Fullerenyl boronic esters: Ferric perchlorate-mediated synthesis and functionalization

Article abstract of DOI:10.1021/ol300398n

Fullerenyl boronic esters have been prepared by a ferric perchlorate-promoted reaction of [60]fullerene with various arylboronic acids. The obtained fullerenyl boronic esters could undergo further functionalization with diols to afford C₆₀-fuse

Full text of DOI:10.1021/ol300398n

ORGANIC
LETTERS
2012
Vol. 14, No. 7
1800–1803
Fullerenyl Boronic Esters: Ferric
Perchlorate-Mediated Synthesis and
Functionalization
Fa-Bao Li,†,‡ Xun You,† Tong-Xin Liu,† and Guan-Wu Wang*,†,§
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of
Soft Matter Chemistry, and Department of Chemistry, University of Science and
Technology of China, Hefei, Anhui 230026, P. R. China, Ministry of Education Key
Laboratory for the Synthesis and Application of Organic Functional Molecules and
School of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062,
P. R. China, and State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou, Gansu 730000, P. R. China
gwang@ustc.edu.cn
Received February 17, 2012
ABSTRACT
Fullerenyl boronic esters have been prepared by a ferric perchlorate-promoted reaction of [60]fullerene with various arylboronic acids. The
obtained fullerenyl boronic esters could undergo further functionalization with diols to afford C60-fused dioxane/dioxepane derivatives. A possible
reaction mechanism for the formation of fullerenyl boronic esters has been proposed.
Since fullerenes have become available in macroscopic
amounts, numerous chemical reactions of fullerenes have
been developed to prepare a plethora of functionalized
fullerene derivatives.1 Various reactions of fullerenes cat-
alyzed or mediated by transition-metal salts, particularly
those of Pd,2 Rh,3 and Mn,4 have attracted extensive
attention.5 Organoboron compounds have been widely
used in organic synthesis.6 Itami and co-workers recently
reported the Pd-catalyzed arylation of [60]fullerene (C60)
with arylboronic acids.2d Itami’s group also developed the
Rh-catalyzed arylation and alkenylation of C60 using
† University of Science and Technology of China.
‡ Hubei University.
§ Lanzhou University.
(1) For selected reviews, see: (a) Hirsch, A. Synthesis 1995, 895. (b)
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Martın, N. Chem. Rev. 2006, 106, 5136. (d) Matsuo, Y.; Nakamura, E.
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(2) Pd: (a) Shiu, L.-L.; Lin, T.-I.; Peng, S.-M.; Her, G.-R.; Ju, D.-D.;
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r
10.1021/ol300398n
Published on Web 03/14/2012
2012 American Chemical Society

organoboron compounds.3h,i They later extended the Rh-
catalyzed arylation to [70]fullerene (C70).3i p-Tolylboronic
acid was also employed to functionalize N-tosyl[1,2]-
aziridino[60]fullerene affording the corresponding fuller-
enyl boronic ester, and the aziridinofullerene was in turn
synthesized by the aziridination of C60 with TsNdIPh.5e
This was the only known example of fullerene diols
protected as a boronic ester. We previously disclosed the
reactions of C60 with nitriles, aldehydes/ketones, and
malonate esters mediated by cheap Fe(ClO4)3 to afford
C60-fused oxazolines,7a C60-fused 1,3-dioxolanes,7b and
C60-fused disubstituted lactones,7c respectively. The reac-
tions of C60 with nitriles7a and aldehydes/ketones7b were
believed to proceed via the Fe(ClO4)3-mediated hydration
reaction to generate Fe(III) complexes I and II, respec-
tively (Figure 1). We conjectured that boronic acids might
coordinate with Fe(ClO4)3 to form Fe(III) complex III,
which could similarly react with C60 and provide the scare
fullerenyl boronic esters in one pot. Herein, we report our
success in the synthesis of fullerenyl boronic esters by the
Fe(ClO4)3-mediated reaction of C60 with arylboronic acids
and further conversion into C60-fused dioxane/dioxepane
derivatives.
reaction of C60 with 1aꢀi are listed in Table 1. As can be
seen from Table 1, fullerenyl boronic esters 2aꢀi were
obtained in 13ꢀ38% yields (57ꢀ93% based on consumed
C60), comparable to the previously reported data for most
monoadducts. Control experiments in the absence of Fe-
(ClO4)3 or heating gave no desired products. It should be
noted that although no bis-adducts could be isolated,
some unknown highly polar byproducts were formed in
some cases. The current reaction was compatible with a
wide variety of functional groups such as bromo, keto,
cyano, and nitro groups, which may be used as synthetic
handles forfurther functionalization. Unfortunately, no or
only trace amounts of fullerenyl boronic esters could be
obtained for arylboronic acids bearing electron-donating
groups such as 4-methylphenylboronic acid and 4-meth-
oxyphenylboronic acid as well as alkyl boronic acids such
as isopropyl boronic acid even in the presence of excess of
reaction reagents or by increasing the reaction temperature
and extending the reaction time.
It is noteworthy that the Fe(ClO4)3-promoted reaction
of C60 with thiophene-2-boronic acid (1j) under the same
conditions produced fullerenyl borate ester 2j0 in 32%
isolated yield (94% based on consumed C60), instead of
the expected fullerenyl boronic ester 2j (Scheme 1).
Scheme 1. Fe(ClO4)3-Mediated Reaction of C60 with Thio-
phene-2-boronic Acid 1j
Figure 1. Proposed reaction intermediates.
To our satisfaction, Fe(ClO4)3 efficiently promoted the
reaction of C60 with a series of arylboronic acids (1aꢀi) to
generate the expected fullerenyl boronic esters. The reac-
tion conditions and yields for the Fe(ClO4)3-promoted
Fullerenyl boronic esters 2aꢀi and borate 2j0 were
1
characterized by HRMS, H NMR, 13C NMR, FT-IR,
and UVꢀvis spectra. All of the 1H NMR spectra exhibited
the corresponding expected signals. In the 13C NMR
spectra of 2aꢀi and 2j0, there were no more than 16 lines
in the range of 136ꢀ149 ppm for the 58 sp2-carbons of the
(4) Mn: (a) Zhang, T.-H.; Lu, P.; Wang, F.; Wang, G.-W. Org.
Biomol. Chem. 2003, 1, 4403. (b) Wang, G.-W.; Zhang, T.-H.; Cheng, X.;
Wang, F. Org. Biomol. Chem. 2004, 2, 1160. (c) Li, C.; Zhang, D.; Zhang,
X.; Wu, S.; Gao, X. Org. Biomol. Chem. 2004, 2, 3464. (d) Wang, G.-W.;
Li, F.-B. Org. Biomol. Chem. 2005, 3, 794. (e) Chen, Z.-X.; Wang, G.-W.
J. Org. Chem. 2005, 70, 2380. (f) Cheng, X.; Wang, G.-W.; Murata, Y.;
Komatsu, K. Chin. Chem. Lett. 2005, 16, 1327. (g) Wang, G.-W.; Yang,
H.-T.; Miao, C.-B.; Xu, Y.; Liu, F. Org. Biomol. Chem. 2006, 4, 2595. (h)
Wang, G.-W.; Li, F.-B.; Zhang, T.-H. Org. Lett. 2006, 8, 1355. (i) Li,
F.-B.; Liu, T.-X.; Huang, Y.-S.; Wang, G.-W. J. Org. Chem. 2009, 74,
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Org. Lett. 2011, 13, 6130.
C
60 skeleton and one peak at 92ꢀ96 ppm for the two sp3-
carbons of the C60 cage, agreeing well with their C2v
molecular symmetry. The observed chemical shifts at
92ꢀ96 ppm are close to those of other 1,2-adducts, of which
the oxygen atom is connected to the C60 skeleton.4hꢀj,7
On the basis of the previously suggested mechanisms for
the reactions of C60 with nitriles7a and aldehydes/ketones7b
in the presence of Fe(ClO4)3, we propose a possible
mechanism for the formation of fullerenyl boronic esters
2 from the Fe(ClO4)3-mediated reaction of C60 with boro-
nic acids (Scheme 2). A chosen boronic acid reacts with
Fe(ClO4)3 to produce Fe(III) complex III accompanied by
the elimination of HClO4. Addition of complex III to C60
generates fullerenyl radical IV, followed by coordination
with another molecule of Fe(ClO4)3 to form Fe(III)
(5) For other recent examples, see: (a) Filippone, S.; Maroto, E. E.;
ꢀ
n-Domenech, A.; Suarez, M.; Martı
Martı
´
´
n, N. Nat. Chem. 2009, 1, 578.
(b) Tzirakis, M. D.; Orfanopoulos, M. J. Am. Chem. Soc. 2009, 131,
4063. (c) Tzirakis, M. D.; Orfanopoulos, M. Angew. Chem., Int. Ed.
2010, 49, 5891. (d) Xiao, Z.; Matsuo, Y.; Nakamura, E. J. Am. Chem.
Soc. 2010, 132, 12234. (e) Nambo, M.; Segawa, Y.; Itami, K. J. Am.
ꢀ
Chem. Soc. 2011, 133, 2402. (f) Maroto, E. E.; de Cozar, A.; Filippone,
ꢀ
S.; Martın-Domenech, A.; Suarez, M.; Cossıo, F. P.; Martın, N. Angew.
´ ´ ´
Chem., Int. Ed. 2011, 50, 6060. (g) Lu, S.; Jin, T.; Bao, M.; Yamamoto
J. Am. Chem. Soc. 2011, 133, 12842. (h) Lu, S.; Jin, T.; Kwon, E.; Bao,
M.; Yamamoto, Y. Angew. Chem., Int. Ed. 2012, 51, 802.
(6) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (b)
Fagnou, K.; Lautens, M. Chem. Rev. 2003, 103, 169.
(7) (a) Li, F.-B.; Liu, T.-X.; Wang, G.-W. J. Org. Chem. 2008, 73,
6417. (b) Li, F.-B.; Liu, T.-X.; You, X.; Wang, G.-W. Org. Lett. 2010, 12,
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Org. Lett., Vol. 14, No. 7, 2012
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Table 1. Reaction Conditions and Yields for the Fe(ClO4)3-
Mediated Reaction of C60 with 1aꢀia
Scheme 2. Proposed Reaction Mechanism for the Formation of
Fullerenyl Boronic Esters 2
groups probably arises from the lower acidity of the
electron-rich arylboronic acids and alkyl boronic acids,8
i.e., lower amount of RB[(OH)Oꢀ], retarding the forma-
tion of Fe(III) complex III and thus inhibiting the subse-
quent process. It is obvious that the mechanism for the
Fe(ClO4)3-mediated formation of fullerenyl boronic esters
is completely different from that for the Pd-2d or Rh-
catalyzed3h,i hydroarylation of C60 although the same
arylboronic acids were used.
The fullerenyl boronic esters are valuable precursors for
further functionalization. The ArBO2 moiety in products 2
was removable and used as a template to introduce other
functional groups. We found that treatment of represen-
tative 2a with ethylene glycol, 1,3-propanediol, and 1,3-
butanediol in the presence of p-toluenesulfonic acid at
150 °C for 1 h afforded the rare C60-fused dioxane/
dioxepane derivatives 3a-c in 92%, 81%, and 84% yield,
respectively (Scheme 3). This transformation should pro-
ceed via the acid-catalyzed transetherization of fullerenyl
boronic esters with diols.
Scheme 3. Reaction of Fullerenyl Boronic Ester 2a with Ethy-
lene Glycol, 1,3-Propanediol, and 1,3-Butanediol in the Pre-
sence of p-Toluenesulfonic Acid
a All reactions were performed in a molar ratio of C60/Fe(ClO4)3
3
6H2O/1 = 1:3:5 at 100 °C for 30 min under nitrogen atmosphere.
b Isolated yield; that in parentheses was based on consumed C60
.
complex V and then intramolecular cyclization with the
loss of a Fe(II) species to afford fullerenyl boronic esters 2.
Control experiments showed that 2j0 could not be obtained
from the Fe(ClO4)3-mediated reaction of C60 with boric
acid. Product 2j0 was likely formed from the spontaneous
loss of the thiophene-yl group from the initially generated
2j under our reaction conditions. The failure for the
reaction with arylboronic acids bearing electron-donating
Products 3aꢀc were also fully characterized. In the 13
C
NMR spectra of 3a and 3b, there were 16 lines in the range
(8) Yan, J.; Springsteen, G.; Deeter, S.; Wang, B. Tetrahedron 2004,
60, 11205.
1802
Org. Lett., Vol. 14, No. 7, 2012

of 136ꢀ151 ppm for the sp2-carbons of the C60 skeleton
and one peak at 86ꢀ92 ppm for the sp3-carbons of the C60
cage, agreeing with their C2v molecular symmetry. In
contrast, the 13C NMR spectrum of 3c displayed 50 lines
in the 135ꢀ152 ppm range for the sp2-carbons of the C60
moiety and two peaks at 91.00 and 92.66 ppm for the sp3-
carbons of the C60 skeleton, consistent with its C1 mole-
cular symmetry.
In summary, fullerenyl boronic esters have been effec-
tively prepared by the reaction of C60 with boronic acids
in the presence of Fe(ClO4)3 under nitrogen atmosphere,
and can undergo further transformation with diols to
afford novel C60-fused dioxane/dioxepane derivatives.
The current protocol provides facile access to fullerenyl
boronic derivatives via a one-step procedure by using
cheap and easily available boronic acids and Fe(ClO4)3.
A possible reaction mechanism for the formation of
fullerenyl boronic esters has been suggested. The study
on other Fe(ClO4)3-mediated reactions of fullerenes is
underway.
Acknowledgment. We are grateful for financial support
from the National Natural Science Foundation of China
(21132007, 20972145, 21102041), National Basic Research
Program of China (2011CB921402), and Specialized Re-
search Fund for the Doctoral Program of Higher Educa-
tion (200803580019).
Supporting Information Available. Experimental pro-
cedures, characterization data, and the 1H NMR and 13
C
NMR spectra of products 2aꢀi, 2j0, and 3aꢀc. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 7, 2012
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