Tandem nucleophilic addition/Diels-Alder reaction of N-butadienyl N,O-ketene silyl acetals with C-60: Stereoselective formation of bicyclic octahydroquinoline-1,2,3,4-tetrahydrobuckminsterfullerenes and combined NMR spectroscopic and computational evaluation of the functionalization reactions

We have studied the reactivity of the N,O-ketene N-1,3-butadienyl-N-alkyl-O-silyl acetals 1a-e with C-60 proceeding through a tandem process to give the adducts 2a-e, The addition order of these tandem reactions has been evaluated. The initial nucleophilic Michael-like addition of the electron-rich N,O-ketene acetal moiety proceeds unusually fast at 25 degrees C, followed by an intramolecularly accelerated Diels-Alder step that is highly diastereoselective. The structures of compounds 2a-e were determined from the H-1 and C-13 NMR shifts and from II-II coupling patterns, while their stereochemistry was deduced from 2D T-ROESY NMR experiments. The proposed mechanism for the nucleophilic addition involves single electron transfer followed by radical anion-radical cation recombination. Computational investigations of the reaction pathways, transition states, and conformational energies have been carried out to corroborate the experimental data.