Nucleophilic Addition to Diradicals Derived From Cycloaromatization of Maleimide-Based Enediynes

Diradical chemistry, typified by the cycloaromatization of enediynes or enyne-allenes, have been extensively explored due to the involved intriguing biological activity and unique mechanistic actions. Because of the essential Myers-Saito cycloaromatization involved in the maleimide-assisted rearrangement and cycloaromatization (MARACA) mechanism disclosed in our recent work, the important object of diradical/zwitterion dichotomy and the reactivity of thermally induced sigma,pi-diradicals were investigated in this work through the combination of experimental and computational studies. Deuterium incorporation experiments demonstrated that the polar product was afforded from the MARACA strategy, in which the diradical and zwitterion reactivities from the cycloaromatization step could both lead to the closed-shell product via the subsequent nucleophilic addition reaction and protonation. Using the density functional theory calculations, the unusual reactivity of the heterosymmetric diradicals to closed-shell zwitterions was examined, and the addition of carbonyl moiety to alpha,3-dehydrotoluene was allowed to occur via a nonplanar cyclic allene transition structure with a small barrier of 9.1 kcal/mol. The observed nonplanar geometry is essential for the necessarily symmetry-breaking action, resulted in the continuous transformation from open-shell diradical to closed-shell zwitterion species during the addition process.

Automated summary

This study investigated the reactivity of diradical/zwitterion dichotomy and the thermally induced sigma,pi-diradicals with the use of experimental and computational methods. The research demonstrated the transformation from open-shell diradicals to closed-shell zwitterions during the addition process, with a notable nonplanar geometry essential for symmetry-breaking action. The results provided insight into the intriguing biological activity and unique mechanistic actions involved in diradical chemistry.