Electron-transfer-induced Diels-Alder reactions of indole and exocyclic dienes: Synthesis and quantum-chemical studies

Photoinduced electron transfer (PET) initiates radical-cation Diels-Alder r eactions between indole and exocyclic dienes. As catalysts, triaryl-pyryliu m tetrafluoroborates are used. The products are [b]-anellated tetrahydrocar bazoles, which are not accessible through neutral cycloadditions. The resul ting structures are of interest because of their DNA intercalating properti es. their antitumor activity, and their importance as building blocks of nu merous indole alkaloids. When the exocyclic dienes are substituted with an aromatic or heteroaromatic ring, the reaction proceeds with complete regios electivity. The mechanism is discussed in detail and the potential-energy h ypersurface of the radical-cation Diels-Alder reaction is investigated with quantum-chemical methods by variation of the distances of the two newly fo rmed bonds. The potential surface demonstrates the nonsynchronous and nonco ncerted reaction pathway. The energies of the different long-bond intermedi ates, which are local minima on the energy surface, have been calculated wi th semiempirical, molecular orbital, and density functional methods. Regio- and diastereoselectivities can be explained and predicted by comparing the se energies. Quantum-chemical calculations lead to a reaction model that is consistent with the observed products.