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, Shunichi Fukuzumi

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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 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.

Original languageEnglish
Pages (from-to)2236-2243
Number of pages8
JournalJournal of the American Chemical Society
Volume122
Issue number10
DOIs
StatePublished - 15 Mar 2000

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