Stepwise bond formation in photochemical and thermal Diels-Alder reactions of C₆₀ with Danishefsky's dienes

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.