Selective electrolytic removal of bis(alkoxycarbonyl)methano addends from C60 bis-adducts and electrochemical stability of C70 derivatives

Article abstract of DOI:10.1002/1521-3765(20000616)6:12<2184::AID-CHEM2184>3.0.CO;2-6

The novel mixed his-adducts of C₆₀, (±)-4-(±)-8 and 9, with a bis(ethoxycarbonyl)methano addend (Bingel addend) and a second addend ([1,2]benzeno, but[2]eno, methaniminomethano, or diarylmethano) bridging 6,6-closed bonds of the carbon sphere were synthesized in two-step reactions. Each bis-adduct was exhaustively electrolyzed at the potential of the second fullerene-centered reduction step, resulting in the selective removal of the Bingel addend (retro-Bingel reaction) to produce the corresponding mono-adducts, which were isolated in yields of over 60%. These results open up the possibility of using the Bingel addend as a temporary protecting and directing group in the construction of multiple adducts of C₆₀ with unusual addition patterns. The Bingel-type mono-adduct of C₇₀ 10 and the constitutionally isomeric bis-adducts 11, (±)-12, and (±)-13 were also included in this investigation. A large difference in the electrochemical behavior between C₇₀, bis-adducts and the corresponding C₆₀ derivatives was observed. Thus, the intramolecular "walk-on-the-sphere" isomerization which occurs readily with Bingel-type bis-adducts of C₆₀ under the conditions of two-electron controlled potential electrolysis (CPE) is only a minor reaction pathway in the series of C₇₀ derivatives. The latter preferentially undergo retro-Bingel reaction.

Full text of DOI:10.1002/1521-3765(20000616)6:12<2184::AID-CHEM2184>3.0.CO;2-6

FULL PAPER
Selective Electrolytic Removal of Bis(alkoxycarbonyl)methano Addends
from C60 Bis-adducts and Electrochemical Stability of C70 Derivatives
Roland Kessinger,[a] Nicolette S. Fender,[b] Lourdes E. Echegoyen,[b] Carlo Thilgen,[a]
Luis Echegoyen,*[b] and François Diederich*[a]
Abstract: The novel mixed bis-adducts
of C60, (Æ)-4 ± (Æ)-8 and 9, with a
ducts, which were isolated in yields of
over 60%. These results open up the
possibility of using the Bingel addend as
a temporary protecting and directing
group in the construction of multiple
adducts of C60 with unusual addition
patterns. The Bingel-type mono-adduct
of C70 10 and the constitutionally iso-
meric bis-adducts 11, (Æ)-12, and (Æ)-13
were also included in this investigation.
A large difference in the electrochem-
ical behavior between C70 bis-adducts
and the corresponding C60 derivatives
was observed. Thus, the intramolecular
bis(ethoxycarbonyl)methano
addend
(Bingel addend) and a second addend
([1,2]benzeno, but[2]eno, methanimino-
methano, or diarylmethano) bridging
6,6-closed bonds of the carbon sphere
were synthesized in two-step reactions.
Each bis-adduct was exhaustively elec-
trolyzed at the potential of the second
fullerene-centered reduction step, re-
sulting in the selective removal of the
Bingel addend (retro-Bingel reaction) to
produce the corresponding mono-ad-
ªwalk-on-the-sphereº
isomerization
which occurs readily with Bingel-type
bis-adducts of C60 under the conditions
of two-electron controlled potential
electrolysis (CPE) is only a minor reac-
tion pathway in the series of C70 deriv-
atives. The latter preferentially undergo
retro-Bingel reaction.
Keywords: electrochemistry ´ elec-
trolysis
´ fullerenes ´ protecting
groups ´ retro-Bingel reaction.
offering the potential for inducing profound changes in the
properties of the molecular carbon allotrope.[11] The Bingel
reaction has also been heavily applied in supramolecular
construction involving fullerene components.[12]
Introduction
Among the various protocols developed for the covalent
functionalization of fullerenes,[1±5] the Bingel reaction[6] and
the azomethine ylide dipolar cycloaddition[7] are the most
widely used methods. The original Bingel reaction and its
modifications[8±10] generally proceed in good yield, and the
bis(alkoxycarbonyl)methano addends (Bingel addends) in-
troduced at 6,6-bonds of the carbon spheres are of high
thermal and chemical stability. The macrocyclization of the
carbon sphere by double Bingel addition of tethered bis-
malonates represents a highly effective method for precisely
positioning organic chromophores such as crown ethers and
porphyrins in close proximity to the fullerene surface, thus
Regioselective multiple functionalization of fullerenes
often requires the temporary introduction of an addend
which protects reactive bonds, directs new incoming addends
into specific positions, and after completion of these tasks, can
be readily removed. Three protocols have been mainly
applied in the past for this purpose. The first one consists of
Diels ± Alder additions of 9,10-dimethylanthracene, which are
thermally readily reversible.[13] The second one is an elegant
multistep protocol which was developed for the removal of
the more stable cyclohexene rings fused to the carbon sphere
by Diels ± Alder addition with buta-1,3-dienes.[14, 15] The third
method consists of the fusion of isoxazoline rings to the
fullerene by dipolar cycloaddition with nitrile oxides and
removal of the addend with [Mo(CO)6] or diisobutylalumi-
num hydride (DIBAL-H).[16]
[a] Prof. Dr. F. Diederich, Dipl.-Chem. R. Kessinger, Dr. C. Thilgen
Laboratorium für Organische Chemie
ETH-Zentrum
Universitätstrasse 16, 8092 Zürich (Switzerland)
Fax : (‡41)1-632-1109
E-mail: diederich@org.chem.ethz.ch
During investigations of the electrochemical stability of
anions of bis(ethoxycarbonyl)methano-functionalized fuller-
enes such as 1,[17] we discovered that this addend can be
electrochemically removed in high yield by exhaustive
electrolytic reduction at constant potential.[18] We named this
[b] Prof. Dr. L. Echegoyen, Dr. N. S. Fender, Dr. L. E. Echegoyen
Department of Chemistry
University of Miami
Coral Gables, FL 33124 (USA)
Fax : (‡1)305-284-4571
novel procedure the retro-Bingel reaction (Scheme 1).[19]
A
E-mail: echegoyen@miami.edu
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2184±2192
showed the characteristic bands around 435 and 700 nm of a
Diels ± Alder mono-adduct.
O
O
[24]
EtO
OEt
Inspired by these preliminary results, we undertook a more
comprehensive investigation of the synthetic scope of the
electrochemical retro-Bingel reaction and the possible use of
the Bingel addend as a protecting and directing group. For this
purpose, a series of mixed C60 bis-adducts (Æ)-4 ± (Æ)-8 and 9
were synthesized and subjected to the electrochemical retro-
Bingel reaction. In addition, we report here investigations on
retro-Bingel reaction
Bingel reaction
1
Scheme 1. The Bingel and retro-Bingel reactions.
first major application of the Bingel-retro-Bingel strategy was
the preparation of pure constitutional isomers of higher
fullerenes, such as C2v ± C78
[17c]
and a new C84 isomer,[20] and,
X
X
gratifyingly, of enantiomerically pure chiral higher fullerenes
f
f
f
such as C- and A-C76[18] and fC- and A-C84.[20]
MeO
OMe
While investigating the retro-Bingel reaction, we also
discovered an isomerization reaction, the ªwalk-on-the-
sphereº rearrangement, where two bis(ethoxycarbonyl)-
methano addends intramolecularly change their positions on
the C60 sphere.[21] This rearrangement occurs when the
electrolytic reduction is interrupted after only two electron-
equivalents have been transferred (Scheme 2).
OEt
O
(±)-4
(±)-5
EtO
trans-3
O
OAc
AcO
Me
N
(±)-6
(±)-7
O
OMe
MeO
O
O
OEt
EtO
O
OEt
O
O
EtO
1) 2-e– CPE
CH2Cl2, Bu4NPF6
EtO
OEt
e and other
trans isomers
MeO
OMe
+
2) Re-oxidation
(±)-8
OEt
EtO
O
O
cis-3
trans-2
OMe
MeO
Scheme 2. The intramolecular ªwalk-on-the-sphereº rearrangement ob-
served during two-electron controlled potential electrolysis (CPE).
Preliminary electrochemical investigations with the mixed
bis-adduct 2 obtained by tether-directed remote functionali-
zation[22] suggested that the chemoselective electrochemical
removal of a Bingel addend from the fullerene sphere in the
OEt
O
e
O
EtO
9
presence of
a
different addend might be possible
(Scheme 3).[23] The reaction was very sluggish, and only traces
the retro-Bingel reaction and ªwalk-on-the-sphereº rear-
rangement of the C70 mono-adduct 10 and the three constitu-
tionally isomeric bis-adducts 11, (Æ)-12, and (Æ)-13, and
compare the results with those obtained for analogous
derivatives of C60.
O
O
O
MeO
1) 5-e– CPE
CH2Cl2, Bu4NPF6
OH
2) Re-oxidation
Results and Discussion
2
3
Scheme 3. First observation of the electrochemical retro-Bingel reaction
of a mixed bis-adduct.[23]
Synthesis: The C70 mono-adduct 10[6a] and the constitutionally
isomeric bis-adducts 11, (Æ)-12, and (Æ)-13[6b, 25] were pre-
pared as previously reported. Starting from the known C60
derivatives 14,[26] 15,[27] 16,[28] and 17,[29] respectively, the mixed
trans-3 bis-adducts (Æ)-4, (Æ)-5, (Æ)-6, and (Æ)-7 were
obtained by Bingel cyclopropanation and isolated in 7 ±
of benzyl alcohol 3 could be isolated. Whereas the unambig-
uous identification of the product was impossible due to low
yield and difficult workup, its UV/Vis spectrum clearly
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L. Echegoyen, F. Diederich et al.
FULL PAPER
14 could the e isomer 9 be isolated in pure form (10% yield)
by simple column chromatography. Among the two possible e
isomers,[22] 9 was isolated exclusively according to 1H and
13C NMR spectroscopy. Bis-adduct (Æ)-8 was obtained by
Diels ± Alder addition to 1 following a previously described[24]
protocol. The low yield of 2% in this reaction is due to
extreme difficulties in the isolation of the pure isomer and not
due to an inherently problematic synthetic pathway. For
analytical purposes and full characterization, all bis-adducts
were completely purified by preparative HPLC (high per-
formance liquid chromatography).[30] The trans-3 isomers used
in the preparative electrochemical experiments contained
small amounts of other regioisomers (trans-2, trans-4, e; a
total of less than 10%) which did not change the outcome of
the retro-Bingel reactions in view of the above-mentioned
scrambling of the regioisomers by the ªwalk-on-the-sphereº
rearrangement.
O
O
O
O
O
O
O
O
O
O
O
O
11
10
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(±)-12
(±)-13
14% yield after extremely tedious column chromatographic
separation from the other regioisomers formed (Scheme 4).
Since the ªwalk-on-the-sphereº rearrangement of bis(alkoxy-
carbonyl)methano addends in C60 derivatives leads to a
scrambling of the regioisomers during the retro-Bingel
reaction,[21] the scope of the latter reaction can be fully
investigated starting from only one of the possible[30] regio-
isomeric bis-adducts. Only in the Bingel cyclopropanation of
Electrochemical investigations of C60 derivatives: All experi-
ments were performed using a home-made cell as previously
described.[31] Constant potential electrolysis (CPE) was car-
ried out under high vacuum in CH2Cl2 with Bu4NPF6 (0.1m) as
supporting electrolyte (see Experimental Section for details).
To determine the potential to be applied, the cyclic voltammo-
grams (CV) of the compounds were recorded. The applied
potential for CPE was typically chosen 100 mV more negative
than the second reduction peak potential. After electrolysis,
the solutions were exhaustively re-oxidized at 0 V before
product analysis.
X
X
The retro-Bingel reaction occurs at the second reduction
potential of bis(ethoxycarbonyl)methano[60]fullerenes. In the
case of 18[32] (Scheme 5), with only one carboxylic group
OEt
O
X
EtO
O
MeO
OMe
O
a)
b)
OEt
14
15
(±)-4 (14%) + 9 (10%)
H
O
O
1) 4-e– CPE
OAc
AcO
CH2Cl2, Bu4NPF6
(±)-5 (7%)
2) Re-oxidation
Me
N
a)
b)
35%
18
(±)-6 (13%)
(±)-7 (8%)
16
17
OMe
MeO
OMe
MeO
1) 6-e– CPE
CH2Cl2, Bu4NPF6
no apparent change
2) Re-oxidation
OMe
MeO
MeO
OMe
17
SiMe3
Me3Si
Br
Br
1) 1-e– CPE
Bu4NI, PhMe, ∆
1
OEt
O
CH2Cl2, Bu4NPF6
decomposition
2) Re-oxidation
EtO
(±)-8 (2%)
O
19
Scheme 4. Synthesis of the mixed C60 bis-adducts. a) EtO2CCH2CO2Et, I2,
DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), PhMe/CH2Cl2; b) EtO2CCHBr-
CO2Et, DBU, PhMe.
Scheme 5. Experiments demonstrating that efficient retro-Bingel reac-
tions require two electron-withdrawing groups at the methano bridge.
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Bis-adducts of C60
2184±2192
Table 1. Selective removal of bis(ethoxycarbonyl)methano addends in mixed
C60 bis-adducts by the retro-Bingel reaction.
directly attached to the cyclopropane ring, the dianion is
stable under bulk electrolysis conditions. However, when
electrolyzed at the third reduction potential, cleavage of the
methano bridge is induced. In this case, the yield of the parent
fullerene, C60, drops remarkably (35%) compared to the yield
obtained in the electrolysis of 1 (Scheme 1), which is typically
higher than 80%.[18] Electrochemical cleavage of the cyclo-
propane ring also occurs when two cyano[19c] groups are
attached. Thus, the presence of two strongly electron-with-
drawing groups (EWG) attached to the cyclopropane ring is
apparently necessary for efficient electrolytic removal of the
addend.
X
X
1) 5-e– CPE
CH2Cl2, Bu4NPF6
OEt
2) Re-oxidation
O
EtO
O
X
In order to further test this hypothesis, mono-adducts 17
and 19[33] were submitted to the conditions of the retro-Bingel
reaction. In the case of 17, which lacks electron-withdrawing
groups at the methano bridge, it is not possible to cleave the
cyclopropane ring electrochemically within the limits of the
potential window of the solvent. Mono-adduct 19, with two
weakly electron-withdrawing (trimethylsilyl)ethynyl groups
attached to the methano bridge, decomposes to an insoluble
solid after reduction to the mono-anion. This decomposition is
faster than a possible cyclopropane ring cleavage, and indeed,
no C60 was detected in the reaction mixture after re-oxidation.
To demonstrate the stability in the reduced state of the
other C60 mono-adducts (which lack Bingel addends), com-
pounds 14 ± 16 were subjected to CPE up to at least the tri-
anionic stage. Upon re-oxidation to the neutral state, only
starting material was present, confirming their stability.
Having established that the mono-adducts 14 ± 17 are stable
under the electrochemical conditions of the retro-Bingel
reaction, we turned our attention to the selective removal of a
Bingel addend from fullerenes in the presence of other
addends. Thus, the five trans-3 bis-adducts (Æ)-4 ± (Æ)-8 were
subjected to the retro-Bingel reaction. The electrochemical
reaction to the corresponding mono-adducts 14 ± 17 and 20[24]
proceeded in most cases with yields over 60% (Table 1).
These yields are encouraging for the use of the Bingel addend
as a temporary protecting and directing group in fullerene
chemistry, since two of the most commonly used addends,
fused pyrrolidine[7, 34] and cyclohexene rings[35, 36] are perfectly
stable under the conditions of the retro-Bingel reaction.
To study the effect of the retro-Bingel reaction on different
regioisomers, the trans-3 and e derivatives (Æ)-4 and 9 were
electrolyzed under similar conditions. The difference in their
electrochemical behavior is only marginal, and the yield of
mono-adduct 14 is practically the same in both conversions,
considering the possible errors introduced during the manip-
ulation of the small quantities of compounds used for the
electrolysis experiments (2 ± 3 mg). In view of the scrambling
caused by the ªwalk-on-the-sphereº rearrangement during
electrolysis, it is reasonable to assume that the same results
may be obtained for the regioisomers of all the other
compounds investigated here.
MeO
OMe
(±)-4
(±)-5
14 (67%)
15 (43%)
OAc
AcO
Me
N
16 (63%)
17 (60%)
(±)-6
(±)-7
OMe
MeO
OMe
MeO
MeO
(±)-8
20 (61%)
14 (65%)
OMe
9 (e isomer)
C60 and 1,1'-(ethane-1,2-diyl) 3,3'-diethyl bis(malonate) and
electrolyzed non-exhaustively at the second reduction poten-
tial. The electrolysis was stopped after 2.5 electrons per
molecule were transferred. After exhaustive re-oxidation at
0 V, the mixture obtained was separated by column chroma-
tography. Analysis of the isolated material showed that in
addition to recovering some of the starting material, both C60
and 22 were also produced (Scheme 6). This is a clear
indication that the leaving malonate remains intact during
the cleavage of the cyclopropane ring.
Electrochemical investigations of C70 derivatives: The C70
mono-adduct 10, similar to the C60 analogue 1, shows chemical
O
O
O
O
O
O
OEt
O
EtO
O
O
O
O
O
1) 2-e– CPE
CH2Cl2, Bu4NPF6
EtO
OEt
Throughout our studies of the retro-Bingel reaction, the
cleaved addend has never been recovered due to its difficult
separation from the electrolyte material. However, its recov-
ery and characterization could be crucial in establishing the
mechanism of the reaction. For this reason, (Æ)-21 was
synthesized by the Bingel macrocyclization reaction between
+ C60 (25%)
2) Re-oxidation
+ (±)-21 (18%)
(±)-21
22 (17%)
Scheme 6. Experiment showing that the malonate residue remains intact
in the cleavage of the cyclopropane ring during the retro-Bingel reaction.
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L. Echegoyen, F. Diederich et al.
FULL PAPER
Table 2. Distribution of products obtained by coulometric CPE of C70
mono- and bis-adducts in CH2Cl2 (‡0.1m Bu4NPF6) and re-oxidation.
reversibility for the first and second reduction steps on the CV
time scale of 100 mVsÀ1. This behavior is different from the
CV response of the previously studied bis{[(ethoxy)car-
2-electron CPE
Starting material
Products [%]
bonyl]methyl}
1,2-methano[70]fullerene-71,71-dicarboxyl-
C70
64
< 1
2
10
11
(Æ)-12
(Æ)-13
ate,[17c, 18] which displayed an electrochemically irreversible
second reduction. The fact that the latter mono-adduct with
four ester groups in the side chains of the methano addend
undergoes decomposition in the di-anionic state much more
readily than its diester counterpart 10 further strengthens our
hypothesis that the retro-Bingel reaction is enhanced by
increasing the electron-withdrawing character of the substitu-
ents at the methano bridge.
10
11
21
3
27
39
< 1
92
4
< 1
< 1
31
1
< 1
< 1
6
(Æ)-12
(Æ)-13
5
±
28
1.5-electron CPE
Starting material
Products [%]
C70
10
11
(Æ)-12
(Æ)-13
10
11
1
< 1
±
81
< 1
7
< 1
94
3
< 1
< 1
71
< 1
< 1
4
CPE carried out on 10 at the second reduction potential
(À1.44 V vs. Fc/FcÀ) was interrupted when charge corre-
sponding to two electrons had been transferred. The solution
was then re-oxidized at 0 V (25% charge recovered). Analysis
of the product mixture showed that the retro-Bingel reaction
had occurred, producing C70 in 64% yield with 21% of the
starting material remaining unreacted (Scheme 7). CPE at the
(Æ)-12
(Æ)-13
1
5
4
20
32
version, producing 3% mono-adduct and practically no
isomerization to other bis-adducts. Compound (Æ)-13, on
the other hand, forms the mono-adduct as the major product
in 39% yield and, similar to 11, shows nearly no tendency to
isomerize. The third constitutional isomer (Æ)-12, again
behaves differently. The main reaction pathway of the dianion
of (Æ)-12 is the retro-Bingel reaction, but a small portion also
isomerizes to the dianions of 11 and (Æ)-13. Whether the
dianion of (Æ)-12 looses directly an addend or isomerizes
preferentially to the dianion of (Æ)-13 which then undergoes
the retro-Bingel reaction could not be decided by our
experiments so far.
O
O
O
1) 2-e– CPE
CH2Cl2, Bu4NPF6
O
2) Re-oxidation
10
(64%)
Scheme 7. Retro-Bingel reaction of C70 mono-adduct 10.
When these results are compared to those obtained with the
corresponding C60 bis-adducts,[18, 21] it becomes obvious that
the electrochemistry of the C70 bis-adducts is quite different.
Either the dianions of the bis-adducts of C70 are stable or the
retro-Bingel reaction is much faster than isomerization.
Isomerization plays only a minor role in the electrochemical
reactions of the dianions of the C70 bis-adducts, whereas it is
the major reaction of the dianions of the Bingel-type bis-
adducts of C60.
The observation that the dianion of bis-adduct 11 is more
stable than the di-anions of its isomers (Æ)-12 and (Æ)-13 is in
full agreement with the results of our previous CV-studies.[17c]
There, it was shown that only the Bingel-type C70 bis-adduct
with an addition pattern corresponding to 11 (but slightly
different malonate addends) has a reversible second reduc-
tion wave, whereas the bis-adducts with addition patterns
corresponding to (Æ)-12 and (Æ)-13, respectively, showed an
irreversible second electron transfer on the CV time scale.
Since the retro-Bingel reaction predominates over the
ªwalk-on-the-sphereº rearrangement under the conditions of
two-electron CPE, it was presumed that in order to effect
isomerization, a lower number of electrons should be trans-
ferred. Thus, the C70 bis-adducts were subjected to coulo-
metrically controlled 1.5-electron-per-molecule electrolysis
(Table 2). Whereas the behavior of 11 did not reflect the
change in conditions, (Æ)-12 underwent retro-Bingel reaction
to a small extent, while still producing only very small
amounts of the two other constitutional isomers. Isomer (Æ)-
13, on the other hand, displayed the most striking example of
rearrangement occurring on the C70 spheroid with the
first reduction potential and between the first and second
reduction potentials transferring 1 and 1.5 electron-equiva-
lents, respectively, resulted in the production of only ꢀ1%
C70. Interestingly, the chromatograms (HPLC) of the products
of these reactions also revealed the presence of traces of the
three constitutional bis-adducts of C70, 11, (Æ)-12, and (Æ)-13.
Such evidence of an intermolecular reaction process was not
observed in similar experiments with bis[bis(dialkoxycar-
bonyl)methano] adducts of C60.[18, 21]
The bis-adducts, 11, (Æ)-12, and (Æ)-13 were subjected to
similar electrochemical conditions as was 10. Recall that
under these conditions, the analogous bis-adducts of C60
isomerize by the ªwalk-on-the-sphereº rearrangement
(Scheme 2).[21] It was thus interesting to learn whether a
two-electron CPE of C70 bis-adducts would also lead to
isomerization, or would it induce addend removal via the
retro-Bingel reaction. The former would possibly provide a
route for the preparation of otherwise unattainable Bingel-
type bis-adducts with addends
β
across b, e, or even k bonds
(Scheme 8).[4, 37]
ε
α
κ
The results of the electro-
chemical transformations of
10, 11, (Æ)-12, and (Æ)-13 by
two-electron CPE, followed by
re-oxidation, are presented in
Table 2. It is evident that 11
undergoes only minimal con-
Scheme 8. The four different
[4, 37]
types of 6,6-bonds in C70
.
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Bis-adducts of C60
2184±2192
generation of (Æ)-12 in 20% yield. Further variations of the
number of electrons transferred per molecule of (Æ)-13
confirmed that at room temperature (298 K) the ideal
condition for isomerization of (Æ)-13 is 1.5-electron CPE.
The 1.5-electron CPE results may be explained using the
known stability of the dianions from the previous experi-
ments. It is assumed that the small quantities of mono-adduct
10 formed during 1.5-electron CPE of (Æ)-12 and (Æ)-13
result from the decomposition of the corresponding dianions.
The dianion of (Æ)-12 is also responsible for the formation of
the rearranged bis-adducts 11 and (Æ)-13 in 3% and 4% yield,
respectively. On the other hand, the mono-anion of (Æ)-12
seems to be stable on the time scale of CPE. The same can be
said for the mono-anion of bis-adduct 11. Compound (Æ)-13
shows again a different behavior. As already pointed out, it
isomerizes to (Æ)-12 under 1.5-electron CPE in 20% yield.
Since the dianion of (Æ)-13 undergoes preferentially the
retro-Bingel reaction, it must be the mono-anion that isomer-
izes slowly to the mono-anion of (Æ)-12. Once this mono-
anion is formed, it is stable and does not further isomerize.
This behavior of the mono-anion of (Æ)-13 explains the fact
that only traces of 11 are formed under these conditions,
which was further confirmed by conducting a 1-electron CPE
on isomer (Æ)-13. In this case, part of the starting material
isomerized to (Æ)-12 (4%), and another small part rearranged
to 11 (1%), whereas nearly no retro-Bingel reaction to the
mono-adduct 10 took place (1%). Most of the starting
material (29%) was recovered unchanged.
three constitutionally isomeric Bingel-type bis-adducts of C70.
They also showed that the retro-Bingel reaction and the
ªwalk-on-the-sphereº rearrangement during coulometric
CPE do not necessarily occur along the same pathway for
Bingel bis-adducts of C60 and C70. Thus, this study provides
another interesting example of the differences in chemical
behavior between the two most abundant fullerenes.
Experimental Section
General methods: Reagents and solvents were purchased reagent-grade
and used without further purification. C60 (purity: >99%) was purchased
from Southern Chemical Group, LLC, Tucker, GA 30085-0527, USA.
Compounds 1[6a], (Æ)-7[30b], 14[26], 15[27], 16[28], 17[29], and 4,4'-dimethoxy-
benzophenone N-tosylhydrazone[41] were synthesized according to liter-
ature procedures. All reactions were performed in standard glassware
under N2. Evaporation and concentration in vacuo were done at water
aspirator pressure, and compounds were dried at 10À2 Torr. Column
chromatography: SiO2 60 (230 ± 400 mesh, 0.040 ± 0.063 mm) from E.
Merck and SiO2-H from Fluka. TLC glass plates coated with SiO2 60 F254
from E. Merck; visualization by UV light. Preparative and analytical
HPLC of C60 and C70 bis-adducts: Knauer HPLC Pump 64 with preparative
pump head and vacuum on-line degasser, electrical injection valve AA
A0619, and variable wavelength monitor from Knauer; all chromatograms
recorded at ambient temperature with a fixed detector wavelength at l ˆ
310 nm; preparative HPLC-column: Macherey-Nagel Nucleosil 100-7 silica
gel (7 mm, 250 Â 21 mm I.D.); analytical HPLC-column: Macherey-Nagel
Nucleosil 100-7 silica gel (5 mm, 250 mm  4 mm I.D.). Analytical HPLC of
electrolysis samples: Waters instrument equipped with a 515 pump and a
476 tunable UV absorbance detector using a Lichrosorb Si 60 (5 mm, 250 Â
4 mm I.D.) column with toluene as eluent. Melting points: Büchi Melting
Point B-540, uncorrected. UV/Vis spectra: Varian Cary-5 spectrophotom-
eter. IR spectra: Perkin Elmer 1600-FTIR. NMR spectra: Bruker AM 500
at 298 K, with solvent peaks as reference. FAB-MS: VG ZAB 2SEQ
instrument, 3-nitrobenzyl alcohol as matrix. Elemental analyses were
performed by the Mikrolabor at the Laboratorium für Organische Chemie,
ETH-Zürich.
It must be remarked that the HPLC analysis of the
electrolysis products of 11, (Æ)-12, and (Æ)-13 showed addi-
tional peaks that do not correspond to C70 or its known
Bingel-type mono- and bis-adducts. These peaks correspond
to as yet uncharacterized species. Intensive efforts are now
underway to isolate and characterize these compounds, which
we are hoping could be the first examples of Bingel bis-
adducts with addends at b-, e-, or k-type 6,6-bonds. The
different reactivity of the anions of Bingel-type bis-adducts of
(Æ)-Diethyl 65,66-dimethoxy-1,2-([1,2]benzeno)-33,50-methano[60]fuller-
ene-61,61-dicarboxylate ((Æ)-4) and diethyl 65,66-dimethoxy-1,2-
([1,2]benzeno)-18,36-methano[60]fullerene-61,61-dicarboxylate (9): To a
solution of 14 (150 mg, 0.175 mmol), diethyl malonate (0.08 mL, 0.5 mmol),
and I2 (189 mg, 0.75 mmol) in PhMe/CH2Cl2 (600 mL/100 mL), DBU
(0.2 mL, 2.0 mmol) was slowly added and the mixture was stirred for 12 h.
Plug filtration (SiO2; CH2Cl2) and column chromatography (SiO2; CH2Cl2)
afforded (Æ)-4 (25 mg, 14%) and 9 (20 mg, 10%). Preparative HPLC
yielded analytically pure (Æ)-4 (eluent PhMe, injection volume 500 mL (c ˆ
2.1 mgmLÀ1), flow rate 8.0 mLminÀ1, retention time 38 min) and 9 (eluent
C
60 and C70 in coulometric CPE adds another example to the
increasing number of experiments in which substantial differ-
ences in the chemical behavior between the two fullerenes
have become apparent.[25, 38±40]
PhMe, injection volume 500 mL (c ˆ 2 mg mLÀ1), flow rate 8.0 mLminÀ1
,
retention time 56 min).
Conclusion
(Æ)-4: Analytical HPLC purity control (eluent PhMe, flow rate
2.0 mLminÀ1, retention time 6.5 min); m.p. >250oC; UV/Vis (CH2Cl2):
lmax (e) ˆ 702 (sh, 300), 622 (sh, 700), 479 (2500), 251 nm
(120000 molÀ1 dm3 cmÀ1); IR (KBr): nÄ ˆ 2978, 2922, 1745, 1478, 1281,
1044, 522 cmÀ1; 1H NMR (500 MHz, CDCl3): d ˆ 7.59 (s, 1H), 7.40 (s, 1H),
4.61 ± 4.57 (m, 2H), 4.45 (q, J ˆ 7.1 Hz, 2H), 4.13 (s, 3H), 4.05 (s, 3H), 1.51
(t, J ˆ 7.1 Hz, 3H), 1.14 (t, J ˆ 7.1 Hz, 3H); 13C NMR (125.8 MHz, CDCl3):
d ˆ 163.57, 163.53, 157.16, 156.23, 156.14, 155.95, 151.81, 151.77, 148.26,
147.99, 147.90, 147.78, 147.74, 147.32, 147.27, 147.25, 147.22, 147.17, 147.12,
146.91, 146.75, 146.33, 146.18, 146.06, 145.95, 145.58, 145.32, 145.30, 145.22,
125.17, 145.02, 144.88, 144.58, 144.34, 144.31, 144.09, 144.08, 144.03, 144.02,
143.73, 143.53, 143.38, 143.34, 143.31, 143.24, 143.04, 142.82, 142.39, 142.37,
142.03, 141.84, 141.72, 141.59, 141.28, 141.11, 141.02, 140.62, 139.65, 139.52,
138.87, 138.76, 138.71, 106.80, 106.75, 71.43, 71.12, 63.55, 63.35, 63.28, 63.22,
The results presented in this study clearly demonstrate that it
is possible to electrochemically remove in high yields a
bis(ethoxycarbonyl)methano addend from the C60 sphere in
the presence of a variety of other addends, including cyclo-
hexene, pyrrolidine, and benzocyclobutene rings fused to 6,6-
bonds. This investigation establishes the bis(alkoxycarbonyl)-
methano addend (Bingel addend) as a versatile new protect-
ing and directing group in fullerene chemistry which is readily
introduced and removed. This protecting/directing group
strategy, coupled with tether-directed remote functionaliza-
tion, should provide access to many new C60 multiple adducts
featuring novel addition patterns and functions.[14b, 15]
‡
63.16, 56.58, 56.51, 51.12, 14.28, 14.14; MS (FAB): m/z (%): 1015 ([M ], 57),
‡
720 (C60 , 100).
9: Analytical HPLC purity control (eluent PhMe, flow rate 2.0 mLminÀ1
,
retention time 9.8 min); m.p. >250oC; UV/Vis (CH2Cl2): lmax (e) ˆ 462
The experiments reported in this paper revealed not only
substantial differences in the electrochemical reactivity of
(5000), 255 nm (50000 molÀ1 dm3 cmÀ1); IR (KBr): nÄ ˆ 2967, 2922, 1744,
Chem. Eur. J. 2000, 6, No. 12
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0947-6539/00/0612-2189 $ 17.50+.50/0
2189

L. Echegoyen, F. Diederich et al.
FULL PAPER
1477, 1455, 1238, 1094, 1044, 522 cmÀ1
;
1H NMR (500 MHz, CDCl3): d ˆ
250), 620 (sh, 700), 475 (3000), 242 nm (142000 molÀ1 dm3 cmÀ1); IR (KBr):
nÄ ˆ 2922, 1745, 1505, 1450, 1280, 1111, 1022, 522 cmÀ1; 1H NMR (500 MHz,
Cl2DCCDCl2, 808C): d ˆ 7.19 (br s, 1H), 7.05 (br s, 1H), 4.58 ± 4.45 (m, 2H),
4.58 (br q, J ˆ 7.1 Hz, 2H), 4.35 ± 4.25 (m, 2H) 4.34 (q, J ˆ 7.1 Hz, 2H), 3.97
(s, 3H), 3.90 (s, 3H), 1.57 (br t, J ˆ 7.1 Hz, 3H), 1.31 (t, J ˆ 7.1 Hz, 3H);
13C NMR (125.8 MHz, CDCl3): d ˆ 163.56 (2  ), 159.00, 157.69, 157.68,
148.71, 148.64 (2 Â ), 148.27, 148.14, 147.57 (2 Â ), 147.48, 146.80, 146.72,
146.00, 145.76, 145.73, 145.47, 145.21, 144.86, 144.48, 144.34 (2 Â ), 144.13
(2 Â ), 143.92, 143.82, 143.63, 143.22, 143.01, 142.53, 142.42, 142.38, 141.82,
141.69, 141.52, 141.44, 140.78, 138.83, 130.07, 129.02, 128.21, 111.53, 71.03,
70.88, 66.07, 65.62, 63.30, 63.13, 56.28, 56.24, 50.81, 45.20, 14.27, 14.11; MS
7.36 (s, 2H), 4.41 (q, J ˆ 7.1 Hz, 4H), 4.04 (s, 6H), 1.37 (t, J ˆ 7.1 Hz, 6H);
13C NMR (125.8 MHz, CDCl3): d ˆ 163.21, 154.30, 154.07, 151.68, 148.48,
147.90, 147.81, 147.23, 146.93, 146.85, 146.56, 146.51, 146.33, 145.18, 145.17,
144.80, 144.76, 144.56, 144.45, 143.89, 143.80, 143.55, 143.16, 142.90, 142.84,
142.58, 142.46, 141.62, 141.02, 140.27, 139.05, 137.66, 106.55, 77.88, 71.68,
‡
70.30, 63.11, 56.50, 50.81, 14.14; MS (FAB): m/z (%): 1014 ([M ], 57), 720
‡
(C60 , 100).
(Æ)-Diethyl
63,64-bis[(acetoxy)methyl]-1,2-(but[2]eno)-33,50-meth-
ano[60]fullerene-61,61-dicarboxylate ((Æ)-5): To a solution of 15 (420 mg,
0.46 mmol) and diethyl 2-bromomalonate (0.11 mL, 0.69 mmol) in PhMe
(350 mL), DBU (0.13 mL, 0.92 mmol) was added and the solution was
stirred for 2 h. Plug filtration (SiO2; CH2Cl2 ! CH2Cl2/MeOH 95:5) and
column chromatography (SiO2; CH2Cl2/MeOH 98:2) gave (Æ)-5 (35 mg,
7%) as the second fraction. Preparative HPLC yielded analytically pure
(Æ)-5 (eluent PhMe/AcOEt 49:1, injection volume 500 mL (c ˆ
2.1 mgmLÀ1), flow rate 8.0 mLminÀ1, retention time 17 min). Analytical
HPLC purity control (eluent CH2Cl2, flow rate 2.0 mLminÀ1, retention
time 7.9 min); m.p. >2508C; UV/Vis (CH2Cl2): lmax (e) ˆ 701 (sh, 290), 630
(sh, 650), 475 (3150), 244 nm (125000 molÀ1 dm3 cmÀ1); IR (KBr): nÄ ˆ 2977,
‡
‡
(FAB): m/z (%): 1043 ([M ], 65), 720 (C60 , 55).
Tetraethyl 1,2:41,58-bis(methano)[70]fullerene-71,71,72,72-tetracarboxy-
late (11), (Æ)-tetraethyl 1,2:56,57-bis(methano)[70]fullerene-71,71,72,72-
tetracarboxylate ((Æ)-12), and (Æ)-tetraethyl 1,2:67,68-bis(methano)[70]-
fullerene-71,71,72,72-tetracarboxylate ((Æ)-13): The bis-adduct mixture 11,
(Æ)-12, and (Æ)-13 was prepared according to the literature.[6b] After
isolation of the mono-adduct fraction from the crude reaction mixture by
column chromatography (SiO2; PhMe), the regioisomeric bis-adducts were
separated by column chromatography (SiO2-H; PhMe/hexane 8:2) to yield,
in the order of elution: 11 (11%), (Æ)-12 (47%), and (Æ)-13 (13%). The
purity of each fraction was checked by HPLC (SiO2, PhMe, flow rate
1.0 mLminÀ1).
2922, 1741, 1367, 1229, 1016, 522 cmÀ1
;
1H NMR (500 MHz, CDCl3): d ˆ
5.24 (br s, 2H), 5.15 (br s, 2H), 4.61 ± 4.57 (m, 2H), 4.45 ± 4.38 (m, 2H), 4.16
(d, J ˆ 14.2 Hz, 1H), 4.10 (d, J ˆ 14.2 Hz, 1H), 3.94 (d, J ˆ 13.9 Hz, 1H),
3.88 (d, J ˆ 13.9 Hz, 1H), 2.10 (s, 3H), 1.94 (s, 3H), 1.51 (t, J ˆ 7.1 Hz, 3H),
1.38 (t, J ˆ 7.1 Hz, 3H); 13C NMR (125.8 MHz, CDCl3): d ˆ 170.93, 170.79,
163.52, 163.47, 148.72, 148.30, 148.28, 148.15, 148.02, 147.59 (2 Â ), 147.56,
147.53 (2 Â ), 147.47, 147.39, 146.79, 146.74, 146.21, 146.05, 145.95, 145.77,
145.49, 145.20, 144.85, 144.47, 144.36, 144.34, 144.20 (2 Â ), 144.14, 144.11,
143.94, 143.81, 143.65, 143.25, 143.10, 143.05, 142.44, 142.00, 141.83, 141.70,
141.54, 141.49, 141.25, 140.76, 139.50, 138.78, 137.62, 136.38 (2 Â ), 71.10,
70.93, 65.76, 65.24, 63.33, 63.15, 62.38, 62.30, 50.90, 43.21, 42.05, 20.09, 20.72,
61,61-Bis(p-methoxyphenyl)-1,2-methano[60]fullerene (17): Compound 17
was synthesized by a slightly modified literature procedure.[30b] C60 (1.2 g,
1.58 mmol) was dissolved in PhMe (600 mL), then BuLi (1.75 mL 1.6m
solution in hexane, 2.8 mmol) was added. A solution of 4,4'-dimethoxy-
benzophenone N-tosylhydrazone in PhMe (100 mL) was added, and the
mixture was heated to reflux for 1 h. After cooling to 208C, plug filtration
(SiO2; PhMe) yielded the crude product which was further purified by
column chromatography (SiO2; PhMe/hexane 3:2 ! 2:1). The mono-
adduct fraction that contained a mixture of the 6,6-closed and 6,5-open
mono-adducts was dissolved in PhMe (200 mL) and heated to reflux for
10 h. Finally, the solution was evaporated to dryness, dissolved in CH2Cl2,
and re-precipitated with hexane to yield pure 17 (550 mg, 37%). Analytical
data are in agreement with the literature.[30b]
‡
‡
14.82, 14.11; MS (FAB): m/z (%): 1077 ([M ], 48), 720 (C60 , 97); C77H24O8
(1077.0): calcd: C 85.87, H 2.25; found: C 85.90, H 2.42.
(Æ)-Diethyl
63-methyl-1,2-(methaniminomethano)-33,50-methano[60]
solution of 16 (260 mg,
fullerene-61,61-dicarboxylate ((Æ)-6): To
a
0.33 mmol), diethyl malonate (0.06 mL, 0.4 mmol), and I2 (126 mg,
0.5 mmol) in PhMe/CH2Cl2 (250 mL/200 mL), DBU (0.15 mL, 1.0 mmol)
was slowly added. After 2 h, diethyl malonate (0.12 mL, 0.8 mmol), I2
(252 mg, 1.0 mmol), and DBU (0.3 mL, 2.0 mmol) were added again and
the solution was stirred for another 2 h. Plug filtration (SiO2; CH2Cl2 !
CH2Cl2/MeOH 9:1) and column chromatography (SiO2; CH2Cl2) provided
(Æ)-6 (40 mg, 13%). Preparative HPLC yielded analytically pure (Æ)-4
(eluent PhMe, injection volume 500 mL (c ˆ 1.9 mg mLÀ1), flow rate
8.0 mLmin, retention time 75 min). Analytical HPLC purity control
1,1'-(Ethane-1,2-diyl) 3,3'-diethyl bis(malonate): Ethane-1,2-diol (250 mg,
4.03 mmol), ethyl 3-chloro-3-oxopropanoate (1.8 g, 12.1 mmol), and C5H5N
(0.96 g, 12.1 mmol) were mixed in CH2Cl2/THF (60 mL/10 mL), and the
solution was stirred for 12 h at 208C. The mixture was washed with
saturated aqueous NH4Cl solution (2 Â ), dried (MgSO4), and evaporated
to dryness. Column chromatography (SiO2; CH2Cl2/AcOEt 3:1) yielded the
desired product (1.013 g, 87%) as a colorless oil. IR (film): nÄ ˆ 2985, 1754,
1733, 1446, 1410, 1328, 1271, 1189, 1151, 1035, 979, 867, 841, 787, 681 cmÀ1
;
1H NMR (200 MHz, CDCl3): d ˆ 4.31 (s, 4H), 4.14 (q, J ˆ 7.5 Hz, 4H), 3.34
(eluent PhMe/AcOEt 9:1, flow rate 1.0 mLminÀ1
, retention time
(s, 4H), 1.22 (t, J ˆ 7.5 Hz, 6H); 13C NMR (50 MHz, CDCl3): d ˆ 165.90,
4.7 min); m.p. >2508C; UV/Vis (CH2Cl2): lmax (e) 700 (sh, 290), 620 (sh,
700), 473 (2500), 247 nm (105000 molÀ1 dm3 cmÀ1); IR (KBr): nÄ ˆ 2968,
2922, 2773, 1743, 1229, 526 cmÀ1; 1H NMR (500 MHz, CDCl3): d ˆ 4.62-4.58
(m, 2H), 4.42 (dq, J ˆ 7.1 Hz, 1.5 Hz, 2H), 4.41 (d, J ˆ 9.2 Hz, 1H), 4.35 (d,
J ˆ 9.2 Hz, 1H), 4.23 (d, J ˆ 9.3 Hz, 1H), 4.17 (d, J ˆ 9.3 Hz, 1H), 2.94 (s,
3H), 1.51 (t, J ˆ 7.1 Hz, 3H), 1.38 (t, J ˆ 7.1 Hz, 3H); 13C NMR (125.8 MHz,
CDCl3): d ˆ 163.53 (2  ), 156.87, 155.82, 155.63, 148.44, 148.09, 148.06,
148.01, 147.98, 147.88, 147.48, 147.46, 147.40 (2 Â ), 147.30, 147.19, 146.47,
146.31, 146.29, 146.15, 146.03, 145.88, 145.46, 145.29, 145.14, 144.99, 144.64,
144.55, 144.39, 144.28, 144.20, 144.16, 144.15, 143.91, 143.82, 143.71, 143.37
(2 Â ), 143.16, 143.10, 143.06, 143.02, 142.50, 142.00, 141.77, 141.69, 141.48,
141.46, 140.80, 139.90, 139.73, 139.47, 139.06, 138.80, 137.91, 136.75, 135.94,
71.10, 70.59, 70.15, 69.46, 63.31, 63.15, 56.57, 51.04, 41.61, 14.27, 14.12; MS
‡
165.77, 62.28, 61.11, 40.82, 13.55; MS (EI): m/z (%): 291 [MH ]; C12H18O8
(290.3): calcd: C 49.65, H 6.25; found: C 49.42, H 6.03.
(Æ)-endo,endo-61,62-(Ethane-1,2-diyl) 61,62-diethyl 1,2:16,17-bis(meth-
ano)[60]fullerene-61,61,62,62-tetracarboxylate ((Æ)-21): To a solution of
C60 (300 mg, 0.416 mmol), 1,1'-(ethane-1,2-diyl) 3,3'-diethyl bis(malonate)
(145 mg, 0.5 mmol), and I2 (264 mg, 1.04 mmol) in PhMe (600 mL), DBU
(0.4 mL, 2.5 mmol) was slowly added under N2 at 208C and the mixture was
stirred for 12 h. Plug filtration (SiO2; CH2Cl2) and column chromatography
(SiO2; CH2Cl2) yielded (Æ)-21 (60 mg, 16%) as red-brown solid. M.p.
>2508C; UV/Vis (CH2Cl2): lmax (e) ˆ 699 (260), 634 (sh, 480), 459 (2140),
314 (34700), 253 nm (99300 molÀ1 dm3 cmÀ1); IR (film): nÄ ˆ 2967, 1745,
1439, 1362, 1230, 1097, 1050, 751, 525 cmÀ1 1H NMR (200 MHz, CDCl3):
;
‡
‡
(FAB): m/z (%): 936 ([M ], 35), 720 (C60 , 50).
d ˆ 5.27 (br d, J ˆ 11.2 Hz, 2H), 4.52 (q, J ˆ 7.1 Hz, 4H), 4.26 (br d, J ˆ
11.2 Hz, 2H), 1.47 (t, J ˆ 7.1 Hz, 6H); 13C NMR (125.8 MHz, CDCl3): d ˆ
163.91, 163.24, 146.71, 146.58, 145.58, 145.56, 145.55, 145.50, 145.43, 145.13,
144.96, 144.90, 144.56, 144.41, 144.14, 143.91, 143.65, 143.36, 142.49, 142.46,
142.45, 141.96, 141.07, 141.06, 140.87, 140.83, 140.55, 135.31, 128.75, 71.80,
(Æ)-Diethyl 64,65-dimethoxy-1,2-methano-33,50-(methano[1,2]benzeno
methano)[60]fullerene-69,69-dicarboxylate ((Æ)-8): To a solution of 1
(470 mg, 0.54 mmol) in PhMe (300 mL), 1,2-bis(bromomethyl)-4,5-dimeth-
oxybenzene (230 mg, 0.71 mmol) and Bu4NI (580 mg, 1.57 mmol) were
added and the mixture was heated to reflux for 16 h. After cooling to 208C,
the mixture was washed with water (3 Â ) and the organic phase dried
(MgSO4). Column chromatography (SiO2; PhMe/CH2Cl2 2:1) afforded five
fractions of which the second contained (Æ)-8 (12 mg, 2%). Preparative
HPLC yielded analytically pure (Æ)-8 (eluent PhMe, injection volume
500 mL (c ˆ 2.0 mg mLÀ1), flow rate 8.0 mLminÀ1, retention time 37 min).
‡
‡
68.56, 63.93, 63.57, 49.80, 14.22; (FAB): m/z (%): 1006 ([M ], 75), 720 (C60
,
100).
Electrochemistry: Fullerene mono- and bis-adducts (2 ± 3 mg) and sup-
porting electrolyte Bu4NPF6 (0.6 g) were added into a home-built two-
compartment electrolysis cell (for full description, see ref. [31]). The cell
was degassed and pumped to 10À6 mm Hg. The solvent, CH2Cl2 (14 mL),
which had also been degassed and pumped to the same pressure, was then
vapor-transferred into the cell, directly from P2O5. Prior to CPE, cyclic
Analytical HPLC purity control (eluent PhMe, flow rate 2.0 mLminÀ1
,
retention time 9.8 min); m.p. >2508C; UV/Vis (CH2Cl2): lmax (e) ˆ 700 (sh,
2190
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0947-6539/00/0612-2190 $ 17.50+.50/0
Chem. Eur. J. 2000, 6, No. 12

Bis-adducts of C60
2184±2192
voltammetry was performed using a glassy carbon- or a Pt-disk working
electrode to obtain the reduction potentials versus a Ag wire pseudo-
reference electrode. The latter was separated from the bulk solution using a
vycor tip. Unless otherwise specified, exhaustive CPE was conducted at
293 K on a Pt mesh (100 mesh, 6.5 cm2) working electrode at 100 mV more
negative than the second reduction peak potential. The solution was then
exhaustively re-oxidized at 0 V. The electrolyte was removed by evapo-
ration of the solvent followed by product extraction with toluene. The
product mixture was then passed through a short column of SiO2 and eluted
with either toluene or carbon disulfide. TLC, HPLC, 1H NMR, and/or UV/
VIS spectroscopy were used to identify the products.
[15] a) F. Cardullo, L. Isaacs, F. Diederich, J.-P. Gisselbrecht, C. Boudon,
M. Gross, Chem. Commun. 1996, 797 ± 799; b) F. Cardullo, P. Seiler, L.
Isaacs, J.-F. Nierengarten, R. F. Haldimann, F. Diederich, T. Morda-
sini-Denti, W. Thiel, C. Boudon, J.-P. Gisselbrecht, M. Gross, Helv.
Chim. Acta 1997, 80, 343 ± 371.
[16] T. Da Ros, M. Prato, F. Novello, M. Maggini, M. De Amici, C. De
Micheli, Chem. Commun. 1997, 59 ± 60.
[17] For previous studies, see: a) C. Boudon, J.-P. Gisselbrecht, M. Gross,
L. Isaacs, H.-L. Anderson, R. Faust, F. Diederich, Helv. Chim. Acta
1995, 78, 1334 ± 1344; b) J.-F. Nierengarten, T. Habicher, R. Kessinger,
F. Cardullo, F. Diederich, V. Gramlich, J.-P. Gisselbrecht, C. Boudon,
M. Gross, Helv. Chim. Acta 1997, 80, 2238 ± 2276; c) C. Boudon, J.-P.
Gisselbrecht, M. Gross, A. Herrmann, M. Rüttimann, J. Crassous, F.
Cardullo, L. Echegoyen, F. Diederich, J. Am. Chem. Soc. 1998, 120,
7860 ± 7868.
[18] For a preliminary communication of parts of this work, see:R.
Kessinger, J. Crassous, A. Herrmann, M. Rüttimann, L. Echegoyen, F.
Diederich, Angew. Chem. 1999, 110, 2022 ± 2025; Angew. Chem. Int.
Ed. 1999, 37, 1919 ± 1922.
[19] For earlier hints at the reversibility of the Bingel reaction, see: a) A.
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1,1'-(Ethane-1,2-diyl) 3,3'-diethyl 2,2-([60]fullerene-1,2-diyl)bis(malonate)
(22): Bis-adduct (Æ)-21 (19.5 mg, 0.019 mmol) was electrolyzed at the first
reduction potential and 1.8 C were transferred to the solution. In a second
reduction step at the second reduction potential, another 2.8 C were
transferred to the solution. After exhaustive re-oxidation at 0 V, 1.9 C were
recovered. Column chromatography (SiO2-H, CH2Cl2) yielded C60 (25%),
(Æ)-21 (18%), and 22 (17%). 22: M.p. >250 oC; UV/Vis (CH2Cl2): lmax
(e) ˆ 686 (280), 484 (2100), 426 (2700), 325 (41000), 259 nm
(141000 molÀ1 dm3 cmÀ1); IR (KBr): nÄ ˆ 2967, 1746, 1233, 526 cmÀ1
;
1H NMR (500 MHz, CDCl3): d ˆ 4.73 ± 4.71 (m, 2H), 4.57 (q, J ˆ 7.1 Hz,
2H), 4.56 ± 4.54 (m, 2H), 4.21 (q, J ˆ 7.1 Hz, 2H), 1.49 (t, J ˆ 7.1 Hz, 3H),
1.30 (t, J ˆ 7.1 Hz, 3H); 13C NMR (125.8 MHz, CDCl3): d ˆ 166.36, 166.10,
163.47, 163.29, 145.29, 145.21, 145.20, 145.13, 145.10, 145.02, 144.92, 144.83,
144.70, 144.69, 144.65, 144.58, 143.90, 143.87, 143.10 (2 Â ), 143.04, 143.03,
142.96, 142.21, 142.19, 141.88, 141.86, 140.97 (2 Â ), 139.37, 138.81, 71.38,
64.39, 63.58, 62.69, 61.73, 51.84, 41.28, 14.22, 14.11; MS (FAB): m/z (%):
‡
‡
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found: C 85.49, H 1.47.
Â
Â
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We thank Mr. S. Siefken for the preparation of the C70-adducts and the
Swiss National Science Foundation as well as the US National Science
Foundation (grant CHE-9816503) for financial support. A donation of pure
C70 by Hoechst AG (Germany) is gratefully acknowledged.
Â
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Received: December 7, 1999 [F2177]
2192
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