Re-Evaluation of Product Outcomes in the Rh-Catalyzed Ring Expansion of Aziridines with N -Sulfonyl-1,2,3-Triazoles.

N -heterocycles are prevalent in pharmaceuticals and natural products, but traditional methods often do not introduce significant stereochemical complexity into the ring. We previously reported a Rh-catalyzed ring expansion of aziridines and N- sulfonyl-1,2,3-triazoles to furnish dehydropiperazines with excellent diastereocontrol. However, later studies employing ketone-containing carbene precursors showed that [3,9]-bicyclic aziridine formation competes with production of the desired heterocyclic scaffolds. In light of these surprising results, our initial findings were re-examined both experimentally and computationally to reveal how noncovalent interactions and restricted bond rotation in the aziridinium ylide intermediate promote this unexpected reaction pathway.