Fullerene-Cation-Mediated Noble-Metal-Free Direct Introduction of Functionalized Aryl Groups onto [60]Fullerene

Article abstract of DOI:10.1021/acs.orglett.8b01295

Aryl[60]fullerenyl cations (ArC₆₀⁺), which were generated by heating aryl[60]fullerenyl dimers (ArC₆₀-C₆₀Ar) to generate aryl[60]fullerenyl radicals (ArC₆₀^?) followed by oxidation using Cu(II) salts (Cu(BF₄)₂(aq)), were reacted with various functionalized aryl boronic acids to produce functionalized 1,4-diaryl[60]fullerenes. This protocol tolerated various functional groups, such as OH, NH₂, COCH₃, and Cl substituents, with yields reaching 93%. C₆₀Ar¹Ar² (Ar² = p-NH₂C₆H₄) was used as a dopant in a photoactive CH₃NH₃PbI₃ layer of a perovskite solar cell.

Full text of DOI:10.1021/acs.orglett.8b01295

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Fullerene-Cation-Mediated Noble-Metal-Free Direct Introduction of
Functionalized Aryl Groups onto [60]Fullerene
Xiao-Yu Yang,† Hao-Sheng Lin,‡ Il Jeon,‡ and Yutaka Matsuo*,†,‡
†Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, 96 Jinzhai Road,
Hefei, Anhui 230026, China
‡Department of Mechanical Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-8656, Japan
S
* Supporting Information
ABSTRACT: Aryl[60]fullerenyl cations (ArC60+), which were
generated by heating aryl[60]fullerenyl dimers (ArC60−C60Ar)
to generate aryl[60]fullerenyl radicals (ArC60•) followed by
oxidation using Cu(II) salts (Cu(BF4)2(aq)), were reacted with
various functionalized aryl boronic acids to produce function-
alized 1,4-diaryl[60]fullerenes. This protocol tolerated various
functional groups, such as OH, NH2, COCH3, and Cl substituents, with yields reaching 93%. C60Ar1Ar2 (Ar2 = p-NH2C6H4) was
used as a dopant in a photoactive CH3NH3PbI3 layer of a perovskite solar cell.
unctionalization of fullerenes has enabled their use in
cations marks a frontier in the chemistry of fullerenes for their
Fvarious practical applications, including energy conversion synthetic modification. The abstraction of one electron from
devices1 and biomaterials.2 Because of the excellent electron
affinity of fullerenes,3 typical fullerene modification reactions
proceed via addition of nucleophilic moieties to fullerene,4 such
as azomethine ylides (Prato reaction),5 malonates (Bingel−
Hirsch reaction),6 and organometallic nucleophiles.7 However,
such reactions generally involve fullerene anion intermediates,
which cause problems with functional group tolerance.
Although ester and ketone groups in substrates have been
used in such nucleophilic reactions,8 there was lower tolerance
for more reactive functional groups such as hydroxy and amino
groups due to protonation of fullerene anion intermediates and
so on. In general, installation of organic groups containing a
hydroxy group onto fullerene has been achieved by using
protection/deprotection strategies.9 Otherwise, reactive func-
tional groups will take part in the reaction.10
organofullerenyl anions to produce organofullerenyl radicals
and the subsequent removal of one more electron to generate
organofullerenyl cations can be regarded as abstraction of two
electrons from the highest occupied molecular orbital
(HOMO), which forms a new lowest unoccupied molecular
orbital (LUMO) offering nontraditional reactivity for new
fullerene modification reactions (Figure 1).
We have previously reported the unique reactivity of
fullerene cation intermediates for intramolecular migration
and cyclization.11,12 For example, thermal cleavage of
alkylfullerenyl dimer RC60−C60R produces alkylfullerenyl
Figure 1. Concept of this work. The newly generated LUMO has
unique reactivity. For example, weak nucleophiles such as aryl boronic
acids can react with the newly formed LUMO.
13,14
•
radical Me3SiCH2C60
,
which can then be oxidized to
alkylfullerenyl cation, Me3SiCH2C60+. This cation undergoes
cyclization to produce dihydromethano[60]fullerene
(C61H2),11a which is an important intermediate in the synthesis
of methanoindene-fullerene, C60(CH2)(indene), which is a
high-performance electron acceptor used in organic solar
cells.11c,d Another example is demethylative cyclization of an
arylfullerenyl cation bearing an o-methoxy group on the aryl
Here, we report an intermolecular coupling reaction between
arylfullerenyl cations and various functionalized aryl boronic
acids to produce functionalized 1,4-diaryl[60]fullerenes, C60Ar-
(Ar-FG) (FG = functional groups, including reactive OH and
NH2 groups). The diaryl[60]fullerenes, C60Ar2, are simple and
robust scaffolds with a 1,4-addition pattern of fullerene
derivatization, accessible using various synthetic method-
ologies.15−19 An advantage of the present reaction is functional
group.12d ArC60 (Ar = o-MeOC6 H4) produces
+
dihydrobenzofurano[60]fullerene and related compounds with
fullerene−oxygen bond formation occurring through intra-
molecular interaction between the fullerene cation and the
oxygen atom of the aryl group. In this way, the use of fullerene
Received: April 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01295
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Table 1. Fullerenyl-Cation-Mediated Coupling Reactions with Functional Aryl Groups To Obtain Functionalized
a
Diaryl[60]fullerenes
a
b
Unless otherwise specified, all reactions were conducted at 120 °C for 16 h with a 1:5:4 molar ratio of dimer/boronic acid/oxidant. Isolated yield.
c
d
Values in parentheses are based on the use of the oxidant CuBr2 instead of Cu(BF4)2(aq). Isolated yield when using Cu(BF4)2(aq) in the presence
of molecular sieves 4 A.
group and water tolerance due to the use of fullerenyl cation
intermediates, which are not reactive toward them. Importantly,
this reaction features novel reactivity of the newly generated
LUMO of the fullerene cations toward aryl boronic acids
bearing a partial negative charge. Furthermore, this reaction can
utilize the commercial availability of various functionalized aryl
boronic acids. We anticipate that this reaction can contribute to
further understanding of cation-mediated fullerene functional-
ization reactions and can provide a route to various
functionalized fullerene derivatives for a wide range of applied
research, without requiring the use of protection/deprotection
protocols.
The present reaction starts from an organofullerenyl dimer,
which can be synthesized in high yield and isolated simply by
silica gel column chromatography.14 Arylfullerenyl dimers (2a
and 2b for Ar = p-CH3OC6H4 and p-tBuC6H4, respectively)
were readily prepared by I2-mediated single-electron oxidation
of arylfullerenyl anions (ArC60−) obtained by deprotonation of
C60ArH with tBuOK.12d Upon heating, the singly bonded dimer
1 underwent homolytic cleavage of the single bond to give an
arylfullerenyl radical (ArC60•) which was then oxidized by a
Cu(II) salt to generate an arylfullerenyl cation (ArC60+).
Subsequently, the arylfullerenyl cation generated in situ
participated in an intermolecular coupling reaction in the
presence of various aryl boronic acids. Thus, 1, hydroxyphenyl
boronic acid 3a (5 equiv), and aqueous Cu(BF4)2 (4 equiv)
were heated in o-dichlorobenzene (o-DCB) at 120 °C under an
inert atmosphere, producing coupling product 1-(4-methox-
yphenyl)-4-(4-hydroxyphenyl)[60]fullerene 4a in 78% isolated
yield (Table 1, entry 1). Note that the use of Cu(BF4)2 (aq)
was superior to the use of CuBr2 in terms of both ease of
operation and product yield. A certain amount of an aqueous
solution of Cu(BF4)2 can be taken by pipetting without
weighing, utilizing the water-tolerance of this reaction due to
the use of fullerene cation intermediates that are stable against
water. In a reaction with CuBr2 previously investigated in our
laboratory, Cu(II) oxidant gave 51% isolated yield, which can
be ascribed to the poor solubility of CuBr2 in o-DCB.
After optimization of the reaction conditions, we further
investigated the scope and limitations of this reaction. Amino
B
DOI: 10.1021/acs.orglett.8b01295
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Letter
group functionalized boronic acid 3b reacted with the fullerenyl
cation to produce derivative 4b in 50% and 53% yields in the
presence of Cu(BF4)2(aq) and CuBr2, respectively, without the
use of protecting groups (Table 1, entry 2). Notably, the acetyl
and chloro groups were also tolerated in this reaction to
produce 4c and 4d, respectively, in the presence of
Cu(BF4)2(aq) as the oxidant (entries 3 and 4). The benzofuran
and thiophene functional groups also could be installed by
using benzofuranyl and thienyl boronic acids to produce 4e and
4f in 89% and 75% yields, respectively (entries 5 and 6). Other
organic substituents (entries 7−10) on the aryl groups could be
used to obtain diaryl[60]fullerenes 4g−j, with the highest yield
reaching 93%.
conditions, and hydroxide attacks the boron atom to generate
tetracoordinated borate Ar2B(OH)3 , which participates in
−
transmetalation to the Pd(II)Ar1(OH) species to give B(OH)4
−
and Pd(II)Ar1Ar2, which subsequently undergoes reductive
elimination (Figure 2c). Similarly, we can surmise that a halide
t
Meanwhile, dimer 2 bearing a Bu group could be used for
this reaction to obtain corresponding diaryl[60]fullerenes 5a−d
under the same reaction conditions (Supporting Information,
SI). We also examined the use of boronic acid pinacol esters
instead of boronic acids and found that they similarly generated
the desired products in satisfactory yield (SI). Generally, the
t
products 5a−d with the Bu group had better solubility than
compounds 4a−j, and all of the products 4 could be easily
separated by silica gel column chromatography, owing to the
polarity of the MeO group.
We noticed that when the reaction system contained
methanol methoxy products C60Ar1(OMe) were produced, as
characterized by NMR and HRMS. In addition, when we added
water to the reaction systems in the absence of boronic acid
substrates, we obtained the known aryl(hydroxy)fullerene
C60Ar1OH.17 These results indicate that the intermediate is
an arylfullerenyl cation Ar1C60+, which can react with methanol
and water to give C60Ar1(OMe) and C60Ar1OH, respectively.
The present reaction to diarylfullerenes can take place in the
presence of water from Cu(BF4)2(aq), indicating that
C60Ar1OH is equivalent to the arylfullerenyl cation in the
presence of a Cu(II) ion.
Bearing in mind the above observations, we next investigated
the limitations of this reaction. When we used an aryl boronic
acid with an alcohol group HOCH2C6H4B(OH)2 instead of a
phenolic group like in 3a, we could not obtain the desired
diarylfullerene, even after considerable experimentation. Both
the alcohol and boronic acid groups in the substrate reacted
with the fullerenyl cation intermediate to give a complex
mixture. Although phenolic OH groups are well tolerated in
this reaction, alcoholic OH groups cannot be used. This is a
limitation of the present reaction.
Another limitation can be seen from the low chemical yield
of 4c. This was due to the low nucleophilicity of the substrate
with an electron-withdrawing acetyl group toward the fullerene
cation electrophile. Interestingly, we obtained a hydroxyfuller-
ene (C60Ar1OH) as a side product from this reaction. This was
attributed to competition between the boronic acid (Ar1(δ−)B-
(OH)2) and water (O(δ−)H2), both of which act as weak
nucleophiles. When we added molecular sieves 4 A to this
reaction system with Cu(BF4)2 (aq), the yield was improved
from 22% to 35%. Overall, we consider this intermolecular
reaction to be competitive, and the reactivity of the
arylfullerenyl cation is in the order alcohol > aryl boronic
acids > water; the electronic properties of the boronic acids
slightly affect this order.
Figure 2. Plausible reaction mechanism of the fullerene-cation-
mediated coupling reaction. (a) Nucleophilic attack of weakly
electronegative aryl groups to the fullerenyl cation. (b) Trans-
metalation analogue between Ar2 and Br− on Ar1C60+. (c) Trans-
metalation between Ar2 and OH− on Pd2+.
coordinates to the boron atom to produce a tetracoordinated
species for exchanging an aryl group and a halide on a borate
and a fullerene cation, respectively, to afford the product
(Figure 2b). However, considering that the reaction proceeds
in the presence of Cu(BF4)2 (not only CuBr2), we propose a
reaction mechanism involving unconventional nucleophilic
attack of weakly electronegative (δ−) aryl groups onto the
fullerenyl cation to generate the product (Figure 2a). This
reaction does not occur with the usual LUMO of neutral C60
and fullerene derivatives, but it can take place with the newly
generated LUMO after removal of two electrons from
organofullerenyl anions (Figure 1). By utilizing this new
frontier orbital, we found a unique reaction that was previously
inaccessible. From the viewpoint of organic chemistry, this
reaction can be regarded as an elementary reaction analogous
to transmetalation in the Suzuki-Miyaura coupling (Ar1−Pd-
OH to Ar1−Pd-Ar2 vs Ar1−C60-X to Ar1−C60-Ar2 with
similarity of Pd2+ and ArC60+). Given the rich synthetic
chemistry of transition-metal complexes, the analogous full-
erene cations here could be promising reactants for the
preparation of various unique fullerene derivatives.
The present reaction can be compared with a palladium-
catalyzed fullerene modification reaction reported by Itami and
co-workers.19b They synthesized C60Ar2 from C60ArH and aryl
halides in the presence of a Pd(II) catalyst and a base. Because
they used aryl halides and a base, their reaction mechanism
involves a fullerenyl anion intermediate. In contrast, the present
reaction mechanism involves a fullerenyl cation intermediate.
This is advantageous for realizing functional group and water
tolerance and avoiding the use of noble metal catalysts.
Based on our results, and a review of the literature, we
propose a plausible mechanism for this fullerenyl-cation-
mediated coupling reaction. In general, Suzuki−Miyaura
cross-coupling reactions with boronic acids occur under basic
C
DOI: 10.1021/acs.orglett.8b01295
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We conducted controlled experiments for further under-
standing of this reaction. We examined the reaction in air, in
the absence of a Cu(II) reagent, and in the absence of a Cu(II)
in air while using dimer 1 and boronic acid 3i. Under the
standard conditions, in the presence of Cu(BF4)2(aq) in argon,
HPLC yield and isolated yield were 98% and 88%, respectively
(Figure S1). In air, the yields dropped somewhat to 84% and
63%, respectively (Figure S2). Without Cu(II) in argon, the
target molecule also formed, but HPLC and isolated yields were
42% and 23%, respectively, and many unidentifiable products
formed (Figure S3). Finally, the reaction in the absence of
Cu(II) in air gave only 5% and 2% HPLC and isolated yields,
respectively (Figure S4). From these data, Cu(II) is not
mandatory, and the reaction can occur with a fullerenyl radical,
but Cu(II) to generate a fullerenyl cation is necessary to obtain
the desired products in high yield. Avoiding oxygen is
preferable for cleaner reactions, even though a small amount
of water does not affect the reaction.
Finally, we demonstrated the potential of our new fullerene
derivatives in an organic electronic application. Fullerene
application has played a crucial part in the field of perovskite
solar cells (PSCs).20 Accordingly, we fabricated PSCs and
showcased the application of amine-functionalized compound
(4b) as a photoactive layer dopant. It has been reported that
the amine group can interact with CH3NH3PbI3 to form a
Lewis acid adduct, passivating trapping sites.21 In addition,
fullerene derivatives inside the photoactive layer are reported to
enhance photovoltaic performance.22 Therefore, we fabricated
inverted PSCs and introduced 4b according to the method in
ref 22. The power conversion efficiency (PCE) of control
device increased from 12.7% to 13.5% upon addition of 4b,
owing to increased current density (JSC) (Figure S5). This
demonstrates that fullerenes prepared by this reaction can
function as a dopant in PSCs, and we are planning further
analysis and more detailed investigations for our future
research.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: matsuo@ustc.edu.cn, matsuo@photon.t.u-tokyo.ac.jp.
ORCID
Il Jeon: 0000-0002-4220-8374
Yutaka Matsuo: 0000-0001-9084-9670
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by high-level human resource funding
in University of Science and Technology of China. Part of this
work was financially supported by Grants-in-Aid for Scientific
Research (JSPS KAKENHI Grant Nos. JP15H05760,
JP16H04187, and 17H06609) and the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01295.
Experimental section including 1H and 13C NMR spectra
and HRMS (PDF)
D
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