Are the pyrazolines formed from the reaction of [60]fullerene with alkyl diazoacetates unstable?

Article abstract of DOI:10.1016/j.tet.2004.02.074

[60]Fullerene-fused pyrazolines 1 were prepared by the reaction of C ₆₀ with alky diazoacetates under the solid-state high-speed vibration milling conditions as well as in toluene solution. Pyrazolines 1 were stable in refluxing toluene and its thermolysis process in 1,2-dichlorobenzene was investigated, the decomposition rates and activation energies of pyrazolines 1 were obtained. The current work demonstrated that the liquid-phase reaction of C₆₀ with alkyl diazoacetates undergoes via 1,3-dipolar cycloaddition pathway at room temperature, or proceeds via carbene mechanism at a temperature of refluxing toluene, thus clarifies the previous ambiguity of its reaction mechanism.

Full text of DOI:10.1016/j.tet.2004.02.074

Tetrahedron 60 (2004) 3921–3925
Are the pyrazolines formed from the reaction of [60]fullerene
with alkyl diazoacetates unstable?
†
,
Guan-Wu Wang,a,b Yu-Jin Li,a,b Ru-Fang Peng,a Zhen-Hua Lianga and You-Cheng Liua,b
,
*
aDepartment of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China
bState Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, People’s Republic of China
Received 22 September 2003; revised 31 January 2004; accepted 23 February 2004
Abstract—[60]Fullerene-fused pyrazolines 1 were prepared by the reaction of C60 with alky diazoacetates under the solid-state high-speed
vibration milling conditions as well as in toluene solution. Pyrazolines 1 were stable in refluxing toluene and its thermolysis process in
1,2-dichlorobenzene was investigated, the decomposition rates and activation energies of pyrazolines 1 were obtained. The current work
demonstrated that the liquid-phase reaction of C60 with alkyl diazoacetates undergoes via 1,3-dipolar cycloaddition pathway at room
temperature, or proceeds via carbene mechanism at a temperature of refluxing toluene, thus clarifies the previous ambiguity of its reaction
mechanism.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
developed because fullerenes have low solubility in
common organic solvents and some unusual fullerene
reactions could only occur in the solid-state reaction.9
Since the first solid-state reaction of C60 with ethyl
bromoacetate and zinc under high-speed vibration milling
(abbreviated as HSVM) conditions was studied in 1996,10
there have been reports on reactions of C60 catalyzed by
various potassium salts, alkaline metals, or solid amines to
prepare fullerene dimers and trimers,11 [4þ2] reaction of
C60 with condensed aromatic compounds,12 with phthala-
zine13 and with di(2-pyridyl)-1,2,4,5-tetrazine,14 reaction of
C60 with dichlorodiphenylsilane and lithium,15 reaction
of C60 with organic bromides and alkali metals,16 reaction
of C60 and N-alkylglycines with and without aldehydes,17
reaction of C60 with active methylene compounds18 under
the HSVM conditions. As a continuation of the mecha-
nochemical reactions of fullerenes under the HSVM
conditions, we have investigated the mechanochemical
reaction of C60 with diazo compounds. Preliminary work on
the HSVM reaction of C60 with 9-diazofluorene has been
described.9b In that case, the pyrazoline intermediate
generating the final methanofullerene product could not be
isolated. However, when we conducted the reaction of C60
with alkyl diazoacetates, we found that the formed pyrazo-
line could be isolated and turned out stable in refluxing
toluene, and thus questioned the mechanism of the
formation of methanofullerene and fulleroid from the
reaction of C60 with alkyl diazoacetates in refluxing toluene
via 1,3-dipolar cycloaddition pathway.2b,5a In this paper, we
report the preparation of fullerene-fused pyrazolines under
the solid-state HSVM conditions as well as in the liquid-
phase solution and their thermolysis behavior.
Nucleophilic additions, free radical additions, carbene
additions, 1,3-dipolar cycloadditions are widely known to
occur with fullerenes.1 The addition of diazo compounds to
[60]fullerene (C60) is one of the first investigated reactions
in fullerene chemistry.2 The reaction of C60 with mono- and
diphenyldiazomethane,2,3 diazomethane and dimethyl-
diazomethane,4 diazoacetates,5 diazomalonates6 and diazo-
amides7 has been reported. Preliminary result of the reaction
of C60 with ethyl diazoacetate was first described by Wudl
in a review,2b and detailed investigation of the reaction of
C60 with alkyl diazoacetates in refluxing toluene was later
on reported by Diederich’s group.5a In these reactions, they
could not obtain the evidence of the formation of the
pyrazoline intermediates.5a The isolated pyrazoline com-
pounds from the reaction of C60 with diazomethane4a and
monoalkyl diazomethanes8 were reported to decompose in
refluxing toluene and at 70 8C or below, respectively. All
these studies seem to support the claim that pyrazolines
formed from the reaction of C60 with diazo compounds are
unstable at a temperature of refluxing toluene or below.
Solvent-free mechanochemical reactions of fullerenes were
Keywords: C60; Diazo compounds; Pyrazoline; Glycine ester; Sodium
nitrite; Thermolysis; Solvent-free; High-speed vibration milling.
*
Corresponding author. Address: Department of Chemistry, University of
Science and Technology of China, Hefei, Anhui 230026, People’s
Republic of China. Tel./fax: þ86-551-360-7864;
e-mail address: gwang@ustc.edu.cn
†
Visiting Scholar from College of Material Sciences, Southwest
University of Science and Technology.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.02.074

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G.-W. Wang et al. / Tetrahedron 60 (2004) 3921–3925
Scheme 1.
2. Results and discussion
absorption at 1560 cm21 for NvN vibration in the IR
spectrum,4a,8 and the Cs symmetry rather than C1 symmetry
inferred from the 13C NMR spectrum all support the
conclusion that the isolated product is 2-pyrazoline 1 rather
than 1-pyrazoline 2.
The reaction of glycine ethyl ester hydrochloride with
sodium nitrite was utilized to prepare ethyl diazoacetate in
situ under the HSVM conditions. A mixture of C60, glycine
ethyl ester hydrochloride and sodium nitrite in a molar ratio
of 1:1:2 was vigorously milled for 30 min under the HSVM
conditions to give pyrazoline 1a in 48% yield (87% based
on consumed C60) (Scheme 1).
Pyrazolines 1a and 1b could be prepared in 51% yield by
liquid-phase reaction of C60 with glycine ester hydrochlo-
ride and sodium nitrite at a molar ratio of 1:5:200 in toluene
at room temperature. But this liquid-phase reaction required
large excess of insoluble reagents and much longer reaction
time to reach comparable yield as under the solvent-free
HSVM conditions. This clearly shows the advantage of the
solid-state HSVM reaction over the liquid-phase reaction.
The structure of the [60]fullerene-fused pyrazoline deriva-
1
tive 1a was determined by MS, IR, UV–vis, and H NMR
spectral data. The APCI MS of 1a gave M2 at 834 as the
base peak. The IR spectrum of 1a showed absorptions at
3250, 1706, and 1546 cm21 for the N–H, carbonyl and
CvN groups, respectively, besides those peaks at 1425,
1172, 573 and 528 cm21 for the C60 skeleton. The sharp
absorption at 425 nm in the UV–vis spectrum is character-
istic for the closed [6,6]-adducts.1 The 1H NMR spectrum of
1a exhibited a quartet at 4.44 ppm and a triplet at 1.47 ppm
of the OCH2CH3 group and a singlet at 7.91 ppm of the
acidic N–H. Due to the very low solubility of 1a, its 13C
NMR spectrum could not be obtained.
Diederich and co-workers reported that the reaction of C60
with alkyl diazoacetates in refluxing toluene afforded a
mixture of methanofullerene and fulleroids, but the pyrazo-
line intermediates were not obtained.5a Our anticipated
products from the reaction of C60 with glycine ester
hydrochloride and sodium nitrite under the HSVM con-
ditions are the mixture of compounds 3, 4 and 5. Therefore
the isolation of pyrazoline 1 was surprising. We then carried
out the reaction of C60 with ethyl diazoacetate at room
temperature in order to see if we could obtain pyrazoline 1a.
To our delight, the reaction of C60 with 10 equiv. of ethyl
diazoacetate in toluene for 10 h at room temperature gave
In order to ascertain the pyrazoline structure for the product
obtained, we employed the glycine ester with long alkyl
chain to increase the solubility of the product and thus
measured its 13C NMR spectrum. [60]Fullerene-fused
pyrazoline 1b was prepared in the same way as 1a by
using glycine octyl ester hydrochloride in place of glycine
ethyl ester hydrochloride. Pyrazoline 1b was fully charac-
terized by the above spectral means. As expected,
satisfactory 13C NMR spectrum of 1b was obtained. Twenty
nine lines including one overlapping line between 135–
148 ppm of the sp2 carbons and two peaks at 88.45 and
77.38 ppm of the sp3 carbons for the [60]fullerene skeleton
of 1b were observed, indicating its Cs symmetry.
2-Pyrazolines 1 were obtained via isomerizations of
1-pyrazolines 2 which were formed directly by the 1,3-di-
polar cycloadditions of alkyl diazoacetates generated in
situ from glycine ester hydrochlorides and sodium nitrite
with C60 (Scheme 2). The observation of acidic proton at
1
about 8 ppm for N–H in the H NMR spectrum, which
disappeared upon the addition of D2O, the lack of the
Scheme 3.
Scheme 2.

G.-W. Wang et al. / Tetrahedron 60 (2004) 3921–3925
3923
pyrazoline 1a (Scheme 3) in 47% yield (96% based on
consumed C60) with no evidence of the formation of
compounds 3a, 4a and 5a. When the reaction was conducted
in refluxing toluene, only bridged fullerenes 3a, 4a and 5a
were obtained, as reported by Diederich’s group. Mean-
while, the HSVM reaction of C60 with pre-formed ethyl
diazoacetate in a molar ratio of 1:1 for only 6 min afforded
pyrazoline 1a as the major product in 45% yield along
with a mixture of compounds 3a, 4a and 5a in 19% yield
(Scheme 3).
Table 1. Decomposition rates at different temperatures and activation
energies of pyrazolines 1a and 1b
Compound
1a
1b
Temperature (8C)
k£102 (h21
160
2.87
170
7.03
180
14.87
160
3.05
170
6.42
180
14.65
)
Ea (kJ mol21
)
134
128
Diederich and co-workers conducted the reaction of C60
with ethyl diazoacetate in refluxing toluene for 7 h and only
obtained methanofullerene 3a and fulleroids 4a and 5a in a
ratio of 1:1:3.5a They stated that both 1,3-dipolar cyclo-
addition followed by rapid loss of N2 as well as thermal
decomposition of diazo compound followed by addition of
the formed carbene could explain the formation of the
methanofullerene 3 and fulleroid compounds 4 and 5. Even
though they did not obtain any evidence of the formation
of pyrazoline intermediates, they could not exclude the
1,3-dipolar cycloaddition mechanism. They proposed that
both mechanisms could actually occur concurrently in the
refluxing toluene.5a We found that when 2-pyrazoline 1a
was heated in refluxing toluene for the same reaction time
(7 h) as for the reaction of C60 with ethyl diazoacetate, only
about 5% of a mixture of 3a, 4a, and 5a was obtained. This
fact along with the thermolysis behavior of pyrazolines 1
indicates that once pyrazolines 1 are formed through the
1,3-diploar cycloaddition under the HSVM conditions or at
room temperature in toluene, it is hard to lose N2 to form
bridged fullerene derivatives 3, 4 and 5 at the temperature of
refluxing toluene. Therefore compounds 3, 4 and 5 resulting
from the reaction of C60 with alkyl diazoacetates in
refluxing toluene should be formed via carbene mechanism.
Our observed transformation of 2-pyrazoline 1 into
methanofullerene 3 and fulleroids 4 and 5 should proceed
through the loss of N2 from 1-pyrazoline 2, which is
generated by unfavorable rearrangement of the conjugated
2-pyrazoline 1 and thus requires high temperature
(Scheme 4).
Methanofullerene 3 and fulleroids 4 and 5 might be formed
through the loss of N2 from 1-pyrazoline 2.2b,5a With
isolated pure 2-pyrazoline 1 in hand, 1 we investigated its
thermal stability and the possible transformation into
compounds 3, 4 and 5 through the loss of N2. 2-Pyrazoline
1a was stable in refluxing toluene. However, when pyrazo-
line 1a was heated in 1,2-dicholorobenzene (ODCB) at
180 8C for 10 h, most of 1a decomposed to a mixture of
compounds 3a, 4a and 5a, as confirmed by HPLC analysis
and 1H NMR measurement. The thermal decomposition
process of pyrazolines 1a and 1b at different temperatures in
ODCB was followed by HPLC analysis on an analytical
Buckyprep column with 326 nm as the detection wave-
length. The concentration of remained pyrazoline 1 after
heating for a certain time was determined by the initial
concentration of 1 and the relative peak areas of 1 and
products 3, 4 and 5. As shown in Figures 1 and 2, ln C vs
reaction time displayed good linear relationship at different
temperatures. The decomposition of pyrazoline 1 was
therefore a first-order reaction. The derived decomposition
rates at different temperatures and activation energies of
pyrazolines 1a and 1b are listed in Table 1.
Scheme 4.
Figure 1. The linear plot of ln C vs reaction time of 1a at temperatures of
160, 170 and 180 8C with initial concentration of 3.0£1024 M.
Since the known 1-pyrazoline-fused fullerenes are reported
to decompose easily in refluxing toluene,4a 70 8C or even
slightly elevated temperature (.20 8C),8 the high activation
energy required for the formation of methanofullerene 3 and
fulleroids 4 and 5 from 2-pyrazoline 1 probably reflects that
the rearrangement of the conjugated 2-pyrazoline 1 to non-
conjugated 1-pyrazoline 2 is highly energy-consuming and
is the rate-determining step.
The temperature of the contents inside the capsule of the
high-speed vibration mill does not reach up to 100 8C,11b
which is lower than the decomposition temperature of
pyrazoline 1. The HSVM reaction of C60 with ethyl
diazoacetate giving a mixture of 1a, 3a, 4a, and 5a indicated
that both 1,3-dipolar cycoaddition and carbene mechanisms
were working. Ethyl diazoacetate may not be stable under
Figure 2. The linear plot of ln C vs reaction time of 1b at temperatures of
160, 170 and 180 8C with initial concentration of 2.7£1024 M.

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G.-W. Wang et al. / Tetrahedron 60 (2004) 3921–3925
our HSVM conditions and part of ethyl diazoacetate may
lose N2 to generate carbene species, which reacts with C60 to
form 3a, 4a and 5a in a ratio (1:1.3:2.7) close to that in
the liquid-phase reaction of C60 with ethyl diazoacetate in
refluxing toluene.
(2.8 mg, 0.04 mmol) was vigorously vibrated for 30 min.
The combined reaction mixture from two runs was
separated on a silica gel column with toluene as the eluent
to give unreacted C60 (13.0 mg, 45%) and pyrazoline 1a
1
(16.0 mg, 48%). 1a: H NMR (300 MHz, CS2/CDCl3) d
(ppm) 7.91 (s, 1H), 4.44 (q, J¼7.2 Hz, 2H), 1.47 (t, J¼
7.2 Hz, 3H); FT-IR (KBr) n (cm21) 3300, 2977, 2952, 2867,
1706, 1546, 1425, 1373, 1329, 1172, 1133, 1098, 1016, 796,
772, 573, 528; UV–vis (CHCl3) lmax (nm) 246, 275, 313,
425, 684; APCI MS m/z 834 (M2).
In summary, [60]fullerene-fused pyrazolines 1 prepared by
the reaction of C60 with alky diazoacetates under the solid-
state HSVM conditions and in toluene solution turned out
stable at a temperature of refluxing toluene. The thermolysis
process at higher temperatures in 1,2-dichlorobenzene
was investigated, the decomposition rates and activation
energies of [60]fullerene-fused pyrazolines 1 were obtained.
The present work establishes that the liquid-phase reaction
of C60 with alkyl diazoacetates undergoes via 1,3-dipolar
cycloaddition pathway at room temperature and proceeds
via carbene mechanism at the temperature of refluxing
toluene, thus clarifies the previous ambiguity of its reaction
mechanism.5a
3.1.2. Reaction of C60 with glycine octyl ester hydro-
chloride and sodium nitrite under the HSVM conditions.
A mixture of C60 (14.4 mg, 0.02 mmol), glycine octyl ester
hydrochloride (6.8 mg, 0.03 mmol) and sodium nitrite
(4.2 mg, 0.06 mmol) was vigorously vibrated for 30 min.
The combined reaction mixture from two runs was
separated on a silica gel column eluted with toluene to
give unreacted C60 (13.0 mg, 45%) and pyrazoline 1b
1
(17.9 mg, 49%). 1b: H NMR (300 MHz, CS2/CDCl3) d
(ppm) 8.01 (s, 1H), 4.39 (t, J¼6.7 Hz, 2H), 1.85–1.75 (m,
2H), 1.43–1.28 (m, 10H), 0.89 (t, J¼6.7 Hz, 3H); 13C NMR
(75 MHz, CS2/CDCl3) d (ppm) 161.78 (COO), 147.63 (1C),
147.42 (2C), 147.15 (1C), 146.30 (2C), 146.25 (2C), 146.01
(2C), 145.95 (2C), 145.90 (2C), 145.69 (2C), 145.50 (2C),
145.21 (2C), 145.18 (2C), 144.36 (2C), 144.32 (2C), 144.20
(2C), 144.09 (2C), 143.93 (2C), 143.05 (2C), 142.88 (2C),
142.79 (2C), 142.56 (2C), 142.34 (2C), 142.29 (2C), 142.18
(4C), 141.83 (2C), 140.35 (2C), 140.47 (2C), 136.30 (2C),
135.86 (2C), 134.72 (CvN), 88.45 (sp3-C of C60 cage),
77.38 (sp3-C of C60 cage), 66.17 (OCH2), 32.04 (CH2),
29.52 (CH2), 29.47 (CH2), 28.94 (CH2), 26.30 (CH2), 22.98
(CH2), 14.36 (CH3); FT-IR (KBr) n (cm21) 3285, 2920,
2850, 1701, 1542, 1462, 1427, 1330, 1169, 1133, 1098,
1020, 974, 772, 574, 527; UV–vis lmax (nm) 246, 275, 313,
425, 685; MALDI-TOF MS m/z 918 (M2).
3. Experimental
3.1. General procedures
1H and 13C NMR spectra were recorded at 300 and 75 MHz,
respectively, in CS2/CDCl3. IR spectra were recorded on a
Shimadzu 8600 FT IR spectrometer. UV–vis spectra were
obtained on a Shimadzu UV-2501 PC spectrometer. APCI
and MALDI-TOF mass spectra in negative mode were
taken on a Finnigan TSQ7000 and a Bruker BIFLEXIII
spectrometer with 4-hydroxy-a-cyanocinnamic acid as the
matrix, respectively. High-performance liquid chroma-
tography analysis was conducted on an Agilent 1100
liquid chromatograph with a diode-array detector using a
Cosmosil Buckyprep column (4.6 mm£250 mm) with
toluene as the eluent. For the HPLC measurements, the
reaction mixture was monitored at 326 nm, and the relative
ratios of C60 and its derivatives’ peak areas were taken as
the relative amounts of C60 and its derivatives. Their actual
amounts should be slightly different due to the slightly
different molar extinction coefficients for C60 and its
derivatives at 326 nm, but will not affect our treatment.
3.1.3. Reaction of C60 with glycine ethyl ester hydro-
chloride and sodium nitrite in toluene solution. A mixture
of C60 (30.8 mg, 0.043 mmol), 5 equiv. of glycine ethyl
hydrochloride (29.7 mg, 0.215 mmol) and 200 equiv. of
sodium nitrite (593 mg, 8.60 mmol) in 30 mL of toluene
was stirred in the dark for two days. The resulting brown
solution was filtrated to remove the excess ester hydro-
chloride and sodium nitrite and evaporated in vacuo. The
residue was separated on a silica gel column with toluene as
the eluent to afford unreacted C60 (12.5 mg, 41%) and
pyrazoline 1a (18.1 mg, 51%).
All solvent-free reactions were performed using a vibration
mill that consists of a capsule and a milling ball made of
stainless steel. The capsule containing the milling ball was
fixed in a home-built vibration arm, which was vibrated
vigorously at a rate of 3500 cycles per minute.11b
3.1.4. Reaction of C60 with glycine octyl ester hydro-
chloride and sodium nitrite in toluene solution. A mixture
of C60 (36.0 mg, 0.05 mmol), 5 equiv. of glycine ethyl
hydrochloride (55.8 mg, 0.25 mmol) and 200 equiv. of
sodium nitrite (690 mg, 10.0 mmol) in 30 mL of toluene
was stirred in the dark for 12 h. The resulting brown solution
was filtrated to remove the excess ester hydrochloride and
sodium nitrite and evaporated in vacuo. The residue was
separated on a silica gel column with toluene as the eluent
to afford unreacted C60 (15.7 mg, 44%) and pyrazoline 1b
(23.4 mg, 51%).
C60 (.99.9%) was purchased from 3D Carbon Cluster
Material Co. of Wuhan University in China. Glycine ester
hydrochloride derivatives were prepared by the reaction of
glycine with corresponding alcohol through the bubbling of
dry hydrogen chloride, and ethyl diazoacetate was prepared
by the reaction of the obtained glycine ethyl ester hydro-
chloride with sodium nitrite according to the reported
procedures.19,20
3.1.1. Reaction of C60 with glycine ethyl ester hydro-
chloride and sodium nitrite under the HSVM conditions.
A mixture of C60 (14.4 mg, 0.02 mmol), glycine ethyl ester
hydrochloride (2.8 mg, 0.02 mmol) and sodium nitrite
3.1.5. Reaction of C60 with ethyl diazoacetate in toluene
solution. A mixture of C60 (30.8 mg, 0.043 mmol) and ethyl

G.-W. Wang et al. / Tetrahedron 60 (2004) 3921–3925
3925
6510–6512. (c) Osterodt, J.; Nieger, M.; Windscheif, P.-M.;
¨
Vogtle, F. Chem. Ber. 1993, 126, 2331–2336. (d) Wilson,
diazoacetate (47.3 mL, 0.43 mmol) in 30 mL of toluene was
stirred for 10 h at room temperature. The brown solution
was evaporated in vacuo, and the residue was separated on a
silica gel column with toluene as the eluent to give the
unreacted C60 (16.0 mg, 51%) and pyrazoline 1a (16.4 mg,
47%).
S. R.; Wu, Y. H. J. Chem. Soc., Chem. Commun. 1993,
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3.1.6. Reaction of C60 with ethyl diazoacetate under the
HSVM conditions. A mixture of C60 (14.4 mg, 0.02 mmol)
and ethyl diazoacetate (2.2 mL, 0.02 mmol) was vigorously
vibrated for 6 min. The combined reaction mixture from two
runs was separated on a silica gel column with CS2 as the
eluent to afford unreacted C60 (4.9 mg, 17%), mixture of
methanofullerene 3a and fulleroids 4a and 5a (6.2 mg,
19%), and then with toluene as the eluent to give pyrazoline
1a (15.0 mg, 45%). The ratio of 3a, 4a and 5a was 1:1.3:2.7
´
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1
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an oil bath at the desired temperature. The thermolysis
process was followed by the HPLC analysis on a Buckyprep
column with toluene as the eluent. The concentration of
remained pyrazoline 1 after heating was determined by the
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products 3, 4 and 5 monitored at 326 nm.
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Acknowledgements
We are grateful for the financial support from National
Science Fund for Distinguished Young Scholars
(20125205), Fund for Innovative Research Groups of
National Science Foundation of China (20321101), Anhui
Provincial Bureau of Personnel Affairs (2001Z019) and
Anhui Natural Science Foundation (00045306). We thank
Professor Koichi Komatsu of Kyoto University for pro-
viding us the high-speed vibration mill and Dr. Yasujiro
Murata of Kyoto University for the spectral measurements
of pyrazoline 1a.
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