[2 + 2] cycloaddition of fullerenes with electron-rich alkenes and alkynes

Article abstract of DOI:10.1021/jo9602231

Photochemical [2 + 2] cycloaddition of C₆₀ with N,N-diethyl-4-methyl-3-penten-1-yn-1-amine (1) afforded the stable C₆₀-fused cyclobutenamine 2 which underwent self-sensitized photooxidation to dihydrofullerenone amide 4 in high yield. Fullerenes C₆₀ and C₇₀ react thermally with tetraalkoxyethylenes to give dihydrofullerene cyclobutanediketals, which undergo a clean cycloreversion to fullerenes on irradiation with visible or UV light. Attempted hydrolysis of the ketals was unsuccessful, but treatment of 1,2-(6161,62,62-tetraethoxycyclobutano)dihydro[60]fullerene (8) with TMSI gave ring-contracted product 11 (by cleavage of the C-C bond to the fullerene fragment) along with reduction product 12.

Full text of DOI:10.1021/jo9602231

5456
J . Org. Chem. 1996, 61, 5456-5461
[2 + 2] Cycloa d d ition of F u ller en es w ith Electr on -Rich Alk en es
a n d Alk yn es
Xiaojun Zhang, Andrew Fan, and Christopher S. Foote*
Department of Chemistry and Biochemistry, University of California, Los Angeles
Los Angeles, California 90095-1569
Received February 2, 1996X
Photochemical [2 + 2] cycloaddition of C60 with N,N-diethyl-4-methyl-3-penten-1-yn-1-amine (1)
afforded the stable C60-fused cyclobutenamine 2 which underwent self-sensitized photooxidation
to dihydrofullerenone amide 4 in high yield. Fullerenes C60 and C70 react thermally with
tetraalkoxyethylenes to give dihydrofullerene cyclobutanediketals, which undergo a clean cyclo-
reversion to fullerenes on irradiation with visible or UV light. Attempted hydrolysis of the ketals
was unsuccessful, but treatment of 1,2-(6161,62,62-tetraethoxycyclobutano)dihydro[60]fullerene (8)
with TMSI gave ring-contracted product 11 (by cleavage of the C-C bond to the fullerene fragment)
along with reduction product 12.
In tr od u ction
adducts. These reactions make use of the electron
deficiency of fullerenes. However, [2 + 2] cycloadditions
of fullerenes are less common. Hoke reported that
benzyne reacted with C60 to give a series of [2 + 2]
adducts, from which the monoadduct was isolated and
characterized.28 We and Wilson et al. have reported
photochemical [2 + 2] cycloaddition of fullerenes with
N,N-diethylpropynylamine (ynamine) and cyclohexenone,
respectively.29-31 Reactions with several other ynamines
have been reported, as have some of the reactions of the
initial adducts.32 The initial cycloadducts of ynamines
with fullerenes are labile and are easily cleaved to
dicarbonyl dihydrofullerenes (DHFs) through an inter-
esting self-sensitized mechanism. In this paper, we
report a further example of photochemical [2 + 2]
cycloaddition of an enynamine to C60 to give a stable C60-
fused cyclobutenamine, and its self-sensitized photo-
oxygenation to a fullerenyl enone. We also report that
tetraalkoxyethylenes, another type of electron-rich olefin,
react with fullerenes thermally to give good yields of DHF
bisketals, which undergo a facile photocycloreversion on
irradiation.
Many methods of functionalizing fullerenes have been
reported, including many that employ cycloaddition.1-3
Among the cycloaddition routes, Diels-Alder4-17 and 1,3-
dipolar cycloadditions18-27 are particularly useful because
they usually give good yields of characterizable mono-
X Abstract published in Advance ACS Abstracts, J uly 1, 1996.
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S0022-3263(96)00223-X CCC: $12.00 © 1996 American Chemical Society

[2 + 2] Cycloaddition of Fullerenes
J . Org. Chem., Vol. 61, No. 16, 1996 5457
Sch em e 1
Sch em e 2
Sch em e 3
in 70% yield based on recovered C60. Detailed spectro-
1
scopic studies support the structure of 2. The H NMR
spectrum of 2 resembles that of 1, but with significant
downfield shifts: the vinyl proton singlet shifts to 6.45
ppm (5.35 ppm in 1) because of the electron-withdrawing
effect of the DHF ring. 13C NMR spectra (125 MHz)
resolved 34 olefinic carbon signals between 157.75 and
116.10 ppm, of which 30 belong to C60 and four belong to
the dienamine moiety. Among the 34 olefinic carbon
signals, 28 have intensity of two and 6 have intensity of
one. This is the correct number and intensity for an
adduct of Cs symmetry with addition occurring at the 6:6-
ring junction, like other such C60 adducts. The two sp3
dihydrofullerenyl (DHF) carbons resonate at 76.10 and
75.42 ppm. Besides the characteristic stretching fre-
quencies for DHFs,34 FTIR spectrum of 2 showed several
absorptions between 1635 and 1684 cm-1, which we
assign to the strained cyclobutene CdC bands. Negative
FABMS gave the expected M- at 871. These spectro-
scopic characteristics resemble similar adducts reported
previously.31,32
Irradiation of a solution of 2 in toluene under a slow
stream of oxygen cleanly produced the fullerenone amide
4 in a self-sensitized photooxygenation of the enamines
like that observed previously.31,32 Fullerenes35,36 and
dihydrofullerenes37,38 have been shown to be excellent
singlet oxygen sensitizers. 1,2-Dioxetane 3 is the most
likely intermediate for the photocleavage. Such unstable
dioxetanes have been characterized in photooxidation of
other systems by low-temperature NMR experiments.39
Compound 4 could also be prepared in similar yield, in
one pot without isolation of 2.
bond: all four methylene hydrogens are nonequivalent
at -40 °C; pairwise averaging occurs at room tempera-
ture, where the two pairs are still broad and are appar-
ently beginning to coalesce to a single group.
[2 + 2] Cycloa d d ition of Tetr a a lk oxyeth ylen es to
C60 a n d C70. Tetraalkoxyethylenes are good π-electron
donors, which are known to react with electron-deficient
alkenes.40 In view of the electron affinity of fullerenes,41,42
we expected that adducts would be formed with these
compounds. Heating a mixture of C70 and tetraethoxy-
ethylene40,43 5 (5 mol equiv) at 100 °C in toluene for 12 h
afforded a monoadduct, 1,9-(71,71,72,72-tetraethoxy-
cyclobutano)dihydro[70]fullerene (6) in 53% isolated yield
(Scheme 2).44
1
The H NMR spectrum of 6 showed four sets of CH2
protons groups (ABX3, details in the Experimental Sec-
tion). The 13C NMR spectrum showed 35 sp2 and 2 sp3
fullerene carbon signals. Among the 35 sp2 carbon
signals, 33 have double and two have single intensity.
Signals at 112.35 and 111.31 ppm are assigned to the
Fullerenone amide 4 was characterized by FAB-MS
and 13C NMR (carbonyls at 169.3 ppm for the amide and
193.4 ppm for the ketone and overall Cs symmetry for
1
two ketal carbons. Both the H and 13C NMR data are
consistent with Cs symmetry from addition at the 1,9
bond of C70. The structure of 6 was also shown by FTIR,
FABMS and UV-vis data.
Similarly, refluxing a mixture of C60 with 5 or tetra-
butoxyethylene40,43 (7) in benzene afforded diketal 8 (69%)
or 10 (59%) respectively, after isolation (Scheme 3).
These compounds were fully characterized by 1H, 13C
1
the fullerene carbons) as well as FTIR and H NMR. It
shows temperature-dependent dynamic behavior in the
1H NMR from restricted rotation around the amide
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5458 J . Org. Chem., Vol. 61, No. 16, 1996
Zhang et al.
Sch em e 4
F igu r e 1. The eight possible regioisomers that can arise by
double addition of 5 at 6:6 ring junctions in C60 and their
symmetry.
NMR, FTIR, UV-vis, and MS data (see Experimental
Section for details).
A small amount of products of multiple addition was
also obtained in reaction of C60 with 5; diadduct 9 was
isolated after two preparative TLC separations. There
are eight possible regioisomers for 9 if one assumes the
second addition also occurs at 6,6-ring junctions (Figure
1). The 1H NMR spectrum (500 MHz) of 9 showed a
multiplet at 4.1 ppm for the methylene protons, and four
sets of triplets between 1.50 and 1.35 ppm. The 13C NMR
of 9 showed 28 signals between 153.9 and 125.11 ppm,
in which two signals have intensity of one, and 26 have
intensity of two, and the remaining one has intensity of
four (from two overlapping peaks of intensity two). Three
sp3 fullerenyl carbons were found; one is twice the
intensity of the others. From the analysis of the sym-
metry of the 1H and 13C NMR spectra, compound 9 is
probably the 1,2:18,36-diadduct.44 The second addition
occurs at the equatorial double bond (Figure 1). This is
consistent with the steric effects shown for C60 poly-
adducts previously,45,46 where attack at the octahedral
sites is preferred.
Diketal 8 could not be hydrolyzed by H2SO4 or HCl in
THF. With CF3COOH in benzene, 8 decomposed to C60.
However, on treatment with trimethylsilyl iodide (TMSI)
in CH2Cl2, 8 gave a ring-contracted product, methano-
fullerene 11, and reduction product 12 in 38% total yield.
Methanofullerenes of structure 11 have been reported
previously by 1,3-dipolar cycloaddition of diazoacetates
to C60 by Wudl3 and Diederich et al.47
Several electron-rich alkynes, including ethyl ethynyl
ether, bis(ethylthio)ethyne, and the alkene tetrakis-
(morpholino)ethylene did not react with C60 either ther-
mally or photochemically.
triplet quencher,49 efficiently inhibits the cycloreversions.
However, neither irradiation of the C60-fused cyclohexane
15 (the Diels-Alder adduct of C60 with benzocyclo-
butenol16) nor cyclopentane 16 (the 1,3-dipolar addition
product of C60 with the ylide25) gave appreciable cyclo-
reversion or decomposition.
Discu ssion
The photophysical properties of fullerenes have been
the subject of numerous investigations,50-52 but their
photochemistry has received less attention. Most at-
tempts to functionalize C60 and C70 have been based on
thermal chemistry, especially cycloadditions. Previous
results have demonstrated that the triplet excited state
of C60 has a reduction potential near 0.98 V (36 kcal/mol
triplet E) vs SCE, and is formed with a quantum yield of
nearly unity.35 Triplet C60 is readily photoreduced by
amines and other donors to the C60 radical anion and
donor radical cation.51,53,54 We believe that the photo-
additions of ynamines to fullerenes proceed via electron
(or at least charge) transfer from the electron-rich
ynamines, followed by rapid collapse of the initial ion pair
or charge-transfer complex to the covalent adducts.
Preliminary attempts to observe transient intermediates
have not been successful.55 It is likely that collapse of
the intermediate in this case is too rapid to allow direct
observation, at least on a 100 ns time scale.
P h ot och em ica l Cyclor ever sion of F u ller en yl
Dik eta ls 6, 8, a n d 10. Diketals 6, 8, and 10 are
thermally stable compounds. However, irradiation with
visible or UV light produced C70 and C60, respectively,
along with dialkyl carbonates 13 and 14 (Scheme 4).
Compounds 13 and 14 are formed through singlet
oxygenation of the tetraalkoxyethylenes,48 which were
produced by cycloreversion of the corresponding DHFs
to the original starting materials. Preliminary studies
indicate that the cycloreversion goes through the triplet
excited states, because oxygen or rubrene, a well-known
Cycloadduct 2 and similar adducts from other ynamines
are unique in that they have a photosensitizer (the DHF)
and an easily photooxidizable group (the enamine) in the
same molecule. Sensitized photooxidative cleavage of
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[2 + 2] Cycloaddition of Fullerenes
J . Org. Chem., Vol. 61, No. 16, 1996 5459
Sch em e 5
enamines is a well-known process and proceeds via an
intermediate 1,2-dioxetane.39,56 Recently, Wudl has re-
ported an elegant example of this process, opening a hole
in the DHF framework by self-sensitized photocleavage
of a fullerene CdC bond connected to a nitrogen.57
It is not surprising that cycloadduct fullerenyl diketals
are formed between fullerenes and tetraalkoxyethylenes,
in view of the electron affinity of fullerenes and electron-
donating ability of tetraalkoxyethylenes. We suspect that
reactions of these very electron-rich alkenes can proceed
directly to a charge-transfer state without light; similar
thermal reactions were observed with the very electron-
rich ynediamines and thioynamines.32
Interestingly, the cycloadducts from the tetraalkoxy-
ethylenes undergo photocycloreversion, giving back C60
and C70 cleanly and efficiently on visible or UV light
irradiation. The photocycloreversion goes through the
triplet excited state, because rubrene and oxygen ef-
ficiently inhibit it. Excitation of the adducts may lead
directly to a charge-transfer state that is reversibly
formed from the starting materials. Further work is in
progress to test these mechanistic hypotheses.
This photocycloreversion has potential applications, for
example, in solubilizing fullerenes with such diketal
adducts as a removable handle, or more interestingly,
as a template to direct multiple additions. Examples of
the use of such templates for preparation of multi-
addended fullerenes in a controlled fashion have just
started to appear in the literature.58-61
It is known that most reactions with C70 occur pre-
dominantly at the 1,9-bond.32,62-65 This has been ex-
plained by the greater curvature and resulting strain at
the polar region. Monitoring the reaction of C70 with 5
by HPLC indicated three other minor peaks at t ) 7.46
min (3%), 5.89 min (4%), and 4.45 min (9%), besides 6 at
retention time t ) 7.95 min (84%). The peaks at 5.89
and 4.45 min were identified as a mixture of di- or
polyadducts adducts by 1H NMR after isolation. The
peak at 7.46 min could not be isolated because of the
small amount present in the mixture. Because its
retention time is so close to that of the monoadduct 6, it
is likely that this peak corresponds to a 7,8-isomer with
a similar polarity to 6. If this is true, the ratio of 1,9 to
7,8 addition would be 28, which corresponds to a differ-
ence in free energy of activation ∆Gq at 293 K of 1.93
kcal/mol, with 1,9-addition more favorable. A recent
study by Cahill et al. measured the equilibration of two
C70H2 isomers, showing a ∆∆G0 at 295 K of 1.4 ( 0.2 kcal/
mol favoring the 1,9-isomer.63
rapidly than the C-O bond because of the strong
electron-withdrawing ability of the DHFs. On treatment
with the strong Lewis acid TMSI, the fullerene bond
migrates to the carbocation, giving methanofullerene 11
after addition of H2O (Scheme 5). We31 and others66,67
have previously observed this type of C-C bond cleavage
in hydrolysis of fullerene derivatives. In a competing
reaction, formation of the iodide followed by halophilic
attack by iodide would give a carbanion next to the
electron-withdrawing cage. Protonation of the carbanion
would produce compound 12. Nucleophilic attack on
iodine in C-I bonds is precedented.68
Con clu sion s
Enynamine 1 undergoes efficient photochemical [2 +
2] cycloaddition to C60. The initial cycloadduct cyclo-
butenamine 2 is much more stable to oxygen than
previously reported ones.31,32 However, like the other
adducts, 2 could also be photooxygenated to the dicar-
bonyl compound via self-sensitization. Since we have
already demonstrated that yndiamines and thioynamines
add to C60 and C70, the reaction of fullerenes with
ynamines is general and preparatively useful. Thermal
cycloaddition of tetraethoxyethylene to fullerenes also
gave high yields of [2 + 2] cycloadducts, which undergo
efficient photochemical cycloreversion through the triplet
excited state. Because these cycloadducts are very
soluble in many organic solvents, this novel photochem-
istry should be useful for reversible solubilization of
fullerenes.61
Hydrolysis of 8 did not produce the diketone as one
would expect for hydrolysis of a diketal. The C-C bond
directly connected to the DHF surface cleaves more
Exp er im en ta l Section 32
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1,2-[61-(Dieth ylam in o)-62-isopr opyliden ecyclobu ten o]-
d ih yd r o[60]fu ller en e (2). A solution of C60 (86 mg, 0.12
mmol) in 50 mL of toluene was sonicated for 5 min and
degassed by bubbling with argon for 20 min. With stirring,
N,N-diethyl-4-methyl-3-penten-1-yn-1-amine (1) (20 mg, 20 µL,
0.12 mmol) was added via a syringe. The mixture was
irradiated with a 300 W Xenon lamp with a K2Cr2O7 filter
(pathlength 3.5 cm, 530 nm) for 50 min. The resulting brown
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5460 J . Org. Chem., Vol. 61, No. 16, 1996
Zhang et al.
reaction mixture was condensed. Column chromatography
13C NMR (90 MHz, CS2/acetone-d6) δ 151.54 (4 C), 147.32 (2
C), 147.20 (4 C), 146.40 (4 C), 146.38 (4 C), 145.91 (2 C), 145.66
(4 C), 145.04 (4 C), 143.62 (2 C), 143.43 (4 C), 143.17 (4 C),
142.79 (4 C), 142.68 (4 C), 142.62 (4 C), 140.83 (4 C), 139.11
(4 C), 110.25 (2 C, C(OEt)2), 76.10 (2 C, sp3 fullerene), 61.63
(4 C, OCH2), 16.10 (4 C, CH3); FTIR (KBr) cm-1 2973 (w), 2892
(w), 1429 (m), 1240 (s), 1177 (s), 1084 (s), 1073 (s), 1042 (s),
600 (m), 554 (w), 542 (w), 527 (s); FAB MS m/ z 925 (M + H,
16), 879 (M - OCH2CH3, 100); UV-vis λmax (CH2Cl2) (ꢀ) 705
(400), 648 (820), 436 (3900), 312 (45 100), 256 (120 100).
1,2:18,36-Bis(t e t r a e t h oxycyclob u t a n o)[60]d ih yd r o-
fu ller en e (9). This compound was obtained in a small amount
by two preparative TLC separations of the multiaddition
adducts in preparation of 8: 1H NMR (500 MHz, CDCl3) δ
4.21-4.03 (m, 16 H, OCH2), 1.48 (t, 6 H, J ) 7.64 Hz), 1.46 (t,
6 H, J ) 7.66 Hz), 1.44 (6 H, J ) 7.64 Hz), 1.38 (t, 6 H, J )
7.62 Hz); 13C NMR (125 MHz, CDCl3) δ 153.85 (2 C), 148.98
(2 C), 148.68, 148.32 (2 C), 148.18 (4 C), 147.51 (2 C), 147.35
(2 C), 147.21 (2 C), 146.77, 146.74 (2 C), 146.70 (2 C), 146.21
(2 C), 145.11 (2 C), 144.70 (2 C), 144.48 (2 C), 144.18 (2 C),
144.10 (2 C), 144.04 (2 C), 143.33 (2 C), 142.97 (2 C), 142.90
(2 C), 142.42 (2 C), 141.96 (2 C), 141.84 (2 C), 141.07 (4 C),
137.81 (2 C), 136.77 (2 C), 109.11, 109.08, 107.88 (2 C), 74.66
(2 C), 73.72, 73.68, 60.52, 60.47, 60.37, 60.04, 15.32 (2 C), 15.27,
15.13; FTIR (KBr) cm-1 2974 (s), 1241 (s), 1179 (s), 1084 (s),
554 (w), 542 (w), 527 (s); FABMS m/ z 1129 (M + H, 30).
1,2-(6161,62,62-Tetr a bu toxycyclobu ta n o)d ih yd r o[60]-
fu ller en e (10). Compound 10 was prepared from tetra-
butoxyethylene (7) in 59% yield in a procedure similar to that
for 8. Physical data for 10: 1H NMR (360 MHz, CS2/CDCl3) δ
4.23 (dt, ABX2, 4 H, J ) 9.28, 6.73 Hz), 4.13 (dt, ABX2, 4 H, J
) 9.28, 6.73 Hz), 1.83 (m, 8 H), 1.60 (sextet, 8 H, J ) 7.30
Hz), 1.05 (t, 12 H, J ) 7.25 Hz); 13C NMR (125 MHz, CS2/
CDCl3) δ 150.55 (4 C), 146.47 (2 C), 146.25 (4 C), 145.55 (4 C),
145.02 (2 C), 144.80 (8 C), 144.19 (4 C), 142.76 (2 C), 142.59
(4 C), 142.34 (4 C), 141.92 (4 C), 141.83 (4 C), 141.77 (4 C),
139.96 (4 C), 138.17 (4 C), 109.15 (2 C, C(OBu)2), 75.27 (2 C,
sp3 fullerene), 64.70 (4 C, OCH2), 32.09 (4 C), 19.78 (4 C), 14.21
(4 C); FTIR (KBr) cm-1 2955 (s), 2900 (s), 1239 (s), 1094 (s),
1076 (s), 1028 (s), 600 (w), 529 (s); FAB MS m/ z 1038 (M + 2
, 25), 1037 (M + 1, 23), 1036 (M-, 20), 720 (C60, 100); UV-vis
λmax (CH2Cl2) (ꢀ) 702 (400), 436 (4200), 312 (43 000), 256
(112 000).
61-Eth oxy-61-(eth oxycar bon yl)1,2-dih ydr o-1,2-m eth an o-
[60]fu ller en e (11) an d 1,2-(61,61,62-Tr ieth oxycyclobu tan o)-
d ih yd r o[60]fu ller en e (12). To a stirred solution of 8 (20 mg,
0.02 mmol) in dry degassed CH2Cl2 was added trimethylsilyl
iodide (4 µL, 0.028 mmol) at 0 °C. The mixture was stirred at
0 °C for 1 h and at room temperature for 4 h. A few drops of
a dilute solution of NaHCO3 and Na2S2O5 were added to
quench the reaction. The aqueous solution was extracted with
15 mL of toluene. The extracts were dried with Na2SO4,
condensed in vacuo, and chromatographed on SiO2. Elution
with petroleum ether-toluene (2:3) gave C60 (3 mg, 22%), 11
(4 mg, 25%), and 12 (2 mg, 13%, contained ca. 15% of 11).
Physical data for 11: 1H NMR (360 MHz, CS2/CDCl3) δ 4.59
(q, 2 H, J ) 7.15 Hz), 4.34 (q, 2 H, J ) 7.00 Hz), 1.59 (t, 3 H,
J ) 7.09 Hz), 1.50 (t, 3 H, J ) 6.99 Hz); 13C NMR (125 MHz,
CS2/CDCl3) δ 165.48 (1 C, CO), 145.70 (1 C), 145.46 (2 C),
145.44 (2 C), 145.38 (2 C), 145.36 (2 C), 145.20 (2 C), 145.04
(2 C), 145.02 (2 C), 144.90 (2 C), 144.88 (2 C), 144.86 (2 C),
144.68 (2 C), 144.17 (2 C), 144.09 (2 C), 143.47 (2 C), 143.35
(1 C), 143.34 (2 C), 143.31 (2 C), 143.25 (1 C), 143.21 (2 C),
142.69 (2 C), 142.51 (2 C), 142.46 (2 C), 142.04 (2 C), 141.44
(2 C), 141.29 (1 C), 140.10 (2 C), 138.73 (2 C), 128.53 (2 C),
127.50 (2 C), 77.19 (sp3 fullerene), 74.61 (sp3 fullerene), 67.56
(OCH2), 62.76 (OCH2), 15.77, 14.78; FTIR (KBr) cm-1 1740 (s),
1260 (m), 1184 (m), 1036 (m), 575 (w), 556 (w), 527 (s); FAB
MS m/ z 852 (M + 2H, 30), 720 (C60, 100); UV-vis λmax (CH2Cl2)
432, 328, 260. Physical data for 12: 1H NMR (500 MHz, CS2/
CDCl3) δ 5.58 (s, 1 H), 4.36 (m, 1 H), 4.24 (m, 1 H), 4.16 (m, 4
H), 1.50 (m, 9 H); 13C NMR (125 MHz, CS2/CDCl3) δ 154.02,
150.93, 150.78, 149.95, 147.90, 147.13, 147.05, 147.01, 146.34,
146.32, 146.22, 146.21, 146.18, 146.10 (2 C), 146.09, 146.05,
145.73, 145.70, 145.67, 145.55, 145.46 (3 C), 145.42, 145.39,
145.27 (2 C), 144.83, 144.75, 144.72, 144.62, 143.31 (2 C),
(hexane-toluene, gradient elution) furnished unreacted C60
,
cyclobutenamine 2 (58 mg, 70%), and enone amide 4 (6 mg).
Enone amide 4 could also be prepared in a one-pot procedure
using oxygen-saturated toluene in ∼60% yield. Representative
physical data for 2: 1H NMR (500 MHz, CS2/CDCl3) δ 6.45 (s,
1 H), 3.83 (q, 4 H, J ) 7.00 Hz), 2.25 (s, 3 H), 2.09 (s, 3 H),
1.48 (t, 6 H, J ) 7.00 Hz); 13C NMR (125 MHz, CS2/CDCl3) δ
157.74 (2 C), 155.97 (2 C), 150.09, 146.67 (2 C), 146.35, 146.21,
146.14 (2 C), 145.72 (2 C), 145.59 (2 C), 145.52 (2 C), 144.91
(2 C), 144.86 (2 C), 144.78 (2 C), 144.74 (2 C), 144.67 (2 C),
144.38 (2 C), 144.06 (2 C), 142.74 (4 C), 142.44 (2 C), 142.42
(2 C), 142.15 (4 C), 142.03 (2 C), 141.80 (2 C), 141.73 (2 C),
141.64 (2 C), 139.85 (2 C), 139.64 (2 C), 139.58 (2 C), 138.32
(2 C), 137.69, 118.44 (dCH), 116.10, 76.10 (sp3 fullerenyl
carbon), 75.42 (sp3 fullerenyl carbon), 43.16 (2 C, CH2N), 25.85
(dCCH3), 21.70 (dCCH3), 13.99 (2 C, CH3CH2); FTIR (KBr)
cm-1 1671 (m), 1636 (m), 1541 (w), (w), 1094 (w), 694 (m), 527
(s); FABMS m/ z 871 (M-, 50), 720 (C60, 100).
1,2-[61-(Diet h yla m id o)-62-isop r op ylid en eca r b on yl]-
d ih yd r o[60]fu ller en e (4): 1H NMR (360 MHz, CS2/CDCl3)
at 25 °C δ 7.41 (t, 1 H, J ) 1.21 Hz, dCH), 4.00-3.20 (b, 4 H,
NCH2), 2.46 (d, 3 H, J ) 1.07 Hz, dCCH3), 2.19 (d, 3 H, J )
1.04 Hz, dCCH3), 1.38 (t, 6 H, J ) 7.03 Hz); at -40 °C 7.41 (s,
1 H, dCH), 5.13 (dt, 1 H, J ) 14.6, 6.80 Hz), 4.01 (dt, 1 H, J
) 12.41, 6.64 Hz), 3.59 (dt, 1 H, J ) 14.7, 7.29 Hz), 3.22 (dt,
1 H, J ) 12.40, 6.20 Hz), 2.45 (s, 3 H, dCCH3), 2.20 (s, 3 H,
dCCH3), 1.36 (t, 6 H, J ) 6.20 Hz); 13C NMR (90 MHz, CS2/
CDCl3, 25 °C) δ 193.37 (CO), 169.31 (NCO), 160.72, 151.89,
147.30, 147.19, 146.18 (2 C), 146.10, 145.92 (2 C), 145.89 (br),
145.33, 145.16 (br, 2 C), 144.28 (br), 142.99, 142.43 (2 C),
141.98, 141.91, 141.53 (br), 141.25, 139.79 (br), 139.36 (br),
122.97 (dCH), 81.15 (sp3 fullerenyl carbon), 75.00 (sp3 fuller-
enyl carbon), 43.58 (NCH2), 42.66 (NCH2), 28.13 (dCCH3),
21.58 (dCCH3), 13.81 (CH3CH2), 11.82 (CH3CH2); FTIR (KBr)
cm-1 2950 (w), 1676 (m), 1636 (s), 1260 (m), 1115 (m), 770 (m),
576 (w), 527 (s); FABMS m/ z 903 (M-, 50), 720 (C60, 100); UV-
vis λmax (ꢀ) (CH2Cl2) 702 (250), 436 (2920), 312 (33 300), 256
(90 000).
1,9-(7171,72,72-Tetr a eth oxycyclobu ta n o)d ih yd r o[70]-
fu ller en e (6). A solution of C70 (20 mg, 0.024 mmol) in 20
mL of toluene was degassed by bubbling with argon for 20 min.
To this solution was added tetraethoxyethylene (5) (40 µL, 0.20
mmol). The mixture was refluxed overnight. Toluene was
removed in vacuo. The crude product was chromatographed
on SiO2. Elution with hexane-toluene (from 10:1 to 1:5) gave
6 (13 mg, 53%): 1H NMR(500 MHz, CS2/CDCl3) δ 4.36 (ABX3,
2 H, J AB ) 9.50 Hz, J AX ) 7.00 Hz), 4.25 (ABX3, 2 H, J AB
)
9.50 Hz, J AX ) 7.00 Hz), 4.03 (ABX3, 2 H, J AB ) 9.50 Hz, J AX
) 7.00 Hz), 3.91 (ABX3, 2 H, J AB ) 7.00 Hz, J AX ) 7.00 Hz),
1.57 (t, 6 H, J ) 7.00 Hz), 1.37 (t, 6 H, J ) 7.00 Hz); 13C NMR
(125 MHz, CS2/CDCl3) δ 156.40 (2 C), 151.98 (2 C), 151.71 (2
C), 151.41 (1 C), 151.28 (2 C), 150.69 (2 C), 150.51 (2 C), 150.15
(2 C), 150.01 (2 C), 149.70 (2 C), 149.64 (2 C), 149.14 (2 C),
148.99 (2 C), 148.94 (2 C), 148.64 (2 C), 147.51 (2 C), 147.36
(2 C), 147.05 (2 C), 146.87 (2 C), 146.81 (1 C), 146.11 (2 C),
145.57 (2 C), 143.41 (2 C), 143.39 (2 C), 143.33 (2 C), 142.54
(2 C), 142.22 (2 C), 141.96 (2 C), 140.71 (2 C), 139.26 (2 C),
134.04 (2 C), 133.57 (2 C), 131.36 (2 C), 131.08 (2 C), 131.03
(2 C), 112.35 (1C, C(OEt)2), 111.30 (1 C, C(OEt)2), 68.27 (1 C,
sp3 fullerene), 66.04 (1 C, sp3 fullerene), 61.28 (2 C, OCH2),
60.98 (2 C, OCH2), 15.72 (2 C, CH3), 15.49 (2 C, CH3); FTIR
(KBr) cm-1 2975 (w), 1515 (m), 1428 (s), 1241 (s), 1088 (s),
1052 (s), 672 (m), 591 (m), 542 (m), 534 (m), 528 (m). FAB
MS m/ z 1044 (M, 6), 840 (C70, 8); UV-vis λmax (CH2Cl2) 664,
532, 460, 396, 308 (sh), 236.
1,2-(6161,62,62-Tetr a eth oxycyclobu ta n o)d ih yd r o[60]-
fu ller en e (8). A solution of C60 (60 mg, 0.083 mmol) in 50
mL of dry benzene was degassed by bubbling with argon for
20 min. To this solution was added tetraethoxyethylene (5)
(90 µL, 0.42 mmol). The mixture was refluxed overnight.
Benzene was removed in vacuo. The crude product was
chromatographed on SiO2. Elution with petroleum ether-CS2
(from 2:1 to 1:3) gave 8 (53 mg, 69%): 1H NMR (360 MHz,
CS2/acetone-d6) δ 4.33 (dq, ABX3, 4 H, J ) 9.40, 7.0 Hz), 4.20
(dq, ABX3, 4 H, J ) 9.40, 6.99 Hz), 1.49 (t, 12 H, J ) 7.00 Hz);

[2 + 2] Cycloaddition of Fullerenes
J . Org. Chem., Vol. 61, No. 16, 1996 5461
Oxygen In h ibition of P h otocyclor ever sion of 6.
toluene solution of 6 (1.0 mM) was bubbled with oxygen for
10 min before irradiation, and a slow stream of oxygen was
continued during irradiation. After 2.5 h, HPLC indicated no
143.25, 143.22, 143.17, 143.01, 142.95, 142.89, 142.78, 142.62,
142.54, 142.53, 142.51, 142.37 (2 C), 142.34 (2 C), 142.18,
141.06, 140.84, 140.66, 140.50, 140.20, 138.68, 138.58, 137.45,
107.75 (OC(OEt)2), 89.51 (OCH), 77.22 (sp3 fullerene), 69.37
(sp3 fullerene), 67.96 (OCH2), 61.22 (OCH2), 59.96 (OCH2),
16.14, 16.06, 15.79; FTIR (KBr) cm-1 1514 (m), 1184 (m), 1120
(m), 667 (w), 575 (w), 527 (s); FAB MS m/ z 881 (M + H, 45),
765 (M + H - 116, 100), 720 (C60, 80).
P h otocyclor ever sion s of 6, 8, a n d 10 were achieved by
irradiation of a solution of the corresponding diketal (1.0-1.2
mM) with a 300 W Xenon (Cermax) lamp (K2Cr2O7 filter, >500
nm) at room temperature for 5.0 h under a slow stream of
argon. HPLC and13C NMR spectra indicated clean formation
of C70 from 6 and C60 from 8 and 10, respectively. 1H NMR
indicated exclusive formation of the diethyl and dibutyl
carbonates 13 and 14, respectively.
A
formation of C70
.
Inhibitions of photocycloreversion of compound 8 and 10
were done in a similar way, and HPLC indicated rubrene (3
-5 mol equiv) and oxygen completely stopped the reversion
reactions.
Ack n ow led gm en t. This work was supported by
Grant NSF Grants No. CHE89-11916 and CHE94-
23027.
Su p p or t in g In for m a t ion Ava ila b le: 1H NMR and 13C
NMR spectra of compounds 2, 6, and 8-10 (9 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from ACS; see any current masthead page for
ordering information.
Ru br en e In h ibition of P h otocyclor ever sion of 6.
A
solution of 6 (1.2 mM) and rubrene (3.6 mM) in toluene was
deoxygenated by argon bubbling for 10 min. This solution was
irradiated with a 300 W Xenon lamp (either with K2Cr2O7 filter
or without) at room temperature, under a slow stream of argon
for 3.0 h. HPLC indicated less than 1% C70 was formed, while
a similar irradiated solution without rubrene gave 61% of C70
at this point.
J O9602231