Thermal and Microwave-Assisted Synthesis of Diels-Alder Adducts of [60]Fullerene with 2,3-Pyrazinoquinodimethanes: Characterization and Electrochemical Properties

Article abstract of DOI:10.1021/jo9701344

[4+2] Cycloaddition reactions of six-membered heterocyclic o-quinodimethanes, generated "in situ" from pyrazine derivatives, to [60]fullerene, either under thermal or microwave irradiation are described. Other microwave assisted cycloadditions involving o-quinodimethanes derived from thiophene were also performed. A comparative study of the activation energy for the boat-to-boat conformational inversion has been carried out by dynamic NMR experiments, the ΔG? values being highly dependent on the nature of the covalently attached heterocyclic systems. Theoretical calculations predict a more planar cyclohexene ring for the five member containing cycloadducts. The cycloaddition process is controlled by the HOMO of the heterocyclic o-quinodimethanes showing a LUMO(C₆₀)-HOMO(diene) energy differences typical for favoured cycloadditions. The redox properties of the novel organofullerenes have been determined by cyclic voltammetry in solution, showing a cathodically shifted first reduction potential values, related to [60]fullerene. Compound 15c bearing two cyano groups exhibited an opposite trend which was accounted for by the lower LUMO energy determined by semiempirical calculations.

Full text of DOI:10.1021/jo9701344

J . Org. Chem. 1997, 62, 3705-3710
3705
Th er m a l a n d Micr ow a ve-Assisted Syn th esis of Diels-Ald er
Ad d u cts of [60]F u ller en e w ith 2,3-P yr a zin oqu in od im eth a n es:
Ch a r a cter iza tion a n d Electr och em ica l P r op er ties
Ursula M. Fern a´ ndez-Paniagua, Beatriz Illescas, Nazario Mart ´ı n,* and Carlos Seoane*
Departamento de Qu ´ı mica Org a´ nica, Facultad de Qu ı´ mica, Universidad Complutense,
2
8040 Madrid, Spain
Pilar de la Cruz, Antonio de la Hoz, and Fernando Langa*
Departamento de Qu ı´ mica Org a´ nica, Facultad de Qu ´ı mica, Universidad de Castilla la Mancha,
Toledo, Spain
Received J anuary 24, 1997X
[4+2] Cycloaddition reactions of six-membered heterocyclic o-quinodimethanes, generated “in situ”
from pyrazine derivatives, to [60]fullerene, either under thermal or microwave irradiation are
described. Other microwave assisted cycloadditions involving o-quinodimethanes derived from
thiophene were also performed. A comparative study of the activation energy for the boat-to-boat
q
conformational inversion has been carried out by dynamic NMR experiments, the ∆G values being
highly dependent on the nature of the covalently attached heterocyclic systems. Theoretical
calculations predict a more planar cyclohexene ring for the five member containing cycloadducts.
The cycloaddition process is controlled by the HOMO of the heterocyclic o-quinodimethanes showing
a LUMO(C60)-HOMO(diene) energy differences typical for favoured cycloadditions. The redox
properties of the novel organofullerenes have been determined by cyclic voltammetry in solution,
showing a cathodically shifted first reduction potential values, related to [60]fullerene. Compound
1
5c bearing two cyano groups exhibited an opposite trend which was accounted for by the lower
LUMO energy determined by semiempirical calculations.
In tr od u ction
Cycloaddition reactions are among the most outstand-
ing and expeditious methods for the derivatization of [60]-
fullerene, which behaves as a good dienophile reacting
1
with a large variety of dienes. In particular, [4 + 2]
cycloaddition reactions afford selectively the adducts on
6
,6-ring junctions which, in some cases, undergo a facile
retro-Diels-Alder reaction as a consequence of the low
thermodynamic stability of the adduct formed.2 Very
stable Diels-Alder cycloadducts have been, however,
prepared by using different substituted o-quino-
F igu r e 1.
3
dimethanes due to the stabilization by aromatization of
the resulting products. Thus, Diels-Alder and hetero-
Diels-Alder cycloadditions of C60 with o-quinodimethane
(
1) and related heterodienes (2) have been previously
4
reported to yield the respective carbo and heterocycle-
fused fullerenes (3 and 4) (Figure 1).
5
X
Abstract published in Advance ACS Abstracts, May 1, 1997.
(
1) (a) Hirsch, A. The Chemistry of the Fullerenes; Thieme Verlag:
Stuttgart, 1994. (b) Diederich, F.; Thilgen, C. Science 1996, 271, 317.
c) Taylor, R.; Walton, D. R. M. Nature 1993, 363, 685. (d) The
(
Chemistry of Fullerenes; Taylor, R., Ed.; University of Sussex: World
Scientific, 1995.
(
2) Hirsch, A. Synthesis 1995, 895.
(3) For a review on o-quinodimethanes, see: Mart ´ı n N.; Seoane, C.;
Hanack M. Org. Prep. Proced. Int. 1991, 23, 237.
4) (a) Kraus, A.; G u¨ gel, A.; Belik, P.; Walter, M.; M u¨ llen, K.
(
Tetrahedron 1995, 51, 9927. (b) Belik, P.; G u¨ gel, A.; Kraus, A.; Walter,
M.; M u¨ llen, K. J . Org. Chem. 1995, 60, 3307. (c) Imahori, H.; Hagiwara,
K.; Akiyama T.; Taniguchi, S.; Okada, T.; Sakata, Y. Chem. Lett. 1995,
2
65. (d) G u¨ gel, A.; Kraus, A.; Spickermann, J .; Belik, P.; M u¨ llen, K.
Angew. Chem., Int. Ed. Engl. 1994, 33, 559. (e) Iyoda, M.; Sultana, F.;
Sasaki, S.; Yoshida, M. J . Chem. Soc., Chem. Commun. 1994, 1929.
F igu r e 2.
(f) Bidell, W.; Douthwaite, R. E.; Green, M. L. H.; Stephens, A. H.;
Turner, J . F. C. J . Chem. Soc., Chem. Commun. 1994, 1641. (g) Zhang,
X.; Foote, C. S. J . Org. Chem. 1994, 59, 5235. (h) Belik, P.; G u¨ gel, A.;
Spickermann, J .; M u¨ llen, K. Angew. Chem., Int. Ed. Engl. 1993, 32,
[4 + 2] Cycloaddition reactions of heteroaromatic
analogues of o-quinodimethane (5) (Figure 2) as dienes
and [60]fullerene have been much less studied due to the
synthetic difficulties in the preparation of the specific
7
8. (i) Belik, P.; G u¨ gel, A.; Kraus, A.; Spickermann, J .; Enkelmann,
V.; Frank, G.; M u¨ llen, K. Adv. Mater. 1993, 5, 854. See also: Meier,
M. S. Chapter 10 in ref 1d.
S0022-3263(97)00134-5 CCC: $14.00 © 1997 American Chemical Society

3
706 J . Org. Chem., Vol. 62, No. 11, 1997
Fernandez-Paniagua et al.
Sch em e 1
heterocyclic precursors needed for the generation “in situ”
of the corresponding heteroaromatic o-quinodimethanes.
with the corresponding 2,3-bis(bromomethyl)pyrazine
derivatives (13a -c), which can be easily prepared, in a
one-step procedure, by condensation of 1,4-dibromo-2,3-
butanedione (12) with 1,2-phenylenediamine (11a ), naph-
6
Although this methodology results very appropriate for
the covalent attachment of heterocycles to the C60 core
connected through two methylene units, it is syntheti-
cally restricted to the generation of these reactive inter-
mediates, some of which have not yet been reported.6
Consequently, only a quite limited number of hetero-
cyclic systems, not directly fused to the C60 cage, are
known. To the best of our knowledge, only the thiophene-
containing derivatives 6-8,7 the very light sensitive in
thalenediamine (11b), and 2,3-diaminobutenedinitrile
14
(
11c) (Scheme 1). 2,3-Pyrazinoquinodimethane deriva-
tives (14a -c) were generated “in situ” by treatment of
,3-bis(bromomethyl)pyrazine derivatives (13a -c) with
2
sodium iodide in o-dichlorobenzene (ODCB) at 130 °C,
according to the method recently reported for the genera-
,8
15
tion of 2,3-quinoxalinoquinodimethane. These reactive
9
oxygen atmosphere furan-linked derivative 9, and the
intermediates (14a -c) were easily trapped as the respec-
tive Diels-Alder adducts by reaction with [60]fullerene
acting as the dienophile (see the Experimental Section)
thiazole ring containing cycloadduct 109 have been
reported. Indole and quinoxaline derivatives have also
been recently described by Eguchi et al.9 Other attempts
(Table 1).
to prepare cycloadducts with the oxazolidine ring at-
tached to the C60 core were unsuccessful.5
An alternative procedure for the preparation of pyra-
zine-containing cycloadducts (15a and 15b) has been
recently reported by Mattay et al.16 In this synthetic
approach the pyrazine ring is generated in the last
synthetic step by condensation of o-phenylenediamine
On the other hand, microwave irradiation has shown
to be a very useful alternative to conventional heating
1
0
as source of energy for chemical reactions.
In this
regard, significative reduction of reaction times and
improved yield of products are generally achieved. Some-
times, microwave irradiation is crucial, because the
reaction fails under conventional heating.11
1
7
with the appropriate fullerocyclohexanedione.
In this work we have used a focused microwave reactor
18
(Maxidigest MX350 from Prolabo). The novel results,
We have recently reported that the Diels-Alder reac-
tion of o-quinodimethane and its derivatives with [60]-
fullerene can be advantageously carried out by using
sultines as precursors of o-quinodimethane under micro-
wave irradiation due to the observed accelerating effect
of microwaves.12 In fact, a wide variety of cycloaddition
together with those obtained for the cycloaddition of
(5) For a recent review on the synthesis of heterocycle-containing
[60]fullerenes, see: Eguchi, S.; Ohno, M.; Kojima, S.; Koide, N.;
Yashiro, A.; Shirakawa ,Y.; Ishida, H. Fullerene Sci. Technol. 1996, 4,
303.
(6) (a) Ando, K.; Takayama, H. Heterocycles 1994, 37, 1417. (b) Chou,
reactions can be successfully performed on [60]fullerene
T.-S. Rev. Heteroat. Chem. 1993, 8, 65. Chou, T.-S.; Chang, R.-C. J .
Org. Chem. 1993, 58, 493. (c) Chaloner, M. L.; Crew, A. P. A.; O’Neill,
P. M.; Storr, R. C.; Yelland, M. Tetrahedron 1992, 48, 8101.
under microwave conditions.13
In this paper we report [4 + 2] cycloadditions of a series
of six-membered heteroaromatic o-quinodimethanes to
(
7) Fern a´ ndez-Paniagua, U. M.; Illescas, B. M.; Mart ´ı n, N.; Seoane,
C. J . Chem. Soc., Perkin Trans. 1 1996, 1077.
8) Ohno, M.; Koide, N.; Eguchi, S. Heterocycl. Commun. 1995, 1,
25.
9) Recent Advances in the Chemistry and Physics of Fullerenes and
(
[60]fullerene together with other cycloadditions of five-
1
membered sulfur containing o-quinodimethanes to C60
obtained by microwave irradiation in comparison with
thermal conditions. The C60-pyrazine and thiophene-
containing adducts are readily prepared in moderate
yields, thus providing an expeditious access to [6,6]-closed
fullerene derivatives covalently attached to different
heterocycles.
(
Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemi-
cal Society, Inc.: New J ersey, 1996; Vol. 3, pp 1233-1243.
(
10) (a) Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. Rev. 1991,
2
0, 1. (b) Abramovich, R. A. Org. Prep. Proced. Int. 1991, 23, 685. (c)
Loupy, A.; Bram, G.; Sansoulet, J . New J . Chem. 1992, 16, 233. (d)
Caddick, S. Tetrahedron 1995, 51, 10403. (e) Strauss, C. R.; Trainor,
R. W. Aust. J . Chem. 1995, 48, 1665.
(11) (a) Motorina, I. A.; Parly, F.; Grieson, D. S. Synlett. 1996, 389.
Dynamic behavior of the different adducts has also
been investigated by using variable-temperature NMR
studies. Considering the electroactive character of these
novel organofullerenes, we have carried out their elec-
trochemical characterization by cyclic voltammetry in
solution. PM3 theoretical calculations were also per-
formed in order to gain some understanding of the
experimental findings.
(b) Huber, R. S.; J ones, G. B. J . Org. Chem. 1992, 57, 5778. (c) D ´ı az-
Ortiz, A.; Carrillo, J . R.; D ´ı ez-Barra, E.; de la Hoz, A.; G o´ mez-
Escalonilla, M. J .; Moreno, A.; Langa, F. Tetrahedron 1996, 52, 9237.
(12) Illescas, B.; Mart ´ı n, N.; Seoane, C.; de la Cruz, P.; Langa, F.;
Wudl, F. Tetrahedron Lett. 1995, 36, 8307.
(13) de la Cruz, P.; de la Hoz, A.; Langa, F.; Illescas, B.; Mart ´ı n, N.
Tetrahedron 1997, 53, 2599.
(14) Bandy, R. B.; Greenblatt, L. P.; J irlobvsky, I. L.; Conklin, M.;
Russo, R. J .; Bramlett, D. R.; Emrey, T. A.; Simmonds, J . T.; Kowal,
D. M.; Stein, R. P.; Tasse, R. P. J . Med. Chem. 1993, 36, 331.
(15) Alexandron, N. E.; Mertzanos, G. E.; Stephanidon-Stephanaton,
J .; Tsoleridis, C. A.; Zaxharion, P. Tetrahedron Lett. 1995, 36, 6777.
For other recent precursors for quinoxalino-2,3-quinodimethanes,
see: Chung, W.-S.; Liu, J .-H. Chem. Commun. 1997, 205.
Resu lts a n d Discu ssion
(
16) Mattay, J . Communication presented to Fullerenes’96. Oxford,
The novel pyrazine-containing cycloadducts (15a -c)
were synthesized by Diels-Alder reaction of [60]fullerene
UK, J uly 1996.
(17) Torres-Garc ´ı a, G.; Mattay, J . Tetrahedron 1996, 52, 5421.

Diels-Alder Adducts of [60]Fullerene
J . Org. Chem., Vol. 62, No. 11, 1997 3707
Ta ble 1. Com p a r a tive Resu lts in Cycloa d d ition s Usin g
Cla ssica l a n d Micr ow a ve P r oced u r es
Ta ble 2. P M3 Ca lcu la ted HOMO a n d LUMO Levels of
C60 a n d Heter ocyclic o-Qu in od im eth a n es
microwavea
classical heating
compd
time (h)
1.25
yield (%)
T (°C)
time (h)
yield (%)
b
b
b
b
b
c
b,7
7
8
16 (21)
23 (28)
26 (37)
22 (52)
9 (42)
reflux
reflux
reflux
reflux
72
17
24
24
5
25 (42)
43 (70)
13 (22)
e
c
b,7
1.16
d
b
1
1
1
5a
5b
5c
0.50
0.50
0.33
d
5 (14)b
d
16 (43)b
reflux
a
Power 60 W. b Based on recovered C60. c Toluene. d ODCB.
e
Power 70 W.
substituted thieno-o-quinodimethanes to [60]fullerene,
7
previously reported under thermal conditions, are shown
in Table 1.
Thermally activated cycloadditions were accomplished
by carrying out these reactions in refluxing ODCB under
argon atmosphere and using the stoichiometry C60:13a -
c:NaI:18-crown-6 (1:1.1:4:3). It is interesting to note that
only compound 15c was optimized in the pyrazine series
by monitoring of the thermal reaction by TLC. The
reaction was concluded when the presence of regioiso-
meric bisadducts could be detected. This result is in good
agreement with the remarkable higher yield obtained
based on recovered C60
. Longer reaction times as in
1
5a ,b result in lower yields due to the formation of
nonisolated higher adducts (see Table 1). Microwave-
assisted cycloadditions were performed under the same
experimental conditions for the preparation of cycload-
ducts 7 and 8 (105 W, monomode) and different experi-
mental conditions for cycloadducts 15a -c (C60:13a -c, 2:1
or 4:1; TBAB as phase transfer catalyst; 60 W monomode)
were used. Both different microwave-assisted experi-
mental conditions were used for the preparation of
organofullerene 7 without significative change in the
obtained yields, although shorter times and lower power
were needed when TBAB was used (see the Experimental
Section). Interestingly, the use of microwave irradiation
led to lower yield with five-membered o-quinodimethanes
1 4
distance (r1-4) between the C and C carbon atoms of
the respective dienes (Table 2). Consequently, cycload-
dition reactions using 2,3-thienoquinodimethanes and
2,3-pyrazinoquinodimethanes are both favored from the
orbital term point of view.
(18, 19), and higher yields were obtained when six-
membered 2,3-pyrazinoquinodimethanes were used as
dienes, related to the classical heating. Only compound
The UV-vis spectra of compounds 15a -c exhibit a
1
5c was obtained in a lower yield although in this case
typical weak absorption band at around 430 nm, char-
large amounts of polyadducts were observed by TLC. The
yields of pyrazine cycloadducts obtained by thermal
activation are comparatively lower than those of thiophene
cycloadducts. In this regard, in addition to the formation
of higher adducts and possible dimerization of the gener-
ated o-quinodimethane, the reaction of the bromomethyl
groups with the heterocyclic nitrogen yielding a pyri-
dinium salt cannot be ruled out.
13
acteristic for a dihydrofullerene structure. The C NMR
spectra of 15a and 15c showed 22 and 19 lines, respec-
tively, consistent with a C2v symmetry for these mol-
ecules. Cycloadduct 15a showed the quaternary sp3-
hybridized carbon atoms of attachment to the substituent
at δ 64.52 and the CH groups at δ 47.46. The quinoxa-
2
2
line carbons appear together with the remaining C60 sp -
carbon atoms between 155 and 127 ppm. Analogously,
cycloadduct 15c showed the quaternary carbons at δ
In order to rationalize these experimental findings we
have carried out PM3 quantum-chemical calculations
6
4.02 and 46.82 with the cyano group at δ 112.85, thus
(Hyperchem from Autodesk) on the heterocyclic o-quin-
confirming the 6,6-ring junction on the C60 cage. The
lower solubility of adduct 15b prevented the registration
of its 13C NMR spectrum.
odimethanes used as dienes in the preparation of cy-
cloadducts 7, 8, and 15a -c, in addition to the parent
unsubstituted pyrazino- and thienoquinodimethanes (16
and 17) (Table 2). From these data it is observed that
the cycloaddition is controlled by the HOMO of the
heterocyclic o-quinodimethanes. The LUMO(C60)-HO-
MO(diene) energy differences are clearly in the range of
energetically favored cycloadditions. On the other hand,
no significative differences were observed from the
Compared to the previously reported thiophene-
7
containing cycloadducts (7, 8) in which the methylene
protons appear as two singlets at around δ 4.6-4.8, the
pyrazine analogues (15a ,b) present two broad doublets
as a consequence of the restricted conformational inver-
sion of the ring at room temperature, and 15c shows a
broad singlet.
(
18) In the early experiments, domestic microwave ovens were used;
The cyclohexene ring has a boat conformation, and the
activation free energy for the boat-to-boat inversion has
been determined by DNMR experiments for the cycload-
this hardware had several limitations for using in laboratory. Home-
made modifications were first performed, and specific designed ap-
paratus have appeared later.

3
708 J . Org. Chem., Vol. 62, No. 11, 1997
Fernandez-Paniagua et al.
F igu r e 3.
similar to those described for carbocyclic compounds,19a
while in compounds 7 and 8 with a five-membered
Ta ble 3. Activa tion F r ee En er gies Deter m in ed for th e
Novel Heter ocyclic Cycloa d d u cts
∆
Gq
∆Gq
c
thiophene ring, T values decrease 80 and 100 deg and
compd Tc (K) ∆ν (Hz) (kJ mol 1)a J AB (Hz) (kJ mol-1)b
-
q
-1
∆G values decrease 15 and 20 kJ ‚mol , respectively
(Table 3).
1
1
7
7
8
8
5a
5b
323
333
245
241
223
231
104.4
126.1
26.2
14.8
51.0
86.7
64.7
66.2
51.3
51.6
45.3
45.9
14.4
14.4
14.7
13.9
15.4
15.4
64.5
66.1
50.2
49.8
44.9
45.8
In order to gain information about the origin of these
conformational differences, the molecular geometries of
adducts 7 and 15a were optimized at the semiempirical
PM3 level. The most relevant bond distances and bond
angles are illustrated in Figure 3.
a
Activation free energies at the coalescence temperatures
q
Both adducts show clearly a boat conformation in the
cyclohexene defined by C(1)-C(6)-C(64)-C(63)-C(62)-
C(61). The calculated bond lengths C(1)-C(6) are 1.59
Å in 7 and 1.58 Å in 15a , slightly shorter than 1.62 Å,
determined by X-ray diffraction for 63,66-dimethyl-64,-
65-diphenyl-1,9-(methano[1,2]benzenomethano)fullerene-
[60] by Rubin et al.1 As expected, the outer angles to
the heterocyclic ring which form part of the boat are
larger for the thiophene (120.4° and 120.9°) than for the
pyrazine ring, which shows a value of 116.6°. Interest-
ingly, calculations indicate that the boat is more planar
in 7 than in 15a ; the torsional angle defined by C(1)-
C(61)-C(62)-S(65) is 148.1° and for C(6)-C(64)-C(63)-
C(67) is 149.3° in 7, while that defined by C(1)-C(61)-
C(62)-N(65) in 15a resulted to be 131.5°, closer to the
value (135°) found by Rubin for the above carbocyclic
according to ∆G ) aT[9.972 + log(T+∆ν)] (ref 20). a ) 1.914 ×
-
2
-1 b
1
0
kJ mol
peratures according to ∆G ) aT[9.972 + log(T+(δν +6J AB)
ref 20).
.
Activation free energies at the coalescence tem-
q
2
2
1/2
]
(
dition with carbocyclic o-quinodimethanes.19 However,
this is the first study on cycloadducts derived from C60
and heterocyclic o-quinodimethanes. The coalescence
9a
q
temperature and ∆G values show a great dependence
on the nature of the heterocyclic system (Table 3).
1
The H-NMR spectra of 15a and 15b at 293 K in CDCl
3
solution display a broad double doublet, indicative of a
dynamic process. At 253 K a sharp AB system is
observed in both compounds (δ
4.4 Hz for compound 15a ; and δ
4.4 Hz for compound 15b). These AB quartets coalesce
B
) 5.29, δ
) 5.36, δ
A
) 4.94, J AB
) 4.94, J AB
)
)
1
1
B
A
at 323 K for 15a and 333 K for 15b.
1
19a
The H-NMR spectrum of 7 at 293 K in CDCl
3
solution
system.
displays two broad singlets which become sharp at 323
K (δ ) 4.76 for CH -61 and δ ) 4.61 for CH -64). At 213
K two AB systems are observed (δ ) 4.82, δ ) 4.74,
) 4.65, δ ) 4.6, J AB
-64), and these quartets coalesce at 241
The high barrier inversion of these compounds (15a ,b)
could be accounted for by the high torsional and angular
2
2
B
A
constraints of the structures due to the rigidity of the
9,21
J
1
AB) 14.7 Hz for CH
3.9 Hz for CH
2
-61 and δ
B
A
)
C
60 core.1
Because the transition state for the boat-
2
to-boat inversion should be at least partially planar, the
observed lower barrier of inversion in the thiophene-
containing adducts can be related to the higher planarity
of the boat and, consequently, a major proximity to the
transition state for the inversion. The intermediate
and 245 K, respectively.
Similarly, compound 8 displays at 293 K two sharp
singlets (δ ) 4.89 for CH
2
-61 and δ ) 4.84 for CH
) 4.90,
) 4.90, δ
-64), and the coalescence is
2
-64).
At 213 K two AB spin systems are observed (δ
B
q
-1
δ
A
) 4.73, J AB ) 15.4 Hz for CH
2
-61; and δ
B
A
)
values for ∆G (61.0 kJ ‚mol ) and planarity (135°) found
by Rubin fit quite well with this suggestion.
4
.61, J AB ) 15.4 Hz for CH
2
produced at 223 and 231 K, respectively.
In order to confirm the influence of the heteroatoms
q
q
It is remarkable that T
c
and ∆G values in 15a and
and electronic factors to the observed ∆G differences,
1
5b, which possess a six-membered pyrazine ring, are
work is in progress on the preparation of some new
derivatives derived from six- and five-membered hetero-
cyclic o-quinodimethanes.
(19) (a) Rubin, Y.; Khan, S.; Freedberg, D. I.; Yeretzian, C. J . Am.
Chem. Soc.1993, 115, 344. (b) Tago, T.; Minowa, T.; Okada, Y.;
Nishimura, J . Tetrahedron Lett. 1993, 34, 8461. (c) Zhang, X.; Foote,
C. S. J . Org. Chem. 1994, 59, 5235.
(21) (a) Ganapathi, P. S.; Friedman, S. H.; Kenyon, G. L.; Rubin, Y.
J . Org. Chem. 1995, 60, 2954. (b) Ohno, M.; Kojima, S.; Shirakawa,
Y.; Eguchi, S. Tetrahedron Lett. 1995, 36, 6899.
(20) Sandstr o¨ m, J . Dynamic NMR Spectroscopy; Academic Press:
London 1982; p 96.

Diels-Alder Adducts of [60]Fullerene
J . Org. Chem., Vol. 62, No. 11, 1997 3709
Ta ble 4. CV Da ta of Ca r bo a n d Heter ocyclic
cage. This is the first case in which the coalescence
temperature and ∆G values for C60-based cycloadducts
a
Cycloa d d u cts
q
compd E1redb
E2redb
E3redb
E4redb Ered (organic addend)
bearing five- or six-membered heterocycles have been
comparatively studied in reference to the six-membered
carbocyclic analogue. Interestingly, the activation ener-
gies found for the six-membered pyrazine adducts (15a ,b)
were very close to that found for the carbocyclic com-
pound (3), and striking lower values were obtained for
the five-membered thiophene adducts, which showed two
different coalescence temperatures for the two different
methylene units present in compounds 7 and 8.
The molecular geometry was optimized for compounds
7 and 15a by semiempirical calculations (PM3), showing
a significative more planar boat for the five-membered
heterocycle containing adduct (7). The electronic struc-
ture for the heterocyclic o-quinodimethanes indicate that
the cycloaddition is controlled by the HOMO of these
dienes, all the cycloadditions being energetically favored.
Finally, the redox properties of the prepared cycload-
ducts were studied by cyclic voltammetry and show
cathodically shifting reduction potential values in com-
parison with C60. Only compound 15c bearing two strong
electron-withdrawing cyano groups showed a different
electrochemical behavior exhibiting a LUMO energy level
lower than the parent [60]fullerene.
C60
3
7
-0.60 -1.00 -1.52
-0.71 -1.14 -1.69
-0.66 -1.17 -1.89
-0.70 -1.16 -2.25
-0.60 -1.00 -1.56
-0.64 -1.04 -1.56
-2.04
-
-
-
c
8
d
1
1
1
5a
5b
5c
-2.10
-
-
-1.94
-1.31; -1.74
e
e
-0.57 -0.97 -1.64
-1.06
a
All potentials in V vs SCE; toluene/MeCN (5:1); 0.1 M
-
1 b
Bu4NClO4; scan rate: 0.2 V s
.
Reduction potentials of the C60
c
d
moiety. Very broad wave. The intensity of the wave suggest a
two electron process. These values could be interchanged.
e
The electrochemical properties of the novel pyrazine-
containing cycloadducts (15a -c) were measured by cyclic
voltammetry at room temperature using toluene/aceto-
nitrile (5:1) as solvent, and the data are shown in Table
together with the thiophene-substituted analogues (7,
). The carbocyclic analogue 3 and [60]fullerene are also
4
8
included for comparison purposes.
From the reduction potential values (Table 4) it is clear
that substitution on the C60 cage does not alter signifi-
catively the acceptor properties of the parent C60. Most
of the studied cycloadducts show four one-electron qua-
sireversible reduction waves corresponding to the reduc-
tion steps of the fullerene moiety. Compounds 15 show
additional waves corresponding to the reduction of the
substituted pyrazine fragments. It has been previously
established that substitution on the C60 cage results in a
raising of the LUMO energy of the organofullerene as a
consequence of the saturation of a double bond.22 In
agreement with this observation, the first reduction
potentials are slightly shifted to more negative values,
related to [60]fullerene. Only cycloadduct 15c showed a
slightly more positive value than the parent C60 mea-
sured under the same experimental conditions. In order
to ascertain these CV data, theoretical calculations (PM3)
were carried out on compound 15c. The LUMO energy
level of 15c (-3.03 eV) lies at a slightly lower energy than
that found for [60]fullerene (-2.89 eV), thus supporting
the experimental electrochemical observations. Although
the difference between the reduction potentials of 15c
and C60 are not very significant, this experimental finding
could indicate the electronic interaction between the
acceptor pyrazine moiety bearing two cyano groups and
the [60]fullerene core.
Work is in progress directed to the preparation of other
five- and six-membered heterocyclic systems in order to
study the effect that the presence of other heteroatoms
has on the geometrical and electronic properties of the
obtained organofullerenes.
Exp er im en ta l Section
Cycloa d d ition Rea ction s by Cla ssica l Hea tin g. Gen -
er a l P r oced u r e. To a refluxing solution of [60]fullerene
(0.125 mg, 0.17 mmol), sodium iodide (0.104 mg, 0.69 mmol),
and 18-crown-6 (138 mg, 0.52 mmol) in ODCB (25 mL) was
added the respective pyrazine derivative (0.19 mmol). The
resulting brown reaction mixture was refluxed for a variable
period of time (24 h for 15a and 15b and 5 h for 15c). The
solvent was removed under vacuum, and the residue was
chromatographed on silica gel using cyclohexane/CHCl as
3
eluent. Further purification was accomplished by washing the
obtained solid three times with methanol.
Ad d u ct 15a : 13% (22% based on consumed C ); FTIR (KBr,
60
cm- ) 1492, 1453, 1033, 758, 747, 526; H NMR (CDCl
1
1
/CS
.92 (br d, J ) 12.5 Hz, 2H), 5.23 (br d, J ) 12.5 Hz, 2H), 7.88
dd, J ) 3.4 and 6.3 Hz, 2H), 8.27 (dd, J ) 3.4 and 6.3 Hz,
) δ
3
2
4
(
2
1
1
H); 13C NMR (CDCl
/CS ) δ 47.46, 64.53, 127.30, 129.35,
29.56, 130.27, 132.64, 135.21, 140.14, 141.55, 141.78, 141.90,
42.42, 142.70, 144.45, 145.28, 145.52, 146.09, 146.31, 147.51,
153.05, 155.17; MS m/ z 876 (M ), 720 (C60); UV-vis (CHCl
3 2
+
Su m m a r y a n d Con clu sion s
3
)
λ
max (nm) 254, 282, 326, 434, 706.
In summary, we have carried out the synthesis of
several cycloadducts of [60]fullerene and six-membered
heteroaromatic o-quinodimethanes under both thermal
and microwave irradiation, in addition to other cycload-
ducts prepared from five-membered sulfur-containing
o-quinodimethanes by microwave irradiation in compari-
son with the previously reported thermal conditions.
Variable-temperature NMR experiments reveal that
these cycloadducts present a flipping cyclohexene ring
with activation free energies depending upon the nature
of the heterocyclic system covalently attached to the C60
Ad d u ct 15b: 5% (14% based on consumed C60); FTIR (KBr,
cm- ) 2954, 2921, 2842, 1466, 1433, 1387, 1262, 1117, 900, 769,
1
1
5
6
8
78, 538; H NMR (CDCl
3
/CS
2
) δ 4.96 (d, J ) 13.9 Hz, 2H),
.35 (d, J ) 13.9 Hz, 2H), 7.67 (dd, J ) 3.2 and 6.6 Hz, 2H),
.22 (dd, J ) 3.2 and 6.6 Hz, 2H), 8.89 (s, 2H); MS m/ z 927
+
(
M ), 720 (C60); UV-vis (CHCl
3
) λmax (nm) 272, 432.
Ad d u ct 15c: 16% (43% based on consumed C60); FTIR (KBr,
-1
1
cm ) 2963, 2920, 1377, 1261, 1096, 1020, 800, 767, 526; H
NMR (CDCl /CS /CS ) δ
) δ 4.98 (bs, 4 H); 13C NMR (CDCl
46.82, 64.02, 112.85, 132.88, 135.15, 140.43, 141.78, 142.09,
3
2
3
2
1
1
λ
42.73, 143.21, 144.30, 144.58, 145.56, 145.76, 146.61, 147.80,
53.70, 157.51; MS m/ z 876 (M ), 720 (C60); UV-vis (CHCl )
3
max (nm) 342, 432, 692.
+
Cycloa d d ition Rea ction s by Micr ow a ve Ir r a d ia tion :
(
22) Suzuki, T.; Maroyama, Y.; Akasaba, T.; Ando, W.; Kobayashi,
Rea ction of C60 w ith Meth yl 2,3-Bis(ch lor om eth yl)th io-
p h en e-5-ca r boxyla te. Meth od A. A solution of C60 (40 mg,
K.; Nagase, S. J . Am. Chem. Soc. 1994, 116, 1359. See also: Eiermann,
M.; Haddon, R. C.; Knight, B.; Li, Q.; Maggini, M.; Mart ´ı n, N.; Ohno,
T.; Prato, M.; Suzuki, T.; Wudl, F. Angew. Chem., Int. Ed. Engl. 1995,
4, 1591. Ohno, T.; Mart ´ı n, N.; Knight, B.; Wudl, F.; Suzuki, T.; Yu,
H. J . Org. Chem. 1996, 61, 1306.
0
.055 mmol), methyl 2,3-bis(chloromethyl)thiophene-5-car-
boxylate (19 mg, 0.079 mmol), and tetrabutylammonium
bromide (TBAB, 224 mg, 0.696 mmol) in toluene (30 mL) was
3

3
710 J . Org. Chem., Vol. 62, No. 11, 1997
Fernandez-Paniagua et al.
irradiated at 60 W for 45 min. The resulting brown reaction
mixture was washed with water, dried, and evaporated to
dryness under reduced pressure. The residue was chromato-
graphed on a silica gel column with hexane/toluene (5:2) as
the eluent, recovering 12 mg of unreacted C60 and 10 mg of 7
min. The resulting brown solution was washed with water,
dried, and evaporated to dryness. The solid residue was
chromatographed on silica gel with hexane/toluene, recovering
14.5 mg of C60 and 16 mg of 15a (26%, 37% based on consumed
C
60). Further purification was accomplished by washing the
(16%, 28% based on consumed C60). Further purification of
obtained solid three times with methanol.
the solids was accomplished by washing three times with
Rea ction of C60 w ith 2,3-Bis(br om om eth yl)ben zo[g]-
qu in oxa lin e. A solution of C60 (50 mg, 0.069 mmol), 2,3-bis-
methanol. The spectroscopical data of adduct 7 thus obtained
7
agree with those previously reported. When the reaction was
(bromomethyl)benzo[g]quinoxaline (51 mg, 0.139 mmol), and
carried out in ODCB, an 11% (17% based on consumed C60
yield was obtained.
)
TBAB (224 mg, 0.696 mmol) in ODCB (30 mL) was irradiated
at 60 W for 30 min. The resulting brown reaction mixture
was washed with water, dried, and evaporated to dryness
under reduced pressure. The solid obtained was purified by
Meth od B. A solution of C60 (50 mg, 0.069 mmol), methyl
2
,3-bis(chloromethyl)thiophene-5-carboxylate (19 mg, 0.079
mmol), KI (86 mg, 0.558 mmol), and 18-crown-6 (61 mg, 0.242
mmol) was irradiated at 105 W for 75 min. The resulting
brown solution was washed with water, dried, and evaporated
to dryness. The solid residue was purified by column chro-
matography on silica gel with hexane/toluene, recovering 12
mg of unreacted C60 and 10 mg of adduct 7 (16%, 21% based
on consumed C60).
column chromatography on silica gel with toluene/CHCl
eluent, recovering 10 mg of unreacted C60 and 14 mg of adduct
5b (22%, 52% based on consumed C60). Further purification
was accomplished by washing the obtained solid three times
with methanol.
3
as
1
Rea ction of C60 w ith 2,3-Bis(br om om eth yl)-5,6-d icy-
a n op yr a zin e. A solution of C60 (50 mg, 0.069 mmol), 2,3-bis-
Rea ction of C60 w ith 2,3-Bis(br om om eth yl)ben zo[b]-
th iop h en e. A solution of C60 (50 mg, 0.069 mmol), 2,3-bis-
(bromomethyl)-5,6-dicyanopyrazine (24 mg, 0.077 mmol), and
TBAB (124 mg, 0.385 mmol) in toluene (30 mL) was irradiated
at 60 W for 20 min. The resulting brown solution was washed
with water, dried, and evaporated to dryness under reduced
pressure. The residue was chromatographed on a silica gel
column with hexane/toluene (1:1) and hexane/toluene (1:5) as
eluent, recovering 40 mg of unreacted C60 and 5 mg of 15c
(bromomethyl)benzo[b]thiophene (19 mg, 0.079 mmol), NaI (86
mg, 0.558 mmol), and 18-crown-6 (61 mg, 0.242 mmol) in
toluene (30 mL) was irradiated at 70 W for 70 min. The
resulting brown solution was washed with water, dried, and
evaporated to dryness under reduced pressure. The residue
was chromatographed on a silica gel column with hexane/
toluene (7:2) as eluent, recovering 8 mg of unreacted C60 and
(9%, 42% based on consumed C60). Further purification of the
solid was accomplished by washing three times with methanol.
1
4 mg of 8 (23%, 28% based on consumed C60). Further
purification of the solids was accomplished by washing three
times with methanol. The spectroscopical data of adduct 8
thus obtained agree with those previously reported.7
Rea ction of C60 w ith 2,3-Bis(br om om eth yl)qu in oxa -
lin e. A solution of C60 (50 mg, 0.069 mmol), 2,3-bis(bromo-
methyl)quinoxaline (44 mg, 0.139 mmol), and TBAB (224 mg,
Ack n ow led gm en t. Financial support from Spanish
DGICYT (PB94-0742 and PB95-0428-CO2) is grate-
fully acknowledged. We are indebted to Drs. J avier
Gar ´ı n and Jes u´ s Orduna for the FAB-MS measurements.
0
.696 mmol) in ODCB (30 mL) was irradiated at 60 W for 30
J O9701344