CuBr/PMDETA-mediated reactions of [60]fullerene with active halides: Preparation of methano[60]fullerene derivatives

Article abstract of DOI:10.1002/ejoc.201200759

An efficient protocol for the synthesis of methano[60]fullerene derivatives linked with a single electron-withdrawing group has been developed through one-step reaction of [60]fullerene with active halides mediated by CuBr/pentamethyldiethylenetriamine. M

Full text of DOI:10.1002/ejoc.201200759

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DOI: 10.1002/ejoc.201200759
CuBr/PMDETA-Mediated Reactions of [60]Fullerene with Active Halides:
Preparation of Methano[60]Fullerene Derivatives
Hai-Tao Yang,*[a] Zong-Yong Tian,[a] Xiao-Jiao Ruan,[a] Min Zhang,[a] Chun-Bao Miao,[a]
and Xiao-Qiang Sun*[a]
Keywords: Fullerenes / Copper / Cycloaddition / Halides
An efficient protocol for the synthesis of methano[60]fuller-
ene derivatives linked with a single electron-withdrawing
group has been developed through one-step reaction of
esters, ketones, amides, and sulfonyl groups could be easily
introduced onto the fullerene skeleton by using this method-
ology. A plausible reaction mechanism, but not the atom
[60]fullerene with active halides mediated by CuBr/penta- transfer radical process, was proposed for the formation of
methyldiethylenetriamine. Many functional groups including
the methano[60]fullerene derivatives.
the substrates are restricted to active methylene compounds
often with two electron-withdrawing groups. Sulfonium
ylides, phosphonium ylides, and silylated nucleophiles need
to be prepared from halide compounds. The diazo com-
pounds always yield fulleroids as byproducts and the carb-
ene is difficult to prepare. Therefore, an easier method for
the preparation of cyclopropanated [60]fullerenes is still
needed. Herein, we report a convenient method for the syn-
thesis of methano[60]fullerenes linked with a single elec-
tron-withdrawing group through reaction of [60]fullerene
with active halide compounds mediated by CuBr/PMDETA
(pentamethyldiethylenetriamine).
Introduction
The fascinating structure and properties of fullerenes
have led to rapid development of their chemistry in the last
two decades. The functionalization of [60]fullerene with
various organic functional groups is an important subject
in fullerene chemistry, as these derivatives can be used in
materials and medicinal applications. Among a large
number of functionalized [60]fullerene compounds, meth-
ano[60]fullerene derivatives have been one of the most in-
tensively studied classes.[1] Since Wudl showed that water-
soluble methano-bridged C60 compounds inhibited certain
HIV enzymes,[2] several methods for the synthesis of meth-
ano[60]fullerenes have been developed (Scheme 1): (1) The
reaction of C60 with halogenated active methylene com-
pounds in the presence of base (Bingel reaction) is one of
the most widely applied reactions in fullerene chemistry to
give various cyclopropanated fullerene derivatives.[3] (2) The
addition of diazo compounds to C60 followed by photolysis
or thermolysis of the pyrazoline compounds produces the
methanofullerenes and fulleroids through extrusion of N2.[4]
(3) The reaction of phosphonium ylides or sulfonium
ylides with C60 gives selectively [6,6]-methanofullerenes.[5]
(4) Fluoride ion mediated reactions of silylated nucleophiles
with [60]fullerene affords selectively [6,6]-methanofuller-
enes.[6] (5) The addition of free carbene to C60 has also been
studied.[7] CuII acetate or MnIII acetate mediated reaction
of carbonyl compounds with C60 to form methanofullerene
as a byproduct has also been reported.[8] Nevertheless, these
reactions have some limitations. As for the Bingel reaction,
Scheme 1. Previous methods for the preparation of methano[60]-
fullerene derivatives.
Free radical reactions of fullerenes were amongst the first
to be investigated and are still attractive protocols to syn-
thesize fullerene derivatives.[9] Most recently, it was reported
the reaction of C60 with active alkyl bromides in the pres-
ence of CoCl2/Mn/ligand/H2O system gave the monoalkyl-
ated [60]fullerene probably through a radical process.[10] On
[a] School of Petrochemical Engineering, Changzhou University,
Changzhou 213164, P. R. China
E-mail: yanght898@yahoo.com.cn
chemsxq@yahoo.com.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200759.
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H.-T. Yang, X.-Q. Sun et al.
FULL PAPER
the other hand, the atom transfer radical reaction (ATR) (12 mL) at 110 °C for 6 h. To our surprise, no anticipated
process has been extensively investigated in polymerization radical addition product was observed. Instead, undesired
and cyclization reactions.[11] A radical species was generated methano[60]fullerene derivative 2a was obtained in 10%
from an organic halide or a related compound through a yield along with 87% of recovered C60 (Table 1, entry 3).
reversible redox process catalyzed by a transition metal Atmospheric oxygen did not show a marked influence on
complex. A number of groups have reported that transition the reaction (Table 1, entry 4). Although no radical ad-
metal-catalyzed atom transfer radical reactions of haloge- dition product was isolated, this procedure provides an ef-
nated compounds with olefins for the formation of C–C ficient method for the preparation of methano[60]fullerenes
bonds.[12] However, there are rare reports on the application linked with a single electron-withdrawing group through the
of this ATR reaction to functionalization of [60]fullerene in direct reaction with halides. However, the conversion effi-
the field of polymer science up to now.[13] We were wonder- ciency and product yield needed to be further improved.
ing whether the carbon radical generated from halide com- When the amount of CuBr and PMDETA were increased
pound by the copper(I) halide could be captured by C60, to 4 equiv., the yield of 2a improved to 45% (Table 1, en-
and generated 1,2- or 1,4 adducts of C60.
try 6). A further increase in the amount of CuBr and
PMDETA to 6 equiv. resulted in generation of more by-
products (Table 1, entry 7). It should be emphasized that
CuBr and the PMDETA ligand were both crucial to the
success of this reaction, as no conversion was observed in
Results and Discussion
Ethyl bromoacetate (1aa) was chosen as a model sub- the absence of either one (Table 1, entries 1 and 2). Replac-
strate, and the reaction of C60 with 1aa was carried out in ing CuBr with CuI, CuCl, or Cu2O resulted in a decrease
the presence of different copper(I or II) salts and ligands in the yield (Table 1, entries 8–10). CuII could not initiate
(Table 1). A mixture of C60 (0.05 mmol) and 1aa (2 equiv.) the reaction under similar conditions (Table 1, entries 11
was treated with CuBr (1 equiv., purified according to a re- and 12). Other ligands such as TMEDA (N,N,NЈ,NЈ-tetra-
ported procedure[14]) and PMDETA (1 equiv.) in toluene methylethylenediamine), DMEDA (N,NЈ-dimethylethyl-
enediamine), TETA (triethylenetetramine), 2,2Ј-bipyridine,
EDTA-4Na (ethylene diamine tetraacetic acid tetrasodium),
DABCO, and PPh3 were also tried. Unfortunately, they
Table 1. Screening of reaction conditions.[a]
were all ineffective in this reaction (Table 1, entries 13–19).
Eventually, the reagent molar ratio of C60/1aa/CuBr/
PMDETA = 1:2:4:4 and a reaction temperature of 110 °C
were chosen as the optimal reaction conditions (Table 1, en-
try 6).
With the optimized conditions in hand, this kind of cy-
clopropanation reaction was extended to various halide
compounds (Table 2). All examined halides 1aa–1k (except
1d and 1l) could be utilized to prepare methano[60]fuller-
Entry Copper salt (equiv.)
Ligand (equiv.)
Time
[h]
Yield
[%][b]
enes 2 in 15–55% yield (41–96% based on consumed C60).
As for the bromoacetate, when R was an aliphatic ester
group the reaction proceeded well and gave good yields of
2 (Table 2, entries 1, 4, and 6), and the tert-butyl group was
found to be superior to both the ethyl and methyl groups.
However, when R was a phenyl ester, no conversion was
observed (Table 2, entry 7). Meanwhile, the halogen atom
of the halide compound also had an influence on this reac-
tion, and the active sequence is I Ͼ Br Ͼ Cl (Table 2, en-
tries 1–5). From the viewpoint of easy preparation and sta-
bility, the bromide was selected for most substrates. α-Hal-
ide ketones were also suitable for this reaction, but a lower
reaction temperature was needed (Table 2, entries 8–12). α-
Bromoaromatic ketones gave better results than did α-
haloaliphatic ketones. The presence of both electron-donat-
ing and electron-withdrawing groups on the phenyl ring was
tolerated, and 2 could be obtained in good yield (Table 2,
entries 8–10). Bromomethylaryl sulfone 1j also participated
in the reaction to afford 2j in moderate yield when the reac-
tion temperature was increased to 140 °C in chlorobenzene
(Table 2, entry 12). The sulfone structure has never been re-
ported to undergo this type of reaction. Secondary amide
1
2
CuBr (1.0)
none
none
6
6
6
6
6
4
4
4
4
36
24
24
12
24
24
24
24
24
12
0
0
PMDETA (1.0)
PMDETA (1.0)
PMDETA (1.0)
PMDETA (2.0)
PMDETA (4.0)
PMDETA (6.0)
PMDETA (4.0)
PMDETA (4.0)
PMDETA (4.0)
PMDETA (4.0)
PMDETA (4.0)
TMEDA (4.0)
2,2Ј-bipyridine (4.0)
DMEDA (4.0)
TETA (4.0)
3
CuBr (1.0)
CuBr (1.0)
CuBr (2.0)
CuBr (4.0)
CuBr (6.0)
CuI (4.0)
10 (87)
11 (81)
26 (84)
45 (85)
47 (55)
33 (90)
29 (84)
15 (77)
0
0
trace
0
0
0
0
0
0
4[c]
5
6
7
8
9
CuCl (4.0)
Cu2O (4.0)
CuBr2 (4.0)
Cu(OAc)2 (4.0)
CuBr (4.0)
CuBr (4.0)
CuBr (4.0)
CuBr (4.0)
CuBr (4.0)
CuBr (4.0)
CuBr (4.0)
10
11
12
13
14
15
16
17
18
19
EDTA-4Na (4.0)
Ph3P (4.0)
DABCO (4.0)
[a] Unless otherwise specified, all reactions were performed with
C60 (0.05 mmol), 1aa (0.10 mmol), and the given amount of CuBr/
ligand in toluene (12 mL) at 110 °C for the designated time. [b] Iso-
lated yield. The yield in parentheses is based on consumed C60.
[c] Performed under a N2 atmosphere.
2
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Preparation of Methano[60]Fullerene Derivatives
1k was also tolerant to this reaction (Table 2, entry 13). PMDETA attacks C60 to produce fullerene radical 4, which
Nevertheless, the primary amide failed to react with C60 reacts with CuBr2 to form 1,2-adduct 5. PMDETA can also
under similar conditions.
act as a base to generate carbanion 6, and subsequent intra-
molecular SN reaction gives methano[60]fullerene derivative
Table 2. CuBr/PMDETA-mediated reaction of C60 with active hal- 2.
ides.[a]
Scheme 2. Possible ATR process for the generation of 2.
To substantiate this mechanism, monoalkylated fullerene
7 was prepared according to a literature procedure.[10] We
expected that bromination of 7 under basic conditions
would generate intermediate 5[17] and that subsequent intra-
molecular SN reaction would afford methanofullerene 2a.
Nevertheless, treatment of 7 with Br2 (1 equiv.) and
PMDETA (3 equiv.) in orthodichlorobenzene (ODCB) at
room temperature for 1 h gave single-bonded fullerene di-
mer 8 in 80% yield (Scheme 3, conditions a). Further inves-
tigation revealed that 7 could be converted into 8 directly
under basic conditions (PMDETA or tBuOK) in the ab-
sence of Br2 (Scheme 3, conditions b and c). This proved
[a] Unless otherwise specified, the molar ratio of C60/1/CuBr/
PMDETA = 1:2:4:4. [b] Isolated yield. The yield in parentheses was
based on consumed C60. [c] Molar ratio of C60/1i/CuBr/PMDETA
= 1:10:4:4. [d] Molar ratio of C60/1j/CuBr/PMDETA = 1:20:4:4,
solvent: chlorobenzene.
All of the known products were confirmed by compari- that conversion of 7 into 8 is a radical process. Under basic
–
son of their spectroscopic data with those reported in litera- conditions, monoanion RC60 (R = CH2CO2Et) generated
ture. The identification of new compounds 2h and 2j was from 7 underwent one-electron oxidation by O2 to afford
fully confirmed by their MS, 1H NMR, and 13C NMR fullerene radical 4. Coupling of the monomer radicals
spectra. As an example, the APCI mass spectrum of 2h would give dimer 8. A similar process for the formation of
1
showed a molecular ion peak at m/z = 834. The H NMR a singly bonded C60 dimer using I2 as the oxidant or cata-
spectrum of 2h showed a singlet at δ = 5.79 ppm for the lyzed by (CuOAc)2 has been reported.[18] If the generation
methine proton, triplets at δ = 4.53 ppm, and quartets at δ of 2 is an ATR process, as shown in Scheme 2, capturing
= 1.53 ppm for the ethoxy group protons. In the 13C NMR radical intermediate 4 with CuBr2/PMDETA would yield 2.
spectrum of 2h, there were two signals at δ = 183.7 and However, when 7 was treated with PMDETA (4 equiv.),
159.7 ppm for the two carbonyl carbon atoms, 29 peaks due CuBr (3 equiv.), and CuBr2 (1 equiv.) at 110 °C for 1 h, all
to sp2 carbon atoms of the C60 skeleton at δ = 136.69– the starting material disappeared and methanofullerene 2a
147.63 ppm, one peak at δ = 71.91 ppm for the sp3 carbon was not detected. Instead, dimer 8 and lactone 9 were iso-
atom of the C60 cage, one peak at δ = 40.96 ppm for the lated in 61% and 20% yield, respectively (Scheme 3, condi-
methine carbon atom, and two peaks at δ = 63.38 and tions d). When tBuOK was used as the base and the
14.25 ppm for the ethoxy group, agreeing well with its Cs amount of CuBr2 was increased to 6 equiv., 7 could be con-
symmetry. Compound 2j was also fully characterized in a verted into lactone 9 (82%) completely after 4 h at 110 °C
similar way and exhibited spectral patterns similar to those (Scheme 3, conditions e). Moreover, 8 also could be trans-
of 2j.
formed into lactone 9[19] in 87% yield upon treatment with
A possible ATR mechanism based on those reported in 5 equiv. of CuBr2 at 100 °C for 3 h (Scheme 3, conditions f).
the literature is depicted in Scheme 2. Carbon radical 3 gen- These results proved that the reaction of [60]fullerene with
erated from halide compound 1 in the presence of CuBr/ active halides under ATR conditions is not a radical pro-
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FULL PAPER
cess, which also could be supported by the fact that no reac- generates 2. In path B, deprotonation of 10 generates carb-
tion was observed when C60 was treated with benzyl brom- anion 13, which attacks C60 to form 14 and subsequent in-
ide (Table 2, entry 15).
tramolecular SN reaction affords 2.
Conclusions
In conclusion, a wide variety of methano[60]fullerenes
linked with a single electron-withdrawing group were syn-
thesized conveniently through the one-step reaction of
active halides with C60 mediated by CuBr/PMDETA. A
possible reaction pathway, but not the ATR mechanism,
was proposed to explain the cyclopropanation of C60.
Supporting Information (see footnote on the first page of this arti-
cle): General methods; experimental procedures for the preparation
of methanofullerenes 2, 8, and 9; spectral data for products 2h, 2j,
1
8, and 9; H NMR and 13C NMR spectra of the products.
Acknowledgments
The authors are grateful for financial support from the National
Natural Science Foundation of China (Nos. 20902039) and the Pri-
ority Academic Program Development of Jiangsu Higher Educa-
tion Institutions.
Scheme 3. Mechanism investigation. Conditions: (a) Br2 (1 equiv.),
PMDETA (3 equiv.), ODCB, r.t., 1 h, 80%; (b) PMDETA
(3 equiv.), ODCB, r.t., 6 h, 50%; (c) tBuOK (2 equiv.), r.t., 30 min,
94%; (d) PMDETA (4 equiv.), CuBr (3 equiv.), and CuBr2
(1 equiv.), ODCB, 110 °C, 1 h, gave a mixture of 8 (61%) and 9
(20%); (e) tBuOK (3 equiv.), CuBr2 (6 equiv.), ODCB, 100 °C, 4 h,
82%; (f) CuBr2 (5 equiv.), ODCB, 100 °C, 3 h, 87%.
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Received: June 7, 2012
Published Online: ᭿
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Job/Unit: O20759
/KAP1
Date: 02-08-12 11:51:36
Pages: 6
H.-T. Yang, X.-Q. Sun et al.
FULL PAPER
Cycloaddition
H.-T. Yang,* Z.-Y. Tian, X.-J. Ruan,
M. Zhang, C.-B. Miao,
X.-Q. Sun* ........................................ 1–6
CuBr/PMDETA-Mediated Reactions of
[60]Fullerene with Active Halides: Prepara-
tion of Methano[60]Fullerene Derivatives
An efficient method for the preparation of
methano[60]fullerene derivatives linked
with a single electron-withdrawing group
has developed through the one-step reac-
tion of [60]fullerene with active halides
mediated by CuBr/pentamethyldiethyl-
enetriamine (PMDETA). A plausible reac-
tion mechanism, but not the atom transfer
radical process, was proposed.
Keywords: Fullerenes / Copper / Cyclo-
addition / Halides
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