An unprecedented [5,6]-open adduct via a direct benzyne-C60 cycloaddition

Article abstract of DOI:10.1016/j.tet.2013.06.073

Cycloaddition is one of the best-studied types of reactions in the organic chemistry of fullerenes, engendering [6,6]-closed adducts in the vast majority of cases. Notwithstanding that a formation of open fulleroid structures is undoubtedly of theoretical interest, no one has demonstrated the conformation of [5,6]-open fulleroid via the direct cycloadditions. Here, we establish that cycloaddition between C₆₀ and benzyne in situ generated from 2-amino-4,5-dibutoxybenzoic acid affords a new type of elusive [5,6]-open structure that is characterized on the basis of NMR, UV-vis spectroscopy, and cyclic voltammetry. Additionally, from density functional theory (DFT) calculations for possible [5,6]-open and [6,6]-closed adducts induced of benzyne-C₆₀ reaction, as expected, features such as the charge distributions, binding characteristics, and frontier molecular orbital levels, are affected by the different binding modes in C₆₀ cage.

Full text of DOI:10.1016/j.tet.2013.06.073

Tetrahedron 69 (2013) 7354e7359
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An unprecedented [5,6]-open adduct via a direct benzyne-C60
cycloaddition
Gyoungsik Kim a, Kyu Cheol Lee a, Jonggi Kim a, Jungho Lee a, Sang Myeon Lee a,
,
Jeong Chul Lee d, Jung Hwa Seo b, Won-Youl Choi c, Changduk Yang a
*
a Interdisciplinary School of Green Energy and KIER-UNIST Advanced Center for Energy, Low Dimensional Carbon Materials Center, Ulsan National
Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea
b Department of Materials Physics, College of Natural Science, Dong-A University, Busan 604-714, Republic of Korea
c Department of Metal and Materials Engineering, Gangneung-wonju National University, Gangneung 210-702, Republic of Korea
d KIER-UNIST Advanced Center for Energy, Korea Institute of Energy Research, Ulsan 689-798, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Cycloaddition is one of the best-studied types of reactions in the organic chemistry of fullerenes, en-
gendering [6,6]-closed adducts in the vast majority of cases. Notwithstanding that a formation of open
fulleroid structures is undoubtedly of theoretical interest, no one has demonstrated the conformation of
[5,6]-open fulleroid via the direct cycloadditions. Here, we establish that cycloaddition between C60 and
benzyne in situ generated from 2-amino-4,5-dibutoxybenzoic acid affords a new type of elusive [5,6]-
open structure that is characterized on the basis of NMR, UVevis spectroscopy, and cyclic voltamme-
try. Additionally, from density functional theory (DFT) calculations for possible [5,6]-open and [6,6]-
closed adducts induced of benzyne-C60 reaction, as expected, features such as the charge distributions,
binding characteristics, and frontier molecular orbital levels, are affected by the different binding modes
in C60 cage.
Received 11 March 2013
Received in revised form 17 June 2013
Accepted 20 June 2013
Available online 28 June 2013
Keywords:
Buckminsterfullerene
Benzyne
[5,6]-Open adduct
[6,6]-Closed adduct
Cycloaddition
Ó 2013 Published by Elsevier Ltd.
1. Introduction
unfavorable [5,6]-double bond in the lowest energy valence bond
(VB) structure is required.4 As a result, the synthesis of [5,6]-
Among a large variety of protocols developed for chemical
modification of buckminsterfullerene,1e4 functionalization by ad-
dition reactions is one of the most powerful methodologies in
fullerene chemistry.3e7 In the fullerenes, the kinetic and thermo-
dynamic products are derived from ‘localized’ addition, that is, 1,2-
addition across 6,6-bonds or 1,4-addition across six-membered
rings, as well as the halodiester anion8 and the azomethine ylide
dipolar cycloaddition9 to 6,6-bonds of the carbon spheres. Fur-
thermore, despite the fact that benzyne typically reacts with
polycyclic aromatic compounds by [4þ2] cycloaddition, the cyclo-
addition of benzyne with C60 favorably occurs at 6,6-ring junctions,
resulting in a [6,6]-closed adduct in which benzyne adds across one
of the interpentagonal bonds (forming a cyclobutene in the process
(see the right of Fig. 1)).10e12 One reason for this type of reactivity is
certainly due to the fact that in a hypothetical [4þ2] adduct one
isomers has hitherto proven to be essentially unobserved.3,4 Up
to now, there are no experimental data available for [5,6]-bridged
compounds with an open transannular bond by the direct cyclo-
additions with C60. In contrast to such principles and results, we
found that open [5,6]-bridged fullerene as an atypical result was
exclusively obtained by the benzyne-C60 reaction (see the left of
Fig. 1). Herein, we report on the structural characteristics and
* Corresponding author. Tel.: þ82 52 217 2920; fax: þ82 52 217 2909; e-mail
address: yang@unist.ac.kr (C. Yang).
Fig. 1. Possible [5,6]-open and [6,6]-closed isomers of the benzyne-C60 cycloaddition.
0040-4020/$ e see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tet.2013.06.073

G. Kim et al. / Tetrahedron 69 (2013) 7354e7359
7355
features of the open [5,6]-structure including density functional
theory (DFT) calculations that support the binding modes in the
fullerene framework.
2. Results and discussion
With respect to a direct cycloaddition of benzyne to C60, in our
initial study, we chose commercially available 2-amino-4,5-
dimethoxybenzoic acid as an in situ generated benzyne substrate.
A solution of C60 in o-dichlorobenzene was treated with 2-amino-
4,5-dimethoxybenzoic acid and isoamyl nitrite at 100 ꢀC for 24 h.
Purification by column chromatography on silica gel afforded
a benzyne-C60 monoadduct as a major product and a regioisomeric
mixture of bisadducts, respectively. The 1H NMR spectrum of the
monoadduct shows a singlet signal at
d 7.57 ppm in the aromatic
region and a singlet resonance at 3.99 ppm corresponding to the
d
methoxy protons, implying the C2v structural conformation (see
Supplementary data). These observations well agree with the
characterization of the [6,6]-closed adduct previously reported by
C
60ebenzyne reaction.11
In the course of our study on the benzyne-C60 reaction, we
proceeded to prepare 2-amino-4,5-dibutoxybenzoic acid (4) as
a benzyne precursor since the fullerene monoadduct with the
methoxy groups on the phenyl ring, after complete drying, pos-
sesses only limited solubility in common organic solvents, leading
to difficulty in facilitating its spectral characterization. The pro-
cedures for the synthesis of the anthranilic acid with two butoxy
chains for the generation of benzyne are outlined in Scheme 1. The
synthetic procedures and characterization data are described
in Supplementary data (e.g., nucleophilic substitution, nitration,
reduction, and saponification). The same procedure, above, using 4
instead of 2-amino-4,5-dimethoxybenzoic acid was adopted for the
synthesis of fullereneebenzyne monoadduct. Chromatography on
silica gel yielded unconverted C60 (first fraction), a monoadduct
(second fraction, 15%, m/z 940þ), and a mixture of bisadducts (third
fraction, 12%, m/z 1160þ). To our surprise, the 1H NMR spectrum of
Fig. 2. 1H NMR (a) and 13C NMR (b) spectra of [5,6]-open regioisomer in 3:1 CS2/ac-
etone-d6.
Such 1H NMR patterns suggest that the product has two geometric
isomers with different conformations of the dibutoxybenzene
moiety, bringing to mind the formation of an open [5,6]-structure.
As shown in Fig. 2b, the 13C NMR spectrum shows 28 carbon
signals corresponding to fullerene carbons in the sp2 region
(
d
¼140e160 ppm) and each of four butoxy peaks in the sp3 region
¼69.55, 69.49, 32.21, 32.09, 20.47, 20.39, 14.80, and 14.72 ppm)
(d
(see HETCOR and HMBC in Supplementary data for further in-
formation). In conjunction with such observations, surprisingly, the
resonances at d 60e80 ppm, assigned to the quaternary bridgehead
carbons of the fullerene cage, which are the sites of attachment of
the benzyne moiety, were not observed, supporting the open
annulene structure. However, the caveat exists that being quater-
nary carbons, their intensity is expected to be very low. This ob-
servation is in opposition to our and other groups’ results
previously studied of the C60edimethoxybenzyne reaction, in
which even with the good dienophile benzyne, C60 does not react as
diene but rather forms a [2þ2] cycloadduct11 since the dienes
prefer to attack the shorter carbonecarbon bond linking two six-
membered rings, rather than the carbon–carbon bond in the
junction between five- and six-membered rings.13e15 However, the
asymmetry of 1H NMR and the absence of sp3 carbon atoms in the
13C NMR spectrum of the fullerene cage forced us to conclude for-
mation of the elusive [5,6]-open structure.
the monoadduct reveals two singlets of equal intensity at d 7.69 and
7.64 ppm produced by the phenyl ring (Fig. 2a). Moreover, the
resonances related to the butoxy protons appear to be non-
equivalent and thus reveal divided peaks, such as two triplets at
4.01 and 3.84 ppm for
two multiplets at 1.79 and 1.69 ppm for
(eOCH2CH2CH2CH3), two multiplets at 1.40 and 1.28 ppm for
a
-methylene protons (eOCH2CH2CH2CH3),
b
-methylene protons
g-
methylene protons (eOCH2CH2CH2CH3), and two triplets at 0.86
and 0.75 ppm for methyl protons (eOCH2CH2CH2CH3), respectively.
Assuming that a direct attack on a s-bond in C60 would be un-
precedented, a plausible hypothesis may be formulated (see
Scheme 2). The benzyne first forms the [2þ2] adduct, which can
facilitate the subsequent rearrangement because the butoxy groups
are more electron releasing than the methoxy groups. Of course,
Scheme 2. The mechanism proposed to explain the formation of [5,6]-open
regioisomer.
Scheme 1. Synthesis of [5,6]-open regioisomer via the benzyne-C60 cycloaddition.

7356
G. Kim et al. / Tetrahedron 69 (2013) 7354e7359
one would have to prove this hypothesis, which would make this
a much more extended study. For example, how do more electron
releasing substituents (dialkylamino) on the benzyne affect this
product formation?
To obtain further additional evidence on the assignment of the
[5,6]-open structure, we subsequently undertook investigations on
its characteristic absorption and electrochemical behavior. Fig. 3
depicts the UVevis absorption characteristics of C60, (6,6)-phenyl-
C
61-butyric acid methyl ester (PCBM), and [5,6]-open regioisomer
in chloroform solution, where the absorption spectra for the pris-
tine C60 and PCBM obtained in this study are listed for comparison
(see Table 1). In the case of the PCBM, the absorption characteristics
for the [6,6]-closed fullerene derivatives including the typical band
at around 430 nm are observed,16e18 reflecting the partially broken
symmetry (C2v) of the fullerene core, relative to pristine C60 (Ih).16,4
On the other hand, the similarity of the absorption features be-
tween pure C60 and [5,6]-open regioisomer is revealed with par-
ticularly the absence of the diagnostic peak at 430 nm for the closed
isomers. An immediate consequence, which stems from lifting
a carbonecarbon bond, located along a hexagonepentagon junc-
Fig. 4. Cyclic voltammograms of C60, PCBM, and [5,6]-open regioisomer.
[5,6]-open regioisomer and C60, which is not unexpected since it is
isoelectronic analogue of C60 (60p-electrons).
Quantum-chemical calculations at the density functional theory
(DFT) level using B3LYP/6-31G (Gaussian 03 package)20e22 were
conducted to establish qualitative charge distribution, binding
characteristics, and frontier molecular orbital levels of [5,6]-open
and [6,6]-closed products. For the [6,6]-closed product (Fig. 5a),
two bridgehead sp3 carbons in the C60 cage are negatively charged
(ꢁ0.188e) and the carbon atoms of the phenyl ring bound to the C60
skeleton are positively charged (þ0.072e), clearly showing that
a resonance effect is donation of the alkoxy side chains as solubil-
izers on the phenylene to C60. As shown in Fig. 5b, a similar trend is
observed in the [5,6]-open adduct, however, each conjoined carbon
between the benzene and C60 exhibits different values in spite of
the structurally identical positions, supporting the broken sym-
metry (see the electrostatic charge distributions in Supplementary
data for further information). This notion is completely consistent
with the nonequivalent resonances in the 1H NMR spectrum. In
tion, is that 60p-electron nature of the fullerene core is largely
conserved in the [5,6]-open isomer.
addition, the standard enthalpies of formation ð
D
HfoÞ were com-
puted from the enthalpies of reaction ð
D
HroÞ of the each fullerene
Fig. 3. UVevis absorption of C60, PCBM, and [5,6]-open regioisomer in CHCl3 solution.
Table 1
Optical data
Compound
Absorption (nm)
C60
PCBM
330, 405, 540, 600(sh), 623(sh)
328, 430, 490, 603(sh), 695
[5,6]-Open regioisomer
319, 417(sh), 540(sh), 600(sh), 690
The cyclic voltammetry (CV) data are summarized in Table 2 (see
Fig. 4). The reduction potentials of PCBM as a [6,6]-closed model are
shifted to more negative values with respect to C60 due to a de-
crease in its number of p
-electrons and release of strain energy.19 In
contrast, the first reduction values reveal similarities between
Table 2
Electrochemical dataa
Compound
Er1ed
Er2ed
Er3ed
C60
PCBM
[5,6]-Open regioisomer
ꢁ1.071
ꢁ1.158
ꢁ1.036
ꢁ1.484
ꢁ1.540
ꢁ1.558
ꢁ1.969
ꢁ2.039
ꢁ2.036
Fig. 5. Calculated Mulliken charges (top; red (negative), green (positive)) and opti-
a
Experimental conditions: values for 0.5 (EpaþEpc) in V versus Fc/Fcþ; 10ꢁ4 to
10ꢁ3 mol/L o-DCB solution; Bu4NClO4 (0.1 M) as supporting electrolyte; Pt wire as
counter electrode; 50 mV/s scan rate.
ꢀ
mized structures (bottom; the unit of bonding length is A (gray: carbon, red: oxygen,
and white: hydrogen atoms)). (a) [6,6]-Closed and (b) [5,6]-open products of the
benzyneeC60 cycloaddition.

G. Kim et al. / Tetrahedron 69 (2013) 7354e7359
7357
reactions. The
D
Hfo were determined 538 kcal/mol for [5,6]-
Hro were
behavior. Additionally, the theoretical study of the possible two
[5,6]-closed and [5,6]-open products for the benzyneeC60 reaction
enables one to access the influence of the binding modes in the
fullerene framework. Altogether, the present work provides an
open and 614 kcal/mol for [6,6]-closed, while the
D
16 kcal/mol for [5,6]-open and ꢁ33 kcal/mol for [6,6]-closed, re-
spectively. Thus, the formation of [6,6]-closed adduct is favored
over by approximately 50 kcal/mol. Note that, for both [5,6]-open
and [6,6]-closed products with methoxy groups, the charged
values of the bridgehead carbons show identity, reflecting the
symmetry (see the bottom of Fig. S2 in Supplementary data). It is
interesting to note that the charge distribution of [5,6]-open
structure is more positively charged when compared to [6,6]-
closed product (see the range of the scale bar) and its reckoned
binding charge (j0.20je) is somewhat lower than that obtained by
the closed product (j0.26je).
example showing
a contrast with [6,6]-closed adduct of
benzyneeC60 previously reported by several research groups. Re-
search in cycloaddition of C60 to benzynes with various sub-
stitutions is currently under way to help us not only prove the
proposed mechanism but also access the correlation of the different
formations between C60 and benzyne compounds.
4. Experimental section
The optimized structures for both the [5,6]-open and [6,6]-
closed products are shown in the bottom of Fig. 5. In the case of
[6,6]-closed junction, the butoxy substituents on the phenyl ring
exhibit a planar geometry with the bonds connecting benzyne to
C60 of 1.53 A and the sp bond of 1.37 A in the cyclobutene for-
mation. An unusually long interior C60 bond length of 1.66 A is
4.1. Materials and instruments
All starting materials were purchased either from Aldrich or
Acros and used without further purification. All solvents are ACS
grade unless otherwise noted. Anhydrous THF was obtained by
distillation from sodium/benzophenone prior to use. Anhydrous
toluene was used as received. 1H NMR and 13C NMR spectra were
recorded on a Varian Mercury Vx 200 MHz, Varian Unity Inova
500 MHz, or Varian VNMRS 600 spectrophotometer using CDCl3 as
solvent and tetramethylsilane (TMS) as the internal standard and
MALDI-MS spectra were obtained from Ultraflex III (Bruker, Ger-
many). UVevis spectra were taken on Cary 5000 (Varian USA)
spectrophotometer. Cyclic voltammetry (CV) measurements were
performed on Solartron SI 1287 with a three-electrode cell in
a 0.1 M tetra-n-butylammonium perchlorate (Bu4NClO4) solution in
acetonitrile at a scan rate of 50 mV/s at room temperature under
argon. A silver wire, a platinum wire, and a platinum disk were used
as the reference electrode, counter electrode, and working elec-
trode, respectively. The Ag/Agþ reference electrode was calibrated
2
ꢀ
ꢀ
ꢀ
noticeable, compared with typical values for benzocyclobutenes
(1.58e1.60 A).23 The bond lengths in the bridged bicyclic system of
ꢀ
2
ꢀ
the [5,6]-open adduct are 1.37 A long for the sp character and
2.46 A long for the distance between two bridgehead carbons in
ꢀ
C
60. Interestingly, for the [5,6]-open product, the bridging CeC
bond between the fullerene (CF) and benzyne (CB) is relatively
ꢀ
shorter (1.48 A) as compared to the [6,6]-closed product (see the
above), implying that the binding of the open structure is slightly
stronger. Besides, the resultant values by using 6-31G(d) are almost
identical to those above (see Fig. S3 in Supplementary data).
In addition, the DFT results reveal considerable dipole moments
of 4.01 D and 5.27 D for [6,6]-closed and [5,6]-open products, re-
spectively. These dipole moments are different compared to their
counterpart products with methoxygroups ([6,6]-closed¼1.48 D and
[5,6]-open¼1.62 D), respectively, implying the different polarizabil-
ity of the fullerene derivatives depending on the degree of the chain
length of alkoxy groups. The calculated stabilization energies are
476.23 kcal/mol for [6,6]-closed and 596.26 kcal/mol for [5,6]-open.
The calculated HOMO energies of ꢁ5.78 eV for both the products are
essentially the same. However, the LUMOs are estimated to
be ꢁ3.16 eV and ꢁ3.19 eV for [6,6]-closed and [5,6]-open products,
respectively (Fig. 6). This suggests that the reductionpotentials of the
functionalized C60 derivatives are sensitive to the binding mode in
fullerene framework. This above study clearly shows the qualitative
effects of the bridge modes in a fullerene cage, i.e., [5,6]-open versus
using
a ferrocene/ferrocenium redox couple as an external
standard.
4.2. Synthesis of methyl-3,4-dibutoxybenzoate (1)
Methyl-3,4-dihydroxybenzoate (10.0 g, 59.4 mmol), n-butyl-
bromide (24.4 g, 178.2 mmol), and K2CO3 (28.7 g, 207.9 mmol) were
added to methanol (250 ml) and refluxed for 24 h. After evapo-
rating solvent, the mixture was then extracted into ethyl acetate
and washed with brine. The crude product obtained was purified by
chromatography on silica with 0e10% ethyl acetate in hexane as
eluent. Isolated yield¼12.8 g (77%) as a colorless liquid. 1H NMR
[6,6]-closed, as a function of the changed fullerene’s
p-system.
(200 MHz, CDCl3)
d
(ppm) 7.64 (dd, J¼8.41, 2.04 Hz, 1H), 7.54 (d,
J¼2.01 Hz, 1H), 6.87 (d, J¼8.44 Hz, 1H), 4.05 (dt, J¼6.56, 6.56,
0.98 Hz, 4H), 3.89 (s, 3H), 1.83 (m, 4H), 1.51 (m, 4H), 0.99 (t,
J¼7.31 Hz, 6H). 13C NMR (CDCl3, 50.2 MHz):
d 166.42, 152.61, 147.91,
122.96, 113.56, 111.27, 68.34, 68.08, 51.32, 30.66, 30.55, 18.65, 13.33.
ESI-MS (m/z): 280 ((M)þꢂ). Elemental analysis: calculated for
C
16H24O4: C, 68.54; H, 8.63; O, 22.83. Found: C, 68.23; H, 8.51; O,
22.53. FTIR (
1275, 1172.
n
/cmꢁ1): 3009, 2971, 2841, 1721, 1602, 1519, 1435, 1298,
4.3. Synthesis of 2-nitro-4,5-dibutoxy-methylbenzoate (2)
Fig. 6. HOMOs and LUMOs of (a) [6,6]-closed and (b) [5,6]-open products.
A flask immersed in a room-temperature ice bath was charged
with methyl-3,4-dibutoxybenzoate (10 g, 35.7 mmol) and acetic
acid (glacial, 52 mL). Over a 15-min period, HNO3 (70%, 54 mL) was
added dropwise with stirring. The orange solution was stirred at
room temperature overnight. The reaction was quenched upon
addition of ice. The cooled mixture was extracted with diethyl
ether, washed with water, NaHCO3, NaOH, and brine and then dried
over MgSO4. The crude product obtained was purified by chroma-
tography on silica with 0e10% ethyl acetate in hexane as eluent.
Isolated yield¼10.7 g (93%) as a light yellow liquid. 1H NMR
3. Conclusion
In conclusion, we have discovered a novel reaction of C60 with in
situ generated benzyne from 2-amino-4,5-dibutoxybenzoic acid,
which affords one monoadduct identified as a [5,6]-open annulene
adduct. The structure of [5,6]-open isomer has been characterized
by 1H NMR and 13C NMR spectra, which are supported by additional
evidence such as UVevis spectroscopy and electrochemical

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G. Kim et al. / Tetrahedron 69 (2013) 7354e7359
(200 MHz, CDCl3)
d
(ppm) 7.44 (s, 1H), 7.04 (s, 1H), 4.09 (dt, J¼6.52,
collected, respectively. The spectral data of monoadduct: 1H NMR
(600 MHz, CDCl3)
(ppm) 7.57 (s, 2H), 3.99 (s, 6H). 13C NMR
6.51, 2.55 Hz, 4H), 3.90 (s, 3H), 1.84 (m, 4H), 1.52 (m, 4H), 0.99 (t,
d
J¼7.44 Hz, 6H). 13C NMR (CDCl3, 50.2 MHz):
d
165.96, 152.01, 140.30,
(150.1 MHz, 3:1 CS2/acetone-d6): After complete drying, the prod-
uct exhibited only limited solubility, which leads to the difficulty in
accessing the proper 13C NMR spectrum. MALDI-MS m/z: 856
((M)þꢂ). Elemental analysis: calculated for C68H8O2: C, 95.32; H,
120.83, 111.28, 107.63, 68.82, 52.62, 30.27, 18.55, 13.21. ESI-MS (m/
z): 325 ((M)þꢂ). Elemental analysis: calculated for C16H23NO6: C,
59.06; H, 7.13; N, 4.31; O, 29.50. Found: C, 59.35; H, 7.35; O, 29.73.
FTIR (
n
/cmꢁ1): 3043, 2982, 2853, 1727, 1581, 1525, 1475, 1456, 1359,
0.94; O, 3.73. Found: C, 95.21; H, 0.85; O, 3.71. FTIR (
2954, 2835, 1596, 1441, 1355, 1234, 1190, 721, 525.
n
/cmꢁ1): 3068,
1270.
4.4. Synthesis of 2-amino-4,5-dibutoxy-methylbenzoate (3)
4.7. Reaction of C60 with 2-amino-4,5-dibutoxybenzoic acid
An round-bottomed flask equipped with a stirring bar was im-
Following the same procedures as the reaction of C60 with 2-
amino-4,5-dimethoxybenzoic acid, C60 (720 mg, 1.0 mmol), 2-
amino-4,5-dibutoxybenzoic acid (450 mg, 1.6 mmol), isoamyl ni-
trite (187 mg, 1.6 mmol), and o-dichlorobenzene (o-DCB, 70 mL)
was placed under argon and stirred. The homogenous mixture re-
action was heated at 100 ꢀC under argon for 24 h. The residue was
chromatographed on silica gel (eluent; toluene/toluene/ethyl
acetate (9:1(v/v))) to give unreacted C60 (86 mg, 12%), monoadduct
(140 mg,15%) as brown-colored powder, and regioisomeric mixture
of bisadducts (140 mg, 12%). Then, each solution was concentrated
in vacuo, redissolved in a minimal amount of toluene and trans-
ferred to a centrifuge tube. The compounds were precipitated with
MeOH, centrifuged, decanted, and collected, respectively. The
spectral data of monoadduct ([5,6]-open regioisomer): 1H NMR
mersed in
a room temperature ice bath and charged with
SnCl2$2H2O (100 g, 441.7 mmol) and HCl (concentrated, 100 mL). A
solution of 2-amino-4,5-dibutoxy-methylbenzoate (9.0 g,
27.7 mmol) in EtOH (30 mL) was added and the mixture was stirred
at room temperature overnight. The mixture was poured into ice
and filtered washed with water. The resulting white solid was taken
up in EtOH (30 mL) and poured into aqueous NaOH, filtered and
dried. Isolated yield¼7.7 g (97%) as a colorless solid. 1H NMR
(200 MHz, CDCl3) d (ppm) 7.34 (s,1H), 6.11 (s,1H), 5.57 (m, 2H), 4.02
(m, 4H), 3.91 (s, 3H), 1.78 (m, 4H), 1.50 (m, 4H), 0.99 (t, J¼7.22 Hz,
6H). 13C NMR (CDCl3, 50.2 MHz): 168.35, 155.02, 146.91, 139.57,
116.06, 99.72, 69.78, 67.71, 50.77, 30.98, 30.48, 18.69, 13.41, 13.32.
ESI-MS (m/z): 295 ((M)þꢂ). Elemental analysis: calculated for
C
16H25NO4: C, 65.06; H, 8.53; N, 4.74; O, 21.67. Found: C, 65.23; H,
(600 MHz, 3:1 CS2/acetone-d6):
d (ppm) 7.69 (s, 1H), 7.64 (s, 1H),
8.35; O, 21.55. FTIR (
n
/cmꢁ1): 3454, 3358, 2997, 2854, 1674, 1598,
4.01 (t, J¼6.25 Hz, 2H), 3.84 (t, J¼6.25 Hz, 2H), 1.79 (m, 2H), 1.69 (m,
1517, 1461, 1390, 1309, 1190.
2H), 1.40 (m, 2H), 1.28 (m, 2H), 0.86 (t, J¼7.45 Hz, 3H), 0.75 (t,
J¼7.45 Hz, 3H). 13C NMR (150.1 MHz, 3:1 CS2/acetone-d6):
d 160.28,
4.5. Synthesis of 2-amino-4,5-dibutoxybenzoic acid (4)
155.51, 154.32, 150.16, 149.55, 149.25, 148.72, 147.30, 147.21, 146.95,
146.90, 146.88, 146.40, 146.98, 145.91, 145.80, 145.50, 145.18, 145.12,
143.68, 146.38, 143.18, 142.85, 142.62, 141.94, 141.79, 140.64, 140.52,
136.18, 135.12, 130.20, 115.78, 114.17, 113.22, 69.55, 69.49, 32.21,
32.09, 20.47, 20.39, 14.80, 14.72. MALDI-MS m/z: 940 ((M)þꢂ). Ele-
mental analysis: calculated for C74H20O2: C, 94.46; H, 2.14; O, 3.40.
Found: C, 94.21; H, 2.33; O, 3.52. The bisadducts are characterized
A round-bottom flask containing 2-amino-4,5-dibutoxy-meth-
ylbenzoate (7 g, 23.7 mmol) was mixed with KOH (4.78,
118.5 mmol) and water (15 mL) and EtOH (20 mL). The solution was
refluxed overnight to give a clear brown solution. After cooling at
ambient temperature for 10 min, water (100 mL) was added to the
flask and the solution was titrated to pH 6 with 1 N HCl. The pre-
cipitate was filtered and washed with water. The solid was treated
with aqueous NaHCO3, washed again with water, and dried in vacuo
over P2O5 overnight (Caution: Do not increase temperature, it was
decomposed.). Isolated yield¼6.3 g (95%) as a colorless solid. 1H
by only MALDI-MS m/z: 1160 ((M)þꢂ). FTIR ( /cmꢁ1): 3037, 2959,
n
2873, 1693, 1604, 1427, 1329, 1067, 734, 573, 511.
Acknowledgements
NMR (200 MHz, CDCl3)
d (ppm) 7.38 (s, 1H), 6.12 (s, 1H), 3.98 (m,
The authors gratefully thank Prof. Fred Wudl (UCSB, USA) for his
fruitful discussions and advices. This research was supported by
Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science, ICT
and Future Planning (2013R1A1A1A05004475, 2010-0019408,
2010-0026916), the New & Renewable Energy of the Korea Institute
of Energy Technology Evaluation and Planning (KETEP) grant fun-
ded by the Korea government Ministry of Knowledge Economy (No.
20123010010140), and the framework of Research and De-
velopment Program of the Korea Institute of Energy Research
(KIER) (B3-2415). The first two authors equally contributed in this
work.
4H), 1.80 (m, 4H), 1.51 (m, 4H), 0.98 (m, 6H). 13C NMR (CDCl3,
50.2 MHz): 172.87, 155.95, 147.87, 139.90, 116.22, 100.60, 99.70,
69.71, 67.95, 21.03, 30.56, 18.83, 13.54, 13.45. ESI-MS (m/z): 281
((M)þꢂ). Elemental analysis: calculated for C15H23NO4: C, 64.03; H,
8.24; N, 4.98; O, 22.75. Found: C, 63.99; H, 8.15; O, 23.01. FTIR (n/
cmꢁ1): 3489, 3373, 2955, 2925, 2854, 1655, 1586, 1501, 1378, 1309,
1230, 1160, 1036.
4.6. Reaction of C60 with 2-amino-4,5-dimethoxybenzoic acid
A
mixture of C60 (720 mg, 1.0 mmol), 2-amino-4,5-
dimethoxybenzoic acid as an in situ generated benzyne precursor
(316 mg, 1.6 mmol), isoamyl nitrite (187 mg, 1.6 mmol), and o-di-
chlorobenzene (o-DCB) (70 mL) was placed under argon and stir-
red. The homogenous reaction mixture was heated at 100 ꢀC under
argon for 24 h. After the reaction mixture was cooled to room
temperature, the solvent was evaporated under reduced pressure.
The residue was chromatographed on silica gel (eluent; tolue-
ne/toluene/ethyl acetate (9:1(v/v))) to give unreacted C60
(100 mg, 14%), monoadduct (150 mg, 17%) as brown-colored pow-
der, and regioisomeric mixture of bisadducts (100 mg, 10%). Then,
each solution was concentrated in vacuo, redissolved in a minimal
amount of toluene and transferred to a centrifuge tube. The com-
pounds were precipitated with MeOH, centrifuged, decanted, and
Supplementary data
1H and 13C NMR spectra, HETCOR and HMBC spectra as well as
additional calculation studies. Supplementary data associated with
this article can be found in the online version, at http://dx.doi.org/
10.1016/j.tet.2013.06.073.
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