
7
d
2
3
4
5
6
7
8
9
6
10
10
9
2g
2h
2i
3g
3h
3i
1
6
The structures of 1,4-bisadducts 3 were characterized by H
NMR, 13C NMR, and high resolution mass spectra (Supporting
Information). The UV−vis absorption spectra of compounds 3
exhibit a characteristic broad absorption band around 450 nm
as shown in Figure S1 (Supporting Information), which further
supports the 1,4-bisadduct structure of 3.8−11 The low
symmetry of 1,4-bisadducts 3 showed an increased absorption
in the visible region compared to [6,6]-phenyl-C61-butyric acid
methyl ester (PC61BM) which is a well-used benchmark
acceptor in bulk heterojunction (BHJ) solar cells.1 For example,
the extinction coefficients of 3a and 3i are 7000 mol−1 cm−1 at
444 and 447 nm, respectively, which are approximately 2.5
times higher than that of PC61BM (2800 mol−1 cm−1 at 431
nm) (Table S1, Supporting Information). The electrochemistry
of 1,4-fullerenes was investigated by electrochemical cyclic
voltammetry (CV) in ODCB (Supporting Information, Figure
S2). The LUMO levels of 3a and 3h having a dibenzylamine or
morpholine group on the C60 core were estimated be −3.64
0
2j
3j
0
a
Reaction conditions: 1a (0.1 mmol), 2 (0.2 mmol), CuBr2 (10 mol
%), ODCB/DMF (10:1, 10 mL), oxygen balloon, 70 °C. Yields of
isolated products. Yields determined by HPLC using C70 as an
internal standard. A less than 2% yield of C60 was obtained in every
example unless otherwise noted. C60 was obtained in 5% yield
b
c
d
reactions were monitored by TLC and HPLC analysis, and the
corresponding 1,4-bisadducts 3 were isolated by using silica gel
column chromatography. In some cases, a small amount of the
fullerene dimer 4a remained, and the HPLC analysis did not
show any amine multiadducts (see Supporting Information). A
variety of acyclic amines, such as benzylamine (2b), N-
methylaniline (2c), diethylamine (2d), and dibutylamine (2e),
were tolerated, affording the corresponding amination 1,4-
bisadducts 3b−e in 54−64% yields (entries 1−4). The
622
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Organic Letters
Letter
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and −3.65 eV, respectively, which are sufficiently low to be
used as the potential n-type materials.
In conclusion, we have described a novel and selective Cu-
catalyzed C−H amination of the monofunctionalized hydro-
fullerenes with various amines. A variety of the new
monoamine-substituted 1,4-bisadducts were synthesized in
good to high yields. The experimental results indicated that
the present reaction proceeds through the formation of a
fullerene monoradical by an amine followed by coupling with
an amine radical generated by a CuBr2 catalyst under an oxygen
atmosphere. The optical properties of the new 1,4-fullerenes
compared to the methanofullerene showed increased absorp-
tion bands in the visible region. This methodology provides a
valuable and general synthetic tool for the synthesis of a new
series of 1,4-fullerenes having various monoamine addends
which are expected to be useful in electronic device
applications.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: tjin@m.tohoku.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Scientific Research (B) from the
Japan Society for Promotion of Science (JSPS) (No.
25288043), 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), and World Premier
International Research Center Initiative (WPI), MEXT
(Japan). We also thank the support of Showa Denko Group
Institute for Advanced and Core Technology. W.S. acknowl-
edges the support of the China Scholarship Council (CSC).
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