Stepwise Bond Formation in Photochemical and Thermal Diels−Alder Reactions of C60with Danishefsky's Dienes

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 photoindu...