Reactions of anionic oxygen nucleophiles with C60 revisited

Article abstract of DOI:10.1021/ol300805p

Reactions of C₆₀ with oxygen nucleophiles of HO^- and CH₃O^- are revisited in PhCN in the presence of PhCH ₂Br. Different from previous results that such reactions lead to the formation of complex mixtu

Full text of DOI:10.1021/ol300805p

ORGANIC
LETTERS
2012
Vol. 14, No. 9
2386–2389
Reactions of Anionic Oxygen
Nucleophiles with C60 Revisited
Wei-Wei Chang, Zong-Jun Li, Wei-Wei Yang, and Xiang Gao*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied
Chemistry, Graduate School of Chinese Academy of Sciences, Chinese Academy of
Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China
xgao@ciac.jl.cn
Received March 29, 2012
ABSTRACT
Reactions of C60 with oxygen nucleophiles of HOꢀ and CH3Oꢀ are revisited in PhCN in the presence of PhCH2Br. Different from previous results
that such reactions lead to the formation of complex mixtures, well-structured C60 oxazolines are obtained when HOꢀ is involved, while di- and
tetraadducts with methoxy and benzyl addends are obtained when CH3Oꢀ is engaged. The reactions are followed by in situ visꢀnear-IR
spectroscopy, which reveals further information for the reactions.
Fullerenes, represented by C60, are electron-deficient
molecules and are favorable for reactions with nucleo-
philes.1 However, the so far well-established nucleophilic
reactions for fullerenes are mostly based on carbon nucleo-
philes, including organometallic reagents,1ꢀ11 a cyanide
anion,12 depronated R-halo esters or ketones for Bingel
reactions,13,14 and transition metal catalyzed arylation and
allylation reactions,15,16 while reactions with oxygen nu-
cleophiles such as hydroxide and alkoxide anions are often
overlooked, since such reactions usually result in complex
mixtures rather than well-structured organofullerenes.1,17ꢀ20
Even though methoxylated C60 adducts can be detected
in ESI MS, no products have been actually isolated and
characterized from the reaction mixtures.20,21 Conse-
quently, the preparations of alkoxylated fullerene de-
rivatives are usually achieved via either substitution
reactions of halogenated fullerenes22ꢀ24 and fullerenol,25
or via photoinduced radical reactions,26 rather than direct
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Wiley-VCH: Weinheim, Germany, 2005.
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r
10.1021/ol300805p
Published on Web 04/19/2012
2012 American Chemical Society

nucleophilic additions to fullerenes. Wang and co-workers
have shown that the presence of O2 is crucial for the
production of well-defined compounds from the reactions
ofC60 withalkoxide anions, where C60-fused 1,3-dioxolane
and tetrahydrofuran derivatives can be obtained.27ꢀ29
Meanwhile, Gan et al. have shown that monomethoxy-
lated and multiple methoxylated C60 derivatives can be
produced via nucleophilic additions of methoxide to C60
derivatives with peroxide adducts.30 However, to the best
of our knowledge, no work has appeared on the prepara-
tion of alkoxylated fullerene derivatives via direct addition
of alkoxide anions to pristine fullerenes to date. Appar-
ently, the reactions of C60 with anionic oxygen nucleophiles
are still not well understood and are very much neglected as
a synthetic approach for fullerene functionalizations.
Herein, we report the reactions of C60 with hydroxide
and methoxide anions. Unexpectedly, C60 oxazolines
(compounds 1 and 2) are formed from the reactions of
HOꢀ with C60 and PhCN via an anion relay mechanism;
while methoxylated fullerenes (compounds 3, 4, and 5) are
formed via the direct nucleophilic additions of a methoxide
anion to C60 followed by subsequent electrophilic addition
of PhCH2Br.
Figure 1. Insituvisꢀnear-IRspectrafor the reaction ofC60 (1.3 ꢁ
10ꢀ4 M) with 3 equiv of TBAOH in PhCN at different reaction
times under deoxygenated conditions at rt.
A broad absorption band at 933 nm appears immediately
following the addition of 1.0 M TBAOH/CH3OH into C60
PhCN solution. As the reaction proceeds, the intensity for
the broad absorption band keeps increasing but is red-
shifted to 962 nm, along with the appearance of new strong
absorption bands at 644 and 710 nm. Komatsu and co-
workers have shown that a dianionic singly bonded C60
ꢀ
intermediate of C60ꢀ;CtC;C60 exhibits a strong ab-
The nucleophilic addition of OHꢀ to C60 was examined
in PhCN with the use of 1.0 M TBAOH (tetra-n-butylam-
monium hydroxide)/CH3OH solution as the OHꢀ source.
The color of the solution changed gradually from purple to
completely green after addition of TBAOH/CH3OH, and
the reaction was finished by quenching with either I2 or
PhCH2Br, leading to compound 1 (43%) or 2 (30%)
respectively (see Supporting Information for details).
The identities of compounds 1 and 2 are established on
the basis of NMR, MS, HPLC, and UVꢀvis characteriza-
tions in light of reported data.31ꢀ34 The formation of C60
oxazolines from OHꢀ addition to C60 in PhCN is quite
unexpected, since no such compounds have been reported
from reactions carried out under similar conditions,21 and
the addition of OHꢀ to C60 has been intended mainly as
an approach to prepare fullerenols.17ꢀ19 The in situ visꢀ
near-IR was therefore carried out to obtain a better under-
standing of the reaction.
sorption at 965 nm,35 which matches well with the 962 nm
absorption band in Figure 1, indicating that a dianionic
singly bonded intermediate is probably present. Previous
work has suggested that 12ꢀ is a singly bonded dianion;33 it
is therefore reasonable to assign the absorption band at
962 nm to 12ꢀ. The assignment is confirmed by the near-IR
spectrum of 12ꢀ (Figure S3), which shows absorption
bands at 963, 710, and 645 nm, in excellent agreement
with the spectrum of the reaction mixture at 60 min as
showninFigure1, indicatingthatthe nucleophilicaddition
of OHꢀ to C60 in PhCN almost exclusively leads to the
formation of a C60 oxazoline dianion. In fact, the yield of
C60 oxazolines obtained from this method is much higher
compared to that obtained via aerobic oxidations of
anionic C60,31,34 even though there are still toluene inso-
luble materials formed from the reaction, which are likely
produced via polymerization of anionic oxygenated C60
species.36,37
•ꢀ
Figure 1 shows the in situ visꢀnear-IR spectra for the
reaction of C60 with TBAOH and PhCN at different
reaction times under deoxygenated conditions at rt.
An absorption band corresponding to C60 is also
shown at 1079 nm,38 which ꢀis probably produced via
electron transfer from ROC60 (R = H or CH3) to C60
as proposed previously.11,21 However, the intensity of thꢀe
peak is rather weak, indicating that the amount of ROC60
is very slight, implying that the reaction mixture of C60,
OHꢀ, and PhCN is an overall dianionic system rather than
a monoanionic one.
(25) Wang, G.-W.; Lu, Y.-M.; Chen, Z.-X. Org. Lett. 2009, 11, 1507–
1510.
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(29) Wang, G.-W.; Lu, Y.-M.; Chen, Z.-X.; Wu, S.-H. J. Org. Chem.
2009, 74, 4841–4848.
We have recently shown that C60 oxazolines can be
produced via aerobic oxidation of C602ꢀ, where 12ꢀ is
(30) Huang, S.; Xiao, Z.; Wang, F.; Gan, L.; Zhang, X.; Hu, X.;
Zhang, S.; Lu, M.; Pan, Q.; Xu, L. J. Org. Chem. 2004, 69, 2442–2453.
(31) Zheng, M.; Li, F.-F.; Ni, L.; Yang, W.-W.; Gao, X. J. Org.
Chem. 2008, 73, 3159–3168.
(35) Tanaka, T.; Komatsu, K. J. Chem. Soc., Perkin Trans. 1 1999,
1671–1675.
(36) Winkler, K.; Costa, D. A.; Balch, A. L.; Fawcett, W. R. J. Phys.
Chem. 1995, 99, 17431–17436.
(32) Li, F.-B.; Liu, T.-X.; Wang, G.-W. J. Org. Chem. 2008, 73, 6417–
6420.
(33) Yang, W.-W.; Li, Z.-J.; Li, F.-F.; Gao, X. J. Org. Chem. 2011,
76, 1384–1389.
(37) Krinichnaya, E. P.; Moravsky, A. P.; Efimov, O.; Sobczak,
J. W.; Winkler, K.; Kutner, W.; Balch, A. L. J. Mater. Chem. 2005,
15, 1468–1476.
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Eaton, S. S. J. Am. Chem. Soc. 1994, 116, 3465–3474.
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Org. Lett., Vol. 14, No. 9, 2012
2387

Scheme 1. Proposed Mechanism for the Formation of C60
Oxazolines via the Reaction of C60 with TBAOH and PhCN
formedvia the keyintermediate of C60ꢀꢀOꢀ.34 Since12ꢀ is
also shown to form during the reaction of C60 with OHꢀ
and PhCN, it is likely that the reaction proceeds via a
similar mechanism with the formation of the key inter-
mediate of C60ꢀꢀOꢀ. A reaction mechanism for the
nucleophilic addition of OHꢀ to C60 in PhCN is therefore
proposed as shown in Scheme 1, where the C60 oxazolines
are formed via an anion relay mechanism initiated by the
nucleophilic addition of OHꢀ to C60.
In addition to the nucleophilic nature, the hydroxide
anion is also a strong base and has been extensively used to
abstract the fullerenyl proton from HRC60 (R = alkyl or
H).39,40 It is therefore likely that C60ꢀꢀOH is formed first
via the nucleophilic addition of OHꢀ to C60, then
C60ꢀꢀOꢀ would be formed via subsequent deprotonation
of C60ꢀꢀOH by OHꢀ owing to its strong basic nature. The
resulting C60ꢀꢀOꢀ would then attack PhCN to form an
anionic imine species, which would then attack back at the
C60 sphere accompanied by a heterolytic cleavage of the
C60ꢀO bond to form 12ꢀ, leading to the formation of 1 or 2
upon quenching with I2 or PhCH2Br. The results indicate
that, for the nucleophilic additions of OHꢀ to fullerenes,
the OHꢀ functions as not only a nucleophile but also a
strong base.
Interestingly, when PhCH2Br was first added into C60
PhCN solution before adding 1.0 M TBAOH/CH3OH
solution, C60 reacted with CH3Oꢀ and PhCH2Br, lead-
ing to methoxylated products 3, 4, and 5, rather than the
reaction involving OHꢀ, with no compound 2 formed.
The reaction was affected by the temperature, and a
yield of 29%, 10%, and 27% was obtained for 3, 4, and 5
when the reaction was performed at 50 °C, which was
better than the case when the reaction was carried out
at rt. It is noteworthy that under such conditions,
PhCH2Br could also react with OHꢀ or MeOꢀ, compet-
ing with C60.
Figure 2. Expanded HMBC NMR spectrum of compound 5.
and 4 (Figure S12) exhibit absorptions at 431 and 443 nm
respectively, which are characteristic for the 1,2-41,42 and 1,4-
adduct of C60.26,42,43 The 13C NMR spectra of 3 and 4
display 28 and 45 resonances respectively for the sp2 carbons
of C60, indicating that they have Cs and C1 symmetry
respectively. The HMBC NMR of 3 (Figure S10) shows
that the methylene protons have cross peaks not only with
the C60 sp3 C-atom (67.2 ppm) bonded to the benzyl but also
with the C60 sp3 C-atom (93.3 ppm) bonded to the methoxy,
confirming that the molecule is a 1,2-adduct for 4, the
resonances for the sp3 C60 carbons bonded to the benzyl
and methoxy groups appear at 59.27 and 80.47 ppm,
which are in good agreement with the reported values for
1,4-(PhCH2)2C60 (60.2 ppm)42,43 and 1,4-(CH3O)RC60
(around 80 ppm),22,25,26 indicating that 4 is a 1,4-adduct.
The accurate MS of 5 (Figure S23) shows that the com-
pound is a tetraadduct with a formula of (C6H5CH2)2-
1
(CH3O)2C60, and the H (Figure S19), 13C (Figure S20),
and HSQC (Figure S21) NMR of the compound are in
good agreement with the formation of a tetraadduct.
Figure 2 shows the HMBC NMR spectrum of 5, which
provides key data for structural elucidation. First, it shows
that the two sets of methylene protons all have correlations
with the most downfield resonance at 166.39 ppm, which
3
are correlated via the JCH couplings, indicating that the
two benzyls are both located next to this sp2 C60 carbon;
Second, the spectrum shows that the ABq I (4.41 ppm) and
the methoxyl protons (4.05 ppm) are both correlated with
the resonance at 87.96 ppm, which is due to the sp3 C60
carbon bonded to the methoxy group, indicating that the
benzyl (ABq I) and the methoxyl are positioned with a 1,2-
configuration. In contrast, no correlation is shown be-
tween the ABq II (4.38 ppm) and either one of the sp3
C60ꢀO carbons (78.08 or 87.96 ppm), indicating that this
benzyl is likely to have a 1,4-addition pattern with respect
to the two methoxy groups.
The HRMS of 3 and 4 indicate that both compounds
have the formula (CH3O)(PhCH2)C60. The configurations
of two compounds are further established with UV/vis and
NMR characterizations. The UV/vis spectra of 3(Figure S6)
Since the two C60 sp3 carbons that bonded to the two
benzyls are connected to the same sp2 C60 carbon, and one
pair of methoxyl and benzyl groups is positioned with
a 1,2-configuration, while the other benzyl is positioned
with a 1,4-pattern relative to both methoxy function-
alities, the only possible structure for the product is
(39) Meier, M. S.; Bergosh, R. G.; Gallagher, M. E.; Spielmann,
H. P.; Wang, Z. J. Org. Chem. 2002, 67, 5946–5952.
(40) Wang, Z.; Meier, M. S. J. Org. Chem. 2004, 69, 2178–2180.
(41) Smith, A. B., III; Strongin, R. M.; Brard, L.; Furst, G. T.;
Romanow, W. J.; Owens, K. G.; Goldschmidt, R. J.; King, R. C. J. Am.
Chem. Soc. 1995, 117, 5492–5502.
(42) Zheng, M.; Li, F.-F.; Shi, Z.; Gao, X.; Kadish, K. M. J. Org.
Chem. 2007, 72, 2538–2542.
(43) Kadish, K. M.; Gao, X.; Van Caemelbecke, E.; Hirasaka, T.;
Suenobu, T.; Fukuzumi, S. J. Phys. Chem. A 1998, 102, 3898–3906.
2388
Org. Lett., Vol. 14, No. 9, 2012

Scheme 2. Proposed Mechanism for the Reaction of C60 with
CH3Oꢀ and PhCH2Br
Figure 3. In situ visibleꢀnear-IR spectra for the reaction of C60
(5.0 ꢁ 10ꢀ4 M) with 5 equiv of CH3Oꢀ and 20 equiv of PhCH2Br
at different reaction times in deoxygenated PhCN at 50 °C.
is crucial for the reaction. The mechanism for the formation
of 5 is proposed as shown in Scheme 2, where the reaction
proceedsvia analternate nucleo- and electrophilicaddition
of CH3Oꢀ and PhCH2Br.
1,15-(CH3O)2-2,4-(C6H5CH2)2C60. Such a configuration has
been shown to be favorable for C60 tetraadducts involv-
ing both 1,2- and 1,4-additions, no matter if the products
The favorable formation of 5 as shown in the HPLC
(Figure S17) indicates that the reaction of C60 with CH3Oꢀ
and PhCH2Br prefers the formation of the symmetrical
indenide intermediate A over the asymmetrical B. Compu-
tational calculations using HatreeꢀFock (HF) methods at
the 6-31G level with the Gaussian 03 program predict that
A is favored over B by ∼6.4 kcal/mol, which may be
rationalized by the higher degree of conjugation of the 10
π-electron indenide anion of A compared to that of B
(Figure S26) due to the higher symmetry of A, where the
bond lengths of the indenide π-conjugation ring are more
averaged compared to the case of B.
are formed via nucleophilic additions to electron-deficient
8,9,11,16
C60
C
or electrophilic additions to electron-rich anionic
60 species.44
Further study with 1,4- and 1,2-(C6H5CH2)(CH3O)C60
shows that 5 can be obtained starting from 1,4-(C6H5CH2)-
(CH3O)C60, while it cannot be obtained from the reaction
starting from 1,2-(C6H5CH2)(CH3O)C60 (Figures S24 and
S25 for HPLC). The results are consistent with previous
reports8,9,11,16 and indicate that 5 is likely formed via an
indenide intermediate monoanion (R3C60ꢀ) as proposed by
Nakamura et al.8,9,11
In summary, nucleophilic additions of OHꢀ and CH3Oꢀ
to C60 are revisited, and well-structured oxygenated C60
derivatives have been isolated and characterized. The
results show that OHꢀ functions as not only a nucleophile
to react with C60 but also a base to depronate the
C60ꢀꢀOH intermediate and form the reactive C60ꢀꢀOꢀ
intermediate, which may further react with the nitrile via
an anion relay mechanism to form fullerooxazolines; as for
the nucleophilic addition of CH3Oꢀ to C60, the results
Figure 3 shows the in situ visꢀnear-IR spectrum of the
nucleophilic additions of CH3Oꢀ to C60 in the presence of
PhCH2Br. A broad absorption band at 931 nm appears
immediately after adding TBAOH/CH3OH into C60
PhCN solution, in which PhCH2Br is present. The absorp-
tion band is essentially identical to that when no PhCH2Br
is present (Figure 1), suggesting that this absorption is only
related to the nucleophilic addition of ROꢀ (R = H, CH3)
to C60. Unlike the case of OHꢀ addition, no shift occurs for
the absorption band at 931 nm as the reaction proceeds,
indicating any further change of the charged state for the
resulting intermediate is unlikely. Absorption bands also
appear at 594 and 541 nm, which are also different from
those for dianionic intermediates as shown in Figure 1. The
results indicate that the intermediates for the CH3Oꢀ
addition to C60 are mostly monoanionic species, which
are subsequently quenched with PhCH2Br. In addition, an
absorption band due toC60•ꢀ isalsoshown at1080nm, but
with a much stronger intensity compared with the case of
nucleophilic addition of OHꢀ to C60 (Figure 1), consistent
with thepresence of bulk monoanionicintermediates inthe
mixture.
ꢀ
show that only monoanionic intermediates of CH3OC60
are produced during the reaction, and it is necessary to
quench the monoanionic intermediates with electro-
philes to obtain well-structured monomethoxylated
and multiple methoxylated C60 derivatives. This work
extends the scope of nucleophiles that are suitable for
fullerene functionalizations and may shed light on devel-
oping new methods for preparing oxygenated fullerenes
via direct nucleophilic reactions.
Acknowledgment. The work was supported by NSFC
(20972150, 21172212) and the Solar Energy Initiative of
CAS (KGCX2-YW-399 þ 9).
Since the reaction is initiated with the formation of
ꢀ
Supporting Information Available. Experimental and
calculation details, and spectra for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
CH3OC60 as indicated by the in situ visꢀnear-IR
spectrum, and no dimethoxy C60 compound is obtained,
it implies that quenching of CH3OC60 with electrophiles
ꢀ
(44) Kadish, K. M.; Gao, X.; Van Caemelbecke, E.; Suenobu, T.;
Fukuzumi, S. J. Am. Chem. Soc. 2000, 122, 563–570.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 9, 2012
2389