Mechanism of the [2 + 2] photocycloaddition of fullerene C60 with styrenes

Article abstract of DOI:10.1021/jo0006223

Stereochemical studies on [2 + 2] photoaddition of cis-/trans-4-propenylanisole (cis-1 and trans-1) and cis-1-(p-methoxyphenyl)ethylene-2-d₁ (cis-3-d₁) to C₆₀ exhibit stereospecificity in favor of the trans-2 cycloadduct in the former case and nonstereoselectivity in the latter. The observed stereoselectivity in favor of the cis-6-d₃ [2 + 2] diastereomer by 12% in the case of the photochemical addition of (E)-1-(p-methoxyphenyl)-2-methyl-prop-1-ene-3,3,3-d₃ (trans-5-d₃) to C₆₀ is attributed to a steric kinetic isotope effect (k(H)/k(D) = 0.78). The loss of stereochemistry in the cyclobutane ring excludes a concerted addition and is consistent with a stepwise mechanism. Intermolecular secondary kinetic isotope effects of the [2 + 2] photocycloaddition of 3-d₀ vs 3-d₁, and 3-d₆ as well as 5-d₀ vs 5-d₁, and 5-d₆ to C₆₀ were also measured. The intermolecular competition due to deuterium substitution of both vinylic hydrogens at the β-carbon of 3 exhibits a substantial inverse α-secondary isotope effect k(H)/k(D) = 0.83 (per deuterium). Substitution with deuterium at both vinylic methyl groups of 5 yields a small inverse k(H)/k(D) = 0.94. These results are consistent with the formation of an open intermediate in the rate-determining step.

Full text of DOI:10.1021/jo0006223

8180
J . Org. Chem. 2000, 65, 8180-8187
Mech a n ism of th e [2 + 2] P h otocycloa d d ition of F u ller en e C60 w ith
Styr en es
Georgios Vassilikogiannakis,† Maria Hatzimarinaki, and Michael Orfanopoulos*
Department of Chemistry, University of Crete, 71409 Iraklion, Crete, Greece
orphanop@chemistry.uch.gr
Received April 24, 2000 (Revised Manuscript Received J uly 31, 2000)
Stereochemical studies on [2 + 2] photoaddition of cis-/trans-4-propenylanisole (cis-1 and trans-1)
and cis-1-(p-methoxyphenyl)ethylene-2-d1 (cis-3-d1) to C60 exhibit stereospecificity in favor of the
trans-2 cycloadduct in the former case and nonstereoselectivity in the latter. The observed
stereoselectivity in favor of the cis-6-d3 [2 + 2] diastereomer by 12% in the case of the photochemical
addition of (E)-1-(p-methoxyphenyl)-2-methyl-prop-1-ene-3,3,3-d3 (trans-5-d3) to C60 is attributed
to a steric kinetic isotope effect (kH/kD ) 0.78). The loss of stereochemistry in the cyclobutane ring
excludes a concerted addition and is consistent with a stepwise mechanism. Intermolecular
secondary kinetic isotope effects of the [2 + 2] photocycloaddition of 3-d0 vs 3-d1, and 3-d6 as well
as 5-d0 vs 5-d1, and 5-d6 to C60 were also measured. The intermolecular competition due to deuterium
substitution of both vinylic hydrogens at the â-carbon of 3 exhibits a substantial inverse R-secondary
isotope effect kH/kD ) 0.83 (per deuterium). Substitution with deuterium at both vinylic methyl
groups of 5 yields a small inverse kH/kD ) 0.94. These results are consistent with the formation of
an open intermediate in the rate-determining step.
In tr od u ction
undergoes exothermic, electrophilic cycloadditions on the
6-6 double bond. Due to the stability of the cycloadducts,
further side-chain chemistry can be applied which is
suitable for preparation of useful and interesting fullerene
derivatives.
Since the discovery of C601 (buckminsterfullerene) and
its preparation in large quantities,2 a large variety of
thermal and photochemical cycloadditions3 have been
studied. C60 behaves like an electron-deficient alkene,
with double bonds located at the junctions of two hexa-
gons (6-6 bonds),4 rather than like an aromatic com-
pound. It is an electronegative molecule, which can be
easily reduced.5 This is reflected theoretically by the
molecular orbital diagram6 of C60 (low-lying triply de-
generate LUMOs), as well as experimentally by the
reversible one-electron reductions up to a hexaanion.7
The relief of strain in the C60 cage (highly pyramidalized
sp2 carbon atoms)8 is the primary driving force for
addition reactions. As a result of these properties, C60
[2 + 2] cycloadditions to C60 are relatively uncommon.
The thermal [2 + 2] cycloaddition of benzyne to C60 was
the first reported example.9 Foote and co-workers re-
ported a possible charge-transfer mechanism for the
photochemical [2 + 2] cycloaddition of electron-rich
ynamines10 to 3C60. The triplet excited state of C60 (3C60)
is formed with a quantum yield near unity and has a
3
reduction potential close to 0.98 V.11 Thus C60 is more
electrophilic than the ground state. They also reported
the thermal [2 + 2] cycloaddition of tetraalkoxyethylenes10c
(a very efficient π-electron donor) to C60 and C70.
Schuster and co-workers have reported a photochemi-
cal [2 + 2] cycloaddition of cyclic enones12 and cyclic 1,3-
diones13 to C60. These photocycloadditions cannot be
achieved by irradiation at 532 nm wavelength where C60
† Present address: Department of Chemistry, The Scripps Research
Institute, 10500 North Torrey Pines Road, La J olla, CA 92037.
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10.1021/jo0006223 CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/02/2000

Photocycloaddition of Fullerene C60 with Styrene
J . Org. Chem., Vol. 65, No. 24, 2000 8181
Sch em e 1. Ster eosp ecific P h otoch em ica l [2 + 2]
Cycloa d d ition of tr a n s-1 a n d cis-1 to C60
Sch em e 2. Ster eoselective P r ep a r a tion of cis-1
a n d cis-3-d 1
is the only light absorbing component,12a while product
yields were improved either by decreasing the concentra-
tion of C60 or by increasing the concentration of enone.
3
These results indicate that C60 does not undergo addition
to the ground-state enone. It was proposed that the
addition of enones to C60 proceeds by a stepwise addition
of the enone triplet excited state to the ground state of
the fullerene, via an intermediate triplet 1,4-biradical,
as occurs in the [2 + 2] photoadditions of enones to
alkenes.14 The regio- and stereoselectivity of the [2 + 2]
photocycloaddition of acyclic enones to C60 was also
recently reported.15
at 4.98 ppm (H2), and two doublets at 6.85 and 7.69 ppm
corresponding to the aromatic hydrogens (Scheme 1). The
photocycloaddition is stereospecific, affording uniquely
one of the two possible diastereomeric [2 + 2] adducts.
If a mixture of diastereomeric adducts had been formed,
We also reported16 the photochemical [2 + 2] cycload-
dition of alkyl substituted 1,3-butadienes to C60. These
substrates are less electron rich than the previously
reported unsaturated substrates that undergo [2 + 2]
1
more H resonances should have been observed. Further-
more, the coupling constant between H1 and H2 (J ) 8.8
Hz) is typical for a trans disubstituted cyclobutane ring.
Thus the trans stereochemistry of the double bond is
maintained in the [2 + 2] adduct trans-2. A small degree
of isomerization of the recovered trans-4-propenylanisole
to the cis analogue (∼2%) was detected by gas chroma-
tography.
To further examine the stereochemistry of this reac-
tion, the opposite isomer cis-4-propenylanisole (cis-1) was
prepared in 96% isomerical purity (Scheme 2). The
synthesis of cis-1 was carried out from cis-1-bromo-2-(p-
methoxyphenyl)ethylene via a Wittig reaction of p-
methoxybenzaldehyde with bromomethylenetriphenylphos-
phorane18 followed by transmetalation with tert-butyl-
lithium and MeI addition.
3
addition to C60. Stereochemical and secondary kinetic
isotope effects studies showed that electron transfer from
the dienes to 3C60 was the likely first step of the reaction,
followed by rapid collapse of the initial open intermediate
to the [2 + 2] adducts.
In this paper, we report the stereochemistry and the
secondary isotope effects of [2 + 2] photocycloaddition
between arylalkenes and C60. These results shed light
on the mechanism of [2 + 2] photocycloadditions of
arylalkenes to C60.
Resu lts
Cycloaddition of cis-1 to C60, under identical photo-
chemical conditions to those of trans-1 and C60, afforded
exclusively the same trans-2 cycloadduct (Scheme 1). The
structure of this adduct was confirmed by matching the
1H NMR spectra of [2 + 2] adducts produced by the
photochemical addition of trans-1 and cis-1 to C60. An
unexpected complete reversion of the double bond ster-
eochemistry of cis-1 in the [2 + 2] adduct trans-2 was
observed. After 30 min of irradiation, a small amount of
isomerization of the recovered cis-4-propenylanisole to
the trans analogue (∼3%) was detected by gas chroma-
tography. Additional irradiation did not increase the yield
of trans-2 (40% based on recovered C60), but rather
resulted in isomerization of the starting alkene. Photo-
chemical cycloreversion of the isolated trans-2 adduct
afforded 90% trans-4-propenylanisole, 10% cis-4-propen-
ylanisole, and C60. Similar photocycloreversion products
from C60 and tetraalkoxyethylenes [2 + 2] adducts have
been reported earlier.10c
A mixture of C60 and a 200-fold excess of trans-4-
propenylanisole (trans-1) did not react when heated for
10 h at reflux in deoxygenated toluene. However, upon
30 min of irradiation at λ > 500 nm with a 300 W xenon
lamp, a reaction product was detected by HPLC on a
Separon C18 reversed-phase column. This stable adduct,
at ambient conditions (no traces of decomposition prod-
ucts were detected by 1H NMR and HPLC after standing
for several days), was isolated by flash column chroma-
tography on SiO2 (2:1 toluene:hexane) and was charac-
terized by 1H NMR to be the [2 + 2] cycloaddition product
1
(trans-2)17 (Scheme 1). The H NMR spectrum of trans-2
exhibits a doublet at 2.01 ppm (-CH3), a singlet at 3.64
ppm (-OCH3), a multiplet at 4.63 ppm (H1), a doublet
(12) (a) Wilson, S. R.; Kaprinidis, N.; Wu, Y.; Schuster, D. I. J . Am.
Chem. Soc. 1993, 115, 8495. (b) Wilson, S. R.; Wu, Y.; Kaprinidis, N.;
Schuster, D. I.; Welch, C. J . J . Org. Chem. 1993, 58, 6548. (c) Schuster,
D. I.; Cao, J .; Kaprinidis, N.; Wu, Y.; J ensen, A. W.; Lu, Q.; Wang, H.;
Wilson, S. R. J . Am. Chem. Soc. 1996, 118, 5639.
(13) J ensen, A. W.; Khong, A.; Saunders: M.; Wilson, S. R.; Schuster,
D. I. J . Am. Chem. Soc. 1997, 119, 7303.
(14) Schuster, D. I.; Lem, G.; Kaprinidis, N. A. Chem. Rev. 1993,
93, 3.
(15) Vassilikogiannakis, G.; Orfanopoulos, M. J . Org. Chem. 1999,
64, 3392.
In an effort to rationalize effectively the stereospeci-
ficity of [2 + 2] photocycloaddition of cis-/trans-4-propen-
ylanisole to C60 (complete retention of stereochemistry
in case of trans-1 and complete reversion in case of cis-
1), cis-1-(p-methoxyphenyl)ethylene-2-d1, cis-3-d1, was
prepared in greater than 96% isomerical purity according
(16) Vassilikogiannakis, G.; Chronakis, N.; Orfanopoulos, M. J . Am.
Chem. Soc. 1998, 120, 9911.
(17) Vassilikogiannakis, G.; Orfanopoulos, M. Tetrahedron Lett.
1997, 38, 4323.
(18) Matsumoto, M.; Kuroda, K. Tetrahedron Lett. 1980, 21, 4021.

8182 J . Org. Chem., Vol. 65, No. 24, 2000
Vassilikogiannakis et al.
Sch em e 3. Non ster eoselective P h otoch em ica l
Sch em e 4. [2 + 2] P h otocycloa d d ition of
â,â-Dim eth yl-p-m eth oxystyr en e to C60
[2 + 2] Cycloa d d ition of cis-3-d 1 to C60
toluene. After 30 min of irradiation, both reactions
afforded [2 + 2] adducts in 30% yield, based on the
recovered C60. Reactions were monitored by HPLC. After
purification of the reaction products by flash column
chromatography (2:1 toluene:hexane), the secondary
kinetic isotope effects kH/kD were measured by integration
to the former synthetic pocedure,18 using D2O instead of
MeI in the last step (Scheme 2).
Irradiation of a mixture of C60 and a 200-fold excess of
cis-3-d1 in deoxygenated toluene at λ > 500 nm afforded
[2 + 2] adducts cis-4-d1 and trans-4-d1 (Scheme 3), within
30 min, in 30% yield based on recovered C60. The [2 + 2]
diastereomers were purified by flash column chromatog-
1
of the H NMR signals of the [2 + 2] products at 5.46
ppm (kH) and 7.72 ppm (2kH + 2kD). A small normal
isotope effect (kH/kD ) 1.08) was found due to isotopic
substitution at the R-carbon of the olefin. The substantial
total inverse secondary isotope effect in the intermolecu-
lar competition between 3-d0 and 3-d3 due to the isotopic
labeling at R- and â-carbon (eq 1), indicates extensive
1
raphy (2:1 toluene:hexane) and were characterized by H
NMR and FAB-MS [m/z 856 (M + 1, 8), 720 (M - 135,
100)]. Two doublets of equal intensity corresponding to
the resonances of H1 and H1’ were observed in the 1H
NMR spectrum at 4.25 and 4.30 ppm, respectively
(J H ) 10.4 Hz, J H ’ ) 8.3 Hz, Scheme 3). Thus, the
3
1H2
1’H2
bond making between C60 and â-carbon of the alkene in
the transition state of the first, rate-determining step.20
Thus, isotopic labeling at the â-carbon gave an R-second-
ary kinetic isotope effect, while isotope substitution at
the R-carbon showed a â-secondary kinetic isotope effect.
Substitution of (kH/kD)â with the value of 1.08 in eq 1 gave
(kH/kD)R2 ) 0.69, which corresponds to (kH/kD)R ) 0.83 per
deuterium atom.19
photochemical [2 + 2] addition of cis-3-d1 to C60 is not
stereoselective.19 Interruption of the reaction after 30 min
1
and H NMR analysis of the unreacted alkene revealed
the formation of 10% of trans-3-d1 (6% isomerization of
cis-3-d1, taking into account the 96% isomerical purity of
the reactant alkene). trans-3-d1 and cis-3-d1 are expected
to be equally reactive with C60. Furthermore, irradiation
at λ > 500 nm of the isolated [2 + 2] adducts afforded
complete cycloreversion to C60 and equimolar amounts
of trans-3-d1 and cis-3-d1.
2
(kH/kD)obsd ) (kH/kD)R (kH/kD)â ) 0.75
(1)
To obtain information on the extent of bond formation
and bond breaking in the transition state, we measured
the intermolecular secondary isotope effects of this [2 +
2] photocycloaddition reaction. For this purpose, we
prepared 1-(p-methoxyphenyl)ethylene-1-d1, (3-d1), and
1-(p-methoxyphenyl)ethylene-1,2,2-d3, (3-d3). Compound
3-d1 was prepared by reduction of p-methoxyacetophe-
none with LiAlD4, followed by dehydration. Compound
3-d3 was prepared by addition of CD3MgI to p-methoxy-
benzaldehyde followed by J ones oxidation, LiAlD4 reduc-
tion, and dehydration.
â,â-Dimethyl-p-methoxystyrene (5) is also photochemi-
cally reactive with C60, forming exclusively the [2 + 2]
adduct 6 (Scheme 4) under the previously described
experimental conditions.
To further study the stereochemistry of the [2 + 2]
addition of p-methoxyarylalkenes to C60, (E)-1-(p-meth-
oxyphenyl)-2-methyl-1-propene-3,3,3-d3 (trans-5-d3) was
prepared by selectively labeling the anti methyl group
with respect to the p-methoxyphenyl moiety. Synthesis
of trans-5-d3 in 97% isomerical purity was accomplished
through the stereoselective formation of methyl (E)-2-
methyl-p-methoxycinnamate, by a Wittig-Horner reac-
tion with p-methoxybenzaldehyde, followed by LiAlD4/
AlCl3 reduction,21 and subsequent chlorination of the
resulting allylic alcohol,22 followed by LiAlD4 reduction
of the allylic chloride23 (Scheme 5). The only nonste-
reospecific step of the synthesis was the conversion of
the allylic alcohol to the allylic chloride, in which 3% of
the Z-isomer was formed.
Photochemical addition of trans-5-d3 to C60 led to the
formation of a mixture of the two possible [2 + 2]
diastereomeric adducts, trans-6-d3 and cis-6-d3. After 45
min of irradiation, a 40% yield of the [2 + 2] adduct was
obtained, based on the recovered C60. The ratio of the
purified [2 + 2] products was measured by integration
b
solvent substrate time (min) conversion,a %
kH/kD
toluene 3-d0/3-d3
toluene 3-d0/3-d1
30
30
30
30
0.75 ( 0.05
1.08 ( 0.05
a
b
Based on recovered C60
proper 1H NMR signals.
.
Determined by integration of the
To measure the intermolecular isotope effects of the
photocycloaddition reactions, equimolar amounts of 3-d0,
3-d1 as well as 3-d0, 3-d3, in 200-fold molar excess to C60,
were dissolved (in separate experiments) in deoxygenated
(20) (a) Matsson, O.; Westway, K. C. Adv. Phys. Org. Chem. 1996,
31, 143. (b) Carpenter, K. B. Determination of Organic Reaction
Mechanisms. Isotope Effects; J ohn Wiley & Sons: New York, 1984;
Chapter 5, pp 83-111.
(21) J orgenson, M. J . Tetrahedron Lett. 1962, 13, 559.
(22) Collington, E. W.; Meyers, A. I. J . Org. Chem. 1971, 36, 3044.
(23) Grdina, M. B.; Orfanopoulos, M. J . Org. Chem. 1979, 44, 2936.
(19) Vassilikogiannakis, G.; Orfanopoulos, M. J . Am. Chem. Soc.
1997, 119, 7394.

Photocycloaddition of Fullerene C60 with Styrene
J . Org. Chem., Vol. 65, No. 24, 2000 8183
Sch em e 5. Ster eoselective P r ep a r a tion of
competition of â,â-dimethyl-p-methoxystyrene versus
analogues deuterated at allylic (5-d6) and vinylic (5-d1)
positions. Alkene 5-d6 was synthesized by thermolysis of
the corresponding â-lactone (Scheme 7).24 The deuterium
content on the allylic position was >96% measured by
1H NMR spectroscopy. Attempts to synthesize 5-d6 by
Wittig coupling of the semistabilized aryl ylide with
acetone-d6 resulted in significant deuterium scrambling
between the allylic and the olefinic hydrogens. Alkene
5-d1 was prepared by Wittig coupling of p-methoxybenz-
aldehyde-1-d1 with triphenylphosphoranylidene isopro-
pane.
tr a n s-5-d 3
Sch em e 6. Ster eoch em istr y of th e P h otoch em ica l
[2 + 2] Cycloa d d ition of tr a n s-5-d 3 to C60
b
solvent substrate time (min) conversion,a
%
kH/kD
toluene 5-d0/5-d6
toluene 5-d0/5-d1
30
30
30
30
0.94 ( 0.05
1.02 ( 0.05
a
b
Based on recovered C60
proper 1H NMR signals.
.
Determined by integration of the
The secondary kinetic isotope effect in the intermolecular
competition of 5-d0 vs 5-d6 was measured by integration
1
of the H NMR signals of the [2 + 2] adducts at 1.87 ppm
Sch em e 7. P r ep a r a tion of 5-d 6 by Th er m olysis of
th e Cor r esp on d in g â-La cton e
(3kH), 2.10 ppm (3kH), and both signals at 5.23 and 5.24
ppm (kH + kD). The measurement of the isotope effect in
the intermolecular competition of 5-d0 vs 5-d1 was achieved
by integration of the 1H NMR signals at 7.74 ppm
(2kH + 2kD), 6.83 ppm (2kH + 2kD), and 5.24 ppm (kH).
Discu ssion
In the present work the [2 + 2] photochemical cycload-
ditions of moderately electron-rich p-methoxyarylalkenes
to C60 are reported. Under identical photochemical ex-
perimental conditions unsubstituted (at the phenyl group)
styrenes are unreactive with C60, which implies a sig-
nificant role of the p-methoxy group in the reaction
mechanism. These photocycloadditions reach an equilib-
rium between the reactants (p-methoxyarylalkenes and
C60) and the products ([2 + 2] adducts). For this reason
∼200-fold excess of the p-methoxyarylalkene is needed
to achieve a satisfactory conversion to the [2 + 2] adducts.
Rapid and complete photolysis of the isolated [2 + 2]
adducts into the starting reactants is consistent with an
equilibrium that is shifted toward the reactants. A small
equilibrium constant Keq of approximately 2.5 × 10-3
was calculated from the recovered C60.
1
of the H NMR methyl resonances at 1.87 and 2.10 ppm
and found to be 56:44 (Scheme 6). 1H NMR examination
of the unreacted arylalkene at the end of the reaction
showed no isomerization. In a control experiment, ir-
radiation of a mixture of C60 and a 3-fold excess of trans-
5-d3 for 1 h showed no isomerization to the cis isomer of
the starting material.
The stereochemistry of the major and the minor adduct
were determined by nuclear Overhauser effect (NOE)
experiments. Upon irradiation of the methyl group at
2.10 ppm of the major adduct, a positive 0.7% NOE was
measured for the benzylic hydrogen. In contrast, irradia-
tion of the methyl group at 1.87 ppm of the minor adduct
gave no similar enhancement for the benzylic hydrogen
(Scheme 6). These results indicate that the major [2 +
2] adduct is cis-6-d3, which corresponds to a change in
stereochemistry relative to the starting alkene.
Ster eoch em ica l a n d Ster eoisotop ic Stu d ies. Re-
gardless of the initial stereochemistry of the double bond
of cis-/trans-4-propenylanisole, the photocycloaddition to
C60 is stereospecific in favor of the trans [2 + 2] adduct.
A possible mechanism that could account for the exclu-
sive formation of the trans-2 adduct includes the forma-
tion of a common dipolar or biradical intermediate
3
between C60 and the arylalkene. Subsequent partial
rotation of the aryl moiety around the former double bond
CR-Câ, leads exclusively to the most thermodynamically
To elucidate the reaction profile, intermolecular sec-
ondary kinetic isotope effects H/D were measured in the
(24) Adam, W.; Baeza, J .; Liu, J . C. J . Am. Chem. Soc. 1972, 94,
2000.

8184 J . Org. Chem., Vol. 65, No. 24, 2000
Vassilikogiannakis et al.
Sch em e 8. P r op osed Step w ise Mech a n ism of th e
P h otoch em ica l [2 + 2] Cycloa d d ition of
p-Meth oxya r yla lk en es to C60
Sch em e 9. Tr a n sition Sta tes TScis a n d TStr a n s of
th e Secon d Step of th e [2 + 2] P h otocycloa d d ition
of tr a n s-5-d 3 to C60
stable trans [2 + 2] adduct (Scheme 8). The unexpected
stereospecificity is attributed to the difference in activa-
tion energy of the diastereomeric transition states in the
second step of the reaction, involving collapse of the
“open” intermediate to the [2 + 2] adduct. These results
thus exclude a synchronous mechanism. The formation
of an open intermediate whose closure is faster than bond
rotation (libration) is also excluded.
Sch em e 10. P ossible Tr a n sition sta tes TSI, TSII,
a n d TSIII of th e Ra te-Deter m in in g Step of th e [2 +
2] P h otocycloa d d ition of p-Meth oxystyr en es to C60
The possibility of a concerted addition of trans-1 and
cis-1 to C60 has been excluded for the following reasons:
(a) Retention of configuration in the cycloadducts would
have been expected because no substantial isomerization
of cis-1 to trans-1 under the experimental conditions was
observed (∼3%). Thus, the stereospecificity would not be
attributed to an unexpected greater reactivity of trans-
1, produced from isomerization (∼3%) of the starting
alkene. (b) The equimolar formation of cis and trans
[2 + 2] adducts in the addition of cis-3-d1 to C60. In the
last case, both initial (cis-3-d1) and isomerized (trans-3-
(3-d0 vs 3-d1) in combination with a substantial inverse
isotope effect (kH/kD ) 0.83, eq 1) for deuterium substitu-
tion at the â-carbon (3-d0 vs 3-d3) requires the formation
of a dipolar or biradical intermediate in the rate-
determining step (first step) through the transition state
TSI (Scheme 10). Furthermore, the small steric inverse
â-secondary kinetic isotope effect in the competition
between 5-d0 and 5-d6 (kH/kD ) 0.94) is in agreement with
3
d1) alkenes are equally reactive toward C60.
The most interesting result is the 12% preference for
cis [2 + 2] adduct formation in the addition of trans-5-d3
to C60.25 Partial rotation of the phenyl group around the
previous CR-Câ double bond in the intermediate leads
to the transition states TStrans and TScis (Scheme 9). It is
reasonable to assume that the conjugation between the
p-orbital of the R-carbon and the aromatic system pre-
vents free rotation around the bond between R-carbon
and the aryl ring (Scheme 9). Due to congestion, TScis
involving nonbonding interactions between the smaller
-CD3 group and the o-phenyl hydrogen is favored. The
resulting steric isotope effect (kH/kD ) 0.78) was at-
tributed to the smaller effective size of the C-D bond
relative to the C-H bond in the methyl groups.20 Remote
steric isotope effects are less common. Mislow and co-
workers26 earlier reported a classical example.
3
bond formation between the â-carbon and C60, in the
rate-determining step (late transition state) of the reac-
tion.
The small positive â-secondary isotope effects derived
from substitution at the R-carbon in competitions be-
tween 3-d0 vs 3-d1 and 5-d0 vs 5-d1 (kH/kD ) 1.02) exclude
the transition states TSII (concerted mechanism) and TSIII
since in these cases an inverse isotope effect would have
been expected (Scheme 10). With respect to the competi-
tion between 5-d0 vs 5-d6, the formation of a positive
charge or spin at the â-carbon in the transition state TSIII
(Scheme 10) would have given a normal and large
â-secondary isotope effect (kH/kD ) 1.05-1.10),20 at-
tributed to a hyperconjugative effect involving the six
hydrogen atoms in 5-d0 vs the six deuterium atoms in
5-d6. Such a normal secondary isotope effect was recently
found in a typical dipolar [2 + 2] cycloaddition of
tetracyanoethylene (TCNE) to 2,5-dimethyl-2,4-hexadi-
ene.27
In ter m olecu la r Secon d a r y Kin etic Isotop e Ef-
fects. Secondary kinetic isotope effects provide a power-
ful mechanistic tool for studying the extent of bond
making and bond breaking in the transition states of the
reactions.20 The small normal isotope effect (kH/kD ) 1.08)
that was found for deuterium substitution at the R-carbon
(25) Hatzimarinaki, M.; Vassilikogiannakis, G.; Orfanopoulos, M.
Tetrahedron Lett. 2000, 41, 4667.
(26) Mislow, K.; Graeve, R.; Gordon, J . A.; Wahl, G. H., J r. J . Am.
Chem. Soc. 1964, 86, 1733.
In conclusion, based on the stereochemical and stereo-
isotopic results, the mechanism of the photochemical

Photocycloaddition of Fullerene C60 with Styrene
J . Org. Chem., Vol. 65, No. 24, 2000 8185
Com p ou n d 6-d 0. 1H NMR (500 MHz): δ 1.87 (s, 3 H), 2.10
(s, 3 H), 3.66 (s, 3 H), 5.24 (s, 1 H), 6.83 (d, J ) 8.7 Hz, 2 H),
7.75 (d, J ) 8.7 Hz, 2 H).
[2 + 2] cycloaddition of p-methoxyarylalkenes to C60
proceeds through the formation of an open intermediate
(dipolar or biradical) between the triplet excited state of
C60 and the ground state of p-methoxyarylalkenes. The
competition between internal rotation and ring closure
of the intermediate in the second step allows rotational
equilibrium to be fully attained before ring closure. The
secondary kinetic isotope effects reveal extensive bond
Com p ou n d s (tr a n s-6-d 3 + cis-6-d 3). 1H NMR (500 MHz):
δ 1.87 (s, 3 H of trans-6-d3), 2.10 (s, 3 H of cis-6-d3), 3.66 (s, 3
H of trans-6-d3 + 3 H of cis-6-d3), 5.24 (s, 1 H of trans-6-d3
+
1 H of cis-6-d3), 6.83 (d, J ) 8.7 Hz, 2 H trans-6-d3 + 2 H of
cis-6-d3), 7.75 (d, J ) 8.7 Hz, 2 H trans-6-d3 + 2 H of cis-6-d3).
The steric isotope effect was measured by integration of
cyclobutanic methyls at 1.87 ppm which corresponds to trans-
6-d3 and 2.10 ppm which corresponds to cis-6-d3.
3
making between C60 and Câ of the p-methoxyarylalkenes
in the first and rate-determining step of the photocy-
cloaddition.
1
Com p ou n d s (6-d 0 + 6-d 6). H NMR (500 MHz): δ 1.87 (s,
3 H of 6-d0), 2.10 (s, 3 H of 6-d0), 3.68 (s, 3 H of 6-d0 + 3 H of
6-d6), 5.23 (s, 1 H of 6-d6), 5.24 (s, 1 H of 6-d0), 6.82 (d, J ) 8.7
Hz, 2 H of 6-d0 + 2 H of 6-d6), 7.74 (d, J ) 8.7 Hz, 2 H of
6-d0 + 2 H of 6-d 6). â-Secondary kinetic isotope effect in the
intermolecular competition of 5-d0 versus 5-d6 was measured
by integration of both signals at 5.23 and 5.24 ppm (kH + kD)
and 1.87 ppm (3kH) or 2.10 ppm (3kH). It is interesting to note
that the cyclobutanic hydrogen of 6-d6 resonates at 5.232, while
6-d0 resonates at 5.240. The chemical shift change, ∆δ ) 0.008
ppm, is attributed to H/D substitution four bonds away from
the resonating hydrogens. Thus, the higher electronegativity
of the six hydrogens compared to six deuteriums causes a
higher diamagnetic deshielding.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Analytical scale HPLC analysis
was done on a Separon C18, 7 µm, 200 mm × 4.6 mm i.d.,
reversed phase column using a mixture of toluene:acetonitrile
(1:1) as eluent, at 1 mL/min flow rate and UV detection at
310 nm. H NMR and 13C NMR spectra were attained using
1
250 and 500 MHz spectrometers and CDCl3 as the solvent.
1H NMR spectra of C60 adducts were attained using a mixture
of CS2:C6D6 (4:1). Geometrical isomeric purities were deter-
mined by 1HNMR and by analytical gas chromatography with
50%-50% phenyl methyl silicone capillary column and FID
detector. FAB mass spectra were obtained on a VG-ZAB-SE
mass spectrometer using m-nitrobenzyl alcohol as the matrix.
Photocycloadditions were achieved using a xenon Variac Eimac
Cermax 300 W lamp. Flash column chromatography was
carried out on SiO2 (silica gel 60, SDS, 230-400 mesh ASTM).
Organic extracts were dried during workup over MgSO4. C60
was gold grade (>99.9% purity) and purchased from Hoechst
Co.
1
Com p ou n d s (6-d 0 + 6-d 1). H NMR (500 MHz): δ 1.87 (s,
3 H of 6-d0 + 3 H of 6-d1), 2.10 (s, 3 H of 6-d0 + 3 H of 6-d1),
3.68 (s, 3 H of 6-d0 + 3 H of 6-d1), 5.24 (s, 1 H of 6-d0), 6.83 (d,
J ) 8.7 Hz, 2 H of 6-d0 + 2 H of 6-d1), 7.74 (d, J ) 8.7 Hz, 2 H
of 6-d0 + 3 H of 6-d1). â-Secondary kinetic isotope effects in
the intermolecular competition of 5-d0 vs 5-d1 were measured
by integration of both signals at 5.24 ppm (kH) and 6.83 ppm
(2kH + 2kD) or 7.74 ppm (2kH + 2kD).
Syn th esis of cis-4-P r op en yla n isole (cis-1) a n d cis-1-(p-
Meth oxyp h en yl)eth ylen e-2-d 1 (cis-3-d 1). These compounds
were prepared according to the procedure shown in Scheme
2.
Gen er a l P r oced u r e for P h otocycloa d d ition s of p-
Meth oxya r yla lk en es to C60. A solution of C60 (15 mg, 0.0208
mmol) and 200-fold excess of p-methoxyarylalkene (4 mmol)
in HPLC grade toluene (30 mL) was stirred and bubbled with
argon for 30 min and subsequently irradiated for 30 min.
Prolonged irradiation did not affect the reaction yields. During
irradiation, the reaction mixtures were cooled with ice water
and the reaction was monitored by HPLC. Toluene was
removed under reduced pressure. In all cases, [2 + 2] products
were separated from the unreacted C60 by flash column
chromatography using a mixture of toluene:hexane (2:1) as
eluent. The retention time of [2 + 2] adducts is longer than
that of C60. After concentration, the residue was sonicated with
hexane. Hexane was decanted from the precipitated solid, and
the remaining volatile substance was removed by pumping the
sample under high vacuum.
Br om om eth yltr ip h en ylp h osp h on iu m Br om id e. A solu-
tion of triphenylphosphine (30 g, 114 mmol) and dibro-
momethane (44.6 g, 256 mmol) in 250 mL of toluene was
refluxed for 24 h. After cooling to 0 °C, the phosphonium salt
was collected as a white precipitant and washed with hot
toluene. The filtrate was heated further at reflux for 24 h,
affording an additional amount of the phosphonium salt. The
total yield of the phosphonium salt was 41.2 g (82% yield). 1H
NMR (CDCl3): δ ) 5.85 (d, J ) 5.8 Hz, 2 H), 7.69 (m, 6 H),
7.80 (m, 3 H), 7.94 (m, 6 H).
cis-1-Br om o-2-(p-m eth oxyp h en yl)eth ylen e. To a cooled
mixture (-78 °C) of bromomethyltriphenylphosphonium bro-
mide (20 g, 45.8 mmol) in dry tetrahydrofuran (THF), under
N2 atmosphere, was added solid tert-BuOH (5.15 g, 46.0 mmol).
The solution was colored yellow due to the formation of
bromomethylenetriphenylphosphorane. The temperature of
the reaction is a crucial factor because at room temperature
the ylide reacts with tert-BuOH which is produced from the
base. To the resulting solution, after 1 h of stirring at -78 °C,
was added dropwise a solution of p-methoxybenzaldehyde (5.44
g, 40.0 mmol) in 15 mL of dry THF and stirring continued for
1 h. Subsequently, the reaction mixture was heated at room
temperature and poured into 50 mL of pentane. Triphen-
ylphosphine oxide (Ph3PO) precipitated out of solution. After
filtration, the solution was concentrated and the residue was
chromatographed (Et2O:hexane ) 2:1) yielding vinyl bromide
The 1H NMR and FAB-MS spectroscopic data for trans-2,
4-d0, (trans-4-d1 + cis-4-d1), 6, (6-d0 + 6-d6), (6-d0 + 6-d1), and
(trans-6-d3 + cis-6-d3) are given below.
Com p ou n d tr a n s-2. 1H NMR (250 MHz): δ 2.01 (d, J )
7.0 Hz, 3 H), 3.64 (s, 3 H), 4.63 (qd, J 1 ) 8.8 Hz, J 2 ) 7.0 Hz,
1 H), 4.98 (d, J ) 8.8 Hz, 1 H), 6.85 (d, J ) 8.6 Hz, 2 H), 7.69
(d, J ) 8.6 Hz, 2 H).
Com p ou n d 4-d 0. 1H NMR (250 MHz): δ 3.62 (s, 3 H), 4.22
(m, 2 H), 5.46 (dd, J 1 ) 10.4 Hz, J 2 ) 8.6 Hz, 1 H), 7.29
(AA’MM’, ∆δ ) 0.85 ppm, J 1 ) 8.6 Hz, J 2 ) 3.1 Hz, J 3 ) 2.1
Hz, 4 H). â-Secondary kinetic isotope effects in the intermo-
lecular competition of 3-d0 vs 3-d1 and 3-d0 vs 3-d3 were
measured by integration at 5.46 ppm (kH) and 7.72 ppm
(2 kH + 2 kD).
1
(6.30 g, 74%) in 96% geometrical purity. H NMR (CDCl3): δ
Com p ou n d s (tr a n s-4-d 1 + cis-4-d 1). 1H NMR (250 MHz):
δ 3.65 (s, 3 H of cis-4-d1 + 3 H of trans-4-d1), 4.21 (d, J ) 10.4
Hz, 1 H of cis-4-d1), 4.29 (d, J ) 8.3 Hz, 1 H of trans-4-d1),
5.50 (d, J ) 10.1 Hz, 1 H of trans-4-d1 + 1 H of cis-4-d1), 7.32
(AA’MM’, ∆δ ) 0.85 ppm, J 1 ) 8.6 Hz, J 2 ) 3.1 Hz, J 3 ) 2.1
Hz, 4 H of trans-4-d1 + 4 H of cis-4-d1). The ratio trans-4-d1:
cis-4-d1 was measured by integration of doublets at 4.21 and
4.29 ppm.
3.82 (s, 3 H), 6.30 (d, J ) 8.1 Hz, 1 H), 6.99 (d, J ) 8.1 Hz, 1
H), 7.29 (AA’MM’, ∆δ ) 0.76 ppm, J 1 ) 8.9 Hz, J 2 ) 2.9 Hz,
J 3 ) 2.1 Hz, 4 H).
cis-4-P r op en yla n isole (cis-1). To a cooled solution (-78
°C) of cis-1-bromo-2-(p-methoxyphenyl)ethylene (3.15 g, 14.8
mmol) in dry THF, under N2 atmosphere, was carefully added
dropwise 9.3 mL of tert-BuLi (1.6 M) in hexane. After 1 h of
stirring at -78 °C, addition of MeI (0.94 mL, 15 mmol)
occurred. The reaction mixture was heated at room temper-
ature, washed with a 5% solution of NaHCO3 and dried over
MgSO4. After evaporation of the solvent, the product was
(27) Vassilikogiannakis, G.; Orfanopoulos, M. Tetrahedron Lett.
1996, 37, 3075.

8186 J . Org. Chem., Vol. 65, No. 24, 2000
Vassilikogiannakis et al.
purified by flash column chromatography using a mixture of
ethanol-1-d1. A 1.60 g (84%) amount of the deuterated alcohol
was isolated. 1H NMR (CDCl3): δ 1.88 (s, -OH), 3.81 (s, 3 H),
6.89 (d, J ) 8.6 Hz, 2 H), 7.30 (d, J ) 8.6 Hz, 2 H).
1-(p-Meth oxyp h en yl)eth ylen e-1,2,2-d 3 (3-d 3). This com-
pound was prepared by dehydration of 1-(p-methoxyphenyl)-
ethanol-1,2,2,2-d4 (1.60 g, 10.2 mmol), using a catalytic amount
of p-toluenesulfonic acid (see the experimental in the synthesis
of 1-(p-methoxyphenyl)ethylene-1-d1). 1H NMR (CDCl3): δ 3.81
(s, 3 H), 7.10 (AA’MM’, ∆δ ) 0.49 ppm, J 1 ) 8.8 Hz, J 2 ) 2.9
Hz, J 3 ) 2.1 Hz, 4H). MS m/z 137 (M+, 100).
hexane and Et2O (4:1) as eluent. 1H NMR (CDCl3): δ 1.89 (dd,
J 1 ) 7.2 Hz, J 2 ) 1.8 Hz, 3 H), 3.81 (s, 3 H), 5.71 (qd, J 1
11.6 Hz, J 2 ) 7.2 Hz, 1 H), 6.38 (d with allylic coupling, J 1
)
)
11.6 Hz, J 2 ) 1.8 Hz, 1 H), 7.07 (AA’MM’, ∆δ ) 0.36 ppm,
J 1 ) 8.8 Hz, J 2 I) 2.9 Hz, J 3 ) 2.1 Hz, 4 H). 13C NMR (CDCl3):
δ ) 14.5, 55.1, 113.5, 125.0, 129.2, 129.9, 130.3, 158.1. HRMS
for C10H12O: calcd 148.0888, found 148.0880.
cis-1-(p-Meth oxyp h en yl)eth ylen e-2-d 1 (cis-3-d 1). This
compound was prepared according to the previously described
procedure for synthesis of cis-1 using D2O instead of MeI
(Scheme 2). 1H NMR (CDCl3): δ 3.81 (s, 3 H), 5.11 (d, J )
10.9 Hz, 1 H), 6.66 (td, J H-H ) 10.9 Hz, J H-D ) 2.6 Hz, 1 H),
Syn th esis of (E)-1-(p-Meth oxyp h en yl)-2-m eth ylp r op -
1-en e-3,3,3-d 3 (tr a n s-5-d 3). This compound was prepared
according to the procedure shown in Scheme 5.
7.10 (AA’MM’, ∆δ ) 0.49 ppm, J 1 ) 8.8 Hz, J 2 ) 2.9 Hz, J 3
)
Meth yl (E)-2-Meth yl-p-m eth oxycin n a m a te. A solution
of methyl diethyl-2-phosphonopropionate (5.6 g, 25.0 mmol)
in dry DME (20 mL) was added to a cooled (0 °C) mixture of
NaH (60% in paraffin oil, 1 g, 27.0 mmol) in 30 mL of dry DME,
under N2 atmosphere. After 1 h of stirring at room tempera-
ture, a solution of (2.8 mL, 23.0 mmol) p-methoxybenzaldehyde
in dry DME was injected. Stirring was continued for 1 h, and
the reaction mixture was quenched with MeOH, poured into
H2O and extracted with Et2O. The combined ether layers were
dried over MgSO4 and concentrated, affording exclusively the
2.1 Hz, 4 H). MS m/z 135 (M+, 100).
Syn th esis of 1-(p-Meth oxyp h en yl)eth ylen e-1-d 1 (3-d 1).
This compound was synthesized by reduction of p-methoxy-
acetophenone with LiAlD4, followed by dehydration.
1-(p-Meth oxyp h en yl)eth a n ol-1-d 1. To a cooled (0 °C)
mixture of LiAlD4 (0.42 g, 10.0 mmol) in dry Et2O, under N2
atmosphere, was added dropwise a solution of p-methoxyac-
etophenone (3 g, 20.0 mmol) in dry Et2O (10 mL). The mixture
was stirred at room temperature for 2 h and quenched at 0 °C
by addition of 0.4 mL of H2O, 0.4 mL of a 15% solution NaOH,
and 1.2 mL of H2O, followed by filtration. The filtrate was
washed with a 5% solution of NaHCO3 and brine, dried over
1
E-ester (3.4 g, 72%). H NMR (CDCl3): δ 2.11 (s with allylic
coupling, J ) 1.0 Hz, 3 H), 3.78 (s, 3 H), 3.81 (s, 3 H), 6.90 (d,
J ) 7.0 Hz, 2 H), 7.36 (d, J ) 7.0 Hz, 2 H), 7.63 (s, 1 H).
(E)-3-(p -Met h oxyp h en yl)-2-m et h ylp r op -2-en -1-ol-1,1-
d 2. To a cooled (0 °C) mixture of LiAlD4 (0.38 g, 9.0 mmol)
and AlCl3 (0.4 g, 3.0 mmol) in dry Et2O (20 mL), under N2
atmosphere, was added dropwise a solution of the E-ester (3.4
g, 16.5 mmol) in dry Et2O (10 mL). After 2 h of stirring at
room temperature, the reaction mixture was quenched at 0
°C with a 2 M solution of HCl and filtered. The filtrate was
washed with brine, dried over MgSO4, and concentrated to give
1
MgSO4 and concentrated to give the alcohol (2.45 g, 80%). H
NMR (CDCl3): δ 1.48 (s, 3 H), 1.88 (s, -OH), 3.81 (s, 3 H),
6.89 (d, J ) 8.6 Hz, 2 H), 7.30 (d, J ) 8.6 Hz, 2 H).
1-(p-Meth oxyp h en yl)eth ylen e-1-d 1 (3-d 1). In a sealed
tube (rotaflon gastight tube) was added 2.45 g (16 mmol) of
1-(p-methoxyphenyl)ethanol-1-d1 and a catalytic amount of
p-toluenesulfonic acid. The neat mixture was heated at 120
°C for 1 h. After cooling to room temperature, the reaction
mixture was dissolved in 50 mL of Et2O and the ethereal layer
was washed with a 10% solution of NaHCO3 and brine. The
organic extracts were dried over MgSO4 and concentrated,
giving a mixture of products. The desired product was sepa-
rated from a less volatile unidentified byproduct by fractional
distillation under vacuum (40 mmHg). 1H NMR (CDCl3): δ
3.81 (s, 3 H), 5.11 (d, J ) 0.9 Hz, 1 H), 5.60 (dt, J H-D ) 2.6 Hz,
J H-H ) 0.9 Hz, 1 H), 7.10 (AA’MM’, ∆δ ) 0.49 ppm, J 1 ) 8.8
Hz, J 2 ) 2.9 Hz, J 3 ) 2.1 Hz, 4 H). MS m/z 135 (M+, 100).
Syn th esis of 1-(p-Meth oxyp h en yl)eth ylen e-1,2,2-d 3 (3-
d 3). This compound was prepared by addition of CD3MgI to
p-methoxybenzaldehyde, followed by J ones oxidation, LiAlD4
reduction, and dehydration.
1
the corresponding E-alcohol (2.1 g, 70%). H NMR (CDCl3): δ
1.88 (s with allylic coupling, J ) 1.1 Hz, 3 H), 3.80 (s, 3 H),
6.44 (s, 1 H), 6.86 (d, J ) 7.0 Hz, 2 H), 7.21 (d, J ) 7.0 Hz, 2
H).
(E)-3-(p -Met h oxyp h en yl)-2-m et h ylp r op -2-en -1-yl-1,1-
d 2 Ch lor id e. To a stirred mixture of 2.1 g (11.7 mmol) of the
E-allylic alcohol and 1.5 mL (12.9 mmol) of 2,6-lutidine, under
N2 atmosphere was added 0.55 g (13 mmol) of LiCl dissolved
in
a minimum amount of anhydrous dimethylformamide
(DMF). On cooling to 0 °C, a suspension was formed, which
was treated dropwise with MeSO2Cl (1 mL, 12.9 mmol). After
10 h of stirring at room temperature, the reaction mixture was
poured into a saturated solution of CuSO4 to remove 2,6-
lutidine, and extracted with Et2O. The organic extracts were
dried over MgSO4 and concentrated to afford the allylic
chloride in 96% geometrical purity (2.0 g, 86%). 1H NMR of
the E-isomer (CDCl3): δ 1.96 (s with allylic coupling, J ) 1.0
Hz, 3 H), 3.79 (s, 3 H), 6.84 (s, 1 H), 6.86 (d, J ) 6.8 Hz, 2 H),
7.21 (d, J ) 6.8 Hz, 2 H).
(E)-1-(p -Met h oxyp h en yl)-2-m et h ylp r op -1-en e-3,3,3-d 3
(tr a n s-5-d 3). To a cooled (0 °C) mixture of LiAlD4 (0.21 g, 5.0
mmol) in dry THF (20 mL), under N2 atmosphere, was added
dropwise a solution of 2.0 g (10 mmol) of allylic chloride in
dry THF (10 mL). After 4 h of stirring at room temperature,
the reaction mixture was quenched at 0 °C by addition of 0.4
mL of H2O, 0.4 mL of 15% NaOH solution, and 1.2 mL of H2O
and then filtered. The organic layer was washed with a 5%
solution of NaHCO3, dried over MgSO4, and concentrated. The
residue was purified by flash column chromatography (petro-
leum ether:ethyl acetate ) 10:1) to afford trans-5-d3 in 96%
geometrical purity (1.2 g, 73%). 1H NMR of the E-stereoisomer
(CDCl3): δ 1.84 (s with allylic coupling, J ) 1.3 Hz, 3 H), 3.80
(s, 3 H), 6.20 (s, 1 H), 6.85 (d, J ) 8.7 Hz, 2 H), 7.14 (d, J )
8.7 Hz, 2 H).
1-(p-Meth oxyp h en yl)eth a n ol-2,2,2-d 3. To a mixture of Mg
(0.61 g, 25.0 mmol) in dry Et2O, under N2 atmosphere, was
added dropwise a solution of 1.25 mL (20.0 mmol) of CD3I in
dry Et2O. The mixture became muddy, indicating the forma-
tion of CD3MgI, which is an exothermic process. After heating
at reflux for 1 h, the mixture was cooled to 0 °C and a solution
of 2.58 g (19.0 mmol) of p-methoxybenzaldehyde in dry Et2O
was added. After 2 h of stirring at room temperature, the
reaction mixture was quenched at 0 °C, by addition of 0.9 mL
of H2O. The organic layer was washed with a 5% solution of
NaHCO3 and brine, dried over MgSO4, and concentrated to
1
give 2.15 g of the alcohol (73%). H NMR (CDCl3): δ 1.88 (s,
-OH) 3.81 (s, 3 H), 4.85 (s, 1 H), 6.89 (d, J ) 8.6 Hz, 2 H),
7.30 (d, J ) 8.6 Hz, 2 H).
p-Meth oxya cetop h en on e-2,2,2-d 3. To a solution of 1-(p-
methoxyphenyl)ethanol-2,2,2-d3 (2.15 g, 13.9 mmol) in acetone
was added dropwise J one’s reagent, until the solution was
colored slightly red. The excess of oxidizing reagent was
quenched with some drops of 2-propanol. The reaction mixture
was poured into 100 mL of Et2O and washed with H2O, a 5%
solution of NaHCO3 and brine, and dried over MgSO4. Solvent
evaporation afforded p-methoxyacetophenone-2,2,2-d3 (1.87 g,
Syn th esis of 1-(p-Meth oxyph en yl)-2-m eth ylpr op-1-en e-
3,3,3,2′,2′,2′-d 6 (5-d 6). This compound was prepared according
to the procedure shown in Scheme 7.
1
88%). H NMR (CDCl3): δ 3.87 (s, 3 H), 6.94 (d, J ) 8.7 Hz, 2
H), 7.94 (d, J ) 8.7 Hz, 2 H).
1-(p-Meth oxyp h en yl)eth a n ol-1,2,2,2-d 4. This compound
was prepared by LiAlD4 reduction of p-methoxyacetophenone-
2,2,2-d3 (1.87 g, 12.2 mmol) according to the experimental
procedure followed in the synthesis of 1-(p-methoxyphenyl)-
2-(p-Meth oxyph en yl)-3-m eth yl-3-h ydr oxybu tan oic Acid-
4,4,4,3′,3′,3′-d 6. To a cooled (0 °C) solution of anhydrous
diisopropylamine (3 mL, 21.3 mmol) in dry THF (20 mL) was
added dropwise 13.7 mL of 1.6 M solution of n-BuLi in hexane,

Photocycloaddition of Fullerene C60 with Styrene
J . Org. Chem., Vol. 65, No. 24, 2000 8187
under N2 atmosphere. After 15 min, 1.57 g (9.4 mmol) of
p-methoxyphenylacetic acid (to be converted into its lithium
R-lithiocarboxylate) was injected as a 1 M solution in anhy-
drous THF to the cooled (0 °C) mixture of lithium diisopropyl-
amide in THF. The resulting mixture was stirred for 1 h at
room temperature. Subsequently, a solution of 0.7 mL (9.5
mmol) of acetone-d6 in anhydrous THF (15 mL) was added at
-78 °C and the mixture was allowed to stir overnight. The
reaction mixture was poured onto ice. After several extractions
with Et2O, the aqueous layer was acidified with 6 M HCl. The
aqueous mixture was extracted with Et2O, and the combined
ether extracts were dried over MgSO4 and concentrated to
provide the â-hydroxy acid (1.85 g, 85%). Approximately 4%
scrambling was observed at the methyl groups 1H NMR
(CDCl3): δ 3.55 (s, 1 H), 3.77 (s, 3 H), 6.84 (d, J ) 6.8 Hz, 2
H), 7.29 (d, J ) 6.8 Hz, 2H).
alcohol was isolated. 1H NMR (CDCl3): δ 3.79 (s, 3 H), 6.87
(d, J ) 6.8 Hz, 2 H), 7.20 (d, J ) 6.8 Hz, 2 H).
p-Meth oxyben za ld eh yd e-1-d 1. To a cooled (0 °C) solution
of pyridinium chlorochromate (6.4 g, 29.7 mmol) in dry CH2-
Cl2 (140 mL) was added dropwise a solution of 2-(p-methoxy-
phenyl)ethanol-1,1-d2 (2.8 g, 19.7 mmol) in dry CH2Cl2 (60 mL).
The reaction mixture was stirred at room temperature for 3
h. Subsequently, the CH2Cl2 was evaporated and the remain-
ing residue was diluted in anhydrous Et2O. After filtration,
the filtrate was concentrated and the residue was purified by
flash column chromatography using petroleum ether and ethyl
acetate (8/1) as eluent, to afford p-methoxybenzaldehyde-1-d1
(1.6 g, 66%). 1H NMR (CDCl3): δ 3.87 (s, 3 H), 6.99 (d, J ) 8.7
Hz, 2 H), 7.82 (d, J ) 8.7 Hz, 2 H). MS m/z 137 (M+, 75).
1-(p-Meth oxyph en yl)-2-m eth ylp r op-1-en e-1-d1 (5-d1). To
a cooled mixture (0 °C) of isopropyltriphenylphosphonium
bromide (5.0 g, 13.0 mmol) in dry THF was added a solution
of n-BuLi 1.4 M in hexane (11 mL), under N2 atmosphere. The
solution turned red due to the formation of triphenylphospho-
ranylidene isopropane. After stirring for 1 h at room temper-
ature, a solution of p-methoxybenzaldehyde-1-d1 (1.6 g, 11.9
mmol) in dry THF was added dropwise. The resulting mixture
was stirred at room temperature for 30 min and poured into
100 mL of hexane. Triphenylphosphine oxide (Ph3PO) precipi-
tated out of solution. After filtration, the solution was concen-
trated and the residue was distilled in a vacuum, affording
3-(p-Meth oxyp h en yl)-4,4-d im eth yloxeta n -2-on e-5,5,5,-
4′,4′,4′-d 6. A solution of 1.85 g (8.0 mmol) of â-hydroxy acid in
dry pyridine was cooled to 0 °C and two mol of benzensulfonyl
chloride (2.05 mL, 16.06 mmol) per mol of â-hydroxy acid were
added. The mixture was well-shaken, sealed, and kept at -10
°C overnight. The reaction mixture was poured into a cooled
(0 °C) saturated solution of CuSO4 to remove pyridine and then
was extracted several times with Et2O. The combined ether
layers were washed with a saturated solution of Na2CO3 and
H2O to remove unreacted â-hydroxy acid, dried over MgSO4,
and concentrated to afford the corresponding â-lactone (1.14
g) in 67% yield. 1H NMR (CDCl3): δ 3.78 (s, 3 H), 4.54 (s, 1
H), 6.88 (d, J ) 7.0 Hz, 2 H), 7.10 (d, J ) 7.0 Hz, 2 H).
1-(p-Meth oxyp h en yl)-2-m eth ylp r op -1-en e-3,3,3,2′,2′,2′-
d 6 (5-d 6). Thermolysis of 5.4 mmol (1.14 g) of 3-(p-methoxy-
phenyl)-4,4-dimethyloxetan-2-one-5,5,5,4′,4′,4′-d6 at 100 °C
under vacuum provoked decomposition to 5-d6 and simulta-
neous distillation. The resulting alkene was purified by flash
column chromatography using a mixture of petroleum ether
and ethyl acetate (10:1) as eluent, affording 0.65 g (3.7 mmol)
1
5-d1 (1.3 g, 67%). H NMR (CDCl3): δ 1.84 (s, 3 H), 1.87 (s, 3
H), 3.80 (s, 3 H), 6.85 (d, J ) 8.6 Hz, 2 H), 7.14 (d, J ) 8.6 Hz,
2 H). MS m/z 163 (M+, 100).
The precursor to ylide, isopropyltriphenylphosphonium
bromide, was prepared by heating neat an excess of isopro-
pylbromide (4.8 mL, 52 mmol) and triphenylphosphine (8.2 g,
31.2 mmol) in a sealed tube for 24 h at 140 °C. The phospho-
nium salt was collected as a white solid and was washed with
hot toluene. 1H NMR (CDCl3): δ 1.29 (dd, J H-P ) 19 Hz,
J H-H ) 6.8 Hz, 6 H), 5.56 (m, 1 H), 7.68 (m, 9 H), 7.95 (m, 6
H).
1
of 5-d6. H NMR (CDCl3): δ 3.80 (s, 3 H), 6.19 (s, 1 H), 6.85
(d, J ) 8.6 Hz, 2 H), 7.14 (d, J ) 8.6 Hz, 2 H). MS m/z 168
(M+, 100).
Ack n ow led gm en t. We thank professor G. J . Kara-
batsos for valuable comments and discussions. This
work was supported by the Secretariat of Research and
Technology Grants ΥΠΕΡ-1995 and ΠΕΝΕ∆-2000.
Syn th esis of 1-(p-Meth oxyph en yl)-2-m eth ylpr op-1-en e-
1-d 1 (5-d 1). This compound was synthesized by reduction of
methyl p-methoxybenzoate with LiAlD4, followed by PCC
oxidation of the corresponding alcohol-d2 and Wittig coupling
of the resulting p-methoxybenzaldehyde-1-d1 with triphenyl-
phosphoranylidene isopropane.
p-Meth oxyp h en ylm eth a n ol-1,1-d 2. This compound was
prepared by LiAlD4 reduction (0.74 g, 17.6 mmol) of methyl
p-methoxybenzoate (3.9 g, 23.4 mmol) according to the experi-
mental procedure followed in the synthesis of 1-(p-methoxy-
phenyl)ethanol-1-d1. A 2.8 g (84%) amount of deuterated
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
cis-1, trans-2, cis-3-d1, cis-4-d1 + trans-4-d1, 3-d1, 3-d3, 4, 6,
trans-5-d3, 5-d6, and cis-6-d3 + trans-6-d3 and NOE difference
spectrum for cis-6-d3 + trans-6-d3. This material is available
free of charge via the Internet at http://pubs.acs.org.
J O0006223