Tandem Nucleophilic Addition/Diels-Alder Reaction of <i>N</i>-Butadienyl <i>N,O</i>-Ketene Silyl Acetals with C₆₀: Stereoselective Formation of Bicyclic Octahydroquinolino-1,2,3,4-Tetrahydrobuckminsterfullerenes and Combined NMR Spectroscopic and Computational Evaluation of the Functionalization Reactions

Abstract: We have studied the reactivity of the <i>N,0</i>-ketene <i>N</i>-l,3-butadienyl-<i>N</i>-alkyl-<i>0</i>-silyl acetals <b>1a-e</b> with C₆₀ proceeding through a tandem process to give the adducts <b>2a-e</b>. The addition order of these tandem reactions has been evaluated. The initial nucleophilic Michael-like addition of the electron-rich <i>N,O</i>-ketene acetal moiety proceeds unusually fast at 25°C, followed by an intramolecularly accelerated Diels-Alder step that is highly diastereoselective. The structures of compounds <b>2a-e</b> were determined from the ¹H and ¹³C NMR shifts and from H - H 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.