Regiocontrolled synthesis of 1,2-Di(organo)fullerenes via copper-assisted 1,4-aryl migration from silicon to carbon

Article abstract of DOI:10.1021/ol202511u

A concise and efficient way to generate fullerene cationic species through the oxidation of a fullerene radical or a fullerene anion with a Cu(II) salt has been developed. It was demonstrated that the cationic fullerene is useful for functionalization of

Full text of DOI:10.1021/ol202511u

ORGANIC
LETTERS
2011
Vol. 13, No. 22
6058–6061
Regiocontrolled Synthesis of 1,2-
Di(organo)fullerenes via Copper-Assisted
1,4-Aryl Migration from Silicon to Carbon
Ying Zhang, Yutaka Matsuo,* and Eiichi Nakamura*
Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
matsuo@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
Received September 19, 2011
ABSTRACT
A concise and efficient way to generate fullerene cationic species through the oxidation of a fullerene radical or a fullerene anion with a Cu(II) salt
has been developed. It was demonstrated that the cationic fullerene is useful for functionalization of fullerene, in particular, for the synthesis of
noncyclic 1,2-di(organo)[60]fullerene derivatives that can be selectively prepared through intramolecular 1,4-aryl migration of an aryl group from a
silicon atom to the fullerene core.
Regioselectivity is a central issue in organic synthesis
and so is important in fullerene functionalization.1,2 Intro-
duction of two organic addends to [60]fullerene, for instance,
the addition of R and R0 groups across a double bond, can
yield 1,2- and 1,4-addition products (Scheme 1a). In full-
erene chemistry, the former has been found to occur only in
the addition of sterically small cyano3 and methyl groups4
(or hydrogen atoms), while the 1,4-adduct is always a major
or exclusive product in the addition of bulkier groups.5ꢀ8
The putative pentadienyl radical, cationic, or anionic inter-
mediate (marked with red color in Scheme 1a) preferentially
produces the 1,4-adduct because of either the inherent nature
of the pentadienyl species to react on the central carbon or
steric hindrance due to the R group or both. Here, we report a
new method to control this regioselectivity by exploiting the
ability of an arylsilane to act as an intramolecular donor of an
aryl group to the carbocationic species (Scheme 1b). This
method provides a synthetic entry to 1,2-di(organo)-
[60]fullerenes that have so far been unavailable except for
cycloaddition products (e.g., C61-butyric acid methyl ester
(PCBM) widely used for photovoltaic research).9
(1) (a) Lamparth, I.; Maichle-Mossmer, C.; Hirsch, A. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1607. (b) Hirsch, A.; Vostrowsky, O.
Eur. J. Org. Chem. 2001, 829. (c) Lamparth, I.; Herzog, A.; Hirsch, A.
Tetrahedron 1996, 52, 5065. (d) Nierengarten, J. F.; Gramlich, V.;
Cardullo, F.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 2101.
(2) Hirsch, A.; Brettreich, M. Fullerenes: Chemistry and Reactions;
WileyꢀVCH: Weinheim, 2005.
(3) Keshavarz-K., M.; Knight, B.; Srdanov, G.; Wudl, F. J. Am.
Chem. Soc. 1995, 117, 11371.
(4) (a) Caron, C.; Subramanian, R.; D’Souza, F.; Kim, J.; Kutner,
W.; Jones, M. T.; Kadish, K. M. J. Am. Chem. Soc. 1993, 115, 8505.
(b) Allard, E.; Riviere, L.; Delaunay, J.; Dubois, D.; Cousseau, J.
Tetrahedron Lett. 1999, 40, 7223.
(5) Subramanian, R.; Kadish, K. M.; Vijayashree, M. N.; Gao, X.;
Jones, M. T.; Miller, M. D.; Krause, K. L.; Suenobu, T.; Fukuzumi, S.
J. Phys. Chem. 1996, 100, 16327.
(6) (a) Matsuo, Y.; Iwashita, A.; Abe, Y.; Li, C. Z.; Matsuo, K.;
Hashiguchi, M.; Nakamura, E. J. Am. Chem. Soc. 2008, 130, 15429.
(b) Matsuo, Y.; Sato, Y.; Niinomi, T.; Soga, I.; Tanaka, H.; Nakamura,
E. J. Am. Chem. Soc. 2009, 131, 16048.
(7) (a) Iwashita, A.; Matsuo, Y.; Nakamura, E. Angew. Chem., Int.
Ed. 2007, 46, 3513. (b) Matsuo, Y.; Zhang, Y.; Soga, I.; Sato, Y.;
Nakamura, E. Tetrahedron Lett. 2011, 52, 2240. (f) Kitagawa, T.;
Sakamoto, H.; Takeuchi, K. J. Am. Chem. Soc. 1999, 121, 4298.
(c) Wang, G.-W.; Lu, Y.-M.; Chen, Z.-X. Org. Lett. 2009, 11, 1507.
(d) Murata, Y.; Cheng, F.; Kitagawa, T.; Komatsu, K. J. Am. Chem.
Soc. 2004, 126, 8874. (e) Tajima, Y.; Hara, T.; Honma, T.; Matsumoto,
S.; Takeuchi, K. Org. Lett. 2006, 8, 3203.
(8) (a) Nambo, M.; Itami, K. Chem.;Eur. J. 2009, 15, 4760.
(b) Nambo, M.; Segawa, Y.; Itami, K. J. Am. Chem. Soc. 2011, 133,
2402. (c) Varotto, A.; Treat, N. D.; Jo, J.; Shuttle, C. G.; Batara, N. A.;
Brunetti, F. G.; Seo, J. H.; Chabinyc, M. L.; Hawker, C. J.; Heeger, A. J.;
Wudl, F. Angew. Chem., Int. Ed. 2011, 50, 5166.
(9) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F.; Yao, J.;
Wilkins, C. L. J. Org. Chem. 1995, 60, 532.
(10) Zhang, Y.; Matsuo, Y.; Li, C.-Z.; Tanaka, H.; Nakamura, E.
J. Am. Chem. Soc. 2011, 133, 8086.
r
10.1021/ol202511u
Published on Web 10/25/2011
2011 American Chemical Society

in a higher yield of the phenylated compound 3b.
Using more electron-rich aryl groups (1c and 1d)
resulted in exclusive formation of the arylated pro-
ducts in high yield.
Scheme 1. 1,2- vs 1,4-Addition via a Pentadienyl Species and
Silicon-Directed Regioselection
Inorganic Cu(II) salts, CuX2 (X = Cl, Br, OTf) are
equally effective,13 while inorganic Cu(I), Fe(III), and
Mn(III) salts were entirely ineffective. The use of 6 equiv
(3 equiv/fullerene) was necessary to achieve full conver-
sion of the starting material (a double amount of copper is
necessary while using fullerene monomer 2). The reaction
can be conducted under either nitrogen or air.14
These results suggest a mechanism that a cationic full-
erene intermediate7 was generated by oxidation of a
fullerene radical with CuCl2, and that the intermediate
reacted with the SiꢀPh bond to produce 3, and with the
SiꢀCH2 bond to produce 4.
Table 1. Copper-Mediated Synthesis of 1,2-Di(organo)fullerene
3 from Fullerene Dimer 1a
We discovered this reaction during the screening of
substrates on the CuCl2-mediated synthesis of methano-
fullerene10 from an arylsilylmethylfullerene dimer 1, which
was synthesized very cleanly in quantitative yield by treat-
ment of the corresponding silylmethylfullerene 2 with a base
and an oxidant as described previously.10 The silylmethyl-
fullerene 2 is available in good yield by addition of a
Grignard reagent to [60]fullerene in the presence of
N,N-dimethylformamide.6 Note that 1 formed as a nearly
1:1 mixture of diastereomers, which equilibrate with each
other upon heating at 100 °C via a fullerene radical.11,12
entry
substrate
Ar
R1
3a (%)
4a (%)
1
2
3
4
1a
1b
1c
1d
Ph
Ph
Me
Ph
Me
Me
49 (3a)
67 (3b)
90 (3c)
92 (3d)
46
26
C6H4-4-OMe
C6H4-2-OMe
a See Scheme 2 and the Supporting Information for conditions.
We started with the arylsilylmethyl[60]fullerene 2 by
removal of the proton attached to the fullerene core with
t-BuOK followed by oxidation of the anion with CuCl2
and treatment with MeOH (Table 2). The reaction took
place in the same manner as the one starting with a dimer
(entries 1ꢀ5), while the overall yield of the products was
lower in this one-step synthesis because of some compli-
cations in the first oxidation step. Quenching the reaction
with water afforded a silanol, which was difficult to purify
because of siloxane formation (entry 6),15 and quenching
with i-PrOH gave a stable isopropoxysilane compound 3g.
In this manner, the substituents of the 1,2-di(organo)
fullerene adducts could be easily modified, which is bene-
ficial for their applications in organic electronics devices.
Scheme 2. CuCl2-Mediated Conversion of Silylmethylfullerenes
1 and 2 to Arylfullerene 3 and Methanofullerene 4
Table 2. Copper-Mediated Synthesis of 1,2-Di(organo)fullerene
3 from Arylsilylmethylfullerene 2a
entry substrate
Ar
R1
R2b
3a (%)
4a (%)
When the (phenyldimethyl)silylmethylfullerene dimer
1a was treated with 6 equiv of CuCl2 at 100 °C in o-
dichlorobenzene (ODCB) and then with MeOH, we
obtained a phenylated compound 3a in addition to the
methanofullerene 4 (Scheme 2, Table 1). Increasing the
number of phenyl groups on the silicon atom (1b) resulted
1
2
3
4
5
6
7
2a
2b
2c
2d
2e
2d
2d
Ph
Ph
Me Me
Ph Me
38 (3a) 52
46 (3b) 34
4-MeOC6H4 Me Me
2-MeOC6H4 Me Me
66 (3c)
trace
78 (3d) trace
2-iPrOC6H4
Me Me
67 (3e)
81 (3f)
trace
trace
2-MeOC6H4 Me Hc
2-MeOC6H4 Me iPr
66 (3g) trace
(11) Cheng, F.; Murata, Y.; Komatsu, K. Org. Lett. 2002, 4, 2541.
(12) (a) Morton, J. R.; Preston, K. F.; Krusic, P. J.; Hill, S. A.;
Wasserman, E. J. Am. Chem. Soc. 1992, 114, 5454. (b) Segura, J. L.;
a Isolated yield. b Anhydrous alcohol was used. c Water was used to
quench the reaction.
´
Martın, N. Chem. Soc. Rev. 2000, 29, 13.
Org. Lett., Vol. 13, No. 22, 2011
6059

The reaction of an allyldimethylsilylmethylfullerene 5
under the same conditions afforded an 18:1 mixture of
1,4- and 1,2-compounds 6 (Scheme 3). We speculate that
the reaction initially gave a 1,2-adduct, which subse-
quently underwent a Cope rearrangement during heating.
Brief attempts to probe the mechanism using a crotyl
group were hampered by the formation of a regioisomeric
mixture of the crotylsilane starting material.
Scheme 3
With possible applications to organophotovoltaic re-
search in mind,6,16 we obtained electrochemical data for
these compounds and compared them with those of PCBM.
The cyclic voltammetry measurement was thus performed
to elucidate the electrochemical properties of the 1,2-
diadducts 3aꢀe,gas well as the 1,4-diadduct 6(the 1,4-isomer
of 6) (Table 3). For all compounds, the voltammograms
exhibited three reversible waves on reduction. The first
reduction potentials for all compounds were more negative
than those for PCBM. The aryl group being directly
attached to a fullerene core, we expected that the LUMO
level of fullerenes 3 could be controlled more readily than in
the case of PCBM derivatives where the phenyl group is
attached to the methano bridge on the fullerene core.
We noted that the compounds 3d, 3e, and 3g bearing an
electron-donating alkoxyl group on the phenyl ring at the
2-position showed a higher LUMO level, ꢀ3.73 eV, than
those where it was at the 4-position (3c), suggesting that
the 2-methoxy group donates electrons much more
Figure 1. (a) Crystal structure of C60(CH2SiMe2OEt)(C6H4-2-
OMe). (b) Crystal packing of C60(CH2SiMe2OEt)(C6H4-2-
OMe) along the b axis.
readily than the 4-methoxy group;a rather unconven-
tional observation in the context of standard organic
chemistry. In addition, 3d bearing a methyldiphenylsilyl-
methyl group also has a LUMO level of ꢀ3.73 eV as
distinct from 3a bearing a dimethylsilylmethyl group,
indicating a significant effect of the difference of one
remote phenyl group. This is also anomalous.
We obtained a black crystal of C60(CH2SiMe2OEt)-
(C6H4-2-OMe) (an ethoxysilane instead of the isopropoxy
compound 3g) suitable for crystallographic analysis
(Figure 1) and gained insight into a possible reason for the
anomalously raised LUMO level for the 2-methoxy com-
pounds (i.e., compound 3e). The crystal structure showed a
close packing of fullerene molecules without solvent mole-
cules. The distance between the oxygen atom of the 2-meth-
oxy group on the phenyl ring and the nearest carbon atom
on the fullerene core was found to be 2.712 A, and the
oxygen lone pair was apparently pointing toward the surface
of the fullerene π-system.
Table 3. Reduction Potentials of Di(organo)[60]fullerenes
Compared with Those of PCBMa
compd
E1/2red1/V
E1/2red2/V
E1/2red3/V
LUMO level/eV
An interesting feature is the fullereneꢀfullerene distance
of 3.026 A (for the nearest neighboring carbon atoms),
which is much shorter than the distance of 3.186 A found for
PCBM
3a
ꢀ1.00
ꢀ1.03
ꢀ1.07
ꢀ1.02
ꢀ1.01
ꢀ1.07
ꢀ1.07
ꢀ1.03
ꢀ1.59
ꢀ1.59
ꢀ1.65
ꢀ1.59
ꢀ1.57
ꢀ1.65
ꢀ1.64
ꢀ1.60
ꢀ2.19
ꢀ2.21
ꢀ2.28
ꢀ2.22
ꢀ2.20
ꢀ2.28
ꢀ2.27
ꢀ2.20
ꢀ3.80
ꢀ3.77
ꢀ3.73
ꢀ3.78
ꢀ3.79
ꢀ3.73
ꢀ3.73
ꢀ3.77
3b
3c
3d
(13) The conversion is the same, but the ratio of 3 and 4 is different.
(14) This is true only when a dimer is used.
(15) Whitnall, W.; Cademartiri, L.; Ozin, G. A. J. Am. Chem. Soc.
2007, 129, 15644.
(16) Kooistra, F. B.; Knol, J.; Kastenberg, F.; Popescu, L. M.;
Verhees, W. J. H.; Kroon, J. M.; Hummelen, J. C. Org. Lett. 2007, 9, 551.
(17) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurisic, A. B.; Yip, C.-T.;
Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nat. Mater.
2007, 6, 521.
(18) (a) Chikamatsu, M.; Nagamatsu, S.; Yoshida, Y.; Saito, K.;
Yase, K.; Kikuchi, K. Appl. Phys. Lett. 2005, 87, 203504. (b) Kusai, H.;
Nagano, T.; Imai, K.; Kubozono, Y.; Sako, Y.; Takaguchi, Y.;
Fujiwara, A.; Akima, N.; Iwasa, Y.; Hino, S. Appl. Phys. Lett. 2006,
88, 173509.
3e
3g
6c
a Potential in volts vs a ferrocene/ferrocenium couple was measured by
cyclic voltammetry in THF solution containing Bu4NþPF6 (0.1 M) as a
ꢀ
supporting electrolyte at 25 °C with a scan rate of 0.05 V/s. Glassy carbon,
platinum wire, and Ag/Agþ electrodes were used as working, counter, and
reference electrodes, respectively. b The values for the LUMO level were
estimated using the following equation: LUMO level = ꢀ(4.8þ E1/2 red1) eV.
See ref 17. c The data is for the 1,4-isomer. The two isomers can be isolated.
6060
Org. Lett., Vol. 13, No. 22, 2011

a structurally related bis(dimethylphenylsilylmethyl)[60]-
fullerene (SIMEF).6 This feature is potentially beneficial
for n-type semiconductors18,19 and is to be examined in
depth in the future.
In summary, we have achieved the selective synthesis of
1,2-di(organo)fullerenes, which have so far been difficult
to obtain. This new method allows us to synthesize these
compounds in two to three steps from [60]fullerene in
40ꢀ75% overall yields. The experimental observations
suggest that the present reaction as well as the cyclopro-
panation that we reported recently10 take place via a
cationic fullerene intermediate generated either from a
dimer or from a fullerene anion by oxidation with Cu(II).
Acknowledgment. This study was partially supported
byKAKENHI(No. 22000008), the Global COE Program,
“Chemistry Innovation through Cooperation of Science
and Engineering”, MEXT, Japan, and Strategic Promo-
tion of Innovative Research and Development from Japan
Science and Technology Agency (JST). Y.M. thanks the
Funding Program for Next Generation World-Leading
Researchers.
(19) (a) Svanstrom, C. M. B.; Rysz, J.; Bernasik, A.; Budkowski, A.;
Zhang, F.; Inganas, O.; Andersson, M. R.; Magnusson, K. O.; Benson-
Smith, J. J.; Nelson, J.; Mons, E. Adv. Mater. 2009, 21, 4398. (b) Zhu, H.; Li,
T.; Zhang, Y.; Dong, H.; Song, J.; Zhao, H.; Wei, Z.; Xu, W.; Hu, W.; Bo,
Z. Adv. Mater. 2010, 22, 1645. (c) Motiei, L.; Yao, Y.; Choudhury, J.; Yan,
H.; Marks, T. J.; van der Boom, M. E.; Facchetti, A. J. Am. Chem. Soc.
2010, 132, 12528.
Supporting Information Available. Synthetic proce-
dures for and characterization of 1,2-di(organo)fullerene
derivatives, X-ray crystal structure, and their electroche-
mical properties. This material is available free of charge
via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 22, 2011
6061