Stepwise bond formation in photochemical and thermal Diels-Alder reactions of C60 with Danishefsky's dienes

Article abstract of DOI:10.1021/ja992848x

Mechanisms of both the photochemical and thermal Diels-Alder reactions of C₆₀ with Danishefsky's dienes are studied on the basis of product stereochemistry, kinetics, and the detection of radical intermediates. A stereochemically defined (1E,3Z)-1-methoxy-2-methyl-3-[(trimethylsilyl)oxy]-penta-1,3-diene (1a) is used as a stereochemical probe in the Diels-Alder reactions with C₆₀. The major Diels-Alder product is trans-adduct (3) rather than cis-adduct (4) in both the photochemical and thermal Diels-Alder reactions. Such stereochemistry indicates that the Diels-Alder reactions proceed by a stepwise mechanism rather than a concerted mechanism. The transient spectra of C₆₀?^- formed in photoinduced electron transfer from 1a to the triplet excited state of C₆₀ (³C₆₀*) have been detected successfully in laser flash photolysis of the 1a-C₆₀ system. The observed rate constants determined from the dependence of the quantum yields on the concentrations of Danishefsky's dienes agree well with those for the photoinduced electron transfer from Danishefsky's dienes to ³C₆₀*. Such an agreement together with the formation of trans-adduct (3) indicates that the photochemical Diels-Alder reaction proceeds via stepwise bond formation in the radical ion pair produced in the photoinduced electron transfer from Danishefsky's dienes to ³C₆₀*. On the other hand, the rate constants for the thermal Diels-Alder reactions of Danishefsky's dienes are 10¹⁶ times larger than those expected for outer-sphere electron transfer from Danishefsky's dienes to C₆₀. Thus, a strongly unsymmetrical orbital interaction is required for the thermal Diels-Alder reaction to yield trans-adduct (3) which would not be produced by a concerted pathway.

Full text of DOI:10.1021/ja992848x

2236
J. Am. Chem. Soc. 2000, 122, 2236-2243
Stepwise Bond Formation in Photochemical and Thermal
Diels-Alder Reactions of C60 with Danishefsky’s Dienes
Koichi Mikami,*,† Shoji Matsumoto,† Yasutaka Okubo,† Mamoru Fujitsuka,‡ Osamu Ito,*,‡
Tomoyoshi Suenobu,§ and Shunichi Fukuzumi*,§
Contribution from the Department of Chemical Technology, Faculty of Engineering, Tokyo Institute
of Technology, Meguro-ku, Tokyo 152-8552, Japan, Institute for Chemical Reaction Science,
Tohoku UniVersity, Aoba-ku, Sendai 980-8577, Japan, and Department of Material and Life Science,
Graduate School of Engineering, Osaka UniVersity, CREST, Suita, Osaka 565-0871, Japan
ReceiVed August 9, 1999
Abstract: Mechanisms of both the photochemical and thermal Diels-Alder reactions of C60 with Danishefsky’s
dienes are studied on the basis of product stereochemistry, kinetics, and the detection of radical intermediates.
A stereochemically defined (1E,3Z)-1-methoxy-2-methyl-3-[(trimethylsilyl)oxy]-penta-1,3-diene (1a) is used
as a stereochemical probe in the Diels-Alder reactions with C60. The major Diels-Alder product is trans-
adduct (3) rather than cis-adduct (4) in both the photochemical and thermal Diels-Alder reactions. Such
stereochemistry indicates that the Diels-Alder reactions proceed by a stepwise mechanism rather than a
concerted mechanism. The transient spectra of C60•- formed in photoinduced electron transfer from 1a to the
triplet excited state of C60 (3C60*) have been detected successfully in laser flash photolysis of the 1a-C60
system. The observed rate constants determined from the dependence of the quantum yields on the concentrations
of Danishefsky’s dienes agree well with those for the photoinduced electron transfer from Danishefsky’s dienes
to 3C60*. Such an agreement together with the formation of trans-adduct (3) indicates that the photochemical
Diels-Alder reaction proceeds via stepwise bond formation in the radical ion pair produced in the photoinduced
3
electron transfer from Danishefsky’s dienes to C60*. On the other hand, the rate constants for the thermal
Diels-Alder reactions of Danishefsky’s dienes are 1016 times larger than those expected for outer-sphere
electron transfer from Danishefsky’s dienes to C60. Thus, a strongly unsymmetrical orbital interaction is required
for the thermal Diels-Alder reaction to yield trans-adduct (3) which would not be produced by a concerted
pathway.
Introduction
an alternative stepwise (open-shell) mechanism for the Diels-
Alder reaction has recently merited increasing attention.6-8
There have been some reports on an electron transfer with
formation of radical ion pairs as primary step of the Diels-
Alder reactions, followed by stepwise bond formation.9,10 In
particular Sustmann et al.11 have reported stereochemical studies
and observation of radical ions for [4+2]-cycloadditions of
strong electron donor-acceptor pairs. There has been consider-
able interest in photocycloadditions via photoinduced electron
transfer as a new potentially important pathway for controlling
synthetic processes which cannot be exploited using a classical
concerted pathway.12,13 However, there has so far been no report
on a thermal or photoinduced electron-transfer pathway in the
Since the preparation of fullerene (C60) in synthetically useful
quantities was established,1 the chemistry of C60 has been rapidly
explored to show a wide variety of reactivities.2 Particularly,
Diels-Alder reactions of C60 with Danishefsky’s dienes3 have
been well-examined4 for fullerene functionalization.2 The Diels-
Alder reactions of C60 are generally believed to proceed via a
thermally allowed concerted (suprafacial) process or a photo-
chemical concerted (antarafacial) process.5 On the other hand,
* Author for correspondence.
† Tokyo Institute of Technology.
‡ Tohoku University.
§ Osaka University.
(1) (a) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D.
R. Nature 1990, 347, 354. (b) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulos,
K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (c) Kroto, H. W.;
Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318,
162. Also see: Osawa, E. Kagaku (Chemistry) 1970, 25, 854; Chem. Abstr.
1971, 74(8), 75689v.
(2) Reviews: (a) Wudl, F. Acc. Chem. Res. 1992, 25, 157. (b) Miller,
G. Chem. Ind. 1993, 226. (c) Taylor, R.; Walton, D. R. M. Nature 1993,
363, 685. (d) Hirsch, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1138. (e)
Hirsch, A. The Chemistry of the Fullerenes; Thieme: Stuttgart, 1994. (f)
Diederich, F.; Isaacs, L.; Philp, D. Chem. Soc. ReV. 1994, 243. (g) ’Sliwa,
W. Fullerene Sci. Technol. 1995, 3, 243. (h) Hirsch, A. Synthesis 1995,
895. (i) Diederich, F.; Thilgen, C. Science 1996, 271, 317.
(3) Reviews: Danishefsky, S. J.; DeNinno, M. P. Angew. Chem., Int.
Ed. Engl. 1987, 26, 15; Danishefsky, S. J. Chemtracts 1989, 273.
(4) (a) An, Y.-Z.; Anderson, J. L.; Rubin, Y. J. Org. Chem. 1993, 58,
4799. (b) An, Y.-Z.; Ellis, G. A.; Viado, A. L.; Rubin, Y. J. Org. Chem.
1995, 60, 6353. (c) Wilson, S. R.; Lu, Q. Tetrahedron Lett. 1993, 34, 8043.
(5) Arce, M.-J.; Viado, A. L.; An, Y.-Z.; Khan, S. I.; Rubin, Y. J. Am.
Chem. Soc. 1996, 118, 3775; Arce, M. J.; Viado, A. L.; Khan, S. I.; Rubin,
Y. Organometallics 1996, 15, 4340; Wilson, S. R.; Lu, Q. Y.; Cao, J. R.;
Wu, Y. H.; Welch, C. J.; Schuster, D. I. Tetrahedron 1996, 52, 5131; Ohno,
M.; Koide, N.; Sato, H.; Eguchi, S. Tetrahedron 1997, 53, 9075 and
references therein.
(6) Houk, K. N.; Gonza´lez, J.; Li, Y. Acc. Chem. Res. 1995, 28, 81.
(7) Sauer, J.; Sustmann, R. Angew. Chem., Int. Ed. Engl. 1980, 19, 779.
(8) (a) Goldstein, E.; Beno, B.; Houk, K. N. J. Am. Chem. Soc. 1996,
118, 6036. (b) Chen, J. S.; Houk, K. N.; Foote, C. S. J. Am. Chem. Soc.
1998, 120, 12303. (c) Beno, B. R.; Wilsey, S.; Houk, K. N. J. Am. Chem.
Soc. 1999, 121, 4816.
(9) Reviews on the mechanism of the Diels-Alder reactions in general:
Houk, K. N.; Evanseck, J. D. Angew. Chem., Int. Ed. Engl. 1992, 31, 682;
Dewar, M. J. S.; Jie, C. Acc. Chem. Res. 1992, 25, 537; Horn, B. A.; Herek,
J. L.; Zewail, A. H. J. Am. Chem. Soc. 1996, 118, 8755; Sustmann, R.;
Tappanchai, S.; Bandmann, H. J. Am. Chem. Soc. 1996, 118, 12555 and
references therein.
10.1021/ja992848x CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/23/2000

Diels-Alder Reactions of C60
J. Am. Chem. Soc., Vol. 122, No. 10, 2000 2237
solution phase fullerene Diels-Alder reaction. Although we
have recently reported a solid-state photoinduced electron
transfer in the fullerene Diels-Alder reaction with anthracenes,14
the energetics for the photoinduced electron transfer in the solid
state is quite different from that in solution where the solvent
plays an important role.
Scheme 1
We report herein the first definitive evidence for stepwise
bond formation in both the photochemical and thermal Diels-
Alder reactions of C60 with Danishefsky’s diene (1a)15 as a
vinylogue of ketene silyl acetal (KSA).16 In the case of the
photochemical Diels-Alder reaction, the stepwise bond forma-
tion via a photoinduced electron transfer can be unequivocally
•-
shown by detection of the transient absorption spectrum of C60
in the visible and near-IR region using laser flash photolysis
combined with dependence of the quantum yields on the
concentrations of Danishefsky’s dienes. The fundamental electron-
transfer properties of Danishefsky’s dienes such as the one-
electron oxidation potentials (E0ox) and the intrinsic barrier for
the electron-transfer oxidation are also determined for the first
time in this study. These data provide the energetic basis to
evaluate the rate constants of thermal outer-sphere electron
transfer from Danishefsky’s dienes to C60. Comparison of the
rate constants for the thermal Diels-Alder reactions of C60 with
Danishefsky’s dienes and those expected for the outer-sphere
electron transfer from Danishefsky’s dienes to C60 provides
valuable insight into stepwise bond formation in the thermal
Diels-Alder reactions.
Table 1. Fullerene Diels-Alder Reaction with Danishefsky’s
Dienes
substrate
condition
time
3 (major)a 4 (minor)a
Results and Discussion
1a (R ) Me) dark, rt
30 min
1 h
6 h
10 min
30 min
5
1
13 (27)
36 (46)
10
3 (6)
33 (43)
Photochemical Diels-Alder Reaction. The fullerene Diels-
Alder reaction was examined with a stereochemically defined
(1E,3Z)-1,4-disubstituted Danishefsky’s diene (1a)17 as a ster-
eochemical probe. A mixture of C60 and 1a in deaerated benzene
was irradiated with high-pressure mercury lamp for 9 h at -30
°C when no thermal reaction would occur in the dark. Formation
of the Diels-Alder adduct 2 was monitored on TLC [Rf ) 0.8
(hexane/ethyl acetate ) 4:1)]. After the treatment with triethyl-
amine/SiO2 suspension and purification by column chromatog-
raphy, two desilylated adducts (3 and 4) were obtained through
hν, rt
18 (27)
dark, -30 °Cb 9 h
no reaction
hν, -30 °Cb
dark, rt
hν, rt
9 h
6 h
6 h
20 (26)
24 (32)
19 (24)
14 (18)
1b (R ) H)
a Values in parentheses are based on recovery of C60. b Toluene used
as a solvent.
the Diels-Alder adduct (2) in 20 and 14% yield, respectively
(Scheme 1). The product (3) was identified as trans with respect
to the 1,4-substituents on the basis of the NOEs between MeO/
H4 (+5.2%) and MeO/Me (-2.4%). The other product (4) was
deduced to be cis by the NOE between H4/H1 (+8.9%) (Scheme
1). The boat conformations of 3 and 4 were fully optimized by
PM3 semiempirical calculations18 (see Supporting Information).
The 1,2-relationships of 3 and 4 were thus deduced to be cis
(10) On the possibility of intervention of electron transfer in the following
cycloadditions, see: (a) The Diels-Alder reactions of electron-deficient
dienophiles with electron-rich dienes: Kiselev, V. D.; Miller, J. G. J. Am.
Chem. Soc. 1975, 97, 4036; Fukuzumi, S.; Kochi, J. K. Tetrahedron 1982,
38, 1035; Sustmann, R.; Korth, H.-G.; Nu¨chter, U.; Siangouri-Feulner, I.;
Sicking, W. Chem. Ber. 1991, 124, 2811; Fukuzumi, S.; Okamoto, T. J.
Am. Chem. Soc. 1993, 115, 11600. (b) [2 + 2] Photocycloaddition of
propenylanisole to C60: Vassilikogiannakis, G.; Orfanopoulos, M. Tetra-
hedron Lett. 1997, 38, 4323. (c) [3 + 2] Cycloaddition of dipolar
trimethylenemethane with electron-deficient olefins: Yamago, S.; Ejiri, S.;
Nakamura, M.; Nakamura, E. J. Am. Chem. Soc. 1993, 115, 5344; Boger,
D. L.; Brotherton, C. E. In AdVances in Cycloaddition; Curran, D. P., Ed.,
JAI Press: Greenwich, CT, 1990; Vol. 2, p 147; Schmittel, M.; Wo¨hrle,
C.; Bohn, I. Chem. Eur. J. 1996, 2, 1031.
3
and trans from the coupling constants, JH1,H2 ) 2.7 and 11.4
Hz, respectively. The boat conformations of fullerene Diels-
Alder products were determined by X-ray analysis.19 The same
trans product (3) was obtained as a single isomer in the
photochemical reaction of C60 with 1a after a short irradiation
time when the thermal reaction was negligible (Table 1). This
indicates that the cis adduct (4) obtained at the prolonged
irradiation time may be formed via the isomerization from the
trans adduct (3) (Table 1). When a mixture of C60 and 1a in
benzene was stirred for 6h, 3 and 4 were obtained in 36 and
33% yield, respectively, after treatment with triethylamine/SiO2
suspension and purification by flash column chromatography
(11) Sustmann, R.; Lu¨cking, K.; Kopp, G.; Rese, M. Angew. Chem., Int.
Ed. Engl. 1989, 28, 1713.
(12) Mu¨ller, F.; Mattay, J. Chem. ReV. 1993, 93, 99.
(13) Sun, D.; Hubig, S. M.; Kochi, J. K. J. Org. Chem. 1999, 64, 2250.
(14) Mikami, K.; Matsumoto, S.; Tonoi, T.; Okubo, Y.; Suenobu, T.;
Fukuzumi, S. Tetrahedron Lett. 1998, 39, 3733.
(15) Mikami, K.; Matsumoto, S.; Okubo, Y.; Suenobu, T.; Fukuzumi,
S. Synlett 1999, 1130.
(16) We have reported the photoaddition reaction of C60 with ketene
silyl acetals via an electron-transfer mechanism: (a) Mikami, K.; Matsu-
moto, S. Synlett 1995, 229. (b) Mikami, K.; Matsumoto, S.; Ishida, A.;
Takamuku, S.; Suenobu, T.; Fukuzumi, S. J. Am. Chem. Soc. 1995, 117,
11134. Also see: (c) Tokuyama, H.; Isobe, H.; Nakamura, E. J. Chem.
Soc., Chem. Commun. 1994, 2753.
(17) Danishefsky, S.; Yan, C.-F.; Singh, R. K.; Gammill, R. B.; McCurry,
P., Jr.; M.; Fritsch, N.; Clardy, J. J. Am. Chem. Soc. 1979, 101, 7001; Myles,
D. C.; Bigham, M. H. Org. Synth. 1992, 70, 231.
(18) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209, 221.
(19) Rubin, Y.; Khan, S.; Freedberg, D. I.; Yeretzian, C. J. Am. Chem.
Soc. 1993, 115, 344; Khan, S. I.; Oliver, A. M.; Paddon-Row: M. N.; Rubin,
Y. J. Am. Chem. Soc. 1993, 115, 4919; Ganapathi, P. S.; Friedman, S. H.;
Kenyon, G. L.; Rubin, Y. J. Org. Chem. 1995, 60, 2954 and references
therein.

2238 J. Am. Chem. Soc., Vol. 122, No. 10, 2000
Mikami et al.
Figure 1. Transient absorption spectra observed in the photochemical
Diels-Alder reaction of C60 (1.0 × 10-4 M) with 1a (2.0 × 10-2 M)
at 100 ns, 1 µs, 2 µs, and 4 µs after laser excitation in deaerated
benzonitrile. Note that there is a clean isosbestic point with which the
•-
yield of C60 can be obtained accurately.
(Table 1). Essentially the same products were obtained in
reactions of C60 and 1a in benzonitrile as well.
•-
The transient absorption spectra of C60 in the visible and
near-IR region are observed by the laser flash photolysis of a
deaerated benzonitrile solution of C60 in the presence of 1a as
shown in Figure 1, where the triplet-triplet absorption band of
the triplet excited state of C60 (3C60*) at 740 nm20 appearing
immediately after nanosecond laser exposure decays, ac-
companied by concomitant appearance of a new absorption band
Figure 2. (a) Decay of the absorbance at 740 nm due to 3C60* and (b)
•-
the rise of the absorbance at 1080 nm due to C60 observed in the
photochemical Diels-Alder reaction of C60 (1.0 × 10-4 M) with 1a
•-
at 1080 nm which is diagnostic of C60 as shown in Figure
(2.0 × 10-2 M) after laser excitation in deaerated benzonitrile.
1.21
The decay of the absorbance at 740 nm due to the triplet-
3
triplet absorption of C60* obeys pseudo-first-order kinetics,
•-
coinciding with a rise of the absorbance at 1080 nm due to C60
as shown in Figure 2. The pseudo-first-order rate constant (kd)
for the decay of 3C60* increases linearly with an increase in the
concentration of 1a (Figure 3).
These results show unequivocally that an electron transfer
3
•- 22
occurs from 1a to C60* to produce C60
.
The rate constant
3
of electron transfer from 1a to C60* is determined from the
linear plot of kd vs [1a] as 3.6 × 107 M-1 s-1
.
•-
The C60 in the triplet radical ion pair produced in the
photoinduced electron transfer from 1a to 3C60* is stable within
the examined time scale in Figure 2. This indicates that the spin
conversion is required prior to the back-electron transfer from
C60•- to 1a•+ in the triplet radical ion pair to the singlet ground
state.
Figure 3. Plot of the pseudo-first-order rate constant (kd) for the decay
of C60* vs [1a].
3
One-Electron Oxidation Properties of Danishefsky’s Dienes.
Since direct electrochemical measurements of Danishefsky’s
dienes are complicated by the irreversible behavior seen with
other organosilanes,23 we have examined the rates of outer-
sphere electron-transfer oxidation from which the fundamental
one-electron oxidation properties can be deduced.
A number of rate constants (ket) for photoinduced electron
transfer from Danishefsky’s dienes (1a and 1b) to the singlet
excited states of electron acceptors (9,10-dicyanoanthracene,
1-cyanonaphthalene, naphthalene, triphenylene, phenanthrene,
and pyrene) and the triplet excited state of C60 are determined
by fluorescence quenching and triplet quenching in deaerated
acetonitrile and benzonitrile at 298 K, respectively. The ket
values and the known values of the one-electron reduction
potentials (E0red) of the excited states20,24 are listed in Table 2.
The ket values for both 1,4-disubstituted Danishefsky’s diene
1a (RdCH3) and the parent Danishefsky’s diene 1b (RdH) in
Table 2 increase with an increase in the E0red value of the oxidant
reaching a diffusion-limited value, a typical feature of photo-
induced electron-transfer processes.25,26
(20) (a) Foote, C. S. Top. Curr. Chem. 1994, 169, 347. (b) Arbogast, J.
W.; Foote, C. S.; Kao, M. J. Am. Chem. Soc. 1992, 114, 2277.
(21) Butcher, E.; Rhodes, C. J.; Standing, M.; Davidson, R. S.; Bowser,
R. J. Chem. Soc., Perkin Trans. 2 1992, 1469; Subramanian, R.; Kadish,
K. M.; Vijayashree, M. N.; Gao, X.; Jones, M. T.; Miller, M. D.; Krause,
K. L.; Suenobu, T.; Fukuzumi, S. J. Phys. Chem. 1996, 100, 16327.
(22) The formation of 1a•+ was not observed in the wavelength region
examined in Figure 1.
(24) (a) Baggott, J. E.; Pilling, M. J. J. Chem. Soc., Faraday Trans. 1
1983, 79, 221. (b) Eriksen, J.; Foote, C. S. J. Phys. Chem. 1978, 82, 2659.
(25) Rehm, A.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 834.
(b) Rehm, A.; Weller, A. Isr. J. Chem. 1970, 8, 259.
(23) Fukuzumi, S.; Fujita, M.; Otera, J.; Fujita, Y. J. Am. Chem. Soc.
1992, 114, 10271.

Diels-Alder Reactions of C60
J. Am. Chem. Soc., Vol. 122, No. 10, 2000 2239
Table 2. Observed Second-Order Rate Constants (kobs) for Electron-Transfer Oxidation of 1a and 1b with One-Electron Oxidants in MeCN at
298 K, Maximum Fluorescence Wavelength (λem), Fluorescence Lifetime (τ), and One-Electron Reduction Potentials (E0red) of Oxidantsa
kobs, M-1 s-1
E0
red
no.
oxidant
λ
em, nm
τ, ns
15.3
8.7
105
37
vs SCE, V
1a
1b
1
2
3
4
5
6
7b
9,10-dicyanoanthracene*
1-cyanonaphthalene*
naphthalene*
triphenylene*
phenanthrene*
pyrene*
442
348
338
400
367
394
1.91
1.73
1.46
1.40
1.28
1.23
1.14
1.4 × 1010
1.2 × 1010
5.4 × 109
3.5 × 109
4.3 × 108
1.4 × 108
2.8 × 107
1.3 × 1010
1.1 × 1010
3.5 × 109
1.8 × 109
8.0 × 107
2.2 × 107
4.1 × 106
61
475
3C60*
3.2 × 104
a Asterisk (*) denotes the excited state. b In PhCN.
dienes (1a and 1b). According to eq 4, the intercept corresponds
to the E0 value of Danishefsky’s diene and the ∆Gq value
ox
0
can be obtained from the slope. Thus, the unknown values of
E0 and ∆Gq can be determined from the intercept and slope
ox
0
of the plots of ∆Gq + E0 vs (∆Gq)-1 by using eq 4,
red
respectively. The E0ox value of 1a (1.29 V) is less positive than
that of 1b (1.36 V) due to the electron-donating effect of the
methyl group of 1a. The ∆Gq values (2.9 kcal mol-1) of 1a
0
and 1b are the same.
The ∆G0 value for photoinduced electron transfer from 1a
et
to C60* is determined as 3.5 kcal mol-1 using eq 3. The
3
equilibrium constant (Ket) for the photoinduced electron transfer
at 298 K is then determined as 2.7 × 10-3 using the relation,
∆G0 ) -RT ln Ket. Although the photoinduced electron
et
Figure 4. Plots of (∆Gq/F)+ E0 vs (F /∆Gq) for electron-transfer
red
•-
transfer is endergonic (Ket < 1), the yield of C60 in the
oxidation of Danishefsky’s dienes 1a (b) and 1b (2) with various
photoexcited-state oxidants: (2) 1-cyanonaphtharene, (3) naphthalene,
(4) triphenylene, (5) phenanthrene, (6) pyrene, and (7) C60 in deaerated
acetonitrile (2-6) or benzonitrile (7) at 298 K.
presence of excess 1a (2.0 × 10-2 M) is evaluated from the Ket
value (2.7 × 10-3) as 87 ( 9%,28,29 which is consistent with
•-
the nearly quantitative formation of C60 in Figure 2.
Stepwise Bond Formation via Photoinduced Electron
Transfer. The quantum yields (Φ) were determined from an
increase in absorbance due to the adduct formation by using a
ferrioxalate actinometer30 under irradiation of monochromatized
light of λ ) 546 nm. The Φ values for the photocycloaddition
reaction of Danishefsky’s dienes (1a and 1b) to C60 in
benzonitrile increases with an increase in the concentration of
Danishefsky’s dienes [1], reaching a limiting value (Φ∞) as
shown in Figure 5. In each case, the dependence of quantum
yields on the concentrations of 1 is expressed by eq 5
The Gibbs activation energy of electron transfer (∆Gq) is
obtained from the ket value by eq 1
∆Gq ) 2.3RT log[Z(ket-1 - kdiff-1)]
(1)
where Z is the collision frequency, taken as 1 × 1011 M-1 s-1
,
kdiff is the diffusion rate constant taken as 2.0 × 1010 M-1 s-1
in acetonitrile, and the other notations are conventional.25,26 The
dependence of ∆Gq on the Gibbs energy change of electron
transfer (∆G0et) has been well-established as given by eq 225
∆Gq ) (∆G0 /2) + [(∆G0 /2)2 + (∆Gq )2]1/2
(2)
et
et
0
Φ ) Φ∞Kobs[1]/(1 + Kobs[1])
(5)
where ∆Gq is the intrinsic barrier that represents the Gibbs
0
which is rewritten as a linear correlation between Φ-1 vs [1]-1
(eq 6)
activation energy when the driving force of electron transfer is
zero, that is, ∆Gq ) ∆Gq0 at ∆G0et ) 0. On the other hand, the
∆G0et value is obtained from the one-electron oxidation potential
of the Danishefsky’s diene donor (E0ox) and the one-electron
Φ-1 ) Φ∞-1[1 + (Kobs[1])-1]
(6)
reduction potential of the excited state of the acceptor (E0
)
red
by using eq 3, where F is the Faraday constant. From eqs 2
and 3 is derived a linear relation between ∆Gq + E0red and (∆Gq
where Kobs is the quenching constant of the excited state of C60
by Danishefsky’s dienes. The validity of eq 6 is confirmed by
the plot of Φ-1 vs [1]-1 as shown in Figure 6. From the slope
and the intercept are obtained the Φ∞ and Kobs values: Φ∞ )
0.042 ( 0.004 and 0.032 ( 0.003, Kobs ) (9.1 ( 1.8) × 102
M-1 and (1.3 ( 0.3) × 102 M-1 for 1a and 1b, respectively.
-1
)
as shown in eq 4.27
∆G0et ) F(E0ox - E0
)
(3)
(4)
red
(∆Gq/F) + E0red ) E0ox + (∆Gq /F)2/(∆Gq/F)
0
Figure 4 shows such linear plots of ∆Gq + E0red vs (∆Gq)-1
(28) The yield of C60 (R ) [C60•-]/[3C60*]) is obtained by the
•-
equilibrium equation, Ket(1 - R)[1a]/[3C60*] ) R2. The [3C60*] value is
determined using the molar absorption coefficient (ꢀmax ) 20 200 ( 2000
M-1 cm-1) reported by Bensasson et al.29
(29) Bensasson, R. V.; Hill, T.; Lambert, C.; Land, E. J.; Leach, S.;
Truscott, T. G. Chem. Phys. Lett. 1993, 201, 326.
for photoinduced electron-transfer reactions of Danishefsky’s
(26) (a) Julliard, M.; Chanon, M. Chem. ReV. 1983, 83, 425. (b) Kavarnos,
G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401.
(27) Fukuzumi, S.; Tanaka, T. In Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, pp 578-
635.
(30) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518.

2240 J. Am. Chem. Soc., Vol. 122, No. 10, 2000
Mikami et al.
Stepwise Bond Formation in the Thermal Diels-Alder
Reaction. The trans product is also obtained as the primary
product in the thermal reaction, while the trans/cis ratio
decreases with prolonged reaction time in the dark (Table 1).
The trans product (3) could have arisen only via a stepwise
bond formation, since it cannot be obtained in a thermal
concerted process because of the orbital symmetry constraint.
Formation of the same trans product (3) in the thermal reaction
as that obtained in the photochemical reaction (Scheme 2),
suggests that the thermal reaction also proceeds via electron
transfer from 1 to C60. The rate constant for electron transfer
from 1 to C60 can be estimated from the Gibbs energy relation
between log ket and ∆G0et as shown in Figure 7, which includes
the data in Table 2. The solid line in Figure 7 is based on eqs
1 and 2. The ∆G0et values for electron transfer from 1a and 1b
to C60 are obtained from the E0ox values of 1a (1.29 V) and 1b
(1.36 V), both of which are determined from the plots in Figure
Figure 5. Dependence of the quantum yields (Φ) on the concentration
of Danishefsky’s dienes (a) [1a] and (b) [1b] for the photochemical
Diels-Alder reaction of C60 (1.3 × 10-4 M) with Danishefsky’s dienes
(1a and 1b) in deaerated benzonitrile at 298 K.
4 and the E0red value of C60 (-0.43 V)32 using eq 3. The ∆Gq
0
•-
value for the electron self-exchange reaction of C60/C60 has
recently been determined as 3.7 kcal mol-1 33 The ∆Gq0 values
.
for photoinduced electron transfer from 1a and 1b to the excited
states of aromatic compounds are 2.9 kcal mol-1 as determined
in the plots in Figure 4. Then, the ∆Gq value for electron
0
transfer from 1 to C60 can be obtained as the average of these
two values, that is, 3.3 kcal mol-1. From these ∆G0et and ∆Gq
0
Figure 6. Plots of Φ-1 vs (a) [1a]-1 and (b) [1b]-1 for the
photochemical Diels-Alder reaction of C60 (1.3 × 10-4 M) with
Danishefsky’s dienes (1a and 1b) in deaerated benzonitrile at 298 K.
values are calculated the rate constant of electron transfer (ket)
from 1a and 1b to C60 using eqs 1 and 2 as 5.2 × 10-19 and
3.4 × 10-20 M-1 s-1, respectively. Such small ket values indicate
that essentially no thermal electron transfer from 1a or 1b to
C60 occurs.
The quenching constant Kobs () kobsτΤ) is converted to the
corresponding rate constant (kobs) for the reaction of 1a and 1b
with 3C60* by using the triplet lifetime (τΤ) of 3C60* (32 µs).16b,31
The kobs values thus obtained are given in Table 2. The kobs
value of 1a ((2.8 ( 0.6) × 107 M-1 s-1) agrees within
experimental error with the ket value ((3.6 ( 0.4) × 107 M-1
s-1) determined directly by the decay of 3C60* and the formation
The rates of thermal Diels-Alder reactions of 1a and 1b with
C60 were determined by monitoring the increase in the adduct
absorbance. The rates obey pseudo-first-order kinetics under
experimental conditions where the diene concentration is greater
than a 10-fold excess of the C60 concentration. The pseudo-
first-order rate constants (k1) increase proportionally with
increasing the diene concentration (Figure 8). The second-order
rate constants of 1a and 1b (kobs) are obtained from the slopes
•-
of C60 (Figure 2). In addition, the kobs value of 1a fits in the
linear plot for the photoinduced electron-transfer reactions of
1a in Figure 4. The kobs value of 1b also fits in the plot for the
photoinduced electron-transfer reactions of 1b in Figure 4. Thus,
the photochemical Diels-Alder reaction proceeds via photo-
of the linear plots as 4.7 × 10-2 and 4.1 × 10-2 M-1 s-1
,
respectively. These kobs values are more than 1016 times larger
than the predicted ket values for electron transfer from 1a and
1b to C60 as shown in Figure 7. Such a huge difference between
the kobs and ket values clearly indicates that the thermal Diels-
Alder reaction proceeds by a strong orbital interaction between
Danishefsky’s dienes and C60 rather than by the electron transfer
in which little interaction is required. The question that must
be addressed is why the trans-adduct 3, which would not be
produced by a concerted HOMO-LUMO interaction, is ob-
tained as the primary product.
3
induced electron transfer from 1 to C60* as shown in Scheme
2.
The photoinduced electron transfer from 1a to 3C60* (ket) gives
the triplet radical ion pair (A) in competition with the decay to
the ground state (kT ) τT-1). The triplet radical ion pair then
provides the singlet radical ion pair to give a diradical
intermediate (B) or a zwitterionic intermediate (C) (kp) in
competition with the back-electron transfer to the reactant pair
(kb). The PM3 semiempirical calculation of 1a•+ with the full
structural optimization indicates that the positive charge density
is the largest on the carbon (C-1) next to the methoxy group
and that the unpaired electron density is the largest on the
terminal carbon (C-4). Thus, in the case of the diradical pathway
(B) in Scheme 2, the initial C-C bond formation should occur
between C-1 carbon of 1a•+ and C60•-, while it occurs between
The optimized geometry and the HOMO orbital are calculated
with density functional theory at the Becke3LYP/6-31G*
level34,35 which has been successfully used to predict geometries
and energies for some Diels-Alder reactions.8 The result is
shown in Figure 9 where the two double bonds of 1a are twisted
(part a) and the two pπ HOMO orbitals at C1 and C4 positions
are not coplanar (part b, where the angle between the two pπ
HOMO orbitals is 41°). The corresponding torsion angle of 1b
becomes slightly smaller (33.2°). Essentially the same result
C-4 of 1a•+ and C60 in the case of the zwitterion pathway
•-
(C). At present, however, it is difficult to distinguish the two
pathways. In any case, the bond formation occurs stepwise, and
thus there is no symmetry restriction for bond formation. The
PM3 calculation gave the heat of formation of 3 (705.9 kcal
(32) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K. M.
J. Phys. Chem. 1992, 96, 7137.
(33) Fukuzumi, S.; Nakanishi, I.; Suenobu, T.; Kadish, K. M. J. Am.
Chem. Soc. 1999, 121, 3468.
(34) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(35) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986.
mol-1), which is about the same as that of 4 (706.2 kcal mol-1
)
as shown in the Supporting Information. Thus, both trans-adduct
3 and cis-adduct 4 were obtained as the final products.
(31) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka, T.; Itoh, S.;
Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 1998, 120, 8060.

Diels-Alder Reactions of C60
J. Am. Chem. Soc., Vol. 122, No. 10, 2000 2241
Scheme 2
Figure 8. Plots of pseudo-first-order rate constant (k1) vs [1a or 1b]
in the thermal Diels-Alder reaction of C60 (1.3 × 10-4 M) with 1a
(O) or 1b (4) in deaerated benzonitrile at 298 K.
Figure 7. Dependence of log kobs on ∆G0 for the electron-transfer
et
oxidation of Danishefsky’s dienes 1a (O) and 1b (4) with various one-
electron oxidants as compared to the thermal Diels-Alder reaction of
1a and 1b (b) with C60 in deaerated acetonitrile or benzonitrile at 298
CH3) was used as a stereochemical probe in both thermal and
photochemical Diels-Alder reactions. The major Diels-Alder
adduct is the trans-diastereomer (3), thus indicating that the
Diels-Alder reactions proceed by a stepwise mechanism, which
is likely to involve electron transfer from 1a to C60 under
photochemical conditions. The transient spectra of C60•-, formed
in the photoinduced electron transfer from 1a to the triplet
excited state of C60 (3C60*) have been detected successfully in
a laser flash photolysis of the 1a-C60 system. The observed rate
constant agrees well with the predicted rate constant on the basis
of the electron-transfer mechanism. More significantly, the rate
constants for the thermal Diels-Alder reactions of Danishef-
sky’s dienes are 1016 times larger than those expected for
ground-state electron transfer from Danishefsky’s dienes to C60.
Thus, a strongly unsymmetrical orbital interaction is required
for the stepwise thermal Diels-Alder reaction to yield trans-
adduct (3), which would not be expected from a concerted
pathway.
K. The solid line is drawn according to eqs 1 and 2, using the E0
ox
values and the averaged ∆Gq value (2.9 kcal mol-1).
0
was obtained by a PM3 calculation.36 It is therefore not
surprising that there is a strongly unsymmetrical orbital interac-
tion between Danishefsky’s dienes and C60, resulting in stepwise
bond formation to yield the trans-adduct (3) as well as the cis-
adduct (4).
Summary
In summary, we have shown a stepwise bond-forming process
via an electron transfer in the fullerene [60] Diels-Alder
reaction with Danishefsky’s diene which has so far been
generally regarded as a concerted process. A stereochemically
defined (1E,3Z)-1,4-disubstituted Danishefsky’s diene 1a (Rd
(36) The angle between the two pπ HOMO orbitals of 1a calculated by
the PM3 method becomes even larger (70°) as compared to the DFT
calculation.

2242 J. Am. Chem. Soc., Vol. 122, No. 10, 2000
Mikami et al.
stirring for 1 h at room temperature, the reaction mixture was then
diluted with Et2O. The suspension was filtered and washed with Et2O
to give crude sodium salt of 1-hydroxy-2-methyl-1-penten-3-one. Then,
the solution of sodium salt obtained as above in water (225 mL) was
acidified to pH ) 5 by dropwise addition of concentrated HCl. The
resultant two-phase mixture was separated and extracted with Et2O.
The combined organic layer was dried over Na2SO4 and evaporated
under reduced pressure to give crude 1-hydroxy-2-methyl-1-penten-3-
one. To a solution of 1-hydroxy-2-methyl-1-penten-3-one in benzene
(150 mL) was added a 60:40 mixture of benzene/MeOH (75 mL) and
p-toluenesulfonic acid monohydrate (140 mg, 0.74 mmol). The dropping
funnel was charged with a 60:40 benzene/MeOH (75 mL). The stirred
solution was warmed to a gentle reflux at which time the solution began
to darken to a light brown color. The distillation temperature was raised
to 59 °C (the boiling point of the benzene/MeOH azeotrope) and
stabilized. The distillate volume was monitored throughout the course
of the reaction. When 40 mL of distillate was collected, the reaction
vessel was replenished with 40 mL of fresh benzene/MeOH mixture.
This cycle was repeated until the starting material was consumed. At
this time any residual benzene/MeOH mixture in the dropping funnel
was discarded, and the funnel was charged with benzene (75 mL). The
reaction volume was maintained by the addition of benzene as the
boiling point of the distillate rose to 79 °C. In this way, a total of 120
mL of benzene was distilled out of the reaction mixture as the reaction
was driven to completion. The reaction mixture was allowed to cool
to room temperature and quenched by the addition of aqueous NaHCO3
solution. The resultant two-phase mixture was separated and extracted
with Et2O. The combined organic layer was dried over Na2SO4 and
distilled under reduced pressure (62 °C/6 mmHg) to give (E)-1-
methoxy-2-methyl-1-penten-3-one in 63% yield. 1H NMR (300 MHz,
CDCl3) δ 1.09 (t, J ) 7.4 Hz, 3H), 1.71 (d, J ) 1.2 Hz, 3H), 2.52 (q,
J ) 7.4 Hz, 2H), 3.85 (s, 3H), 7.23 (q, J ) 1.2 Hz, 1H). 13C NMR (75
MHz, CDCl3) δ 8.4, 9.0, 30.5, 61.3, 117.0, 159.3, 200.9.
(1E,3Z)-1-Methoxy-2-methyl-3-[(trimethylsilyl)oxy]-penta-1,3-di-
ene (1a). To a solution of (E)-1-methoxy-2-methyl-1-penten-3-one (6.4
g, 49.9 mmol) and triethylamine (17 mL, 122 mmol) in Et2O (30 mL)
was added trimethylsilyl trifluoromethanesulfonate (9.69 mL, 49.9
mmol) at 0 °C. After stirring for 3 h, the organic layer was poured into
saturated NaHCO3 solution and extracted with Et2O. The combined
organic layer was washed with brine and dried over Na2SO4. The
solution was filtered and distilled under reduced pressure (65 °C/6
mmHg) to give (1E,3Z)-1-methoxy-2-methyl-3-[(trimethylsilyl)oxy]-
Figure 9. (a) Becke3LYP/6-31G* optimized structure of 1a and (b)
the HOMO orbital.
Experimental Section
General. 1H and 13C NMR spectra were measured on a Varian
GEMINI 300 (300 MHz) spectrometers. The light source used for high-
pressure mercury lamp was a Riko UVL-100P (100 W) and the
irradiation was performed through a Pyrex vessel. Analytical thin-layer
chromatography (TLC) was performed on a glass plate precoated with
silica gel (Merck Kieselgel 60 F254, layer thickness 0.25 mm). Silica
gel chromatography was performed on a Silica Gel 60 (70-230 mesh)
purchased from Kanto Chemical Co., Inc. All experiments were carried
out under nitrogen or argon atmosphere unless otherwise noted. Hexane
and ethyl acetate were purchased from Kanto Chemical Co., Inc.
Benzene was distilled from sodium benzophenone ketyl under argon
atmosphere.
1
penta-1,3-diene in 84% yield. H NMR (300 MHz, CDCl3) δ 0.20 (s.
9H), 1.64 (d, J ) 6.9 Hz, 3H), 1.66 (d, J ) 1.3 Hz, 3H), 3.63 (s, 3H),
4.73 (q, J ) 6.9 Hz, 1H), 6.32 (q, J ) 1.3 Hz, 1H). 13C NMR (75
MHz, CDCl3) δ 0.5, 10.2, 11.5, 60.0, 101.8, 112.9, 145.6, 150.0.
Procedure for the Fullerene Diels-Alder Reactions with Di-
methyl Danishefsky’s Diene To a solution of C60 (10 mg, 0.0139
mmol) in deaerated benzene (10 mL) was added (1E,3Z)-1-methoxy-
2-methyl-3-[(trimethylsilyl)oxy]-penta-1,3-diene (1a, 13.7 mg, 0.0683
mmol), and the reaction mixture was either irradiated with a high-
pressure mercury lamp or stirred in the dark. The reaction mixture was
poured into a suspension of silica gel ( ∼3 g) in hexane (20 mL) and
ethyl acetate (5 mL) with triethylamine (2 mL). After stirring for 30-
50 min, the reaction mixture was concentrated under reduced pressure.
The residue was separated by flash silica gel chromatography (hexane,
then hexane/ethyl acetate ) 40:1-5:1 and triethylamine as an additive)
to give the desilylated Diels-Alder adducts.
Preparation of Danishefsky’s Diene (1a). (E)-1-Methoxy-2-
methyl-1-penten-3-one. A 1-L, three necked, round-bottomed flask
equipped with a mechanical stirrer, calcium sulfate drying tube, and
100-mL dropping funnel was charged with benzene (280 mL), 55%
NaH dispersion (13.3 g, 305 mmol) and MeOH (0.3 mL, 7.4 mmol).
The flask was immersed in an ice bath, and the dropping funnel was
charged with a mixture of 3-pentanone (31.9 mL, 300 mmol) and ethyl
formate (24.1 mL, 300 mmol). The mixture was added dropwise to
the cooled, stirred suspension over 1.5 h. During the addition, there
was a visible evolution of gas, and a paste-like precipitate formed. After
trans-Adduct (3): 1H NMR (300 MHz, C6D6) δ 1.57 (d, J ) 7.2
Hz, 3H), 1.89 (d, J ) 6.6 Hz, 3H), 3.43 (s, 3H), 3.68 (dq, J ) 2.7, 7.2

Diels-Alder Reactions of C60
J. Am. Chem. Soc., Vol. 122, No. 10, 2000 2243
Hz, 1H), 4.62 (d, J ) 2.7 Hz, 1H), 5.12 (q, J ) 6.6 Hz, 1H). 13C NMR
(75 MHz, C6D6) δ 209.41 (CdO), 155.73, 155.23, 155.13, 154.78,
147.97, 147.92, 147.74, 147.11, 146.92, 146.84, 146.66, 146.61, 146.53,
145.85, 145.80 (br), 147.72 (br), 145.69, 145.30, 144.96, 144.90, 143.63,
143.05, 143.00, 142.58, 142.42, 142.26 (br), 142.21, 142.03, 141.95,
141.92, 141.81, 141.72, 141.69 (br), 140.75, 140.61, 139.35, 139.27,
138.30, 136.79, 136.48, 135.33, 134.74, 92.40 (C60-C-OMe), 70.26,
67.10, 62.94 (OMe), 50.18 (C60-C(Me)-CdO), 47.71 (OdC-
C(Me)-C(OMe)), 13.80, 13.48. UV-vis (λmax, CHCl3) 701, 433, 370
nm. FAB-MS m/z ) 849 ([M + H]+), 848 ([M]+), 720 ([M -
C7H12O2]+). HRMS for C67H12O2 calcd 848.0837, found 848.0855.
absorption spectrum by the addition of 1a. The pseudo-first-order
relation was obtained for the triplet decay rate constants (kd) in the
absence and presence of 1a, and the concentrations of 1a. The
quenching rate constants kq were determined from the slope of a linear
plot of kd vs [1a].
Quantum Yield Determinations. A standard actinometer (potassium
ferrioxalate)30 was used for the quantum yield determination of the
photoinduced Diels-Alder reaction of C60 with 1. Square quartz
cuvettes (10 mm i.d.) which contained a deaerated benzonitrile solution
(3.0 cm3) of C60 (1.3 × 10-4 M) with 1 at various concentrations (8.6
× 10-4 - 1.7 × 10-2 M) were irradiated with monochromatized light
of λ ) 546 nm from a Shimadzu RF-5000 fluorescence spectropho-
tometer. The light intensity of monochromatized light of λ ) 546 nm
was determined as 4.99 × 10-9 einstein s-1 with the slit width of 5.0
nm. The photochemical reaction was monitored by means of a Hewlett-
Packard 8453 diode-array spectrophotometer. Since the thermal cy-
cloaddition reactions of C60 (1.3 × 10-4 M) with 1a as well as 1b
proceed slowly at 298 K, the quantum yields for certain concentrations
of 1a or 1b were determined from the increase in absorbance due to
photochemically produced C60 adducts at 434 nm (ꢀmax ) 3.7 × 103
M-1 cm-1 in benzonitrile) by subtracting the small change in the
absorbance due to the thermal reaction from the total absorbance change.
To avoid the contribution of light absorption of the products, only the
initial rates were determined for determination of the quantum yields.
Kinetic Measurements. The kinetic measurements were performed
using Hewlett-Packard 8452A or 8453 diode array spectrophotometers
which were thermostated at 298 K. Rates of Diels-Alder reactions of
Danishefsky’s dienes with C60 in deaerated benzonitrile at 298 K were
monitored by following an increase in absorbance due to the adducts
at 434 nm (ꢀmax ) 3.7 × 103 M-1 cm-1 in benzonitrile) under pseudo-
first-order conditions where the diene concentrations were maintained
at more than 10-fold excess of the C60 concentration (1.3 × 10-4 M).
The pseudo-first-order plots were linear for three or more half-lives
with a correlation coefficient greater than 0.999.
cis-Adduct (4): 1H NMR (300 MHz, C6D6/CS2) δ 1.61 (d, J ) 6.9
Hz, 3H), 1.74 (d, J ) 6.9 Hz, 3H), 3.41 (dq, J ) 11.4, 6.9 Hz, 1H),
3.66 (s, 3H), 4.20 (q, J ) 6.9 Hz, 1H), 4.92 (d, J ) 11.4 Hz, 1H). (in
C6D6) δ 1.55 (d, J ) 6.9 Hz, 3H), 1.77 (d, J ) 6.9 Hz, 3H), 3.36 (s,
3H), 3.50 (dq, J ) 11.4, 6.9 Hz, 1H), 3.73 (q, J ) 6.9 Hz, 1H), 4.54
(d, J ) 11.4 Hz, 1H). 13C NMR (75 MHz, C6D6/CS2) δ 207.49 (Cd
O), 154.58, 153.83, 152.46, 152.02, 148.90, 147.91, 147.77, 147.56,
146.86, 146.83, 146.80, 146.75, 146.70, 146.57 (br), 146.51, 146.47,
146.43, 146.13, 146.10, 145.79 (thresh high), 145.75 (br), 145.68,
145.62, 145.57, 145.03, 144.87, 144.84, 144.71, 143.37, 143.09, 143.04,
142.98, 142.67, 142.56 (br), 142.48, 142.03, 141.96, 141.80, 141.75
(br), 141.16, 140.10, 139.52, 139.31, 136.11, 136.06, 134.94, 134.70,
86.91 (C60-C-OMe), 70.25, 67.16, 63.08 (OMe), 50.89 (C60-C(Me)-
CdO), 49.03 (OdC-C(Me)-C(OMe)), 14.94, 13.84. UV-vis (λmax
,
CHCl3) 696, 431, 381 nm. FAB-MS m/z ) 849 ([M + H]+), 848
([M]+), 720 ([M - C7H12O2]+). HRMS for C67H12O2 calcd 848.0837,
found 848.0817.
Theoretical Calculations. Theoretical calculations were performed
by means of the MOPAC program (version 6) which is incorporated
in the MOLMOLIS program by Daikin Industries, Ltd. The PM3
Hamiltonian was used for the semi-empirical MO calculations.18 Final
geometries and energetics were obtained by optimizing the total
molecular energy with respect to all structural variables. The heats of
formation (∆Hf) were calculated with the restricted Hartree-Fock
(RHF) formalism using a key word “PRECISE”. The density-functional
theory (DFT) calculations were performed at the Becke3LYP/6-31G*
level34,35 with GAUSSIAN 94.37
Procedure for the Fullerene Diels-Alder Reactions with Parent
Danishefsky’s Diene To a solution of C60 (10 mg, 0.0139 mmol) in
deaerated benzene (10 mL) was added (E)-1-methoxy-3-[(trimethylsi-
lyl)oxy]-buta-1,3-diene (1b, 11.8 mg, 0.0683 mmol), and irradiated with
high-pressure mercury lamp or stirred in the dark. The reaction mixture
was treated by the same workup procedure in the reaction with dimethyl
Danishefsky’s diene to give the Diels-Alder product.4c
Laser Flash Photolysis. A deaerated benzonitrile solution of C60
(1.0 × 10-4 M) and 1a (2.0 × 10-2 M) was excited by a Nd:YAG
laser (Quanta-Ray, GCR-130, 6 ns fwhm) at 532 nm with the power
of 32 mJ. A pulsed xenon flash lamp (Tokyo Instruments, XF80-60,
15 J, 60 ms fwhm) was used for the probe beam, which was detected
with a Ge-APD module (Hamamatsu, C5331-SPL) after passing through
the photochemical quartz vessel (10 mm × 10 mm) and a monochro-
mator. The output from the Ge-APD module was recorded with a
digitizing oscilloscope (Hewlett-Packard 54510B, 300 MHz). Since the
purple solution of C60 in benzonitrile disappeared by each laser shot
(532 nm, 32 mJ) in the presence of 1a, the transient spectra were
recorded using fresh solutions in each laser excitation. All experiments
were performed at 295 K.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (No. 11166241,
11133232) from the Ministry of Education, Science, Culture,
and Sports, Japan.
Supporting Information Available: The optimized struc-
tures and heat of formation of 3 and 4 calculated by the PM3
method (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA992848X
(37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision D.4;
Gaussian, Inc.; Pittsburgh, PA, 1995.
For the quenching experiments of 3C60* by 1a, the irradiation laser
wavelength of C60 was 532 nm which excites C60 selectively. The
solution was deoxygenated by argon purging for 10 min prior to the
measurements. Relative intensities of triplet-triplet absorption spectrum
at maxima (740 nm) were measured for benzonitrile solutions containing
C60 (1.0 × 10-4 M) and 1a at various concentrations. There was no
change in the shape but there was a change in the lifetime of the T-T