C O M M U N I C A T I O N S
Scheme 4. Photochemical Cycloaddition of (E,Z)-3 to C60
Scheme 6. Proposed Mechanism for the Formation of syn-7
Scheme 5. Proposed Mechanism for the Formation of syn-4
deuterium-labeled 6sinitially a mixture of E:Z ) 70:30s
completely isomerized to a ratio of E:Z ) 50:50 in the product.
Cycloadduct syn-7 was purified by flash column chromatography
and was fully characterized by all of the above-mentioned NMR
techniques. These results suggest that the [2 + 2] photocyclo-
3
addition of an alkene to C60 proceeds through a photoinduced
3
electron-transfer step between the cyclopropylalkene and C60,
which leads to a biradical intermediate, as shown in Scheme 6.
In conclusion, we have shown here, by using a hypersensitive
molecular mechanistic probe, that photocycloaddition of dienes or
alkenes to C60 proceeds via a biradical (not dipolar) intermediate,
which is preceded by an electron-transfer step between the diene
3
or the alkene and C60.
Acknowledgment. Dedicated to the memory of Professor
Christopher S. Foote. The financial support of the Greek Secretariat
of Research and Technology (PYTHAGORAS II, 2005 and
HRAKLITOS 2002) is acknowledged.
Supporting Information Available: Detailed experimental pro-
cedures and NMR spectra. This material is available free of charge via
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3
geminate radical anion of C60. In this step, an isomerization of
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Coupling of the rearranged radical cation RC3b with radical anion
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photocycloaddition is highly stereospecific since among the 16
possible stereoisomers of 4, only the syn-4 product was formed.
Also, in product syn-4, the thermodynamically most stable E
configuration of the dienyl moiety is formed although the starting
material 3 was a mixture of E:Z ) 70:30 stereoisomers. If the
rearrangement of cyclopropylcarbinyl radical in RC′3a would be
the case, product 5, through its dipolar intermediate I2, would be
expected to be formed (Scheme 5). The former pathway is excluded
since 5 was not observed. Furthermore, we examined the addition
of cyclopropyl alkenes to C60. Similar to that in cyclopropyl diene
3, the photocycloaddition of cyclopropyl alkene 69 to C60 gave the
rearranged five-membered adduct syn-7 (Scheme 6), under similar
experimental conditions. Addition to C60 with the double bond of
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(9) For the preparation of 3 and 6, see the Supporting Information.
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