Stereochemistry and isotope effects of the [2 + 2] photocycloadditions of arylalkenes to C60. A stepwise mechanism

Full text of DOI:10.1021/ja970916e

7394
J. Am. Chem. Soc. 1997, 119, 7394-7395
Although thermal [π2s + 2a] cycloadditions are symmetry
Stereochemistry and Isotope Effects of the [2 + 2]
Photocycloadditions of Arylalkenes to C60. A
Stepwise Mechanism
π
allowed,14 they occur rarely if at all. In contrast, suprafacial [2
+ 2] photochemical cycloadditions are allowed and proceed
easily with retention of stereochemistry. To test the stereo-
chemistry on the [2 + 2] cycloaddition of arylalkenes to C60,
cis-1-(p-methoxyphenyl)-ethylene-2-d1, cis-1-d1, was prepared
in greater than 96% isomeric purity.15,16 This substrate is ideal
for this purpose because it bears a deuterium cis to the aryl
group, which allows elucidation of the stereospecificity on the
reaction.
Georgios Vassilikogiannakis and Michael Orfanopoulos*
Department of Chemistry, UniVersity of Crete
71409 Iraklion, Crete, Greece
ReceiVed March 24, 1997
In the absence of light, a mixture of C60 and a 200-fold excess
of cis-1-d1 did not react when heated for 12 h at reflux in
deoxygenated toluene. Upon irradiation at λ > 500 nm, with
a Xenon lamp Variac Eimac Cermax 300W, a 30% of the [2 +
2] adduct, as monitored by HPLC, was obtained after 30 min
based on the recovered C60. The adduct, which was stable at
room temperature (no traces of decomposition or cycloreversion
1
Since the discovery of the C60 (buckminsterfullerene) and
its isolation in large quantities,2,3 a remarkable array of its
reactions have been studied in the last six years. Whereas many
[4 + 2]4-6 and [3 + 2]4,7-9 cycloadditions have been reported,
whose monoadducts have been isolated and well characterized,
only a few [2 + 2] cycloadditions to C60 are known10-12 and,
other than product isolation and characterization, little is known
of their mechanism. For example, Foote and co-workers
reported recently that the [2 + 2] photocycloaddition of
ynamines to the triplet excited state13 of C60 possibly proceeds
through a charge transfer mechanism.11c However, as they
stated, attempts to observe transient intermediates have not been
successful.
1
products were detected by H NMR and HPLC after standing
for several days), was purified by flash column chromatography
1
(toluene/hexane 2/1) and characterized by H NMR17 (Figure
1) and FAB-MS [m/z 856 (M + 1, 8), 720 (M - 135, 100)]. A
synchronous cycloaddition is expected to give only one product,
cis-2-d1. However, two doublets of equal intensity due to H1
1
and H1′ were observed in the H NMR spectrum at 4.3 and
4.25 ppm, respectively (JH1H2 ) 10.4 Hz, JH1′H2′ ) 8.3 Hz).
Therefore, the lack of any stereoselectivity mitigates against a
synchronous one-step mechanism for this cycloaddition reaction.
We report here for the first time the stereochemistry and the
secondary isotope effects of a novel [2 + 2] photocycloaddition
between arylalkenes and C60. These results shed light on the
mechanism of [2 + 2] photocycloadditions of arylalkenes to
C60.
(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R.
E. Nature 1985, 318, 162.
(2) Kratschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett.
1990, 170, 167.
(3) Scrivens, W. A.; Bedworth, P. V.; Tour, J. M. J. Am. Chem. Soc.
1992, 114, 7917.
(4) Hirsch, A. Synthesis 1995, 895-913.
(5) (a) Kra¨utler, B.; Puchberger, M. HelV. Chim. Acta 1993, 76, 1626.
(b) Rubin, Y.; Khan, S.; Freedberg, D. I.; Yeretzian, C. J. Am. Chem. Soc.
1993, 115, 344. (c) An, Y. Z.; Anderson, J. L.; Rubin, Y. J. Org. Chem.
1993, 58, 4799. (d) Linssen, T. G.; Du¨rr, K.; Hirsch, A; Hanack, M. J.
Chem. Soc., Chem. Commun. 1995, 103. (e) Khan, S. I.; Oliver, A. M.;
Paddon-Row, M. N.; Rubin, Y. J. Am. Chem. Soc. 1993, 115, 4919.
(6) (a) Gu¨gel, A.; Kraus, A.; Spikermann, J.; Belik, P.; Mu¨llen, K. Angew.
Chem., Int. Ed. Engl. 1994, 33, 559. (b) Iyoda, M.; Sultana, F.; Sasaki, S.;
Yoshida, M. J. Chem. Soc., Chem. Commun. 1994, 1929. (c) Zhang, X.;
Foote, C. S. J. Org. Chem. 1994, 59, 5235.
(7) (a) Suzuki, T.; Li, Q.; Khemani, K. C.; Wudl, F.; Almarsson, O¨ .
Science 1991, 254, 1186. (b) Prato, M.; Suzuki, T.; Wudl, F.; Lucchini,
V.; Maggini, M. J. Am. Chem. Soc. 1993, 115, 7876. (c) Isaacs, L.;
Diederich, F. HelV. Chem. Acta 1993, 76, 2454. (d) Prato, M.; Bianco, A.;
Maggini, M.; Scorrano, G.; Toniolo, C.; Wudl, F. J. Org. Chem. 1993, 58,
5578. (e) Diederich, F.; Isaacs, L.; Philp, D. J. Chem. Soc., Perkin. Trans.
1 1994, 391.
(8) (a) Isaacs, L.; Wehrsig, A.; Diederich, F. HelV. Chim. Acta 1993,
76, 1231. (b) Prato, M.; Li, Q.; Wudl, F.; Lucchini, V. J. Am. Chem. Soc.
1993, 115, 1148. (c) Tokuyama, H.; Nakamura, M.; Nakamura, E.
Tetrahedron Lett. 1993, 34, 7429. (d) Maggini, M.; Scorrano, G.; Prato,
M. J. Am. Chem. Soc. 1993, 115, 9798. (e) Maggini, M.; Scorrano, G.;
Bianco, A.; Toniolo, C.; Sijbesma, R. P.; Wudl, F. J. Chem. Soc., Chem.
Commun. 1994, 305.
(9) (a) Vasella, A.; Uhlmann, P.; Waldraff, C. A.; Diederich, F.; Thilgen,
C. Angew. Chem., Int. Ed. Engl. 1992, 31, 1388. (b) An, Y. Z.; Rubin, Y.;
Schaller, C.; McEivany, S. W. J. Org. Chem. 1994, 59, 2927. (c) Anderson,
H. L.; Faust, R.; Rubin, Y.; Diederich, F. Angew. Chem., Int. Ed. Engl.
1994, 33, 1366.
(10) Hoke, S. H., II; Molstad, J.; Dilattato, D.; Jay, M. J.; Carlson, D.;
Kahr, B.; Cooks, R. G. J. Org. Chem. 1992, 57, 5069.
(11) (a) Zhang, X.; Romero, A.; Foote, C. S. J. Am. Chem. Soc. 1993,
115, 11024. (b) Zhang, X.; Romero, A.; Foote, C. S. J. Am. Chem. Soc.
1995, 117, 4271. (c) Zhang, X.; Fan, A.; Foote, C. S. J. Org. Chem. 1996,
61, 5456.
(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. J. Org. Chem. 1993, 58, 6548.
1
A plausible mechanism that could account for the H NMR
data includes the formation of dipolar or diradical intermediate
in a rate determining step (see below), followed by fast rotation
of the aryl moiety around the former double bond, leading to
the [2 + 2] adducts cis-2-d1 and trans-2-d1. This intermediate
may or may not be preceded by an electron or charge transfer
complex between the triplet excited state of C60 and the alkene.
(14) Hoffmann, R. W.; Woodward, R. B. The ConcerVation of Orbital
Symmetry; Academic Press: New York, 1970.
(15) Alkenes cis-1-d1, 3-d1, and 3-d3 were prepared as follows: Reaction
of 4′-methoxybenzaldehyde with bromomethylenetriphenylphosphorane gave
cis-1-bromo-2-(p-methoxyphenyl)ethylene in >96% isomeric purity. Trans-
metallation of cis-1-bromo-2-(p-methoxyphenyl)ethylene with tert-butyl-
lithium followed by D2O quench gave cis-1-d1 in >96% isomeric purity.
1H NMR (CDCl3) δ 3.81 (s, 3H), 5.11 (d, J ) 10.9 Hz, 1H), 6.66 (td, JHH
) 10.9 Hz, JDH ) 2.6 Hz, 1H), 6.86 (dd, J1 ) 6.8 Hz, J2 ) 2.1 Hz, 2H),
7.35 (dd, J1 ) 6.8 Hz, J2 ) 2.1 Hz, 2H). MS m/z 135 (M+, 100). Reduction
of 4′-methoxyacetophenone with LiAlD4, followed by dehydration gave
1
alkene 3-d1. H NMR (CDCl3) δ 3.81 (s, 3H), 5.11 (d, J ) 0.9 Hz, 1H),
5.6 (dt, JDH ) 2.6 Hz, JHH ) 0.9 Hz, 1H), 6.86 (dd, J1 ) 6.8 Hz, J2 ) 2.1
Hz, 2H), 7.35 (dd, J1 ) 6.8 Hz, J2 ) 2.1 Hz, 2H). MS m/z 135 (M+, 100).
Addition of CD3MgI to the p-methoxybenzaldehyde followed by PCC
oxidation gave 4′-methoxyacetophenone-d3 in good yield. Subsequent
LiAlD4 reduction of 4′-methoxyacetophenone-2,2,2-d3, followed by dehy-
dration gave alkene 3-d3. 1H NMR (CDCl3) δ 3.81 (s, 3H), 6.86 (dd, J1 )
6.8 Hz, J2 ) 2.1 Hz, 2H), 7.35 (dd, J1 ) 6.8 Hz, J2 ) 2.1 Hz, 2H), MS m/z
137 (M+, 100).
(13) Previous results have demonstrated that the triplet excited state of
C60 has a reduction potential near 0.98 eV (36 Kcal/mol) VS SCE and is
formed with a quantum yield of nearly unity: Arbogast, J. W.; Darmanyan,
A. O.; Foote, C. S.; Rubin, Y.; Diedrich, F. N.; Alvarez, M. M.; Anz, S. J.;
Whetten, R. L. J. Phys. Chem. 1991, 95, 11.
(16) Matsumoto, M.; Kuroda, K. Tetrahedron Lett. 1980, 21, 4021.
(17) 1H NMR of the [2 + 2] adduct from 3-d0 and C60: (CS2:C6D6 3:1),
δ 3.62 (s, 3H), 4.22 (m, 2H) 5.46 (dd, J1 ) 10.4 Hz, J2 ) 8.6 Hz, 1H),
7.29 (AA′ MM′, J1 ) 8.6 Hz, J2 ) 3.1 Hz, J3 ) 2.1 Hz, 4H).
S0002-7863(97)00916-5 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 31, 1997 7395
Table 1. Intermolecular Kinetic Secondary Isotope Effects of
Photocycloaddition of 3-d0/3-d3 and 3-d0/3-d1 to C60
b
solvent
substrate
t, min
convn,a %
kH/kD
toluene
toluene
3d0/3d1
3d0/3d3
30
30
30
30
1.08 ( 0.05
0.75 ( 0.05
1
a Based on recovered C60. b Determined by H NMR integration of
the proper hydrogen signals.
well as 3-d0 and 3-d3, were measured by integrations of the
proper 1H NMR signals of the [2 + 2] products. These results
are summarized in Table 1.
A small normal isotope effect (kH/kD ) 1.08) was found for
deuterium substitution at the R-carbon (3-d0 vs 3-d1). The
observed inverse secondary isotope effect kH/kD ) 0.75 between
3-d0 and 3-d3 can be factored into R-secondary, for deuterium
substitution at the â-carbon, and â-secondary, for deuterium
substitution at the R-carbon according to eq 1. Substitution of
Figure 1. 1H NMR spectrum of [2 + 2] adducts cis-2-d1 and trans-
2-d1 from C60 and cis-1-d1.
(kH/kD)â for the value of 1.08 (Table 1) results to (kH/kD)2
)
R
0.69 which corresponds to a (kH/kD)R ) 0.83 per deuterium.
Interruption of the reaction after 30 min and analysis of the
unreacted alkene showed the presence of about 10% of trans-
1-d1 in the recovered cis-1-d1. Furthermore, irradiation at λ >
500 nm of the isolated products cis-2-d1 and trans-2-d1 gave
C60, cis-1-d1 and trans-1-d1 (cycloreversion products). These
results, which eliminate a concerted photocycloaddition, are
consonant with a dipolar or a diradical intermediate via an
electron or charge transfer mechanism.
(kH/kD)observed ) (kH/kD)2 (kH/kD)â ) 0.75
(1)
R
These results require the formation of a dipolar (or diradical)
intermediate in a rate determining step as shown by transition
states TSI and TSII. Substantially inverse R-secondary isotope
effect kH/kD ) 0.83 (normalized per deuterium) indicates
extensive bond making between C60 and Câ of the alkene at
the transition state (Câ is changing from sp2 at the ground state
to sp3 in the transition state). Furthermore, the presence of a
small normal â-secondary isotope effect (kH/kD ) 1.08) in the
competition of 3-d0 with 3-d1 indicates no change in the sp2
hybridization of the CR carbon in going from the ground state
(double bond) to the transition state (sp2 carbocation or radical)
and is consonant with little or no bond making at the CR in the
transition state. The two isotope effects, when taken in
conjuction, exlude the formation of transition state TSIII
(concerted mechanism), because in that case substitution at either
CR or Câ would have given an inverse isotope effect.
To probe this mechanism further and obtain information on
the extent of bond making and bond breaking in the transition
state, we measured the intermolecular secondary isotope effects
of this [2 + 2] photocycloaddition reaction. To this end, we
prepared the alkene 1-(p-methoxyphenyl)ethylene, 3-d0, and its
deuterated analogs 1-(p-methoxyphenyl)ethylene-1-d1, 3-d1, and
1-(p-methoxyphenyl)ethylene-1, 2, 2-d3, 3-d3.15,16
In conclusion, the photochemical [2 + 2] cycloaddition of
arylalkenes to C60 occurs by a two-step mechanism, involving
the formation of a dipolar or diradical intermediate in the rate
determinining step.
Acknowledgment. We thank Dr. K. Prassides for taking the FAB
MS at Sussex University. This work was supported by YΠEP-1995
and NATO (Grant No. 931419). The financial support of M & S
Hourdakis SA. is also acknowledged. Dedicated to Professor G. J.
Karabatsos on the occasion of his 65th birthday.
Equimolar ratios of 3-d0/3-d1 as well as 3-d0/3-d3, in 200-
fold molar excess to C60, were dissolved (in separate experi-
ments) in deoxygenated toluene. Upon irradiation, 30% of the
[2 + 2] adduct was obtained after 30 min, in each reaction,
based on the recovered C60. The reaction was monitored by
HPLC on a reverse phase column. After purification of the
reaction product by flash column chromatography (hexane/
toluene) the secondary isotope effects kH/kD, which are the
results of intermolecular competition between 3-d0 and 3-d1 as
Supporting Information Available: Spectra for cis- and trans-1-
d1, 3-d1, 3-d3, cis- and trans-2-d1, 2-d0 and 2-d0 and 2-d2 (9 pages).
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