Highly functionalized bicyclo[2.2.2]octenone-fused [60]fullerenes from masked o-benzoquinones and C60

Article abstract of DOI:10.1021/ol000198s

The Diels-Alder reactions of masked obenzoquinones (MOBs) with [60]fullerene, affording novel and highly functionalized bicyclo[2.2.2]-octenone-fused [60]fullerene derivatives, are described.

Full text of DOI:10.1021/ol000198s

ORGANIC
LETTERS
2000
Vol. 2, No. 18
2909-2912
Highly Functionalized
Bicyclo[2.2.2]octenone-Fused
[60]Fullerenes from Masked
o-Benzoquinones and C
60
Chi-Feng Yen, Rama Krishna Peddinti, and Chun-Chen Liao*
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu-300, Taiwan
ccliao@mx.nthu.edu.tw
Received July 26, 2000
ABSTRACT
The Diels−Alder reactions of masked o-benzoquinones (MOBs) with [60]fullerene, affording novel and highly functionalized bicyclo[2.2.2]-
octenone-fused [60]fullerene derivatives, are described.
Since the discovery1 and macroscale preparation2 of C60,
numerous reactions have been developed to explore the
potentially rich and diverse chemistry of this new and
spherical allotrope of carbon.3 Although a number of
protocols have been devised, cycloaddition reactions4 proved
to be one of the most outstanding and expeditious methods
for the functionalization of [60]fullerene. The [4 + 2]
cycloaddition is of special significance in this context.4,5 C60
with a low-lying LUMO6 behaves as a good dienophile,
reacting with a large variety of dienes, particularly with
electron-rich dienes, to afford selectively the adducts on 6,6-
ring junctions. In some cases these cycloadducts undergo a
facile retro-Diels-Alder reaction as a result of their low
thermodynamic stability.3a,7 However, cycloreversion could
be prevented by stabilizing the cycloadducts through incor-
poration of the formed double bond into an aromatic ring,
as in the cases of o-quinonedimethanes8 and isobenzofuran,9
by the extrusion of CO,7a or by converting the double bond
into a single bond, as in the cases of Danishefsky’s diene,6b
2-silyloxy-1,3-butadienes,10 and 4-hydroxytropones.11a
Recently, electron-deficient dienes such as tropones,11 1,3-
dienes bearing an electron-withdrawing group,10b,12 and
2-pyrone13 were employed in the Diels-Alder reactions of
electron-poor polyolefin C60, and the cycloadducts were
found to be quite stable. In this communication we report
(1) Kroto, H. W.; Heath, J. R.; O′Brien, S. C.; Curl, R. F.; Smalley, E.
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Y. Angew. Chem., Int. Ed. Engl. 1992, 31, 1101. (c) Taylor, R. The
Fullerenes; Kroto, H. W., Walton, D. R. M., Eds.; Cambrige University
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Fullerenes; Thieme: Stuttgart, 1994. (e) Hirsch, A. Top. Curr. Chem. 1999,
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Org. Chem. 2000, 65, 3395 and references therein.
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Commun. 1997, 27, 1475.
10.1021/ol000198s CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/16/2000

the Diels-Alder reactions of masked o-benzoquinones
(MOBs), which are known as electron-deficient dienes, with
C60 to obtain novel and highly functionalized bicyclo[2.2.2]-
octenone-fused [60]fullerenes.
the [4 + 2] cycloaddition with C60 at three different
temperatures. While the reactions performed at 30 and 60
°C produced the desired cycloadduct 3a in poor yields
together with a dimer of 1a, the reaction carried out at 110
°C resulted in the exclusive formation of dimer of 1a. In
contrast to 1a, MOBs 1b and 1c exhibited improved
reactivity toward C60 at 110 °C and afforded the adducts 3b
and 3c in acceptable yields.
To broaden the scope of these reactions, we have prepared
a series of stable MOBs 1d-j. Thus, MOBs 1d-j were
produced at 0 °C in MeOH by the oxidation of the
corresponding 2-methoxyphenols 2d-j in the presence of
DAIB and were isolated in very good to excellent yields.23
The Diels-Alder reactions of MOBs 1d-h were carried out
at different temperatures in toluene to furnish cycloadducts
3d-h22b (Scheme 1), and the results are summarized in Table
1.
Masked o-benzoquinones, a class of cyclohexa-2,4-di-
enones, can be easily generated in situ by the oxidation of
readily available 2-methoxyphenols with hypervalent iodine
reagents such as diacetoxyiodobenzene (DAIB) and bis-
(trifluoroacetoxy)iodobenzene in MeOH. Over the past 10
years we have been investigating the inter-14-17 and intramo-
lecular14,18 Diels-Alder reactions of MOBs, and their
synthetic potential19 has been explored. Despite the fact that
MOBs are electron-deficient, they readily undergo Diels-
Alder cycloadditions with both electron-poor16 and electron-
rich20 dienophiles. It occurred to us that if MOBs, being
electron-deficient dienes, participate in the [4 + 2] cycload-
dition with [60]fullerene, easy access to stable bicyclo[2.2.2]-
octenone-fused [60]fullerenes could be achieved. Therefore,
work was carried out in this direction.
Unlike other cyclohexa-2,4-dienones, MOBs are highly
reactive and readily undergo dimerization,16,21 leaving less
room for their isolation (Figure 1). To circumvent this
Scheme 1
Figure 1. Structures of MOBs and their dimers.
obstacle, the Diels-Alder reactions could be carried out by
slowly generating MOBs in the presence of a dienophile.
However, owing to the solubilty problems associated with
C60 under the reaction conditions for the in situ generation
of MOBs, it was not possible to perform the cycloaddition
as mentioned above. Alternatively, toluene solutions of
unstable MOBs, which are generated in MeOH at 0 °C, can
be used for the cycloaddition step. The reactive MOB 1a
was first generated, and its toluene solution22a was used for
The gross structures of adducts 3a-h were established
1
from their IR, UV-vis, H (600 MHz) and 13C NMR (150
MHz), DEPT, and low- and high-resolution FAB-MS spectral
analyses. The cycloadducts 3a-h exhibit IR absorption at
ca. 526 cm-1 and a weak band in UV-vis spectra at ca. 432
nm, indicating the characteristic features of dihydroful-
lerenes.7a,24 In the 13C NMR spectra, the fullerene bridgehead
(14) Chu, C.-S.; Lee, T.-H.; Liao, C.-C. Synlett 1994, 635.
(15) Chen, C.-H.; Rao, P. D.; Liao C.-C. J. Am. Chem. Soc. 1998, 120,
13254.
(16) Liao, C.-C.; Chu, C.-S.; Lee, T.-H.; Rao, P. D.; Ko. S.; Song, L.-
D.; Shiao, H.-C. J. Org. Chem. 1999, 64, 4102.
(17) Hsieh, M.-F.; Rao, P. D.; Liao, C.-C. Chem. Commun. 1999, 1441.
(18) Chu, C.-S.; Lee, T.-H.; Rao, P. D.; Song, L.-D.; Liao, C.-C. J. Org.
Chem. 1999, 64, 4111.
(19) Liu, W.-C.; Liao C.-C. Chem. Commun. 1999, 117 and references
therein.
(20) Gao, S.-Y.; Lin, Y.-L.; Rao, P. D.; Liao, C.-C. Synlett 2000, 421.
(21) Andersson, G.; Berntsson, P. Acta Chem. Scand. B 1975, 29, 948.
(22) General Procedure. (a) To a suspension of DAIB (3.3 equiv) and
KHCO3 (7 equiv) in MeOH (3 mL) was added a solution of phenols 2a-c
(3 equiv) in MeOH (2 mL) in one portion at 0 °C under nitrogen. After 5
min of stirring, the reaction mixture was diluted with CH2Cl2, washed with
brine, and dried over anhydrous MgSO4. The thus obtained solution was
further diluted with toluene (10 mL), C60 (1 equiv., 50 mg, 0.069 mM)
was added, and the low boiling solvents were removed under reduced
pressure. Then the reaction was continued as mentioned in Table 1. After
the reaction was complete, the reaction mixture was concentrated under
reduced pressure, and the residue was purified by silica gel column
chromatography with toluene as an eluent to furnish the pure cycloadducts
3a-c. (b) A solution of MOBs 1d-j (1.2 equiv) and C60 (1 equiv, 50 mg,
0.069 mM) in toluene (1,2-dichlorobenzene for the reactions at 180 °C)
(10 mL) was stirred at the appropriate temperature over a period of time
(Table 1). The solvent was removed under reduced pressure at room
temperature [removal of toluene at above room temperature provided
monoadducts 3d-h in low yields as a result of (further) formation of
bisadducts with unreacted MOBs under high concentrations!], and the
residue was purified as in the case of 3a-c to afford pure adducts 3d-h.
(23) For the synthesis of MOBs 1e and 1f, see: Lai, C.-H.; Shen, Y.-L.;
Liao C.-C. Synlett 1997, 1351. Synthesis of other stable MOBs will be
published elsewhere.
(24) (a) Taylor, R.; Walton, D. R. M. Nature 1993, 363, 685. (b) Hirsch,
A.; Gro¨sser, T.; Skiebe, A.; Soi, A. Chem. Ber. 1993, 126, 1061. (c) Isaacs,
L.; Wehrsig, A.; Diderich, F. HelV. Chim. Acta 1993, 76, 1231.
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Org. Lett., Vol. 2, No. 18, 2000

Table 1. Bicyclo[2.2.2]octenone-Fused [60]Fullerene Derivatives 3 from the Diels-Alder Reactions of MOBs 1 and C60
a All reactions were carried out with MOBs (3 equiv for 1a-c or 1.2 equiv for 1d-i) and C60 (1 equiv) in toluene unless otherwise specified; also see
ref 22. b Temperature refers to oil bath. c Yields of pure and isolated adducts are based on consumed C60. d Bisadducts were also obtained. e Part of MOB
1
was aromatized. f Reaction was performed in 1,2-dichlorobenzene. g Observed in H NMR spectrum of the crude reaction mixture.
quaternary sp3 carbon atoms appeared at ca. δ 66 and 69
(3a-e and 3g) or ca. δ 68 and 72 (3f and 3h), which
indicates the closed transannular bond, confirming the 6-6
ring junction on the C60 cage.7a,25 Of the 58 sp2 carbons of
the fullerene framework that are diastereotopic in nature
owing to the introduction of chiral centers in the fused
bicyclo[2.2.2]octenone system, 13C NMR spectra of 3a-h
showed only 49-56 peaks in the region δ 135-156 because
of overlap of the rest of the signals. In the mass spectra, the
parent ions of the cycloadducts are clearly observed, although
the major peak in each spectrum is at m/z 720 corresponding
to C60+. In addition, the bicyclo[2.2.2]octenone system also
showed characteristic signals at ca. δ 200 for carbonyl and
ca. δ 97 for quaternary carbon with gem dimethoxy groups
in 13C NMR for the adducts 3a-h.
From Table 1, it is clear that the yields of the cycloadducts
depend on the nature of MOBs and the reaction conditions
1
employed. The H NMR (400 MHz) spectra of the crude
Org. Lett., Vol. 2, No. 18, 2000
2911

samples revealed the compositon of the compounds formed
during the reaction. In the cases of more reactive MOBs 1a-
c, which are unstable, large amounts of dimers were obtained
along with fullerene derivatives; however, no unreacted
MOBs were noticed. It is worth mentioning that MOB 1a,
which exhibited very good reactivity with many other
dienophiles,14-17 failed to produce 3a in good yields in the
cycloaddition with C60. This diminished reactivity of 1a
appears to be due to electronic reasons. The stable MOBs
1d-f produced the Diels-Alder cycloadducts 3d-f in higher
yields at 30 °C. The reactions of 1d-f at higher temperatures
resulted in lowering the yields of 3d-f and increasing the
yields of bisadducts of C60 as a result of the enhanced Diels-
Alder reactivity of 1d-f. The stable MOBs 1g and 1h require
higher temperatures in order to furnish the adducts 3g and
3h, respectively, in high yields, presumably because of the
bulky ketal group. While MOB 1d produced noticeable
amounts of dimer at low temperatures, MOBs 1e-h did not
dimerize under the reaction conditions.
and electronic factors. These results illustrate that 5-substi-
tuted MOBs are more stable and exhibit less Diels-Alder
reactivity toward less reactive/bulky dienophiles. Our find-
ings are in accordance with Andersson’s observations on the
Diels-Alder reactions of 5-methoxy-MOB.26
Initially it was assumed that the low yields observed in
some reactions might be due to the cycloreversion of the
adducts 3. To verify this assumption, cycloadduct 3e was
taken in toluene and 1,2-dichlorobenzene and stirred at 110
and 180 °C, respectively, for 24 h. However, 3e was found
to be unchanged, and neither the dimer of MOB nor
bisadduct/higher adducts were observed, indicating that no
retro-Diels-Alder reaction had taken place.
In conclusion, we have shown that masked o-benzoqui-
nones, which are reactive cyclohexa-2,4-dienones, undergo
Diels-Alder cycloadditions with [60]fullerene to produce
hitherto unknown, stable, and highly functionalized bicyclo-
[2.2.2]octenone derivatives. In light of the mild conditions
and considerable generality, these reactions are certainly
noteworthy. Because of their high functionality, compounds
3 will allow easy access to other derivatives. Such further
derivatization is underway in our laboratory.
MOB 1i, having substitution at position 5, did not
participate in the Diels-Alder reaction with C60 in toluene.
1
However, the H NMR (400 MHz) analysis of the crude
reaction mixture from the reaction in 1,2-dichlorobenzene
at 180 °C showed signals corresponding to the cycloadduct
3i. The MOB 1j (Rn ) 5-CO2Me) did not give the Diels-
Alder adduct even at 180 °C, presumably because of steric
Acknowledgment. Financial support from the National
Science Council (NSC) of the Republic of China is sincerely
acknowledged. R.K.P. thanks NSC for a postdoctoral fel-
lowship.
(25) (a) Belik, P.; Gu¨gel, A.; Spickermann, J.; Mu¨llen, K. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 78. (b) Khan, S. I.; Oliver, A. M.; Paddon-Row:
M. N.; Rubin, Y. J. Am. Chem. Soc. 1993, 115, 4919. (c) Maggini, M.;
Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115, 9798. (d) Diederich,
F.; Jonas, U.; Gramlich, V.; Herrmann, H.; Ringsdorf, H.; Thilgen, C. HelV.
Chim. Acta 1993, 76, 2445. (e) Kra¨utler, B.; Puchberger, M. HelV. Chim.
Acta 1993, 76, 1626.
Supporting Information Available: 1H and 13C NMR
and DEPT spectra for compounds 3a-h. This material is
available free of charge via the Internet at http://pubs.acs.org.
(26) Andersson G. Acta Chem. Scand. B 1976, 30, 403.
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